Radiological Sciences Dictionary

RadiologicalSciencesDictionary

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David J. Dowsett MSc PhDPreviously Chief Physicist at the MaterUniversity Hospital, University CollegeDublin, IrelandCurrently Consultant Medical Physicist tovarious hospitals and veterinary hospitalsin Ireland

RadiologicalSciencesDictionary

DAVID J DOWSETT

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First published in Great Britain in 2009 byHodder Arnold, an imprint of Hodder Education,an Hachette UK Company, 338 Euston Road, London NW1 3BH

http://www.hoddereducation.com

© 2009 David J Dowsett

All rights reserved. Apart from any use permitted under UK copy-right law, this publication may only be reproduced, stored or trans-mitted, in any form, or by any means with prior permission inwriting of the publishers or in the case of reprographic productionin accordance with the terms of licences issued by the CopyrightLicensing Agency. In the United Kingdom, such licences are issuedby the Copyright licensing Agency: Saffron House, 6–10 KirbyStreet, London EC1N 8TS.

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Whilst the advice and information in this book are believed to betrue and accurate at the date of going to press, neither the author[s]nor the publisher can accept any legal responsibility or liability forany errors or omissions that may be made. In particular, (but with-out limiting the generality of the preceding disclaimer) every efforthas been made to check drug dosages; however it is still possiblethat errors have been missed. Furthermore, dosage schedules areconstantly being revised and new side-effects recognized. Forthese reasons, the reader is strongly urged to consult the drug com-panies’ printed instructions before administering any of the drugsrecommended in this book.

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ISBN 978-0-340-94167-6

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Dedication

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Contents

Preface ix

Acknowledgements xi

List of abbreviations xiii

Number Prefix 1

A 3

B 22

C 38

D 69

E 93

F 110

G 129

H 140

I 151

J 176

K 178

Contents

L 182

M 196

N 216

O 224

P 230

Q 259

R 263

S 283

T 315

U 332

V 339

W 344

X 350

Y 354

Z 355

References 357

Radiological Sciences Dictionary ix

PrefaceThe Radiological Sciences Dictionary is meant as an easy referencebook for all hospital staff employed in diagnostic imaging, includingestablished radiologists, nuclear medicine physicians, doctors intraining, physicists, radiographers and technicians. Having under-stood the basic meaning and application of any particular keyword,then wider knowledge should be gained by consulting specialistbooks. Keywords in the book are linked so that a particular topiccan be followed by reference to all relevant keywords. This isdesigned to aid revision and also act as a guide when exploring aparticular subject. In many instances, keywords are further definedby showing worked examples. A particular word may be associatedwith a given topic (e.g. mammography, digital image filtering, radi-ation dose measurement, etc.). Relevant words are then either insmaller sans serif font when used in the descriptive text or identi-fied at the end of the word’s description. The author has attemptedto add brief details of clinicians and scientists (alive and dead) whohave, over the years, made significant contributions to diagnosticimaging. Apologies are given for unintended omissions.

Since new words are regularly being applied to the radiologicalsciences, recommendations and suggested additions would begratefully received by the publishers. Errors may have made theirappearance and the publishers would also welcome corrections andconstructive comments. All material used will be recognized infuture editions of the dictionary.

DJDOctober 2008

Disclaimer: Drugs, contrast media, radiopharmaceuticals and othercompounds for human use, identified in this dictionary, may not berecognized by individual countries. Relevant national drug adminis-trations and organizations should be consulted.


Radiological Sciences Dictionary xi

AcknowledgementsWhen compiling this dictionary, I had many discussions withexperts in the various disciplines. Amongst the doctors and scien-tists I wish to thank for their useful comments and advice are GregFoley, Cari Borràs, Roger Price, Ng Kwan Hoong and Sarah Walker. Iam indebted to Susie Bond and Alyson Thomas for their help andadvice when editing the manuscript.


Radiological Sciences Dictionary xiii

ABC automatic brightness controlac alternating currentACPI advanced configuration and power

interfaceADA alternated delay acquisitionADC apparent diffusion coefficient;

analogue to digital converterADRF adiabatic demagnetization in the

rotating frameADSL asymmetric digital subscriber lineAFP adiabatic fast passageAGP accelerated or advanced graphics portAIUM American Institute of Ultrasound in

MedicineALARA as low as reasonably achievableALARP as low as reasonably practicalALI annual limit on intakeAPI application program interfaceARP adiabatic rapid passageASA American Standards AssociationASCII American Standard Code for

Information InterchangeASIS aromatic solvent-induced shiftATM asynchronous transfer modeBBB blood–brain barrierBGO bismuth germanateBq becquerelCBF cerebral blood flowCFOV central field of viewCi curieCIN contrast-induced nephropathyCOM component object modelCPU central processor unitCR computed radiographyCSI chemical shift imagingCT computed tomographydB decibelsDGC depth gain compensationDPFGSE double pulsed field gradient spin-echoDVT deep vein thrombosisECRP endoscopic retrograde cholangiopan-

creatographyEDTMP ethylene-diamine-tetramethylene-

phosphonateEIDE enhanced integrated drive electronicsELR excess lifetime riskemf electromotive forceERC endoscopic retrograde cholangiographyFDA Food and Drug AdministrationFFD focus-to-film distance

FOV field of viewFPF false positive fractionFS focal spotFSE fast-spin echoGFR glomerular filtration rateGI gastrointestinalGy grayHSE Health and Safety ExecutiveHU heat unitIAEA International Atomic Energy AuthorityIEC International Electrotechnical

CommissionIFD image file directoryIR infraredITLC instant thin layer chromatographyIVC intravenous cholangiographyIVC inferior vena cavakW kilowattLAR lifetime attributable riskLLE loss of life expectancyLOR line of responseLUT look up tableMB megabyteMiB mebibyteMIP maximum intensity projectionMIRD medical internal radiation doseMP magnetization preparationMRA magnetic resonance angiographyMRCP magnetic resonance cholangiopancre-

aticographyms millisecondsMSA multiple system atrophyMTF modulation transfer functionNEMA National Electrical Manufacturers

AssociationNIC network interface cardNIST National Institute of Standards and

TechnologyNMR nuclear magnetic resonanceNRPB National Radiological Protection Boardns nanosecondsOD optical densityOFD object-to-film distanceOLE object linking and embeddingOTR osmotoxicity ratioPD Parkinson’s diseasePDA personal digital assistantsPDF probability density functionPOP Post Office protocolPRP pulse repetition period

List of abbreviations

xiv Radiological Sciences Dictionary

PSP progressive supranuclear palsyPTC percutaneous transhepatic cholan-

giographyRad radiansRBC red blood cellsRBE relative biological effectivenessRCP radiochemical purityREID risk of exposure-induced deathRF radiofrequencyRMS root mean squareROC receiver operating characteristicROI region of interestRPTL reverse phase thin layer chromatographySAR specific absorption rateSCLC small cell lung cancerSCSI small computer systems interfaceSDP slice dose profileSID source to image distance

SNR signal-to-noise ratioSPECT single photon emission computed

tomographySPGR spoiled gradient recalledSPT selective population transferSPTA spatial peak time averaged intensitySSR surface-shaded reconstructionsSTP standard temperature and pressure;

shielded twisted pairSv sievertSVC superior vena cavaSVS single volume spectroscopyTA Time-averaged powerTAD time of analogue to digital conversionTGC time gain compensationTMS tetramethylsilaneTPF true positive fractionTSE turbo-spin echo

Radiological Sciences Dictionary 1

Number prefix

over UTP wiring using a star configuration witha hub at the centre (see 100BASE-T).

10BASE2 (clin) An IEEE 802.3 media standard fora 10 Mbits per second baseband, 185 m persegment ethernet LAN.

10BASE5 (clin) An IEEE 802.3 media standard fora 10 Mbits per second baseband, 500 m persegment ethernet LAN.

10-day rule (dose) An early recommendation fromthe ICRP for examinations involving the lowerabdomen of females. Radiation exposure is leastlikely to pose any hazard to a developing embryoif carried out during the 10-day interval followingonset of menstruation.

28-day rule (dose) Patients should be routinelyasked if there is any chance they may be preg-nant. If the answer is ‘No’, the radiologicalexamination can proceed; if there is any uncer-tainty, the clinician or technologist is asked tocheck the date of the last menstrual period. Ifthis is overdue, the examination should bedelayed. This is the so-called ‘28-day rule’.

64-bit architecture (comp) This enables 64-bitcomputing on server, workstation, desktop andmobile platforms when combined with supportingsoftware. Improves performance by allowingsystems to address more than 4 GB (gigabytes)of both virtual memory and physical memory.Currently most processors for server and work-station platforms support 64-bit computing.With the introduction of dual core processors, mostdesktop and workstation processors are also64-bit capable and provide 64-bit support for:

● address space;● general purpose registers;● integer size.

They also allow up to one terabyte (TB) of addressspace, AMD multi-core Opteron® and Athlon® 64processors are currently available (see Pentium).

100BASE-T (comp) The IEEE 802.3u standardEthernet specification for 100 Mbps (fast ethernet) transmission using UTP cable (see10BASE-T).

100BseOT (comp) Also called fast ethernet, it is a 100 Mbps (megabits per second) baseband

2D display (ct) Visual display of a 2D distribution,e.g. the reconstructed axial image of a com-puted tomography (CT) scan displaying CT val-ues as grey-scale information.

3D display (ct) Visual display of a 3D distributionby combining 2D axial slice data. Various meth-ods such as shading, movement or perspectiveare used to create a subjective 3D impression.

2D-FFT (mri) Two-dimensional fast Fourier trans-form (FT). The standard image reconstruction forthe axial plane from the phase and frequencyencoding data using a fast Fourier Transform.

2π-geometry (phys) The geometrical efficiency of aflat surface detector (gamma camera).

3D-FFT (mri) The 2D-FFT can be extended to a 3Dsequence by exciting the whole sample volumewithout using a slice gradient.

3D-MPRAGE (mri) 3D magnetization preparedrapid gradient echo. T1-weighted contrast fromSiemens (see FLASF, SPGR, FSPGR, HFGR, RE

spoiled, T1-FEE, STAGE-T1W).3D GRE (mri) 3D gradient echo.3D MP (mri) 3D magnetization prepared rapid.3D slab (mri) Measurement MR volume for 3D

imaging. The 3D slab is divided into partitions.3D imaging (mri) An entire measurement volume

using the 3D slab instead of single slices.Additional phase encoding in the z-axis (slice)direction provides information.

3-Phase supply (phys) The distributed a.c. powersupply having three phases separated by 120°.The 3-phase European voltage is 440 V the USvoltage 230 V. Equipment with high electrical

rating (CT, MRI and most fixed x-ray generators).3D shim (mri) Limits the local shim defining a 3D

volume. The local magnetic field distribution isthen determined in this volume. More precisethan MAP shim providing better fat saturationand more precise spectroscopy.

3D Turbo SE (mri) TurboSE as a 3D sequencegives T2-weighted images with thin slices andalmost isotropic voxels.

4π-geometry (phys) The geometrical efficiency of aspherical detector (see dose calibrator).

10BASE-T (comp) The IEEE 802.3I standardspecification for 10-Mb ethernet transmission

2 Radiological Sciences Dictionary

100BseOT–510k

network standard. It supports different types ofcabling: 100BaseTX uses twisted pair, while100BaseFX uses fibreoptics.

180° interpolation (ct) A type of algorithm for z-axis interpolation which utilizes a range of2 � 180° projection angles within the meas-ured spiral CT data (see z-interpolation).

360° interpolation (ct) A type of algorithm for z-axis interpolation which utilizes a range of2 � 360° projection angles within the meas-ured spiral CT data (see z-interpolation).

510k (legis) See FDA 510k.

A


A2–acceleration (linear)

AA2 (stats) The area under an ROC (receiver operat-

ing characteristic) curve, often used as an indexof detectability or diagnostic accuracy. Strictly,A2 refers only to ROCs with binormal form, butoccasionally it is used more loosely to indicatethe area under any ROC curve. Perfect A2 � 1.0and random decisions correspond to A2 � 0.5.

Az (stats) The area under an ROC curve, useful asan index of detectability or diagnostic accuracy.Strictly, this refers only to ROC curves withbinormal distribution (see binormal ROC curve,but more loosely can describe the area underany ROC curve. Values of Az and the index da

are related by the expression ,where Φ represents the cumulative standard-normal distribution function. For perfect detec-tion Az � 1.0 and for purely random decisionsAz � 0.5. Az can be used as the value indicatinga test sensitivity for a certain specificity rangingfrom 0 to 1, or, as the value of t specificity for acertain sensitivity ranging from 0 to 1.

A-mode (us) A display that gives echo strengthversus time of arrival (distance). A display pres-entation of echo amplitude versus depth (usedin ophthalmology).

ABC (xray) Automatic brightness control (see auto-

matic exposure control).α/β ratio (dose) The radiation dose at which the

linear and quadratic components of cell killingare equal; a measure of the cell survival curveand a measure of sensitivity of a tissue ortumour to fractional radiation doses.

abdominal aortography (clin) Radiography of theabdominal aorta including its major branchesfrom the diaphragm to the bifurcation. The pelvicarteries are often investigated at the same time(see percutaneous abdominal aortography).

abscissa (math) The horizontal x-axis of a graphusually holding the independent variable. Thevertical or y- co-ordinate is called the ordinate.

absorbed dose (D) (dose) The definition given byICRP60 is the mean energy imparted byionizing radiation to matter (e.g. the body) ofmass m in a finite volume V. The fundamentaldose quantity given by

where is the mean energy imparted by ionizing radiation to the matter in a volume

dε

Dm

�dd

ε

ε

A daz � Φ( / )2

element of mass dm. The SI unit for absorbeddose is joule per kilogram (J kg�1) and it istermed gray (Gy). The earlier definition referredto a point dose, but the ICRP60 revised defini-tion is the average dose over the tissue volume.Conversion factor: The gray is equivalent to100 rads; 1 rad is equivalent to 10 mGy.

absorbed fraction (dose) The fraction of any givenradiation absorbed by a target. Used in the MIRD(medical internal radiation dose) scheme.

absorption (phys) The complete loss of energy inan absorbing material (chem) incorporatedwithin a substance (liquid or solid) (see adsorp-

tion, accretion).absorption coefficient (phys) (see linear absorp-

tion coefficient).absorption mode (mri) Component of the signal

that yields a symmetric, positive-valued lineshape.

abundance (nuclear) (chem) The ratio of the num-ber of atoms of a specific nuclide to the totalnumber of atoms of all the natural nuclides pres-ent (sometimes expressed as a percentage).Hydrogen is a mixture of two isotopes 1H(99.9852% abundant) and deuterium 2H (0.0148%abundant); both are stable. Potassium has threenaturally occurring isotopes: two stable 39K(93.1%) and 41K (6.88%) and the unstable, butlong-lived, isotope 40K (�0.08% abundant).(nmed) The fractional abundance of gamma pho-tons actually emitted (maximum 1000) dependson a variety of secondary reactions; internal con-

version being the most important.ac (alternating current) (phys) An electrical

power supply where magnitude of the electro-motive force (emf) and current varies with time(alternates with time). A complete alternationbeing a cycle and the number of cycles per sec-ond (hertz) is the frequency. European fre-quency is 50 Hz, North America is 60 Hz. Highfrequency x-ray generators operate at 5–20 kHz(see emf, generator).

acceleration (angular) (α) (phys) Acceleration(angular) is the rate of change in angular veloc-ity; related to acceleration (linear) (a) by α � a/r. Itis a vector quantity and has dimensions of T�2

given in units of degree s�2, revolutions s�2 orrad s�2.

acceleration (linear) (phys) Rate of change invelocity with respect to time. Acceleration (a) ismeasured in metres per second per second and is a vector quantity, where a � velocity


acceleration (linear)–acoustic noise

A change/time taken. This is relevant to anode

construction design; a smaller mass anode willreach operating speed faster (see momentum,velocity (linear)).

acceleration factor (mri) A factor by which fasterimaging pulse sequences, such as multipleecho imaging, reduce total imaging time com-pared to conventional imaging sequences, suchas spin echo imaging.

accelerator (nmed) A device that acceleratescharged particles (e.g. protons, electrons) tohigh speed in order to produce ionization ornuclear reactions in a target; often used for theproduction of certain radionuclides or directlyfor radiation therapy. The cyclotron and the lin-ear accelerator (LINAC) are types of accelerators.

ACCESS® (comp) A database program packagesupplied by Microsoft.

access time (comp) Time taken for a device toaccess data. Quoted in milliseconds (ms) for hard

disks and nanoseconds (ns) for memory. Thetiming is usually an average value. Togetherwith transfer rate, it measures the performanceof hard disks.

accretion (chem) A surface reaction where achemical compound unites with a surface (bonescan agent on bone surface) and does not pen-etrate further, so can be detached.

accuracy (math) Description of how a series ofmeasured values is close to the true value. Thecloseness of a measured value, m, of a quantityto the true value, t. Measured as the percentageof the difference as (m � t )/t � 100 and givenas a percentage error.

based on triiodinated benzoic acid as 3-(acetyl-amino-2,4,6, triiodobenzoic acid. Salts: sodiumor meglumine acetrizoate (see Urokon).

achromatic (image) Without colour (see colour scale).acoustic absorption (us) The loss of sound

energy by attenuation and scatter.acoustic absorption coefficient (αa (us) The over-

all attenuation coefficient α � αa � αs (the sum ofabsorption and scatter coefficients); the unitsare either m�1 or cm�1. Also called intensityabsorption coefficient. The absorption coeffi-cient varies with frequency. The frequencywhere absorption is maximum is called therelaxation frequency.

acoustic amplitude (us) This is measured fromthe zero (crossover) point of the sine-wave to itspeak in mV or μV.

acoustic frequency (f) (us) The compression andrarefaction events translated as a sine wavewhose frequency range is 2.5–15 MHz in clini-cal ultrasound.

acoustic impedance (Z) (us) The product ofmaterial density and speed of sound in amedium. The unit is kg m�2 s�1, also termedthe rayl, and is usually calculated as:

where κ is compressibility ρ the material den-sity and c the speed of sound in the medium.Some values are:

Material Speed of α (dB cm�1) Z (kg m�2 s�1)(�10�6) sound at 1 MHz

(m s�1)

Air 330 0.0043Water 1492 0.002 1.48Fat 1470 0.6 1.38Blood 1570 0.18 1.61Brain 1530 0.85 1.55Soft tissue 1500 1.0 1.63Liver 1549 0.9 1.65Muscle 1560 1.2 1.65Eye lens 1620 2.0 1.85Bone 4080 20.0 6.1

(see compressibility).acoustic intensity (us) The power per unit area

as W or mW cm�2.acoustic noise (mri) Gradient coil vibrations creat-

ing sound waves. Caused by the interactions ofthe pulsed magnetic field of the gradient coilwith the main magnetic field. (us) Sound pres-sure registered on a logarithmic scale assound-pressure level in decibels (dB).

Z c� �ρρκ

AccuracyRepeated dose readings from a calibrated source of110.8 MBq give measured activities of 100.8, 100.3,99.6, 100.1, 100.8 MBq. The average measured activityis therefore 100.3 MBq; the percentage error is then(110.8� 100.3)/110.8 � 9.4% (an accuracy of 90.6%).

Precision does not necessarily imply accuracysince if a true reading of exactly 5.0 is measuredgiving a series of very high precision readings of4.9876, 4.8631 and 5.2144, then the result,though precise, is nonetheless very inaccurate.A less precise measurement which returns: 5.0,5.0 and 5.0 may be less precise but very accu-rate. (see diagnostic accuracy, quality control).

acetrizoic acid/acetrizoate (derivatives) (cm)The first water-soluble x-ray contrast medium

A


acoustic output (limits)–actinides/actinoids

acoustic output (limits) (us) The Food and DrugAdministration (FDA) (1976) recommended thefollowing acoustic power limits:

Value Heart Opth Fetus

ISPPA (Wcm�2) 190 28 190ISPTP (Wcm�2) 310 50 310ISPTA (mWcm�2) 430 17 94ISATA (mWcm�2) 430 17 94

(see ultrasound intensity (pulsed)).acoustic parameters (us) Pressure density, tem-

perature, impedance.acoustic power (us) The sound energy trans-

ferred per unit time through the whole crosssection of the beam; measured in watts m�2.Time averaged power (TA) equals:

Potential maximum practical values:

Display mode TA (mW) ISPTA (mWcm�2)

B-mode 350 1000M-mode 350 �1000Duplex (pulsed �400 �1000

Doppler)CW Doppler 25–90 20–600

The spatial peak time averaged intensity (ISPTA) is the time-averaged power relating peakpower to pulse width and duty factor (see ultra-

sound intensity (pulsed)).acoustic pressure (p) (us) The value of the total

pressure minus the ambient pressure. Thepressure difference from normal pressureinduced by sound wave. Unit is the pascal;1 Pa � 1 N m�2 or bars, where 1 Pa � 10 μbar;a typical range is 0.06–1.5 Mpa.

acoustic reflection coefficient (R) (us) If theincident radiation is perpendicular to the sur-face then:

If the incident radiation is angled (angle of inci-dence and angle of reflection �90°) then theacoustic reflection coefficient is dependent onthe angle of incidence:

acoustic transmission coefficient (T) (us)The unreflected ultrasound beam passes as a

R ��

�

� �

� �

Z cos Z cos Z cos Z cos

i t

i t

2 1

2 1

2θ θθ θ

⎛

⎝⎜⎜⎜⎜

⎞

⎠⎟⎟⎟⎟

RIIr

i� �

�

�

( )( )Z ZZ Z

2 1

1 2

2⎛

⎝⎜⎜⎜⎜

⎞

⎠⎟⎟⎟⎟

Total power per frameFrame duration

transmitted beam. The efficiency of transmis-sion of the incident beam at right angles to asmooth surface:

It is related to the reflection coefficient as:T � (1 � R).acoustic variables (us) These include pressure,

density, temperature and particle motion; thequantities that vary with location and time in asound wave.

ACPI (comp) Advanced configuration and powerinterface. The latest standard that specifieshow computer power can be efficiently man-aged. Features include activating devices onlywhen required and reducing clock speed whenfull processing power is not needed.

acquisition matrix (image) The number of inde-pendent data samples in each direction. Forsymmetric sampling, will roughly equal theratio of image field of view to spatial resolution

along corresponding directions (depending onfiltering and other processing). May be asym-metric and of different size than the size of thereconstructed image or display matrix. (mri)The number of independent data samples inthe phase-encoding and frequency-encoding

directions.acquisition time (A) (mri) The time taken to

acquire image frame using the spin echo

sequence. Depends on:

● number of phase encoding steps M (matching thematrix size);

● signal averaging N (usually two but can begreater);

● time to repeat, TR.

for a 256 � 256 matrix M � 256, so for a sig-nal averaging of 2 and TR � 500 ms, the acqui-sition time is 4.2 minutes.

acquisition window (mri) Time in pulsesequence during which the signal is recorded.This is sometimes called the TAD (time of ana-logue to digital conversion).

actinides/actinoids (elem) The group of ele-ments in the periodic table in the atomic number range 90 (thorium) to 103 (lawren-cium). It includes elements useful to radiology,

A M N TR= × ×

TIIt

i� �

�

�4Z ZZ Z( )

1 2

1 22


actinides/actinoids–adaptive technology

A such as: 92 (uranium), 95 (americium) and 98(californium).

Z Element

89 Actinium90 Thorium91 Protactinium92 Uranium93 Neptunium94 Plutonium95 Americium96 Curium97 Berkelium98 Californium99–103

activation analysis (phys) A method of chemicalanalysis used for the detection of very smallconcentrations of elements in a sample.Following nuclear (neutron) bombardment, cer-tain atoms are rendered radioactive and maybe detected by their characteristic gamma radi-ation (peaks).

active shielding (mri) Magnetic shielding throughthe use of secondary shielding coils designed toproduce a magnetic field that cancels the fieldfrom primary coils in regions where it is notdesired. These coils may be inside the magnetcryostat. Can be applied to the main magnet or tothe gradient magnetic fields. This also applies tothe gradient field system using counterwoundcoils to reduce eddy currents (see magnetic shield-

ing, self-shielding and room shielding).active shimming (mri) Shimming or adjusting

the homogeneity of the main magnet by varyingthe currents in shim coils.

active transport (nmed) Passage of an agent(radiopharmaceutical) across a cell membraneby being incorporated into an energy depend-ent metabolic process (iodine uptake in thethyroid, 99mTc uptake in salivary glands) (seepassive transport).

ActiveX (comp) A set of interactive technologiesdeveloped by Microsoft, combining OLE (objectlinking and embedding) and COM (componentobject model). Unlike Java, ActiveX is not a pro-gramming language, but a set of instructionson how an object should be used.

activity (dose) Average number of spontaneousnuclear transformations per unit time. Measuredas the becquerel (Bq) in disintegrations per sec-ond or one radioactive transformation per sec-ond (1 s�1 or one disintegration per second)(ICRP60). The non-SI unit is the curie (Ci) where

1 Ci 3.7 � 1010Bq 37 GBq1 mCi 3.7 � 107Bq 37 MBq1 μCi 3.7 � 105Bq 37 kBq1 nCi 3.7 � 102Bq 370 Bq1 Bq 2.703 � 10�11Ci 27.03 pCi1 kBq 2.703 � 10�8Ci 27.03 nCi1 MBq 2.703 � 10�5Ci 27.03 μCi1 GBq 2.703 � 10�2Ci 27.03 mCi1 TBq 2.703 � 101Ci 27.03 Ci

activity concentration (unit) Measured in Bq m�3.A conversion factor exists for non-SI unitswhere 1Ci L�1 � 3.7 � 1013 Bq m�3.

activity-time curve (nmed) The plot of activityper unit time obtained from a region of interest(ROI) for a set of dynamic frames.

AcuTect® (nmed) Commercial preparation of apci-

tide (Diatide Inc).Acutest® (nmed) See apcitide.ADA (mri) Alternated delay acquisition.adapter (comp) The device that connects a piece of

equipment to the network and controls the elec-trical protocol for communication with that net-work; also called network interface card (NIC).

adaptive array (ct) Commercial detector array;those with detector elements of unequal width(the adaptive array detector), a major drawbackwith the adaptive array design. Narrow detectorelements are close to the centre; the width of thedetector rows increases with distance from thecentre. The adaptive design avoids dead spaces;with the prepatient collimator a typical slice com-bination would be: 2 � 0.5 mm, 4 � 1 mm, 4 �2.5 mm, 4 � 5 mm, 2 � 8 mm and 2 � 10 mm.

(see linear array).adaptive response (dose) A cellular response to

radiation which increases the resistance of thecell to a subsequent radiation exposure.

adaptive technology (comp) An Intel technology(supported in adapters and switches) that auto-matically and dynamically customises productperformance to match network-operating con-ditions, thus helping to optimize network performance.

Collimation

4 � 5.0 mm

4 � 1.0 mm

2 � 0.5 mm

2 � 8.0 mm

2 � 10.0 mm

A


ADC (A to D conversion)–agreement state (USA)

ADC (A to D conversion) (di) See analogue to dig-

ital converter.A to D conversion (ct) See analogue-to-digital

converter.ADC map (mri) Apparent diffusion coefficient.

Images are acquired with two or more b-values.Using equation e�(bD) then D is computed foreach point in the image to give the ADC map,which removes artefacts in a T2-weighted image

called ’T2 shine through’ an emphasis whichcan obscure the diffusion image.

Adenoscan® (nmed) A cardioactive drug manu-factured for use as an adjunct to 201Thalliummyocardial perfusion scintigraphy. Its effectivehalf-life is about 10 s.

added filtration (ct) Thin metal sheets (com-monly aluminium or copper) used for increas-ing the filtration of the x-ray spectrum emittedby the x-ray source in addition to the intrinsic

filtration.adiabatic change (phys) Expansion or compres-

sion of a gas where no heat enters or leaves thesystem. Adiabatic expansion therefore cools thegas and is used as a method for gas liquefac-tion (helium recycling in superconducting mag-nets) (see cryogen).

adiabatic fast passage (AFP) (mri) Rotation ofthe macroscopic magnetization vector by sweepingthe frequency of the radiofrequency (RF) pulseor strength of the magnetic field through reso-nance (Larmor frequency) in a time period whichis short compared to the relaxation times. Usedfor spin inversion.

adiabatic rapid passage (mri) See adiabatic fast

passage.adrenal arteriography (clin) Selective injection

of the adrenal arteries with iodine contrastmaterial. Complete demonstration of both adre-nal glands usually requires selection of thethree supplying arteries on each side: thesuperior adrenal arteries, the middle adrenalarteries and the inferior adrenal arteries.

adrenal venography (clin) Radiography usingselective retrograde injection of contrast materialinto the adrenal vein.

ADRF (mri) Adiabatic demagnetization in therotating frame.

ADSL (comp) Asymmetric digital subscriber line.Generally referred to as DSL. Use standard tele-phone lines and narrow frequency bandsachieving 8 Mbps for ADSL version 2� at dis-tances 4 km from the server.

adsorption (phys) The taking up of a substanceon the surface of another (see absorption).

adverse reactions (contrast medium) (cm) Ioniccontrast materials associated with adversereactions in patients. Most of these reactionsare mild and include sneezing, coughing, rhini-tis, conjunctival edema, mild urticaria, pruritus,vomiting and light-headedness. Severe reac-tions occur in approximately 1 of 500 injectionsof ionic contrast medium. The death rate associ-ated with ionic contrast material injectionranges from 1 in 40 000 to 1 in 100 000 injec-tions (see toxicity).■ Reference: Dawson and Clauss, 1999.

aerosol (nmed) An air suspension of particulatematerial ranging in size from 0.2 to 2.0 μmmedian diameter. When labelled with aradionuclide, it is used as a gas substitute inlung ventilation scintigraphy (see Technegas®).

a.f. (phys) See audio frequency.a.f.c. (phys) See automatic frequency control.AFP (mri) See adiabatic fast passage.afterglow (xray) Prolonged emission of light, a

feature of phosphorescence, but a degradingfeature in fluorescent phosphors (detectors andintensifying screens). (ct) Temporal signal decay

after a short radiation pulse. The sinteredceramic composite phosphor used in ceramic

detectors, chosen for multislice detectors have arapid decay time of 106 s. Luminescence after-glow should be �0.1% at 100 ms after x-raycessation; too high a value results in imageblurring.

a.g.c. (us) See automatic gain control.aggregate (stats) The value of a single variable

resulting from the combination of data for anumber of variables.

AGP (comp) Accelerated or advanced graphics port.Most computers are provided with a number ofPCI slots for adding external hardware. The AGPslot will, however, award special priority, partic-ularly video-processing speed. An extremely fastexpansion slot and bus design for high perform-ance graphics cards. Provides hardware functionsfor two-dimensional and three-dimensional sothe host central processor unit (CPU) has lessinvolvement with image production. 2 � AGP haspeak transfer rates of 512 MBytes/s and 4 � AGPsupplies 1.1 GBps (see graphics card).

agreement state (USA) (nmed) any state whichthe US Nuclear Regulatory Commission or theAtomic Energy Commission has concluded an


agreement state (USA)–aliasing

A effective agreement under Section 247b of theAtomic Energy Act of 1954.

air (material)

Effective atomic number (Zeff) 7.78Density (ρ) kg/m3 1.293 kg/m3

In addition to water vapour, the composition ofair at sea level is:

Gas Percentage

Nitrogen 78.08Oxygen 20.95Argon 0.93Carbon dioxide 0.03Neon 0.0018Helium 0.0005Krypton 0.0001Xenon 0.00001

air gap (xray) A technique which maintains a gapbetween subject (patient) and image surface(film cassette). This reduces scattered radiationreaching the film surface, but increases geo-metrical unsharpness.

air-kerma (dose) The energy released from all ion-izing events in a volume of air. The unit is C kg�1

or Gy. The quotient of dE/dm, where dE is thesum of the initial kinetic energies of all the chargeparticles produced by the ionizing event in amass of air dm. The quotient of dEtr/dm, wheredEtr is the sum of initial kinetic energies of all thecharged ionizing particles liberated by unchargedionizing particles in a mass of air dm (ICRU, 1980).Examples of typical air kerma rates are:

90 kV x-rays at 1 m 43–52 μGy mA s�1

28 kV x-rays 90 μGy mA s�1

Nuclear medicine patient at 1 m 16 μGy � h�1

(bone scintigram)

(see kerma, absorbed dose).air scan (ct) Test procedure with no object in the

gantry; used for checking uniformity in detectorchannel sensitivities and also providing basicdata set for calibrating the detectors.

ALARA (dose) As low as reasonably achievable.Making every reasonable effort to reduce radi-ation levels below the stated dose limits, togive adequate safety and appropriate diagnos-tic accuracy at reasonable cost and recognizingsocial factors. (us) For diagnostic ultrasound,combinations of thermal or mechanical indices anddwell times are considered optimum combinationof acoustic output and dwell time, needed toachieve the required diagnostic information(see ultrasound safety, optimization).

ALARP (dose) As low as reasonably practical. Thelimiting factor being existing facilities due forupdate or replacement (unofficial term).

albumin (nmed) Serum albumin constitutes 55%of blood plasma. Soluble in water but formsinsoluble coagulate when heated.

Generic name 125I-RISACommercial names Isojex®

Nonimaging category Plasma volumeGeneric name 99mTc-HAS

99mTc-MAACommercial names LyoMAA¨

Imaging category Blood pool

Albumoscint® (nmed) Bristol Myers Squibb ver-sion of albumin for labelling.

algebraic reconstruction (di, ct) Historically theoldest image reconstruction method based ondata cells with scanning movements in a finitenumber of equally spaced angles. An iterativeimage reconstruction cycle corrects horizontal,diagonal and vertical estimates (see back projection).

algorithm (stats) A set of well-defined rules orstatements as a result of a systems analysisforming the plan for a computer program.

ALI (dose) See Annual limit on intake.aliasing (di) An artefact resulting from the viola-

tion of Shannon’s equations requirements; signalcontributions with higher frequencies mayaffect the frequency range below the Nyquist

frequency in the form of aliasing artefacts. Therecorded signal will appear to be at a lower fre-quency. The diagram shows three samplingrates (A) exceeds the Nyquist frequency, (B) is atthe Nyquist frequency and (C) is sampled belowthe Nyquist frequency which shows aliasing.

Sampling frequency

A


aliasing artefact–241Americium (Am)

aliasing artefact (ct) An artefact resulting fromthe violation of Shannon’s sampling theorem

requirements; signal contributions with higherfrequencies may affect the frequency rangebelow the Nyquist frequency in the form of alias-ing artefacts in the computed tomography (CT)axial image. The display artefact due to aliasingwithin the measured projections; appears inthe reconstructed images as fine streaks andweb-like patterns. There is a loss of fine detail(petrous bone) in CT images. (mri) Seen whenthe measurement object is outside the field of

view (FOV), but still within the sensitive volumeof the main coil. The consequence of samplingin which any components of the signal that areat a higher frequency than the Nyquist limitwill be ‘folded’ in the spectrum so that theyappear to be at a lower frequency. In Fouriertransform imaging this can produce an appar-ent wrapping around to the opposite side of theimage of a portion of the object that extendsbeyond the edge of the reconstructed region.(us) In spectral and pulsed Doppler, determinesupper limit of Doppler signal frequencies. False Doppler shift information from a pulsed-Doppler or colour-flow instrument when trueDoppler shift exceeds half the pulse repetitionfrequency.

alkali metals (elem) Elements of group 1 in theperiodic table: lithium, sodium, potassium, rubidium,caesium and francium. All are highly reactive.

Almén, Torsten. Swedish radiologist who in1968 developed non-ionic contrast media byreplacing the ionic carboxyl radical in triiodi-nated benzoic acid derivatives. Metrizamide

(Amipaque) was the first non-ionic contrastagent. Second generation products are iopami-

dol, iohexol and Hexabrix (ioxaglate).alpha (α) (phys) Symbol used for absorption, alpha

particle, angular acceleration.alpha-decay (nmed) A process of radionuclide

decay, where the nucleus loses two neutronsand two protons in the form of a heliumnucleus:

alpha particle (α) (nmed) At the moment of emis-sion, an alpha particle consists of the nucleus ofthe helium atom or which is a4 2He +

24He

Example: Ra Rn ( MeV)88226

86222

24 4 78→ α� .

ZA

ZAX Y→ �

� �24

24α

tightly bound group of two protons and twoneutrons. Alphas have a mass of 4 amu and acharge of 2. As an alpha particle slows downthrough matter, it acquires two orbital electronsto become a neutral atom of helium . Thealpha particle forms a charged beam in cyclotron

reactions.aluminium (Al) (elem)

Atomic number (Z) 13Relative atomic mass (Ar) 26.98Density (ρ) kg/m3 2710Melting point (K) 932Specific heat capacity J kg�1 � K�1 913Thermal conductivity W m�1 � K�1 237K-edge (keV) 1.56Relevance to radiology: Filter material for x-ray tube

a.m. (phys) See modulation (amplitude).AMBER (xray) A technique used in chest radiogra-

phy which varies exposure to each part of thechest so that the image receives optimum pho-ton density for both the lung fields and medi-astinum. The AMBER system projects 21adjacent collimated beams in a linear arraybacked by its own detector. Each segment ofthis horizontal array is modulated by the inten-sity reaching the detector; in the most radio-opaque area (mediastinum) receives the mostexposure and the least radio-opaque area(intercostal lung) receives only 10% of the avail-able intensity.

ambient dose equivalent (H �d) (dose ICRU)The dose equivalent d in a radiation point fieldthat would be produced by expanded andaligned fields in the ICRU sphere at a depth of10 mm on the radius vector opposing the direc-tion of the aligned field. The unit of ambientdose equivalent is joule per kilogram (J kg�1)and its unit is the sievert (Sv) (see directional dose

equivalent.)AMD® (comp) Advanced Micro Devices Inc.241Americium (Am) (elem) Used as a marker

source in scintigraphy and also as a calibrationsource for laboratory instruments.

Atomic number (Z) 95Relative atomic mass (Ar) 243Density (ρ) kg/m3 13670Melting point (K) 1267K-edge (keV) –

Relevance to radiology: radionuclide 241Am calibration source

24

2He


241Americium (Am)–analogue to digital converter (ADC)

A ProductionDecay 241Am T½ 433 yr (α, γ 60 keV) scheme (α) → 239Pu T½ v. longGamma 4.2 � 10�3 � mSv � h � 1 MBq�1 rayconstant @ 1 mDecay 0.0016 y�1

constant

amidotrizoate (cm) Ionic monomer as medrizoatesodium amidotrizoate produced commerciallyas Gastrografin and Urografin.

Amipaque® (cm) A non-ionic extracellular fluidcontrast medium as metrizamide introduced byNycomed in 1977. Since it is unstable in solution,it requires reconstitution in water prior to use.

Compound Viscosity Osmolality Iodine (cP) (mOsm/kg) (mg I mL�1)

Metrizamide 2.9 at 25, 272 1701.8 at 37

Metrizamide 12.9 at 25, 480 3006.2 at 37

Ampère, André Marie (1775–1836) Frenchmathematician, chemist and physicist whosename is given to the SI base unit of electric cur-rent (ampere, amp).

ampere (A or amp) (unit) A measure of electricalcurrent; defined as a constant current main-tained in two parallel conductors under idealconditions 1 metre apart, in a vacuum, produc-ing a force between the conductors of 2 � 10�7 N m�1. One ampere or one coulomb persecond represents 6.24 � 1018 electrons s�1.Smaller currents are measured in milliamps(mA; 10�3A) and microamps (μA; 10�6A). X-raytube current is measured in mA and x-rayexposure measured as the product mA �

second (mAs) so: 1 mAs �1 � 10�3 coulomb(1millicoulomb; 1 mC).

ampere second (As) (xray) the unit used todescribe the total electron flow through an x-ray tube. It is a measure of x-ray exposure usu-ally expressed in terms of milliampere secondsor mAs. 1mAs �1mC (10�3 coulomb).

amplification (us) The process by which smallvoltages are increased to larger ones using anamplifier.

amplifier (xray) An electronic circuit for eitherincreasing signal amplitude (e.g. ultrasoundecho) or signal charge (e.g. ionization chamber).

amplifier noise (xray) Electronic/thermal noisegiven by the components of an amplifier.

94239

94240

94241

95241Pu (n, ) Pu (n, ) Pu Am β�⎯ →⎯⎯

amplitude (phys) The peak height of a waveformmeasured from the crossover point (zero) to thepeak which is commonly a sinusoid.

1

0

Time

A

Cycle

�1

(us) Maximum variation of an acoustic variableor voltage (see a.c., RMS, sine wave).

amplitude modulation (a.m.) (di) Typically seenin a video signal where the carrier is VHFwhose amplitude is varied in sympathy withthe signal information.

amplitude resolution (us) The accuracy withwhich the smallest returning echo can bemeasured. Interference signals (noise) superim-posed on these signals may be regarded as thelimiting factor. In the near field accuracy is typ-ically 0.0001% corresponding to 100 dB SNR atgreater depths it reduces to 20 dB or 10% accu-racy. (xray) See pulse height analyser.

analogue analog (ct) A signal type representingvoltage, current or phase; the measured valueof a physical quantity which can adopt arbi-trary values within a certain continuous range.(us) The procedure or system where data arerepresented by proportional, continuously vari-able, physical quantities (varying voltage, cur-rent or phase).

analogue filter (di) A filter consisting of elec-tronic components (resistors, capacitors orinductors) that can be tuned to accept or rejectparts of an analogue signal.

analogue signal (di) Characterized by either avarying current or voltage, although frequencyand phase can also be treated as analoguecharacteristics. Analogue signals are prone toboth interference and distortion.

analogue to digital converter (ADC) (comp) Anelectronic device which samples the incominganalogue (voltage) waveform, converting thesampled amplitude to a binary value for subse-quent computer processing. Conversion of an


65% 9.3 at 20°, 1530 306Meglumine- 5.0 at 37°;

diatrizoate

angiography (clin) X-ray examination of a bloodvessel, usually done with injection of a contrastmedium through a catheter inserted through anartery in the groin. Magnetic resonanceangiography (MRA) uses the magnetization sta-tus or local velocity of the blood for imaging.Represents all vessels in the blood volume.Various views can be subsequently recon-structed (MIP) from three-dimensional data vol-umes (see digital subtraction angiography, DSA).

angioplasty (clin) Recanalization of a blood ves-sel using either balloon inflation or stent place-ment directed by fluoroscopic imaging. (seepercutaneous transluminal angioplasty (PTA), percuta-

neous transluminal coronary angioplasty (PTCA)).Angiovist282® (cm) Ionic x-ray contrast agent

manufactured by Berlex Laboratories. Thegeneric name is meglumine diatrizoate. Angiovist292 has an increased iodine content with corre-sponding increase in osmolality and viscosity.


Meglumine 6.1 at 25°, 1400 282diatrizoate 4.1 at 37°Meglumine 5.9 at 25°, 1500 292diatrizoate 4.0 at 37°

angle of incidence (us) Angle between beamdirection and the perpendicular axis.

angle of reflection (us) For non-perpendicularbeams equals the angle of incidence (see reflec-

tion, Snell’s law).Angström, Anders Jonas (1814–74) Swedish

physicist. The angstrom non-SI unit measuresthe wavelength of light.

angstrom (Å) (unit ) Non-SI unit measuring wavelength;replaced by the SI unit, the nanometer 1nm�10Å.

angular acceleration (α) (phys) The rate ofchange in angular velocity; related to linearacceleration (a) where α � a/r or v2/r.

angular displacement (θ) (phys) The angularchange in angular motion defined as where x is the linear displacement and r theradius of angular motion. Since r (the radius) isfixed then . Angular displacement is in radians (rad) where 360°�2π rad and

.1 57 3 180 rad rad= ° = °. / π

θ x

θ = x r/

A


analogue to digital converter (ADC)–angular displacement (θ)

analogue input signal (usually voltage as vary-ing amplitude) into a digital signal which can beprocessed by a digital computer.

analysis of variance (Anova) (stats) A techniquefor investigating how the variability in a set ofmeasurements can be ascribed to differentcauses. A one way analysis of variance tests forthe null hypothesis where all samples havebeen drawn from the same population. A two-way analysis of variance tests for two factors.Analysis of variance may also be applied to non-parametric ranked data.

anaphylactoid/idiosyncratic reactions (cm)Idiosyncratic reactions for contrast media arethose that mimic anaphylactic responses. Theyrange from mild, including hives and mild bron-chospasm, to severe, including sudden death;proposed mechanisms include histaminerelease, complement cascade activation anddirect central nervous system effects; these arenot purely allergic reactions because no circu-lating IgG antibodies to contrast material can beisolated in patients with a history of idiosyn-cratic contrast material reaction. The most com-mon severe anaphylactoid reactions associatedwith intravascular contrast media administra-tion include tachycardia, hypotension and someelement of bronchospasm. Lower incidence fornon-ionic than ionic contrast media.■ Reference: Dawson and Clauss, 1999.

anechoic (us) Echo free. A region of the imagethat does not return echoes (e.g. large vessels).

Anger, Hal, O (1920–2005) US scientist whoinvented and developed a practical stationary-imaging device for nuclear medicine in 1958:the Anger or gamma camera.

angiocardiography (clin) Radiographic imagingof the great vessels and chambers of the heartimmediately following rapid injection of con-trast material. Anatomy and function of theheart chambers is revealed in left ventricularangiography together with right and left coro-nary angiography. Single plane or bi-planecine-angiocardiography or real-time digitalimaging equipment is used.

AngioCis® (nmed) A commercial preparation forlabelling red blood cells with 99mTc(CIS/Schering).

Angiografin® (cm) Generic name for meglumine

diatrizoate. Ionic contrast medium manufacturedby Schering AG containing 65% megluminediatrizoate.


angular displacement (θ)–annual limit on intake (ALI)

A Angular parameters

Angle θ (rad)Angular velocity ω (rad s�1)Angular acceleration α (rad s�2)Moment of inertia ITorque τ � IαAngular momentum L � IωWork W � τθPower P � τωKinetic energy

angular frequency (ω) (mri) Frequency of oscil-lation or rotation (radians s�1). The symbol is ωso ω �2π f, where f is in Hz (see sine wave).

angular momentum (phys) A vector quantitygiven by the vector product of moment of iner-tia and angular velocity. This remains constantuntil an external force (torque) changes thedirection of rotation causing precession.Atomic nuclei possess intrinsic angularmomentum termed spin. (mri) In the absence ofexternal forces, the angular momentumremains constant, with the result that anyrotating body tends to maintain the same axisof rotation. In the presence of a torque appliedto a rotating body in such a way as to changethe direction of the rotation axis, the resultingchange in angular momentum results in preces-

sion. All atomic nuclei with an odd number ofnucleons possess an intrinsic angular momen-tum (spin) measured in multiples of Planck’s con-

stant. This is coupled with the magnetic dipolemoment. Electrons and protons possess anintrinsic magnetic moment that is distinct fromany ‘orbital’ motion, associated with and pro-portional to the ‘spin’ angular momentum ofthe particle.

The electron orbit of a hydrogen atom

The electron moves in a circular orbit at constant velocity. Given that the radius r of the orbit �0.5�10�10m

Linear velocity of electron �2.2�106m s�1

Since ω � v/r � 2.2�106/0.5�10�10

Angular velocity ω � 4.4 � 1016rad s�1

Angular acceleration α � v2/r � 9.7 � 1022ms�2 (or s�1s�1)

Angular momentum of electron

Angular momentum (L) of orbiting electron in hydrogenatom, radius r � 5.29�10�11mRest mass of electron m is 9.11 �10�31kgLinear velocity v � 2.2 �106m s�1

Angular momentum L � mr2ω, where ω is the angularvelocity �v/r � 4.4 �1016rad s�1 so angularmomentum �1.054 � 10�34kg m2s�1

12

2Iω

angular sampling (nmed) The sampling intervalfor single photon emission computed tomogra-phy (SPECT) imaging.

angular velocity (ω) (phys) Circular motiondefined from the angle through which theradius moves in radians per second (rad s�1).Angular velocity is related to linear velocity vby ω�v/r. For a constant speed the angularvelocity (or frequency), ω, is the number of com-plete revolutions per second. One revolution is2πr, so angular velocity can be expressed asω � 2π/t or 2πv rad s�1; the time t to completeone revolution. (mri) The symbol ωo representsthe angular frequency of precession.

The rotating anode of an x-ray tube.

For an anode rotating at constant angular velocity ω (orangular frequency) with radius (r) 100 mm (0.1 m),mass (m) 2.0 kg, rotating at 9000 revolutions perminute. Its angular velocity, moment of inertia, angularmomentum and linear velocity are:

Angular velocity: ω � 9000�2π�1/60�942.5 rad s�1

alternatively ω�v/r�942.5 rad s�1

Moment of inertia:Angular momentum: L�Iω�9.425 kg m2s�1

Linear velocity: 2πr � (rpm/60)�94.25 m s�1,alternatively v�ω r�942.5�0.1m �94.25 m s�1

(or just over 200 mph)

anisoplanasy (xray, nmed) Resolution varyingover the image intensifier or gamma camera face.

anisotropic (mri) Describing a three-dimensionalproperty of a material (crystal lattice, nervepathways) or parameter (thermal, electrical con-

ductivity) that varies in all directions (see diffusion

tensor imaging).annihilation event (nmed) Annihilation events

are not always emitted at 180°, since thepositron–electron pair is not completely at restduring annihilation. Such angular deviationsmay be of the order of 0.25° and can degradethe spatial resolution of the image.

annihilation radiation (phys) The electromag-netic radiation resulting from the mutual anni-hilation of two particles of opposite charge(positron and negatron or electron) giving twophotons 180° opposed each of energy 511keV(see positron).

annual limit on intake (ALI) (dose) The derivedlimit for amount of radioactive material taken inby the body (ingestion or inhalation) per yearthat would cause him/her to exceed the ICRPannual dose limit for a radiation worker. TheALI is the smaller value of intake of a given

I mr� �12

2 20 01. kgm

A


annual limit on intake (ALI)–anode cooling

radionuclide in a year that would result in acommitted effective dose equivalent currentlyset at 20 mSv per year. (ICRP 30 (Part 1)). Otherlegislation (10 CFR 20.1003.) states that the ALIis the smaller value of intake of a givenradionuclide in a year by the reference manthat would result in a committed effective doseequivalent to 5 rems (50 mSv) or a committeddose equivalent to 50 rems (0.5 Sv) to any indi-vidual organ or tissue. Some ALIs of isotopes incommon use would be:

Nuclide ALI (Bq)

32P 3 � 107

51Cr 7 � 108

99mTc 9 � 108

125I 2 � 106

131I 9 � 105

(see annual reference levels of intake (ARLI)).annual reference levels of intake (ARLI)

(nmed) The activity of a radionuclide that, takeninto the body during a year, would provide acommitted effective dose to a person repre-sented by the reference man, equal to 20 mGy.The ARLI is expressed in becquerels (Bq).

annular array (us) The ultrasound transducersarranged in concentric circles.

anode (xray) The positive electrode of athermionic device. Commonly applied to the x-ray tube (see compound anode).

anode angle (xray) Typically the focal track onthe anode disk is at an angle to the imping-ing stream of electrons arriving from thecathode. It is angled with respect to the exitwindow that when viewed from the plane ofthe image in the direction of the central ray,the optically effective focal spot appears smalland more or less square. In general, thesmaller the anode angle, the wider the focaltrack which increases the loadability. Anglesize also influences the field size at a givensource to image distance (SID). Field sizeincreases with anode angle as does the effec-tive focal spot size which will degrade imageresolution, so large area radiography isobtained at the expense of resolution; con-versely, smaller anode angles give a smallerfield size but a better resolution.

anode construction/assembly (xray) The con-struction of the x-ray tube anode uses varioushigh refractory metals, separately and asalloys. The body of an x-ray anode is typically

a disk consisting of an alloy of titanium, zirconium and molybdenum. The target isabout 0.7–1.0 mm thick tungsten/rheniumalloy. Common anode constructions are:

X-ray tube Construction Heat capacity(target)

Mammography Mo (Mo) 100 kJMo (Mo and Rh) 120 kJW (W and Mo) 135 kJ

Conventional Mo (Re/W alloy) 160 kJFluoroscopy/CT Mo alloy �C backing 1.0 MJ

(Re/W alloy)Ti/Zr/Mo alloy 1.8 MJ

(Re/W alloy)

anode cooling (xray) Measures the heat lostby the x-ray tube and determines the tubeworkload (loadability). Depends on anode heat

storage and temperature difference betweenthe anode ta and the surrounding medium tm(air, oil), since heat loss is proportional to t4

then tloss�ta4�tm4. The greatest heat lossoccurs at high temperature differences.Doubling the values of ta and tm increasesheat loss by �16. Heat loss from the anode ismeasured as joules per minute (J m�1) and ismost effective when the anode is operatingnear its maximum rating (falling load princi-ple). Tube housing is also involved andforced convection is sometimes necessary toimprove heat loss. The cooling curve showsthe number of heat units (HU) remaining perminute. The heat stored in an anode massdepends on its heat capacity; expressed inheat units or joules.

200

Anode cooling

150

100

50

00 1 2 3 4 5

Minutes

Hea

t uni

ts (

KH

U) 850 HU s�1

500 HU s�1

400 HU s�1

If the maximum heat capacity of the anode is 100 000 HU (from the graph shown in anode

cooling) then at a rate of 850 HU s�1 this can be


anode cooling–antecubital vein

A exceeded in 2.5 minutes continuous exposure.For long exposure times (fluoroscopy), thermalequilibrium occurs when the heat generated bythe electron beam is balanced by factors influ-encing heat loss (radiation, conduction, convec-

tion). The heating curves show that equilibriumwould be achieved for this tube at about500 HU s�1, levelling out at the maximum heatrating of 100 kHU (see heat storage, tube rating).

anode current (xray) Electrons emerging fromthe filament are accelerated towards the anodeby the applied high voltage. The anode currentis influenced by the space charge effect, fila-ment current and applied high voltage.

anode heat capacity (AHC) (xray) May beexpressed in kilowatts (kW) or heat units (HU)where 1Ws �1.35 HU (see heat capacity).

anode heating (xray) Considerable heat is pro-duced by bombarding the anode with an elec-tron beam. This heat is removed by radiation

from the anode to the enclosure wall (glass ormetal) and also by conduction along the bear-ing or by direct oil cooling. Reflector platesbehind the anode prevent excessive radiantheat from reaching the rotor-bearing mecha-nism. Excessive heating of the anode willvaporize the target giving a rough surface sodegrading focal spot geometry and reducing x-ray output by photon scatter; it will also causebearing damage. The vaporized anode material(tungsten) deposited on the tube window willincrease beam filtration (increased HVL), lower-ing image contrast.

anode heat storage (xray) For an x-ray tube,anode heat storage is the product of anodemass, specific heat and temperature rise. Themaximum heat capacity of the anode may beexceeded if the rate of heat production is high.The heat energy deposited in the target limitsthe workload and depends on:

● exposure time;● disk rotation;● focal track length;● focal spot size.

Early methods of measuring x-ray tube powerused heat units which were the product of:

● tube kV � tube mA � time (single phase);● tube kV � tube mA �1.35 (3-phase)● tube kV � tube mA � time �1.4 (constant cur-

rent/high frequency generator).

An increase applied to 3-phase or constantpotential (or high frequency) supply allows fortheir improved efficiency. Conversion: HU �

0.71 � joules; joules � 1.41 � HU. A conven-tional anode construction would have a typicalheat storage capacity of 100 kHU this wouldincrease to a few MHU for heavily loaded tubes(fluoroscopy and CT). Current metal/ceramictubes can have a heat storage approaching5 MHU and a continuous load of 7 kW (see rating).

anode load capacity (ct) Maximum permittedvalue of the instantaneous and of the meanlong-term x-ray tube power; the instantaneousmaximum value for the tube current for a giventube voltage is determined by the melting pointof the anode material and thus depends on theactual tube temperature, size of the focal spot(FS) and the duration of the electron beam tar-geting the anode’s surface. For short scantimes, the permitted x-ray tube loading dependsmainly on the tube’s heat capacity. For longerscan times, the tube capacity is limited by therate of heat transfer from the anode away fromthe tube housing (see rotating anode).

anode size (xray) Anode diameter determines theheat rating of the x-ray tube and therefore itsthermal loading. The disk mass and surfacearea also plays an important part. A largeranode diameter at the same rotational speedoffers a longer track length of target and so theheat generated is spread over a greater area ofmetal. Larger disks therefore take higher load-ing (higher loadability) and are used for highpower applications such as fluoroscopy and CT.A larger disk diameter increases the heat capac-

ity and also the area radiating heat, but there ispotential mechanical damage in the largeranode due to localized expansion. This is pre-vented by cutting radial slots into the anode;these are stress-relieved anodes.

Anode diameter and power ratingFor a100 mm diameter anode with 7 mm track width:

Mean radius: (100�7 mm) � 93 mmTrack length � 2π � 93/2 � 292 mmAnode rotation speed: 3000 rpmComplete target area exposed every 60/3000 �

0.02s

antecubital vein (clin) Vein located in front of theelbow in each arm, commonly used for bloodsampling or injection.

A


antegrade cystography–apcitide

antegrade cystography (clin) Antegrade urogra-phy performed by introducing contrast mediuminto the bladder.

antegrade pyelography (clin) See antegrade urog-

raphy.antegrade urography (clin) Radiography by per-

cutaneous injection of contrast agent with nee-dle or catheter into renal calices or pelvis(antegrade pyelography) or into bladder (ante-grade cystography).

antenna (mri) A coil or length of wire attached toan RF receiver as a pick-up device for RF signals.A device to send or receive electromagnetic radi-ation. In the NMR context, it is preferable to thinkof fields rather than electromagnetic radiation,as it is the magnetic vector alone that couplesthe spins and the coils, and the term coil shouldbe used instead (see coils (MRI)).

anti-aliasing (comp) A technique used to adjustjagged diagonal lines and curves in imagescaused by false frequencies (aliases) to makethem look smoother. Making transitionsbetween separate pixels more gradual.

antibody (monoclonal MAbs) (nmed) Antibodiesof uniform structure targeting a single anti-genic determinant. Produced by a clone anti-body population (homogenous population).Developed by Cesar Milstein and George Kohlerin 1975.

antibody (polyclonal PAbs) (nmed) Antibodiesof mixed structure targeting multiple antigenicdeterminants (see carcinoembryonic antigen (CEA)).

anticoagulant (clin) Agents preventing bloodcoagulation, such as heparin, ethylenedi-aminetetraacetic acid (EDTA).

antigen/antibody reaction (nmed) A mechanismin which a radiolabelled antibody binds to asurface tumour antigen to form an insolubleantigen/antibody complex, permitting imagingof the tumour.

anti-isowatt (xray) See isowatt.antimony Sb (elem)

Atomic number (Z) 51Relative atomic mass (Ar) 121.75Density (ρ) kg/m3 6680Melting point (K) 903.7Specific heat capacity J kg�1K�1 205Thermal conductivity W m�1 K�1 18K-edge (keV) 30.4

anti-neutrino (phys) See neutrino.anti-reflective coating (imaging) A coating

on video monitor screens to reduce light

14 λ

reflections at certain angles. Anti-reflectivecoatings are also found between the phosphorlayer and plastic support on a variety of inten-sifying screens and image phosphor plates toreduce reflection of scattered light from thesupport surface.

anti-scatter grid (xray) A collimating device forreducing scattered radiation reaching the imageplane (film, image intensifier). Consisting of thinlead strips (typically 36–70 μm) separated by x-ray transparent spacer material (paper, kevlar,carbon fibre, etc.). The height of the grid andspacer thickness determine grid performance.

D

h

Lead foil

Plastic or carbon fiberspacers

(ct) A collimator system mounted in front of thedetector in order to remove scattered radiation.Constructed from thin metal sheets alignedtowards the focal spot and almost completelyabsorbs those parts of the incoming radiationwhich do not originate from the focal spot (seegrid ratio, grid factor).

aortography (clin) Imaging the aorta and itsbranches using an iodine-based contrastmedium. Percutaneous transfemoral catheteri-zation is the usual method employed; failingthis, then a percutaneous transaxilliaryapproach is made. The contrast material isinjected via a catheter placed into the thoracicaorta (thoracic aortography) or abdominal aorta(abdominal aortography).

aortography (retrograde) (clin) Imaging theabdominal aorta, injecting contrast mediuminto the femoral artery and the visceral vesselsafter temporary occlusion of the artery distal tothe injection site; percutaneous transfemoralcatheterization is commonly employed.

apcitide (nmed) The preparation 99mTc-apcitide isthe technetium complex of the polypeptide,apcitide, a small-molecule synthetic peptideand has a high affinity and selectivity for theGPIIb/IIIa receptor that is expressed on the


apcitide–argon (Ar)

A membrane surface of activated platelets. Playsa major role in platelet aggregation and throm-bus formation. AcuTect® targets acute venousthrombosis in the lower limbs.

aperture (optic) The relative aperture of a lens isdefined as the ratio d/f where d is the diameterof the lens and f is the focal length. The lensaperture can be controlled by an iris diaphragmbehind the lens. Aperture is expressed as an f-number. An aperture of f-8 requires a �16longer exposure time than f-2, but has agreater depth of field. (us) Size of the transduceror size of a number of transducer elements (seedepth of field).

aperture (angle) (us) The angle between anintensity contour and the face of the trans-ducer; a measure of the lateral resolution. It canbe modified by signal delay in multi-elementtransducers (lateral focusing), but becomeslarger in the far field.

aperture (dynamic) (us) Change in aperture sizeduring transmission to maintain focus withdepth (see focus (dynamic)).

AP file (comp) Also called virtual memory, this is adisk file used as a main memory supplement.Program code and data are written to it inpages and swapped into main memory whenrequired.

Apgar score (stats) A scoring system used tograde neonate status.

API (comp) Application program interface. A com-mon interface that allows programs to makeuse of services provided by the operating sys-tem or other applications. Winsock, for exam-ple, is an API that allows Windows and otherprograms to talk to TCP/IP for Internet access.

apodisation (us) Removal of sound beam sidelobes by using gaussian beam profile.

(mri) Multiplication of acquired MR data by afunction reducing at higher spatial frequenciesso reducing artefacts near edges, due to trunca-

tion and Gibbs phenomenon.apoptosis (dose) A process of programmed cell

death following radiation or other damage.a posteriori (stats) Analysis suggested after the

event by examination of collected data.a priori (stats) Analytical predictions made before

data are collected.applet (comp) A small program that runs within a

larger program. Cannot function as stand-aloneprograms. Designed to be executed from withinanother program. Applets are embedded inmany Microsoft products, such as Windows andExcel. An applet can be a Java program which isdesigned to run only on a web page. Applets dif-fer from applications in that they are governedby a strict security protocol. Applets are furtherrestricted and can only read and write datafrom the same domain from which they areserved.

approximation image (di) Original image, fil-tered by a smoothing scaling function at thecoarsest scale.

APT (mri) Attached proton test.archiving (xray) The medium- and long-term

storage of patient details. An image data foreasy retrieval, which includes paper, film andelectronic methods.

Arcitumomab (nmed) (CEA-Scan®, Immunomedics)Murine monoclonal antibody Fab’ fragmentlabelled with 99mTc. Targets a variety of carcino-mas particularly in the gastrointestinal (GI) tract,including also Crohn’s disease and inflammatorybowel disease.

area (unit) The SI unit is the metre squared (m2).Conversions for non-SI units:

1barn 10�28m2

1m2 104cm2 (106mm2)1km2 106m2

1acre 4.04686 � 103m2

1 hectare 104m2

The barn is the effective cross section innuclear reactions and is equal to 10�28m2.Dose-area-product (DAP) meters use either gray orroentgen in a more practical cm2 area (see metre).

area dose product (F) (dose) See dose area product.argon (Ar) (elem) An inert gas used as a filling for

electric lamps and gas radiation detectors.Commonly found as a pressurized filling for thedose calibrator. Cheaper than xenon.

Main � Side lobes

Transducer face Transducer face

Singlemainlobe

afterApodization

A


argon (Ar)–arteriography

Atomic number (Z) 18Relative atomic mass (Ar) 39.95Density (ρ) kg/m3 1.784Melting point (K) 83.7K-edge (keV) 3.2Relevance to radiology: inert gas fill for ion-chamber

Gas Abundance (%) Density (kg m�3)

Air 100 1.293Nitrogen 78 1.25Argon 0.93 1.784Xenon 10�5 5.88

41Argon (nmed) Isotope of argon having a half-life of 110 minutes. Produced mainly from theactivation of argon in the air which cools theoutside surfaces of reactor vessels and theirshields or present in the cyclotron room. 40Argas molecules in the room air are activated bythe reaction:

As a result, radioactive noble gas 41Ar isreleased through a stack into the atmosphere.Being inert, it is not reconcentrated by biologi-cal systems.

ARP (mri) Adiabatic rapid passage.array (us) A group of piezo-elements (typically

several hundred) that are electronicallyswitched to transmit and receive ultrasoundsignals. (image) A block of computer memoryusually reserved for image data and treated asan image matrix.

array (adaptive) (ct) See Adaptive array.array (linear) (ct) See linear array.array coil (mri) RF coil composed of multiple sep-

arate elements that can be used individually(switchable coil) or used simultaneously. Whenused simultaneously, the elements can eitherbe electrically coupled to each other (‘coupledarray coils’) with common transmission lines(‘mutual inductance’) or electrically isolatedfrom each other (‘isolated array coils’), withseparate transmission lines and receivers andminimum effective mutual inductance and with the signals from each transmission lineprocessed independently or at different frequencies.

array processor (comp) Optional component ofthe computer system specially designed tospeed up numerical calculations like thoseneeded in magnetic resonance imaging; dedi-cated computer for reconstructing the imagematrix.

40 41 12Ar(n, ) Ar ; ( = 1.29 MeV, T = 1.83 h)γ γ

ARSAC (nmed) Administration of RadioactiveSubstances Advisory Committee, whichadvises the UK Secretary of State for Healthand authorizes doctors to administer radioac-tivity to patients for diagnosis, therapy andresearch.

arsenic (As) (elem)

Atomic number (Z) 33Relative atomic mass (Ar) 74.92Density (ρ) kg/m3 5730Melting point (K) 1090K-edge (keV) 11.8Relevance to radiology: Used as a doping agent for

semiconductors.

artefact (xray) An image distortion or unwanteditem caused by a fault in the machine (cassette,ADC, etc.) or software (program corrupted). Therandom fluctuation of intensity due to noisecan be considered separately from artefacts. Animage distortion or unwanted item caused by afault in the machine (cassette, ADC, etc.) or soft-ware (program corrupted).

artefact (structured noise) (ct) The appearancein the CT image of details not present in thescanned object. The main components ofstructured noise are due to a form of partial vol-

ume artefact and to beam hardening. Botheffects usually result in streaking artefacts,which are observed in regions of high contrastwhen there is a sharp discontinuity in objectdensity, such as at air–tissue, air–bone andmetal–tissue boundaries. Streaking will alsoarise from mechanical misalignment withinthe scanner and, in clinical practice, frompatient motion and the use of high-densitycontrast media. Image artefact can appear asfalse structures (aliasing artefact, beam hardening

artefact, motion artefact, structured noise or partial

volume artefact) or as a corruption of the CTnumbers due to errors in the reconstructionalgorithm.

arteriography (clin) Imaging an artery or arteriesby using an iodine-based contrast medium usinga percutaneous arterial needle puncture or place-ment of a percutaneous intra-arterial catheter.According to the puncture site or the organ to bevisualized, different terminologies are used, suchas peripheral or femoral, hepatic or renal trans-femoral arteriography. Arteriography is appliedfor investigating adrenal arteriography, bronchialarteriography, cerebral arteriography, coeliacarteriography, pancreatic arteriography, renal


arteriography–ATAPI

A arteriography, splenic arteriography, vertebralarteriography (see angiography).

arthrography (clin) The radiographic examinationof skeletal joints, with specific intra-articularinjection of contrast medium (gas or air) oriodine contrast material into the joint space.Displays cartilage and limits of the joint cavity.Non-ionic water-soluble contrast media arecommonly used. Magnetic resonance imaginghas made arthrography of the knee joint almostobsolete, and has also dramatically decreasedthe use of this invasive technique in other joints.

artificial intelligence (comp) A branch of com-puter science that tries to instruct a computerto learn from experiences or mistakes in orderto improve its performance. Examples are chessprograms and computer diagnosis using Bayes’theorem on conditional probability.

ASA (international) (film) Abbreviation forAmerican Standards Association. A film speedrating taking an arithmetic scale, taking animage luminance range of 20:1 (1.3 range log10).The speed is based on the exposure required togive a shadow density of 0.1 above fog. DIN andBS standards also adopt this standard.

ASCII (comp) American Standard Code forInformation Interchange. An encoding systemwhich converts keyboard characters andinstructions into binary for computer operation.

aseptic area (nmed) An isolated room, con-structed to comply with the highest standardsof air filtration (class I BS5295).

ASIS (mri) Aromatic Solvent-Induced Shift.as low as reasonably achievable (dose) See

ALARA.aspect ratio (di) The ratio between one image

plane and the other. A 35-mm slide has anaspect ratio of 3:2 and a 14-inch monitor isapproximately 5:4.

assembly language (comp) A type of computerlanguage which is complex but enables themost efficient way of programming the com-puter, particularly for speed. Assembly lan-guages have the same structure and set ofcommands as machine languages, but theyenable a programmer to use alpha numericlabels instead of numbers. High-level lan-guages such as FORTRAN, Java or C.Programmers still use assembly languagewhen speed is essential or when they need to

perform an operation that is not possible in ahigh-level language.

assumed radiation risks (dose) There is mostlyagreement between ICRP60 (1991a) and NCRPreport 116 (1993).

Workers 4 � 10�2 Sv�1 for fatal cancer0.8 � 10�2 Sv�1 non-fatal cancer detriment

0.8 � 10�2 Sv�1 for severe genetic effectsPublic 5 � 10�2 Sv�1 for fatal cancer

1.0 � 10�2 Sv�1 non-fatal cancer1.3 � 10�2 Sv�1 for severe genetic effects

Embryo/foetus approximately 10 � 10�2 Sv�1

astatine At (elem) A radioactive halogen, occur-ring naturally from the radioactive decay ofthorium and uranium.

Atomic number (Z) 85Relative atomic mass (Ar) 210Density (ρ) kg/m3 –Melting point (K) 575K-edge (keV) 95.7Relevance to radiology: alpha emitter used for

radiotherapy

211 Astatine:

Nuclear data 211 AtHalf life 7.2 hDecay mode α 5.8 MeVDecay constant 0.09625 h�1

Photons (abundance) 6(γ) 96keV–1.0MeV

asymmetric sampling (mri) Collecting more datapoints on one side of k-space origin than onthe other. With fewer k-space data points, ashorter echo time can be attained. Also, asym-metric acquisition in any phase encoding direc-tion followed by partial-Fourier reconstructiongives a reduced imaging time.

asynchronous transmission (comp) A type oftransmission in which each character is trans-mitted independently without reference to astandard clock. The data are commonly trans-mitted one character at a time to the receivingdevice, with intervals of varying lengthsbetween transmissions. ADSL and most dial-up modem communications uses asynchro-nous communication. (see ATM, synchronous

transmission).ATAPI (comp) Advanced technology attachment

packet interface. A standard for connecting aCD-ROM drive to an EIDE (enhanced integrateddrive electronics) adapter. This simplifies instal-lation of CD-ROM drives.

A


Athion–attenuation coefficient (sound)

Athion (comp) The processor produced by thecompany AMD. A direct competitor for Intel’srange of Pentium processors.

ATM (asynchronous transfer mode) (comp) Ahigh-speed networking technology that trans-fers packets of data to transmit various kindsof information (voice, video, data). ATM pro-vides standards for 25 Mbps and 155 Mbpstransmission speeds. Current ATM cells are 53bytes long containing a 5-byte header and a48-byte payload packet. The header containsinformation for data to reach the appropriateend point for a specified priority. The payloadcontains any type of information, voice, videoor data.

atmosphere (unit) A non-SI unit of pressurewhere 1 atm �1.013 � 105Pa (101.3 kPa).Equivalent to 760 mmHg or 14.696 psi. Somegas detectors (CT) use xenon at 20 atmospheresequivalent to 2.0 MPa or nearly 300 psi.

atom (phys) The basic unit of matter consisting ofa single nucleus surrounded by one or moreorbital electrons. The atom is electrically neu-tral and can be stable or unstable.

atomic mass number A (phys) The total numberof protons and neutrons in a nucleus A � Z �N.Shown below (top left) above the atomic number

value:

(see atomic weight).atomic mass unit (phys) See relative atomic

mass.atomic number Z (phys) The proton number in

the nucleus of an atom. The atomic numberalso equals the number of orbiting electrons ina neutral atom. This has the symbol Z in theconnotation

where A is the atomic mass number and N theneutron number.

AZ NX

AZ NX

Common stable isotopes are given in the exam-ple with IUPAC group numbers 13–16 showingthe change of elements with different atomicnumbers (bottom left) (see isotopes, isotones, iso-

diapheres, isobars).atomic weight (phys) See relative atomic mass.attenuation (ct) The decrease in the intensity of

the x-ray beam when passing through matter;the extent of attenuation is a property of thematerial which is exposed to radiation and theenergy of radiation. It is quantitativelydescribed by the linear attenuation coefficient;(mri) Reduction of power by passage throughan absorbing medium or electrical component.Commonly expressed in decibels. (xray) Theprocess by which radiation is reduced in inten-sity when passing through matter. It is a com-bination of absorption and scattering events.(us) Reduction of power, e.g. due to passagethrough a medium. Attenuation in electricalsystems is commonly expressed in dB as theproduct of:

● attenuation coefficient (dB cm�1);● pulse length (cm);● 0.5�frequency (MHz);● path length in dB.

Soft tissue has an approximate attenuation of1.0 dB cm�1 at 1MHz (see linear attenuation coeffi-

cient, linear absorption coefficient).attenuation coefficient (sound) α (us) The

speed of sound is frequency independent butα is influenced strongly by frequency. Theattenuation coefficient is the sum of the indi-vidual coefficients for scatter and absorption.Fractional change in intensity is given in deci-bels as μ � 4.3 αdB m�1; the decibel attenua-tion coefficient. The attenuation coefficient isroughly proportional to frequency as μ � kfwhere k � dB m�1MHz�1. For soft tissues, thisis 70 dB m�1 at 1 MHz or α � 0.1 dB mm�1 so3 cm of tissue will reduce the intensity by 50%;bone will give α � 1.3 dB mm�1 attenuation.

13 14 15 16

boron carbon nitrogen oxygen

aluminium silicon phosphorus sulphur

gallium germanium arsenic selenium3480Se,33

75As,3274Ge,31

69Ga,

1632S,15

31P,1428Si,13

27Al,

816O,7

14N,612C,5

11B,


attenuation coefficient (x-ray)–autoscan (autoscanning)

A attenuation coefficient (x-ray) (xray) See linear

attenuation coefficient.attenuation profile (ct) Spatial distribution of the

total x-ray attenuation of the object to be exam-ined measured at a certain position of the x-raysource. Intensity profiles from detected photonsare converted into attenuation profiles (inverse ofintensity). These are then converted to log values,so a quantity proportional to attenuation is avail-able for back projection (see CT number).

attenuator (mri) Device which reduces a signal bya specific amount, commonly given in dB.

audio frequency (phys) Range of sound frequen-cies from 20 to 20 000 Hz. The human voicefrequency is between 300 and 3500 Hz. Thebeat frequency in Doppler CW has a range in theaudio frequency band.

Auger Pierre (1899–1993) French physicist.Demonstrated from cloud chamber photographsthat the main photoelectric electron was accom-panied by characteristic auger electrons. Theseplay an important part in radiation dosimetry.

auger electron (phys) Electron ejected from K orL shell as an alternative to characteristic radia-tion emission. These electrons play an impor-tant role in estimating internal dosimetry,particularly at the cellular level (DNA damage)(see photoelectric effect, radiation weighting factor).

autocorrelation (image) Information about a sig-nal’s periodicity reveals information in thepresence of noise. This is obtained by makingamplitude measurements at two different timesseparated by a delay t, finding their productand averaging over the time of the recording:

where x(t) and x(t�τ) are the two amplitudemeasurements taken t intervals apart in awaveform of length T. Autocorrelation is verysensitive to the presence of periodicity in thetime domain (signal) in the presence of noise. Itindicates how well a shifted noise imageresembles or correlates with the unshiftedimage. The Wiener spectrum is the Fourier trans-

form of the autocorrelation function and viceversa. (us) Used in most colour-flow instrumentsfor obtaining mean Doppler shift frequency. TheDoppler signals along one image line are com-pared or correlated with those from the follow-ing transmit pulse (see Fourier analysis).

R tTx T

T

( ) lim��

→ ∫1

0

x(t) x(t ) dτ τ

autocorrelation function (image) The autocorre-lation function is the ensemble average of thejoint second moment of any image process. Thenoise autocorrelation function is defined ingeneral as:

Cn(xi, xj � (n(xi)�n(xj))

where n(i) is the noise at location i. For additivenoise, this is equivalent to the autocovariancefunction, which is the autocorrelation about themean. The autocorrelation function is theFourier transform of the Wiener spectrum.

autocovariance (image) See autocorrelation function.autoradiography (xray) A process where low

energy beta emitters (14C, 3H) are incorporatedinto a living system, organ or tissue beforebeing prepared histologically for microscopicdisplay. A photo-emulsion is applied to the sur-face of the thin section specimen and, after aperiod of hours or days, it is developed to reveallocal areas of beta activity as exposed silvergrains. Resolution is subcellular. Image platetechnology is now replacing film emulsion.

automatic brightness control (ABC) (xray) Seeautomatic exposure control.

automatic exposure control (AEC) (xray) Alsoknown as automatic brightness control (ABC)where the generator output is terminatedwhen a preset radiation exposure is reached.When used for fluoroscopy, this gives a con-stant dose rate at the image intensifier face(typically within 0.2–0.5 μGy s�1), maintainingthe consistency of image quality. Either a pho-todiode or photo-multiplier monitors the output-screen of the image intensifier; feed-back alters kV and mA (see iso-watt).

automatic frequency control (AFC) (xray) Anelectronic circuit that holds the frequency of agenerator to within very narrow limits. This isan important component of a high frequency x-ray generator where a slight variation in frequency would give a change in voltage.

automatic gain control (xray) Adjusts video sig-nal for constant display brightness in conjunc-tion with the automatic brightness control.

autoregressive filter (di) An alternative to theFourier process which is applied when only themost prominent features are to be extracted.The Fourier process is non-selective and willextract every feature.

autoscan (autoscanning) (us) The electronic ormechanical steering of successive ultrasonic

A


autoscan (autoscanning)–axis of rotation

pulses or series of pulses, through at least twodimensions.

autosomal (dose) Concerning chromosomes otherthan sex chromosomes.

auto-transformer (xray) This component of thex-ray generator allows fine adjustment of thea.c. supply voltage to the x-ray generator, com-pensating for voltage drop in the main supply.

auto-tuning (nmed) Automatic gain control ofphotomultiplier response with time by monitor-ing photopeak variations. Used as a method formaintaining gamma camera uniformity ofresponse. (mri) Automatic tuning optimizationmatching the RF coils for various loading condi-tions. The tuning elements are typically vari-able capacitance diodes (varactors) and areprogram controlled.

average CT value (ct) Arithmetic mean of CTnumbers within a region of interest. The meanor average CT number for an object detail canbe determined with high precision, since thesingle values are corrupted by quantum noise.

average glandular dose (mamm) Referenceterm (ICRP 1987) for radiation dose estimationfrom x-ray mammography, i.e. the averageabsorbed dose in the glandular tissue (exclud-ing skin) in a uniformly compressed breast of50% adipose, 50% glandular tissue. The refer-ence breast thickness and composition shouldbe specified (see mean glandular dose).

average lifetime (T-) (nmed) A term describing

radioactive decay. Average lifetime is calcu-lated by summing all the individual lifetimesand then dividing this by the total number ofnuclei involved:

Describes the time course of the number of unde-cayed nuclei, or the amount of the radioactivesubstance that is present at any time after meas-urements have started.

averaging (mri) Mean value of measured signalsin a slice; improves signal-to-noise ratio.

averaging filter (spatial) (di) A method forreducing random noise in a signal by addingtogether repetitive signals. The structured sig-nal strength will improve, while the randomnoise will cancel out. Applying this technique

TT

T� � �1

0 6931 44

λ ..

to image noise reduction. The simplest formsimply adds the pixel values in a small block(2 � 2 or 3 � 3) and constructs a new imagematrix from the mean values. Resolution is lostif larger blocks are taken (see frame averaging).

averaging filter (temporal) (di) A recursive filter forhandling image frames. The general algorithm is:

A version of non-recursive filtering applied toimage frames N. A running average is performedon the image data which improves SNR by

averted dose (dose) The radiation dose pre-vented or avoided by applying countermea-sures; the difference between the potentialdose if the countermeasure(s) had not beenapplied and the actual dose.

Audio video interleave (AVI) (comp) Designedby Microsoft to combine audio and video in asingle track or frame to keep them synchro-nized. Files are compressed by only recordingthe differences between each frame.

Avogadro’s constant (unit) The number of particlesin 1 mole which is 6.022 � 1023. The currentaccepted value issued by the National Institute ofStandards and Technology (NIST) is 6.0221415(�0.0000010) � 1023. Avogadro’s constant isused for calculating osmolarity, specific activity of aradionuclide and substance electron density.

axial (us) In the direction of the transducer axis(sound–travel direction).

axial plane (ct) Anatomical plane orthogonal tothe longitundinal axis (z) of the human bodyand parallel to the x/y plane.

N .

DDk

Dknew

old in� �

axial resolution (us) See resolution (axial).axial spatial resolution (ct) See spatial resolution.axis of rotation (ct) Axis about which the CT

gantry rotates (both x-ray tube and the detec-tor) during the measurement.

Right Left

Right

Axial plane

Left

(Anterior)

(Anterior)

(Posterior)

(Posterior)


B

Bo–banding

BBo (mri) A conventional symbol for the constant

magnetic (induction) field in a nuclear magneticresonance (NMR) system. The Ho units of mag-netic field strength ampere per meter should bedistinguished from Bo, unit of magnetic induc-tion in tesla.

B1 (mri) The conventional symbol for RF (radio frequency) magnetic induction field opposed toBo (the symbol H1 was previously used); consid-ered as two opposing rotating vectors in aplane transverse to Bo. At the Larmor frequency,this vector rotating in the same direction as theprecessing spins will interact strongly with thespins to give the NMR signal.

B-mode (us) An early brightness display giving a cross-sectional (axial) image. Replaced bygrey scale displays.

b-value (mri) This, together with the diffusioncoefficient D, are factors describing the relativesignal intensity S in diffusion weighted imaging

(DWI). The diffusion sensitivity parameter bequals:

where λ is the gyromagnetic ratio, G the strengthof the gradient pulse, δ the duration of the pulseand δ the time between the two pulses. Themagnitude of b is measured in mm2s�1 and canreach 10 000 in current magnetic resonanceimaging (MRI) machines. Gradient amplitude,slew rates and duty cycles during echo planar imag-

ing (EPI) are important in order to achieve highb values. The relative signal intensity is thengiven by:

S � e�bD

where D is the diffusion coefficient.■ Reference: Schaefer et al., 2000.

backbone (comp) The part of the network thatcarries the heaviest traffic; it connects LANS,either within a building or across a city orregion. It is the top level of a hierarchical net-work; the central connection. The main data busfor data transference. The Internet has a back-bone although not existing as a virtual routingpath. Also the network that joins servers toother servers, multiple concentrators or both.Smaller networks may not carry a backbone.

λ δδ2 2 2

3G Δ �

⎛⎝⎜⎜⎜

⎞⎠⎟⎟⎟

Either fibreoptic cable or RG-6 is used for thebackbone. Smaller networks using backbonetechnology, use RG-58 to connect betweenbridges, routers and concentrators.

background radiation (phys) Cosmic and radia-tion from natural radioactive sources. (nmed)Radiation originating in sources (environmentalor artificial) other than the activity of primaryconcern. Measured as becquerels (Bq).

back projection (di) Summation or linear super-position of projections, of light or x-rays throughan object, to form a simple tomographic imageslice. (ct) A method for reconstructing a sectionaldisplay from a series of radial projections. Thesummation or linear superposition of projec-tions, of light or x-rays through an object, toform a simple tomographic image slice. In com-puted tomography (CT) the mathematical proce-dure for the reconstruction of the axial image.This method suffers from serious image blurringso spatial filtration (convolution) of the raw data isnecessary before back projection in order toreduce artefacts (see filtered back-projection).

back scatter (xray) Radiation scattered ordeflected from its path into the sensitive volume of a detector.

back up (comp) Copying data from one storagemedium to another, i.e. from hard disk to floppy

disk. It is a protection against data destruction.balanced gradients (mri) Rewinding the gradi-

ent fields so that the total time-amplitude areais zero. A gradient waveform acting on any sta-tionary spin at resonance between two consec-utive RF pulses and return it to the same phaseit had before the gradients were applied.

balanced steady-state free precession (bSSFP)(mri) A gradient echo (GRE) sequence which pro-duces image contrast weighted by the T2/T1ratio; gives higher signal to noise ratio andreduced artefacts compared to SSFP. Uses balanced

gradients which returns magnetization to the original phase before gradients were applied;increases the acquired signal and reduces arte-facts. Typically, TR is short compared to the T2values of the tissues of interest, TE is intermedi-ate. The flip angle of 45° to 90° produces a T2/T1-weighted SSFP image (see True-Flash, True-FISP).

banding (image) Bands of discrete colour or tonethat appear when a printer is unable to repro-duce a smooth graduation from one colour toanother. Instead there are noticeable jumpsbetween one value and the next.

band-pass filter–barium contrast studies

band-pass filter (di) Also called pass-band filter. Asignal filter that passes a particular band of fre-quencies relatively strongly (see filtering (signal)).

band-stop (notch) filter (di) See stop-band filter,filtering (signal).

bandwidth (image) A general term describingrange of frequencies in a signal. Usually definesas the �3 db points of a pass-band or stop-band fil-

ter. A large bandwidth signal transmits a largevolume of information (video signal), while a nar-row bandwidth contains a restricted volume.Large bandwidths also increase noise content.The bandwidth of a pure sine-wave is very narrow and can be represented as a pulse in the frequency domain. (comp) The maximumamount of data that a network cable can carry,measured in bits per second (bps). The larger thebandwidth, the more information the networkcan handle. General KBPS standards for modemare 14.4, 28.8, 33.6 and 56 kB. ISDN is usually64, 128 or 256 kB. ADSL and DSL are generallyfaster than ISDN and sometimes faster thancable. Cable connections are usually 500 kB or1 MB. T1 is 1.5 MB and T3 is 45 MB. Satelliteuplinks are usually between 25 and 80 MB (seereceiver bandwidth, transmission bandwidth, Q, delta

(δ) pulse). (mri) The centre frequency of the RFtransmitted pulse selects the slice location andthe RF transmitter bandwidth (frequency spread)determines the slice thickness. The frequencyencoding gradient depends on the field of viewand the receiver bandwidth which covers theselected field of view; typically 4 to 32 kHz. (us)Range of frequencies contained in an ultrasoundpulse; expressed as the difference between the

Frequency

f1 f2

Tran

sduc

er r

espo

nse

most widely separated frequencies f1 and f2 atwhich the transmitted acoustic pressure spectrumis 0.71 of its maximum value (�3 dB).

bar (unit) Barometric pressure; a non-SI unit ofpressure where 1 bar is 105Pa, approximately750 mmHg or 0.987 atm. 1 millibar (mbar) is 10�3

bar or 100 Pa (see atmosphere, pascal, pressure).Baritop® (clin) A barium sulphate x-ray contrast

medium for the gastrointestinal (GI) tract. A whitesuspension of barium sulphate BP 100% w/v.

barium (Ba) (elem) An alkali metal used as barium

sulphate (BaSO4) as barium contrast studies for x-rayexaminations of the stomach and alimentarycanal.

Atomic number (Z) 56Relative atomic mass (Ar) 137.34Density (ρ) kg/m3 3600Melting point (K) 1000K-edge (keV) 37.4Relevance to radiology: As barium sulphate (BaSO4) as acontrast medium for x-ray examinations of the stomachand guts. Barium sulphate in this context is often calledsimply ‘barium’.

133Barium

Production

Decay scheme

(e.c.) 133BaPhotons (abundance) 81 (0.34)

303 (0.18)356 (0.62)

Half-life 10.8 yearsDecay constant 0.063 y�1

Gamma ray 7.7 �10�2mSv h�1MBq�1

constant at 1 mUses Calibration isotope emulating

131iodine

133 12

133

10 8 81 356Ba y ( keV)

Cs stable

T . ,γ

→

92235

56133

36101 2U(n, ) Ba Kr nf ( )� �

barium contrast studies (clin) Barium sulphate,being highly insoluble, is a non-toxic x-ray con-trast medium as an aqueous suspension admin-istered either orally or via the rectum in order toimage the complete alimentary tract. The single-

contrast barium enema is a less-preferredmethod to the double-contrast barium enema forthe evaluation of the anatomical configuration ofthe colon. This technique uses barium sulphatesuspensions and a less radiopaque secondmedium for a double-contrast or ‘see-through’effect, achieved by combining the barium enemawith insufflation of air (or carbon dioxide) (seesmall bowel enema).

B



B

barium enema–Bateman equation

barium enema (clin) Barium contrast study of thelower colon using a water suspension of bar-ium sulphate as the contrast medium.

barium follow through (clin) Performed after thestomach and duodenum have been examined,an image is taken 1–3 h later. This provides anassessment of stomach emptying.

barium meal (clin) Oral administration of bariumsulphate for visualizing upper gastro-intestinaltract (stomach and duodenum). The examination may be done as a single- or double-contrastexamination (see barium contrast studies).

barium sulphate (chem) A highly insoluble com-pound of barium (solubility approximately0.22 mg per 100 cm3 water) used in a watersuspension for barium contrast studies involvingthe alimentary tract. Common to all preparationsis the suspension of 1 g mL�1; high-densitypreparations of 2.5 g mL�1 are tailored to specialrequirements (see Baritop). (dose) In its impurestate as barite (native BaSO4) which is yellow/brown and used as a plaster material for radi-ation (x-ray room) shielding.

barium swallow (clin) Oral administration of bar-ium sulphate aqueous suspension for radio-graphic examination of hypopharynx andoesophagus. Both single- and double-bariumcontrast examinations are used.

Barkla, Charles Glover (1877–1944) Englishphysicist. Extensive work with x-ray scatteringrelated atomic number with the number of itselectrons. Observed characteristic x-rays from K and L shells. He was awarded the Nobel prizefor physics 1917.

barn (phys) Unit of reaction cross-section for describing particle bombardment; one barn �10�24cm2; one millibarn �10�27cm2.

barrel distortion (xray) Display distortion mostcommonly seen in image intensifiers due tocurved input face; its converse is seen as pin-cushion distortion.

Bartlett window (di) A filter for smoothing thetruncated result so that it is free of oscillationsthat would introduce false peaks into theresultant spectrum. A triangular window pro-viding a sharp main peak that quickly decays tozero (see Hanning filter).

BASE (mri) BAsis imaging with SElective inversion-prepared.

baseband (comp) A network transmission technique that uses voltage to represent data.The most common type of network, data are

transmitted digitally, each wire carrying onesignal at a time. A single signal, unmodulatedsignal of digital information over relativelyshort distances; the converse is broadband,Ethernet, ARCNet and Token Ring are basebandnetworks which use the entire bandwidth of cable to carry a single digital data signal.This limits such transmission to a single form ofdata transmission, since digital signals are notmodulated.

base-fog (film) Background density film reading.Its magnitude should not exceed an opticaldensity of 0.2. The value increases with poorfilm storage conditions (see characteristic curve).

baseline (math) A generally smooth backgroundcurve with respect to which either the integralsor peak heights of the resonance spectral lines in the spectrum are measured. (mri)Non-activated image, in contrast to activated image. Background signal from which thepeaks rise.

baseline correction (mri) Processing of the spec-trum to suppress baseline deviations from zerothat may be superimposed on desired spectrallines. These deviations may be due to broadspectral lines.

baseline rates (dose) The annual disease inci-dence in a population in the absence of radia-tion exposure to the source under study.

baseline screen (mamm) A woman’s firstscreening attendance (also prevalent screen),which may be part of a prevalent or incidentscreening round (see prevalent screening round).

baseline shift (us) Movement of the zero doppler-shift frequency or zero flow speed line up ordown on a spectral display.

baseline value (stats) The value that is used forcomparison when no absolute limiting value ispresent.

baseband (comp) A network transmission tech-nique that uses voltage to represent data.

baseline value (math) The value that is used forcomparison when no absolute limiting value ispresent.

BASIC (comp) Beginners all-purpose symbolicinstruction code. A very user-friendly program-ming language (unlike assembler). Versions areGW Basic, Quick Basic.

basic units (phys) See Systeme international (SI).Bateman equation (nmed) Describes

parent:daughter decay in a radionuclide generator.

B


Bateman equation–beam

where Ad(t) is the daughter activity at time t; λp

is the decay constant of parent; λd is the decayconstant of daughter; Ap is the initial activity ofparent; and Ad is the initial activity of daughter.For the 99Mo/99mTc generator, the decay constantfor the parent λp is 0.693/67 or 0.01033 h�1; thedaughter λd is 0.693/6 or 0.1155 h�1.

(see decay constant, generator).bathtub curve (stats) A predictive curve descrip-

tive of a quality assurance plan where theprobability of breakdown is highest at thebeginning and end of a machine’s useful life.

(see mean time between failures (MTBF), mean dow

time (MDT), mean time to recovery).baud (comp) The speed of a modem, or other serial

device, attached to and communicating with acomputer. A variable unit of data transmissionspeed (as one baud per second). Often confusedwith bits per second (bps or bs�1); these are

100

80

60

40

20

Equipment lifetime

Bre

akdo

wn

(%)

0

0 20

1

0.5

0.140

Time (hours)

Rel

ativ

e ac

tivity

(99

Mo

and

99mT

c)

99mMo99mTc

60 80

A A e p e d A e dd td

d pp

t td

t( ) �

�� λ

λ λλ λ λ( )

different. Only at low bit rates are they equal(300 baud �300 bps), but at higher speeds, i.e.V22 modem 1200 bps is 600 baud. The typicalspeed on the network of phone company lines is53.6 kB, though this can be increased by usingvarious data compression techniques.

baud rate (comp) The number of signals that canbe sent along a communications channel everysecond. In common usage; often confused withbits per second (bps). Modem speeds are usu-ally quoted in bps.

Bayes’ theorem (stats) Statistical inference draw-ing conclusions or diagnosis/prognosis underly-ing experimental or clinical data. A statisticalprocedure for revising and updating the predic-tion of some disease state in the light of newevidence. Used for suggesting the best ‘next-diagnostic test’, previous knowledge beingavailable. The prevalence of the disease gives theunconditional probability P(A). The uncondi-tional probability of a successful test is P(B). IfP(A|B) represents the conditional probability ofevent A (disease present) conditional on event B(positive test for this disease). The sensitivity ofthis test gives the conditional probability of apositive test amongst patient population withthe disease designated P(B|A). Then

The specificity of the test gives the conditionalprobability of the test giving a negative result(B ) in a population without the disease (A orcontrol group); represented by P(B |A ). Theunconditional probability of a positive testresult P(B) is therefore

{P(B|A) � P(A)} � {P(B|A ) � P(A )} or

sensitivity � prevalence � (1 � specificity) �(1 � prevalence).

so the complete Bayes’ theorem can be stated as:

predicting the probability of the test giving apositive result in a mixed population containingthe disease.

beam (us) Region containing continuous-wavesound; region through which a sound pulsepropagates.

sensitivity prevalencesensitivity prevalenec

( specificit

�

�

� �1 yy) ( prevalence)� �1

P(A|B)P(B|A) P(A)

P(B)�

�


B

beam area–beam homogeneity

beam area (us) Cross-sectional area of a soundbeam.

beam axis (us) A straight line joining the points ofmaximum pulse intensity integral measured atseveral different distances in the far field. Thisline, calculated according to regression rules, isto be extended back to the transducer assembly

surface.beam cross-sectional area (us) The area on the

surface of a plane perpendicular to the beam axis

consisting of all points where the pulse intensity

integral is �25% of the maximum pulse intensity

integral in that plane. For situations in which the relative acoustic pressure waveform does not change significantly across the beam cross-

sectional area, the beam cross-sectional areamay be approximated by measuring the areaon the surface of a plane perpendicular to thebeam axis consisting of all points where theacoustic pressure is �50% of the maximumacoustic pressure in the plane. Unit, cm2.

beam filtration (xray) The use of thin metal foil to remove low energy components from a poly-energetic beam.

beam former (us) The part of an instrument thataccomplishes electronic beam scanning,apodization, steering, focusing and aperturewith arrays.

beam hardening (xray) Filtration of a polychro-matic beam by the preferential absorption oflower energy photons in tissue, with a subse-quent increase in effective energy. The associ-ated artefacts are of particular significance incomputed tomography.

beam hardening artefact (ct) Errors in theabsolute CT number values due to beam hard-ening; beam hardening artefacts appear when

Position along slice

Slice center

CT

num

ber

beam hardening correction (ct) Preprocessingalgorithm for compensating errors due to beam

hardening in the measured projection data; thebeam hardening correction is based onassumptions concerning the composition of atypical object (head or abdomen).

beam homogeneity (xray) Comparison betweenpoly-energetic beam and mono-energeticbeam of the same effective energy (Eeff). Ahomogenous photon beam (gamma source)would be attenuated in a simple exponentialfashion by increasing absorber thickness; thehalf value layer (HVL) reduces beam intensity tohalf. A second HVL will bring the intensity to0.25. For a continuous spectrum (poly-energeticbeam) of x-rays, the primary and secondaryHVL are not equal since lower energy photonsare preferentially removed. The simple exponen-

tial law is not obeyed.

HVL 1

Single energy

HVL 2

0.0

1

0.5

0.25

0.1

Pho

ton

fluen

ce

0.5 1.0 1.5 2.0

Filter thickness (mm)(a)

2.5

Beam hardening (from data given under CT

number)At 100 kV the reference for water is 0.1707, but if thebeam effective energy changes to 105 kV in the cen-tre of the profile, the μ for tissue changes to 0.1750then:

and not 31 which it should be. This gives the cuppingeffect.

New CT value ��

�0 1750 0 1707

0 170725

. ..

the actual object differs significantly from theassumptions made by the reconstruction beam

hardening correction with respect to the x-rayattenuation. Critical regions are the base of theskull and pelvic structures.

B


beam homogeneity–BEIR

beam intensity (phys) The number of photonsper unit area (photon fluence).

beam quality (xray) The penetrating power of thex-ray beam and subjectively describes theshape of the continuous spectrum. Beam qual-ity is estimated from the half-value layer.Changing the kilovoltage changes beam qualitysince the penetrating power alters and theincreased cut-off point (kVp) changes the spec-trum shape; the effective energy also changes.Other factors: filtration and high voltage supplycharacteristic (single, three-phase or constantvoltage supply) also changes beam quality.Tube current, although it changes the quantityof radiation has no effect on the spectrum’seffective energy, so has no influence on thequality of the beam (see equivalent energy).

beam width (us) This changes with depth andimages of small objects will have their widthdistorted close to and at a distance from thetransducer face. The focal region of the trans-ducer has the narrowest width where objectswill be accurately displayed. (ct) The width ofthe x-ray beam within the scan plane meas-ured in the direction orthogonal to the beam’sdirection; the beam width is determined by thescanner geometry, focal spot size and effectivedetector width (see slice width).

bearings (xray) Rotational support for x-ray tube anode. Either ball or sleeve bearings areused.

beat frequency (phys) If two waveforms haveslightly different frequencies, then phase dif-ferences will change with time and wave inter-ference alternate between constructive anddestructive. These alterations of intensity cause

HVL 1 HVL 2

Poly-energy

0.0 0.2 0.4 0.6 0.8

1

0.5

0.25

0.1

Pho

ton

fluen

ce

Filter (mm)(b)

an overlaying beat frequency. The beat frequency is the difference between the twooriginal wave frequencies. The frequency vari-ation modulates the amplitude of the resultantwave which will have a frequency equal to theaverage frequency of the two waves.

Beck, Robert N (1928–2008) US engineer/sci-entist, whose early work (1961) on the designand theory of gamma ray scintillation opti-mized the imaging performance of both recti-linear scanners and gamma cameras.

Becquerel, Antoine Henri (1852–1908)French physicist, discovered that Becquerelrays were a property of atoms and had bychance discovered radioactivity in 1896.

becquerel (unit) A measure of radionuclide activ-ity in disintegrations per second (1 dps �1 Bq).Multiples are kBq, MBq and GBq. 1 Ci is3.7 �1010 Bq (37 GBq); 1 mCi is 37 MBq; 1 μCi is37 kBq (see activity, curie).

Beer’s Law (phys) As an extension of Lambert’s

law. It was shown by Beer that the linear absorp-

tion coefficient for a solution is directly propor-tional to its concentration:

I � Io � 10�abc

where a is the linear absorption coefficient, bthe thickness of absorber (path length) and c isthe concentration or density of absorbing mate-rial (see nuclear decay).

BEIR (dose) Biological effects of ionizing radiation.The committee (Washington, DC: National


B

BEIR–biexponential decay

Academy of Sciences) has produced several reference works:■ References: BEIR 1972, 1980, 1988, 1990, 1998,

2006.

bel (B) (phys) See decibel (dB).Bell, Alexander Graham (1847–1922)

Scottish-born American inventor. The bel (deci-bel) is the logarithmic comparative measure ofpower, intensity and amplitude or voltage gain.

bEPI (mri) Blipped echo planar imaging.Bernoulli, Daniel (1700–82) Swiss mathemati-

cian who pioneered the field of hydrodynamics.Bernoulli effect (us) Pressure reduction in a

region of high-flow speed.Bernoulli’s principle (phys) The relationship

between pressure and velocity in a flowingmedium. Where velocity is high, pressure islow and where velocity is low, pressure is high.The principle has wide implications for vascularimaging (see cavitation, flow).

beryllium (Be) (elem)

Atomic number (Z) 4Relative atomic mass (Ar) 9.01Density (ρ) kg/m3 1800Melting point (K) 1550Relevance to radiology: Lightest metal for x-ray tubewindows

Bessel correction (stats) While the mean value of asample of n items is unbiased, the standard devi-ation is considerably influenced by sample size.The expected variance in a sample of n items s2

is related to the population variance σ2 as:

The factor (n � 1)/n is Bessel’s correction whichgives the best estimate of the population variance.

Bessel function (math) Functions which are solu-tions of the second-order differential equations.They are used in heat conduction problems.

beta decay (phys) The unstable nucleus loses apositive or negative beta particle. Since betadecay produces a continuous spectrum and nota line spectrum, like alpha and gamma decay, itwas proposed by Pauli in 1930 (see Pauli,

Wolfgang) that another particle was emittedwhich shared the energy of emission. In 1932,Enrico Fermi proposed the decay process whichincluded an anti-neutrino:

01

11

1n p e v→ � ��

sn

n2 21

��

�⎛⎝⎜⎜⎜

⎞⎠⎟⎟⎟ σ

where is the anti-neutrino. An example is:

The mass number of the parent and daughter isthe same mass number (isobars).

n → p � e� � (β� decay with anti-neutrino)

p → n � e� � v (β� positron decay with neutrino)

p � e� → n (electron capture, EC)

(see neutrino, positron, beta plus decay).beta particle (phys) A negative or positive parti-

cle (see positron) emitted during nuclear decay.The negative particle is identical to the elec-tron, but since the electron is not part of thenuclear construction, it is assumed that betadecay consists of ‘creating’ an electron from theavailable decay energy; this electron is thenimmediately ejected from the nucleus.Determination of charge e and charge to massration e/m confirmed them to be negative elec-trons. They are emitted from nuclei that haveneutron excess and have a continuous energyspectrum (see beta decay, beta plus decay).

beta-plus decay (β�) (nmed) A mode of decayresulting from nuclear instability because of aneutron-deficient condition. A high-energy,positively charged electron (symbol β�) emit-ted by an unstable nucleus as a result of a neutron-deficient condition in the nucleus,along with a neutrino. The beta-plus particleundergoes an annihilation reaction with a freeelectron (see positron decay).

Bexxar® (nmed) 131Iodine labelled antibody tositu-momab, which is a murine IgG2a lambdamonoclonal antibody directed against CD20antigen found on the surface of normal andmalignant B-lymphocytes.

bias (stats) Deviation of results from an expectedconclusion.

bicisate (nmed) A 99 mTc-ECD complex commer-cially available as Neurolite®.

bidirectional (us) Indicating Doppler instrumentscapable of distinguishing between positive andnegative Doppler shifts (forward and reverseflow).

biexponential decay (nmed) Decay of a mixtureof two radioisotopes with different half-livesgiving a fast (biological) and slow (physical)exponential decay rate.

v

1532

1632P S→ � �β

v

B


biexponential decay–binding energy

bilateral imaging (mri) Used for symmetricanatomic features; imaging both organs oranatomic features in the same imaging session.

biliary contrast media (oral) (cm) Examples oforal cholecystographic contrast media containiocetamic acid, iopanoic acid, salts of ipodate ortyropanoate. The contrast media is absorbedintestinally, carried to the liver where it entersthe hepatocytes and is conjugated with glu-curonic acid, which increases its water solubil-ity and decreases its fat solubility. It is excretedinto the bile canaliculi. With intact hepatic andcystic ducts, the contrast medium flows into thegallbladder where it is concentrated by theresorption of water through the gallbladderwall (see cholegraphic contrast agents).

biliary contrast media (intravenous) (cm)Meglumine salts of iodipamide or iotroxic acids areused for intravenous cholangiography. They aretransported by binding to plasma proteins com-peting with bilirubin for binding sites on albu-min. Intravenous cholangiographic media havehigh water solubility, are not conjugated in theliver and are excreted unchanged in the bile, sothe intrahepatic bile ducts and common bileduct are clearly imaged. No reabsorption of thecholangiograhic media occurs in the intestines(see cholegraphic contrast agents).

Bilibyk® (cm) See cholegraphic contrast agents,Biloptin, Cholebrin.

Biligrafin® (clin) A 30% concentration of meglu-

mine iodipamide, iodine content 150 mg cm�3,manufactured by Schering AG used for cholecys-

tangiography. A 50% concentration is also avail-able. Iodine concentration 150 mg I mL�1 (seecholegraphic contrast agents, Biliscopin).

1

0.1

0.010 10 20 30

Time (relative)

Fast

Slow

Two components

Rel

ativ

e ac

tivity

(lo

g. s

cale

)

40 50 60

Biligram® (cm) A 35% solution of meglumine iogly-

camide manufactured by Schering AG for cholan-

giography and cholecystography. It is excreted inthe bile with minimal enterohepatic recircula-tion. Biligram-Infusion® has 17% meglumine

ioglycamide delivering 85 mg I mL�1.


Meglumine 1.57 at 20 85 1.05 at 37ioglycamideMeglumine 2.36 at 20 176 1.86 at 37ioglycamide

(see cholegraphic contrast agents).Bilimiro® (cm) Generic name iopronic acid.

Monomeric hepatobiliary x-ray contrast for oraluse (see Biloptin, Cholebrin, Telepaque).

Biliscopin® (cm) Generic name iotroxic acid. Lowtoxicity replacement for Biligrafin. Biliscopin(Schering) is a commercial preparation of meg-lumine iotroxate for cholecystocholangiography(infusion cholegraphy).

Compound Viscosity Osmolality Iodine (cP) (mOsm/kg) (mg I L�1)

Meglumine 1.3 at 20°C 290 50iotroxate(50 mg I mL�1)

Bilopaque® (cm) See cholegraphic contrast agents.Biloptin® (cm) Brand name (Schering) for sodium

iopodate, a capsular contrast medium for oralcholecystography, delivering 60–68% iodine by weight. Solu-Biloptin® contains calcium ipodate (see cholegraphic contrast agents).

bimodal distribution (stats) A probability or fre-quency distribution with two modes.

binary (comp) Numbers written in base 2. Thefundamental counting system used by comput-ers where 1 is ON and 0 is OFF. (Example:001 �1 decimal, 111 �7 decimal, 01010 �10decimal) (see bit, byte).

binder (xray) Minimum concentration of resinmaterial with very high transparency used as asubstrate for intensifying screen phosphors.Currently represents 10% of the weight of thephosphor mix.

binding energy (phys) The energy which isassociated with an orbital electron. High for K-shell electrons and low for L, M and N.Electronic binding energies are of the order 10to 100 keV in heavy atoms. This is the energynecessary to free an electron from its orbit.


B

Tungsten has a K-shell energy of 69.5 keV andan L-shell energy of 12 keV. Copper has a K-shell energy of 8.9 keV and carbon a K-shellenergy of 284 eV (see K-edge).

binomial distribution (stats) The distribution ofthe probabilities of the various possible num-bers of successes (0 to n), when n independenttrials are carried out. The probability generatingfunction is (p �q)n. The mean or expectation ofthe distribution is given by np and the standarddeviation by .

binormal ROC curve (stats) A receiver operator char-

acteristic (ROC) true positive fraction (TPF) where:

TPF � Φ(a � b � ZFPF),

where Φ represents the cumulative standard-normal distribution function, ZFPF indicates thenormal deviate which corresponds to the falsepositive fraction (FPF), and a and b are vari-ables. Most empirical receiver operating curves(ROCs) can be approximated to this form.

Binz, Arthur (1868–1943) German chemist whofirst synthesized organic iodine compounds ofpyridine in 1925, later used by Räth as x-raycontrast medium Uroselectan (see Räth, Swick).

bioassay (dose) Any laboratory procedure for esti-mating the nature, activity, location or retentionof a radionuclide/agent in the body by in vivo/invitro measurement of material excreted or other-wise removed from the body.

biodistribution (nmed) Distribution of chemical(e.g. radionuclide or contrast agent) in a livinganimal. Ideally, the highest chemical concentra-tion should be in the organ or tissue of interest.

biological half-life (nmed) See half life (biological),half life, half life (effective).

BIOS (comp) (Basic input/output system). A set ofprograms encoded on a ROM chip. Programs(stored in ROM) that handle the start-up opera-tions on computers. It enables the computer toperform basic input/output instructions duringboot-up. The BIOS advises the operating sys-tem about the presence and type of harddrives, keyboard, processor etc. (see boot sector).

biplanar (xray) A system using two C-arm fluoro-scopy units which have independent position-ing. Commonly used in cardiac studies.

biplane angiography (clin) Angiocardiographyin two planes at 90° or in paired orthogonalplanes or synchronous fluoroscopic acquisitionin two planes. Common procedure for cardiacimaging.

npq

biplanar transformation (di) Converting an ana-logue filter into a digital filter.

BIR British Institute of Radiology.BIRD (mri) Bilinear rotation decoupling.birdcage coil (mri) A volume coil designed to

produce a homogeneous B1 field with multipleparallel conductors around the surface of acylindrical volume. These are tuned so that atresonance there is a resulting homogeneous B1

field. When the field is circularly polarized, thestructure can be used as a quadrature coil.

bismuth Bi (elem)

Atomic number (Z) 83Relative atomic mass (Ar) 208.98Density (ρ) kg/m3 9800Melting point (K) 544.4K-edge (keV) 90.5

bismuth germinate (BGO) (rad) Bismuth germi-nate Bi4Ge3O12 has high stopping power (highefficiency), high spatial resolution and are 50%more efficient than NaI (Tl) crystals; they arenot hydrophilic. However, BGO is inferior in thefollowing areas:

Light output 15% of NaIDecay time 300 nsEnergy resolution �20% (poorer than NaI)Dead time Long

The inferior time resolution causes larger acci-dental detections and greater deadtime. BGOdetectors are best suited for imaging isotopeswith long half-lives, such as 18F and 11C (seePET detectors).

bistable (us) Having two possible states (e.g. onor off; white or black).

bistable display (us) Display in which allrecorded spots have the same brightness.

bit (comp) A word coined for ‘binary digit’. Eachbit in a computer memory can represent 1 or 0.The fundamental computer instruction.

bit depth (comp) See display resolution.bit range (ct) Number of bits used to store the

value of a CT value numerical entity; typicallyusing a 12-bit with sign, corresponding to therange of Hounsfield units from �1024 throughzero (air) to �3071 (see byte).

bitmap (comp) Bitmapped (or raster) graphics areimages, rather than characters, composed of bits.Each dot on the page has a one-to-one corre-spondence, or mapping, to a digital bit stored in

binding energy–bitmap

B


bitmap–blood cell

computer memory. The typical file extension fora Microsoft Windows bitmap file uses the exten-sion of .bmp. A bitmap file defines an image(such as the image of a scanned page) as a pat-tern of dots, or pixels.

bit specification (comp) The size of the computer’sinternal word or working register. The amount ofdata that can be used simultaneously. The typi-cal size is 32-bit, but 64 and 128 are used bysome mainframe computers. The computer speedincreases proportionately with bit specification.

bit speed (comp) Bits per second (bps). The meas-urement of data transfer speed in communica-tion systems.

bivariate distribution (stats) The combined dis-tribution of two random variables. The normalcurve is an example which shows a bivariatenormal distribution (see normal distribution).

bleeding (image) A print distortion where adja-cent colours run and merge into one another,sometimes caused by excess ink or over-absorbent paper.

Blackett, Patrick Maynard Stuart(1897–1974) English physicist who confirmedthe existence of the positron.

Bloch, Felix (1905–1983) Swiss born Americanphysicist who developed the technique ofnuclear magnetic resonance in the 1940s. Hereceived the Nobel prize for physics in 1952.

Bloch equations (mri) A set of classical equations ofmotion for the macroscopic magnetization vector.They describe the precession about the magneticfield (static and RF) and the T1, T2 relaxationtimes. Solution for the three magnetic vectors is:

Mx � e�t/T2 � cos wot

My � e�t/T2 � cos wot

Mz � e�t/T2 � cos wot

showing that M will exhibit a spiralling preces-sion. Longitudinal relaxation time T1 is the timerequired for Mz to increase from zero to 1�e�1

and the transverse relaxation time T2 is thetime for Mx,y to decay from e�1.

blood (biochemistry) (clin) Normal values

Albumin 2–48 g/LBilirubin 3–14 pmol/LCalcium 2.3–2.8 mmol/LChloride 5–105 mmol/LCholesterol 3.6–6.6 mmol/LCortisol (am) 300–800 nmol/LCreatinine 88–133 μmol/LGlobulin 18–39 g/L

blood (biochemistry) (Contd.)

Glucose (fasting) 3.3–5.6 mmol/LIron 1 3–31 μmol/LLead <2μmol/LMagnesium 0.7–1.0 mmol/LOsmolality 275–295 mmol/kgPhosphate 0.6–1.5 mmol/LPotassium 3.4–5.0 mmol/LSodium 134–146 mmol/LThyroxine (T4) 70–140 nmol/LTotal protein 60–80 g/LUric acid 0.15–0.48 mmol/L

blood (hemodynamic values) (clin) Normal values

Central venous pressure 0–8 mmHg(CVP)/right atrium

Right ventricle systolic 15–30 mmHgRight ventricle end diastolic 0–8 mmHgPulmonary artery systolic 15–30 mmHgPulmonary artery end diastolic 3–12 mmHgPulmonary artery mean 0–15 mmHgCardiac output 4–6 L/min(stroke volume �heart rate)

blood–brain barrier (BBB) (clin) The physiologi-cal interface between the blood, at the level ofthe capillaries and the brain parenchyma, whichis the primary site of exchange between bloodand the surrounding tissues. Brain capillarieslack fenestrations, unlike other capillaries pre-venting free passage of materials not necessaryfor brain metabolism but specific carrier systemsenable active transport. Some lipid-soluble(lipophiliic) compounds can diffuse across (e.g.alcohol, nicotine and certain drugs, HMPAO).Passage through the undamaged blood–brainbarrier by contrast media (radiographic and MRI)has not been demonstrated.■ Reference: Dawson and Clauss, 1999.

blood cell (clin) A general term covering redblood cells (erythrocytes about 7 μm) whichrepresent 45% of the blood volume (5 �106 permm3). White cells (leukocytes) are present insmaller quantities (8 �103 per mm3) havingdimensions of 9–15 μm. White cells are furthermade up of different cell types.

Men Haemoglobin (Hb) 13.5–18.0 g/dlHaematocrit 0.4–0.54Red cell count 4.5–6.5 �1012 L

Women Haemoglobin (Hb) 11.5–16.0 g/dLHaematocrit 0.37–0.47Red cell count 3.9–5.6 �1012 L

Both sexes White cell count 4.0–11.0 �109/LPlatelet count 150.0–400.0 �109/LMean cell volume 77–93 fI


B

blood cell labelling (nmed) Different labellingtechniques are used for erythrocytes andleukocytes. Leukocytes may be labelled witheither:

●111In as oxine or tropolone,

●99 mTc as exametazime (HMPAO).

Both 111In and 99 mTc leukocytes localize in sitesof inflammation and infection. Differencesbetween the two chelating agents exist; one ofthe most significant is that 111In-oxine cannotbe used to label leukocytes in the presence ofplasma because of the higher affinity of 111Infor transferrin than for oxine. 111In-tropolone isa stronger chelating agent and using it doesnot require the removal of plasma prior to celllabelling. 99 mTc-exametazime is able to crossthe cell membrane of leukocytes. Once insidethe cell, its structure alters to the hydrophilicform and the 99 mTc becomes trapped withinthe cell. 99 mTc-exametazime leukocytelabelling can be performed in the presence ofplasma. Erythrocytes/red blood cells arelabelled in vivo using a prior injection ofpyrophosphate by 99 mTc as pertechnetate.Alternatively, they can be labelled in vitro bywithdrawing the pyrophosphate-preparedblood and adding 99 mTc or using 51Cr with freshblood.

blood coagulation (contrast medium) (cm)All contrast media interfere to some extent withcoagulation and clotting factors. In standarddoses, this interference is usually clinicallyinsignificant. It is clear that ionic agents have amore profound effect on inhibiting blood coagulation. The lessened anticoagulant effectsof nonionic contrast agents, however, do notappear to increase the risk of thrombo-embolic events associated with angiographic procedures■ Reference: Dawson and Clauss, 1999.

blood flow (us) Depends on the vessel length and radius, blood viscosity and pressure differ-ence according to Hagen–Poiseuille’s equation.The SI unit is m�3 s�1 if pressure is measuredin pascals, viscosity in Pa�s and radius/lengthin metres. Doppler volume flow (Q) can be cal-culated as TAVmean � A where TAVmean is theangle corrected time averaged amplitudeweighted for blood velocity and A is the cross-sectional area.

blood gases (clin) Normal values:

Arterial Venous

pH 7.35–7.45 7.31–7.41PCO2 4.7–5.8 kPa 5.5–6.8 kPaHCO3 21–25 mmol/L 2–29 mmol/LBase excess 22–�2 0–�4PO2 10.6–13.3 kPa 4.0–5.3 kPaO2 saturation 95–99% –

blood oxygen level (mri) See BOLD.blood pool (clin) The intravascular space.blood velocity (us) The table lists peak and

mean blood velocities. The limits of currentDoppler flow velocity measurements are about0.3 cm s�1, but this depends on the ultrasoundwavelength.

Vessel Vessel Peak Mean diameter velocity velocity(mm) (cm s�1) (cm s�1)

Arteries 25 100 25Arterioles 0.04–0.1 1–2 1End arterioles 0.02–0.04 0.2–0.3 0.2Capillaries 0.003–0.008 0.02–0.05 0.05

blood volume (clin) Measured in millilitres perkilogram body weight:

Mean Range standard deviation

Whole blood 69.8 53.2–86.4 8.3Erythrocytes 22.7 20.5–34.9 3.6Plasma 42.1 30.5–53.7 5.8

Bluetooth® (comp) A telecommunications industryspecification describing how certain types ofmobile phones, computers and personal digitalassistants (PDAs) can be interconnected using ashort-range wireless connection technology.The maximum range is 10 m; realistically, con-siderably less than that, but not required as lineof sight. Data can be exchanged at a rate of1 megabit per second (up to 2 Mbps in the sec-ond generation of the technology). Built-inencryption and verification give minimal secu-rity. Bluetooth is also used for wireless key-boards, mice and similar pointing and inputdevices. Bluetooth networks feature a dynamictopology called a piconet or PAN. Piconets con-tain a minimum of two and a maximum of eightBluetooth peer devices; data are synchronized.A transceiver chip included within each device,

blood cell labelling–Bluetooth®

B


Bluetooth®–Boltzmann distribution

transmits and receives in a frequency band of2.45 GHz. There are up to three voice channelsthat are available. Each device has a unique 48-bit address from the IEEE 802 standard,somewhat similar to an Ethernet address.

Blu-ray® disk (comp) An optical disk storage for-mat giving high-definition video and data stor-age. It uses a blue-violet laser (405 nm) used toread/write. Significantly more data can bestored on the same dimension disk than on theDVD format, which uses a red (650 nm) laser. A dual layer Blu-ray disk can store 50 GB,which is almost six times the capacity of a duallayer DVD. The first mass-produced Blu-raydisk was released by Sony in 2006. It recordedboth single and dual layer.

Blumgart, Herman (1896–1977) US physician.Probably performed the first medical investigationusing a radionuclide (radon in solution), when hemeasured circulation transit time in 1927.

Blumlein, Alan Dower (1903–42) Englishengineer who worked with the partnership ofRCA and EMI pioneering the first electronicscanning television. He also patented auto-matic gain control, stereophony, versions of thecathode ray tube and shielded co-axial cable.

blurring (xray) See unsharpness.BMP (image) Microsoft Window’s native bitmap

format BMP supports indexing through four-and eight-bit palettes. Rather than store allthree RGB values, a palette is added to the BMPformat, so each pixel becomes the index to thispalette avoiding the necessity for storing themuch longer RGB value. This look-up table

approach is more efficient for handling eight-bit images of 256 colours. Only satisfactory forimages with limited numbers of colours.Indexing each colour then needs a palette ofmore than 16 million colours, requiring a mas-sive built-in palette. BMP was not accepted as auniversal 24-bit Truecolor standard, because itis inefficient for the most demanding taskssuch as handling photographs and print out-put. The only compression BMP offers is run

length encoding (RLE) (see TIFF etc.)BMS (mri) Bulk magnetic susceptibility.BNMS (nmed) British Nuclear Medicine Society.body coil (mri) A general body coil is installed in the

magnet and functions as both a transmit/receivecoil. It has a large measurement field, but doesnot have the high signal-to-noise ratio of specialcoils.

body surface area (clin) This is computed fordose or GFR calculations. Combines the individ-ual’s weight and height with the formula:

S � W0.425 � H0.725 � 71.84 or

S � log W � 0.425 � log H � 0.725 � 1.8564

where S is body surface area in cm2, W is weightin kg and H is height in cm (see surface area).

boil off rate (mri) Rate of cryogen evaporation insuperconducting magnets, usually shown aslitres per hour. The boil off rate increases duringramping and with eddy currents in thecryoshields using pulsed field gradients. Typicalboil off rates are 0.03–0.075 L/h. In calculatingcryogen consumption, additional transfer andfilling losses have to be considered.

BOLD effect (mri) Blood oxygenation leveldependent contrast. Oxygenated haemoglobinhas a smaller magnetic susceptibility thandeoxygenated haemoglobin. Transverse mag-netization in blood vessels decays more slowly.This BOLD effect extends T2 and T2*, measura-ble as an increase in signal in the blood volumeunder examination.

BOLD imaging (mri) BOLD imaging uses localchanges in blood flow to indicate the currentlevel of activity in a region of the brain. Localconcentrations of oxygen associated withchanges in blood flow are measured in theBOLD effect.

Boltzmann constant (phys) This is calculated asthe molar gas constant divided by Avogadro’snumber and equals 1.38 � 10�23 JK�1. Theaverage thermal energy of a molecule at anabsolute temperature T is k � (T/2) for eachdegree of freedom. This equation shows howincreasing entropy corresponds to an increas-ing molecular randomness (see entropy).

Boltzmann distribution (phys, mri) If a systemconsisting of particles in thermal equilibriumexchanges energy in collision, then the relativepopulation (number) of particles N1 and N2 attwo particular energy levels of E1 and E2 isgiven by:

where k is the Boltzmann constant and T theabsolute temperature. This equation suggeststhat in an NMR system at room temperature,the difference in numbers of spins aligned with

NN

e E E kT1

2

[( )/ ]1 2� � �


B

and against the magnetic field is slightly morethan one in one million. The slight excess in thelower energy state is the basis of the net mag-netization and resonance phenomena.

Boltzmann, Ludwig (1844–1906) Austrianphysicist who was responsible for many origi-nal ideas, extending Maxwell’s theory, blackbody radiation and atomic theory. In 1877, hepresented Boltzmann equation relating thermo-dynamic entropy S and statistical distributionof molecular configurations W, so that S � k logW, where k is the Boltzmann constant. This for-mula is inscribed on his tombstone in Vienna(see Boltzmann distribution).

bolus (clin) A circumscribed small volume of fluid(see Oldendorf technique).

bolus examinations/tracking (mri) Uses a con-

trast agent. A small amount of contrast agenttransported by blood flow whose spread is fol-lowed. A method of displaying the movingspins after tagging them (locally altering theirmagnetization).

bone (compact) (mat)

Effective atomic number (Zeff) 14.0Density (ρ) kg/m3 1650 kg/m3

bone (spongy) (mat)

Effective atomic number (Zeff) 12.3Density (ρ) kg/m3 1650 kg/m3

Bone thermal index (TIB) (us) The thermal index

for exposure where the ultrasound beampasses through soft tissue and its focal regionbecomes incident on a bone surface.

boot (comp) The process a computer goes throughwhen it starts up. BIOS loads the operating system

from the hard or floppy disk.boot sector (comp) The section of the disk (hard

disk or floppy) that holds information definingthe characteristics of the disk. It also hold ashort assembly language program that begins the process of loading the operating system

(boot-up disk).boron B (elem)

Atomic number (Z) 5Relative atomic mass (Ar) 10.81Density (ρ) kg/m3 2500Melting point (K) 2600

Bose, Satyendra, Nath (1894–1974) Indianphysicist, involved in the evolution of statisticalquantum mechanics (Bose–Einstein statistics)

for integral spin particles. Also contributed tothe work on x-ray diffraction.

BOSS (mri) BimOdal Slice Select RF pulse.bound electron (phys) For photon electron inter-

actions, a bound electron is defined as onehaving binding energies considerably higherthan the photon energy (see binding energy, pho-

toelectric event, Compton scatter).boundary layer (us) Sound reflection and trans-

mission occur at boundary layers betweenmedia with different acoustic impedances.

bounded-square output power (us) Power emit-ted in the non-autoscanning mode from thecontiguous 1 cm2 of the active area of thetransducer through which the highest ultra-sonic power is being transmitted. The symbol isWo1�1 and its unit: milliwatt, mW.

Bouwers A. Dutch engineer/physicist who in 1930produced the first rotating anode x-ray tube.

box-and-whisker plot (boxplot) (stats) Amethod of plotting data, useful in patient doseassessment, consisting of two lines (whiskers)drawn between the extreme values (smallestdose observed and largest dose observed) anda box drawn showing upper and lower quartile

and the median value.

Boyle’s law (phys) Part of the gas laws which statesthat pressure is inversely proportional to volumeat constant temperature. Boyle’s law states thatgas volume is inversely proportional to pressure:

and as gas density is proportional to pressure,then sound velocity is independent of pressurechanges (see Charles’ law).

VP

1

30

25

20

15

10

Y V

alue

s

5

Group A Group B Group C Group D

0

Boltzmann distribution–Boyle’s law

B


bow tie filter–brightness

bow tie filter (ct) See form filter.BP MR (mri) BiPhasic MR imaging.Bq (phys, nmed) Symbol for becquerel, the unit of

activity in disintegrations per second. Multiplesare kBq, MBq and GBq. The non-SI unit is thecurie (Ci, mCi and μCi).

braking circuit (xray) used to control rotatinganode braking after exposure.

breakthrough (99Mo) (nmed) A quality control

measurement for 99 mTc purity. A small quantityof the parent radionuclide 99Mo may be elutedwith the 99 mTc and can deliver an undesirableradiation burden to the patient. The NationalPharmacopea limit (USA and British USP or BP)is 37 kBq/37 MBq 99 mTc (0.1% of the 99 mTcactivity) and there should not be more than185 kBq in any patient dose.

breakthrough (aluminium) (nmed) The supportmaterial in the generator column is alumina(Al2O3). Soluble aluminium ions can be includedwith the 99 mTc-eluate. Freshly eluted 99 mTc forradiolabelling should be used since Al concen-tration increases as the 99 mTc decays (specific

activity falls). The Pharmacopea limit (USP, BP) is20 μg Al/ml 99 mTc eluate for fission-producedgenerators.

breast compression (mamm) The application ofpressure to the breast during mammography soas to immobilize the breast and to present alower and more uniform breast thickness to thex-ray beam. Maximum value 130–200 N.

bremsstrahlung (nmed) Also called braking radi-ation. X-rays are produced when a chargedparticle undergoes deceleration as a result ofthe attractive or repulsive force exerted on it asit approaches an electropositive nucleus. Theclose passage of bombarding electrons to thetarget nucleus (tungsten, molybdenum) pro-duces x-ray emission as a continuousbremsstrahlung spectrum. The intensity (I) ofthe bremsstrahlung spectrum varies with theenergy of the electron (E) on the target and itsatomic number (Z) as:

I Z E2 or I Z kV2

The complete spectrum is never available sincethe x-ray tube window absorbs (filters) thelowest energies. Efficiency (η) ofbremsstrahlung production depends on:η �k �E �Z; where k is a constant (1.1 �10�9

for tungsten). The production efficiency for

tungsten and molybdenum, over a range ofenergies, are:

Efficiency (η)

Tungsten (Z �74)20 kV 0.162%60 kV 0.48%100 kV 0.814%140 kV 1.14%

Molybdenum (Z �42)20 kV 0.092%

brick (shld) Typical density 1800 kg m�3 bakedclay solid building brick. Suitable for structuralshielding material in diagnostic x-rays rooms.External walls mostly double brick thicknesswith or without cavity.

Thickness (mm) Pb-equivalent (mm Pb)

150 1.0260 2.0340 3.0

■ References: Sutton and Williams, 2000; NCRP,

2004.

bridge (comp) Any hardware device that connectstwo physically distinct network segments, usually at a lower network layer than would arouter; the two terms are often interchanged. Adevice that connects two networks at the OSI

data link layer and passes data between them;equivalent to a two-port switch. Also a devicethat connects two local-area networks (LANs) ortwo segments of the same LAN. The two LANsbeing connected can be alike or dissimilar. Abridge can connect an Ethernet with a token-ring

network. Bridges, unlike routers, are protocolindependent; forwarding packets without analyzing and rerouting messages, so they arefaster than routers but less versatile.

bright blood effect (mri) An effect of slow bloodflow in MRI. Vascular spins are completelyreplaced by unsaturated spins during repetitiontime TR. In gradient echo sequences, the signalis maximum. The effect is used in bright bloodimaging of the heart for dynamic display ofblood flow, an effect that is similar for TOFangiography (see BOLD).

brightness (phys) Light intensity. The eye is sen-sitive to a narrow frequency range of electromag-

netic spectrum, about 3 �105 GHz or 400–780 nm,corresponding to the visible colour range. Theamplitude of the vibration corresponds to per-ceived brightness, typically from 10 to 104 lux(see contrast, Weber–Fechner law).


B

brightness gain (flux gain) (xray) Ratio of imagebrightness from the output of an image intensifier

to brightness at the input phosphor as:

From the available x-ray flux and the total lightproduced by the output screen, typical valuesare 50–60.

broadband (comp) A high speed cable networktransmission technique that uses radio fre-quencies. A broadband cable is typically sharedwith other networks or services, such as televi-sion or teleconferencing. Current data trans-mission speeds for domestic use are downloadat 2 Mbps and upload at 500 kbps. These canbe exceeded for specific installations, i.e. med-ical data transmission.

broad beam (xray) An uncollimated radiationbeam (x-ray) where scatter coincidence events caninterfere with detection. Radiation shielding

should take this into account. The half value layer

is larger than for narrow beam measurements(see collimation).

broad beam transmission factor (shld) Anempirical model describes the broad beamattenuation (B) of x-rays through an absorber:

where x is the absorber thickness and α, β, γthe particular aborber fitting parameters for thepolynomial.

11

� �

�β βα

αγα

γ⎛⎝⎜⎜⎜

⎞⎠⎟⎟⎟

⎡

⎣⎢⎢

⎤

⎦⎥⎥exp( )

( )

x/

00.0

0.2

0.4

0.6

0.8

1.0

1 2Absorber thickness

Broad beam

Narrow beam

Tran

smis

sion

3 4

light photons from output phosphorlight photons at photo-caathode

bromine Br (elem)


bronchial arteriography (clin) Selective injec-tion of the intercostal arteries with the contrastmedium.

brachio-cephalic aortography (clin) Selectivecatheterization for subclavian artery occlusion.

bronchography (clin) Radiography of the tra-cheobronchial tree using iodinated contrastmedium. Superseded by high resolution CT.

browser (comp) A method used for searching adatabase for quickly locating information; anidea extended to the Internet which approxi-mates to a large database. The www is a hyper-

text-based system that uses browsers forextracting specific information. Browsersenable comprehensive program searchingbetween documents throughout the world-wide web by clicking on highlighted keywords.Current browser packages are Mosaic (the firstgraphics browser and outdated), NetscapeNavigator, Mozilla Firefox and Microsoft InternetExplorer.

bruit (us) Audible sound (using a stethoscope)originating in vessels with turbulent flow.

Bucky, Gustav (1880–1963) German engineerwho in 1913 introduced a stationary metalanti-scatter grid. In 1914, he included a mech-anism for moving the grid during exposure (seePotter).

Bucky (xray) Shortened expression meaning themoving anti-scatter grid.

0

100

10�1

10�2

10�3

10�4

10�5

10�6

1 2 3Lead thickness (mm)

Bro

ad b

eam

tran

smis

sion

50kVp125kVp

4 5

brightness gain (flux gain)–Bucky

B


Bucky factor–byte

Bucky factor (xray) Exposure increase factorwhen using an anti-scatter grid or ratio of inci-dent to transmitted radiation.

bug (comp) A malfunction due to an error in theprogram or a defect in the equipment.

burst (us) A cycle or two of voltage variation.burst-excited mode (us) A mode of operation by

which a transducer is driven by a cycle of alter-nating driving voltage.

bus (comp) The internal pathway for signals mov-ing inside the computer. A multi-wire connec-tion that carries data between different parts ofthe computer or between computers. Buses forPCs are commonly ISA (obsolete), VESA andcurrently PCI which is the most efficient (seetopology (network)).

bus master (comp) An intelligent device, such asa PCI adapter card, that can gain control of thebus and use it to transfer data without involv-ing the processor.

bus network (comp) A decentralized local areanetwork shared by a group of computers(nodes). The bus signals are only used by aselected computer that responds to a uniqueaddress code. Used by AppleTalk® and Ethernet.Failure of one computer does not disrupt thenetwork.

Butterworth filter (di) Approximates to an idealfilter with maximally flat passband falling grad-ually toward the edge and passing through

(equivalent to �3 dB) at the cut-off frequency. The transfer function of a Butterworthfilter of order n is:

where D0 is the cut-off frequency. TheButterworth filter does not have a sharp dis-continuity between passed and filtered fre-quencies and although it has a smoothing action

HD Du v

u v on( )

( )( / ),,

��

11 2

1 2/

this filter design passes some high frequenciesso preserving edge detail.

The frequency response of the Butterworth filteris maximally flat (has no ripples) in the pass-band, and rolls off towards zero in the stopband(see filtering (signal), ideal filter, Chebyshev filter).

BW (phys, mri, us) See bandwidth.by-product material (nmed) Radioactive mat-

erial produced by nuclear fission (fission prod-ucts) or by neutron activation in a nuclearreactor or similar device (see carrier free, radiopar-

maceuticals (reactor)).bystander effect (dose) The response of unirradi-

ated cells triggered by signals received fromirradiated neighbouring cells.

byte (comp) A unit of storage capacity made upfrom 8 bits (23). This will hold any number from 0 to 256. The byte is the composite buildingblock common to all computers. Several bytesmake up a word. Memory and disk sizes are usu-ally quoted in kilobytes (1024 bytes), megabytes

(1024 k-bytes), giga-bytes (1024 M-bytes). Bytespeed in bytes per second (Bps) and bit speed inbps. However, nomenclature has become con-fusing and the differences between 1000 and1024 bytes have been clarified (see kilobyte).

Filter size

Filter shapes

Mag

nitu

de o

f filt

er e

ffect

1.0

0.5

0.0


C

C/C��–CAMELSPI

CC/C�� (comp) A computer language commonly

used for the creation of professional-gradeapplications. UNIX is primarily written in C. C isdistinguished from other computer languagesby using pointers (variables that point at loca-tions in memory). Developed by Dennis Ritchiein the early 1970s, C�� is a variation based onC that uses objected-oriented programming(OOP) design principles. The language is a prod-uct of Microsoft technology and is part of theVisual Studio Development package.

C-arm (xray) A fluoroscopy design found both insmall mobile and large fixed units. The imageintensifier is fixed in-line with the x-ray tube ona C structure which is sometimes cantileveredgiving oblique views.

cache memory (comp) An area of memory used tostore a copy of information recently read from orwritten to a hard disk or located on the CPU chip.Temporary memory for storing information thatis accessed most frequently. Typical cache mem-ory is a small block of high-speed memorylocated between the CPU and the main memory.A cache memory can significantly improve dataaccess speed. The Pentium processor has 8 k-byte of built in cache. The computer speed canbe further increased by using a secondary cacheformed on the DRAM or hard disk. Browsers alsouse a cache to store web pages so that the usermay view them again without reconnecting tothe web (see duo core, quad core).

cadmium (Cd) (elem)


cadmium telluride (CdTe) (phys) Semiconductormaterial used as a detector.

Atomic number (Z) 48, 52K-edge(s) 26, 32Electron/hole pair 4.4 eVEfficiency 0.85–0.88Uses: As a small volume room temperature photondetector probe for in vivo activity and also in the form ofcadmium–mercury–telluride as a photon detector. Alsocadmium–zinc–telluride and cadmium–mercury–tellurideare used as semiconductor detectors.

caesium (elem) See cesium.

calcium (Ca) (elem)

Atomic number (Z) 20Atomic weight (Ar) 40.08Density (ρ) 1540 kg/m3

Melting point (K) 1120K-edge (keV) 4.03

45Calcium

Production 44Ca(n,2n) 45CaHalf-life 163 daysDecay mode β � 0.257 MeVPhotons (abundance) Pure beta

calcium binding (contrast medium) (cm) Calcium-binding characteristics inherent in ionic contrastmaterials and enhanced by the addition of chelat-ing agents play a major role in the induction ofadverse reactions. Sudden reductions in ionic cal-cium can interfere with the electrical conductionmechanisms of the cardiovascular system andlead to severe reactions.■ Reference: Dawson and Clauss, 1999.

calcium disodium edentate (chem) See chelate.calcium sulphate (chem) See plaster.calcium tungstate (CaWO4) (chem) Phosphor mat-

erial used for intensifying screens. Now mostlysuperseded by more efficient compounds used asa detector material in conjunction with a spectrallymatched photodiode, in some CT machines (seedetectors (CT), rare earth screens, intensifying screens).

calibration (CT-scanner) (ct) Measurement fordetermining the individual detector channelsensitivity for each detector element of a CT sys-tem; the calibration is performed usually basedon an air scan or an appropriate test phantom.Correction procedures are used to take accountof variations in beam intensity or detector effi-

ciency in order to achieve homogeneity withinthe field of view (CT number accuracy).

californium (Cf) (elem)

Atomic number (Z) 98Atomic weight (Ar) 252Density (ρ) 15.1 g/cm3

Melting point 900°C

252Californium

Half-life 2.6 yearsDecay mode Fission α 6.11 MeVDecay constant 0.2665 y�1

Photons (abundance) Neutrons γ 64–590 keVRelevance to radiology: Neutron source for activationanalysis and therapy.

CAMELSPI (mri) Cross-relaxation appropriate formini-molecules emulated by locked spins.

C


Cameron John R (1922–2005)–capacitor charge/discharge

Cameron John R (1922–2005) American medicalphysicist. Founding member of the AmericanAssociation of Physicists in Medicine (AAPM).Developed thermoluminescence for dosimetryand devices for measuring bone densitometry.Founded Radiation Measurement Inc (RMI).

Campbell-Swinton AA Scottish engineer cred-ited with making the first radiograph followingRöntgen on January 7, 1896 of inanimateobjects, later published in Nature.

cancer (induction) (dose) Most information con-cerning the induction of cancer in humans asthe result of radiation exposure comes from theJapanese atomic bomb survivor data and x- andgamma-radiation used in therapy. This hassupplied information confirming the inductionof malignancies due to exposure from low radi-ation doses. Radiation-induced cancers and thenaturally occurring cancers in a population areidentical, the presence of an increase due toradiation can only be achieved by statisticalmeans on very large population groups. A con-siderable latency or latent period may elapsebetween radiation exposure and cancer appear-ance. It is assumed there is no threshold for theinduction of malignant changes (see latency

period, leukaemia, dose–response curve).candela (cd) (unit) A measure of luminous inten-

sity of a source emitting monochromatic radia-tion of frequency 540 � 1012 hertz in a givendirection and with an intensity of 1/683 wattper steradian. Measured as candela m�2 (cd m�2).

Object Luminance (cd m�2)

White surface, sunlight 3 � 104

Viewing, light box 2 � 103

Flat panel (medical) 400–500Flat panel (office) 350–450White level, video screen 200Flat panel (minimum) 170Reading light 30Black level, video screen 0.1White surface, moonlight 0.03

Cannon, Walter B (1871–1945) Americanphysiologist who pioneered the use of x-rays in1896 to investigate the gastrointestinal tract in the dog and human oesophagus.

canonical forms (di) Use of expressions or equa-tions in a form which is regarded as standard.For instance, the ellipse, parabola and quadriccone are classified by canonical form of theirequations.

capacitance (phys) The measure of the ability ofa capacitor to store charge where C � Q/V,where C is capacitance (farad), Q is the charge(coulomb) and V is the voltage between theconductors or plates. The unit of capacitance isthe farad. In practice, micro-, nano- and pico-farads are used (μF, nF, pF).

capacitor (phys) Two conductors separated by aninsulator (dielectric). The ratio of charge to poten-tial difference is the capacitance C � Q/V, meas-ured in farads. The capacitor is used to storecharge (capacitor discharge mobile x-ray sets)or to give a time constant as part of a capacitor–resistor or capacitor–inductor circuit. In NMR,inductors and capacitors are used to tune thetransmitter and receiver coils to the nuclear reso-nant frequency (see farad, Geiger counter).

capacitor charge/discharge (phys) This obeys theexponential law. Capacitor charging described as:

Vt � 1 � Vo � e�t/RC

and discharge, described by:

Vt � Vo � e�t/RC

where the voltage level Vt at any particulartime t depends on the initial voltage level Vo

and the product of resistance and capacitancetermed the time constant. If R is in ohms and Cin farads, then RC is in seconds. After thecharge of a period equivalent to one time con-stant, the voltage has increased to 1�1/e of itsinitial value (63%) and discharged to 1/e of itsvalue (37%). The process of time-constantsforms the design of time base generators inmost electronic circuits (timer devices, videoscan rates, etc.) and critical time measurementin magnetic resonance imaging (MRI).

Time (relative)

Discharge

Charge

Sto

red

char

ge (

rela

tive)


C

capillary blockade–carrier frequency

capillary blockade (nmed) Physical trapping ofaggregated albumin particles by capillaries in the lungs, permitting performance of pul-monary perfusion imaging (see MAA).

capromab pendetide (nmed) See ProstaScint®.carbon C (elem)

Atomic number (Z) 6Relative atomic mass (Ar) 12.01Density (ρ) kg/m3 2300Melting point (K) �3800Specific heat capacity J kg�1 K�1 709Thermal conductivity W m�1 K�1 129Relevance to radiology: Seen mainly as carbon fibrematerial in table construction and added to x-ray tubeanodes to increase mass with minimum weight.

11Carbon (nmed) A positron emitter used forpositron emission tomography (PET).

Production (cyclotron)Decay scheme (β�) 11C 11C T½ 20.4 m (β�, 2γ

511 keV) → 11B stableHalf life 20 mDecay constant 0.0693 min�1

Half value layer mm PbUses 11C useful for imaging drug

receptor sites with PET

14Carbon (nmed) A pure β� emitter

Production (reactor)Decay scheme (β�) 14C 14C T½ 5730 y

(β�, 156 keV) → 14N stableHalf life 5730 yDecay constant 0.00012 y�1

Maximum range (water) 0.3 mm, (air) 220 mmUses Sometimes used as an

autoradiographic tracer andmetabolic test. 11C useful forimaging drug receptor siteswith PET

14Carbon (labelled urea) (nmed) As labelledurea (PyTest®). Oral test for the diagnosis ofHelicobacter pylori (H. pylori).

carbon fibre (phys) Fine filaments of pure carbonbonded with resin to form extremely strong lowattenuation material. At 70 kVp, a carbon fibre x-ray table will absorb approximately 14% fewerphotons than a plastic table and a carbon fibrecassette 10–30% less depending on design. Atlower energies (mammography), the attenuationfigures are less (see Kevlar).

carcinoembryonic antigen (CEA) (nmed) Locallysecreted antigen that may enhance uptake oflabelled antibodies.

carcinogenesis (nmed) Induction of cancer byradiation or any other agent (a somatic effect).

714

614N(n,p) C

714

611

510

611

N(p, C or B(d,n) Cα )

Cardio-Gen82® (nmed) A 82Sr/82Rb generator (see82Rubidium).

Cardiolite® (nmed) In 1982, catonic 99 mTc com-plexes containing isonitrile ligands weredescribed. Further developments into isonitrilcompounds gave a commercial (DuPont Pharma,Bristol Myers Squibb) kit for the preparation of99 mTc-sestamibi, 99 mTc-(MIBI)�6 (Sestamibi or2-methoxy-isobutyl-isonitrile) is taken up intothe myocardium in proportion to blood flow andbound to the mitochondrial membrane. It doesnot distribute like thallium and remains boundfor many hours (see thallium, Myoview).

Cardiotec® (nmed) See terboroxime.carotid angiography (clin) Where selective injec-

tions are a problem in the presence of arterioscle-rosis then direct puncture and catheterization ofthe common carotid is sometimes used. ColourDoppler sonography and magnetic resonanceangiography (MRA) are largely replacing carotidangiography for diagnostic purposes and pre-operative evaluation of atherosclerotic disease.

carotid arteriography (clin) Arteriography usingiodine contrast material for visualization of thecarotid arteries usually performed from a femoralapproach using percutaneous catheterization.Carotid angiography techniques are used fordemonstrating the anatomy of the circle of Willis.Mostly replaced by colour Doppler ultrasonogra-phy and MRA for diagnostic purposes and pre-operative evaluation of atherosclerotic disease(see cerebral arteriography).

Carr–Purcell (CP) sequence (mri) Spin-echosequence of 90° followed by a 180° pulse to pro-duce a train of spin echoes; useful for measuringT2 relaxation times.

Carr–Purcell–Meiboom–Gill (CPMG) (mri)Modification of a Carr–Purcell sequence with a90° phase shift in the rotating frame of refer-ence (further 90° pulse) between the 90° pulseand the subsequent 180° pulses in order toreduce accumulating effects of imperfections.Suppression of effects of pulse error accumula-tion can alternatively be achieved by switchingphases of the 180° pulses by 180°.

carrier free (nmed) Only containing the chemicalof interest (e.g. radionuclide). A carrier-free staterepresents the highest specific activity (intrinsic).

carrier frequency (di) The fundamental fre-quency which is modulated by an input signaleither by influencing amplitude or frequency(see amplitude modulation, frequency modulation).

C


Cassen, Benedict (1902–72)–CDMA

Cassen, Benedict (1902–72) US scientist whodeveloped the rectilinear scanner for nuclearmedicine imaging in 1951 (see Mayneord).

CAT (CT) (ct) More commonly CT, since it nowapplies to any plane (coronal, saggital, oblique)(see computed tomography).

cataract (dose) A deterministic effect, the thresh-old dose being 2–10 Gy for acute exposure or achronic exposure of 150 mGy y�1 over manyyears.

catheters (clin) These may be either radiolucentor radio-opaque. Catheters are manufactured insterile disposable packs either straight withside holes or in predetermined shapes forselective angiography. Usually designated inFrench sizes.

cathode (phys) A negative charged nickel supportfor the filament.

cathode assembly (xray) The complete structurewithin the x-ray tube responsible for produc-tion and control of the electron beam. It consistsof the filament(s) and surrounding preciselyshaped cathode cup. A separate negative volt-age can alter the beam shape in dual spottubes or form an inertia-free switch for pulsedx-ray work (fluoroscopy and CT).

cathode cup (xray) A cup-shaped nickel depres-sion in the cathode that holds the filament(s)whose sharp edges create an electrostatic fieldwhich focuses the electron beam on to theanode target.

cathode rays (phys) Electrons will travel througha vacuum from a cathode emitter (filament) to apositively charged electrode (anode). Theseelectrons were given the name ‘cathode rays’and could be deflected by electrostatic or mag-netic fields (see cathode ray tube, flying spot).

cathode ray tube (xray) The basic vacuum tubedisplay where an electron beam traces out a scanning motion by either electrostatic ormagnetic deflection, under time base control, togive a visible signal on a phosphorescent phos-phor or a raster scan image. Replaced by flat

panel displays.cavitation (us) Gas bubble production due to rare-

faction events in a liquid. Production anddynamics of bubbles in sound (see ultrasound

cavitation).cavity ion dose (Jc) (dose) The ion dose produced

by photon or electron radiation in an air-filledcavity surrounded by matter (see ion dose, stan-

dard ion dose).

cavography (clin) Angiography of the inferiorvena cava usually by catheterizing the commonfemoral vein. Combined pelvic and caval veno-graphy may be performed by simultaneousinjection of contrast medium through cathetersinserted into both common femoral veins at thegroin (see superior vena cava).

CBF (rCBF) (clin) Noninvasive measurement of cere-bral blood flow, regional cerebral blood flow andregional cerebral blood volume (rCBV) is importantclinically. Nuclear medicine techniques include133Xenon washout and PET. 99mTc labelled com-pounds (Ceretec™, Neurolite™) are also used forCBF studies. Major drawbacks are the poor res-olution and exposure to ionizing radiation.Autoradiographic studies include 14C-iodo-antipyrine. Fast MR imaging techniques now giveimages of tissue flow kinetics. Measured as milli-litre per minute per 100g tissue (brain); typicalvalues being 20–30mL/min/100g, although thiscan be exceeded. Stable-xenon computed tomog-raphy is also used (see Kety–Schmidt technique).

CCD (imag) See charge coupled device.CD (comp) Compact disk. An optical recording for-

mat on a metal foil disk (commonly 120 mmdiameter), protected by plastic. Music, speechand digital data are permanently encoded inthe form of pits or impressions, placed in a spi-ral track on one side of the disk. Informationretrieval is by semiconductor laser focused onthe track and modulated by the impressions.The CD-ROM is a read-only storage mediumholding 650 M-bytes with a data transfer rateof 600 k-Bps and a typical seek time of 195 ms.The CD-RW (CD-Rewritable) is able to write andover-write data many times. CD-R is a one-time writing process capable of storing550–650 M-bytes. Used for archiving and canbe read by CD-ROM drives.

CD-R (comp) See CD.CD-ROM (comp) See CD.CD-RW (comp) See CD.CDD (imag) See contrast detail diagram.CDMA (comms) (Code division multiple access).

CDMA is a digital cellular technology. UnlikeGSM, CDMA does not assign a specific frequencyto each user. Instead, every channel uses the fullavailable spectrum and the frequency is dividedusing codes. CDMA has been implemented in800 and 1900 MHz systems in various countriesaround the world. Currently three noncompati-ble mobile wireless protocols are vying for the


C

wireless mobile market. They are GSM, TDMA

and CDMA.CEA (clin) Carcinoembryonic antigen. The expression

of this tumour-associated antigen increases in a variety of carcinomas. It can be shed anddetected in the serum by a variety of specific(monoclonal) and nonspecific antibodies. Labellingthese antibodies enables tumour sites to beimaged (see carcinoembryonic antigen, arcitumomab®).

CE-FAST (mri) Contrast-enhanced FAST (PickerMedical Inc.); contrast-enhanced Fourier acquiredsteady state; commonly used for imaging cere-brospinal fluid (see CFAST, SSFP, DE-FGR, True-FISP,PSIF, ROAST, T2-FEE, E-SHORT, STERE).

CE-FFE-T1 (mri) Contrast-enhanced fast fieldecho gradient-spoiled GRE sequence (PhilipsT1-weighted); same pulse sequence as SiemensFLASH.

CE-FFE-T2 (mri) Contrast-enhanced fast fieldecho (T2-weighted) name of CE-FAST tech-nique used by Philips.

CE FLASH (mri) Contrast-enhanced FLASH. A fastT2-weighted imaging sequence utilizing re-focused transverse coherences.

cell killing (dose) The killing of somatic cells byradiation in a rapidly dividing cell population (e.g.blood, gut epithelium) becomes manifest in a fewhours or days after exposure. In a slowly dividingcell population (e.g. nerve tissue), cell death maynot be seen for months or years. If enough cellsare killed, the function of the organ or tissue maybe impaired (see deterministic effect).

cell modification (dose) Dose response as aresult of specific DNA changes called ‘neoplas-tic transformation’. A characteristic of this is thepotential for cellular proliferation.

cell sequestration (nmed) The removal of dam-aged red blood cells from circulation by thespleen (heat-treated labelled red blood cells forspleen imaging).

cellular transformation (dose) Where a normalcell population is transformed into a malignantcell population by treatment with a carcino-genic chemical or radiation.

celsius (°C) (phys) A measure of temperature; for-mally degree centigrade.

Celsius Kelvin Fahrenheit

�273.15°C 0 K �459.67°F0°C 273.15 K 32°F

100°C 373.15 K 212°F

CE MRA (mri) Contrast-enhanced MR angiography.

centre frequency (us) Defined as:

fc � (f1 � f2)/2

where f1 and f2 are bandwidth frequencies. Thesymbol is fc and the unit is Hertz (Hz).

central field of view (CFOV) (nmed) This is 75%of the diameter of the useful field of view(UFOV). Generally, the CFOV represents the areaoccupied by the organ being imaged.

central processor unit (CPU) (comp) The centralmicroprocessor, e.g. Pentium II, Alpha II, PowerPC.

central radiopharmacy (nmed) A facility thatdispenses radiopharmaceuticals to a number ofhospital users or institutions.

central ray (ct) The x-ray beam emitted from thefocus and passing through the centre of theexit window of the x-ray tube that intersectsthe axis of rotation.

centre frequency (di) The frequency at whichthe peak gain occurs. (us) As:

where f1 and f2 are frequencies within thebandwidth. Unit: Hertz (Hz).

centre of rotation (COR) (nmed) The algorithm ofbackprojection requires accurate placement ofthe centre of detector rotation (e.g. gamma cam-era head) before data acquisition. MisplacedCOR will give reconstruction artefacts. Adjustedso that 0° and 90° and 180° and 360° positionsalign before SPECT data acquisition.

central radiopharmacy (nmed) A facility thatdispenses radiopharmaceuticals to a number ofhospital users or institutions.

centripetal acceleration (phys) A mass m movingwith a circular motion of radius r with uniformvelocity v has an acceleration toward the centrecalled the centripetal acceleration (a) acting radi-ally inward, where a �v2/r. The inward force thatmust be applied to keep the body moving in acircle is the centripetal force (F) where F �m(v2/r).It is sometimes convenient to picture a counter-acting force acting radially outward, termed the‘centrifugal force’.

ff f

c ��1 2

2

CDMA–centripetal acceleration

CT fan beam support and x-ray tubeCentripetal/centrifugal forces on a CT x-ray tube andsupport of mass 100 kg and 0.6 m radius revolving at0.5 s or 2 revolutions per second.

Force � ma � m(v2/r)Velocity � 2πr � 2 m/sCentripetal or centrifugal force is

(100 � (7.54)2)/0.6 � 9466 N

C


cepstrum (di) Whereas auto-correlation is theFourier transform of the power spectrum, the cep-strum is the transform of the logarithm of thepower spectrum. It more clearly indicates repe-tition of periodic signals.

ceramic scintillator (ct) Compounds containingyttrium, cesium, lanthanum, cadmium, tanta-lum, gadolinium and tungsten, are commonlyused and have a high density (�5 g/cm3). Goodspectral linearity requires scintillator response tobe linearly proportional to the x-ray energychanges between 90 and 140 keV. Solid scin-tillator materials are synthesized from transi-

tional elements with additional doping agents

used as a detector material in multislice CT scanners. Compounds used are gadoliniumorthosilicate and yttrium aluminium perovskitesuitably doped with cerium or europium. Thesehave considerable detection efficiency advantagesover simple scintillator compounds. Rare-earth-doped yttrium gadolinium oxide ceramic scin-tillators (Y(x) Gd(y) Eu2�O3) can be processedinto a transparent cubic form with high purity�99.99% and density of 5.92 g cm�3; exhibitluminescence in the 600–900-nm range. Themain europium emission peak is at 610 nm andmatches the higher spectral response of the Siphotodiode. Approximately 99% of the x-rayphotons are stopped in a 3-mm-thick scintillator.The light output of Y(x) Gd(y) Eu2�O3 is 2.5times higher than CdWO4, but about 30% lowerthan that of CsI:Tl. High luminescent efficiency of�10.3% and a major peak emission occurs at awavelength of about 610 nm and high lighttransmission in the 550–700 nm region. Doping

materials are used to reduce luminescent afterglow

below 0.1% at 100 ms without adversely affect-ing the other key scintillator properties. Importantspecifications for a CT ceramic detector are:

Requirements Acceptable values

Large dynamic range 103–106

High quantum absorption �90% (ideally 100%)efficiency

High luminescence efficiency Ideally 100%Good geometric efficiency 80–90%Small afterglow �0.01% 100 ns after

end of irradiationGood homogeneity Purity �99.99%Uniform response of all �0.1% difference

detector elements High precision machineability �10 μm

(see detectors).

cerebral angiography (clin) Radiography of vessels supplying the brain including extra-cranial vessels. The contrast medium is intro-duced percutaneously or through selected vessel by catheterization. The basic cerebralangiogram can demonstrate different parts ofthe cerebral vasculature depending on catheterplacement. Selective cerebral angiography isdivided into carotid arteriography and vertebralarteriography.

cerebral arteriography (clin) Demonstrates thevertebral and basilar artery and its majorbranches (PICA, ICA, superior cerebellar artery,posterior cerebral artery) (see cerebral angiography).

Ceretec® (nmed) A commercial preparation(Amersham) of exametazine in kit form for labellingwith 99 mTc, also known as HMPAO. The labelledcompound readily passes the intact blood–brainbarrier and is retained in brain tissue probablydue to intracellular conversion to a hydrophiliccomplex. The uptake closely matches regionalcerebral blood flow (rCBF) (see HMPAO).

cerium (Ce) (elem)

Atomic number (Z) 58Relative atomic mass (Ar) 140.12Density (ρ) kg/m3 6800Melting point (K) 1070K-edge (keV) 40.4

141Cerium (nmed) Principle gamma 145 keV andT½ 32.5d. used as a calibration source havingan energy very close to 99 mTc.

144Cerium

Half-life 284 daysDecay mode β�

Decay constant 0.00244 d�1

Photons (abundance) 134 keV (0.1)Uses Simulates 99 mTc gamma energy

and is sometimes used as acalibration source (see 57cobalt)

Uses in radiology: Simulates 99 mTc gamma energy and issometimes used as a calibration source.

cesium/caesium iodide (Cs) (elem)

Atomic number (Z) 55Relative atomic mass (Ar) 132.9Density (�) kg/m3 1870Melting point (K) 301.6K-edge (keV) 35.9

cesium/caesium iodide (CsI) (chem) A phosphorcompound found as the input phosphor forimage intensifier tubes (CsI:Na) or, in its undopedform, as scintillation detectors attached to photodiodes.

cepstrum–cesium/caesium iodide (CsI)


C

137Cesium/137caesium (nmed) Used as a calibra-tion source for laboratory instruments.

Production

Decay scheme (β�) 137Cs 137Cs T½ 30 y (β,γ 662 keV)

→ 137Ba stablePhotons (abundance) 32–38 keV (0.07)

662 keV (0.851)Decay constant 0.02296 y�1

Gamma ray constant 8.7 � 10�2 mSv hr�1MBq�1

@ 1 mHalf value layer mm PbUses A high energy source for

calibration and radiationsafety tests

cesium/caesium iodide (CsI) (chem) A phosphorcompound found as the input phosphor forimage intensifier tubes (CsI:Na) or in itsundoped form as scintillation detectors attachedto photodiodes.

CFAST (mri) Cerebrospinal fluid-artefact suppres-sion technique (Toshiba). Reduction of motion-induced phase shifts during time of the gradientspin echo (TE) (see GMR, GMN, FLOW-COMP, MAST,FLAG, GMC, FC, STILL, SMART, GR).

CFOV (nmed, rad) See central field of view.CFM (us) Colour flow map or colour flow image (CFI)

where the Doppler frequency is represented as acolour scale for each displayed pixel.

CGI (comp) Common gateway interface. A pro-gramming standard that permits visitors to fillout form fields on a web page and allows thatinformation to interact with a database, possiblycoming back to the user as another web page.CGI may also refer to computer-generatedimaging in which computer programs createstill and animated graphics.

c.g.s. (units) Metric measurement of centimetre,gram, second. Replaced by the m.k.s. system(metre, kilogram, second) used in SI units.

chain reaction (phys) A single nuclear fission ini-tiating a series of nuclear transformations. Acritical chain reaction 1:1 further transforma-tions; a subcritical chain reaction produces1:�1 further transformations; a supercriticalchain reaction produces 1:�1 further transfor-mations (see nuclear reactor).

channel (us) An independent element, delay andamplifier path.

characteristic curve (film) The graph plottingoptical density against log exposure and pro-ducing a sigma plot whose position and gradi-ent determine the characteristic of film speed

92235

55137

3797U(n, Cs Rb)f ) (�

and contrast (latitude) (see dynamic range, film

gamma).

characteristic dose rate (dose) Defined differentlyaccording to application: (1) For x- and gammaray studies, it is the air kerma rate K100 producedat the axis of the useful radiation beam withoutany scattering body present, at a distance of 1 mfrom the radiation source and a field size of10 � 10 cm. (2) For x-gamma radiation, the max-imum value of the water/energy dose rate D100

measured in water or water-equivalent phantomwith the same geometry as (1). The photonequivalent dose rate for radiation protection isthe value HX100 (see dose rate constant).

characteristic modulation (image) This is meas-ured as the modulation transfer function value at1 Lp/mm as is used as a guide for the overallresolution particularly for image intensifiers.

characteristic radiation (xray) Produced as aresult of an electron vacancy in the K, L or M shellcaused by either photon (photoelectric reaction)or electron (x-ray tube) bombardment. The elec-tron vacancy in the K and L shell causes an elec-

tron cascade producing an emission spectrum(extending into the ultraviolet region) of charac-teristic radiation whose energy range dependson the element being bombarded. Characteristicx-ray energies for tungsten and molybdenum areseen in the continuous spectrum and characteristicx-rays are produced during electron capture decay.Characteristic K-energies:

Tungsten 69.5 and 59.3 keVRhenium 71.6 and 61.1 keVMolybdenum 20.0 and 17.3 keV

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.00

0 1 2 3

5 10 15 20

Sensitometer step

Opt

ical

den

sity

(O

D)

log E

137Cesium/137caesium–characteristic radiation

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characteristic radiation–chemical shift

Charles’ law (phys) Part of the gas laws and statesthat volume and temperature are proportionalat constant pressure. From Charles’ law at con-stant pressure V T; since V 1/ρ where ρ isgas density, then 1/ρ T so at constant pres-sure sound velocity is proportional to (seeBoyle’s law).

Chebyshev filter (di) This gives a ripple effect inthe passband. Its stopband performance is supe-rior with a sharper transition. A Chebyshev fil-ter is preferable to the Butterworth if some rippleis acceptable (see filtering (signal)).

chelate (chem) An inorganic complex where a lig-and is bonded to a metal ion at two or morepoints. A ligand linking at two points is biden-tate; others are tridentate and tetradentate.Ethylenediamine can form two bonds with a transition metal. Common chelating agents areethylenediamine tetra-acetic acid (EDTA) and canform four or six bonds with both transition andmain group metal ions. DTPA: Chelating agentsare added to iodine contrast media (EDTA) inorder to inactivate heavy metals.

chelating agent (nmed) A chemical compoundwhose molecules can form several bonds to a single metal ion, so forming a chelate. Chelationcombines a metal as a complex where the metalion is part of a ring. The larger number (polyden-tate) of ring-closures surrounding the metal ionincreases the stability.

chemical energy (phys) The potential energyliberated in a chemical reaction; the extent ofreactivity between a functional group with agiven reagent. Most commonly demonstratedby batteries both primary and secondary (seeelectron affinity).

chemical shift (mri) The change in Larmor fre-quency of a nucleus due to molecular binding;

Chebyshev filter

Ripple

T

For high-Z elements (e.g. lead) the photoelectric

effect (PE) can be seen only as an attenuationprocess since the high energy characteristic radi-ation (88 keV) can escape taking part of the PEenergy with it. Low-Z elements emit only verylow energy x-rays which are entirely absorbed,so the PE effect shows complete absorption.

charge Q (electric) (units) The integral of theelectric current with respect to time (A s�1) andis measured in coulombs. The electron chargeis 1.602 � 10�19C.

Capacitor chargeAt a voltage V the charge Q on a capacitor having acapacitance of C farads is Q � CV. Stored energyJ � ½QV � ½CV2. For a capacitor of 1000 μF chargedto 220 V the stored energy is 50 joules.

charge (electrostatic) (phys) This is the integral ofthe electric current with respect to time and ismeasured in coulombs (C). The electron charge is1.602 � 10�19C. At a voltage V, the charge Q ona capacitor having a capacitance of C farads isQ � CV. Stored energy J � ½QV � ½CV2. For acapacitor of 1000 μF charged to 220 V the storedenergy is 50 joules.

charge coupled device (CCD) (clin) A semicon-ductor camera where a photosensitive surface isdivided into many thousands of separate islandsof photodiodes arranged in rows and columns.Position-sensitive array of photosensitive semi-conductor material either as a linear device or asa matrix. A replacement for vacuum tube videocameras. The electron charge capacity for eachpixel is 150 000. CCD cameras are commonlyused with image intensifiers; their resolutiondepends on the image intensifier field size.

Matrix size Pixel size (μm)

512 � 512 19 � 191024 � 1024 19 � 192048 � 2048 14 � 14

Electrons liberated by light photons are cap-tured in a charge-coupled layer of the semicon-ductor. Each line of stored information is shiftedelectronically and read out as a direct digitalsignal. CCD frame area arrays:

CCD resolution: Lp/mm

Intensifier (Lp/mm) 5122 10242 20482

40 cm (3.6) 0.64 1.3 2.530 cm (4.0) 0.85 1.7 3.423 cm (4.6) 1.1 2.2 4.415 cm (5.2) 1.7 3.4 6.8


C

caused by local alteration in the magnetic field.It is measured in parts per million relative to areference compound. Chemical shifts makepossible the differentiation of molecular struc-ture in high-resolution magnetic resonancespectroscopy (MRS). The amount of shift is pro-portional to the magnetic field strength and iscommonly specified in parts per million (PPM)

(see magnetic resonance spectroscopy).chemical shift artefact (mri) This occurs with

gradient echo sequences due to slight differ-ences in resonant frequency between fat andwater (approximately 3.5 ppm). Results in ashift in the fat images toward the water image.Visible at the tissue edges as contour artefacts(see chemical shift spatial offset).

chemical shift imaging (mri) An image of arestricted range of chemical shifts correspondingto individual line spectra. A magnetic resonanceimaging technique that provides mapping of theregional distribution of chemical shifts corre-sponding to individual spectral lines or groups oflines. The chemical shift can be treated as anadditional dimension to be reconstructed.

chemical shift reference (mri) A compound withrespect to whose frequency the chemical shiftsof other compounds can be compared. A stan-dard compound used as a reference line inspectroscopy. Proton spectroscopy (1H) usestetramethylsilane (TMS). Phosphorus spec-troscopy (31P) uses phosphoric acid. The stan-dard can either be internal or external to thesample; the internal is preferable since theneed for possible corrections due to differentialmagnetic susceptibility between an externalstandard and the sample being measured.

Fat

Water

Chemical shift � (ppm)�4 �2 0 2 4 6 8

chemical shift spatial offset (mri) Image artefact

of apparent spatial offset of regions with differ-ent chemical shifts along the direction of the fre-

quency encoding gradient, a similar effect may befound in the slice selection direction.

chemisorption (nmed) The surface binding of anadministered radiopharmaceutical (e.g. bindingof 99 mTc MDP to the surface of bone tissue).

chemotoxicity (cm) This refers to the mechanismresponsible for causing the toxic effects of con-trast media that cannot be explained by othermeans. Chemotoxic reactions relate both to thehyperosmolality of the contrast material and totoxic effects inherent in the contrast material,including the cation. The toxicity of intravascu-lar and intrathecal iodine contrast agents is dueto chemical structure and chemotoxicity affectsmost organs. Chemotoxicity is expressed ashigh protein binding, hydrophilicity/lipophilicity,high tendency for histamine release, inhibition ofbiological function (enzyme damage) and coag-

ulation effects. Toxicity of intravascular andintrathecal iodine contrast agents is due tochemical structure. Chemotoxic reactions affectvirtually all organ systems. Contrast media withhigh chemotoxicity will have a low LD50 regard-less of osmolality.■ Reference: Dawson and Clauss, 1999.

ChemSat (mri) Chemical saturation GE. Spectralpre-saturation to reduce MR signal intensity offat (see FATSAT, SPIR).

CHESS (mri) CHEmical Shift Selective imaging. Apulse train used in single voxel slice selectivemagnetic resonance spectroscopy involving a sin-gle frequency selective excitation 90° RF pulsefollowed by a dephasing gradient (spoiling gradi-

ent or spoiler). This removes unwanted magne-tization from the spin system. Useful inspectroscopy to selectively excite and dephaseoverwhelming water signals and typically pre-cedes STEAM and PRESS pulse sequences.

chip (comp) See central processor unit.chipset (comp) Provides the basic or core func-

tionality of a device. Chipsets can be found insound cards, graphics cards and almost everyother piece of computer hardware.

chi-squared test (�2 test) (stats) A test of howwell observed data fit a theoretical distribution.The chi-squared test is actually a test of thenull hypothesis that the data are a samplebeing drawn from a parent distribution fittingthe theoretical distribution being tested.

chemical shift–chi-squared test

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chlorine (Cl) (elem)

Atomic number (Z) 17Relative atomic mass (Ar) 35.45Density (ρ) kg/m3 3.21Melting point (K) 172.1K-edge (keV) 2.82

cholangiography (clin) Radiographic examinationof gall bladder and bile ducts using contrastmedia. Intravenous cholangiography (IVC) hasbeen largely replaced by direct cholangiography,such as percutaneous transhepatic cholangiogra-phy (PTC) or endoscopic retrograde choledochog-raphy (ERC). MR cholangio-pancreaticography(MRCP) using T2-weighted images without anycontrast material is replacing diagnostic invasivecholangiography in the majority of cases.

cholangio-pancreatography (clin) Radiographyof the bile ducts and pancreas.

Cholebrin® (clin) Monomeric hepatobiliary x-raycontrast medium for oral use (Nycomed,Amersham). Generic name iocetamic acid (seeBiloptin).

Cholecis® (nmed) A CIS/Schering preparation for99 mTc-mebrofenin.

cholecystography/cholecystangiography(clin) Radiography of the gall bladder usingcontrast medium either through the cystic ductor intravenously, replaced in some instances byERC or PTC. Water-soluble contrast mediumcan be taken orally; this remains the preferredmethod for gallstone detection, but its role hasbeen diminished by high quality ultrasoundimaging.

cholecystocholangiography (clin) Infusioncholegraphy.

cholecystostomy (clin) Drainage of the gall blad-der, usually performed for inflammatory andobstructive biliary disease in critically illpatients; removal of gallstones.

cholegraphic contrast agents (cm) There aretwo major groups; oral cholegraphic media andintravenous cholegraphic media. In general, oralcontrast media are ionic monomers and intra-venous contrast media are ionic dimers. Protein

binding reduces glomerular filtration in the kid-neys so the major excretion pathway is the liverwhere the hepatocytes, combine these oralcompounds with glucuronic acid and so activelyexcrete into the bile. Intravenous cholegraphicagents are also excreted into the bile, but are subject to different chemical pathways.

Cholegraphic media are excreted and concen-trated in the bile rather than eliminated by thekidneys because of their very high degree ofprotein-binding. This factor causes ionic chole-graphic agents to have a higher chemotoxicitythan urographic contrast media.

Generic Commercialname name

Oral cholegraphic Sodium Biloptin®

media iopodateCalcium iopodate Biloptin®

Iopanoic acid Telepaque®,Cistobil®

Iocetamic acid Cholebrin®

Iopronic acid Bilimiro®

Iobenzamic acid Bilibyk®

Sodium tyropanoate Bilopaque®

Intravenous Meglumine Biliscopin®

cholegraphic media iotroxateMeglumine iodipamide Biligrafin®,

Cholografin®

Meglumine ioglycamate Biligram®

Meglumine iodoxamate Endobil®

Choletec® A commercial preparation of 99 mTcmebrofenin (Bracco Diagnostics) and is an imin-oddiacetic acid (HIDA) derivative. The structureis (2,2 -[[2-[(3-bromo-2,4,6-trimethylphenyl)-amino]-2-oxoethyl]imino] bisacetic acid. Thelabelled injected activity is cleared through thehepatobiliary system. Hepatic duct and gall-bladder activity occurs in about 10–15 minutes.

Cholografin® (cm) Intravenous preparation(Bracco) of meglumine iodipamide (52%) for intra-venous cholangiography and cholecystography.Iodine concentration 257 mg I mL�1 (see chole-

graphic contrast agents).chromatic aberration (phys) When white light is

refracted by a medium (glass), a coloured spec-trum is formed due to the different velocitiesgiven by each wavelength: c � f � λ. Red lightis deflected least. Chromatic aberration is seenin some uncorrected high definition lenses.

chromatography (chem) A process for separatingand analyzing molecules and compoundsaccording to differences in the speed withwhich they travel in solution up an absorbingsurface. Used for quality control techniques inradiopharmacy.

chromatography (nmed) High performance liquidchromatography (HPLC) formerly known as highpressure liquid chromatography, is increasinglybeing used for the ultimate purification of

chlorine (Cl)–chromatography


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radiopharmaceuticals and has found use inanalysis. HPLC can separate all known andlikely impurities in one analysis and is able togive chemical, as well as radiochemical, purityinformation.

chromatography (nmed) (Thin layer/paper,TLC/PC) These methods, which include reversephase thin layer chromatography (RPTLC) andinstant thin layer chromatography (ITLC) havebeen the main vehicle for the determination ofradiochemical purity. The procedures are easyto set up and simple to use. They usuallyemploy mixtures of commonly available sol-vents and chemicals, but can suffer from theinability of any one system to separate out allthe likely impurities. It is the ability to quantifyradiochromatograms which has made thesetechniques the method of choice in the analysisof radiopharmaceuticals.

Chromitope® (nmed) Sodium chromate prepara-tion for labelling red blood cells (Bracco).

chromium (Cr) (elem)


51Chromium (nmed) As sodium chromate, usedfor determination of red blood cell (RBC) volumeor mass. RBC survival time and evaluation ofblood loss.

Production

Decay scheme (e.c.) 51Cr 51Cr (γ 320 keV) → 51V

stableGamma ray constant 4.7 � 10�3mSv hr�1 GBq�1

@ 1 mHalf-life 28 daysALI 1480 MBq (40 mCi)Half value layer 1.7 (mm Pb)

(see Chromitope®).CIDNP (mri) Chemically induced dynamic nuclear

depolarization.cine-camera (xray) A hard copy attachment to

the output of an image intensifier to capture fastcardiac movement at 50–150 frames per sec-ond using 35 mm pan-chromatic film (see cine

angiography, cine film).cine imaging (mri) A set of images collected

throughout the cardiac cycle where each imageis acquired in a fixed portion of the cycle. When

2450

2451Cr(n, ) Crγ

the images are replayed in a closed loop, theydemonstrate cardiac pulsation, blood or cere-brospinal fluid flow.

circular polarization (CP) (mri) Excite or detectspins using two orthogonal transmit and/orreceive channels/coils. Gives an improvementin SNR over a linearly polarized coil as a receiv-ing coil.

CISC (comp) Complex instruction set computer.Processors that use a large instruction set formanipulating data. CISC computers take severalclock cycles to perform a single operation.

CISS sequence (mri) Constructive interference in the steady state (Siemens); combines flow compensation with a DESS signal. Strong T2-weighted 3D gradient echo technique withhigh resolution. Two acquisitions with differentexcitation levels are performed internally and arethen combined. Slower than balanced steady-state

free precession as it uses a longer TR (repetitiontime). Streaking between dissimilar tissues(banding) is prevented.

Cistobil® (cm) See cholegraphic contrast agents.class (stats) A group or collection (values) having

a common characteristic (disease or popula-tion). One of a set of mutually exclusive, pre-established categories to which an object canbe assigned.

classified person (dose) A worker who has beendesignated in accordance with local radiationsafety regulations as likely to receive 3/10th ofany relevant dose limit.

clean area (nmed) A room with a good standard ofhygiene supplied with filtered air (class III BS5295)(see aseptic area, HESPA, laminar flow cabinet).

Clear-Pb® (dose) An acrylic plastic sheet producedby Victoreen Inc., containing 30% by weight oflead. Produced in the following thicknesses:

Thickness Weight Pb-equivalent (mm) (kg m2) (mm Pb)

7 13 0.312 22 0.518 33 0.822 40 1.035 64 1.546 84 2.0

This material is shatterproof and can be easilymachined (see shielding (lead-glass)).

client (comp) Any computer system that requestsa service of another computer system. A work-station requesting the contents of a file from a file

chromatography–client

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server is a client of the file server. A client/serverarchitecture allows many people to use the samedata simultaneously. The program’s main compo-nent (the data) resides on a centralized server,with smaller components (user interface) on eachclient (see server).

client/server network (comp) Communicationstructure between a client (fat client, thin client)and server in a network.

clean area (nm) A room with a good standard ofhygiene supplied with filtered air (class IIIBS5295) (see aseptic area).

clinical trial (stats) A prospective or a priori clinicalstudy designed to demonstrate the effect of aparticular procedure, drug or diagnostic imagingtechnique on a selected group of patients.Sample size and methods of measurement playan important part, as well as a suitable controlgroup.

clock speed (comp) A measure of how manytimes a second the processor cycles, measuredin MHz. Current clock speeds can range between500 and 1000 MHz.

clutter (us) Noise in the doppler signal that is gen-erally caused by high-amplitude, doppler-shifted echoes from the heart or vessel walls.

CMOS (comp) The complimentary metal-oxidesemiconductor is a battery-powered memorychip situated on the motherboard that main-tains the clock settings and stores a record ofthe current system configuration.

CMYK (image) Cyan, magenta, yellow and black:the four basic process colours used in conven-tional colour printing. By overlaying or ditheringcombinations of these four inks in different pro-portions, a vast range of colours can be created.

CNR (C/R) (image) See contrast-to-noise ratio.coagulation (cm) All contrast media interfere to

some extent with coagulation and clotting fac-tors. In standard doses, this interference is usu-ally clinically insignificant. It is clear that ionicagents have a more profound effect on inhibit-ing blood coagulation. The lessened anticoagu-lant effects of nonionic contrast agents, however,do not appear to increase the risk of throm-boembolic events associated with angiographicprocedures.

coaxial cable (comp) A network cable with goodnoise immunity, also known as coax or thick-net.In coaxial cable, a single wire is surrounded byinsulation and a woven copper braid that acts asshielding against electrical noise. There are two

types: (1) Thick coaxial cable (also called ‘thick-net’), the original Ethernet connection requires anAUI connector. Over time, thick coax proved to beexpensive, difficult to install and clumsy, mostlydue to its large size. (2) Thin coaxial cable (alsocalled ‘thin Ethernet’ or ‘thinnet’) became themost popular cabling.

cobalt (Co) (elem)

Atomic number (Z) 27Relative atomic mass (Ar) 58.93Density (ρ) kg/m3 8900Melting point (K) 1765Specific heat capacity J kg�1 K�1 421Thermal conductivity W m�1 K�1 100K-edge (keV) 7.7Relevance to radiology: 57Co as 99 mTc calibrationsubstitute. 58Co for in vitro tests involving cyano-cobalamine. 60Co using the two close gammas forenergy resolution calibration.

57Cobalt (nmed)

ProductionDecay scheme 57Co T½ 270 d (γ 122, 136 keV) (e.c.) 57Co → 57Fe stableDecay constant 0.00255 d�1

Gamma ray constant 1.6 � 10�2mSv hr�1 GBq�1 at1 m

Half value layer mm Pb

Days Fraction remaining

10 0.97530 0.92660 0.85890 0.794

150 0.681180 0.631

Pb (mm) Attenuation factor

0.06 0.50.5 10�1

1.4 10�2

16.0 10�3

35.0 10�4

58Cobalt (nmed)

ProductionDecay scheme 58Co T½ 71.3 d (γ 511, 811 keV)(β�, e.c.) 58Co → 58Fe stableDecay constant 0.00978 d�1

Gamma ray 1.5 � 10�2mSv hr�1 GBq�1

constant at 1 mHalf-life 71.3 daysHalf value layer mm Pb

2658

2758Fe(p,n) Co

2656

2757Fe(d,n) Co

client–58Cobalt


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60Cobalt (nmed)

ProductionDecay scheme 57Co T½ 70.8 d (γ 1.173, 1.333 keV)(β�) → 60Ni stableDecay constant 0.13149 y�1

Gamma ray 3.6 � 10�2mSv h�1GBq�1

constant at 1 mHalf-life 70.8 daysHalf value layer mm Pb

Cockcroft, John Douglas (Sir) (1897–1967)British physicist who with Ernest Walton in1932 was the first to produce, artificially, aradioisotope by bombarding lithium with pro-tons using a specially built particle accelerator,also verifying for the first time Einstein’s mass-energy equivalence. Cockcroft and Walton wereawarded the Nobel prize for physics in 1951.Cockcroft was director of Air Defence Researchand director of the UK Atomic ResearchEstablishment at Harwell. Elected FRS in 1936.Knighted in 1948.

Cockcroft–Walton effect (phys) In 1932, JohnCockcroft and Ernest Walton in Cambridge, UK,bombarded 7Li with artificially accelerated pro-tons to form 4He:

1H1 � 7Li3→ 2 � 4He2 � 17.3 MeV

This was the first artificially induced nuclearreaction and the first demonstration of nuclearfusion. It was also the first experimental confir-mation of Einstein’s formula: ΔE � Δmc2.

code number (shielding) (shld) An early methodof specifying lead or lead-equivalence; relatedto the weight per unit area in pounds persquare foot (lb ft�2). Examples for lead sheet are:

Thickness (mm) Code Weight (kg m�2)

1.32 3 15.001.80 4 20.402.24 5 25.402.65 6 30.103.15 7 35.703.55 8 40.30

coded aperture (nmed) Stationary collimators, usu-ally a multi-holed design, which produce imagedata that need some form of decoding. Thereconstruction algorithms necessary for codedaperture imaging are complex. Other coded aper-ture methods are applied to gamma cameraimaging where the collimator is removed and an

2759

2760Co(n, ) Coγ

intermediate semiconductor array are used forscatter detection. The decoded image data areobtained by iterative reconstruction techniques.Sensitivity without the collimator increases cam-era sensitivities by up to 15 times; the resolutionis degraded, however.

coefficient (math) A numerical constant used as amultiplier for a variable quantity when calcu-lating the magnitude of a physical property.The coefficient μ is a linear attenuation coefficient

which when multiplied by the absorber thick-ness gives the total attenuation.

coefficient of expansion (phys) See expansion.coefficient of variation (stats) An absolute

measure of dispersion as the standard deviation

expressed as a percentage of the mean.coeliac arteriography (clin) Arteriography of the

arterial supply to the liver, spleen and stomach.Alternative imaging using CT, MRI and ultrasound(colour Doppler and duplex scanning) havereduced the need for coeliac arteriography.

coherence (mri) Maintenance of a constant phaserelationship between waveforms. Loss of phasecoherence in the spins result in decrease intransverse magnetization and loss of MR signal.In the quantum mechanical description of mag-netic resonance, coherence refers to a transi-tion between different states of the spinsystem (see multiple quantum coherence).

coherent (mri) A state of a spin sample in whichall spins in a voxel are in-phase.

coherent scatter (phys) Also called elastic scat-tering. A photon interaction with the atom as a whole. The photon is scattered predominantlyin the forward direction with no loss of energy.Its probability is proportional to Z2 and inverselyproportional to energy E. It is therefore reducedfor soft tissues and high kV. It represents about10% of events at diagnostic energy levels (seeCompton scatter).

cohort labelling (nmed) Red blood cell (RBC)labeling using a radionuclide (i.e. 55Fe), that isincorporated into the haem portion of haemo-globin by nascent RBCs during haemopoiesis,so that all cells are the same age.

coil (mri) Single or multiple loops of wire (or electri-cal conductor, such as tubing, etc.) designedeither to produce a magnetic field from currentflowing through the wire, or to detect a changingmagnetic field by voltage induced in the wire.

coil loading (mri) The interaction of the patientwith the RF coil due to magnetic induction and

60Cobalt–coil loading

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dielectric losses causing shifts in the resonant fre-

quency and damping of the coil’s resonance andhence reduction of the quality factor (Q).

coincidence counting (nmed) Two opposingdetectors are operated in coincidence to detectsimultaneous emissions of two 511-keV pho-tons created by annihilation of electrons bypositrons.

coincidence detection (nmed) Registering a co-incident event depends on the two 180° opposedphotons being detected by the system within aspecified time interval known as the ‘coincidencewindow’ (typically 10–20 ns). Opposed detectorsused in PET imaging for registering the 180° dualgamma rays from a positron emitter decay. Allevents (E) that are found in coincidence can betrue events (T ), random events (R) or scatterevents (S). These events are related by the formula:

E � T � R � S

True events represent good data. Random eventscan be estimated and corrected, and scatterevents can be rejected. Location of the coincidentevent is assumed to be along the path betweenthe two detectors. However, this is complicated inpractice since not all events found in coincidenceare true coincidence events.

coincident radiation (nmed) A positron β�

undergoes mutual annihilation with an electrone� yielding two 180° opposed gamma rays of0.511 MeV; these rays are termed ‘coincidentradiation’.

cold spot imaging (nmed) Imaging a region ofnonradioactivity, or relatively low radioactivity,completely surrounded by high levels ofradioactivity.

collective dose (dose) See collective effective dose.collective effective dose (S) (dose) (ICRP60) The

effective dose related to exposed groups orpopulations. It is calculated as the product ofthe average effective dose E

—and the population

number N: S � NE—. The S value relates to effec-

tive whole body dose. A valuable referencemeasure is the percentage collective dose;common diagnostic values are represented inthe pie chart obtained from an exposed patientpopulation. This indicates the relative contribu-tions from each investigation, identifying thehigh-dose procedures (CT and contrast studies);the ‘other’ studies include chest extremitiesand nuclear medicine).

coil loading–collimator

collective equivalent dose (ST) (dose) (ICRP60)The equivalent dose related to exposed groupsor populations. It is calculated as the product ofaverage equivalent dose and populationnumber N. The ST value relates to a specificorgan or tissue: The unit is the man-

sievert (see cancer induction, equivalent dose).S NHT T�

HT

Collective effective doseThe mean effective dose for inhaled radon is estimated as 0.8 mSv, so for the UK population of 60 million S would be 48 000 man-Sv y�1. The UnitedStates with 260 million S would be 200 000 man-Svy�1. Medical x-rays deliver a mean effective dose ofabout 0.5 mSv per year giving S values of 30 000 and130 000 man-Sv y�1, respectively.

Collective equivalent doseThe mean equivalent dose to the lung for inhaledradon is estimated as 6.6 mSv, so for a UK populationof 60 million ST would be 396 000 man-Sv y�1. TheUnited States with 260 million ST would be 1 716 000man-Sv y�1

collimation (xray) Geometrical limitation of theextent of the radiation beam in the z-direction.

collimator (xray) Geometrical limitation of theextent of the radiation beam in the z-direction;earlier name cones. (nmed) The device in frontof the camera crystal that accepts gamma pho-tons only from a particular angle. Common col-limators have a parallel hole design that onlyaccept gamma photons perpendicular to thecrystal face. (ct) CT scanners are commonlyequipped with collimators between the x-raytube and the patient (pre-patient collimation)defining the slice and dose profile according tothe requested slice thickness giving geometricallimitation to the extent of the x-ray beam in thez-direction; some machines employ further col-limation between the patient and the detectorarray (post-patient collimation) for improvingthe slice sensitivity profile; additional comb-shapedcollimators close to the detector array in multi-slice scanners decrease the effective detectorelement width and so improve geometrical res-olution. In conventional scanning, the slice sensi-

tivity profile (SSP) and the dose sensitivity profile

(DSP) coincide precisely. The width of this pro-file, measured as full width at half maximum

(FWHM) is then the collimated slice thickness.Anti-scatter collimators between the individualdetector elements, oriented towards the focus,


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have a width of typically 0.1–0.2 mm, while theseparation of elements in the z-direction isapproximately 0.1 mm.

colloid (nmed) A suspension of solid particles ofsizes between 1 and 100 nm. There are severalcolloid materials, sulphur, albumin and antimonysulphide, used for imaging the reticuloendothe-lial system: liver, spleen and bone marrow. Tincolloid provides 80% of particle sizes between0.05 and 0.6 μm. Sulphur colloid provides parti-cles between 20 and 200 nm. Antimony sulphide(Sb2S3) provides particles between 3 and 30 nm.

Generic name 99 mTc sulphur colloid, 99 mTc-albumin colloid

Commercial names CIS-Sulphur Colloid® NycomedSC®, Microlite®

Imaging category RES (liver/spleen) gastric emptying, GI bleeding

(see Nanocol).colour change (contrast medium) (cm) The main

reasons for colour changes are: byproducts ofmanufacture and chemical decomposition. Theyellow/brown discolouration is attributed toorganic compounds and not free iodine.■ Reference: Dawson and Clauss, 1999.

colour flow display/imaging (us) A 2D represen-tation of blood flow (velocity). The Doppler signalis colour coded according to velocity and super-imposed on a grey-scale ultrasound image sup-plied by a linear or phased array transducer. Thepulsed Doppler transducer is incorporated in thearray. Colour coded (blue/red) according to direc-tion can also be displayed (see Doppler (colour)).

colour scale (di) Converting an image of count rateor signal strength into a colour-coded map usuallyby means of a look-up table. A rainbow scale typ-ically represents low signal in blue and maximumas white with intermediate colours in a rangefrom green, red, orange and yellow. A thermalscale uses an orange–yellow–white spectrum.

colour vision (clin) The human eye uses two setsof light sensitive cells: rods and cones. Rods aresensitive to low light levels, but are not coloursensitive. Cones are much less sensitive to lowlight levels, but have a higher resolving powerand are colour sensitive.

Coltman JW American engineer who producedthe first electronic fluoroscopic tube in 1948.

column (nmed) The solid matrix within a radio-nuclide generator where the parent radionuclideis chemically or physically bound.

collimator–committed radiation-weighted dose, HT(τ)

columns (mri) Frequency-encoded portion of themeasurement matrix (see rows).

comet tail (us) A series of closely spaced rever-beration echoes.

committed dose equivalent (HT50) (dose) Thedose equivalent to organs or reference tissuesthat will be received by an individual over a50-year period from the initial intake.

committed effective dose (E(τ)) (dose) (ICRP60) Ifthe committed organ or tissue equivalent doses aremultiplied by the appropriate tissue weightingfactors and then summed, the result will be thecommitted effective dose: where HT (τ) is the committed equivalent dose intissue T, wT is the weighting factor for tissue T,and (τ) is the integration period in years; 50 yearsfor adults, and up to 70 years for children. Theunit is the sievert (Sv).

E wT HT( ) ( )τ τ� �∑

Committed effective doseThe mean effective dose for inhaled radon is estimatedas 6.6 mSv � 0.12 wT � 0.8 mSv (whole body), givingan E(τ) value for a 50-year period of 40 mSv. A radiationworker receiving, on average, 1.5 mSv y�1 with a work-ing life-time of 40 years, the E(τ) would be 60 mSv.

(see effective dose, collective effective dose).committed effective dose equivalent (HE50)

(dose) The sum of the products of the weightingfactors wT to each organ or tissue and the HT50to these organs or tissues so:

committed equivalent dose (HT(τ)) (dose)(ICRP60) Committed radiation-weighted dose;subsidiary dosimetric quantity of equivalentsingle organ or tissue dose over a time period,where t is the integration time in years takenas 50 for adults and 70 for children followingan intake of radioactive material. Defined as:

The unit is the sievert (Sv).

HT HT t dt( ) ( )τ � ∫

H wT HE T50 50� �∑ .

Committed equivalent doseThe mean equivalent dose to the lung for inhaledradon is estimated as 6.6 mSv, giving a HT(τ) value for50 years as 330 mSv.

(see equivalent dose, collective equivalent dose).committed radiation-weighted dose, HT(τ) (dose)

See committed equivalent dose.

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communications–compression (image)

communications (di) Transfer of informationbetween computers or computer networks. Animportant parts of a PAC’s system (see network,PACs).

compact disk (comp) See CD.compartmental localization (nmed) Placement of

a radiopharmaceutical in a fluid space and thenimaging that fluid space (e.g. a ventilation studywith 133Xe or a cisternogram with 111In DTPA).

compensation (us) Equalization of received echoamplitude differences caused by differentattenuations for different reflector depths; alsocalled depth gain compensation (DGC) or timegain compensation (TGC).

compiler (comp) A special program that takes theinstructions written in a programming lan-guage and turns them into machine code that a PC processor can understand.

complex numbers (math) There is no solution forx where x2 � 1 � 0. This problem is solved byintroducing an ‘imaginary’ number i so i2 � �1.This develops as the equation a � ib in physics,where a and b are ‘real’ numbers and i is .Sometimes j and q are used instead of i (seeimpedance, Euler’s formula, Fourier analysis).

compliance (us) Distensibility; nonrigid stretcha-bility of vessels.

compliance (screening) (stats) The proportion ofthe population who present for screeningamongst the total invited number.

composite (us) Combination of a piezoelectricceramic and a nonpiezoelectric polymer used inthe ultrasound transducer construction.

composite colours (image) Colours formed bymixing various quantities of cyan, magenta,yellow and black.

composite excitation (mri) Tissue excitation by aseries of pulses rather than by a single radio-frequency (RF) pulse. Composite excitation pro-vides the sum of signals to produce netexcitation of a target tissue. Applications includeselective excitation of water, or of tissues notsubject to magnetization transfer.

�1

compound anode (xray) The highest thermo-mechanical demands are placed on a rotatinganode. It was originally manufactured from puretungsten, but now has a sandwich formation ofvarious metals including tungsten, molybde-num, rhenium and zirconium (TMZ). By alloyingrhenium and tungsten, an improvement in theelastic properties of the 1–2 mm thick target isobtained, greatly increasing its wear properties.A disk base of molybdenum and zirconiumincreases heat capacity for the same mass.

composite colours (image) Colours formed bymixing various quantities of cyan, magenta,yellow and black.

composite video (comp) A video signal in whichthe luminance (brightness), chrominance (colour),blanking pulses, sync pulses and colour burstinformation have been combined using one ofthe coding standards (NTSC, PAL, SECAM).

compressibility (us) Ability of a material to bereduced to a smaller volume under externalpressure.

compression (phys) The converse of expansionby heating and can only be observed in gases,since solids and liquids are incompressible.Compression increases the gas density so moremolecules collide with the surface and thereforemore kinetic energy is turned into heat. Theheating effect by compression is the converse ofexpansion by heating and can only be observedin gases, since solids and liquids are incom-pressible. (us) Reduction in differences betweensmall and large amplitudes; region of high den-sity and pressure in a compressional wave.

compression (image) (image) Image compres-sion improves both storage and transmission.Lossless and ‘lossy’ techniques are applieddepending on acceptable image quality. Currentmethods employ JPEG algorithms based on a dis-

crete cosine transform giving compression ratios of�50:1 (see Table below). Other techniquesemploy run-length encoding, wavelet and fractal

algorithms (see PACS).

Image format File size (MByte) Property

Uncompressed TIFF 14.1 3 channels of 8 bitsUncompressed 12-bit RAW 7.7 1 channel of 12 bitsCompressed TIFF 6.0 Lossless compressionCompressed 12-bit RAW 4.3 Nearly lossless compression100% Quality JPEG uncompressed 2.3 Hard to distinguish from80% Quality JPEG 1.3 Sufficient quality for 10 � 15 cm prints60% Quality JPEG 0.7 Sufficient quality for websites20% Quality JPEG 0.2 Very low image quality


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compression (tissue)–Compton scatter

compression (tissue) (xray) x-ray absorptionobeys Nx � N0 � e�μx, where Nx is the exit flu-ence and N0 the incident beam intensity, μ is thelinear attenuation coefficient and x the absorber (tis-sue) thickness. Compression reduces tissue vol-ume, decreasing both absorption and scatter.This improves both image quality and tissuedose. Compression reduces object to film distanceso reducing geometrical unsharpness and localizesthe organ so reducing movement unsharpness.Compression therefore significantly influences:

● tissue dose;● image resolution;● image contrast;● geometrical unsharpness;● movement unsharpness.

compression paddle (xray) A rectangular trans-parent plastic plate parallel to the support platecarrying the imaging surface (film cassetteholder) which compresses the breast using aremote control device. The maximum compres-sion force is 200 N (approximately 20 kg).

compressibility (κ) (us) A factor used for calcu-lating acoustic impedance and depends on theelastic modulus K (stress/strain) so that κ � 1/K.

Compton, Arthur H (1892–1962) US physicistwho first observed and analyzed scattering ofelectromagnetic radiation in 1922. Awardedthe Nobel prize for physics in 1927.

Compton continuum (nmed) The part of the gammaspectrum from the low energy region to theCompton edge which represents loss of energy inthe detector material from Compton scatter events.

Compton edge (nmed) The edge of the Comptoncontinuum in the gamma spectrum which

Photon energy (keV)

Compton edge

Backscatter

Photopeak

Rel

ativ

e in

tens

ity

represents maximum energy loss due to 180°scatter events (see gamma spectrum).

Compton effect (phys) See Compton scatter.Compton plateau (phys) Broad, flat portion of a

pulse-height spectrum corresponding to Comptonscattering of photons in a scintillation detectorcrystal, patient and surroundings.

Compton scatter (phys) An interaction betweenan incident photon with an orbital electron in a partially elastic collision, transferring part ofits energy to the electron and causing it to beejected from its orbit. The residual gamma-rayenergy is deflected at an angle opposite that ofthe ejected electron. The scattering of electro-magnetic radiation requires that the electronabsorbs energy from the photon and reradiatesit as scattered radiation. The frequency of thescattered radiation is smaller than the incidentfrequency, so accordingly the wavelength islonger and the photon energy smaller. In orderto preserve energy and momentum the wave-length before λ1 and after λ2 then the relation-ship is:

where h/mec is the Compton wavelength forelectrons. Substituting the constants gives thevalue 2.426 � 10�12m. The equation demon-strates that the change in wavelengthΔλ � λ2 � λ1 is not dependent on the incidentwavelength. The change in energy ΔE � E1 �E2

is given by:

mec2 � 8.19 � 10�14 J or 0.511 MeV so re-arranging gives:

At low energies, the scattered photon retains alarge fraction of the available energy as thephoton energy increases, the recoil photonretains the larger fraction. For a 90° Comptonevent a 10 keV photon loses 1% of its energy tothe Compton electron; a 100 keV photon loses16% and a 1 MeV photon 66%.

E E2

1

1 0 511 1�

� � �(E / ) ( )1 . cosθ

ΔEE

m ce

� �12

2 1( cos )θ

λ λ θ2 1 1� � �h

m ce( cos )

C


Compton scatter–concrete

(see photon).Compton wavelength (units) See Compton effect.computed radiography (CR) (image) An erasa-

ble x-ray imaging device based on x-ray exci-tation of a phosphor layer and subsequentimage data reading with an infrared (IR) laser(photostimulable luminescence); a methoddeveloped commercially by Fuji in 1983. Thephotostimulable phosphor plate (also known asan image plate, storage phosphor plate or com-puted radiography (CR)) demonstrates phos-phorescence or photoluminescence so is able tostore x-ray energy and later, when stimulatedby an IR laser, frees the energy as emitted light.The light signal is detected by a photomulti-plier tube, and the output electrical signal isdigitized to form an image matrix. The finalresult is a digital projection radiograph (seeThoravision, direct radiography).

computed tomography (CT) (ct) A term originallyapplied to x-ray CT but can now be applied toother tomographic imaging techniques (nuclearmedicine) where a series of ray sums are col-lected from a 360� sampling and then backpro-jected to obtain an axial section image. X-ray

10 100Photon energy (keV)

Sca

tter

coef

ficie

nts

(rel

ativ

e)

Electron

Photon

Total

100

10

1

1000

Scattered photon

Recoil electron

Incident photon�

�

Atom L

K

E1

E2

computed tomography produces separate axialsectional images (transverse slices). Computedaxial tomography is entirely different from linear

tomography and produces radiological images astransaxial sections of the body without any inter-sectional interference or blurring. The methodwas first developed in a commercial x-raymachine in the UK by Godfrey Hounsfield in 1973.

computed tomography dose index (ct) See CTDI.computer aided diagnosis (CAD) (image) A par-

ticular feature extracting algorithms used as apreliminary protocol for eliminating or detect-ing a specified image pattern. Used in mam-mography and chest radiology.

concatenation (mri) Distributing slices into multi-ple measurements. Prevents crosstalk in thecase of short slice distance.

concentration (iodine) (cm) Both viscosity andosmolality of a contrast medium are related to theconcentration or strength of the contrast medium.This is usually given in terms of concentration ofiodine, indicating the concentration in milligramsof iodine per millilitre (mg/mL I).

concrete (shld) High density concrete 2350kg m�3.Load bearing must be at least 150-mm thick, typ-ical density 2400kg m�3. Has a lead equivalent(Pb-eq) similar to barium plaster. Relevant details:

Lead (11340 kg m�3) Concrete (2400 kg m�3)

0.5 mm 50 mm1.0 mm 81 mm2.0 mm 162 mm2.5 mm �200 mm3.0 mm 244 mm4.0 mm 325 mm

Concrete is a cheap structural shielding mat-erial. Its density can vary between 2400 and

250

200

150

100

50

00 1 2

Sheet lead (mm)

Con

cret

e th

icne

ss (

mm

)

3 4


C

3200 kg m�3 and concrete blocks are typically1750 kg m�3. Modern hospital ceiling construc-tion currently employs 200 mm prestressedconcrete sections.

conditional probability (stats) The probabilitythat a disease (A) exists given the positive resultof a test (B). Described as P(A|B). An expressionused in Bayes’ theorem (see prevalence).

conductance (electric) (phys) Defined as theratio I/V and is therefore the inverse of resist-ance being 1/R. The unit of conductance is thesiemens (S).

conduction (electrical) (phys) Depends on freeelectrons whose drift constitutes an electricalcurrent. This has a slow velocity; a current den-sity of 10 amps in 1 mm2 cross-section wireshows an electron velocity of 6.25 mm s�1.

conduction (heat) (units) Thermal conductivity isdefined as the heat flow per second per unit areaper unit temperature gradient. Thermal conduc-tivity measured in W m�2 K�1 is called a ‘U’ value.

Silver 419Copper 385Tungsten 200Molybdenum 150Glass �1.0Water 0.6Air 0.02Oil �0.15

Since the ratio of thermal and electrical con-ductivity is the same for all metals, it suggestselectrons are also thermal carriers.

conduction (light) (phys) The phenomena of thepropagation of light can be interpreted ad-equately by wave theory. Light is propagated bytransverse waves since the vibrations are per-pendicular to the direction of travel. Light inter-acts passively in transparent material, the maininteractions being reflection and refraction.

conduction (sound) (phys) Sound travels through afluid as a longitudinal wave (waves in which theparticles of the transmitting medium are displacedalong the direction of propagation; in solids, lon-gitudinal, transverse and torsional waves exist.Propagation of sound into a surrounding mediuminvolves disturbance in the medium, but becauseof a certain lag will have a finite speed. The speedc equals where E is Young’s modulus and ρthe density. For longitudinal waves, where k is the bulk modulus. Sound waves in gastravel in longitudinal wave.

c k� /ρE/ρ

conduction (thermal) (phys) The speed of heattransfer. Measured in watts per metre per degreekelvin (or Celsius) W m�1K�1 (�C�1) as thermalconductivity, characteristic of the material, inde-pendent of size or shape. Generally the heat Qgained (or lost) by an object: Q � mc ΔT, where mis the mass of the object, c its specific heat capac-ity and ΔT its temperature change. A loss of heatQ to the surroundings is ΔT � Q/mc, the temper-ature fall ΔT of a small mass of material is greaterthan a large mass of material at the same tem-perature. The rate of cooling depends on natureand surface area, in addition to its temperature,mass and specific heat capacity. The table liststhermal conductivities of some important sub-stances to radiology; silver has the highest value(427 J kg�1K�1) and air has one of the lowest0.02 J kg�1K�1.

Substance Specific Thermal heat conductivity (J kg�1 K�1) (W m�1K�1)

Water 4200 0.59Oil 2130 0.15Aluminium 910 237Graphite 711 �30Titanium 523 23Copper 386 401Zirconium 280 22Molybdenum 246 140Rhenium 138 48Tungsten 136 178Glass 67 0.9–1.3

cones (clin) See colour vision; (xray) See collimation.cone angle (ct) Aperture angle of a cone beam

measured in the direction of the system axis of

Detector z-axis

concrete–cone angle

C


rotation. The data sets recorded by any singlerow in cone beam geometry will become increas-ingly inconsistent with increasing distance fromthe central plane. Artefacts will result wheneverthe cone angle exceeds a few degrees. Beyondfour detectors, image distortion becomes pro-nounced and reconstruction algorithms are neededto correct for the divergent beam angles.

cone beam (ct) The transition from scanning oneor only a few slices to data acquisition for anentire field requires the transition from fanbeam to cone beam geometry. With single-detector scanners, the beam is always central tothe detector and encounters the same tissuesection. Increasing the number of detectors inthe z-axis increases the angular volume and thebeam dimension resembles a cone rather than aflat plane.

cone-beam artefacts (ct) An acquisition usingcone beam geometry causes diameter decreasetowards the top of the cone. These conic profilesare not treated in the same way during interpo-lation and this generates an artefact similar to a partial volume effect. Instead of a perfect cir-cle, an ellipse-like reconstruction is displayed;more apparent when the cone has a large topangle, or when large pitches are used (i.e. skull).

confidence interval (stats) That interval withinwhich a parameter of a parent population is cal-culated to have a stated probability of being posi-tioned. The larger the sample size, the smallerthe confidence interval. Commonly used for qual-ity control charts that have confidence limits.

confidence limits (stats) The lowest and highestestimate of a parameter, that is statisticallycompatible with the data. For a 95% confidenceinterval, there is a 95% chance that the intervalcontains the measured parameter.

conformity (declaration) (us) A statement madeby the submitter that a particular device wastested and meets the requirements of a recog-nized standard. It should clearly specify the following:

1 Any element of the standard that was notapplicable to the device.

2 If the standard is part of a family of standardswhich provides collateral and/or particular parts,a statement regarding the collateral and/or par-ticular parts that were met.

3 Any deviations from the standards that wereapplied.

4 What differences exist, if any, between thetested device(s) and the device to be marketedand a justification of the test results in thoseareas of difference.

5 Name and address of any test laboratory or cer-tification body involved and a reference to anyaccreditations of those organizations.

Conray® (cm) Ionic monomer; a commercialpreparation (Malinckrodt/Tyco Healthcare Inc)of meglumine iothalamate. Produced as Conray30, 43, 400 also as Cysto-Conray®. Cardio-Conray has 400 mg I ml�1.


Meglumine 6 @ 25°C 1400 282iothalamate 4 @ 37°C

conservation of energy (phys) The total energyin a closed system is always constant; this isthe first law of thermodynamics. Energy maybe converted into any other (mechanical intoelectrical in ultrasound; kinetic into electro-magnetic radiation in the x-ray tube). Therewill always be a loss of energy in the form ofheat (�95% in the case of x-ray production).This law applies to both inanimate (mechanical)objects and living organisms (see Compton scat-

ter, photo-electric reaction).conservation of momentum (phys) Conservation

of linear momentum states that if no externalforces act on a system of colliding objects. Thetotal momentum of the objects is the same beforeand after the collision. Conservation of angularmomentum states that the angular momentumabout an axis of a given rotating body or systemof bodies is constant and no external torque actsabout that axis (applications in MRI) (see Compton

scatter, photo-electric reaction).constants (phys) Certain physical constants have

special importance on account of their univer-sality or place in fundamental theory. The con-stants commonly required in the radiologicalsciences are:

Speed of light, c 2.997 � 108 m s�1

Planck’s constant, h 6.626 � 10�34 J sElectron rest mass, me 9.109 � 10�31kgCompton wave length, λc 2.426 � 10�12mGyromagnetic ratio, γ 2.6752 � 108s�1T�1

Avogadro constant, N 6.022 � 1023mol�1

cone angle–constants


C

constraint (dose) (ICRP60) A restriction on the pre-dicted dose to persons from a defined source toensure dose limits are not exceeded. The doseconstraint is set at a fraction of the dose limitgained from general knowledge of exposurefrom the source and is seen as an optimization.Diagnostic radiology practice has a recom-mended dose constraint of 5 mSv per year (seejustification).

constructive interference (us) Combination ofpositive or negative pressures.

contamination (nmed) Radioactive material presentin undesired locations; a source of background orpossible hazard requiring decontamination.

contingency table (stats) A table of observa-tions/measurements cross-referencing a set ofvariable. The chi-squared test is commonly dis-played this way.

continuous mode (us) Continuous-wave mode.continuous spectrum (xray) Polyenergetic spec-

trum (see bremsstrahlung spectrum, beta decay).continuous variable (stats) A measurement not

restricted to fixed values (i.e. kidney function(glomerular filtration rate, GFR) or blood pressure).

continuous wave NMR (CW) (mri) Achievingnuclear magnetic resonance by continuouslyapplying RF excitation to the sample and slowlysweeping either the RF frequency or the mag-

netic field through the resonant values; largelysuperseded by pulse MR techniques.

continuous wave (us) A wave in which cyclesrepeat indefinitely; not pulsed.

continuous-wave Doppler (us) A Doppler deviceor procedure that uses continuous-wave ultrasound.

contour artefact (mri) Chemical shift artefact.contrast (CT) (ct) The absolute difference between

the CT (Hounsfield) numbers of adjacent regionsor structures within an image.

contrast (film) (image) Measured as the film gamma.This is not a property only of the film emulsion, italso depends on development temperature andtime of the developer cycle. Contrast approachesa limiting value beyond which further develop-ment will have no effect (see characteristic curve,optical density) (see first figure in next column).

contrast (image) (phys) A measure of machineperformance (e.g. CT or MRI) depending on thesize and object density. Image contrast can berepresented by a contrast detail diagram (CDD)and contrast-to-noise ratio (see second figure innext column).

contrast (MRI) (mri) The contrast behaviour inmagnetic resonance imaging is complex. Contrastfactors are generally divided into intrinsic andextrinsic, although they can interact and influ-ence one another. Some of the factors are:

Intrinsic Extrinsic

Proton density Field strengthT1 relaxation Gradient strengthT2 relaxation Field homogeneityChemical shift Surface coil typeBlood flow RF pulse sequenceTissue type Repetition time

Echo timeFlip angleContrast agents

contrast (radiographic) (Cr) (phys) A measure ofimage density differences by reference to thebackground (D1) and object density D2 so:

This is influenced by image (film) gamma andwindowing (see contrast-to-noise ratio).

CD D

Dr ��2 1

1.

D2

D2

D1

D1

D2 � D1

D1

C �

4

3

3

1

00.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Exposure

Contrast

Optical density

Res

pons

e

constraint–contrast (radiographic) (Cr)

C


contrast (subject) (Cs) (phys) Concerns beam inten-sity differences (DC) within the absorber (patientvolume, organ). If the incident beam (I1) is attenu-ated when travelling through absorber (I2); subjectcontrast Cs is simply I1 � I2. The parametersaffecting subject contrast are: kVp, attenuationcoefficient, electron density, absorber thicknessand radiation scatter. Subject contrast should beoptimized for best radiographic contrast.

contrast (visible) (phys) This is measured fromthe differences in the log of two intensities I1and I2, so visible contrast is (log I1 � log I2). Thesensitivity to low contrast peaks at a brightnesslevel of 10–100 lux.

contrast agents (mri) See contrast (MRI), contrast

media (MRI). (us) These enhance blood velocitysignals, so improving vascular structures anddetecting low velocities. They are gas-filledmicrobubbles (diameter � 10 μm) which respondto pressure amplitudes of the ultrasound pulseby expanding and contracting oscillations.Oscillating amplitudes at low ultrasound intensi-ties are linearly proportional to the excitationpressure. They also allow evaluation of perfusiontextures and perfusion time courses in tissueregions (e.g. tumour parenchyma). Contrastagents are further used for:

● tissue differentiation in the liver;● heart function;● signal enhancement in transcranial applications;● paediatric urine reflux detection from bladder to

kidney.

Definity®, Sonovist®, Imagent®, Optison® andSonoVue® are used for haemodynamic studiesand are various perfusion patterns associatedwith specific tissue properties. Contrast harmonic

imaging shows the harmonics of the echoes fromoscillating microbubbles.

contrast detail diagram (CDD) (image) An exam-ple of a display threshold visibility assessment,unlike an ROC analysis which is an assessment ofdetectability. Observers see simple objects ofdifferent sizes and contrast levels (typically cir-cles) created by diagnostic imaging devices (CT,nuclear medicine and digital x-ray). They areasked to record their minimum diameter or min-imum contrast that they perceive at a givenconfidence level and results are plotted on acontrast-detail diagram in which contrast (orlog-contrast) on the y-axis is plotted againstdiameter (or log diameter) on the x-axis.

contrast-enhancement (mri) Contrast-enhancedangiography utilizing the T1 reduction of mov-ing blood through vessels.

contrast gradient (film) The slope of the film’scharacteristic curve; measured as the gamma (seesensitometry).

contrast improvement factor (xray ) Improvementof contrast when an anti-scatter grid is used.

contrast medium/material (cm) The imagingprocedure decides the most appropriate con-trast medium. For x-ray imaging barium sul-phate, iodinated compounds provide positivecontrast agents and gas O2 and CO2 providenegative contrast; extracellular fluid contrastmedia (ECF-CM) for angiography, urography, myel-

ography, etc. Tissue specific-CM for cholangiogra-

phy and macromolecular-CM confined to thevascular space or blood pool. Water-soluble CMis subdivided into renal excretion and biliaryexcretion agents. Ionic and nonionic com-pounds are available. General intravascularcontrast media must have optimum osmolarityand minimum chemotaxis including ioniceffects. Magnetic resonance imaging, paramagnetic,diamagnetic and ferromagnetic compounds areused, shortening T1 and T2. Ultrasound imag-ing (sonography) uses encapsulated microbub-bles. For nuclear medicine (molecular imaging),tissue-specific radionuclides or labelled agents

1000

100

10

10 1 2 3 4 5 6

Con

tras

t diff

eren

ce (

rela

tive)

Small detail visibleat high contrast

Large detail visibleat low contrast

Detail size (cm)

contrast (subject) (Cs)–contrast medium/material


C

are used (see contrast medium, diatrizoate com-

pounds and proprietary names).■ Reference: Dawson and Clauss, 1999.

contrast medium (gaseous) (cm) Carbon dioxideis indicated intra-arterially for renal angiography.The gas displaces blood and acts as a negativecontrast medium. There is an absence of nephro-

toxicity and allergic reactions. Subsequently theCO2 rapidly dissolves and is respired.

contrast medium (MRI) (cm) intravascular MRIcontrast agents used in clinical imaging are para-

magnetic compounds acting indirectly on T1 andT2 relaxation processes through alteration of thelocal magnetic environment, shortening both T1and T2, the T1 shortening process dominatingand produces enhancement. An MRI contrastagent has magnetic susceptibility, defined as theratio of induced magnetism to that of the mag-netic field applied. There are four categories ofmagnetic behaviour:

1 diamagnetic;2 paramagnetic;3 superparamagnetic;4 ferromagnetic.

The most common paramagnetic substance isgadolinium and its compounds. Positive contrastagents cause a reduction in the T1 relaxationtime (increased signal intensity on T1-weightedimages), appearing white on the MRI display.These are compounds containing gadolinium,manganese or iron. These elements haveunpaired electron spins in their outer shells andlong relaxation times.

Generic name Commercial name

Gadopentetate dimeglumine Magnevist®

Gadoteridol Primovist®, Prohance®

Gadoterate meglumine Dotarem®

Gadobuterol Gadovist®

Mangafodipir trisodium Teslascan®

Gadodiamide Omniscan®

Negative contrast agents are colloidal or smallparticle aggregates, typically superparamag-netic iron oxide (SPIO). These agents producespin–spin relaxation effects causing local fieldinhomogeneities, which give shorter T1 and T2relaxation times appearing predominantly darkon the MRI display. These compounds usuallyconsist of a crystalline iron oxide core and ashell of dextran, polyethyleneglycol, producing

a very high T2 signal. Colloids smaller than300 nm cause strong T1 relaxation. T2-weightedeffects are predominant.

Generic name Commercial name

Ferucarbotran Resovist®

A smaller group of negative agents are perfluorocarbonswhich exclude hydrogen atoms (protons).

■ Reference: Elster, 1994.

contrast medium (tolerance) (cm) A balancedaddition of sodium and calcium to respect theelectrophysiology of cardiac cells and improvecontrast media tolerance in cardiac patients,especially during angiocardiographic proce-dures (see anaphylactic reactions, biodistribution,blood–brain barrier, distribution coefficient, excretion/

elimination half-life, osmolality/osmolarity, pH value,pharmacokinetics, protein binding, tissue-specific,toxicity, viscosity).

contrast medium (ultrasound) (cm) These aremicrobubbles commonly contained in a carbo-hydrate envelope. These act as echo enhancersas the ultrasound beam passes from blood togas in the microbubble. The enhanced echo sig-nal is proportional to the magnitude in changeof the acoustic impedance. Microbubbles aresmaller than the capillary bed (lung and cere-bral) to prevent embolization. Reflectivity is pro-portional to the particles diameter to the fourthpower (D4) and the concentration of microbub-bles which are eventually retained by the liver.The success of ultrasound contrast materialdepends on its prolonged persistence in thevascular bed; however, certain targeted ultra-sound contrast media are used for visualizingthe liver and spleen (see Sonovue®).

contrast resolution (low) (image) A measure ofthe ability to discriminate between structureswith slightly differing attenuation properties (CTnumber). It depends on the stochastic noise andis usually expressed as the minimum detectablesize of detail discernable in the image, for afixed percentage difference in contrast relativeto the adjacent background.

contrast-to-noise ratio (CNR) (image) Ratio of thedifference between two regions by reference totheir signal to noise ratio: CNR � SNR1 � SNR2.Although radiographic contrast differences maybe large, they may not be as visible as smallercontrast differences having lower backgroundnoise levels.

contrast medium/material–contrast-to-noise ratio (CNR)

C


contrast ratio (image) A low contrast measure.Output light intensity before and after a 10%central region is shielded (see veiling glare).

contrast resolution (us) Ability of a grey-scaledisplay to distinguish between echoes ofslightly different amplitudes or intensities.

control group (stats) A population group not sub-jected to the test being applied. This is ideally agroup of normals/members of the public.

controlled area (dose) (ICRP73) The control ofoccupational exposure in medicine can be sim-plified by the designation of workplaces intocontrolled and supervised areas. A controlledarea is where under normal working conditionsthere is a possibility of exposure and workersare required to follow well-established rulesand practices designed to reduce exposure.Commonly defined as a room or location, withlimited access, where maximum surface expo-sure level is 7.5 μSv hr�1 or 3/10 of any maxi-mum could be exceeded.

controlled trial (stats) An investigation using anagreed method of analysis on a group of patientswith a matching control group (i.e. age, sex).

controller (comp) A circuit board that links thehard disk and the motherboard.

convection (thermal) (phys) The transfer of heatin air or fluid by the movement of the mediumitself. There are two types:

1 Natural or free convection where the motion ofthe medium is due to the presence of the hotbody giving rise to temperature and thereforedensity differences.

2 Forced convection where movement of themedium is maintained by mechanical means(e.g. a fan).

X-ray tube housing utilizes both sorts, liquid (oiland water) and air. These media can undergofree or forced convection depending on heatoutput.

convection (thermal: forced) (xray) Seeconvection.

converging collimator (nmed) A gamma cameracollimator which gives a magnified image, butalso distorts with depth.

conversion efficiency (xray) The percentage ofenergy deposited in a detector that contributesto the output signal (electrical pulse or lightpulse).

convex array (us) See linear array.

convolution (ct) A mathematical operationapplied to two functions f (x) and k(x), where thelatter is called the convolution kernel; the resultg (x) is defined as:

g(x ) � f(x ) * k(x ) � J f (t ) k(x � t )dt

Commonly the * symbol is used as an abbrevia-tion for the convolution integral. The back-projected matrix is subjected to a filter mask, orconvolution kernel, whose contents are multi-plied with the back-projected image. The maskis a small symmetrical matrix containing a set ofnumbers having positive or negative values. Allthe values are summed and the result placed inthe central pixel of the image array. The filtermask is then shifted one column and the com-plete process repeated. The signal data are firstlogarithmically amplified to correct for transmis-sion absorption, then beam hardening correc-tions are made. Each projection then undergoesthe convolution process before back projection,the type of convolution filter can be chosen bythe user, giving either image smoothing or edgeenhancement. (image) For one-dimensional sig-nals (e.g. renogram, bolus transit), convolution isthe distribution of one function in accordancewith a law specified by another function. If theoriginal input signal is a delta function f(x) yieldingan output signal g (x ) influenced by the system’simpulse response h(x ). It is assumed that the sys-tem is linear invariant. The process of deconvolu-tion is the reverse, where f (x ) is extracted fromg (x ) with a knowledge of h(x ). It is applied torenal transit times where a blood clearancecurve undergoes deconvolution to yield the origi-nal spike injection peak.

convolution differencing (mri) A method of sup-pressing broad underlying spectral lines inorder to emphasize narrower spectral lines.Strong smoothing of the spectrum (e.g. bysevere negative exponential weighting of thetime data) will suppress the narrow lines, butminimally affect very broad ones: subtractingsuch a smoothed spectrum from the originalwill largely remove the contributions from thebroad lines. This provides a means of baseline

correction. A form of spectral unsharp masking issometimes used.

convolution kernel (image) A small square matrixused as an image filter. Can be small (3 � 3)having minimal effect or large (32 � 32) havingmaximal effect. (ct) A selected function for

contrast ratio–convolution kernel


C

filtered backprojection, which can be varied tosuit contrast detail (see image filter, convolution).

cookie (comp) A file that records information aboutspecific details when accessing the world wideweb. A message sent by a web server to a browserand stored on the client hard disk. Typically smallfiles are downloaded to the workstation whenbrowsing certain web pages. Cookies hold infor-mation that can be retrieved by other web pageson the site. The information usually identifies userpreferences for tailoring responses.

Cooley–Tukey (math) The two mathematicianswho developed the fast Fourier transform.

Coolidge, William David (1873–1975) Americanphysical chemist. In 1908, he discovered how to manufacture ductile tungsten and in 1916replaced the cold cathode in x-ray tubes with aheated cathode. The Coolidge tube was the pre-cursor for all modern x-ray tubes.

cooling curve (xray) Measures the heat lost by thex-ray tube and determines the tube workload(loadability). Depends on anode heat storage andtemperature t, since heat loss is proportional tot4, measured in joules per minute. Tube housingis also involved and forced convection is some-times necessary to improve heat loss (see heat

storage, tube rating, anode cooling).coordinate system (ct) Cartesian x, y and z-coor-

dinate system of the CT system; the x/y-planeis the transverse or section-slice plane inanatomy (this is not the case when the gantryis tilted), the x-axis points in the lateral direc-tion, the y-axis is oriented in the anterior–posterior or posterior–anterior direction; the z-axis is orthogonal to the scan plane and parallel to the table movement.

COPE (mri) Centrally ordered phase encoding. Arespiratory compensation technique.

x-axis

Multislice detector

z-axis

y-ax

is

copper (Cu) (elem)

Atomic number (Z) 29Relative atomic mass (Ar) 63.54Density (ρ) kg/m3 8930Melting point (K) 1356Specific heat capacity J kg�1 K�1 385Thermal conductivity W m�1K�1 401K-edge (keV) 8.9Relevance to radiology: Electrical conductor and x-raybeam filter

64Copper (nmed) β� energy 656 keV

Production (cyclotron)Decay scheme 64Cu T½ 12.7 hr

(β�) 64Cu (β�, 2γ 511 keV) → 64Ni stableHalf value layer 4.1 mm Pb (0.511 MeV γ)Generator derived positron emission isotope for PETimaging

CORE (mri) COnstrained RE-construction. Methodof reconstructing artefact-free super resolutionimages.

Cormack, Alan MacLeod American medical physi-cist who pioneered the first computerized axialscanner independently of Godfrey Hounsfield,sharing the Nobel prize in 1979.

coronal plane (ct) Anatomical plane orthogonalto the transverse (axial) and the sagittal plane; inthe CT coordinate system the coronal plane isusually oriented parallel to the z-plane.

coronary angiography (clin) Radiography ofmyocardial vessels by introducing contrastmedium into a selected vessel (typically femoralor brachial route or directly into aorta).

coronary arteriography (clin) Right heart arteri-ography: Using the fluoroscopic technique, acatheter is usually inserted into the medianbasilica or saphenous vein and advanced intothe right atrium. Left heart arteriography: Thecatheter, under fluoroscopic control, is advancedvia the femoral or axillary artery mostly using aSeldinger technique then advanced along theaorta into the chambers of the left heart andcontrast medium injected.

Right Left

RightCoronalplane

Left

(Anterior) (Posterior)

2864

2964Ni(p,n) Cu

convolution kernel–coronary arteriography

C


correlation–coupling

correlation (stats) An interdependence betweenpairs of measurements/variables.

correlation coefficient (stats) A measure ofassociation between two random variables. Ifone variable changes with the other then theyare said to be correlated.

correlation time (mri) The characteristic timebetween significant fluctuations in the localmagnetic field experienced by a spin due tomolecular motions. For values of the correlationtime, such that the magnetic field as a functionof time has large Fourier components near theresonance frequency, the TI relaxation time willbe shortened.

cos (math) Symbol used for cosine in trigonometry.cosine transform (di) See discrete cosine transform.couch/table increment (ct) Distance by which

position of patient couch (table) is changedbetween individual slices in serial scanning orthe distance the couch position is changed dur-ing one 3600 rotation of the tube during helicalscanning.

coulomb C (units) The SI unit of charge; onecoulomb is the charge carried by 6.24 � 1018

electrons and one ampere is equal to one coulombper second (C s�1); the charge transferred by acurrent of one ampere in one second; each elec-tron has a charge Q of 1.6 � 10�19C so:

Radiation exposure can be measured ascoulombs per kilogram (C kg�1) where oneroentgen (1 R) is 2.58 � 10�4 C kg�1 and onemilligray (mGy) is 2.9452 � 10�5 C kg�1 or29.452 μC kg�1.

IQt

Q I t� � �or .

electron has a charge (Q) of 1.6 � 10�19C. Asthe force is proportional to charge F Q thenevery charge produces an electric field equal toF/Q. The magnitude of the coulomb field obeysthe inverse square law (see cathode cup).

Council Directive 92/85/EEC (dose) (EEC)Directive of October 19, 1992 introducingmeasures to encourage improvements in thesafety and health at work of pregnant workersand workers who have recently given birth orare breastfeeding.

Council Directive 96/29/Euratom (dose) (EEC)Directive of May 13, 1996 which lays down theBasic Safety Standards for the protection of thehealth of workers and the general publicagainst the dangers arising from ionizing radi-ation. Repealing previous directives with effectfrom May 13, 2000.

Council Directive 97/43/Euratom (dose) (EEC)Directive on health protection of individualsagainst the dangers of ionizing radiation inrelation to medical exposure. Referring topoints: justification, optimization, responsibilities,procedures, training, equipment, special prac-tices, special protection during pregnancy andbreastfeeding, potential exposure, estimates ofpopulation doses and inspection. RepealingDirective 84/466/Euratom on May 13, 2000.

count density (image) A term usually applied tonuclear medicine images, where 100 k, 5 M,etc. represents the total count for a particularstudy. The count density for a study dependson whether the organ under examination isuniform (liver, lung) or localized (bone). A largercount density would be necessary for a uniformobject so that pixel values N remain high andnoise low (since

count loss (nmed) See dead time.count rate (intrinsic) (nmed) Five measurements

are made to characterize the count rate per-formance of gamma cameras (1) input count ratefor a 20% count loss; (2) maximum count rate; (3) incident compared to observed count rate; (4) intrinsic spatial resolution at 75 000 cps(observed) and (5) intrinsic flood field uniformityat 75 000 cps (observed).

counts per second (cps) (rad) A measure ofactivity (see becquerel).

coupled array coils (mri) Array coils, the signalsfrom whose elements are electrically combinedbefore processing.

coupling (mri) See spin–spin coupling.

SNR N.

Radiation exposureFor an exposure 100 mA for 0.5 s (50 mAs) so sinceI � 0.1 Cs�1 and Q � 0.05 C or 50 mC (millicoulombs)so 1 mAs is equivalent to 1 mC.

Coulomb, Charles Augustine de (1736–1806)French physicist who measured the force ofmagnetic and electrical attraction. The SI unitfor the quantity of charge is the coulomb (C).

coulomb field (xray) A force of attraction orrepulsion due to an electric field interaction.The electric field surrounding a point charge oredge. Electrostatic field responsible for shapingthe electron beam in an x-ray tube. Each


C

coupling constant (mri) Spectral lines are splitby spin–spin coupling into multiplets whose fre-quencies are separated depending on the valueof coupling constant J. The magnitude of J isindependent of magnetic field strength. The unitis the Hertz (Hz). A measure of the frequencyseparation of spectral lines by spin–spin cou-pling into multiplets.

coupling gel/medium (us) Soft grease providescontinuous interface excluding air; a gel usedto provide a good sound path between a trans-ducer and the skin by eliminating the airbetween the two.

CP (mri) See Carr–Purcell.CP coil (mri) Circularly polarized transmission or

receiver coil with two orthogonal transmissionand/or receiver channels; the receiver coil givesa better signal-to-noise ratio than linearlypolarized coils.

CPMG (mri) Carr–Purcell–Meiboom–Gill. Method usedin SE imaging in which phase of RF is variedbetween pulses.

c.p.s. (rad) See counts per second.CPU (comp) Central processing unit comprising the

computer micro integrated circuit (micro-chip)and immediate ancillary circuits. Usually visibleas a small block on the motherboard with itsown heat sink.

critical angle (phys) The angle of incidence oflight when proceeding from one medium toanother with a different refractive index. Lightincident at a greater angle suffers total internalreflection. Important consideration for fibreoptic

transmission. Considering the respective veloc-ities in the incident and refractive medium (ci

and ct, respectively) then:

This last statement is Snell’s law and the constantk is the index of refraction of one medium withrespect to the other medium. When k � 1, thereis an angle of incidence where sin θi � k thenthe above equation gives θt � 1 or 90° and therefracted beam is parallel to the surface; θi isthen the critical angle (θλ) which depends onlyon the velocity of ultrasound in the two mediaZ1 and Z2. If θi � θλ or sin θi � k, then sin θt � 1,which is not possible, so no refracted beamexists only a reflected one. Total reflection isimportant to fibreoptics (see angle of reflection).

sinsin .

θθi

t

i

t

CC k� �

(see Snell’s law).Cranial bone thermal index (TIC) (us) The ther-

mal index for ultrasound exposure, where theultrasound beam passes through bone at thebeam entrance.

Crookes, William (1832–1919) English chemistand physicist who studied cathode rays usingvacuum tubes of his own design (Crookes’tubes); the forerunner of the cathode ray tube.Discovered thallium from its spectral lines.

crossed-coil (mri) RF coil pair arranged with theirmagnetic fields at right angles to each other insuch a way as to minimize their mutual mag-netic interaction.

cross-correlation (us) A rapid technique fordetermining time shifts in echo arrival; a tech-nique used to determine flow speeds withoutusing the doppler effect.

cross-over (cross talk) (ct, mri) Slice to sliceinterference in computed tomography (CT) andMRI. Cross-over between slices in CT is due tobeam collimation and in MRI is due to imperfecttruncation (see unsharpness (radiographic)).

cross talk (mri) Signals from close adjacent slicesaffect one another. Caused by a slice profile that isnot ideal. Cross-talk effects T1 contrast. Curedcommonly by using an interleaved slice sequence.(us) Leakage of strong signals in one directionchannel of a doppler receiver into the other chan-nel; can produce the doppler mirror-image artefact.

crossed-coil (mri) An RF coil pair arranged withtheir magnetic fields at right angles to eachother to minimize mutual magnetic interaction.

CrossXBeam® (CRI) (us) A GE Healthcare tech-nique for acquiring and combining ultrasoundinformation in real-time to reduce speckle andclutter, and to improve the continuity of specularreflectors. This reinforces aspects of the imagedata and reduces noise to form a single image.

cryogen (mri) Liquid nitrogen or more commonlyliquid helium, which enables superconductivity tooccur in MRI superconducting magnets. Liquefiedgas (helium or nitrogen) used to maintain super-conducting magnets in a superconducting state.Helium is liquid at 4.2K or �268.95°C.

Air

Glass

�

coupling constant–cryogen

C


cryomagnet (mri) See superconducting magnet.cryo-shielding (mri) Cooling a metal cylinder

surrounding the He-vessel in a superconduct-ing magnet, so reducing cryogen boil-off.

cryostat (mri) A thermally insulated enclosure formaintaining a constant low temperature filledwith liquid helium. There is commonly a recy-cling device for reducing helium loss.

crystallization (phys) Formation of crystals from a saturated solution or by evaporation.Contrast media may crystallize from concen-trated solutions.■ Reference: Dawson and Clauss, 1999.

CSF (clin) Cerebrospinal fluid.CSF SE (mri) Contiguous-slice fast-acquisition

spin echo. FSE/RARE variant.CSI (mri) See Chemical shift imaging.CSMEMP (mri) Contiguous slice MEMP.CT (ct) See computed axial tomography.CTDI (CT dose index) (ct) The basic measurement

as defined by the US Federal Regulation 21CFR1020.33(C) represents the dose at the centre orperipheral point on a head or body phantomfrom a single scan and results from absorption ofthe x-ray beam over a distance of approximatelyseven slices. The CTDI is normalized to a stan-dard mAs product and weighted for differentlocations within the scan plane (CTDIw). The dose

profile from a single CT scan spreads beyond theintended section thickness. The area beyond thesection thickness being the penumbra. The CTDIattempts to represent the dose distribution out-side the section thickness, taking a representa-tive 14 slices in air or the phantom as:

This represents the dose D(z), for a single sectionat a given position z along the scan axis. Thisvalue is then divided by the intended sectionthickness per image (T) and the number of imageslices per scan n. Measurements are taken froma plastic (PMMA tissue equivalent) phantom 16and 32 cm in diameter (head/body). CTDI valuesare given in mGy per 100 mAs for 360° scan atthe machines stated in kVp.■ Reference: Code of Federal Regulations, 2005;

European Commission’s Radiation Protection

Actions.

(See CTDI100, CTDIFDA, CTDIw).CTDI(air) (ct) Value of CTDI determined free in air. In

practice, it is convenient to use a pencil ionization

CTDI mGy��

�17

7

nTD dzz( )∫

chamber with an active length of 100mm so as toprovide a measurement of CTD1100 (mGy to air).

CTDIvol (ct) The absorbed dose within the scannedvolume. Whereas CTDIw represents the weightedabsorbed dose per slice (x- and y-axis), theCTDIvol represents the average absorbed doseover the x-, y- and z-axes; it is similar to the mul-

tiple scan average dose (MSAD) and is independentof scan length. It is derived from CTDIw as:

where pitch is 1, then CTDIvol � CTDIw and dose

length product (DLP) � CTDIvol � scanned length.CTDIw (ct) A weighted version of CTDI taking sur-

face and centre dose readings of the head orbody phantom, gives an adequate basis forspecifying reference doses for CT. Defined byIEC 601-2-44CDV, these measurements of aweighted CTDIw represent the average dose toa single slice:

where subscripts c and p denote central orperipheral measurement from the phantom. A normalized mAs value (typically 100 mAs) isused. The CTDIw will vary with slice width andis an estimate of the average dose over a singleslice in a CT dosimetry phantom that is used forcomparison of performance against a referencedose value set for the purpose of promotingoptimization of patient protection.

Calculation of CTDIwFrom measurements made using a PMMA headand body phantom the following readings areobtained:

CTDI head (16 cm) CTDI body (32 cm)

Centre Peripheral Centre Peripheral

16.2 18.1 4.9 10.925.2 27.7 7.8 16.4

Using the formula:

CTDI CTDI CTDI.c .pw � �13

23100 100

CTDI CTDI CTDIw c p� �13

23100 100. .

CTDICTDIpitchvol

w�

CTDI mGy.��

��1T

D dzz( )∫

cryomagnet–Calculation of CTDIw


C

Calculation of CTDIw–CT number

The weighted dose figures are obtained

CTDIw head (16 cm) CTDIw body (32 cm)

110 17.2 8.8130 26.6 13.4

CTDI100 (ct) A more representative dose measure-ment than other CTDI measurements. The CTDI ismeasured over a fixed 100-mm length instead ofthe 14 arbitrary slices. The value is taken over afixed length of integration using a pencil ioniza-tion chamber with an active length of 100 mm.Provides a measure of the basic CTDI, integratedover a standard 100-mm length expressed interms of absorbed dose to air (mGy):

where S is the slice width, the dose D(z) beingintegrated over �50 mm by the 100-mm pen-cil detector. The subscripts for the CTDI meas-urement include (mAs) indicating the exposurevalue used to normalize the result (typicallydoses are stated for a 100 mAs value); the sub-script 100 denotes a 100-mm scan length anda or p indicate that measurements were madein air or a PMMA phantom. In addition, thephantom size 16 or 32 cm should be stateddepending on either head or body values (seeCTDI, CTDIFDA, CTDIw).

CTDIFDA (ct) A slice dose measurement allowing forscattered radiation falling outside the 100-mmdetector length (CTDI100). In order to overcomethis problem, the FDA requires acquisition of 14nominal slice widths. In multiple slice machines, itis necessary to modify the calculation of theCTDI100 equation to take account of the numberof simultaneously acquired slices (M) so:

This formula allows comparison betweenmachines acquiring simultaneous slices of 2, 4,8 or more slices.

CT dosimetry (ct) Phantoms: Cylinders of poly-methyl-methacrylate (PMMA) used for standardmeasurements of dose in CT, having a diameterof 16 cm (head phantom) or 32 cm (body phan-tom) and a length of at least 14 cm. The phan-toms are constructed with removable inserts

n a p zmm

mm

S MD dzCTDI /( , ) ( )*

.100 50

501�

�

�

∫

(mAs) CTDI /( , ) ( )100 50

501a p zmm

mm

SD dz�

�

+

∫

parallel to the axis to allow the positioning of adosemeter at the centre and 1 cm from the outersurface (periphery) (see CTDI, CTDI(air), CTDIw).

CT dose index (CTDI) (ct) See CTDI.CT dosimetry phantom (ct) Cylinders of poly-

methylmethacrylate (PMMA) used for standardmeasurements of dose in CT, having a diameterof 16 cm (head phantom) or 32 cm (body phan-tom) and a length of at least 14 cm. The phan-toms are constructed with removable insertsparallel to the axis to allow the positioning of adosemeter at the centre and 1 cm from theouter surface (periphery) (see CTDI, CTDI(air),CTDIw).

CT number (ct) The comparative attenuation valuethat represents the mean x-ray attenuation asso-ciated with each elemental area (voxel) of the CTimage. These values are expressed in terms ofHounsfield units (HU) and scaled with respect towater as:

where μ is the effective linear attenuation coeffi-

cient for the x-ray beam. The CT number scale isdefined so that water has a value of 0 HU andair a value of �1000 HU; bone can exceed�3000 HU. Since values are scaled to water theHousfield unit is independent of beam energy(voltage).

CT number (Hounsfield unit) tissue water

water�

�μ μμ

CT number calculation

80 kV 100 kV 150 kV

μmuscle 0.1892 0.1760 0.1550μwater 0.1835 0.1707 0.1504

Beam hardening: At 100 kV, the reference for water is0.1707, but if the beam effective energy changes to105 kV in the centre of the profile, the μ for tissuechanges to 0.1750 then:

and not 31 which it should be (see cupping effect).

New CT value ��

�0 1750 0 1707

0 170725

. ..

At kev:

At kev:

80 10000 1892 0 1835

0 183531

100 10000 176

��

�

�

. ..

. 00 0 17070 1707

31

150 10000 1550 0 1504

0 150431

��

��

�

..

. ..

At kev:

C


CT fluoroscopy (ct) This application requires rapidimage reconstruction of the object currently inthe scan field of view. Movement of the patientcouch is under the operator’s control. As theimages are updated up to 12 times per second,the systems are capable of providing real-timefeedback for a range of procedures, particularlyduring interventional procedures.

cumulated activity (nmed) The total number ofradioactive disintegrations that occurs in the tar-get organ. This is dependent on injected activity,biological half-life and physical half-life.

cumulative error (di) The gradual build up oferror in an involved computation from eithertruncation or round-off.

curie (nmed) A non-SI radioactivity measurementhaving a disintegration rate of 3.7 � 1010Bq asdisintegrations per second, 37 GBq. Other equiv-alents are 1 mCi � 37 MBq and 1 μCi � 37 kBq.

Curie, Marie (née Sklodowska) (1867–1934)Polish born French scientist, stimulated byBecquerel’s discovery investigated pitchblende.In 1898, she separated polonium and later thesame year radium. She was awarded the Nobelprize for physics 1903 and for chemistry in1911 (see Joliot-Curie).

Curie, Pierre (1859–1906) French physicist.Discovered piezo-electricity in 1880. Observedthe loss and variation of magnetism with tem-perature (the Curie point and Curie’s law) andpioneered instrumentation for measuring ioniz-ing radiation and established that the rays fromradium contained positive, negative and neu-tral particles. He was awarded the Nobel prizefor physics 1903.

curie point (us) Temperature at which an elementmaterial loses its piezo-electric properties.

current (phys) A flow of electric charge in a sub-stance which can be solid, liquid, gas or plasma.The charge carriers may be electrons, holes orions. The magnitude of the current is given bythe amount of charge flowing in unit time,measured in amperes. One ampere (C s�1) repre-sents about 6.28 � 1018 electrons per second ora rate of flow of charge of one coulomb per sec-ond. Direct current (DC) is given by batteries,dynamos and rectified power supplies and isused as the final power supply for radiologyequipment whether driving high voltage x-raytubes or low voltage semiconductor circuits. For a current (I) in amperes (A), the charge in coulombs (C) and time (t) in seconds then

CT fluoroscopy–current density (j or J)

Q � I � t where 1 A � 1 C s�1 (6.24 � 1018 elec-trons s�1). X-ray tube current is measured in mA,and x-ray exposure is measured in mA per unittime mA, seconds mAs, so 1 mAs �1 � 10�3

coulomb or 1 milliC.current (alternating) (phys) Alternating current

(AC) is produced by generators/alternators andelectrically oscillating circuits. It is more easilycontrolled than DC and is used for transmittingelectrical power over long distances, eventuallyappearing as the domestic mains supply. Powerstation generators are capable of deliveringmany thousands of volts at a very high current.Transmission voltages sometimes exceed500 000 volts. High voltage transmission reducespower loss, which can be reduced by eitherincreasing the supply voltage or reducing cableresistance. Transmission uses a three-phase AC;each phase of the three-phase supply is sepa-rated by 120° and transmits electrical energywith much greater efficiency. Domestic supply iscommonly single phase. Voltage stepdown isachieved with a transformer.

Power lossFor a cable of resistance 2 Ω supplying a 100 W lamp:

● At 110 volts, �1 amp is flowing so the power lost inthe cable is I2R � 1 � 2 � 2 watts or 2% of thelamp power is lost in the cable.

● At 220 volts, �0.5 amp is flowing and the power lostin the cable is 0.25 � 2 � 0.5 watts or 0.5% of theavailable power is lost.

Power transmissionSupplying 1 MW to a small town over a cable with aresistance of 10 Ω can use:

● 500 V at 2000 amps or● 10000 V at 100 amps or● 500000 V at 2 amps

(all delivering the required 1 MW).

Power loss is I2R so at:

● 500 V, there is total power lost in the cable;● 10 000 V, 10% of the power is lost;● 100 000 V, 0.1% of the power is lost.

Note Power transmission uses highest practical volt-age to minimize power loss.

current density (j or J) (phys) The flow of elec-tric charge or current through a conductor,either gas, metal semiconductor or tissue. The


C

current density (j or J)–cystourethrography

unit is ampere per square metre: A/m2. Theamount of current flowing through a given unitcross-sectional area of a current carryingmedium; either a conductor or radiation beam.

curvilinear array (us) See linear array.cut-film (film) 100 mm or 105 mm square film

stock. Used mostly in conjunction with animage intensifier (cut-film camera).

cut-off frequency (di) The �3 dB point is arbi-trarily called the cut-off frequency of low andhigh pass filters (see bandwidth).

CW (mri) See continuous wave.cyanocobalamin (nmed) 57Cobalt labelled for

oral administration. Diagnosis of perniciousanaemia and defects of intestinal vitamin B12

absorption.

Generic name 57Co-cyanocobalamineCommercial names Rubratope® (Bracco), Dicopac®

(Amersham GE)Non-imaging diagnostic kit

cycle (Hz) (phys) Complete transition of waveformmeasured from identical points (zero crossing orpeaks).

cycles per pulse (us) A typical ultrasound pulse

contains one to three cycles.cyclotron (phys) A charged particle accelerator

where the beam travels in a circular path. Usedfor manufacturing short-lived positron emitters:

Nuclide Half-life

15O 2.1 m38K 7.6 m11C 10 m13N 10 m18F 110 m

(see ion source, Lawrence EO, positron emission

tomography).cyclotron (beam production) (nmed) See ion

beam (cyclotron), hydrogen, helium, deuterium).

CYMK (comp) Cyan, yellow, magenta, keystoneblack. The four basic colours used by inkjet andlaser colour printers. A huge range of coloursare created by a combination of overlaying anddithering.

cystic duct cholangiography (clin) Radiographyof the biliary system by introducing contrastmedium through the cystic duct.

cystogram (clin) Radiographic demonstration ofbladder filled with contrast medium. Micturatingcystogram performed when urinating.

cystography (xray) Imaging the bladder by introducing contrast media through a urethralcatheter. Demonstrates anatomic and patholog-ical changes in male/female bladder or urethra and the presence of ureteric reflux (seeurethrography).

Cystografin® (cm) Commercial preparation(Bracco) meglumine diatrizoate (30%) injectioncontaining 141 mg I mL�1 for retrograde cysto-urethrography.

Cysto-Conray® (cm) Commercial preparation ofionic meglumine iothalamate (Malinckrodt/TycoHealthcare Inc) containing 81 mg I mL�1 for ret-rograde cystography or cystourethrography.

cystoscopic urography (clin) See retrograde

urography).cystourethrography (clin) Micturating or voiding

cystogram. Radiography of the bladder andurethra during voiding following filling of thebladder with contrast medium, either intra-venously or by means of retrograde catheteri-zation; performed during the phase of bladderemptying. Ascending urethrogram is where thecontrast medium is instilled slowly via catheter.Descending micturating cystourethrography iswhere the contrast medium is instilled viacatheter and the bladder adequately filled toinduce micturation. Images of posterior urethrataken during voiding (see retrograde urography,urethrography).

D


d –database

Dd (image) In signal detection theory, an effective

signal-to-noise ratio (SNR) that applies only tosituations in which the decision variable arisesfrom one of two normal distributions having equalvariances, but different means. In such situ-ations, the receiver operating characteristic(ROC) curve is symmetric about the diagonal ofthe unit square in which it is plotted and isgiven by the mathematical expression:

TPF � Φ(d ZFPF),

where Φ represents the cumulative standard-normal distribution function and ZFPF representsthe normal deviate that corresponds to thefalse-positive fraction (see binormal ROC curve).

da (image) An effective SNR, more general than d

that applies to situations in which the decision

variable arises from one of two normal distribu-tions having different means and, generally,different variances. In such situations, the ROCcurve depends on two parameters, a and b, andis given by the mathematical expression:

TPF � Φ(a � b ZFPF),

where Φ represents the cumulative standard-normal distribution function and ZFPF representsthe normal deviate that corresponds to false-positive fraction,

D (max) (image) See film sensitometry.D (min) (image) See film sensitometry.Do (dose) The dose that gives (on average) one

lethal event per cell reducing survival to 37% ofits previous value (see LD50).

DAC (dose) See derived air concentration. (comp)Abbreviation for the electronic circuit digital to

analogue converter.dacryocystography (clin) Imaging the lacrimal

ducts injecting contrast medium into the lumen.Almost entirely replaced by nuclear medicine,computed tomography (CT) and magnetic reso-nance imaging (MRI) techniques.

dalton (D) (unit) The atomic mass unit a.m.u. equalto 1/12 of the mass of an atom of 12 carbon.Equivalent to 1.66 �10�27kg or 931 MeV. Usedprincipally for measuring biochemical mol-ecules, commonly expressed as kilo- and Mega-daltons. A protein shell is of the order 13 MDa.

damped oscillation (phys) See decay (signal).damping (us) Material attached to the rear face

of a transducer element to reduce pulse duration.

d a ba � �2 1 2/( )

damping coefficient (mri) Decreasing periodicfunction, e.g. free induction decay process.

DAP (dose–area–product) (dose) Values for somerepresentative reference values (NRPB 1990).

Examination DAP (mGy cm2)

Mean Maximum

Lumbar spine (AP) 3200 5000Lumbar spine (LAT) 3400 7000Lumbar spine (LSI) 3600 5000Chest (AP) 200 300Chest (PA) 150 250Chest (LAT) 500 1000Abdomen (AP) 3800 7000Pelvis (AP) 3300 7000Skull (AP/PA) 800 1500Skull (LAT) 500 1000Thoracic spine (AP) 1800 3000Thoracic spine (LAT) 3300 6000Intravenous urography 16 000 40 000

dark current (xray) Background current duringno input conditions. Present in photomultipliers

and image intensifiers.dark fluid imaging (mri) A preparation pulse sat-

urates the blood and displays cardiovascularanatomy. The TurboIR technique has a longereffective echo time and longer inversion timewhich suppresses fluid signals. Bright fluid signals using conventional T2 contrast arerevealed by this dark fluid technique (see FLAIR).

DAT drive (comp) Digital audio tape. Large storagecapacity digital tape for archiving. A helical scanmagnetic tape storage with a capacity of2–12 G-bytes uncompressed (native) data andup to 24 G-bytes compressed data. Data transferrates are from 183 k-bytes s�1 to 2 M-bytes s�1.It is a common back-up medium for computer-disk data (see storage (bulk)).

data acquisition (comp) Collection of electronicsignals which can be analogue (voltage/ampli-tude varying waveform) or digital (nuclearmedicine or x-ray photon events), processed(e.g. intensification or amplification) and per-haps preprocessed (voltage compression orpulse clipping) before being used to form theimage (film or digital matrix). Losses occur at allstages of data acquisition and handling.

data acquisition system (DAS) (ct) Electronicsystems of the CT machine responsible fordetector signal amplification, integration andanalogue to digital conversion.

database (comp) A program that stores informationso that it can be easily retrieved by searching


database–dead time

D

and sorting facilities. A collection of similarinformation stored in a file, such as a database ofaddresses. This information may be created andstored in a database management system(DBMS).

data compression (di) See compression (image).data interpolation (ct) For sequential CT, slice sen-

sitivity profiles (SSPs) are approximately rectan-gular with widths equal to the section width;however, in spiral/helical acquisition they areextended and more peaked. The SSP representsthe resolution element along the z-axis andsignificantly influences 3D resolution recon-struction. Since the patient is moved duringdata acquisition, the information obtained is dis-torted over the body volume depending on theslice thickness and table increment per rota-tion. This data inconsistency causes artefacts,but can mostly be compensated by data inter-polation. The interpolation methods most com-monly employed work from either the full dataset (360°) or a half data set (180°) (see 180° inter-

polation, 360° interpolation).data matrix (ct) These CT numbers are stored in

computer memory and represent a volume sliceelement or voxel. The matrix store must hold arange of voxel values from �1000 to �4000.This requires a memory location of 16 bitsincluding a ‘sign’ bit positive or negative.

data set (stats) A collection of numbers that usually have one common property.

DaTSCAN® (nmed) A radio-iodinated cocaineanalogue 123I-ioflupane (Amersham/GEHealthcare Inc).

daughter (phys, nmed) Decay product of a ‘parent’ radionuclide (see radionuclide production

(generator)).dB (phys) Abbreviation for decibel.dB/dt (mri) Time derivative rate of change of

magnetic field with respect to time. Using alter-nating magnetic fields/electrical fields pro-duced in conductive material (soft tissue);related to MR safety limits.

d.c. (phys) Direct current, as supplied by batteriesand rectified power supplies to drive electricalor electronic circuits.

DCE (comp) Data circuit terminating equipment. A device used to connect two DTEs over a network.

DCL (comp) Data circuit terminating equipment. Adevice used to connect two DTEs over a net-work. A modem is a DCE.

DDR1/DD1 RAM (comp) Double data rate RAM isused in some graphics cards, having 184 pins;replaced by DDR2.

DDR2/DD2 RAM (comp) Currently the most common computer memory; significant improve-ments over the DDR1 RAM specification dou-bling the transfer rate. It has 240 pins and so isnot a replacement for DDR1 unless the mother-board is changed.

DREF (dose) See dose and dose-rate effectiveness factor.de Broglie, Louis-Victor Pierre Raymond 7th

Duke (1892–1987) French physicist interpretedEinstein’s work on the photoelectric effect intowave theory suggesting that the electron canexhibit wave-like properties. He was awardedthe Nobel prize for physics in 1929.

de Broglie’s equation (phys) A photon or particle(electron) of mass m moving with a velocity vwill exhibit wave-like properties, having awavelength λ �h/mv, where h is Planck’s con-stant (see photon).

DE FGR (mri) Driven equilibrium fast gradientrecalled acquisition in the steady state.

dead time (nmed) The time required for a detectorsystem to process (count or reject) the signalfrom a single photon interaction, during whichadditional photon interactions will be missed.The count rate capability of a detector system(gamma camera) is limited by the decay time ofthe scintillator, about 240 nS for NaI(Tl), alongwith its charge amplifier and associated converterelectronics. The measured count rates from agamma camera fall short of expected count rate.It is therefore expressed as a figure representinga 10, 20 or 30% count loss. The common valueis taken at 20%. If Ni is the incident number ofgamma photons and Na is the photon countsrecorded, then the dead time τ is given as:

The dead time calculated from this equation fora 20% loss at 200 000 cps typically giving avalue of just over 1 μs. Two types of detectordead time are distinguished:

1 Paralyzable, where further photon events serveto increase the total dead time.

2 Non-paralyzable where further events areignored, the detector operating again after a set time.

τ �ln N

N

a

i

a

N

D


dead time–decontamination

Most gamma cameras behave as paralyzabledetectors over the clinical count range.

decay (nmed) The rearrangement of a nucleusresulting in emission of one or more different par-ticles, thereby returning the nucleus to stabilityand resulting in formation of an atom (the daugh-ter) different from the original atom (the parent).

decay (signal) (phys) In a simple harmonic motionsin(x), the amplitude A of the vibration does notremain constant, but decreases exponentiallywith time t due to loss of energy (damping) fol-lowing the expression A � sin(x) � e�t. Thisdecay signal is shown by all electrical andmechanical signals whose energy input is notconstant. Examples are seen in the ultrasoundtransducer, free induction decay (MRI) and thedamped response of an electronic oscillator.

decay constant (λ) (nmed) For an unstablenucleus (radionuclide), the number of atoms Ndisintegrating per second is dN/dT which is

1.0

0.5

0.37

Time

Sig

nal s

tren

gth

0.0

�0.5

�1.0

200

150

100

50

00 50 100 150

Expected count rate (kcps)

Rec

orde

d co

unt r

ate

(kcp

s)

200

20% Loss

directly proportional to the number of atoms atany one time so dN/dT � �λN, where λ is thedecay constant characteristic for the atom. If No

represents atoms present at time zero then thenumber at time t is: Nt � N0 � e�λt. Half-life is . It follows that

. So the decay con-stant of any radionuclide . It iscommon to express decay constants as ‘per sec-ond’ so that comparisons can be made easily.

decibel (dB) (units) The comparative measure ofpower, intensity and amplitude or voltage gain;a ratio of relative powers or intensities. It usesa logarithmic scale as:

where Io is the reference sound level commonlyfixed at 10�12 W m�2 at 1 kHz at audible levels.A 3-dB change halves or doubles the soundintensity. A sound level of 20 dB is ten timesmore intense than a 10-dB sound level.Comparisons of amplitude or pressure differ-ences uses a factor of 20 as:

Here, a change of 6 dB halves or doubles theamplitude or pressure. The attenuation given bya material is quoted as dB mm�1; this is theattenuation coefficient α (see Bell, Alexander Graham).

decimal place (math) The figures to the right ofthe decimal point giving a specified degree ofaccuracy (see significant figures).

decision criterion (stats) In signal detection theory, the critical value that separates therange of decision-variable outcomes associatedwith ‘negative’ decisions from the range asso-ciated with ‘positive’ decisions.

decision variable (stats) In signal detection theory, a statistical quantity that is assumed tounderlie decisions made under uncertainty. For‘detection’ tasks that involve two mutuallyexclusive (‘positive’ and ‘negative’) alternatives,the decision is assumed to be made by compar-ing the value of the decision variable to a criti-cal value (i.e. decision criterion).

decontamination (nmed) The reduction of surfaceactivity after an accidental spill of radionuclide,by either physical means or isolation, allowingphysical decay for short-lived nuclides. Theminimum surface activity varies and local rules

dB � 20 10log .AAo

dB � 10 10logIIo

λ � 0 693 12

. /TN N e t Tt � � ��

00 693

12

. /T1

21 2 0 693� / or /λ λln .


decontamination–density (�)

D

should be consulted for acceptable thresholdswhich usually relate to fractions of maximumdoses (typically 0.1).

deconvolution (math) See convolution.decoupling (mri) Decoupling involves (1) removing

the multiplet structure in a particular resonancedue to spin–spin coupling; (2) preventing mutualinductive coupling between coils by detuning.Decoupling can take the form of active decoup-ling where an externally controlled switchingcircuit is used to detune the nonselected coils orpassive decoupling where RF energy from thetransmitter pulse is used to switch diodes todetune the appropriate coil.

decubitus view (clin) Imaging performed withthe x-ray beam horizontal ‘across the table’.Named according to the patient side viewed:Right or left lateral decubitus.

deep dose equivalent (Hd) (dose) External wholebody dose equivalent at 1 cm depth. The doseequivalent to any organ, except the eye. Someauthorities still retain the recommendationsgiven by ICRP26 of 500 mSv, but modificationshave been made to this.

DEFAISE (mri) Dual-echo fast-acquisition inter-leaved spin echo. A double-echo variant of RARE.

default (comp) The predefined configuration of asystem or an application. In most programs, thedefaults can be changed to reflect personalpreferences, such as font or paragraph indents.

DE FGR (mri) Driven equilibrium fast gradientrecalled acquisition in the steady state.Commonly used for imaging cerebrospinal fluid(see SSFP, CE-FAST, True-FISP, PSIF, ROAST, T2-FEE,E-SHORT, STERE).

defocussing (mri) See dephasing.degree (unit) Defined as a plane angle which the

central area of a circles cuts from 1/360th partof the circumference. 1° �π/180 rad or approx-imately 0.01745 rad (see radian).

degrees of freedom (stats) The number of freevariables in a system. Commonly given as n �2,where n is the number of points. For the t-test,the number of degrees of freedom is n �1. (phys)One of the separate ways in which a moleculecan have energy. A gas will have three degreesof freedom: kinetic energy from motion frommotion in three directions 90° to one another.

delta (Δ) pulse (di) An infinitely narrow (spike)input. The ideal requirement to measure point

spread or line spread function of a system (seeDirac function).

delay time (mri) See trigger delay (TD).demodulation (phys) The reverse of modulation,

where the signal is separated from its carrierwave by a demodulator, RF-detector or ultra-sound detector circuit.

demodulator (phys) Another term for detector, byanalogy to broadcast radio receivers. An electroniccircuit which separates a single signal from amixed signal. (us) An electronic circuit which sep-arates a single signal from a mixed signal.Frequently a phase detector (quadrature phasedetector). The echo signal is more easily processedby mixing with a reference signal (frequency, f )obtained by an inbuilt oscillator. After processing,rectification separates the signal and a fast Fouriertransform (FFT) calculates the Doppler frequenciesfrom the demodulated Doppler signal sampled atintervals where T � 1/PRF (pulse repetition fre-quency). Maximum frequency detectable is theNyquist limit as ½ PRF. (mri) Electronically sepa-rating mixed signals. A part of the nuclear mag-netic resonance (NMR) signal receiver thatconverts the raw signal to a lower frequency foranalysis. This is also phase sensitive (quadrature

demodulator) and will give phase information(detecting phase encoded RF signals).

densitometer (film) Measures film density inoptical density units (see optical density).

density (ρ) (units) The physical density of amaterial (ρ) is:

The SI unit is kg m3, although non-SI g cm3 isoften used. A conversion factor of 103 is used.

ρ �mass

volume

Density50 g of aluminium displaces 18.5 cm3 of water so itsdensity is:

ρ � �50

18 52 70 27003 3

.. g cm or kg m

The density of elements important to radiologyusually follow their atomic number (Z), butthere are exceptions as shown in the table.

Z Material Density kg m�3

Air 1.225Water 1000Muscle 1000Fat 900Bone 1650–1800

(Contd.)

D


density (�)–derived air concentration (DAC)

density (ρ) (Contd.)

Z Material Density kg m�3

13 Aluminum 270026 Iron 787029 Copper 890042 Molybdenum 10 20073 Tantalum 16 60074 Tungsten 19 32076 Osmium 22 48079 Gold 19 30082 Lead 11 340

density (optical) (OD) (image) The logarithm ofthe ratio of the intensity of perpendicularlyincident light (Io) on a film to the light intensity(I ) transmitted by the film:

Optical density differences are always measuredin a line perpendicular to the light source axis.

density (relative) (units) Water has a density of1000 kg m�3 (1 g cm�3) and this liquid is used asa reference for density measurements. The rela-tive density for aluminium is therefore just 2700.The density of a substance divided by the densityof water is the specific gravity or relative density;for a gas the relative substance is usually air.

density resolution (ct) The capability to displaysmall differences in tissue densities (see low

contrast resolution).depletion (barrier) layer (elec) A region

between a p–n junction (see depletion junction).dephasing (mri) Phase differences occurring

between precessing spins after RF excitation,causing a decay in transverse magnetization.Caused primarily by spin–spin interaction andinhomogeneity in the magnetic field. Can also becaused by switching specific gradient fields (flowdephasing).

dephasing gradient (mri) Magnetic field gradientpulse used to create spatial variation of phaseof transverse magnetization. It may be appliedprior to signal detection in the presence of amagnetic field gradient with opposite polarity(or of the same polarity if separated by a re-focusing RF pulse) so the resulting gradient echo

signal will represent a more complete samplingof the Fourier transform of the desired image(see spoiler gradient pulse).

depletion junction (elec) About 10�3mm widewhich exists without a biasing voltage. Whenthe junction is reversed biased (positive pole

OD � log .10IIo

to n and negative to p), the depletion widthincreases preventing current conduction.Ionizing radiation causes electrons and holes toform and a small current signal is obtained.This is the semiconductor version of the gasionization chamber (see semiconductor).

Depreotide (nmed) 99mTc depreotide is an aminoacid peptide binding for somatostatin receptorstype 2, 3 and 5. It has a high affinity to lungcancer.

Generic name 99mTc depreotideCommercial names Neotect®

Imaging category Somatostatin receptor lungmasses

depth of field (image) A circle of 0.25 mm viewedfrom 250 mm is just visible as a point source.This is termed ‘the circle of least confusion’ andis used as a standard of reference in photo-graphic images. It corresponds to an angle of�0.001 radians subtended by the eye. Thisreference is used to determine the ‘in focus’distance covered by a lens and is dependent onthe lens f-number. Smaller f-numbers producelarger depths of field. An important factor withcameras in cine-fluoroscopy and film recording.

depth of image (us) See image depth.depth of response (us) The point from the trans-

ducer face where echoes are �50 dB.depth gain compensation (us) See time gain

compensation.depth pulses (mri) Multiple RF pulses with an

inhomogeneous RF field, so acquiring data fromonly selected regions within the field. Providesa one-dimensional localization along isocon-tours of the magnetic field.

derating (derating factor, derated) (us) A factorapplied to acoustic output parameters intendedto account for ultrasonic attenuation of tissuebetween the source and a particular location inthe tissue. As referred to in this document, theaverage ultrasonic attenuation is assumed to be0.3 dB cm�1MHz�1 along the beam axis in thebody. Derated parameters are denoted with asubscript ‘3’. Unit is decibel per centimetre permegahertz, dB cm�1 MHz�1 (see mechanical index).

derated spatial peak time average intensity(us) The largest value in an ultrasound beam ofany derated time averaged intensity.

derived air concentration (DAC) (dose) Theannual limit on intake (ALI) of a radionuclide dividedby the volume of air inhaled by a reference


derived air concentration (DAC)–detector array

D

person in a working year estimated as2.4 �103m3. The unit of DAC is Bq m�3.

detective quantum efficiency (DQE) (image)Comparison of input and output signal to noiseratios. Signal to noise ratios vary at every stageof image production. The SNRin represents sub-ject contrast. The SNRout represents the detec-tion process then:

The SNR of the incident photon flux N is dueto quantum noise. The SNR of the detector isdependent on photon absorption, which is influ-enced by μ thickness and conversion efficiency.

(see signal to noise ratio, noise equivalent quanta).detector (phys) A gas, solid or liquid which yields a

signal (electrical or light) when radiation inter-acts within its volume. (ct) A single element of adetector array, which produces an electrical orlight signal in response to stimulation by x-rays.Current detectors are pressurized xenon ionchambers and solid scintillators of either cad-

mium tungstate or rare earth oxide ceramic, opti-cally bound to photodiodes spectrally matchedto the scintillator. Conversion efficiency andphoton capture for the solid detector approaches99%. (mri) A circuit of the receiver that demodu-lates the RF signal converting it to a lower fre-quency signal. Most detectors now used arephase sensitive (e.g. quadrature demodulators)giving phase information about the RF signal.(us) A demodulator or circuit for separating theoriginal sound signal from the carrier wave (seeceramic detector).

detector array (ct) The entire assembly of detec-tors, including their interspace material, arranged

0.5

60

50

40

30

20

DQ

E

10

0

1.0 1.5 2.0 2.5 3.0Spatial revolution Lp mm�1

Screen/Film

CR

a-Se DR

Csl DR

N

DQE �SNRSNR

out

in

⎛

⎝⎜⎜⎜⎜

⎞

⎠⎟⎟⎟⎟

2

Derived air concentration131Iodine has an ALI of 9 �105. The DAC value is:

9 10

2 4 10375

5

33�

��

..Bq m

derived reference air concentration (DRAC)(nmed) The annual reference limit on intake(ARLI) of a radionuclide divided by the volumeof air inhaled by reference man in a workingyear. The unit of DRAC is Bq m3.

derived units (phys) See Systeme International (SI).designated standard mode (us) Consist of the

following specific operating modes: A-mode, B-mode, M-mode, PW Doppler, CW Doppler and colour Doppler.

des Plantes, Bernard George Ziedses(1902–93) Dutch radiologist who producedfirst subtraction images in 1930, displayingonly the contrast filled vessels. He also intro-duced radiographic tomography. The idea ofproducing subtraction images dates back to the1930s, when this Dutch radiologist producedsubtraction images using plain films. From the‘mask’ image, i.e. the image of the object justbefore the contrast medium is injected, he pro-duced a positive copy, on to which the imageswith contrast medium were overlaid to co-incide, thus producing a subtraction image onlydisplaying the contrast-filled vessels.

DESS (mri) Double-echo steady state (Siemens)combining FISP and PSIF. A 3D gradient echo dur-ing which two different gradient echoes (FISPand PSIF) are acquired during TR. During imagereconstruction, the strongly T2-weighted PSIFimage is added to the FISP image. Applied tojoint imaging giving good contrast for cartilage.

destructive interference (us) Combination ofpositive and negative pressures.

detail resolution (us) Ability to image fine detailand to distinguish closely spaced reflectors (seeaxial resolution, lateral resolution).

detail transfer function (math) A descriptor forhow the imaging system averages, displaces orblurs the input signals before they are detectedin the output signal. For linear, shift invariant

systems, the detail transfer function is the pointspread function (PSF) or, in frequency space,the optical transfer function (OTF).

D


detector array–detector width

along an arc or circumference (depending onscanner technology) of a circle centred on theaxis of rotation.

detector channel (ct) A single detector element(see detector spacing, detector width).

detector channel sensitivity (ct) The sensitivityof a specific single detector element; the chan-nel sensitivity as measured is crucial formachine calibration; slight variations in thedetector sensitivity will lead to ring artefactswithout.

detector dead-time (nmed) A gas detector has adead time measured in fractions of a second.The dead time of a scintillation detector ismeasured in nanoseconds. A typical value forNaI(Tl) is 250 ns. The dead time of a gammacamera includes crystal and electronics and canbe measured by using two activities Aa and Ab,the combined activity Aab exceeding the countrate of the detector. The dead time t is:

The count rate for 20% loss is 1/t ln (1.25) �

0.223/t. For a 2 μs dead time, the count rate for a 20% loss is about 110 000 cps (seedead time).

detector efficiency (phys) The factors that influ-ence detector efficiency are:

● geometry;● absorption;● background radiation;● collimation;● dead time;● conversion efficiency.

Geometry depends on 4π or 2π design. Theabsorption depends on both density and detec-tor thickness. Background radiation must below for distinguishing small activities.Collimation shields the useful detector surface.A poor dead time will render the detector inef-ficient at high count rates. Conversion effi-ciency measures signal output for energydeposited. (ct) The ratio between the number ofevents recorded by the detector and the num-ber of x-ray photons incident on the detector.The detector efficiency is the product of theabsorption efficiency for the photon energyused and conversion efficiency (x-ray photon tolight photon conversion). Approximately 98%

tA

A AA A

Aab

a b

a b

ab�

�

�

( )ln .2

for a 120 kVp x-ray spectrum. The ratiobetween the number of pulses recorded andthe number of x-ray photons incident on thedetector (see 4π geometry, 2π geometry).

detector element (ct) The section of a detectorelement orthogonal to the direction of the inci-dent radiation which has a rectangular shape;the length of the side of the rectangle which isparallel to the scan plane determines the effec-tive detector width and so determines thebeam width; the other side of the rectangle,oriented in the z-direction, defines the maxi-mum available slice width.

detector material (ct) Solid detectors shouldhave a high relative light output (luminescent

efficiency) giving high signal-to-noise ratios,and have low values for afterglow. Early CTdetectors were thallium-activated CsI and self-activated CdWO4. They had a high luminescentefficiency, but they exhibited high values ofafterglow (�0.3%).

detector (ceramic) (ct) See ceramic scintillator.detector offset/quarter shift (ct) The x-ray tube

focal spot can be shifted in order to doubledetector resolution. Sample spacing in the scanplane can also be improved by quarter ray off-set to improve ray sampling within the projec-tion; opposing projections will be offset byone-half the detector width. If the central ray isoffset by one fourth of the sampling distancefrom the centre of rotation then this ray willagain be shifted by one fourth of the samplingdistance in the opposite direction after 180°rotation. This allows interlacing of opposingprojections and so doubling sampling fre-quency which improves resolution and reducesaliasing artefacts.

detector sensitivity (phys) The sensitivity of adetector is commonly related to the absorptionproperties of soft tissue. Air and certain phos-phors (LiF) have almost an identical photonenergy response as soft tissue; these detectormaterial are deemed tissue equivalent (see detec-

tor efficiency).detector spacing (ct) Distance between the cen-

tres of two adjacent detector elements; thedetector spacing determines the sampling dis-tance for a single projection (see detector

offset/quarter shift).detector width (ct) In a detector array, the dis-

tance between the two opposite faces of anysingle detector.


deterministic–device driver

D

deterministic (dose) A model where all eventsare inevitable consequences of antecedentcauses. An effect seen at a defined point (countrate, dose, etc.) resulting in loss of organ func-tion due to cell damage. Deterministic effect has athreshold below which no effect is seen.

deterministic effect (tissue reactions) (dose)(ICRP60) A type of radiobiological effect dis-played by a group of cells which is character-ized by a severity that increases with dose,above a certain threshold. Below that thresh-old, the effect is not observed. Examples arevisual impairment due to cataract formation,temporary or permanent loss of fertility, loss ofglandular excretion (salivary, thyroid), skin ery-thema and fetal damage. Deterministic effectsare also called ‘tissue reactions’ (see sterility,cataract, hemopoiesis).

deterministic threshold (dose) Threshold doses(acute and chronic) for some deterministiceffects (ICRP 92 2003c 2005(draft)):

Tissue Single Annual and effect dose (Sv) dose (Sv y�1)

TestesTemporary sterility 0.15 0.4Permanent sterility 3.5–6.0 2.0

OvariesSterility 2.5–6.0 �0.2

LensDetectable opacities 0.5–2.0 �0.1Cataract 5.0 �0.15

Bone marrowHaemopoiesis, 0.5 �.4

depression

detriment (dose) (ICRP73) A measure of the totalharm that would eventually be experienced byan exposed group and its descendants from aradiation source. The combination of probabilityand severity of harm or health detriment. Theprobability of causing a level of total harmjudged to be equivalent to one death that causesa loss of 15 years lifetime. Tissue-weighting fac-tors are derived from this definition (see weighting

factors (tissue)).detriment coefficients (dose) (ICRP73) The nom-

inal coefficients are average values for thewhole population of equal numbers of malesand females of all ages (except for breast andovary). They apply to moderately low dosesand dose rates associated with diagnostic pro-cedures. The remainder tissue is 0.59 and thehereditary detriment figure for gonads is 1.33.

Organ or tissue Nominal detriment coefficient (% per Sv)

Bone marrow 1.04Colon 1.03Stomach 1.00Lung 0.80Breast (female) 0.73Ovary 0.29Bladder 0.29Oesophagus 0.24Liver 0.16Thyroid 0.15Bone surface 0.07Skin 0.04

deuterium (2H) (chem) Symbol D. A naturallyoccurring stable isotope of hydrogen having anucleus consisting of one proton and one neutron(2H; heavy hydrogen). It has 0.015% abundanceand is obtained from water by electrolysis. D2O orheavy water has a density, boiling point andfreezing point greater than H2O. D2O is used as a moderator in nuclear reactors and clinically to estimate body water content (see hydrogen,tritium).

developer (film) (image) A reducing compound ofhydroquinone or metol (commonly hydroquinoneand phenidone) variety which magnifies thelatent film image. The action of the developer onexposed and unexposed silver halide grains is dis-tinguished by rate. Exposed grains develop morerapidly than unexposed grains. The latent imageacts as a catalyst. Density resulting from develop-ment of unexposed grains is fog. Commercialdeveloper commonly consists of:

● a reducing/developing agent;● a preservative/antioxidant (usually sodium

sulphite);● an alkali to keep the pH �10;● a restrainer; commonly potassium bromide.

Hardeners are also present (gluteraldehyde)and anti-fogging agents.

deviation (stats) The percentage differencebetween measured value and prescribed value.(maths) The percentage of difference betweenmeasured value (m) and prescribed value (p)according to (m/p �1) �100% (see accuracy).

device driver (comp) A small software packagethat will let the operating system (and programsrunning with it) control a particular output hardware device (e.g. keyboard, monitor, printer,modem, monitor, graphics card or CD-ROM drive).

D


DFSE–DICOM

DFSE (mri) Double fast spin echo.DFT (mri) Discrete Fourier transform.DGC (us) Depth gain compensation (see time gain

compensation).diagnostic accuracy (imaging) (stats) The ability

to detect disease in members of a chosen popu-lation (from a hospital clinic or screening group).It depends on (1) the incidence of disease in thepopulation and (2) the sensitivity of the test. Theconfidence in any assessment of diagnosticaccuracy depends on the gold standard used tojudge the results; whether they are abnormal(true positive (TP)) or negative (true normal (TN)).The most reliable gold standard is the autopsy,but other references are acceptable with certainprovisos (laboratory findings, biopsy or phantomtest object). Results from the imaging processare tabulated as positive (I�) or negative (I�)findings, comparing these with the known pos-itive and negative values from the gold standard(G� and G�). False-negative (FN) and false-positive (FP) are then discovered.

G� G� Totals

I� TP FP TP �FPI� FN TN FN �TNTotals TP �FN FP �TN Total

diagnostic reference levels (dose) (ICRP73)These are recommended by the ICRP and areusually the absorbed dose in air or in a tissueequivalent sample. In nuclear medicine, it willbe the administered activity. The diagnosticreference level is intended to reduce unusuallyhigh patient doses. It is inappropriate to usethem for regulatory or commercial purposes.They are obtained from a percentile point on anobserved distribution of doses to patients.Advisory dose levels set by professional bodiesto prompt local reviews of practice if consis-tently exceeded.

diamagnetism (mri) A material with a small neg-ative magnetic permeability which decreases thelocal magnetic field and has negative magnetic

susceptibility. Water and oxygen rich compoundsare examples (see contrast medium).

diatrizoic acid/diatrizoate compounds (cm)The first fully substituted tri-iodobenzene con-trast medium developed in the 1950s. Water-soluble ionic contrast medium, monomeric saltsof tri-iodinated benzoic acid. Joining a set ofalmost identical analogous compounds iothala-

mate, metrizoate, iodamide and ioxithalamate.Mixtures of meglumine and sodium salts of dia-trizoic acid have been used.

Compound Viscosity (cP) Osmolality Iodine (mOsm/kg) (mg I ml�1)

Diatrizoate 4.2 at 37°C 1570 300

The proportions of the two salts are adjusted togive the most suitable medium for the requiredclinical examination. The anion is the radio-opaque portion, but both anion and cation areosmotically active; therefore, the solution willbe hypertonic to plasma leading to toxicity.Diatrizoate may be used as an intravascularcontrast agent or as an oral contrast mediumfor the gastrointestinal tract (see Angiografin®,Hypaque®, Renografin®, Urografin®, Urovison®,Gastrografin®, Gastrovist®, Gastrovision®).

DICOM (comp) Digital imaging and communica-tions in medicine is a standard for handlingimaging data, developed by the AmericanCollege of Radiologists (ACR) and NationalElectrical Manufacturers Association (NEMA).The DICOM standard enables the transfer of dig-ital medical images and corresponding informa-tion, independent of device and manufacturer. Italso provides an interface between hospital

Breast screeningA screening population of 60 000 gives 3000 suspectimages of which 300 are true-positives, 2700 false-positives and 50 false-negatives. So,

G� G� Totals

I�� 300 2700 3000I� 50 56950 57 000Totals 350 59 650 60 000

From this table calculations can be made for sensitiv-ity, specificity, general accuracy, predictive positiveaccuracy (PPA), predictive negative accuracy (PPN).

These basic results can be used for receiveroperator curves (ROC analysis) and an estimateof improvement in diagnostic accuracy due tointegrated imaging or double reading of mammo-grams. Diagnostic accuracy has also been usedin a different way to indicate the proportion ofmedical diagnoses that prove to be correct,thereby depending not only upon thedetectability of the disease, but also upon itsprevalence and the particular decision criterionthat is adopted (see Bayes’ theorem).


DICOM–diffusion weighted imaging (DWI)

D

systems (HIS) and radiological systems (RIS)based on other standards. DICOM deals withimaging equipment, printers, picture archivaland communication systems (PACS), and alsooffers other functions such as film printing or CDburning. DICOM Version 3 (2007) provides pro-gram protocols for integrating various imagedata formats between imaging and nonimagingmodalities, devices and systems (see HL7, IHE).

Dicopac® (nmed) 57Co/58Co-labelled cyano-cobalamine preparation for Schilling test

(Amersahm/GE Healthcare).dielectric (phys) A substance commonly found in

capacitors that can sustain an electric field andact as an insulator. Substances have differentdielectric constants or permeability.

diethylene triamine pentaacetic acid (DTPA)(nmed) See DTPA.

differential uniformity (nmed) The intrinsic dif-ferential non-uniformity of sensitivity Ud for agamma camera is Ud � �C/M � 100% where�C is the maximum difference in countsbetween two adjacent pixels and M is thelarger of the two counts. Typical current values,for nonuniformity in central regions, are �2.5%and for useful field of view 2.8% (see integral

uniformity, uniformity (intrinsic)).differentiation (dose) Stem cells entering a path-

way of cell division where the daughter cellsacquire specialized functions.

diffraction (phys) Bending of a beam at the edgeof an absorbing surface into the shadow area ofthe surface.

diffusion (image) See unsharpness (radiographic). (mri)The process by which molecules or other parti-cles migrate due to their continuous randomthermal motion (Brownian molecular movement).Particles/molecules move from areas of higherconcentration to areas of lower concentration.The diffusion in a homogeneous medium showsa Gaussian distribution. The variance (σ2) of thisdistribution depends on the diffusion coefficientand time interval t so σ2�2(D � t ) where D is thediffusion coefficient characterized by themedium viscosity (3 �10�9m2s�1 for water at37°C). The effect is exponential: so e(�b D).Diffusion of water molecules along a field gradi-ent reduces the MR signal. With equal concen-trations there is a statistical balance, but in areasof lower diffusion (diseased tissue), signal loss isless intense, and the display from these areas isbrighter. The diffusion image provides a sensitive

technique for measuring diffusion of small mol-ecules in specific tissue types; the diffusion maybe directed (i.e. along myelin sheaths). This pref-erential diffusion is anisotropic diffusion.

diffusion coefficient (mri) See diffusion.diffusion image (mri) Random diffusion of small

molecules (i.e. water) in tissue due to thermalprocesses. Diffusion imaging can be presentedas diffusion weighted imaging (DWI) or diffusion

tensor imaging (DTI).■ Reference: Elster, 1994; Hagmann et al., 2006.

diffusion sensitivity (mri) See b-value.diffusion tensor imaging (DTI) (mri) Uses

diffusion-weighted images of the brain and tensor field mathematics to produce maps ofindividual fibre tracts, determining their direc-tion and course from their origin in the whitematter to their connection in the cortical greymatter. Isotropic diffusion (i.e. cerebrospinalfluid) can be described simply by the diffusioncoefficient. Anisotropic diffusion (i.e. fibre direc-tion) requires tensor mathematics in order todescribe the direction of dominant diffusionpathways. The direction of greatest diffusion isgiven by the eigenvector of the diffusion tensor.If diffusion gradients are applied in six or moredirections then a tensor can be calculated thatgives a three-dimensional picture of dominantdiffusion pathways. This can identify cerebralwhite matter lesions.■ Reference: DaSilva et al., 2003.

diffusion weighted imaging (DWI) (mri) Providesinformation on the viability of brain tissue. Inany tissue sample undergoing a 90°/180°selected gradient pulse sequence, the echo signal is not received from water proton spinsthat are moving (the normal state in healthy tissue); this results in darker areas in the image.If the proton spins are relatively stationary, thedephasing effect of the gradient pulses cancelsgiving a stronger echo and brighter areas in theimage. Image contrast therefore depends on themotion of water protons, which may be substan-tially altered by disease. DWI uses fast (echo-planar) imaging technology so is resistant topatient motion. Imaging time ranges from a few seconds to 2 minutes. It requires high-performance magnetic field gradients for its opera-tion. The primary application of DWI has been inbrain imaging where it can differentiate acutestroke from other processes that are associatedwith neurological deficit. DWI has also assumed

D


diffusion weighted imaging (DWI)–dipole–dipole interaction

an essential role in the detection of acute braininfarction.■ Reference: Schaefer et al., 2000.

DIGGEST (mri) Direct imaging of local gradientsby group echo selection tomography.

digital circuit (elec) An electronic circuit usingsemiconductor devices (transistors, diodes) aslogical switches. The basis of all computers andlogic devices.

digital filter (image) In a general sense, a devicefor selecting any particular frequency or set offrequencies. Usually confined to a system whichtransmits a certain range of frequencies rejectingall others. It acts on a sampled version of a sig-nal using a discrete logical hardware or softwareelements. Digital filtering can take place in eitherthe time domain or frequency domain. The ideal fil-ter would have a rectangular shape; unity trans-mission in the passband and zero transmission inthe stopband, but this is not achieved in practice(see Butterworth filter, Chebyshev filter).

digital radiography (di) An imaging procedurewhere the intensity of the x-ray beam or singlephoton events are recorded on an image matrixeither from an image intensifier, image phosphorplate or CT detector array (see direct radiography).

digital scan converter (us) Computer memorythat stores echo information.

digital subtraction angiography (DSA) (xray) Animaging technique where digital x-ray imagesbefore and after iodine contrast injection aresubtracted using computer procedures yieldinga difference image of the vascular structurealone. DSA systems use digital fluoroscopy/fluorography systems. A reference mask imageis first collected followed by serial images con-taining contrast material. A DSA processing unitis designed in such a way that two separateimage memories hold the mask and the imagewith contrast medium, respectively. The maskimage is stored in memory and the subtractionof the mask image from the contrast imagesmade in an arithmetic unit, from which theresult is passed on to an image processing anddisplay unit.

digital to analogue converter (DAC) (image) Anelectronic circuit which converts digital signalsinto a matching voltage waveform. This canalso be part of a digital to video converterresponsible for the video display on a computersystem. The bit size of a DAC determines theaccuracy of high-resolution displays.

digital versatile disk (phys) See DVD.digitization (image) Process of conversion of con-

tinuous analogue signals, such as the detected MRsignal (voltage) into numbers. This is carried outwith an analogue to digital converter. Typically, thevoltage is measured (sampled) at particular dis-crete times and only voltages within a particu-lar range and separated by a certain time can be distinguished. Voltages beyond these ranges are said to exceed the dynamic range ofthe digitizer.

digitization noise (image) Noise introduced intosignals by the analogue to digital converter(ADC) either by electronic noise or by the limi-tation of digital resolution (bit depth). Alsocalled quantization noise.

digitizer (comp) See analogue to digital converter.dimeric (cm) Linking two monomer rings gives a

compound with more iodine per unit, resultingin a larger molecule with up to six iodine atomsattached. The increased iodine improves x-rayabsorption and consequently the contrasteffect in the image. The more effective dimericcontrast agents reduce chemotoxicity and otheradverse reactions.

dimeric ionic contrast agents (cm) See ionic

dimer.DIMM (comp) Dual inline memory module.

A memory board that is effectively a doublesingle in-line memory module (SIMM). Its 64bit-wide bus allows single modules to beinstalled in Pentium systems.

diode (phys) An electronic device (thermionic orsemiconductor) which passes current in onedirection only.

diopter (phys) A method of quoting focal lengthof a lens system as the reciprocal of a meter.One diopter has a focal length of 100 cm, fourdiopters 25 cm, etc. Higher diopter figures haveshorter focal lengths. Sometimes used for spec-ifying camera performance.

diploid (dose) Relates to cells having a double setof chromosomes, usually relates to all mam-malian cells except gametes.

dipole (phys) A system of two equal and oppositecharges or poles influencing a very small dis-tance. The product of charge and distance is thedipole moment. Electric and magnetic dipolemoments exist. (mri) See magnetic dipole.

dipole–dipole interaction (mri) The interactiondue to their magnetic dipole moments,between a nuclear spin and its neighbours.


dipole–dipole interaction–disk transfer rate

D

An interaction contributing to relaxation times,which in solids and viscous liquids results inbroadening of the spectral lines.

dipole field (mri) The field pattern produced by aclosely spaced positive and negative electriccharge or a north and south magnetic pole. Atdistances, it is large compared to the dipolelength. The field falls off as the third power ofthe distance away from the charges or polesproducing it.

dipyridamole (clin) Indirect vasodilator used incardiac stress imaging. Increases serum adeno-sine levels.

Dirac, Paul Adrien (1902–1984) British mathe-matical physicist. Produced relativistic waveequations to explain electron spin and discov-ered the possibility of negative energy states,which he interpreted as antimatter, anticipat-ing electron/positron pair formation. He alsopredicted the existence of the magnetic mono-pole. Awarded Nobel prize for physics in 1933with Schrödinger for work in quantum theory.

Dirac function (image) Theoretically a perfectimpulse signal, where x is defined as zero for allvalues except for a central point where x has aninfinitely high value, Δ(x � xo) located at xo.

direct radiography (image) A multilayer digitalx-ray detector consisting of a thin film transistor

(TFT) array behind either a 500-μm amorphousselenium x-ray detector or phosphor material. Apixel size of 140 �140 μm is currently avail-able. A full field 35 �43 cm has a matrix2560 �3072. The selenium detector requires abias voltage and incident x-rays generate electron-hole pairs in the selenium. Thesecharges are collected by the storage capacitorsconnected to the thin-film transistors. Thephosphor material requires a photodiode arrayconnected to the TFT. The digital image can bedirectly viewed, unlike image plate devices (seefill-in factor).

CslGd2O2S

X-ray beamdirection

TFT/Capacitorlayer

Glass substrate

directional dose equivalent (H d, Ω) (dose) (ICRU)The dose equivalent at a point that would be pro-duced by an expanding field in the ICRU sphereat depth d on a radius Ω. For weakly penetrat-ing radiation, a depth of 0.07 mm for the skinand 3 mm for the eye are employed; for stronglypenetrating radiation a depth of 10 mm isemployed. The unit is joule per kilogram (J kg�1).Unit of measurement is the sievert (Sv).

directory (comp) A list of files stored in the computer.

direct splenoportography (clin) See splenopor-

tography.discrete cosine transform (di) Selecting the real

coefficients of a Fourier series (cosine terms) foruse in a fast Fourier transform.

discrete Fourier transform (DFT) (image) SinceFourier transformation is carried out by com-puter, the summing is over a finite or discretenumber of data points rather than the integraldescribed by the Fourier transform. The discreteFourier transform is:

and the inverse, which gives the original signal is:

Euler’s formula allows this to be expressed as thecomplex sum of cosine and sine transforms.

discrete variable (stats) Data points, measure-ments having only integer values, i.e. numberof patients, births, number of lesions.

DISE (mri) Driven inversion spin echo.DISIDA (nmed) See disofenin.disk (comp) A general term covering various

types of media used to store program and datafiles on a permanent basis (see bulk storage,CD-ROM)

disk (floppy) (comp) See bulk storage.disk (hard) (comp) See bulk storage.disk spindle speed (comp) The speed of the disk

medium. Hard drives are restricted to onespeed which can be 3600–3880, 4500, 5400and 7200 r.p.m. Disk speed directly influencestransfer rates (see disk transfer rate).

disk transfer rate (comp) The speed at whichdata is moved to and from the disk media. Hard

disk transfer rates increase from the inner diam-eter to the outer diameter of the disk surface.

f x F u e jwx N

u

N

( ) ( ) .� ��

�/

0

1

∑

F u f x e jwx N

x

N

( ) ( )� � �

�

�1

0

1

N/∑

D


disk transfer rate–DMSA(V)

The disk transfer rate is dependent on the diskspeed (r.p.m.) and the data density on the diskin bits per inch (b.p.i.).

disofenin (nmed) Diisopropyl iminodiacetic acid.Diisopropyl-IDA kit for the preparation of 99mTc-disofenin for injection, A radiopharmaceuticalfor cholescintigraphy.

Generic name 99mTc disofenin (DISIDA)Commercial names Hepatolite®-CISImaging category Hepatobiliary

dispersion (us) An ultrasound pulse spreads outas it passes through medium losing high fre-quencies. Pulse height decreases as pulsewidth increases. Depends on the property ofthe medium. The spread of a distribution. Seestandard deviation, quartile).

displacement (angular) (θ) The angular changein angular motion defined as θ �x/r, where x isthe linear displacement and r the radius ofangular motion. Since r (the radius) is fixed thenθ x. Angular displacement is in radians (rad)where 360° � 2π rad and 1 rad � 57.3° �

180°/π rad.display resolution (LCD) (comp) See flat panel

display.display matrix (ct, image) The array of rows and

columns of pixels in the displayed image, typi-cally between 512 �512 and 1024 �1024.It may be equal to or larger than the size of the reconstruction matrix due to interpolation

procedures.display window (ct) Selectable range, by the

operator, within the CT number scale displayedon the screen and using the full range ofbrightness levels. The display window is usu-ally defined according to the window widthand window centre (off-set). Pixels outside thedefined values are displayed as either white(above) or black (below the chosen window).

distortion artefact (mri) Image distortions causedby inhomogeneity of the magnetic field, gradi-ent nonlinearity, or ferromagnetic materials inproximity to the examination.

distribution coefficient (cm) See partition/distribu-

tion coefficient.disturbed flow (phys) Flow that cannot be

described by straight, parallel stream lines.dithering (image) A half-toning method where

several dots of the primary colours are printedin various patterns to give the impression of a

larger colour spectrum. The printing process ofsimulating additional colours or shades by mix-ing available colours and varying dot sizes andspacing. A half-toning method where severaldots of the primary colours are printed in vari-ous patterns to give the impression of a largercolour spectrum; the printing process of simu-lating additional colours or shades by mixingavailable colours and varying dot sizes andspacing.

diverging collimator (nmed) A multiple-holecollimator whose holes diverge with a focalpoint behind the camera crystal. The divergingcollimator reduces patient images, permittingimaging of patients whose width is greaterthan the diameter of the camera crystal.

divergent angle (θ) (us) The angle describing farfield divergence (see Fraunhofer zone).

DLL (comp) Dynamic link library. A small programthat can be shared between several taskssimultaneously. An essential component of adevice driver. A program module that containsinstructions common to different applications.Instead of including these in a program, theDLL can be called as required, loaded intomemory and run. As the link is dynamic, themajority of DLLs can be unloaded when nolonger needed, so saving memory resources.

DMA (comp) Direct memory access. A process forfast data retrieval from a device such as a harddisk that writes it into main memory withoutinvolving the processor, thus freeing it up forother tasks.

DMF (dose) See dose-modifying factor.Dmin Dmax (image) See film sensitometry.DMSA (nmed) Meso-2,3-dimercapto-succinic acid

reconstituted with 99mTc. Localization and eval-uation of various kidney diseases. Because ofbinding to plasma proteins, clearance is mainlythrough tubular absorption, showing a specificaffinity for the renal cortex. Renal accumulationincreases for 6 hours when 20–35% of theinjected activity resides in each kidney. DMSA(V)

targets tumour sites.

Generic name DMSA (III)DMSA (V)

Commercial names Succimer® (MediPhysics Inc)Amersham DMSA

Imaging category Static renal function

DMSA(V) (nmed) Labelling DMSA with 99mTc undermoderate reduction conditions, i.e. alkaline pH


DMSA(V)–Doppler (power)

D

and low concentration of Sn(II), leads to the for-mation of a 99mTc(V) – DMSA complex which hasproved suitable for tumour imaging.

DMSSFP (mri) Double-mode steady state free precession.

DNA (clin) Desoxyribonucleic acid. A type ofnucleic acid found in the nucleus of the cell.The other nucleic acid RNA or ribonucleic acidis found in the cytoplasm and in small amountsin the nucleus. Radiation damage to this struc-ture causes cellular malfunction, but there areelaborate systems of repair processes that haveevolved for correcting DNA damage.

domain (comp) Represents an IP (internet proto-col) address or set of IP addresses that comprisea domain. The domain name appears in URLs toidentify web pages or in email addresses. For example, [email protected],where ‘centralhospital.com’ is the domainname. Each domain name ends with a suffixthat indicates what top level domain it belongsto. These are: .com for commercial, .gov for gov-ernment, .org for organization, .edu for educa-tional institution, .biz for business, .info forinformation, .tv for television, .ws for website.Domain suffixes may also indicate the countryin which the domain is registered: .ie forIreland, .de for Germany, etc. No two partiescan ever hold the same domain name.

dominant (xray) Applied mainly to fluoroscopywhere the region of the object (patientanatomy) which is of diagnostic interest isselected in the field of view of the image inten-sifier. The automatic exposure device (dose lim-iting) coincides with this area. In a radiograph,this region should maintain a specific averageoptical density (including mammography).

door (shielded) (xray) A fire-rated lead-lineddoor set which should comply with BS476 Part 8(1972) and Part 22 (1987) (see lead shielding).

DOPING (mri) Double pulse interlaced echo imaging.doping (phys) The addition of a known quantity

of impurity to either a semiconductor to alter itselectrical characteristics or to a phosphor toimprove its efficiency or alter its emission spec-trum. For scintillator compounds used in theconstruction of x-ray detectors, appropriatedoping improves the detector performance: sig-

nal decay and absorption efficiency.doping agents (phys) Elemental impurities added

to phosphor crystal structures in order to intro-duce traps in the forbidden energy band.

Application Phosphor Doping element

Intensifying screens LaOBr Tb, TmGdOS TbYTaO Nb

Image intensifier CsI NaphosphorNuclear medicine NaI TlComputed radiography BaFX Eu

(X �Cl, Br or I)Thermoluminescent LiF Mg, Tidosimetry

CaSO4 Dy

Doppler, Christian Johann (1803–1853) Austrianmathematician and physicist. Noticed theapparent difference between the frequency atwhich sound waves leave a source and that atwhich they reach an observer caused by therelative motion of the observer and source.

Doppler (colour) (us) This display uses echolocation and Doppler shift to give a display ofblood flow on a grey scale anatomical back-ground. Time-shift colour flow imaging theDoppler effect is not used, but echo arrival timeshifts are used for determining reflector motion.In each case, the Doppler shift or time shiftvariation is colour coded to give a blood flowdisplay either towards or away from the trans-ducer. Imaging frame rates are decreased dueto the added computational time (see Doppler

(power)).Doppler (continuous) (us) See Doppler (CW).Doppler (CW) (us) Continuous wave Doppler. A

transducer having separate transmitter andreceiver transducer crystals active continu-ously. No depth sensitivity but able to giveaudible Doppler velocity signals in a portablemachine (see pulsed Doppler).

Doppler (power) (us) The difference betweencolour and power Doppler is that power Dopplermaps colour to power value rather than themean frequency. The machine uses autocorrela-tion detectors to produce a colour power map.The main advantage is that noise is restricted toa uniform low level and so the colour signalstrength is increased with an overall gain of10–15 dB over the conventional colour Dopplershowing increased sensitivity to small flow dif-ferences. Noise reduction is obtained mainly byframe averaging which tends to affect the per-sistence of the image and introduce movementartefacts due to low frame rates.

D


Doppler (pulsed PW)–Doppler shift

Doppler (pulsed PW) (us) Measuring the Dopplerfrequency by pulsing the ultrasound beam(pulsed wave Doppler). A single transmit/receive transducer is depth sensitive and theuser can select a region of interest (sensitivearea). A sample gate is opened to accept echosignals from a specified depth. The pulse repeti-

tion frequency (PRF) is critical for accurate depthselection.

Doppler (reflector speed) (us) Commonly refersto flowing blood cells. The reflector speed ofblood cells (20–200 cm s�1) is much less thanthe speed of sound in soft tissue (1540 m s�1).

Propogation speed Doppler shift�

�2f cos( )θ

Duplex transducer

Doppler transducer

Sensitivearea

Image field of view

Doppler effect (us) Frequency change of areflected sound wave as a result of reflectormotion relative to the transducer. The fre-quency shift between transmitted and reflectedultrasound wave f2 � f1 as Δf, described by:

where c is the propagation velocity of sound inblood and v is the blood velocity. The angle (θ)between the transducer and skin surface is theangle of insonation. The value 2 represents thedouble Doppler effect, once from the movingblood cells and again when the transducerreceives the echo.

Doppler sample volume (us) The sensitive vol-ume from which the Doppler signal is obtained.The position within the beam is dependent onpulse repetition frequency (PRF) (see sample volume).

Doppler shift (us) This is calculated as the differ-ence between echo frequency and incident fre-quency. Both are measured in MHz. It can alsobe calculated from the transducer incident fre-quency f (MHz), the reflector speed (v) in cm s�1

and propagation speed (c). Also in cm s�1 as:

2f vc

� � cosθ

Δffc

v� 2 ( cos )θ

0 10 20 30 40

12�103

10�103

8�103

6�103

4�103

2�103

50

Doppler angle (O)

Dop

pler

shi

ft (H

z)

10 MHz tranducer0.1 ms�1

60 70 80 90

Doppler–reflector speedFor a 5 MHz (f �5000 kHz) transducer used at anangle of 45° (cos(θ) �0.707) gives a Doppler shift frequency of 900 Hz (0.9 kHz); the reflector speed is:

Since distance to reflector surface is 154 000/2 theequation can be simplified to:

77000 0 95000 0 707

19 6 1�

�� .

.. cm s

15400 0 92 5000 0 707

19 6 1�

� �� .

.. cm s

Doppler angle (us) The angle between the soundbeam and the flow direction. If the anglebetween the transducer and vessel is zero (par-allel) the Doppler shift is maximum; at 90° it iszero (graph).

Doppler shiftA 5 MHz Doppler signal from a transducer placed atan angle of 60° (0.5). It measures a blood velocity of50 cm s�1 (speed of sound in soft tissue �154 000cm s�1). The Doppler shift frequency is:

2 5 10 50 0 5154 000

16236� � � �

�( ) .

.Hz


Doppler (spectral)–dose calibrator

D

Doppler (spectral) (us) A time (x-axis) versusvelocity magnitude display calculated from theDoppler shift on the y-axis. The common veloc-ity spread is intensity coded by pixel brightness.Spectral Doppler assesses organ perfusion iden-tifying stenoses and occlusion. It also evaluatesheart function. Volume blood flow can beassessed directly or indirectly from the Dopplerspectrum as:

Q � TAVmeanA.

TAVmean is the amplitude-weighted averagingover all instantaneous flow velocities registeredin the sample volume. A is the cross-section ofthe vessel at the sampling site obtained from thevessel diameter measured in the B-mode image.

DOS (comp) Disk operating system. An operating

system designed for early IBM-compatible PCs.dosage (nmed) The amount of radiopharmaceuti-

cal administered to a patient, measured in bec-querels (Bq).

dose (entrance) (dose) The dose or dose rate atthe surface of the patient or absorber. Any doselevel measured at the surface will include afraction back-scattered from deeper tissue lay-ers: the back scatter fraction (BSF). Entranceexposure and entrance surface dose (ESD) dis-tinguish with and without BSF correction,respectively. ESD is a measure for individualradiographs (see dose area product).

dose (exit) (dose) The air kerma measured at theabsorber surface opposite the beam. Typicallythis is maintained at a constant level in order toprovide a mean image density or optimum inputrate at the image intensifier face (see entrance dose).

dose (organ) (dose) See mean glandular dose.dose (patient) (dose) Measured as:

● absorbed dose in air (air-kerma); as a dose-area

product;● surface dose using tissue equivalent dosimeters;● depth dose (CTDI);● patient activity levels (nuclear medicine).

(see diagnostic reference levels).dose (x-ray) (xray) From an exposure giving a pho-

ton fluence of 3 �1010 photons cm�2 at 60 keVeff

delivers an approximate dose of 10 mGy (1 R).

(see dose – area product, diagnostic reference levels,photon fluence, photon flux).

dose and dose-rate effectiveness factor(DDREF) (dose) (ICRP60) A factor that general-izes the usually lower biological effectiveness(per unit of dose) of radiation exposures at lowdoses and low dose rates as compared withexposures at high doses and high dose rates.

dose area product (DAP) (Fa) (dose) Used in x-rayexaminations to control radiation exposure topatients. It is the dose integral of the air-kermaKa over the intersecting surface S of the usefulradiation beam:

The SI unit for the dose area product is Gy m2,replacing R cm2 where 1 R cm2 is approximately0.87 cGy cm2 or 0.87 μGy m2. The DAP is a meas-ure of the complete study. National protocolsgive recommended dose quantities for entrance

surface dose (ESD) for individual radiographs anddose – area-product (DAP) for complete examina-tions. The doses are reference levels intended totrigger department investigations if they areroutinely exceeded.

Entrance surface dose (UK)

Study ESD (mGy)

Lumbar spine, AP 10Lat 30LSJ 40Abdomen, AP 10Pelvis, AP 10Chest, PA 0.3Lat 1.5Skull, AP 5

Dose area product (UK)

Study DAP (Gy cm�2)

Lumbar spine 15Barium enema 60Barium meal 25Intravenous urography 40Abdomen 8Pelvis 5

■ Reference: IPSM/NRPB/CoR, 1992.

(see dose (entrance)).dose calibrator (nmed) A device (ionization

chamber) for measuring the amount of radio-active material within a container. Used to ver-ify dosage prior to administration. It iscommercially available as a ‘dose calibrator’.

F K dSa aS

� � .∫

Dose, x-rayFrom the data given in photon exposure. The photon fluence for a chest radiograph is 8.3 �108

photons cm�2. Then the equivalent dose is8.3 �108/3 �1010 �0.2 mGy (0.027 R).

D


dose calibrator–dose length product (DLP)

Having a cylindrical 4π construction used formeasuring radionuclide activity levels innuclear medicine. The activity range covered iscommonly a few MBq to many GBq.

dose coefficient (dose) Alternative for dose perunit intake, can describe other coefficients link-ing quantities or concentrations of activity todoses or dose rates.

dose constraint (dose) An agreed restriction on theindividual dose from a radiation source, whichserves as an upper limit on the dose in optimiza-

tion of protection for that source. For occupationalexposures, the dose constraint reduces the indi-vidual dose limits considered in the process ofoptimization. For public exposure, the dose con-straint is an upper limit on the annual doses thatmembers of the public should receive from theplanned operation of any controlled source.

dose descriptor (ct) Measurable parameter, such asCTDIair, CTDIw, etc. or DLP, from which the effective

dose or the organ dose delivered to a patient in aCT examination can be estimated, or the perform-ances of different CT scanners can be compared.

dose distribution (ct) The spatial distribution ofdose as a function of the position within thescan plane (CTDIw). For CT, the ratio of surfacedose and the dose at the centre is closer to onethan for projection/conventional radiographybecause of the rotating x-ray source around thepatient during the examination.

dose–effect curve (dose) See dose – response curve.dose efficiency (ct) Number that quantifies the

amount of dose absorbed by the detectors as afraction of the total dose reaching the detector;overall dose efficiency of a CT scanner isdescribed as the product of three factors:

1 geometric efficiency of the detector array;2 detector efficiency;

3 x-ray beam z-axis efficiency.

The results given in the table show an improve-ment in the overall dose efficiency for thin sliceswhen progressing from the 2-slice, 4-slice and16-slice scanner. This improvement is mainlydue to the wider beam width used to acquirethe thin slices in the 4- and 16-slice scanners.(See Table below)

dose equivalent (H) (dose) (ICRP26) The productof the absorbed dose at a point in tissue and radi-

ation quality factor Q. So the product: grays(Gy) �Q �H, where H is measured in sieverts(Sv). The previous unit the rem (non-SI) isreplaced by the sievert (SI). This term has beenreplaced by the equivalent dose which measuresabsorbed dose over a specified area.

dose length product (DLP) (ct) Dose measure-ment used as an indicator of overall exposurefor a complete CT examination. Allows perform-ance comparisons. The associated dose – lengthproduct (DLP) for a complete examination, canbe derived as:

where i represents each scan sequence form-ing part of an examination, and CTDIw is theweighted CTDI for each of the N slices of thick-ness T (cm) in the sequence and C the exposurein mAs (as fractions of 100 if the CTDIw wasstandardized to 100 mAs). For spiral acquisitionthe formula changes to:

For each study or scan sequence, A is the tubecurrent (mA) and t the study acquisition time. Itis also convenient to modify this formula toaccommodate multislice scanners using CTDI100.The scattered radiation component increasesslightly with multislice machines increasing the

DLP mGy cm� � � �CTDI T A twi

∑ ( )

DLP mGy cm� � � �CTDI T N Cwi

∑ ( )

100

16

4

2

90

80

70

60

50

40

300 1 2 3 4 5 6

Slice thickness (mm)

Dos

e ef

ficie

ncy

(%)

7 8 9 10

Scanner Slice number Geometric efficiency (%) z-axis efficiency (%) Overall dose efficiency (%)

Dual 2 �mm 80 63 50Quad 4 �mm 78 72 54Sixteen 16 �1mm 75 93 65


dose length product (DLP)–dose profile/slice dose profile (SDP)

D

overall dose. European guidelines for CTDIw andDLP values are obtained as 75th percentile fig-ures from a European wide CT survey.

Organ CTDIw (mGy) DLP (mGy/cm)

Head 60 1050Face and sinuses 35 360Vertebral trauma 70 460Chest 30 650HRCT of lung 35 280Liver 35 900Abdomen 35 800Pelvis 35 600

■ Reference: Tsapaki et al., 2001.

dose limitation (ICRP) (dose) A controlling factorregulating radiation practices which ensures anequitable distribution of individual benefits anddetriment.

dose limits (environmental) (shld) The USEnvironmental Protection Agency’s generallyapplicable environmental radiation standards in40 CFR Part 190 shall comply with those stan-dards. Environmental Conservation Law: Subpart380.5: Radiation dose limits for individual members of the public; 380-5.1: Dose limits forindividual members of the public; 380-5.2:Compliance with dose limits for individual mem-bers of the public. The total effective dose equiv-alent to individual members of the public doesnot exceed 0.1 rem (1 mSv) in a year. Unrestrictedarea in the environment from external sourcesdoes not exceed 0.002 rem (0.02 mSv) in any onehour; in an unrestricted area, the dose fromexternal sources would not exceed 0.002 rem(0.02 mSv) in an hour and 0.05 rem (0.5 mSv) in ayear. The principal European limit for radiationexposure was 0.5 mSv yr�1 (1977 ICRP values).This is based on a mortality risk of 5 �10�6 perannum. This has been reduced to 0.3 mSv yr�1.

dose limits (staff) (dose) (ICRP73) These apply toworkers and members of the public who requireto be in an area where radiation is being used.Dose limits do not apply to patients, providedthat these doses have been justified.

Application Occupational Public

Effective dose 20 mSv per year, 1 mSv y�1

averaged over 5 years

Equivalent doseEye lens 150 mSv 15 mSvSkin 500 mSv 50 mSv

Extremities 500 mSv –

The use of optimization in the work place andthe ALARA principle now makes these doselimits less important.

dose-modifying factor (DMF) (dose) The ratio ofradiation doses with and without modifyingagents, causing the same level of biologicaleffect.

dose modulation (ct) Currently the two methodsare chosen by manufacturers for modulatingthe x-ray beam are essentially:

● A prior scoutview of the scanned area to alter alook-up table for modulating the x-ray beamduring the intended scan: Whole body-dosemodulation uses a scoutview to vary mA alongthe patient and during rotation. Prior to theaxial scan a look at attenuation along one ortwo scout views is made this mapping made tovary the mA in each slice accordingly.

● A continuous slice by slice sampling and modu-lation of the x-ray beam: This uses feedbackduring the scan study to vary x-ray tube mAalong the patient and during rotation, accordingto the attenuation seen in the previous rotation.The tube mA gradually changes in response toanatomy in real time.

dose–mortality curve (dose) The exposure of apopulation to a whole body dose of radiation canbe plotted as dose versus mortality. This gener-ally gives a sigmoid shape curve where theupper and lower portions of the curve are poorlydefined. The midpoint of the curve at the 50%survival mark is the median lethal dose (LD50).

dose profile/slice dose profile (SDP) (ct)Representation of dose as a function of positionalong a line perpendicular to the tomographic

100

75

50

25

0

2 4 6Dose (Sv)

LD50

Pop

ulat

ion

mor

talit

y (%

)

D


dose profile/slice dose profile (SDP)–dose–response curve

plane. The dose as a function of the positionalong the z-axis resulting; due to x-ray scatter,the slice dose profile (SDP) is always broaderthan the slice sensitivity profile (SSP) in spite ofdetector collimation.

dose rate (dose) Amount of energy absorbed pergram per unit time. Unit is sievert per unit timeper gram of tissue (see kerma).

dose rate constant (nmed) This replaces the spe-

cific gamma ray constant (Γ) according to ICRU33,although the two terms are not identical. Forradiation therapy, the dose rate constant Gd ofa radionuclide, emitting photons is the quotientKd � r2 and the activity A. The symbol Kd is theair kerma rate which would be produced by allthe photons of energy E � d at a distance rfrom a point radiation source of activity A, if theradiation were neither absorbed nor scatteredat the source or in any other matter:

The SI unit being Gy m2s�1Bq�1. The selectionof the lower energy limit d expressed in keV,depends on the application. The dose rate con-stant GH is defined for radiation protection,where Hx is used in place of the air kerma rateKd. The energy threshold is 20 keV for allnuclides:

The practical exposure rate constant of a radio-nuclide is given as mGy cm2MBq�1h�1 (SI units)or mrad cm2mCi�1h�1 (non-SI units). A variationis cGy�1U�1 where U represents the unit of air

GH r

AHx�

� 2

GK r

Add�

� 2

1.0

FWHM0.5

0.0

�6 �4 �2 0Slice dimension (mm)

Sig

nal s

tren

gth

2 4 6

Slice sensitivity(SSP)

Slice dose(SDP)

kerma strength 1 μGy m2h�1. Conversion factorsto convert exposure dose rate constants(mGy cm2MBq�1h�1) to specific gamma ray con-

stants (mGy h�1MBq�1 at 1 m) are available.

Nuclide mSv hr�1m�1 GBq�1

57Cobalt 1.6 �10�2

60Cobalt 3.6 �10�1

111Indium 8.4 �10�2

131Iodine 5.7 �10�2

99Molybdenum 4.1 �10�2

99mTechnetium 1.7 �10�2

■ Reference: Siegel et al., 2002.

dose–rate–effectiveness factor (DREF) (dose)A correction factor which allows for the low-dose nonlinear response for low linear energytransfer (LET) radiation when interpreting low-dose effects from high-dose observations. Thisis used to project cancer risk determined athigh doses and high dose rates to the risks thatwould apply at low doses and low dose rates(ICRP 60, 1991). In general, cancer risk at theselow doses and low dose rates is judged, from acombination of epidemiological, animal andcellular data to be reduced by the value of thefactor ascribed to DDREF. It reduces by a factorof 2 (current ICRP value) the probability coeffi-cients seen at high dose rates. Used for doseestimations below 200 mGy or �100 mGy h�1.

dose–response curve (dose) Also called dose–effect curve. Generally there are three specificresponses at low doses:

1 linear response;2 quadratic;3 linear-quadratic.

The general form includes both linear andquadratic terms for defect (cancer) induction (α1

and α2), as well as β1 and β2 coefficients forexponential cell killing. The product of thepolynomial component:

(α0 � α1D � α2D2 � αnDn)

and the exponential form, , combine.Coefficients α (determining the curvature of theslope) are selected to give the best fit to thedata. At low doses, the exponential effect issmall and cell killing is not present (due to cel-lular repair mechanisms). The polynomial com-ponent then dominates in its simpler forms oflinear, quadratic or linear-quadratic depending

e D D( )� �β β1 22


dose–response curve–dosimetric terms (population)

D

on cell type. None of the curves shows a thresh-old below which no effect is seen.

(see linear non-threshold hypothesis).■ Reference: Brill, 1982.

dose–survival curve (bacteria) (dose) The lossof proliferation in a single cell organism (bacte-rial colony) is related as a simple exponential todose over a very wide range of exposure. A typ-ical LD50 is 200 Sv.

dose–survival curve (mammalian) (dose) Cellsurvival to a single dose of radiation. The curvecharacteristics are described by two parametersDo which describes the slope of the exponentialportion of the curve after the shoulder. The sizeof the shoulder (which represents cellular repaireffectiveness) is measured by extrapolating thestraight portion of the curve (plotted on log/linear scales) upwards to the vertical axis where

0 200

100

50

10

Sur

viva

l (%

)

400 600Radiation dose (Sv)

800

LD50

Radiation dose

Org

an/ti

ssue

res

pons

e

Linear quadratic

Quadratic

Low doseresponse

Linear

it intersects at N. The curve for sparsely ionizingradiation (low LET) is given by:

where N is the extrapolation number at zerodose. Do is the dose at 37% survival or thereciprocal of the slope, typically 1–2 Gy. Dq isthe 100% survival shoulder.

(see dose – response curve).dose–threshold hypothesis (dose) A specified

dose above background below which it ishypothesized that the risk of excess cancerand/or heritable disease is zero.

dosimetric terms (individual) (dose) The tablegives the complete updated family introducedin ICRP60. Separate detailed descriptions aregiven under the headings identified in 1–5.

Measure Unit Derivation

1. Absorbed gray (Gy) E/m J kg�1

dose, D2. Equivalent Sievert (Sv)

dose, HT

3. Effective dose, Sievert (Sv)E (effective dose equivalent, EDE)

4. Committed Sievert (Sv) HT(τ)� HT � τequivalent dose, HT(τ)

5. Committed Sievert (Sv) Eτ � E � τeffectivedose, Eτ

dosimetric terms (population) (dose) See collec-

tive equivalent dose, collective effective dose.

E H wT T� �∑

H D wT T R� �∑

0

101

100

10�1S

urvi

val

10�2

10�3

10�4

3 6 9 12 15 18Dose (Gy)

Do

Dq

N

S e D D No� � � �1 1( )/

D


dosimetry–downtime

dosimetry (dose) Calculation of dose received byan organ or tissue or whole body from ionizingradiation. For x-ray studies, this determines theenergy dose in body tissue which cannot bedirectly measured in the body cavity, but canbe calculated from the energy dose produced ina small air probe (thimble dosimeter). The iondose is measured from which the dose in airand thus the energy dose can be calculated.The ion dose is frequently measured using tis-sue equivalent thermoluminescent detectors. Theenergy dose is calculated from the measure-ment and conversion factors (f-factor) listed intables (see absorbed dose, equivalent dose, effective

dose, MIRD).dosimetry phantom (dose) Cylinders of poly-

methylmethacrylate (PMMA) used for standardmeasurements of dose in CT, having a diameterof 16 cm (head phantom) or 32 cm (body phan-tom) and a length of at least 14 cm. The phan-toms are constructed with removable insertsparallel to the axis to allow the positioning of adosemeter at the centre and 1 cm from theouter surface (periphery).

dot-matrix (comp) A printer where the charactersand graphics are formed from a grid of dotsproduced by wire pins striking the paperthrough a fabric ink-ribbon. A common designuses 24 pins as found in line printers producingcontinuous documentation (patient documentsand labels) where very fast printing is essentialand top quality is less important (see laser

printer, ink jet printer, bubble jet printer).dot pitch (display monitor) (image) Monitor dot

pitch is measured as the distance between dotsof the same colour (red, green or blue) andtherefore refers to pixel dimensions; typicalvalues are 0.28, 0.25 or 0.22 mm. Colour moni-tors are rated as pixels per inch (ppi) which istypically 72 to 96 ppi (see flat panel display).

dot pitch (printer) (image) Dot pitch on a printerrefers to dot density as dots per inch (dpi) typi-cally 300, 600 or a commercial printers qualityof 1200 dpi (see display resolution).

Dotarem® (cm) Commercial preparation (Guerbet)of gadoteric acid, a paramagnetic ionic MRIcontrast agent. (See Table below)

double blind (stats) A method for avoiding bias inclinical trials. Single-blind trial has only onegroup (scientist or patient) knowing the treat-ment. In double-blind trials, neither group knowthe distribution of drug or treatment protocol.

double contrast enema (clin) Removal of barium

enema and introducing air into the rectum,showing fine detail of the rectal and colonmucosa.

double contrast sequence (mri) Turbo spin echo(SE) counterpart to double echo sequences. Forshort pulse trains only echoes for PD and T2-weighted images having the phase-encodinggradient are measured. The echoes that deter-mine resolution are used in both raw datamatrices (echo sharing), reducing the number ofechoes required. More slices can be acquired forthe TR period; giving a lower specific absorptionrate (SAR).

double echo sequence (mri) Spin echo (SE)sequence with two echoes; obtained withoutincreasing the measurement time since they areproduced from the first echo of a T2-weighteddouble echo sequence.

double-oblique slice (mri) Rotating an obliqueslice about one axis in the image plane.

double reading (mammography) (clin) Two cli-nicians reviewing the same mammograms canimprove both sensitivity and specificity. If a single reading yields a sensitivity of 80% anda specificity of 70%, then double reading usingthe statistics described in integrated imaging,condition (1) yields sensitivity 96% and speci-ficity 91%, and for condition (2) yields sensitiv-ity 99% and specificity 97%.

downscatter (nmed) Scattered photons of lesserenergy resulting from Compton scattering ofprimary gamma emissions of a radionuclide.

downtime (comp, xray) The percentage of timeduring which a piece of equipment is routinelynot working or broken; a measure of equipmentreliability. Zero downtime implies 100% relia-bility. An annual reliability of 90% means adowntime of 10% or 36.5 days. A 99% reliabil-ity means a downtime of 3.6 days and 99.99%gives a downtime of 1 hour per year and99.999% just over 5 minutes. Mission critical

Compound Concentration (mg mL�1) Viscosity (cP) Osmolality (mosm/kg)

Gadoterate meglumine 279.32 3.2 at 20°C 1350 Gd-DOTA 2.0 at 37°C


downtime–dual focal spots

D

applications demand very small downtime val-ues (see MTBF).

DPD (nmed) A preparation as 99mTc-labelled 2,3dicarboxypropane-1,1-diphosphonate pro-duced from diphosphonopropane-dicarboxylicacid. Used as a diagnostic bone imaging agent.Reported to show higher normal/abnormalbone ratios than MDP or HMDP.

dpi/DPI (comp) Dots per inch. A common measureof dot pitch, the resolution on a printer, scanneror display.

DPSF (mri) Diffusion/perfusion snapshot flash.DQE (image) See detective quantum efficiency.DRAM (comp) Dynamic random access memory.

The most common and cheapest form of com-puter memory which uses one capacitor andtransistor to store one bit of information. Beingdynamic, the capacitor needs refreshing everyfew milliseconds or it will lose its charge (losinginformation).

Draximage® (nmed) See gluceptate.DREF (dose) See dose rate effectiveness factor.drip infusion cholangiography (clin) Commonly

used when oral techniques have failed. Contrastmaterial introduced by intravenous infusion.

drip infusion urography (clin) For demonstrat-ing space-occupying lesions within the kidneyand outlining the whole extent of the ureters.Direct intravenous injection using drip infusionover an extended period.

driver (comp) See device driver.DSA (xray) See digital subtraction angiography.DSL (comp) Digital subscriber line. A method of

connecting to the Internet via a phone line. ADSL connection uses copper telephone lines,but is able to relay data at much higher speedsthan modems and does not interfere with tele-phone use (see ADSL).

DSO2 (dose) A revised dose system developed forestimating gamma and neutron exposure undera large variety of situations and which allowsthe calculation of absorbed dose to specificorgans. DSO2 improved on the DS86 system.

DS86 (dose) An early dose system developed forestimating gamma and neutron exposure undera large variety of situations which allowed thecalculation of absorbed dose to specific organsfor members of the Life Span Study.

DT (dose) See absorbed dose.DTE (comp) Data terminal equipment. An end

device on a communications circuit – a com-puter terminal or PC.

DTI (mri) See diffusion tensor imaging.DTPA (nmed) Diethylene-triamine-penta acetic

acid usually present as a calcium or sodium saltcomplex as monocalcium trisodium diethylene-triamine penta-acetic acid, a chelating sub-stance containing five acetate groups whichforms a stable complex with metallic ions, typi-cally 99mTc or 111In. It is a strong chelating agent

more commonly found as a radiopharmaceutical for99mTc labelling when it forms a glomerular filtra-tion rate (GFR) agent for renography. DTPA alsoforms complexes with 111In for cerebrospinalfluid (CSF) studies or as a method for labellingantibodies with 111In-DTPA as an intermediate.

Generic name 99mTc-DTPACommercial names Techeplex Bracco®

DTPA-CIS®

Draximage DTPA®

Mallinckrodt DTPA®

Amersham DTPA®

Imaging category Renal, ventilation V/Q

(mri) Gd-DTPA and Mn-DTPA are used as para-magnetic contrast agents (see EDTA).

DTR (comp) Data terminal ready. An RS232C cir-cuit that is activated to let a DCE know when aDTE is ready to send or receive data.

dual-energy CT (DECT) (ct) A machine design(Siemens) that exposes the same slice simulta-neously with two different x-ray beam ener-gies: 140 kVp, then 84 kVp. Dual-energy CTpromises additional diagnostic informationincluding bone mineral content, improvedbeam hardening correction and display quality.

dual energy subtraction (xray, image, ct)A method for removing hard and soft tissue con-tributing to a subtracted image. X-ray energiesof 60 and 110 kVp are commonly employed.

dual focal spots (xray) A target (x-ray) carrying asingle focal spot is restrictive since many appli-cations require two focal spot sizes; for generalapplications and a second smaller focal spot forhigher resolution or magnified radiographs (i.e.mammography or spot imaging). Two methodsare used for providing two focal spot sizes onthe same anode. (1) A single filament refocusesthe electron beam electrostatically varying thenegative voltage on the cathode cup, so bom-barding a smaller area on the anode track. (2) A double filament, each one directed to adifferent angled target, which requires a dualtrack anode to give two focal spot sizes.

D


dumb terminal–dynamic focusing

dumb terminal (comp) A monitor and keyboardthat displays information only (as opposed to theprocessing capability of a PC); usually connectedto a mainframe or local area network (LAN).

duo core processors (comp) Two independentprocessors (CPUs) in one package run at the samefrequency and share 4 MB of L2 cache. It has a1.066 MHz bus performing parallel processing.

duplex (comp) A method of data transmission.Full-duplex allows a packet of information to betransmitted and received at the same time.Half-duplex allows packets to be either trans-mitted or received, but not at the same time.

duplex imaging (us) Combining a grey-scaleimage with a colour Doppler image.

duplex instrument (us) An ultrasound instru-ment that combines gray-scale sonographywith pulsed Doppler and, possibly, continuous-wave Doppler.

duty cycle (us) The percentage of time the pulseoccupies in the operational cycle:

The percentage or fractional measure of thetime that the pulse occupies in the transmitreceive cycle. It increases with increasing PRF.Time fraction that the pulse is on. Typical value5 ms, range 1–10 ms. Both PD and PRP aremeasured in microseconds (μs) to give the DF asa unitless fraction (see ultrasound (safety)). (mri)The time during which gradient switching canbe run at maximum power. Based on the totaltime (as a percentage), including the cool-downphase (see SAR).

duty factor (DF) (us) The product of the pulseduration and the pulse repetition frequency fora pulsed waveform. A typical pulsed sequenceconsists of an ‘on-time’ or mark (m) and an ‘off-time’ or space (s). The duty factor is then:

DVD (comp) Originally meaning digital video diskthen digital versatile disk. A DVD disk is thesame size as a CD but offers enlarged storagedue to its double-sided storage capacity anddouble-layer design. Capacities of 17 G-Bytesare possible, each layer holding 4.7 G-bytes.Smaller indentations and shorter wavelengthlasers are used. Using MPEG2 compression,

DF ��

�m

m s100%.

Duty cyclePRP

PD PRF/ ms�

��

�PD1000 1000

1000 .

extensive audio and video information can bestored. DVD-ROM is a read-only device similarto the CD-ROM, but DVD-RAM is a read/writeversion, which is completely reusable. Regionalcoding (currently six regions are recognized) isintroduced to protect DVD data content (e.g. filmvideos, regional variations of software), so thatAmerican disks will not play in Europe or Japan,etc. The various storage capacities for DVD are:

● 2.6 GB single sided;● 3 GB single sided;● 4.7 GB single side/single layer;● 5.2 GB doubled sided for DVD-RAM;● 6 GB double sided for DVD RW;● 8.5 GB single sided/double layer;● 9.4 GB double sided/single layered;● 17 GB double sided/double layered.

The latest disks have two layers and DVDdrives use a laser with two focal points to readeach one. Each layer holds 4.7 GB, so a dual-layer, dual-sided disk will store 17 GB of data(see Blu-ray® disk).

dwell time (us) The amount of time that the trans-ducer is actively transmitting ultrasound power.

DWI (mri) See Diffusion-weighted imaging.dynamic aperture (us) The beam width at the

focus point is limited by the size of the trans-ducer or groups of transducers (elements), theirfocal length and wavelength. The beam widthis controlled by the aperture increasing theaperture (number of apertures) with increasingfocal length, so maintaining constant focalwidth during transmission.

dynamic focusing (us) When a transducer arrayreceives echoes, the receiving focus depth may

Dynamic aperture


dynamic focusing–dysprosium (Dy)

D

be continuously increased by altering receiverdelay as the transmitted pulse travels forward.The continuously changing echo receiver win-dow used for dynamic focusing increasesimage resolution with depth.

dynamic imaging (us) Rapid-frame-sequenceimaging; real-time imaging.

dynamic range (xray) The dynamic range carriedby the beam when considering subject contrastis the ratio of intensity with no attenuation tothe maximum tissue attenuation. Overall atten-uation obeys the general equation Iout � Iin e�μx.The dynamic range to register all the attenua-tion information is Iin/Iout, where Iin is unity.Typical film dynamic range (latitude) rangesfrom 1:10 to 1:100. Image plate and directradiography detectors have dynamic rangesapproaching 1:10 000.

Dynamic focusing

Focus 1

Focus 2

Focus 3

(us) The range of echo intensities. This can be upto 100 dB at the input amplifier, 60–80 dB at thetime gain compensation (TGC) and 50 dB aftercompression. Dynamic range. Ratio (in decibels)of the largest power to the smallest power that asystem can handle; ratio of the largest to thesmallest intensity of a group of echoes. (mri)Range of signal intensities distinguished in animage or spectrum. If the signal dynamic rangeis too great, the need to keep the highest inten-sities from overloading the digitizer may result inthe weaker features being lost in the digitizationnoise. This can be dealt with by using an ana-logue to digital converter with a larger range ofsensitivity or by using techniques to reduce thedynamic range, e.g. suppressing the signal fromwater in order to detect the signal from lessabundant compounds.

dynamic scanning (ct) A method of obtaining CTscans in rapid sequence so as, for example, tofollow the passage of contrast material throughvessels or tissue, or to decrease examinationtime.

dynamic study (xray, nmed) A study where acertain number of time frames are collected(see renogram).

dyne (dyn) (phys) a centimetre – gram – second(cgs) unit of force. 1 dyn �10�5N.

dynode (elec) An electrode in a vacuum tubedevice whose function is to excite secondaryelectron emission (see photomultiplier).

dysprosium (Dy) (elem)

Atomic number (Z) 66Relative atomic mass (Ar) 162.50Density (ρ) kg/m3 8500Melting point (K) 1680K-edge (keV) 53.7Relevance to radiology: Doping agent for glass andphosphors.

Dynamic rangeMammography, where Eeff is 20 keV, x �5 cm andμ �0.76. The dynamic range is:

Chest x-ray, where Eeff is 100 keV, x �20 cm andμ �0.17. The dynamic range is:

10 17 30 1 164

e� � �( . ) .:

10 76 5 1 45

e� � �( . ) :

(ct) For an acceptable wide dynamic range, theADC must be capable of responding to a largevariation in tissue attenuation values; the ADCwhich must be capable of registering 106 steps.The dynamic range represents the largest sig-nal (no attenuation) to the smallest signal (max-imum attenuation) that can be detected.

Dynamic range, CTFor a 20-bit ADC, the dynamic range is 1:106

(220�1 048 576) the uncertainty in voltage measure-ment is or 1000 representing 0.1%. The atten-uation coefficient for soft tissue (μ) at 150 kev is 0.155and water is 0.1504. A variation of 0.1% in μ is0.000155:

(0.155155 � 0.1504)/0.1504 � 32

(0.154845 � 0.1504)/0.1504 � 30

A variation exceeding 0.1% will significantly affect CTnumber calculation.

106

E


e–eddy currents

Ee (math) Symbol for the transcendental number

2.718282 used as the base for the natural log-arithm (see exponential).

e-mail (comp) Electronic mail. An important facil-ity offered by the Internet giving the ability tosend/receive messages, pictures and computerdata (files). The email address has the [email protected] or [email protected],where country xx has a two letter identifier (seedomain).

ECD (nmed) 99mTc-labelled ethyl-cycteinatediethylester or 99mTc-bicicisate. Has similaruptake kinetics to HMPAO, but remains stablein vitro for up to 6 hours after reconstitution.

ECG-gated image reconstruction (ct) A methodfor cardiac imaging freezing cardiac motion byretrospectively reconstructing CT images forselected heart phases. Correlating each meas-ured projection with the recorded electrocar-diogram (ECG) signal.

ECG triggering (ct) Triggering computed tomog-raphy (CT) scan data collection using the ECGsignal.

echo (mri) The point where spins (transverses mag-netization) come back in phase and produce anMR signal. An echo can be produced by a gradientreversal (gradient echo) or by the use of a 180°refocusing pulse. (us) The returning ultrasoundsignal reflected from a surface within the body ofthe patient. The time of arrival of these echoes isused to calculate the depth of tissue interface.

echo amplitude (us) This is determined by thestructure and composition (acoustic impedance)of the reflecting surface used to determine thebrightness (grey-scale) of the ultrasound display.

Echogen® (cm) Ultrasound contrast agent usingperfluorocarbons instead of air, produced bySonus Pharmaceuticals (see Echovist®, Levovist®).

echo offset (mri) Adjustment of radio frequency(RF) spin echo and gradient echo to be non-coincident in time, so as to create phase differ-ences between the signals from different spectra(e.g. fat and water). The magnitude of the result-ing phase difference will be equal to the productof the difference in frequency of the spectrallines and the difference in the echo times (TE).

echo planar imaging (EPI) (mri) Extremely rapidimaging sequence where the complete imageis obtained using a single selective excitationpulse. Field gradients are switched periodically to

generate a series of gradient echoes. The FID isdetected while switching the y-gradient mag-net with a constant x-gradient. The Fourier trans-

form of the spin-echo sequence then supplies theimage of the selected plane. Several artefactsare generated, the resolution is limited and it issensitive to magnet in-homogeneities (seeTurboFLASH, ultrafast gradient echo).

echo sharing (mri) Used in double contrast

sequences. Echoes that determine the imageresolution are used in both raw data matrices.

echo spacing (mri) The distance between twoechoes (e.g. turbo-spin echo (turbo-SE) or EPI

sequences). A short echo space produces shortsequence timing and fewer image artefacts.

echo time (us) The arrival time of echoes is usedfor locating the depth of objects. One ultra-sound pulse yields one scan line along whichreturns echoes from objects at various depths.Axial resolution determines the ability to sepa-rate close echoes.

Echo timeIf the speed of sound in soft tissue is �7 μs cm�1

(1500 m s�1) then echo-times from objects 12 and20 cm from the transducer (allowing double distancesfor transmission and reception) are:

2 � 12 � 7 � 168 μs and 2 � 20 � 7 � 280 μs.

(mri) (TE) Time between the excitation pulse of asequence and the resulting echo used as theMR signal. Determines image contrast. The timebetween the 90° pulse and the echo peak is theecho time TE. Additional 180° pulses can createadditional echoes. Gradient field reversal can takethe place of the 180° pulse to yield gradientechoes.

echo train (mri) Applied to multi-echo sequences,where two or more echoes are acquired, eachof which obtains a different phase-encodingdirection.

Echovist® (clin) Saccharide-based ultrasound con-trast agent containing microbubbles of air, man-ufactured by Schering AG and introduced in1991. Due to limited in vivo stability after injec-tion, the microbubbles dissolve during lung tran-sit so it is particularly useful for right ventricleimaging (see Levovist®).

ED50 (stats) Median effective dose.eddies (us) Regions of circular flow patterns pres-

ent in turbulence.eddy currents (mri) Electric currents induced

in a conductor by a changing magnetic field


eddy currents–effective dose (E)

E

(e.g. gradient fields) or by motion of the conduc-tor through a magnetic field. A source of image

artefacts and concern for safety in rapidlyswitched gradients used in superconducting mag-

nets. A source of concern about potential hazardto subjects in very high magnetic fields or rap-idly varying gradient or main magnetic fields. A problem in the cryostat of superconductingmagnets.The influence of eddy currents on gra-dient fields can be reduced by eddy current com-

pensation and shielded gradient coils.eddy current compensation (mri) A means of

reducing the incidence of eddy currents onpulsed gradient fields by employing an electri-cal pre-emphasis in the gradient amplifiers.Multiple time constants are commonly used tocorrect for eddy current effects in various struc-tures of the MR system (cryoshields and RF-shields).

edentate (calcium/disodium) (cm) A chelating

agent used as a stabilizer in some contrast mediapreparations.

edge (image) A set of pixels with values signifi-cantly different from pixels on the opposite sideof the edge resulting in different grey levels(see penumbra).

EDGE (comm) (Enhanced data rate for global systemfor mobile (GSM) communications evolution).EDGE refers to five technologies in developmentthat are aimed at GSM networks. There are dis-agreements about whether Edge should bedescribed as a 2.SG or a 3G technology. Edgepromises a maximum theoretical rate of 384 Kbps(kilobits per second).

edge enhancement (di) Applying a differentiat-ing filter to an image which will exaggerateany sharp changes in count density difference.

edge-packing (nmed) High-count density arte-fact that appears at the periphery of all Anger-type gamma camera images.

edge oscillation (mri) Truncation artefact (see Gibbs

artefact).edge response function (ERF) (ct) See edge spread

function.edge spread function (ESF) (ct) The image of a

high-contrast edge positioned orthogonal to theimaged plane. Objects made from polymethyl-methacrylate (PMMA) or aluminium within awater bath can be used in order to measure theESF of a CT system if the object provides at leastone even boundary plane oriented orthogonalto the scan plane.

Edison, Thomas A (1847–1931) American inven-tor. After testing 8000 substances, discoveredcalcium tungstate as a fluorescent material forx-ray inten-sifying screens in 1896.

EDMP (nmed) See samarium.EDO (comp) Extended data out RAM. The current

memory used for computers offering improvedperformance. To read a word of computer mem-ory it must be pre-charged first. EDO memoryspeeds up this sequence by pre-charging thenest word while still reading the current word.

EDTA (nmed, mri) Ethylenediamine-tetra-aceticacid having a formula:

(CH2COOH)2 � N � (CH2)2 � N � (CH2COOH)2.

A chelating agent widely used for sequestering di- and trivalent metal ions (M), such as 99mTc,111In or gadolinium.

EFAST (mri) Blood flow artefact suppression andmotion artefact reduction technique (Toshiba)motion artefact reduction techniques. Spatialpre-saturation to reduce MR signal intensity in specific locations (see SAT, REST, PRE-SAT,PRESAT, SATURATION).

effectance (dose) (ICRP60) See effective dose.effective detector width (ct) Width of a single

detector element within the scan plane pro-jected on to the isocenter according to thescanner geometry (see detector element).

effective dose (E) (dose) Also termed effective doseequivalent (EDE). It is the product of the equivalent

dose and a tissue weighting factor (ICRP60). It istherefore a doubly weighted absorbed dose for allthe irradiated tissues:

where DT is the absorbed dose; wR and wT are theweighting factors for radiation and tissue, respec-tively; HT the equivalent dose. The effective dose isa whole body dose equivalent allowing compar-isons to be made between different radiologicalprocedures. This calculation indicates that the riskof fatal cancer associated with this pattern of irra-diation corresponds to the risk of fatal malignancyand serious hereditary harm from a dose equiva-lent of 0.825 mSv received uniformly throughoutthe body. In comparison, a high voltage chestradiograph delivers an equivalent dose of about0.2 mSv to the lung and rib cage giving an effec-tive dose of 0.05 mSv (see weighting factor (tissue)).

E D w w

E w H

T R T

T T

� � �

� �

( ) or∑∑ .

E


effective dose (E)–effective slice thickness

roughly doubles x-ray beam intensity alongwith an increase in effective energy. The effec-tive energy of a moderately filtered x-ray spec-trum is approximately , so 100 kVp wouldtranslate as 70 kVeff.

effective focal spot (xray) The projected dimen-sions from the angled anode target as calculatedfrom the line-focus principle. The effective focal spotfor general radiography is 0.6–1.2 mm and formammography 0.4 mm. The dimensions of theeffective focal spot differs over the image plane.

effective half-life (nmed) The time required forthe amount of a radionuclide deposited in a liv-ing organism to be diminished 50% as a resultof the combined action of radioactive decay andbiological elimination.

effective repetition time (TReff) (mri) Repetitiontime (TR) is not fixed during cardiac triggering soit is determined by the time interval for the trig-ger. The effective repetition time TReff estab-lished by the trigger interval fluctuates with thecardiac rhythm.

effective slice thickness (ct) The total effect ofthe collimation, pitch and interpolation asdescribed by the slice sensitivity profile (SSP). TheFWHM of this profile is commonly used as ameasure for the effective slice thickness. Withthe 180° interpolation, the FWHM of a pitch 1acquisition is similar to that of a conventionalscan with the same collimation. For largerpitches, the FWHM increases linearly. While theSSP of the conventional scan has very steepedges, the SSP of the spiral multi-slice scan ismore bell shaped, so the area of anatomy thatcontributes to the image is wider.

10 20 30 40

A

B

Eeff

50 60 70X-ray photon energy (keV)

Pho

ton

abun

danc

e

80 90 100

Eeff

23 kVp

Effective doseA routine nuclear medicine lung scan using 99mTc-MAA returns the following equivalent organ doses:

● lung tissue: 5 mSv● liver: 1.5 mSv● ribs: 1 mSv● ovaries: 0.15 mSv.

The weighting factors (ICRP60) for these tissues are0.12, 0.05, 0.12 and 0.20, respectively, so the effec-tive dose (E) is:

(5 � 0.12) � (1.5 � 0.05) � (1 � 0.12) � (0.15 � 0.2)

E � 0.825 mSv.

This serves as an estimate of the whole body dosefrom a 99mTc-MAA perfusion lung scan.

effective dose (CT) (ct) This is a useful indicatorof patient radiation risk, although it is also notparticularly suitable as a reference dose quan-tity since it cannot be measured directly and itsdefinition may be subject to further changes.Broad estimates of the effective dose (He) maybe derived from the DLP as:

He � EDLP � DLP

where EDLP is the region or tissue specific nor-malized effective dose in mSv mGy�1cm�1.Multiplying factors for obtaining the effectivedose He from the DLP values.

Organ DLP EDLP He (mSv)

Head 1050 2.3 � 10�3 2.4Chest 650 1.7 � 10�3 1.1Abdomen 800 1.5 � 10�3 1.2Pelvis 600 1.9 � 10�3 1.1

■ Reference: Hidajat et al., 1999.

effective dose equivalent (EDE) (nmed) Seeeffective dose.

effective echo time (TEeff) (mri) The periodbetween the excitation pulse and the echo.Both the contrast and signal-to-noise ratio of theimage are determined mostly by the timing ofthe echo at which the phase-encoding gradient

has the smallest amplitude. The echo signal inthis case undergoes minimal dephasing andhas the strongest signal.

effective energy (Eeff) (xray) The modal point onthe continuous x-ray spectrum, identified on thecurves as Eeff. The beam’s effective energy canbe influenced by changing kVp and filtration.The intensity Q of the beam is seen to increaseas Q kV2; a 30% increase in kilovoltage


effective slice thickness–electrical energy

E

efficiency (conversion) (xray) The efficiency ofsignal (e.g. light) production in a detector.

efficiency (geometrical) (phys) The fraction ofthe isotropic emission collected by the detectorsurface.

efficiency (quantum) (phys) A measure of pho-ton absorption.

EFG (mri) Electric field gradient.EFOMP (phys) European Federation of Medical

Physics.EFR (mri) See electron spin resonance.EHIDA (nmed) A radiopharmaceutical from the

family of HIDA complexes (N-[N -(2,6 dimethyl-phenyl) carbamoyl-methyl] iminodiacetic acid).The diethyl variant is EHIDA used for investi-gating hepatic function and for scintigraphicimaging of the hepatobiliary system.

EIDE (enhanced IDE) (comp) An improved versionof IDE that supports large hard disks, fasteraccess speeds and DMA. It is two controllers pro-viding primary and secondary channels whichhandle two devices each and can also talk to CD-ROMs and tape drives.

eigen value (image) Literally ‘characteristic value’.A set of values associated with a square matrixwhose columns are called eigen vectors. It iscommon to use the symbol λ for eigen values.

eigenvector (math) Characteristic vector (Germaneigen, characteristic). A vector x correspondingto a given eigenvalue of a square matrix (seetensor, diffusion image).■ Reference: Nelson, 2003.

Einstein, Albert (1879–1955) Albert Einsteinwas born in Ulm, Germany and died in Princeton,USA. In 1905, he published papers on lightquanta, Brownian motion and special theory of

1.0

FWHM0.5

0.0

�6 �4 �2 0Slice dimension (mm)

Sig

nal s

tren

gth

2 4 6

Sensitivity(SSP)

Dose(SDP)

relativity (E � mc2). He published papers on gen-eral relativity in 1913 and 1916. He wasawarded the 1921 Nobel prize for physics for hiswork on the photo-electric effect (see Cockroft-

Walton, pair production, positron).EISA (comp) Extended industrial standard archi-

tecture. A non-IBM design of PC more advancedthan AT and XT machines. It is a competitor tomicrochannel and ISA.

elastic collision (phys) In which the kineticenergy of the colliding bodies is the same aftercollision as before. No energy change.

elastic scattering (phys) An interaction betweenan incident photon and the atomic field. Thereis no loss of energy.

elasticity (phys) The property of a body whichtends to resume its original size and shapeafter being deformed. The modulus of elasticityis the ratio of stress to strain. Young’s modulusis defined as:

Other definitions apply to bulk modulus andshear modulus.

ELD (mri) Energy level diagram.electric charge (units) Electric charge (Q) coulomb

(C), where 1 C � 6.24 � 1018 electrons; thecharge/electron 1.6 � 10�19C (see charge).

electric field intensity (E) (phys) The funda-mental quantity for describing electric field, it isdefined in terms of the force exerted by thefield on a stationary charge. The unit is newtonper coulomb N C�1. An important factor playinga part in electron beam focusing in x-ray tubesand gas multiplication in Geiger tubes (see elec-

tric field strength).electric potential or electromotive force (emf)

(phys) Defined in terms of the electric potentialenergy difference or work done in moving aunit charge between two points. The NMR sig-nal is measured as an emf across the terminalsof the receiver coil. Unit, the volt (V) (see electrical

energy).electrical conductivity (phys) The reciprocal of

resistivity. It is also defined as current densitydivided by electrical field strength. Measured insiemens (S) m�1.

electrical energy (phys) A form of potential energy,obtained when a quantity of electricity (elec-trons) moves between two points (positive and

applied load per unit areaincrease in length

.

E


electrical energy–electrical resistance (Ω)

When comparing electrical energy, W, producedby a direct current with an alternating currentflowing through the same load requires a meas-ure of the root mean square for the alternatingcurrent.

electrical power (DC) (phys) Electrical power Prelationships are derived as:

where 1 J s�11 � 1 watt. Power loss is calcu-lated by restating Ohm’s law as V � IR. ThenP � I � (IR) or P � I2R. Similarly, since I � V/Rthen P � V � (V/R) or P � V2/R, which givesthree standard formulas relating power withvoltage, current and resistance. Since all con-ductors have some electrical resistance, poweris always lost during transmission.

Energy deliveredTime taken

sot

J s 1PIVt

IV� � �

negative) which have a potential difference. A potential difference of 1 volt exists when 1joule is generated when 1 coulomb movesbetween two terminals so 1 volt � 1 JC�1.Energy W � QV joules. If 5 coulombs movesbetween points having a potential difference of2 volts then 10 J is generated.

Energy for x-ray unit 50 kV at 50 mAsSince Q � I � t then W � IVt joules, 50 000 volts(50 kV) at 50 mAs (50 mA for 1 s), energy consumed:

0.1 � 50 kV � 0.5 J � 2500 J.

electrical field strength (E) (units) The intensityof an electric field at a given point exerted bythe field at that point. Measured in volts m�1

(V m�1). The electrical field strength increasesat sharp edges or projections; a factor used inthe x-ray tube cathode cup.

electrical heating (phys) The heating effect ofelectric current depends on:

● the electric current being passed, I;● the resistance of the circuit, R;● the time spent, t.

The following relationships can be established:

1 For a constant current, I, and fixed resistance,the heat produced is proportional to time spent.

2 For a constant resistance and fixed time, theheat produced is proportional to I2. Joules’ lawsof electrical heating summarize these findings.The heat developed in a wire is proportional to:● the time spent;● the current squared, I2;● resistance of the conductor (wire), P � I2Rt J.

electrical interaction (phys) A repulsive forceacting between electric charges of like sign andforce of attraction between dissimilar electriccharges.

electrical power (AC) (phys) The measurement inalternating current circuits is only valid whencurrent and voltage waveforms are in phase; thecase with purely resistive circuits (heaters andlighting). Electrical loads that include inductors(motors, transformers) the voltage and currentwaveforms are not in phase. The degree ofphase-shift is expressed by the power factor(cos ϕ) or phase angle. The relationship betweentrue power P measured in kW and apparentpower S measured in kVA is: P � S � cos ϕ.

Electrical energyEnergy gainEnergy W (1 volt � 1 JC�1)Since Q � I � t, then W � IVt JFor 50 000 volts (50 kV) at 50 mAs (50 mA for 1 s),energy consumed 0.1 � 50 kV � 0.5 J � 2500 J.

Electric powerA 100 W electric light bulb at 220 volts consumes:w/v � 0.45 amp and 110 volts consumes 0.9 amp.Commercial electric power is measured in kilowatts (kW).Electrical energy consumed is expressed as kW � timewhich is the kW � hour. So 4 � 100 W lamps burningfor 8 hours consume: 100 � 4 � 8 � 3.2 kW hour inde-pendent of supply voltage (1 kWh � 3.6 � 106J).

Electrical power 1 J � 1 W s

watt (W) W � EQ � I2R t � V2t/R � V I t

kilowatt (kW) 1 W � 1 J s�1

1 kWh � 3.6 � 106 J.

(see electrical units).electrical resistance (Ω) (phys) One ohm (Ω)

maintains a current of one amp at one volt sothat: R � V/I. Variations of this basic formulaare: I � V/R and V � IR. If a is the cross-sectional area of a conductor, then providingthe length L is constant, its resistance is pro-portional to cross-sectional area. So that,

If the diameter of the wire is doubled thenresistance decreases by a quarter.

RL

a 2 .


electrical resistance (Ω)–electromagnetic spectrum

E

Electrical resistance (ohm, Ω) I � V/RR � V/IV � IRP � V2/R

electrical units (phys) The basic SI units andtheir relationships are:

Measure Parameter Relationships

Charge (Q) Coulomb (C) 1 C � 6.24 � 1018 eElectron 1.6 � 10�19C

chargePotential Volt (V) 1 V � 1 J C�1

difference (PD) 1 V � 1 W A�1

Energy (E) Joule (J) J � QVE � W/Q

Electron 1 J � 6.24 � 1018 eVvolt (eV) 1 eV � 1.6 � 10�19 J

Current (I) Ampere (A) Q � I � t1 A � 1 C s�1 (6.24 �

1018e s�1)P � IVP � I2R

electrolyte (phys) A substance that conductselectricity in solution or in the molten state dueto the presence of ions. Sodium chloride disso-ciates into ions when dissolved in water or in amolten state:

NaCl→ Na� � Cl�.

electromagnetic field (phys) The influence thatelectromagnetic radiation exerts on anotherobject in close proximity or at a distance.

electromagnetic induction (phys) When a mag-net is moved within a coil a current is inducedin the coil. The coil can be moved producing thesame effect. This is the principle behind thedynamo and alternator, producing electricenergy from mechanical energy. Conversely ininduction by current, if two coils are placedclosely together and one energized by switch-ing a battery in circuit causing a growth of amagnetic field then this induces a current inthe second coil. The current in one must bechanging to induce a current in the second coil.This is the principle behind the transformer.

electromagnetic radiation (phys) Radiationpropagated through space or material in whichelectric and magnetic fields are vectors whichare 90° opposed. Unaffected by external elec-trical or magnetic fields and absorption followsan exponential law. Speed in vacuum c is a con-

stant: 2.99792 5 � 108m s�1 or approximately3 � 108m s�1. Each frequency band has

unique properties, but the wave character isidentical having an electrical and magneticcomponent. Wavelength can range from sev-eral kilometres to 10�14m. The energy contentof an electromagnetic wave is a multiple of thebasic quantum: the photon, where:

E � hf

f is frequency in Herz (Hz � cycles s�1) and h isPlanck’s constant. Wave energy increases withfrequency. The frequencies f of some electro-magnetic radiations are:

Red light 3.7 � 1014 HzBlue light 7.5 � 1014 HzUltraviolet 4 � 1015 HzX-rays 5 � 1017 HzGamma radiation 5 � 1019 Hz

In comparison, radio and television waves (FM and UHF) are much lower down the scaleand have frequencies between 80 MHz and 1GHz(80 � 106 to 1 � 109Hz).

Measure Equation Values

Velocity, c c � f λ c � 3.0 � 108 m s�1

Wavelength, λ λ � c/f λ in m or nmFrequency, f f � c /λ f in HzEnergy, E E � hc/λ hc � 1.24�10�6eV

electromagnetic spectrum (phys) A spectrum ofelectromagnetic radiation represented either asincreasing frequency or decreasing wave-length. Electromagnetic radiation exhibits thefollowing approximate values of wavelengthand frequency:

Wavelength (m) Frequency (Hz)

Radio 105 to 10�3m 5 � 105 to 1011

Infrared 10�3 to 10�6m 1012 to 5 � 1015

Visible 4 to 7 � 10�7m 5 � 1015 to 1016

UV 10�7 to 10�9m 1016 to 1018

X-rays 10�9 to 10�11m 1018 to 5 � 1019

Gamma 10�11 to 10�14m �1019

Radiation energy as electron voltUltra violet frequency � 4 � 1015 Hz

Red and Blue light have wavelengths of 700 and400 nm, with energies of 1.77 and 3.1 eV, respectively.X-rays have a continuous spectrum from 60 to 120 keV,their wavelength ranges from 0.02 to 0.0099 nm and afrequency range from 1.5 to 3.0 � 1019Hz.

λ ��

� �

�

3 0 104

10 75

16

815.

nm

eV.E

E


electromotive force (E)–electron density

electromotive force (E) (phys) Electromotiveforce of an electrical source (battery, dynamo orgenerator), measured in volts. A device with ane.m.f. E passing a charge, Q, through a circuitliberates electrical energy, QE. Basic electricaldefinitions derive from this as charge, Q, is a steady current, I, for time, t. Then energyW � QE � IEt. Total power P � W/t � EI (seepower).

electron (phys) Negative charged elementary par-ticle, e�. The fundamental charge on the elec-tron is 1.6 � 10�19C. and electron rest mass is9.1 � 10�31kg. Since E � mc2, the rest mass isequivalent to 8.19 � 10�14 J or 0.511 MeV (seeelectron volt).

electron (bound) (phys) For photon–electroninteractions (photoelectric effect, Compton scatter), abound electron is one having a binding energyhigher than the incident photon. In higher Zmaterials (calcium, barium, iodine) the K and Lshells are treated as ‘bound’ and the orbitalsabove M are ‘free’ with respect to the x-ray pho-ton energy (see electron (free)).

electron (free) (phys) Those obital electronswhose binding energies are low compared tothe photon energy. In low Z materials (e.g. softtissue), any electron outside the K-shell can betreated as ‘free’ in C, N and O for the diagnosticx-ray range (see electron (bound)).

electron (rest mass) (phys) This is also a funda-mental constant being 9.1 � 10�31kg.

electron affinity (phys) Either the energy requiredto remove an electron from a negative ion toform a neutral atom or the ability of oxidizingagents to capture electrons.

electron beam (phys) The stream of electronsgenerated by thermionic emission from a hotmetal or metal oxide. The beam produced isusually narrow and consists of high-velocityelectrons whose intensity is controlled by thefilament temperature.

electron beam CT (EBT, EBCT) (ct) Alternativedesign for a CT scanner which functions with-out any moving mechanical parts. The electronbeam is deflected and steered at high speedaround the patient. The beam strikes an anodering and produces a fan of x-rays.

electron capture (phys) A decay process seen in‘neutron-poor’ nuclides (cyclotron produced)where a proton is converted into a neutron bycapturing an electron from the K-shell and liberating a neutrino.

This has a similar end result as positron emission.with the emission of Hg

x-rays (68–82 keV).electron cascade (phys) Filling vacancies in elec-

tron orbits (normally K- or L-shell vacancies) byelectrons in higher (less energetic) orbits.Accompanied by characteristic radiation andAuger electrons.

electron charge (e) (units) This is the charge onthe electron 1.602 � 10�19C so one coulombrepresents 6.24 � 1018 electrons.

81201

80201Tl (e.c) Hgα

11

0 01

1p e n� � → .

The energy gained by electron beam in ax-ray tubeIf 3 � 1017 electrons representing 50 mA (1.6 � 10�19Ceach electron) move along an x-ray tube at 120 keV,then energy gained is

E � QV Joules� (3 � 1017) � (1.6 � 10�19) � (120 � 103)� 5760 joules

electron density (phys) The number of electronsper gram shows a difference between densematerials (bone, lead, etc.) and less dense mate-rials (water, soft tissues, etc.). Hydrogen has thehighest electron density, so substances contain-ing more hydrogen (water) cause greater scatter.The number of electrons per gram is: (N � Z)/A,where Z is the atomic number, N is Avogadro’s num-

ber and A the atomic mass. The probability of aCompton event can be estimated from the numberof electrons per cm3 which is:

Density (g cm3) � electrons g�1.

Material Density Electrons Electrons (kg m�3) (g�1 � 1023) (cm�3 � 1023)

Hydrogen 0.09 6 5.4 � 104

Air 1.225 3.01 3.8 � 1020

Water 1000 3.34 3.34Muscle 1000 3.36 3.36Fat 910 3.48 3.16Bone 1650–1850 3.00 5.55

The charge of an electron beamIf Q coulombs flows during t seconds then:

Q � I � t coulombs.

X-ray exposure for 100 mA at t � 0.5 s (50 mAs), thenI � 0.1 coulombs s�1 and Q � 0.05 coulombs or 50milliCoulombs (mC). So x-ray exposure in mAs isequivalent to mC.


electron emission–element (ultrasound)

E

electron emission (phys) The liberation of elec-trons from a surface. They can be liberated by (1) thermionic emission resulting from the hightemperature of the surface; (2) photoelectric emis-

sion resulting from photon bombardment; (3) field

emission resulting from intense electric fields; and(4) electron emission due to nuclear decay.

electron paramagnetic resonance (EPRI) (mri)See Electron spin resonance.

electron spin resonance (ESR) (mri) magneticresonance phenomenon involving unpairedelectrons, e.g. in free radicals. The frequen-cies are much higher than corresponding NMR frequencies in the same static magneticfield.

electron traps (phys) Locations within the forbid-

den band introduced by impurities that eitherhold electrons falling from the conduction zone

or provide further electron events after a partic-ular event has finished (see energy bands).

electron volt (eV) (units) A measure of radiationenergy, since the joule is too large a quantitybeing 6.24 � 1018eV or conversely 1 eV isequal to 1.6 � 10�19 J. Wavelength and energycan be converted since c � λf. This functioncan then be substituted for f to give:

Then hc � (6.62 � 10�34) � (3.0 � 108) � 1.98 �

10�25 Jm. Expressed in electron volts (eV) pro-viding the wavelength is given in metres (m);alternatively, the answer can be given in kilo-electron volts (keV), if the wavelength is innanometres:

Equivalent values of the electron volt are:

1 eV � 1.6 � 10�19 J

h � 4.13 � 10�15eV.

E

E

��

�

�1 2375 10

1 2375

6.

.

λ

λ

eV or

(nm) keV.

Ehc

�λ

electronic focal spot (ct) Targeted area of inter-section of the electron beam from the cathodeand the surface of the x-ray tube anode (seefocal spot).

electronic noise (ct) Contribution of the elec-tronic devices (thermal) of the data acquisitionsystem to the sampling noise or error of theattenuation CT measurements.

electrophoresis (nmed) Utilizes the property ofions to move under the influence of an electriccharge. Components of a sample will move atdifferent rates along a strip of paper or gelmedium according to their charge and ionicmobility. This procedure is used in a number ofapplications and the electrophoretograms aremeasured in the same manner as thin layerand paper chromatograms.

electrostatics (phys) A branch of physics thatdeals with static or very high voltages (rele-vant to x-ray generators and x-ray tubes). Thestatic electric charge arises when electrons aretransferred from one object to another. Anobject with excess electrons has a negativecharge and conversely an object having lostelectrons has a positive charge. The basicmeasurement of charge is the coulomb (C),where one coulomb represents 6.24 � 1018

electrons.element (ultrasound) (us) The piezoelectric com-

ponent of a transducer assembly.

Units of work and energySI joule (J) 1 J � 1 W s � 1 N m � 1 m2kg s�2

eV 1.60218 � 10�19 JkeV 1.60218 �10�16 JMeV 1.60218 �10�13 J

The energy equivalence of an electron voltThe electron velocity v at 1 volt is 6 � 105 m s�1

The rest mass, m, is 9.1 � 10�31kg so:

This is the energy equivalent of 1 eV.

12

0 5 9 1 10

6 10 1 6 10

2 31

5 2 19

mv � � �

� � �

�

�

. .

.

( )

( ) or J.

Electron velocity at 100 kV is:

Velocity v is 1.88 � 108m s�1 at 100 kV which is about two-thirds the speed of light. Relativisticeffects (mass–velocity) play a small part in electronvelocity.

1 6 10 1 10 1 6 10

1 6 10

19 5 14 12

2

14

. .

. ..� � � � �

� �

� �

�

( ) J so mv

E


elliptic filter–emulsion

elliptic filter (di) Alternative type of Chebyshev fil-

ter with equal ripples in the stopband and pass-band. Digital elliptical filters offer the sharpestpossible passband/stopband transitions.

elongation velocity (us) Velocity of a particleabout its equilibrium position. Typical value is35 mm s�1.

eluate (nmed) The solvent carrying the decay prod-uct when a generator is eluted. Removing thedaughter radionuclide from a generator using asuitable solvent. Partial elution will improve thespecific activity per unit volume.

emergency reference level (rad) Radiationdoses to the general public from ionizing radia-tion likely to be averted by introducing counter-measures after accidental exposure.

e.m.f. (phys) See electromotive force.emission (field) (phys) Emission resulting from

intense electric fields at the surface of a mat-erial. If the metal has a high negative potentialwith respect to an external electrode, electronscan escape, particularly from sharp points.Application to radiology is seen in x-ray tubesand charging equipment for selenium imageplates (see direct radiography).

emission (photoelectric) (phys) Emission resultingfrom the irradiation of a material by electromag-netic radiation: light, x- and gamma radiation. Forcertain solids (semiconductors), electrons are lib-erated when the photon energy exceeds a cer-tain photoelectric threshold. Selenium, silicon andgermanium exhibit this effect and are used as pho-

todetectors. Higher energy photons (x-rays,gamma rays) cause a photoelectric effect in allmaterials.

0

100

80

60

40

20

02 4 6

Eluate volume (cm3)

Per

cent

age

tota

l act

ivity

8 10 12

emission (secondary) (phys, xray) Resulting frombombardment of material by electrons responsi-ble for secondary electron emission in x-raytubes and the broadening of the focal spot.

emission (thermionic) (phys, xray) As a sub-stance is heated, the thermal energy gainedincreases the excitation of the atoms. At a cer-tain temperature, the outermost electrons gainsufficient energy to escape from the atom. Theamount of heat energy required is the workfunction. The electrons emitted are inverselyrelated to the work function:

Tungsten 4.53Sodium 1.8Oxide coating 1.0–1.3

The filaments of x-ray tubes are therefore oxidecoated (see space charge).

emission temperature (phys) The available elec-tron density from a heated filament (emissioncurrent density) depends on the filament tem-perature and work function of the filamentmaterial. The graph shows that small changesin filament temperature induce large changesin emission current.

emulsion (film) A complex of silver halides. Silverchloride, bromide and iodide are used in variousmixtures to alter the sensitivity of the film. Theyhave cuboid crystal structure. Photosensitivityincreases with added silver iodide, but rarelyexceeds 8%. Emulsion formation involves:

● silver nitrate reaction with alkali halides ingelatine;

● emulsion ripening, influencing grain size;

2000

10.000

1.000

0.100

0.010

Em

issi

on c

urre

nt d

ensi

ty

0.001

3000Filament temperature (K)


emulsion–energy bands

E

● sensitive centre formation;● optical sensitizers added, extending spectral

sensitivity.

emulsion ripening (film) After silver halide grainformation, the suspension in gelatine under-goes a series of ripening processes whichintroduces a small proportion of silver sulphide;these act as sensitivity centres influencing thespeed of the film.

encryption (comp) The process of transmittingscrambled data so that only authorized recipi-ents can unscramble it. Commonly used toencrypt, and so scramble, credit card informa-tion when purchases are made over theInternet.

Endobil® (cm) A contrast medium for intravenouscholangiography containing meglumine iodoxa-mate (Bracco) (see cholegraphic contrast agents).

Endografin® (cm) Commercial preparation (ScheringInc) of 70% meglumine iodipamide.


70% meglumine 55 at 20°, – 350iodipamide 18 at 37°C 840

Endomirabil® (clin) Generic name iodoxamic acid.Endorem® (clin) Magnetic resonance imaging

(MRI) contrast agent manufactured by Guerbet.endoscopic retrograde choledochography (ERC)

(clin) The nonsurgical removal of biliary calculiusing papillotomy followed by stone retrieval.

endoscopic retrograde cholangiopancreato-graphy (ERCP) (clin) A procedure using anendoscope for evaluating the biliary tree andpancreatic ducts by endoscopic retrograde can-nulation of the papilla of Vater. Water-solublecontrast medium injected via a catheter underfluoroscopic control; opacified ducts are imagedin multiple projections. More invasive than MRItechniques (see magnetic resonance cholangiopan-

creatography (MRCP)).endoscopy (clin) Study of internal organs by

means of fibreoptic instruments.energy (physics) The capacity to do work and in

so doing to create heat. Work and the amount ofheat are physical quantities having the samedimensions and measured in a common unit,the joule (J); the units of energy are also meas-ured in joules. Energy may take the form ofeither potential energy (the capacity to do workdue to position) or kinetic energy (the capacity to

do work due to motion). Joule equivalents andconversions are:

SI units

1 joule (J) 1 Ws1 Nm1 m2kg s�2

1 eV 1.602 � 10�19 J

Non-SI units SI equivalent

1 erg 1 cm2g s�2 10�7 J1 kWh kW � h 3.6 � 106 J1 Btu 1.05 � 103J

energy (effective) (Eeff) (xray) Defined as themodal energy of a polyenergetic beam (see x-ray

(spectrum), energy (equivalent)).energy (equivalent) (Eeq) (xray) See equivalent

energy.energy (fluence) (�) (xray) Measured in

MeV cm�2. For a monoenergetic beam this is:

For an exposure of 60 kV at 5 mAs (about 1.5 �

1014 available photons) over 1000cm2 this wouldbe (1.5 � 1011) � 0.06 � 9 � 109MeVcm�2.

energy (flux) (�) (xray) Measured inMeV cm�2s�1. For a monoenergetic beam, thisis photon flux � enery (MeV). For a polyener-getic x-ray beam, the proportion of each energy(Ei) per unit time must be considered giving the sum:

(see photon fluence, photon flux, energy fluence).energy absorption coefficient (μen) (phys) The

linear attenuation coefficient combines the totalattenuation due to absorption (μen) and scatter(μtr). The energy absorption coefficientdescribes the energy absorbed by a material,omitting that lost by scatter. It is defined as:

where Ea is the average energy absorbed (seeenergy transfer coefficient, mass energy absorption

coefficient).energy bands (phys) Ranges of energies that

electrons can have in a solid (crystal or metal).

μ μen � �Ehv

a

Ψ Ψ� � � �( ) MeV cm sEi2 1∑ .

Ψ ��Number of photons energy (MeV)

Unit area.

E


energy bands–enhancement

The sharply defined levels of electrons becomebands of allowed energy. In solid compoundsor crystals, the energies of the orbitals changeslightly so single energy levels become rangesof energy called ‘energy bands’. Electrons moveacross into empty bands by changing from onequantum state to another. The bands are:

● The ‘valence band’, where the valence elec-trons are found.

● The ‘conduction band’, where there are bothelectrons and spaces for more electrons. Theelectrons are mobile and materials which havea conduction band are the only ones which canconduct electricity at room temperature.

● The ‘forbidden band’ is the range of energiesbetween two energy bands which is not occu-pied by electrons. The electron must cross thisband gap to occupy a higher electron band. Theband gap is large in insulator materials, verynarrow in semiconductors and does not exist atall in conductors.

These bands are relevant to radiology since theyplay a significant role in semiconductor electronicsand scintillator radiation detector operation.

energy conversion (phys) In a given system, thetotal amount of energy is always constantalthough the energy may change from oneform to another (i.e. electrical energy into lightand heat energy). The total amount of energy isobtained by adding the kinetic and potentialenergies. From the equation E � mc2 amass/energy relationship can be calculated sothat 1 kg mass is equivalent to 9 � 1016 J and1 gram is equivalent to 9 � 1013J.

Conduction

Valency

ENERGYGAIN

ENERGYLOSS

Forbiddenzone

Traps

energy level (mri) In a magnetic field, each spincan exist in one of a number of distinct stateshaving different energies. This number isdetermined by the spin quantum number.

energy (photon) (phys) See photon energy.energy resolution (nm) The detector’s ability to

differentiate photon energy. Measured as fullwidth at half maximum (FWHM) of the energyphotopeak. Energy resolution differs dependingon detector type, proportional counter, scintillation

detector or semi-conductor detector. A typicalvalue for a gamma camera would be 10%, for asingle 7.5 � 7.5 cm NaI(Tl) detector crystal 5%and for a HpGe semiconductor detector �1%.

energy transfer coefficient (μtr) (phys) The linear

attenuation coefficient combines the total attenu-ation due to absorption (μen) and scatter (μtr). Theenergy transfer coefficient is a measure of theenergy transferred from primary photons tocharged particles in a scatter interaction. A related quantity kerma (K) is the energy trans-ferred per unit mass of the material. For mono-energetic photons of energy, hν:

where Ψ is the energy fluence (see energy absorp-

tion coefficient, mass energy transfer coefficient).enhanced IDE (EIDE) (comp) An improved ver-

sion of IDE that supports large hard disks, fasteraccess speeds and DMA. It is actually two con-trollers providing primary and secondary chan-nels which handle two devices each and canalso talk to CD-ROMs and tape drives.

enhancement (us) Increase in echo amplitudefrom reflectors that lie behind a weakly attenu-ating structure.

K tr�μρ

⎛

⎝⎜⎜⎜⎜

⎞

⎠⎟⎟⎟⎟Ψ

Energy/mass conversion: nuclear powerstationFuel consumption (m) of a commercial 600 MWnuclear reactor per day (8.64 � 104 seconds).

or just over 0.5 g of fuel per day! Compared withmany thousand kilograms of coal or oil consumed byconventional power stations.

E (joules) ( ) ( )� � � �

� �

600 104 8 64 104

5 184 10

.

. .113

5 184 10

3 105 76 102

13

8 24

J.

( )kgm

E

c� �

�

�� .

.


enriched uranium–EPI factor

E

enriched uranium (phys) 238-Uranium wherethe content of 235-uranium has been increasedabove the natural occurrence of 0.7% byweight.

ensemble length (us) Number of pulses used togenerate one colour-flow image scan line.

enteroclysis (clin) Introduction of dilute bariumsulphate contrast into the small bowel follow-ing passage of a nasojejunal tube (see small

bowel enema).enthalpy (H) (phys) This is a thermodynamic

function of a system involving energy U, pres-sure p and volume V, so that: H � U � pV. Theheat absorbed in a system equals the increasein enthalpy.

entrance beam dimensions (EBD) (us) Thedimensions of the �12 dB beam width wherethe beam enters the patient. For contact trans-ducers, these dimensions can be taken as thedimensions of the radiating element, if sostated. Unit, centimetre (cm).

entrance dimensions of the scan (EDS) (us)For autoscan systems, the dimensions of the area of the surface through which thescanned ultrasound beams enter the patient,consisting of all points located within the�12 dB beam width of any beam passingthrough that surface during the scan. Unit, centimetre, (cm).

entrance exposure rate (EER) (dose) A measureof the fluoroscopy patient dose rate at the sur-face. Routinely performed by using tissueequivalent absorbers.

entrance surface air kerma (ESAK) (dose) Airkerma value measured in free air, withoutback-scatter, at a point in a plane correspon-ding to the entrance surface of a specifiedobject, e.g. patient skin or phantom.

entrance surface dose (dose) Absorbed dose in air including back-scatter, measured at apoint on the entrance surface of the patient orphantom (see air-kerma, dose (entrance), dose area

product).entrance window (ct) The metallic front covering

of a detector on the side of the incident radia-tion. The entrance window protects the detec-tor surface from mechanical or chemical(oxidation) damage, but inevitably causes adecrease in the detector dose efficiency.

entropy (phys) A thermodynamic concept follow-ing from the second law of thermodynamics. Allchanges in a closed system result in an increase

in entropy which can be seen simply as anincrease in disorder.

envelope (us) A smooth curve tangent to andconnecting the peaks of successive cycles of awaveform.

environmental exposure (man-made) (dose)Originating from artificial sources, such as med-ical exposure and nuclear power, but almostentirely of medical exposure to x-rays. A smallerproportion is due to nuclear medicine and ther-apy radionuclides and �1% to discharges fromnuclear power or fall-out from nuclear weapontesting.

Man-made Dose (mSv) %

Medical 500 21.00Nuclear 13 0.55Occupational 9 0.36Air travel 8 0.34Total 530 22.25

environmental exposure (natural) (dose)Caused by all radiation of natural origin: alpha,beta, gamma, x-ray, cosmic ray, etc. The total ofnatural and man-made is typically 2.5 mSv,although this has a wide range. The largestexposure to respiratory tissue comes fromradon, produced by the decay of 238U series.226Radium decays by alpha decay to 222Radon,which then decays to 218Polonium.

Natural Dose (mSv) %

Cosmic 310 13Terrestrial 380 16Radon 800 33Internal 370 15Total 1860 77

Eovist® (clin) A liver-specific MRI agent manufac-tured by Schering.

EPC (mri) Echo phase correction.EPI (mri) Echo planar imaging. A rapid acquisition

of a train of separately phase-encoded gradientechoes to produce a fully resolved image after asingle excitation.

EPI factor (mri) The number of gradient echoes ofan echo planar imaging (EPI) sequence,acquired after a single excitation pulse (typi-cally 64–128). An EPI factor of 128 means ameasurement time 128 times faster than a nor-mal gradient echo sequence.

E


epoch–error correction protocol

The graph shows an equivalent energy of 40keV.This should not be confused with the effective

energy which is the x-ray spectrum modal point.

erbium (Er) (elem)

Atomic number (Z) 68Relative atomic mass (Ar) 167.26Density (�) kg/m3 9000Melting point, (K) 1770K-edge (keV) 57.4Relevance to radiology: x-ray beam K-edge filter

169Erbium (nmed) A radionuclide as citrate forsynovectomy.

Half life 9.3 daysDecay mode β� 0.34 MeVDecay constant 0.0745 day�1

Photons (abundance) 116 keV(weak)

ERCP (clin) Endoscopic retrograde cholangiopancre-

atography. Using an endoscope to investigateand cannulate the ampulla of Vater and radi-ographically visualize the pancreatic, hepaticand common bile ducts using contrast medium(see relevant contrast medium).

erg (phys) A non-SI unit of energy, where 1 ergequals 10�7 joules (see energy).

Ernst angle (mri) The flip angle (�90°) of a gradi-ent echo sequence which generates the maxi-mum signal for a given tissue. The Ernst angle:

αE � e�(TR/T1).

The optimum flip angle is 90° only when TR ismuch greater than T1.

error correction protocol (comp) A techniqueused in modems to cancel out extraneous

1 10 100

100

10

1

0.11000

Photon energy (keV)

Line

ar a

ttenu

atio

n co

effic

ient

(m

cm

�1 )

epoch (mri) In functional magnetic resonanceimaging (fMRI), a portion of the MR signal meas-urement during which the stimulus presenta-tion or response task is similar or unchanged.

EPP/ECP (comp) Enhanced parallel port/extendedcapabilities port. Improved parallel port whichprovides transfer rates of over 2 MB (megabytesper second) and bidirectional operation. Thelatter is mainly used by printer monitoring soft-ware as it can receive status information fromthe printer while sending it data. EPP mode isdesigned for other devices, such as zip drives.

equations of state (phys) Characteristic equa-tions showing the relationship between pres-sure, volume and thermodynamic temperaturein a system. Many equations of state have beenproposed (van der Waals equation, Clausiusequation, Debye equation).

equivalent dose (HT) (dose) Also the equivalentorgan dose. The absorbed dose averaged over atissue or organ (rather than at a point as in theprevious dose equivalent) and weighted for radi-ation quality by a radiation weighting factor(ICRP60). The equivalent dose in tissue, T, for asingle radiation type, R, having a weightingfactor wR, is then given by HT � wR � DT,where DT is the absorbed dose averaged overthe tissue or organ. Since wR is dimensionless,the unit for the equivalent dose is the same asfor absorbed dose, J kg�1; the sievert (Sv).

Equivalent doseAn absorbed dose of 100 mGy is measured in tissuefor gamma (wR � 1) and alpha (wR � 20) radiation.The equivalent doses are then 100 mSv and 2 Sv,respectively.

(see weighting factor (radiation), effective dose).equivalent energy (xray) Defined as the energy

of the mono-energetic beam which gives thesame half-value layer (HVL) as the x-ray spec-trum. The single energy whose photons wouldbe attenuated to the same extent as those ofthe mixed energies of the x-ray beam continu-ous spectrum.

Equivalent energyA 100 kVp x-ray spectrum having an HVL of 5 mm Algives an attenuation coefficient of:

μ � � �0 6931 38 1.

. .HVL

μ cm


error correction protocol–event location (PET)

E

electrical noise and repeat unsuccessful onlinetransmissions.

erythema (dose) A few days after exposure toabout 8 Gy, the skin may redden. This is earlyerythema which increases during the first week,but starts fading at the end of about the tenthday. The main erythematous reaction thenbecomes maximal on the 15th day after expo-sure and lasts 20–30 days. The main erythemareaction involves the epidermis, the underlyingskin strata and blood vessels. On regeneration,the erythema disappears leaving dry or moistdesquamation.

ESE (phys) See entrance skin exposure.ESD (phys) See entrance surface dose.ESP (mri) Echo spacing: time between echoes in a

FSE sequence.ESR (phys) See electron spin resonance.E-SHORT (mri) Short repetition technique based

on echo steady-state free precession com-monly used for imaging of cerebrospinal fluid(see SSFP, DE-FGR, CE-FAST, True-FISP, PSIF, ROAST,T2-FEE, STERE).

Ethernet (comp) A decentralized tree or bus topol-ogy for computer networking developed by XeroxPARC, using a specific protocol for transmittingdata. Collisions are detected and retransmitted. A shared media technology, Ethernet broadcastspackets to as many as 1024 nodes on a networksegment via twisted pair, coaxial or fibreopticcabling. The general classification of Ethernetencompasses 10 Mbps Ethernet networks and100 Mbps fast Ethernet networks, promising toinclude 1000 Mbps (gigabit Ethernet) networks.Currently, Ethernet is by far the most widelydeployed local area network (LAN) accessmethod, followed by token ring. ARCNET and FDDI

are other network designs (see fast Ethernet, giga-

byte Ethernet).ETL (mri) Echo train length. Number of echoes per

TR in FSE sequence.Euler’s formula (math) Enables the Fourier

transform to be expressed in complex form,extracting real and imaginary roots (phase andamplitude) so:

e�jωx � cos(ωx) � jsin(ωx)

where j is (sometimes represented byi)and ω is 2�f: f being frequency, cos θ � j sin θ isreferred to as cis θ (see complex numbers).

EURATOM Formed from the signing of the EuratomTreaty in 1957 laying down uniform safety

�1

standards and issuing directives and guidelinesfor radiation safety in the workplace. The majordirectives (recent and historical) relevant to med-ical applications of ionizing radiation are:

● 97/43/EURATOM (Patient Protection); MedicalExposure Directive;

● 96/29/EURATOM (Basic Safety Standards);● 84/467 EURATOM (Revised Basic Safety

Standards);● 84/466/EURATOM (Patient Protection);● 80/836 EURATOM (Basic Safety Standards).

These were the foundation for the legislationadopted by European member states (see Ionising

Radiation Regulations).europium (Eu) (elem)

Atomic number (Z) 63Relative atomic mass (Ar) 151.96Density (\rho;) kg/m3 5200Melting point (K) 1100K-edge (keV) 48.5Relevance to radiology: doping agent for phosphors. This isa rare earth element used as a dopant in some intensifyingscreens (BaSrSO4:Eu) and image plate thermoluminescentmaterials (BaXF:Eu).

evaporation (phys) The change of liquid to vapour.Liquids vary in the ease with which they change.Liquids which evaporate easily have a low boil-ing point, e.g. volatile liquids. Latent heat (LH) isneeded for the change from liquid to vapour andthis heat is absorbed (taken) from the surface(cooling). Liquids with large latent heats (water)remove heat more effectively.

Substance LH (vaporization) (MJ kg�1)

Water 2.260Alcohol 0.850

even echo rephasing (mri) Rephasing occurringwhen constant velocity spins return to thesame starting phase they had after the initialRF excitation, as a result of the application of aneven number of gradient pulses. This may alsoresult from the application of multiple gradientecho pulses following the RF pulse.

even–odd rules (nmed) Rules that are used topredict nuclear stability based on whether thenumbers of neutrons and protons are even orodd. These rules are not very reliable.

event location (PET) (nmed) The distance travelledby the positrons before annihilation; the path

E


event location (PET)–expansion

distance increases with the positron energy,leading to a degradation in spatial resolution. Thisdistance is limited by the maximal positronenergy of the radionuclide and the density of thetissue. A radionuclide that emits lower energypositrons yields superior resolution. Activity inbone yields higher resolution than activity in softtissue, which in turn yields higher resolutionthan activity in lung tissue. Approximate valuesare shown in the table.

Positron nuclide β� Energy (MeV) Range (mm)

18F 0.24 1.011C 0.38 1.613N 0.49 2.115O 0.73 2.882Rb 1.4 5.6

(see time of flight (TOF)).exact framing (di) Recording the entire circular

image of an image intensifier or gamma camera(see over-framing).

exametazine (nmed) See HMPAO.EXCEL® (comp) A versatile spreadsheet program

produced by Microsoft.excess absolute risk (dose) The rate of disease

or mortality incidence seen in an exposed pop-ulation compared to the control data in anunexposed population. Commonly expressed asthe additive excess per Gy or per Sv.

excess relative risk (dose) The rate of disease in anexposed population divided by the rate of diseasein a control unexposed population, expressed as afraction (unit � 100%). Commonly expressed asthe excess relative risk per Gy or per Sv.

exchange diffusion (nmed) Diffusion of a radio-pharmaceutical into a tissue and then exchangeof a chemical group on the radiopharmaceuticalfor a different chemical group on the tissue; oneSF-fluoride ion exchanges with the hydroxidegroup on hydroxyapatite in bone tissue to formfluoroapatite.

excitation (mri) Putting energy into the spin sys-tem: if a net transverse magnetization is pro-duced. A magnetic resonance signal can beobserved.

excitation energy (phys) Energy required tochange an atom or molecule from its ground stateto a specified excited state, without ionization.

excitation pulse (mri) The spin equilibrium in themagnetic field distorted by a brief RF pulse. Thehigher the energy of the excitation RF pulse,

the higher the expansion of magnetization. Thefinal expansion of the magnetic field after theRF pulse is the flip angle.

excited state (phys) The state of an atom or mol-ecule having a higher energy state than itsground value. Electromagnetic radiation has suf-ficient energy to cause excitation states in biolog-ical materials which may rupture chemical bonds.

exclusion (dose) Excluding a particular categoryof exposure from measurement on the groundsthat it is not considered amenable to controlthrough the regulatory instrument in question.

excretion/elimination half-life (contrastmedium) (clin) Glomerular filtration is themajor excretory route a typical half-life valuebeing 1.5 hours, increasing to 10 hours or morein cases of renal failure.

exemption (dose) Decision by a regulatory bodythat a source or practice can be exempt fromsome or all aspects of regulatory control on thebasis that the exposure (including potential expo-sure) due to the source or practice is too small towarrant the application of those aspects or thatthis is the optimum option for protection irrespec-tive of the actual level of the doses or risks.

exit dose (dose) See dose (exit).expansion (phys) Expansion in metals is meas-

ured as a coefficient of linear expansion definedas the increase in length, per unit length, for atemperature change of 1 K. The coefficient ofexpansion is a percentage so has no units.

Solid Expansion (10�6 K�1)

Lead 29.0Aluminium 25.0Steel 12.0Copper 17.0Molybdenum 5.0Tungsten 4.0Glass 9.0–12.0Glass (pyrex) 3.0Invar 0.9

The small expansion shown by molybdenum,tungsten, pyrex glass and Invar make theseideal metals for x-ray tube construction.

Expansion of metal1 metre of steel increases its length to 1.10 m whenthe temperature rises by 90 K, the linear coefficient ofexpansion is:

1 101 000 90

1 22 10 5 1..

.�

� � � �K


exponent–extra corporeal shock lithotripsy

E

exposure (nmed) The incidence of ionizing radia-tion on living or inanimate material. Also, ameasure of the ionization produced in a speci-fied mass of air by x- or gamma radiation,which may be used as a measure of the ioniz-ing radiation to which one is exposed. Whenusing SI units, air kerma is often used in placeof exposure. Air kerma has the units of J kg�1

(Gy). In conventional units, the special unit ofexposure is the roentgen (R). An exposure of 1 Rcorresponds to an air kerma of 8.7 mGy (seekerma, gray, roentgen).

exposure factors (xray) These are the parametersaltered either manually or by the automaticexposure control to achieve the optimum imagedensity for a particular imaging medium. Theimage surface can be film-screen, image intensi-

fier or image plate. Four factors are usually con-sidered: kilovoltage, tube current, exposure time anddistance (FFD). These factors are interdependent,e.g. to maintain the same exit dose when the kVis increased from 60 to 70 kV, the mA should bereduced by (60/70)4 or 0.54; an increase by10 kV requires the tube current to be halved orthe exposure time halved for the same current.High kV studies can therefore be performed atfaster exposure times (see isowatt).

exposure time (xray) Duration of emission ofradiation by the x-ray tube (seconds) for anindividual slice in axial scanning or total acqui-sition time for helical scanning.

external focus (us) A focus produced by a lensattached to a transducer element.

extracellular (clin) External to the cells of an organor tissue.

extra corporeal shock lithotripsy (clin) Focusedshock waves, for the fragmentation of renal cal-culi, were first applied by Chaussy and Brendelin 1980. The shock waves employed in clinicalpractice are produced by lithotriptors whichgenerate acoustical pulses with very fast pres-sure rise times and duration (nanoseconds). Themaximum pressures attained are between 500and 1500 bar. Three types of lithotripster areavailable commercially:

● High pressure (1200 bar), with a small focal areausing ultrasound (piezoelectric transducers) asmanufactured by Wolf, EDAP, Diasonics.

● High pressure (500–1000bar), having a large focalarea generated by spark gaps as manufactured byDornier, Direx, Technomed and Medstone.

exponent (math) Indicator for decimal point shift(left or right) in scientific notation, or in anexponential function xn, then n is the exponent.

exponential (math) Most commonly used whenthe exponent is the power of e; the exponentialof x is ex.

exponential distribution (math) A continuousdistribution with probability density functiongiven by f(x) � e�λx, where the mean is 1/λ andthe variance 1/λ2.

exponential weighting (mri) Used in spectroscopy,multiplication of the time-dependent signal databy an exponential function, , where t istime and tc is the time constant, chosen to eitherimprove the signal-to-noise ratio (with a nega-tive tc) or decrease the effective spectral linewidth in the resulting spectrum (with a positivetc). The use of a negative tc to improve signal tonoise ratio (SNR) is equivalent to line broadeningby convolving the spectrum with a Lorentzianfunction.

exposure (dose) Measured in roentgen (R) or air-kerma, where 1 mR � 8.7 μGy. Exposure is alsoa timed exposure to x-rays measured in milliampere-seconds. The exposure in mGyfrom an x-ray exposure can be approximatelyestimated from the formula:

A 1 R incident x-ray exposure (8.7 mGy) with aneffective energy of 30 keV has a fluence of1.3 � 1010 photons cm�2 (see equivalent dose,effective dose).

0 52

2. .��kV mAsD

e t tc�( / )

Radiation exposureIllustrating the benefits of high kV imaging. A highenergy radiograph having a focus film distance (FFD)of 200 cm, at 110 kVp 2 mAs gives a surface dose of:

A low energy radiograph using 70 kVp requires anincrease in mAs estimated by (70/110)4 or �6. Theequation for the same FFD is now:

which is a �2.5 increase in surface dose.

0 570 12

2000 73

2

2. .�

�� mGy

0 5110 2

2000 30

2

2. .�

�� mGy.

E


extra corporeal shock lithotripsy–eye dose equivalent

● Low pressure (�500 bar), with a medium-sizedfocal area using electromagnetic principles asmanufactured by Siemens.■ Reference: Chaussy et al., 1980.

extrafocal radiation (xray) The greatest source ofradiation from the x-ray tube comes from theoptically effective focal spot; radiation also origi-nates from other regions within the tube, outsidethe focus. This radiation component is referred toas extrafocal radiation. A fraction of the electronsare reflected back from the landing position onthe target, falling back towards the anode andlanding outside the focus, giving rise to extrafocalradiation. The strongly emitting focus of the x-raytube is surrounded by a weak but extensivesource of extrafocal radiation. Radiation originat-ing from the optical focus provides a sharplydefined projection image of the object, but theextrafocal radiation superimposes an unsharpprojection of the object, so degrading image

contrast. The intensity ratio of sharp image tosuperimposed unsharp image depends on therelative intensities of the focal and extrafocalradiation. The use of an antiscatter grid does notreduce the amount of contributing extrafocalradiation.■ Reference: Buchmann, 1994.

extrametazine (nmed) See HMPAO, Ceretec.extrapolation (math) Estimation of a value of a

variable beyond known values.extrinsic efficiency (nmed) The ratio of gamma

photons traversing the volume of a scintillationdetector to the total number emitted by aradionuclide.

extrinsic resolution (nmed) See resolution (extrinsic).eye (resolution) (image) Visual resolution.eye (response) (image) See visual response.eye dose equivalent (dose) External exposure

of the lens of the eye. The dose equivalent at0.3 cm depth.


F

f-factor–false-positive fraction (FPF)

Ff-factor (dose) A factor used for converting exposure

in air to exposure in tissue. The mass absorptioncoefficient (μa/ρ) only concerns photoelectricabsorption, which is responsible for tissue radi-ation dose:

The f-conversion factor is then:

Using a conversion factor expresses f- in termsof grays:

This converts C kg�1 exposure in air to expo-sure in tissue as Gy.

f a

a( )grays

tissue

air

/

/34 �

μ ρ

μ ρ

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

Absorbed energy kg tissueAbsorbed energy kg air

�

�

1

1 .

Grays kg tissue beam energy� � �1 μρa

.

(body temperature) as 96°F; on this scale waterfreezes at 32°F and water boils at 212°F. Thecelsius scale, C � 5(F � 32)/9 (see kelvin).

fall out (dose) The nuclear weapons tests carried outin the 1950s and 1960s have deposited about 3 tonnes of 239Pu globally. Other nuclides are 14C,90Sr, 137Cs and with some 99mTc added fromnuclear reprocessing effluent. The average effec-tive dose from fallout is 10 μSv y�1 (80 μSv y�1 inthe 1960s). The collective effective dose is about560 man-Sv (UK) and 2600 man-Sv (USA).

falling load (xray) Using the maximum electricalrating to give a shorter exposure time. The dia-gram shows that by running the tube at maxi-mum output: 500 mA at 0.06 s followed by300 mA at 0.15 s, followed by 200 mA at 0.125 sachieves a 100 mAs exposure time of 0.33 s; afixed 100 mA tube current would have taken1.0 s. Very fast exposure times (�0.5 s) do notlend themselves to falling load exposures.

Bone

Muscle

Air

Fat

f-Fa

ctor

Photon energy (keV)

100

1010 100

FA (mri) Flip angle.FacE (mri) Free induction decay (FID) acquired

echoes.factorial (stats) The product of all the integers from

1 up to and including a given integer. Symbol n!Thus 5! is 5 �4 �3 �2 �1 �120.

FADE (mri) FASE acquisition double echo. TrueFISP technique sampling both FID and SE/STE.

fahrenheit (phys) A temperature scale invented byGD Fahrenheit (1686–1736) who took the lowesttemperature of ice/salt as zero and the highest

0.33 sFalling load

600

500

400

300

200

100

00.01 0.1

Time (s)

1

100 mAat 1 s

Tube

cur

rent

(m

A)

false-negative fraction (FNF) (stats) The condi-tional probability of deciding that in anobserved data set the fraction of patientswhere a diagnostic test suggests the disease isabsent is in fact present (see ROC analysis).

false-positive fraction (FPF) (stats) The condi-tional probability of deciding that an observeddata set (e.g. image) was generated by a spec-ified state (e.g. that a specified disease waspresent) when, in fact, that state was absent.False-positive fraction is equal to one minusthe ‘specificity’ index often used in the medicalliterature to indicate the ability of a diagnostictest to produce ‘negative’ results when the dis-ease of interest is absent (see ROC analysis).

FAME (mri) Fast-acquisition multi echo acronym forFSE/RARE technique used by Picker Medical Inc.

fan angle (ct) Angle covered by the fan of x-rayswithin the scan plane (see fan beam system).

fan-beam system/fan beam (ct) The geometry ofa third-generation machine describing the fixedassembly between x-ray tube and detectorarray which rotates together. The geometry ofthe fan beam is determined by collimation. Twotypes of fan beam designs: third-generationscanners, in which the x-ray tube and detectorarc rotate simultaneously; fourth generationscanners or ring detector (now discontinued),where the x-ray tube rotates only.

Faraday shield (mri) Electrical conductor, typicallya copper mesh, interposed between transmitterand/or receiver coil and patient to block outelectric fields. A Faraday cage, constructed fromcopper mesh, surrounds a magnetic resonance(MR) imaging machine in order to shield thefeint nuclear magnetic resonance (NMR) signalsfrom radio interference.

Farmer FT (1912–2004) British medical physi-cist who developed an accurate ion-chamberdosimeter for measuring dose in x-ray exposure;the Farmer dosimeter.

far zone (us) The region of a sound beam in whichthe beam diameter increases as the distancefrom the transducer increases; also called far field.

FASE (mri) Fast spin echo. FSE/RARE variant.FAST (mri) Fourier acquired steady state. A pulse

sequence used by Picker Medical Inc. similar toGRASS and FISP. Enhanced intensity; rewindingof phase-encoding and no intentional spoiling(see FGR, GFEC, F-SHORT, SSFP).

fast Ethernet (comp) A high-bandwidth net-working technology based on the IEEE802.3Ethernet standard (100BASE-7); supports100 Mbps performance, a 10-fold increase overoriginal 10 Mbps Ethernet (10BASE-7).

fast Fourier transform (FFT) (math) The numberof complex multiplications and additions requiredto complete the Fourier transform is proportionalto N2. The number of multiply and add opera-tions can be made proportional to N log2N rep-resenting a considerable saving in computationaltime. This is the fast Fourier transform proposedby Cooley and Tukey. (ct) The preferred methodfor the reconstruction algorithm for filtered backprojection. The data in each profile are treatedas a mixed frequency, and the entire imagereconstruction then takes place as a series ofamplitudes in the frequency domain. After fil-tering, each modified projection is added to thesum of the previous filtered back projections(see Fourier analysis).■ Reference: Cooley and Tukey, 1965.

fast neutrons (phys) Neutrons with energiesgreater than 0.1 MeV having velocities ofapproximately 4 �106 m s�1.

fast spin-echo (mri) Increasing the acquisition ofthe basic spin-echo acquisition by acquiringonly part of the data and synthesizing theremainder by half-Fourier matrix techniques. Onlythe positive phase-encoding steps are acquiredand a mirror image copy of the positive data is

X-ray tubeFS 1.6 mm

510 mm �

42°

Main beam diaphragm

Secondary diaphragm

Detectors

(see fan angle, cone beam).fanolesomab (nmed) Murine IgM monoclonal

antibody directed against the CD15 antigenexpressed on the surface of polymorphonuclearneutrophils (PMNs), eosinophils and mono-cytes; 99 mTc-fanolesomab (NeutroSpec®) indi-cates infection sites.

FAQ (comp) Frequently asked question(s). Thisterm generally refers to a document posted onthe Internet or elsewhere for the specific pur-pose of assisting new users.

far field (us) See Fraunhofer zone.farad (units) Storage capacity of a capacitor is

equal to the ratio of charge (coulombs) to therise in potential (volts) as A s V�1 (C �A s);equivalent to C V�1. Capacitance is typicallyexpressed in microfarads (μF).

F


FAME–fast spin-echo


fast spin-echo–FDA

F

created. A complete image can then recon-structed. Scan time is halved.

fat (mat) A component, with muscle, of soft tissue.

Effective atomic number (Zeff) 6.46Density (ρ) kg/m3 916

FAT (comp) File allocation table. A table held on afloppy or hard disk that tells the operating sys-tem the location of data and the order in which itis stored. Using 16-bit addresses it can only sup-port disk sizes of 2 G-byte, whereas FAT32 uses32-bit addresses and supports hard disk sizes upto 278 (terabytes) (see FAT16, NTFS, Windows).

FAT16 (comp) File allocation table. This is themap that Windows maintains on each disk vol-ume. FAT16 uses 32 k-byte clusters on 2 G-byte drives from 1966 onwards, keeping thecluster count below 65 536 imposed by 16 bits.This inefficient storage protocol means thateven small files occupied this full 32 k diskspace (see FAT32, NTFS, Windows).

FAT32 (comp) This file allocation table uses 4 k-byte clusters on drives up to 8 G-bytes and8 k on drives up to 16 G-bytes. FAT32 is incom-patible with Win95 disk compression (see FAT16,NTFS, Windows).

fat client (comp) A client station that performsmost of the application (image) processing andvery little or none performed by the server.

fatal exception (comp) An error message gener-ated by the processor when it detects invalidcode, invalid data or illegal instructions beingaccessed by a program. It frequently causes‘blue screen of death’ and generally requires acomputer reboot.

fat saturation (FatSat) (mri) Suppressing the fatcomponent in an MR signal. The protons in thefat are saturated by frequency-selective RFpulses. The saturation affected by magnet homo-geneity. Chemical shift values are 3.5 ppm (seepresaturation).

fat suppression (mri) The signal comprises thesum of water protons and fat protons. Varioustechniques are used for suppressing the fatsignal (see fat saturation, FATSAT).

FATE (mri) A spin-echo version of FLASH with two180° pulses.

FATSAT (mri) FAT SATuration. The mixed water/lipid spectrum has a separation of only 3–4 ppmcausing a chemical shift artefact in gradient echopulse sequences. Spin echo pulse sequences do

not produce these phase errors. Gradient echo

pulse sequences which lack the 180° rephas-ing pulse show fat and water resonance alter-nating in phase (approximately 6.6 ms for a 1Tmagnet) cancelling signals from pixels contain-ing both water and fat. Visible as a black border in tissue interfaces, such as muscle. Apresaturation signal is applied causing a reduc-tion in the MR signal intensity from fat; verynarrow bandwidth RF pulses can selectivelysaturate the fat peak and remove its influence.Requires very high magnetic field homogeneity.Examples of FATSAT pulse sequences use shortinversion recovery (STIR). Higher field strengthshave greater bandwidths which cover the chem-ical shift 62–189 Hz due to fat/water (see SPIR,ChemSat).

Fat/water separationThe 3–3.5 ppm fat/water separation in a 1T field represents:

(3 � 10�6)42.576 MHz � 127.7 Hz.

0.5 T would give separations of 62 Hz; 1.5 T wouldgive separations of 189 Hz. A gradient field of 0.1 mTcm�1 for a 0.5 T field has a bandwidth of 120 Hz(2562 matrix). A gradient field of 0.15 mT cm�1 for a1.0 T field has a bandwidth of 374 Hz (2562 matrix).

FC (mri) Flow compensation.FDD (xray) Focus to diaphragm distance which

influences the field of view (FOV) and thepenumbra magnitude, p, as:

where FDD is focus to diaphragm distance andD the diaphragm setting (see FFD).

FDA (gov) The United States Food and DrugAdministration. An agency within the Departmentof Health and Human Services. The FDA promotesand protects the public health by helping safeand effective products to reach the market. It alsomonitors products for continued safety after theyare in use.The FDA consists of:

● Center for Biologics Evaluation and Research(CBER)

● Center for Devices and Radiological Health (CDRH)● Center for Drug Evaluation and Research (CDER)● Center for Food Safety and Applied Nutrition

(CFSAN)

FOV DFFDFDD

p FSFFD FDD

FFD� � � �

�;

F


FDA–ferromagnetic

● Center for Veterinary Medicine (CVM)● National Center for Toxicological Research (NCTR)● Office of Chief Counsel● Office of the Commissioner (OC)● Office of Regulatory Affairs (ORA).

FDA 510(k) (us) A guidance document whichprovides information for manufacturers seekingUS marketing clearance of diagnostic ultrasoundsystems and transducers. It provides guidancein the preparation of a regulatory submission tothe US FDA.

FDDI (comp) The acronym for fibre distributeddata interface, a standard for fibreoptic cable.Category 7 fibre, supports 100�Mbps data. Datatransmission speeds greater than 1 Gbps persecond, or 100 million bits per second, with thistype of cable. The network uses one fibre cablefor transmitting and another for receiving to min-imize interface problems. Connecting into a fibrecable is difficult, making fibre a very secure net-work technology (see fibreoptics).

FDG (nmed) Fluoro-deoxyglucose; 18F-fluoro-2-deoxyglucose (18F-FDG). A glucose analoguelabelled with 18F that becomes trapped in thecell glucose cycle enabling metabolic activity to be imaged with positron emission tomogra-phy (PET). This can detect active metabolicprocesses and the morphologic features associ-ated with them in a single examination. Therole of 18F-FDG PET has been proven in a vari-ety of cancers.

FE (mri) Field Echo, general sequence, PickerMedical Inc., Toshiba (see FFE, GRE, MPGR, GRECO,PFI, GE, Turbo-FLASH, TFF, SMASH, SHORT, STAGE).

feathering (image) A term used to describe printedtext quality. Feathering occurs when depositedink follows the contours of the paper. Dependingon the viscosity of the ink, the rougher the grainof the paper the more pronounced the feathering.

feature (image) A characteristic of an object; some-thing that can be measured and that assists inclassification of the object (e.g. size, texture,shape).

feature space (image) In pattern recognition, adimensional vector space containing all possi-ble feature vectors (patterns).

FEDIF (mri) Field echo difference (Picker MedicalInc.) for GRE sequence with water and fat sig-nals out of phase.

feedback (elec) Applied to electronic circuitrywhere a fraction of the output signal is returnedto the input. If the phase of the feedback aug-ments the input signal, this is positive feedbackand causes possible instability and oscillation.If the feedback phase decreases the input sig-nal, this is negative feedback which stabilizesthe device (amplifier).

FEER (mri) Field even echo by reversal.femto (phys) Prefix of unit measurement 10�15

(femtoamps sometimes encountered in detectors).Feridex® (cm) An aqueous colloid of super-

paramagnetic iron oxide as injectable ferumox-ides taken up by the reticuloendothelial sys-tem. Gives a reduced signal on normal liver inT2-weighted images. Consists of 11.2 mg Fe/mLand 61.3 mg of manitol. Osmolality 340 mosm/kg.

Fermi, Enrico (1901–54) Italian physicist.Performed fundamental work with Dirac, estab-lished the theory of neutrino production duringbeta decay which causes the continuous betaspectrum and lifetimes of the nuclei. Responsiblefor the first controlled nuclear assembly inChicago in 1942. He studied induced radio-activity using neutrons that formed the founda-tion for the production of artificial radioisotopesin nuclear reactors. He was awarded the Nobelprize for physics in 1938.

ferromagnetic (mri) A substance, such as iron,that has a large positive magnetic susceptibil-ity. Outer orbital shells have unpaired elec-trons, which give them magnetic susceptibility.When placed in an applied magnetic field,these compounds show an induced positive

Cellmembrane

CH2OH

CH2OH

OH

D-glucose

FDG

Glucosemetabolic

cycle

Blocked

O

O

18F

OH

H

H

OH

H

H

OH

H

OH

H

OH

H

OH

H

H

H

Vascularspace


ferromagnetic–field size

F

magnetic moment resulting in an attractiveforce (see contrast agents (MRI)).

FESUM (mri) Field echo with echo time set forwater and fat signals.

FFD (xray) The film to focal spot distance. Animportant parameter in geometrical unsharp-ness and field size. Now replaced by the term‘focus to image distance (FID)’ in order to encom-pass digital imaging (see unsharpness, FDD).

FFE (mri) Fast field echo, general sequence,Philips. Fast field echo steady-state GREsequence acronym used by Philips (see GRE,MPGR, GRECO, FE, PFI, GE, Turbo-FLASH, TFF, SMASH,SHORT, STAGE).

FFF (mri) Fast Fourier flow.FFP (mri) Fast Fourier projection.FFT (math) See fast Fourier transform.FGR (mri) Fast gradient recalled acquisition in the

steady state. Fast GRASS, GE, enhanced inten-sity; rewinding of phase-encoding and nointentional spoiling (see GRASS, FISP, FAST, GFEC,F-SHORT, SSFP).

fibreoptics (phys) Transmission of light along asmall transparent fibre. The speed of light in a vacuum is 3 �105km s�1. However, when itenters a transparent medium (glass), the speedof light is less by a factor of about 1.5 (2 �

105km s�1). This factor is the refractive index ofthe material. Light passing from one medium toanother changes in speed and causes the lightto bend (refraction). Under certain conditions,the light ray will be reflected back into thedenser medium: this is total internal reflection.Light rays in glass are totally internallyreflected if their angle of incidence is increasedbeyond a critical angle (42°). Coherent fibreswhere the position of the fibres is identical atthe start and the end of the bundle length(endoscopes, image intensifier output). Taperingthe fibre bundle can either minify or magnifythe image. Signal losses are about 0.5 dB km�1

resulting in a small signal loss of about 10%per km. Signals can be transmitted over about50 km without amplification (see FDDI, Ethernet).

Fick principle (clin) Establishes that the cardiacoutput can be calculated as:

By extension, this principle can be applied tocardiac output or organ perfusion measurementusing any indicator (radionuclide) providing the

Total body oxygen consumptionArterial O venous O2 2�

.

indicator substance is inert (see Kety–Schmidt

principle).FID (mri) See free induction decay. (image) See

focus–image–distance.field echo (mri) See gradient echo.field emission (phys) See emission (field).field gradient (mri) The magnitude of the gradi-

ent winding in mT m�1. Typical current valuesare 20–40 mT m�1 (see slew rate, magnetic field

gradient).field inhomogeneity (mri) See homogeneity.field lock (mri) A feedback control used to main-

tain the static magnetic field at a constantstrength, usually by monitoring the resonancefrequency of a reference sample or line in thespectrum.

field of measurement (FOM) (ct) Region or volumefor which complete data sets can be acquired;the size depends on scanner geometry (gantrytilt) and the fan angle; the object to be scannedhas to be within the FOM to avoid artefacts dueto data inconsistencies (see truncation error).

field of view (FOV) (image) The extent of thearea visible on the film or display. The FOValters in fluoroscopy with zoom setting and innuclear medicine with collimator types and inradiography with diaphragm setting. (ct) Themaximum diameter of the reconstructed image.

field strength (mri) The main magnet strengthmeasured in tesla (T) (see magnet).

field size (xray) In general, the smaller the anode

angle the wider the focal track which increasesthe power rating; however, angle size alsoinfluences the field size of the x-ray beam at agiven source to image distance (SID). Field sizeincreases with anode angle, as does the effec-tive focal spot size which will degrade image res-

olution, so a large area radiograph would beobtained at the expense of resolution.Conversely, a smaller anode angle would give a

θ θ

F


field size–film gamma

smaller field size, but a better resolution. Thechoice of anode angle depends on the applicationrequired and the SID. Larger focal spots havehigher heat rating or loadability (see heel effect).

field uniformity (nmed) Acceptable planar imag-ing nonuniformity in the central field of view isabout 3%, but SPECT requires uniformity �1%.

filament (xray) The electron emissive surface in anx-ray tube. It is manufactured from drawn tung-sten wire and is part of the cathode assembly.The filament is heated by a low voltage supplyderived from the a.c. (when it has a low frequencyripple) or from the constant potential generator(zero ripple). Single- and dual-filament designsare used according to x-ray tube type and focal

spot requirements (see emission (thermionic)).filament cup (xray) Part of the cathode assembly

surrounding the filament which concentratesthe negative charge so shaping the beam.

filament current (xray) The x-ray tube filamentcurrent is supplied by a low voltage trans-former. Controlling factors are maximum oper-ating filament temperature and filament size.Filament current is increased for low kVp work(mammography) to maintain tube current andcompensate for the space charge effect. In prac-tice, filament current is kept in a standby modebetween exposures (about 5 mA) and increasedto operating currents (4.5–5.5 A) for exposures.

file (comp) A section of information stored on diskand given a name.

file extension (comp) A DOS three letter codewhich identifies file characteristics, e.g. .doc,.txt, .tif, .exe.

file server (comp) See server.fill-in factor (image) Direct radiography thin film

transistor (TFT) image capture. The potentialimage artefact caused by gaps between TFTdetectors are corrected by bending the fieldlines (see direct radiography).

filling factor (mri) A measure of the geometricalrelationship of the RF coil and the object being studied, affects the efficiency and signaldetection, thereby affecting the signal-to-noise

ratio. Achieving a high filling factor requires fitting the coil closely to the patient.

film (xray) The first recording medium used inradiography. Three film types exist: monochro-matic (blue light sensitive), orthochromatic (blue/green light sensitive) and panchromatic (sensi-tive to the complete visible spectrum).

film badge (dose) A double emulsion dental filmused as a personal dosimeter. Placed in aholder complete with metal filters it can indi-cate radiation type (soft x-ray, gamma, beta,neutrons) and approximate energy. It is a per-manent record of personal dose; an assemblycontaining unexposed photographic film andone or more absorbers, worn by those workingwith radiation sources. When the film is devel-oped, an estimate of the dose and type of radi-ation can be made. Film dosimeters are nottissue equivalent (see TLD, dosimeter).

film base (film) A polyester material supportingthe emulsion, sometimes coloured blue.

film contrast (film) An indication of latitude ordynamic range. Obtained from the film or imagedetector gamma (see film gamma).

film emulsion (film) See emulsion.film formatter (laser) (film) A method for record-

ing image data on film using a scanning laserbeam. Typically, a 14 �17 in (350 �430 mm)film is digitized to 4260 �5182 pixels; eachpixel dimension being 0.082 mm (82 μm).

film gamma (image) A measure of film contrast;the rate of change of density with exposure orΔD/Δ log E. This is expressed as the slope ofthe characteristic curve, taken as the straightmiddle portion of the curve. If two pairs of den-sity D and exposure logE values are taken inthis range then:

This ratio is the film gamma, which is also thetangent of the angle between the straight part

Gamma ( )log log

γD DE E

2 1

2 1

�

�

Direct (selenium)

Indirect (scintillator)


film gamma–filter

F

of the curve and the logE axis. If γ � 1, thenthere is a proportionality between exposure anddensity (1:1); if γ � �1, then doubling log Eincreases density by a factor �2 (increased lat-

itude, poorer contrast); if γ � �1, then densitychanges increase faster (good contrast, poor lati-tude). The steeper the curve, the higher the con-trast and the smaller the latitude or dynamic range.Film contrast depends on emulsion type and alsoon development conditions (temperature) andthe period of time spent in the developer (filmprocessor speed).

film screen (film) See intensifying screen.film sensitometry (film) A laboratory procedure

for exposing a film to a standard grey scaleusing a sensitometer and measuring its responseby means of a densitometer.

film speed (image) Film sensitivity to exposure,measured for an optical density � 1. In the fol-lowing diagram, film A is the faster:

Opt

ical

den

sity

Exposure (Log E)

Film speed

A B

3

2

1

00 1 2 3

(see characteristic curve, contrast (film)).film grains (film) The crystals of silver halide which

separate from the silver salt solution and aremixed with the gelatine base. Conventional(mixed grain size), T-grain (flattened profile tothe grains) and cubic grain geometry are cur-rently used in radiography emulsions.

film processor (film) A method for the daylightprocessing of x-ray film accepting the unopenedcassette and after a fixed period of time (typi-cally 90s or 3minutes) providing a fully processedand dried film (see film formatter).

film response (film) The film response to x-raysas its mass absorption coefficient is shown inthe graph. The film sensitivity to x-rays issuperimposed against an arbitrary scale show-ing maximum sensitivity over the diagnosticenergy range (see film badge).

3

Dmax

ToeDmin

Shoulder

Log exposure (subject contrast)

Base � fog

Opt

ical

den

sity

(ob

ject

ive

cont

rast

)

2

1

00 1 2 3 4

Photon energy (keV)

Ag K-edge

10

10

1

0.1

0.01100

Mas

s ab

sorp

. coe

ff. (

m�

2 kg

)

Sensitivity

(see optical density).film viewer (film) A uniform light source for

viewing transparent x-ray film images. Usually2000 cd m�2 for conventional film and higherfor mammography.

filter (image) Any process which alters the relativefrequency content. Can be achieved with ananalogue (conventional electrical) filter; removehigher frequency components to prevent alias-ing in digitizing. Filtering can also be carriedout numerically on the digitized data. (ct)

F


filter–filtering (signal)

Mathematical procedure used for the convolution

of the attenuation profiles and the consequentreconstruction of the CT image.

filter (K-edge) (xray) A high atomic number (Z)metal foil having a K-edge absorption in the diag-nostic energy range. Common materials for con-ventional K-edge filtering are erbium, whichreduces patient dose but reduces beam intensity.Mammography uses molybdenum, palladiumand rhodium.

Signal noise is removed by restricting frequencycomponents. Filters are usually specified in termsof its frequency response. Filters allow frequencycomponents of the input signal lying within a cer-tain band (the passband), and stop, or at leastattenuate, components within the stopband(s).The three filter types are:

● a low pass filter, where the passband extendsfrom zero frequency to some chosen ‘cutoff’value;

● a high pass filter, where the passband extendsfrom the chosen cutoff frequency up to themaximum frequency dictated by the samplingrate (the applet assumes a sampling rate of8000 samples/s, so the maximum frequency is4000 Hz);

● a band pass filter, where the lower and upperpassband limits can have any values between 0and 4000 Hz, although obviously the upper limitshould be greater than the lower. The stopbandattenuation is the minimum acceptable attenua-tion, in dB, within the stopband. The transitionbandwidth is the maximum acceptable frequencywidth of the transition band, in hertz, separatingthe passband from the stopband. The choice ofwindow function is determined mainly by thestopband attenuation requirement.

Window function Minimum stopband attenuation (dB)

Rectangular 21Hanning 44Hamming 54

A Hamming filter function would give an attenu-ation of at least 50 dB throughout the stopband.

X-ray photon energy (keV)

0 20 40 60 80 100

Pho

ton

abun

danc

e

Erbium filterK-edge 57.5 keV

Tungsten target

filter (kernel) (di, ct) mathematical procedureused for the convolution of the attenuation pro-files and the consequent reconstruction of theCT image (see kernel).

filtered back projection (comp) An image recon-struction technique applied to axial tomogra-phy to create images from a set of multipleprojection profiles. The projection profiles arebackprojected to produce a 2D or 3D image. Theprojection profiles are processed by convolvingthem with a suitable filter kernel in order toremove high frequency reconstruction noise.(ct) Method for image reconstruction, which canbe divided into two steps: backprojection of themeasured attenuation profiles and then convo-lution (see algebraic reconstruction).

filtering (signal) (di) An ideal filter provides a stepresponse to signal frequencies. Four types arecommonly employed: a low pass filter where all fre-quencies from 0 to a certain point are transmittedunchanged, a high pass filter where all frequenciesfrom 0 to a certain point are blocked, a pass-band

filter where a frequency window is preferentiallytransmitted and a stop-band filter where frequen-cies within a chosen window are blocked. (image)

Low pass

1.0

1.0

1.0

1.0

High pass

Band pass

Band stop

Signal

Signal

Signal Signal

Signal


filtering (signal)–FISP

F

The filter order mainly determines the width ofthe transition bandwidth; the higher the order,the narrower the transition between the pass-band and stopband which gives a sharper cutoffin the frequency response. The transitionbandwidth (the filter cutoff sharpness) can beimproved by increasing the order of the filter.

filtering (spatial) (di) Applying a function to animage matrix (kernel) which will affect all the pixels in a linear, nonlinear or weighted fashion.Simple filters are smoothing or edge enhancement.

filtering (temporal) (di) Frame averaging orrecursive filtering takes the average value froma small series of images (typically four) whichreduces image noise.

filtration (ct) Application of metal sheet foil on theexit port of the x-ray tube; filtration removes thelower-energy of the polychromatic x-ray spectrum

and increases the effective energy. Thus the sur-face radiation exposure of the patient is reducedwithout significant decrease of the measuredsignal. The remaining x-ray distribution is lessprone to beam hardening, so that the beamhardening correction is less complicated andbeam hardening artefacts are less frequent.

filtration (added) (xray) Additional filter material(aluminium or copper) added in order to removelow energy x-rays. This can be up to 2.5 mmaluminium for conventional tubes with anadded 0.25 Cu for chest x-ray, fluoroscopy andCT x-ray tubes (see half value layer).

and �0.1 mm Al for mammography tubes withberyllium windows.

filtration (total) (xray) This is the sum of inherent �fixed filtration. Total filtration should be at least1.5 mm aluminium for a 80 kVp beam energy.

firewall (comp) A system designed to preventunauthorized access to a network. All informa-tion entering or leaving the network is interro-gated and rejected if it fails to meet establishedcriteria. A firewall can also be used by a com-pany to control what resources outside the net-work can be accessed by employees.

FireWire® (comp) A very fast serial interface forconnecting external devices. Also known as 1394or iLink, it supports up to 63 devices and speedsof 400 Mbps (megabits per second). First imple-mented on the Apple Macintosh, it is more com-plex than a universal serial bus (USB) and suitesdevices that require high-speed transfer rates.

FIRFT (mri) Fast inversion-recovery Fourier transform.

firmware (comp) Software that is permanentlystored on read-only memory (ROM). It providesmany devices, such as printers, modems andtape drives, with basic instructions and, whereFlash-ROM is used, the code can be changedby downloading a new set of instructions.

first pass (nmed) A fast dynamic study (i.e. heart/lungs), which follows transit of a bolus of activity(labelled blood cells); involves collecting fastdynamic frames while a bolus is in transit throughthe major heart chambers (see blood cell labelling).

FISP (mri) Fast imaging with steady-state preces-sion; a steady-state GRE sequence. Siemen’sname for GRASS (GE). The generic name beingSteady State GRE with free induction decay(FID) sampling, where the remaining transverse

coherence is preserved. The sequence has thephase-encoding gradient balanced after thereadout period prior to the next RF pulse. TrueFISP has all three gradients balanced. Dephasingthe transverse magnetization due to the phase-encoding gradient is resolved by a negativegradient after data acquisition and before thenext RF pulse. The net effect is the same fromone TR interval to the next and both the longi-tudinal and transverse magnetization reachequilibrium, contributing to a steady state sig-nal. The advantages with FISP are:

● short imaging time;● high signal to noise ratio (SNR);

0 20 40

1.5 mm AI

0.25 mm Cu

60 80

Pho

ton

dens

ity

filtration (inherent) (xray) The filtration offeredby the x-ray tube glass envelope, the insulatingoil and any glass insert material. It is expressedas an equivalent thickness of aluminium and istypically 0.5–1 mm Al for conventional tubes

F


FISP–flip angle

● 3D imaging possible;● strong T1 or T2* contrast.

A fast imaging sequence sensitive to the effects of magnetic field inhomogeneities and imperfections in the gradient waveforms (seeFLASH, GRASS, FGR, FAST, GFEC, F-SHORT, SSFP

PSIF).fission (phys) See nuclear fission.fission products (phys) Stable and unstable

nuclides from a fission reaction. Some exam-ples are:

Nuclide Emission Half-life Fission yield (%)

90Sr Beta 28 years 699Mo 740 keVγ 66 hours 6131I 364 keVγ 8 days 3133Xe 81 keVγ 5 days 6.5137Cs 662 keVγ 30 years 6

fixer (image) A thiosulphate compound whichremoves unexposed silver halide; this is typi-cally ammonium thiosulphate (NH4)2S2O3.

The development process is stopped by acidi-fying the thiosulphate fixer with acetic acid (pH4.0–6.0). Unfortunately, this causes instabilityin the thiosulphate ion which breaks down toyield sulphite ion and sulphur:

The sulphite ion breaks down further to yieldsulphur dioxide.

FLAG (mri) Flow-adjustable gradients (Philips).Reduction of motion-induced phase shifts dur-ing TE (see GMR, GMN, FLOW-COMP, CFAST, MAST,GMC, FC, STILL, SMART, GR).

FLAIR (mri) Fluid attenuated inversion recovery.IR technique using long TI value to null signalfrom liquids. It requires a long TR so has longimaging times (see dark fluid imaging, RARE).

FLARE(mri) Fast low-angle recalled echoes.FSE/RARE variant using low flip angles.

2 4 2

42 3 2 2 3

3 2

S O H H S O

H 2O S

� �

� � �

�

�

→

→

Ag Br S O AgS O insoluble BrAgS O S OAgS

� � � �

� �2 3 2 3

2 3 2 3

→→→

( ) ( )( )( 22 3 2O soluble and

removed) (

) �

flare (xray) Light dispersion at the input phosphorsurface (see image intensifier).

FLASH (mri) Fast low angle shot. Rapid gradient-

echo imaging sequence. Siemen’s name forspoiled gradient. Contains a spoiler gradient afterthe readout period to disperse residual transverse

magnetization; T1-weighted image contrast. As trans-verse magnetization is dephased prior to eachRF pulse, only the longitudinal magnetization

reaches a steady state. FLASH and SE contrastare very similar. T1 weighting increases withflip angle. Equivalent to SPGR or spoiled GRASS

(GE Medical), where the coherence of the trans-verse magnetization is spoiled or disrupted (seeFISP, GRASS, SPGR, FSPGR, HFGR, RE spoiled, 3D-ME-

RAGE, STAGE-T1W).flash memory (comp) A non-volatile computer

memory that can be electrically erased andreprogrammed; it is non-volatile so does notlose stored data when the power supply isremoved. Primarily used for memory cards andUSB flash drives (memory stick, flash stick, etc.)for data storage/transfer between computersand digital devices (cameras, MP3 players, etc.).It is a specific type of EEPROM (electrically eras-able programmable read-only memory) that iserased and programmed in large blocks. Currentlyavailable up to 8 GB capacity.

flat-panel display (comp) A low voltage replace-ment for cathode ray tube (CRT) displays, con-sisting of a TFT array, backlight and colour dotmatrix. Clinical quality displays have higherresolution than conventional computer displays.Typical specifications for 2–6 million pixel displays are:

2 M 3 M 6 M

Matrix size 1600 �1200 2048 �1536 3280 �2048Pixel size 0.270 mm 0.207 mm 0.1995 mmBit depth 12 30 colour 12Luminance 900 cd m�2 1000 cd m�2 800 cd m�2

Contrast 700:1 900:1 800:1ratio

Flexiled® (shld) Flexible lead/PVC sheeting.flicker (di) Video displays can cause visible flick-

ering if their scan times are too short. Scantimes below 20 have serious visible flickerinterference.

flip angle (mri) Amount of magnetization vectorrotation produced by RF pulse. Flip angles of15–30° used in fast acquisition sequences.


flip angle–flow effects

F

Only a part of the longitudinal magnetization isconverted to transverse magnetization reduc-ing the recovery period. A net signal advantageis achieved with short TR intervals.

bulk motion of the liquid, a potential-energydensity (ρgy) and pressure energy density (P)arising from the moving liquid. The net energydensity contained in a flowing system (blood) istherefore constant throughout the vessel.Poiseuille flow is concerned with steady laminar

flow of a liquid of viscosity η, through a vesselof circular cross-section with radius r, length land a pressure difference p1 � p2. The quantityof liquid Q flowing per second is:

The flow varies as the fourth power of the radius:a slight increase produces a large change in flowrate.

flow artefact (mri) A motion artefact generated bylocal signal changes during measurement.Examples would be the inflow intensity of avessel perpendicular to the image plane whosemovement changes periodically due to pulsatileblood flow. In sectional body imaging, ghostingappears in the aorta. Turbulent blood flow inthe heart results in smearing of the image.

FLOW COMP (mri) Flow compensation GE, Toshiba.Reduction of motion-induced phase shifts dur-ing TE (see GMR, GMN, CFAST, MAST, FLAG, GMC,FC, STILL, SMART, GR).

flow compensation (mri) Means of reducing floweffects, such as gradient moment nulling. Foroverriding the (GMR) signal loss caused by spinmovement, both moved and unmoved spins arerephased. Additional gradient pulses of a suit-able size and duration are applied.

flow dephasing (mri) Nulling the signal fromflowing blood by the application of specificallyapplied gradient fields (see dephasing).

flow effects (mri) Motion of material being imaged,particularly flowing blood, can result in signalincrease (flow-related enhancement) or decreaseor displacement (image misregistration). Causedby time-of-flight effects (wash-out or wash-indue to motion of nuclei between two consecu-tive spatially selective RF excitations). The incon-sistency of the signal resulting from pulsatileflow can lead to artefacts in the image, whichcan be reduced by:

● synchronization of the imaging sequence (car-diac gating);

Qp p r

l�

�πη

( ).1 2

4

8

20

40

60

80

100

00 30 60 90

Sig

nal s

tren

gth

(%)

Flip-angle (degrees)

floating point (comp) A method of writing realnumbers as a � 10n or aEn, where a � 1, but�0.1 and n is the integer exponent directingdecimal point position �, left; �, right. For example 0.564E2 � 56.4, but 0.564E1�0.0564.Floating point format is a common notation usedin computing.

flood image (nmed) Image of gamma cameradetector flooded with a uniform source of photons.

floppy disk (comp) A small disk either 5.25 in(obsolete) or 3.5 in, which can be removed andused on other machines (see storage (bulk), hard

disk, jazz disk, super disk, zip disk).flops or FLOPS (comp) Floating point operations

per second. Mega-flops and giga-flops areused.

flow (mri) Nuclei from liquids moving into anexcited slice-region can be distinguished fromstatic tissues. (phys) In a steady laminar flow ofliquid, a pressure difference exists and so mustcontain energy by virtue of the work done:

where v is the velocity and y the fluid heightdifference. This is Bernoulli’s principle whichrelates kinetic energy density with the( )1

22ρv

P v gy� � �12

2ρ ρ constant

F


flow effects–fluorine (F)

● suppression of the blood signal with saturation

pulses;● reduction of phase shifts with gradient moment

nulling.

flow encoding (mri) Phase encoding obtains infor-mation regarding the direction and velocity ofmoving fluid material (blood, cerebrospinal fluid(CSF) etc.). Provides quantitative information onthe velocity of blood flow by:

● bright blood effect;● inflow amplification;● jet effect;● signal elimination;● washout effect.

(see flow quantification).flow enhancement (mri) The increased intensity

that may be seen due to flowing blood as aresult of loss of saturated spins from theimaged slice; signal increase given by flowingblood due to washout of saturated spins (seesaturation).

flow quantification (mri) Flow measurementsusing phase contrast to examine large vessels oras part of an extensive cardiovascular study.Flow measurements enable noninvasive evalu-ation of blood flow.

flow-related enhancement (mri) The increase inintensity that may be seen for flowing blood orother liquids with some MR imaging techniques,due to the washout of saturated spins from theimaging region (see saturation).

flow rephasing (mri) A phenomenon commonlyseen in blood vessels whose flow is within animaging plane and having a component of theirflow in the frequency encoding direction. Theincrease in flow signal on even echoes (secondand fourth) is even echo rephasing; the loss ofsignal on the odd echoes (first and third) iscalled ‘odd echo dephasing’. These echo signaleffects are seen during the frequency encodinggradient.

flow sensitivity (mri) Phase contrast angiography.The blood velocity at which the phase differencebetween flow compensating and flow encoding

scans is 180°.flow void (mri) Low signal in regions of flow. The

lack of refocusing a spin echo sequence inblood which is excited by the 90° pulse, but not by the 180° pulse. For a gradient echo

sequence, this is caused by the dephasing ofblood signal.

fluence (phys) Radiation intensity per unit area(see photon fluence, photon fluence (energy)).

fluorescence (phys) A luminescence phenomenon,as seen in radiology and clinical applications,first named by George Stokes after the mineralfluorite, a strongly fluorescent mineral. Causedby absorption of some form of radiant energy,such as ultraviolet radiation or gamma or x-raysceasing immediately, or very shortly after, theradiation causing it ceases. The simplest modelfor an inorganic scintillator involves crystal impu-rities and lattice defects providing energy levelsin normally forbidden region. The process is:

1 The traps in the forbidden energy band are fullbefore irradiation.

2 Photons dislodge electrons from the valencyband with sufficient energy to displace theminto the conduction band.

3 The holes created in the valency band attractelectrons from the traps and light with shortemission times (100–200 ns) is emitted.

4 Equilibrium in the phosphor material is estab-lished when displaced electrons in the conduc-tion band fill the empty traps.

Electron de-excitation occurs almost sponta-neously, and emission from a luminescent sub-stance ceases when the exciting source isremoved. The quantum yield φ of a fluorescentsubstance is defined by:

Light in the U/V visible wavelength is emittedfrom a substance under stimulation or excita-tion by other forms of electromagnetic radiationof shorter wavelength. Photons are emittedonly while the stimulation continues or for avery short time after cessation (dead-time), dif-fering from phosphorescence, where photonemission continues for a time after the excita-tion has ceased. Organic and inorganic fluores-cent substances are found in radiology as α, β,γ and x-ray detectors, as well as coatings forvideo displays (flat panel displays, as well as CRT),intensifying screens, gamma cameras) (see energy

bands).fluorine (F) (elem)

Number of photons emittedNumber of photons absorbed

.


fluorine (F)–focal spot (real)

F

Atomic number (Z) 9Relative atomic mass (Ar) 19.0Density (ρ) kg/m3 1.7Melting point (K) 53.5Relevance to radiology: Element of the halide family(Z �9), useful in radiology as the positron radionuclide18F which labels deoxyglucose, without distorting themolecule, forming fluoro-deoxyglucose, used forimaging metabolic sites (heart and various tumours).

18Fluorine (nmed)

Production(cyclotron)Decay scheme 18F (β�, 2γ 511 keV) → 18O stable(β�) 18FHalf-life 110 minutesDose rate 0.143 μSv m2MBq�1hr�1

constantHalf value 4.1 mm Pblayer

96.73% positron emission and electron capture3.27%. The positron energy is 635 keV. A majorradionuclide in positron emission tomography(PET).

18Fluorine tracers (nmed) The production of 18Ftracers is typically by substitution with 18F.Fluoride trapped from 18O water and purifica-tion by high performance liquid chromatograpy;currently available compounds from automatedcyclotron analyzers are:

Tracer Application

18F long chain fatty Anaerobic metabolismacids, 18F FTHA18F fluoromisonidazole Hypoxia18F methylbenperidol Dopaminergic D2 receptor18F methylspiperone Dopaminergic D2 receptor18F fluorostradiol Steroid metabolism18F altanserine Seratonergic S2 receptor18F FLT fluoro-L-thymidine DNA sysnthesis18F fluoro-deoxyglucose Glucose metabolism(FDG)

fluoro-deoxyglucose (nmed) See FDG.fluoroscopic CT (ct) Continuous imaging by CT to

control or to guide a diagnostic or therapeuticintervention.

fluoroscopy (xray) A technique which uses animage intensifier to give a real time x-ray imageon video, cine-film or small format cut film (seeColtman, image intensifier).

flux (phys) Radiation fluence per unit time (seephoton flux, photon flux (energy)).

1020

918

816Ne F or O F( , ) ( , )d p nα 9

18

flux gain (phys) Gain in light intensity betweeninput and output phosphors (electronic gain ofimage intensifier).

flying focal spot (ct) A process where the num-ber of measurement channels is doubled byrapid deflection of the x-ray tube focal spot foreach projection increasing the image resolu-tion. Achieved by electromagnetically deflect-ing the electron beam within the x-ray tube.For each focus position, two measured interlacedprojections result, since the detector continuesto move continuously. The sampling frequencyis doubled enhancing the spatial resolution.

FM (phys) See modulation (frequency).fMRI (mri) Functional magnetic resonance imag-

ing. Measures changes in cerebral blood flowand cerebral blood oxygenation related to neu-ronal activity; also demonstrates function of theheart and other organs.

focal length (us) Distance from a focused trans-ducer to the centre of a focal region or to thelocation of the spatial peak intensity.

focal region (us) Region of minimum beam diam-eter and area.

focal spot (xray) The target area on the anodewhich forms the real focal spot of the electronbeam. The anode angle determines the projectedor effective focal spot size of the x-ray beam(sometimes called the apparent focal spot). Focalspot size influences the sharpness of the imageand the tube rating (loadability). Due to the rota-tion of the anode, the electronic focus traces outa ring-shaped focal track on the surface of theanode disk. The size of the effective focal spot isdetermined by the line focus principle (see dual

focal spots, focal spot (effective), focal spot (real)).focal spot (effective) (xray) The projected

dimensions from the angled anode target ascalculated from the line-focus principle. The effec-tive focal spot for general radiography is0.6–1.2 mm and for mammography 0.4 mm.The dimensions of the effective focal spot dif-fers over the image plane.

focal spot (real) (xray) The rectangular area onthe anode, bombarded by the electron beam. Thearea and angle of the real focal spot determinesthe effective focal spot. Single filaments areused in x-ray tubes having a single anode tar-get; dual-sized focal spots can be obtained byaltering the electron beam size. Dual filaments,operated in parallel on a single focal spot, areused in mammography tubes to overcome the

F


focal spot (real)–focus

space charge limitations; these are operated asthe single filament to obtain dual focal spotsizes. Separate dual focal spots are also used(fluoroscopy), but are operated independentlyand focused on separate differently angled tar-gets to give dual focal spots. Focal spots can bemoved to slightly different positions on the tar-get surface by control coils which surround thex-ray tube; used in some CT machines as flying

focal spots to increase image resolution (see focal

spot (effective), filament).

Side view Endview

Acousticlens focus

Electronicfocus

sometimes present. Multifocal tracks are eachsupplied by a separate filament.

focal zone (us) Length of the focal region. This isthe narrowest region of the ultrasound profile.Two focal zones are present in a multi-elementultrasound array. The focal zone across thewidth of the transducer decides slice width andis controlled by a shaped lens with a fixedradius. The focal zone parallel with the trans-ducer face depends on aperture size and signaldelay. This decides the lateral resolution.

Real FS 11° Anode

Effectivefocal spots

Real FS 20°

focal spot size (ct) While the electronic focal spot

has a nonquadratic shape on the anode surface,the tube is installed on the fan beam assemblyso that the optical focal spot approximates aquadratic shape at the centre of the detectorarray; for CT, the focal spot size does not needto be small, since a finite beam width due to anincreased focal spot size is suitable for the sup-pression of aliasing artefacts.

focal surface (us) The surface which contains thesmallest of all beam cross-sectional areas of afocusing transducer assembly. Unit: centimetresquared (cm2).

focal track (xray) The target area on the circum-ference of a rotating anode x-ray tube. Dual tracks giving two focal spot sizes (two dif-ferent angles) or multiple focal tracks carryingdifferent target material (mammography) are

(see focus (ultrasound)).focus (xray, us) Determines the slice thickness by

shaping the crystal or matching layer, or anelectronic focus can influence lateral resolution.Concentration of a sound beam into a smallerbeam area than would exist otherwise (seefocal spot).

Curved lens

Flatlens

Radius r cm

r � 0.3

r � 0.5

Flat transducer


focus (dynamic)–Fourier analysis

F

focus (dynamic) (us) When a transducer array isreceiving echoes, the receiving focus depthmay be continuously increased by alteringreceiver delay as the transmitted pulse travelsforward. The contiguously changing echoreceiver window is dynamic focusing whichincreases image resolution with depth.

focus–image–distance (FID) (image) A measurefrom the focal spot to the imaging surface (film,image plate or digital image surface).

focusing coils (xray) These control the electronbeam geometry magnetically, using coils whichcontrol and focus the beam. Cathode ray andcamera tubes both used x and y-axis coils togive focused scanning geometry. Some x-raytubes for computed tomography (CT) can havebeam steering which shifts the x-ray spotbetween two focused points on the target.

focusing cup (xray) See cathode assembly.focusing electrodes (xray) High voltage electrodes

accelerating electrons (see image intensifier).foetus (exposure risk) (dose) The mean foetal

dose limit (ICRP60) is 5 mSv over a period of 1year. The added risk to the foetus in utero for a5 mSv exposure is 0.05%. The normal incidenceof no abnormality being 95.93% or an approxi-mate abnormality incidence of 1 in 24 livebirths. Deterministic threshold of between 200and 400 mGy for severe mental retardationresulting from foetal exposure. Lower doses of10 mSv have been a suggested risk. (mri) Atpresent, there is no known evidence linkingMRI exposures to disorders in embryogenesis,but high gradient slew rates should be recon-sidered (20 T s�1 rise time �10 ms NationalRadiological Protection Board (NRPB) in the UKand �6 T s�1 in the United States). Fieldstrengths are limited to 2.0–2.5 T.■ Reference: Doll and Wakeford, 1997.

fog level (film) See characteristic curve.foot (ft) (phys) Non-SI, imperial measure of length

where:

1 ft 0.3048 m (30.48 cm)1 m 3.28 ft (39.37 in)1 ft2 0.093 m2

1 m2 10.76387 ft2

footprint (us) Area of transducer in contact withthe patient. Also applied to certain computerhardware.

forbidden band (phys) See energy bands.

force (F) (phys) A vector quantity where F � ma,the quantity m is the mass of a body and a itsacceleration. Since a � F/m, then the accelera-tion of the body is directly proportional to theresultant force acting on it and that the accel-eration is inversely proportional to the mass ofthe body. This is a point considered with x-rayanode design; lighter anodes accelerate fasterto their working speed and, if greater surfacearea is required, graphite is used as it is a verylight material which adds very little mass.

Mammography compression paddleStated maximum range (F ) is 150–200 newtons. Sincemass m � F/a and acceleration due to gravity a �

9.8 m s�2, the actual pressure felt by the patient undermaximum compression is equivalent to 15–20 kg.

(see weight).forced convection (xray) See convection.form filter (ct) Device for x-ray beam filtration;

the thickness of the form filter increases withthe distance from the central ray, so that thedifference in intensity measured at the detectorbetween rays through the centre of the objectand peripheral rays, which experience low orno attenuation, is decreased. Applying form fil-ters reduces the intensity of scattered radiationfrom peripheral parts and therefore patientdose is decreased. A common name is the‘bow-tie’ filter due to its shape.

forward bias (phys) See junction diode.Fourier Jean (1768–1830) French mathemati-

cian and scientific adviser to Napoleon inEgypt. Developed analysis of complex wave-forms into simple sine waves.

Fourier analysis (math) A method of waveformanalysis. Any periodic function (sine-wave, squarewave, etc.) is a summation of sinusoidal compo-nents consisting of fundamental and harmonicfrequencies of the system (ultrasound pulse, FID,etc.). The simplest example is the sine wavewhere:

X(t ) � A � sin(ωot � θ).

A is a constant for the peak amplitude and ωo isthe angular frequency (2πf where f is in Hz); θ isthe phase angle in radians. A Fourier serieswhere each harmonic component repeats itselfcan be represented as:

x(t) � Ao � A1 sin(ωot � θo) � A2 sin(2ωot � θo)� An sin(nωot � θn)

F


Fourier analysis–Fraunhofer zone

or

The summation is the Fourier series and theanalysis of the composite periodic function intosimple harmonic components is its Fourieranalysis. A complimentary process, the inverseFourier transformation allows regeneration ofthe original signal (see discrete Fourier transform,fast Fourier transform (FFT)).

Fourier space (mri) Raw data matrix axes, kx andky dividing the matrix into four squares. Theplane spanned by the two axes is the Fourierspace or k space.

Fourier’s theorem (math) This states that it ispossible to synthesize or construct any signal(one-dimensional function f(x)) as a summationof a series of sine and cosine terms of increas-ing frequency (see Fourier transform).

Fourier transform (FT) (math) A measure of therelative amplitude of the frequency compo-nents of a signal x takes the exponential nota-tion developed in Euler’s formula to give theFourier transform F(u).

where ω�2πf. The variable x represents time,so functions in a time domain. The transform Frepresents frequency so exists in the frequency

domain. Given F, it is possible to recover theoriginal time domain function as the inverse:

This can also be used as a 2D transform forimage analysis and reconstruction (see discrete

Fourier transform, Fourier’s theorem).Fourier transform imaging (mri) MR imaging

techniques in which at least one dimension isphase-encoded by applying variable gradient

pulses along that dimension before reading theMR signal with a magnetic field gradient perpen-dicular to the variable gradient. The Fouriertransform is then used to reconstruct an imagefrom the set of encoded MR signals An imagingtechnique of this type is spin warp imaging.

f x F u e duj x( ) ( ) .� � �

��

�ω∫

F u f x e dxj x( ) ( )� � ��

��

�ω∫

x A n t nt n o nn

n

( ) sin( ) ( , , , )� � � ��

��

ω θ 12 31

…∑

FOV (nmed) Field of view. Central field-of-view(CFOV) of a gamma camera detector, correspon-ding to 75% of the detector diameter. Useful fieldof view (UFOV) of a gamma camera detector cor-responding to 95% of the detector diameter.Typical values for intrinsic flood field uniformity:

Field UFOV (%) CFOV (%)

Integral �2.5 �2.2Differential �2.0 �1.5

Fowler JF British medical physicist and radiobiol-ogist. Pioneer of the investigation of radiationbiology and dose response for plant and mam-malian tissue.

fraction (math) Consists of a numerator n anddenominator d (or divisor): n/d, a vulgar fractionis written as , a decimal fraction as 0.5. Aproper fraction is a vulgar fraction that has anumerator smaller than the denominator ;an improper fraction is a vulgar fraction havingthe numerator bigger than the denominator .

fractional bandwidth (image) Bandwidthdivided by operating frequency.

fractional error (math) The inaccuracy associatedwith measurement. If original value is Vo andmeasured or nominal value Vn, then fractionalerror f is: (Vn�Vo)/Vo (see accuracy, precision).

fractionated dose (dose) A total dose delivered insuccessive fractions with periods of days orweeks between each fraction. Contrast withprotracted doses given by irradiation continu-ously over a long period.

frame (image) A single image from a series eitheranalogue (cine-angiography) or digital (nuclearmedicine, digital subtration angiography (DSA))(see interlacing).

frame averaging (image) If image frames (F ) aresummed, the composite image signal to noise isimproved as , so improving image contrastat the expense of image resolution.

frame rate (us) Number of complete scannedimages per second. (image) The number ofcomplete matrices completed per second.

frame relay (comp) A high-speed, low-latencypacket switching technology, based on aswitched virtual network topology; popular forLAN-to-LAN connections (see FDDI).

Fraunhofer zone (us) That region of the field inwhich the acoustic energy flow proceedsessentially as though coming from a point

F

62( )

12( )

24


Fraunhofer zone–freeze frame

F

source located in the vicinity of the transducer

assembly. (For an unfocused transducer assembly,the far field is commonly at a distance greaterthan S/πλ, where S is the radiating cross-sectional

area and λ is the acoustic wavelength in themedium.) The far field divergence related towavelength and transducer diameter as:

sin .θλ

� �0 612r

⎛⎝⎜⎜⎜

⎞⎠⎟⎟⎟

RF pulse on the receiver. Signal is induced bythe RF excitation of the nuclear spins and thatdecreases exponentially without external influ-ence at a characteristic time constant T2*.

free radicals (phys) An atom or group of atomsthat have unpaired valency electrons. They areextremely reactive and are responsible for mostcellular damage (DNA, cell membranes) causedby radiation exposure. Ionization is defined asthe formation of free radicals and does not applyto simple dissociation of molecules into ionsseen in other non-radiation events (e.g. NaCl →Na� and Cl�). Water molecules, the most com-mon constituent of tissue, enter a state of exci-tation, forming free radicals (Ho and OHo). Theseare highly reactive and are responsible for indi-rect protein damage. Ionization of the water canoccur (H2O → H� � OH�). Both indirect anddirect reactions can lead to self-perpetuatingchain reactions. A chain reaction may occurcausing further damage in adjacent molecules.Radiation damage to living systems occursalmost exclusively by free-radical productionand not by the direct ionizing event.

Fresnel zone

Fraunhofer zone

0.5 cmθd

(see Fresnel zone).FRE (mri) Field reversal echo (see GRE).free electron (phys) See electron (free).free induction decay (FID) (mri) The signal

induced by an RF excitation of the nuclearspins; decreases exponentially without externalinfluence at a characteristic time constant T2*.When transverse magnetization, Mxy, is pro-duced (e.g. 90° RF pulse), a transient MR signalwill be produced decaying toward equilibriumMo with a characteristic time constant T2 orT2*. In practice, the first part of the FID is notobservable due to residual effects of the exciting

Direct and indirect damageDirect damage where R � protein molecule

1 RH* → Ro � Ho

2 Ro � O2→ ROo2 (peroxy radical)

3 ROo2 � RH→ RO2H�Ro (return to start of 2)

Indirect damage: (the radiolysis of water)

4 H2O*→ H2O� � e�

5 H2O � e�→ H2O�

6 H2O�→ H� � OHo

7 H2O�→ Ho � OH�

The ions OH� and H� are removed since they recom-bine to form water:

H� � OH� H2O.

Ho and OHo have unpaired electrons so are free radi-cals and extract hydrogen from organic molecules:

8 RH � OHo→ Ro � H2O9 RH � Ho→ Ro � H2

which joins the chain reaction in (1) above.

FID

37% T2*

Time

Mx,y

FREEZE (mri) Respiratory selection of phase-encoding steps, Elscint. Respiratory orderedphase encoding (see RESCOMP, RSPE, PEAR).

freeze frame (image) An ability to hold in storageand display a single image frame from a seriesof frames.

F


frequency (Hz)–fringe field (magnetic)

frequency (Hz) (phys) For electromagnetic wave-forms, the relationship between frequency f,wavelength λ and velocity c, is:

f � c/λ

c � fλ

λ � c/f.

(us) Measured in cycles per second. Typicalultrasound values 2–10 MHz.

Frequency (MHz) Wavelength (mm)

2.0 0.743.5 0.425.0 0.307.5 0.20

10.0 0.15

frequency domain (math) When a periodic sig-nal is broken down into its frequency compo-nents (e.g. MR spectroscopy), the signal existsin the frequency domain (see Fourier transform).

frequency encoding (mri) Encoding MR signalsusing a steady magnetic field gradient. Withoutother position encoding (phase), the Fourier

transform is a projection profile of the object.During data acquisition, a gradient is applied inone spatial direction, giving nuclear spins withlinearly increasing precession frequencies. The MRsignal is then a mix of frequencies which mustbe filtered individually. In the row direction, thelocation of the nuclear spin can be reconstructedfrom the frequency; this is the frequency encod-

ing axis. The perpendicular axis is the phase-encoding axis.

frequency offset (mri) Difference between thegiven signal frequency and a reference frequency.

frequency response (image) The output signalspectrum is the product of the input frequencyspectrum and the system’s gain together with itsphase shift. The impulse response characterizes asystem in the time domain; the frequency responsecharacterizes a system in the frequency domain.

frequency response function (math) Describinga circuit in the frequency domain by applyingan arbitrary frequency input and measuringthe Fourier transform of its output. The outputsignal spectrum is the product of the input fre-quency spectrum and the system’s gain, togetherwith its phase shift. The impulse responsecharacterizes a system in time domain; the fre-quency response characterizes a system in the frequency domain. The identified frequency

component or frequency response functionuniquely defines the system (amplifier, filter) in the frequency domain (see impulse response

function).frequency selective RF pulse (mri) An RF pulse

having energy only within a specified fre-quency range. Commonly used for slice excita-tion or for selective saturation pulses.

frequency tuning (mri) The RF system fre-quency is set to the resonant frequency of tissuein the main magnetic field (Larmor frequency).

Fresnel zone (us) or ‘near zone’. This dimensionis dependent on transducer diameter d andaperture as:

The length of the Fresnel zone is proportionalto the square of the transducer diameter andinversely proportional to wavelength; increas-ing transducer frequency extends the Fresnelzone. Also the Fresnel zone length depends on

where α is half width (or radius) of the trans-ducer. Diversion angle (Fraunhofer zone) dependson sin�1(0.61λ/α).

freeze frame (di) An ability to hold in storage anddisplay a single image frame from a series offrames.

frequency (Hz) (us) Cycles per second. Typicalultrasound values 2–10 MHz.

frequency spectrum (us) The range of frequen-cies present. In a Doppler instrument, the rangeof Doppler shift frequencies present in thereturning echoes.

friction (phys) Energy loss commonly seen as anincrease in system heat. Since friction alwaysopposes motion, a moving body experiences africtional force. This is not confined to solids; itis also experienced by fluids and gases causedby viscous drag between layers of molecules.

FRF (mri) Field reversal echo.fringe field (magnetic) (mri) Region surrounding

a magnet. The earth’s magnetic fringe field istypically 0.05–0.1 mT. Due to the physicalproperties of magnetic fields, they form closedfield lines. Depending on the magnet construc-tion, the returning flux will penetrate largeopen spaces (unshielded magnets) or will be

αλ

2

d24λ

.

full-width-at-tenth-maximum (FWTM) (phys)Sometimes used as an additional measurementwith the FWHM to measure the degree of scat-tered radiation entering the photopeak or, inthe case of spatial resolution, the collimatorpenetration or light diffusion within the scintil-lation detector.

function (math) A variable that can take a set of val-ues, each of which is associated with the value ofan independent variable or variables. The nota-tion f(x) � y reads that y is a function of x.

functional groups (nmed) Small group of linkedatoms with chemically active bonds. These areimportant in nuclear medicine labelling reactions.

functional imaging (mri) See fMRI.fuse (elec) A short length of wire acting as a cir-

cuit weak-link which breaks if excessive cur-rent is present that could damage electricalequipment. Wire fuses have a relatively longdead time and are not suitable for protectionagainst fast pulsatile current surges.

fusion (nuclear) (phys) See nuclear fusion.fuzzy logic (comp) A method of analyzing a data

set which considers the probability of setmembership. A measurement that cannot bedefined precisely but is judged relatively, e.g.hot/cold, loud/quiet. The term was coined in1965 by Lofti Zadeh (USA). In radiology, it findsa use for controlling x-ray tube loadability anddose measurement in CT or fluoroscopy.

FWHM (nmed) See full-width-half-maximum.


fringe field (magnetic)–FWHM

F

confined largely to iron yokes or through sec-ondary coils (shielded magnets).

Frost, Edwin Brant American professor ofastronomy asked by his brother (a clinician) tocarry out the first radiograph on a patient onFebruary 3rd 1896 (see Campbell-Swinton).

FS (mri) Fast scan.FSE (mri) See Fast spin echo.F-SHORT (mri) Steady-state gradient echo with

spin echo sampling. Short-repetition techniquebased on free induction decay (Elscint).Rewinding of phase-encoding and no intentionalspoiling (see GRASS, FGR, FISP, FAST, GFEC, SSFP).

FSPGR (mri) Fast spoiled gradient recalled.Gradient-echo imaging techniques T1-weighted contrast (GE) (see FLASH, SPGR, HFGR,RE spoiled, 3D-ME-RAGE, T1-FEE, STAGE-T1W).

FTP (comp) File transfer protocol. A means of trans-ferring files from one computer to another acrossthe Internet, and one of the principal tools that isavailable on the Internet (the three other keyfunctions are e-mail, news groups and the web).

full-scan interpolation (ct) Synonymous with360° interpolation.

full-wave rectifier (elec) The preferable method forrectifying an a.c. waveform since both halves ofthe cycle contribute to the d.c. output. Full waverectification applies to single phase (�4 rectifiers)and three-phase (�12 rectifiers) supplies.

full-width-at-half-maximum (FWHM) (phys) Acommonly used measure for resolution of peakevents such as point spread function in visualimage separation, energy resolution measuredon the photopeak of a scintillation detector, NMRspectral lines and slice profiles in tomography.(ct) Distance on the abscissa of a 1D or 2D distri-bution between the points where the functionreaches one half of its maximum value.

Profile distance

1.0

0.0

0.1

0.5

Sig

nal s

tren

gth

FWHM

FWTM

G


Gx, Gy, Gz–67Gallium

GGx, Gy, Gz (mri) Conventional symbols for mag-

netic field gradient. Used with subscripts todenote spatial direction component of gradient,i.e. direction along which the field changes.

gadobuterol (cm) Generic name for Gd-DO3A-buterol and paramagnetic magnetic resonanceimaging (MRI) contrast agent (see Gadovist®).

gadolinium (Gd) (elem) Lanthanum element

Atomic number (Z) 64Relative atomic mass (Ar) 157.25Density (ρ) kg/m3 7900Melting point (K) 1585K-edge (keV) 50.2Relevance to radiology: As rare-earth scintillators forintensifying screens. Gd2O2S:Tb. MR contrast agents ina chelated form, such as DTPA, because of its effect ofstrongly decreasing the TI relaxation times.

153Gadolinium

Nuclear data Emission

Half-life 242 dDecay mode β�

Decay constant 0.0028 day�1

Photon (abundance) 97 (0.40)103 (0.59)

As a dual energy source for bone mineral estimationand for attenuation correction fitted to gammacameras.

gadolinium contrast medium (mri) Lanthanideelement used in its trivalent state. Used as theactive component of some paramagnetic con-trast agents; strongly decreases the TI relax-ation times of the tissues to which it hasaccess. It is given in a chelated form, such asDTPA (see contrast agent (MRI)).

gadolinium oxy-orthosilicate (GSO) (phys)Cerium-activated gadolinium oxyorthosilicate(Gd2OSiO4:Ce or GSO). This scintillator gives abetter energy resolution and higher light out-put than LSO. It also has better attenuation for511 keV photons; its light decay time is slightlyhigher.

gadolinium phosphor (xray) Gadolinium com-pounds used in the manufacture of rare earthintensifying screens in the form of Gd2O2S:Tb;these emit light in the green spectrum (seeintensifying screen).

gadoteridol (clin) Generic name for magnetic res-onance image (MRI) contrast agent Prohance®.

Gadovist® (cm) Commercial (Schering) prepara-tion of gadobutrol Gd-BT-DO3A, a paramag-netic contrast agent.

Compound Concentration Osmolality (mg mL�1) (mosm/kg)

Gadobutrol 0.5 and 1.0 mol/L 557 and 1603(Gd-BT-DO3A)

gain (elec) Increasing amplification by eitherpotentiometric adjustment or by altering feed-back. Certain charge amplifiers have unity gain,merely acting as impedance matching devices;ratio of amplifier output to input electric power.(us) Ratio of amplifier output to input electricpower.

gain control (us) The signal amplitude changedby either altering the amplitude of the trans-mitted pulse or the gain of the receiving cir-cuits. The first method increases transmittedpower and image depth, the second methodincreases displayed noise levels.

gallium (Ga) (elem)

Atomic number (Z) 31Relative atomic mass (Ar) 69.72Density (ρ) kg/m3 5950Melting point (K) 302.9K-edge (keV) 10.36Relevance to radiology: As a low melting point alloylubricating x-ray tube bearings.

67Gallium (tumour imaging)

ProductionDecay scheme (e.c.) 67Ga 67Ga (γ 93 185 300 keV) →

67Zn stablePhotons (abundance) 91 (0.032)

93 (0.376)185 (0.212)209 (0.024)300 (0.168)394 (0.047)

Half-life 78 hoursDecay constant 0.00885 h�1

Gamma ray constant 2.2 � 10�2 mSv hr�1

GBq�1 at 1 mHalf value layer 0.7 mm Pb, 47 mm waterRadiation attenuation factors for lad (67Ga).

Pb (mm) Attenuation factor

0.66 0.54.10 10�1

12.0 10�2

25.0 10�3

10�4

3068

3167Zn p,2n Ga( )


67Gallium–gamma spectrum

G

Hours Fraction remaining

0 1.006 0.95

12 0.9024 0.8136 0.7348 0.6560 0.5972 0.5384 0.4896 0.43

120 0.35144 0.28168 0.23

67Gallium citrate (nmed) Binds to the intracellu-lar lactoferrin of leukocytes and is transportedto infection sites where it is deposited whenthe leukocytes excrete some of their lactoferrin.Gallium is believed to localize in inflammatorylesions by diffusing across ‘leaky’ capillariesinto the extracellular space and binding to iron-binding proteins. An iron analogue that reactswith iron-binding proteins including:

● transferrin: a primary transport protein for 67Gawithin the circulatory system;

● lactoferrin: stored within specific leukocyte gran-ules and is released by the leukocytes at sites ofinflammation;

● ferritin: an intracellular protein, which medi-ates uptake of iron/gallium within bacteria;

● siderophores: low molecular weight compoundsproduced by bacteria that also mediate uptakeof iron/gallium within bacteria.

68Gallium (nmed) Generator derived, it decays bypositron emission and is used for positronemission tomography.

Production (generator) 68Ge/68Ga

Decay scheme (β�) 68Ga 68Ga (β� 2.9 MeV, 2γ 511 keV) → 68Zn stable

Eluent 0.005 M EDTAHalf-life 1.13 hoursHalf value layer 4.1 mm Pb (511 keV)

gamma (γ) (math) A symbol used to indicate:chemical shift, electrical conductivity, gyromagnetic

ratio, gamma photon, slope of film characteristiccurve (film gamma), mathematical function, Γ,and specific gamma ray constant (dose con-stant), Γ.

gamma camera (nmed) A position-sensitiveimaging device invented by Anger Hal O, thatdisplays the distribution of radioactivity within

a source or body. Uses a bank of photomulti-pliers behind a large thin NaI:Tl scintillationdetector to given positional information with theaid of collimation. It records the quantity anddistribution of photons emitted by the radioac-tive material in the area of interest. The princi-pal imaging device used in nuclear medicine.

gamma decay (phys) A nuclear decay processinvolving the emission of gamma radiationeither as a single or multiple event.

gamma fit (math) A method used for approximat-ing the shape of a dilution (time–activity) curve,so that mixed curves can be separated forquantitation.

gamma radiation (phys) Electromagnetic radia-tion emitted during gamma decay. The rangeextends to several MeV. The diagnostic imagingenergy range is roughly 80–200 keV.

gamma-ray dose rate constant (nmed) Thisconstant, which has the symbol F, representsthe radiation dose rate (mR/hr) present whenan unshielded 1 mCi source is positioned 1 cmfrom a detector. Each radionuclide has a char-acteristic F value (see specific gamma ray constant).

gamma spectrum (nm) For gamma energies�100 keV, the absorption in NaI(Tl) is predomi-nantly photoelectric. Iodine escape peak due toabsorption of photoelectron from iodine x-raysmainly from front surface of the crystal. Thephotopeak corresponds to absorption of photo-electrons from the gamma photoelectric event;depends on scintillator crystal dimensions. At �200 keV, most scintillation pulses occur in the photopeak. Between 200 keV and 1 MeVCompton events are more appreciable and the

Time

Area 2Area 1

Dilution curve

Act

ivity

G


gamma spectrum–Gastroview®

Compton edge, C, is visible. The maximumenergy when the photon is scattered through180° is then:

Compton peak � E2/(E � 256).

For a gamma energy of 662 keV (137Cs), thepeak of the Compton continuum would be447 keV. The backscatter peak is due to incom-plete absorption; no iodine escape peak is visi-ble. At energies greater than 1.02 MeV, pair

production becomes a prominent feature.

(see detector spatial resolution).gantry (ct) Scanner structure containing the x-ray

tube, collimators and the detector array.gantry aperture (ct) Diameter of the physical

opening of the gantry through which the patientis moved for the examination.

gantry rotation time (ct) The time for a full 360°rotation of the gantry around the patient table.Single-section helical CT scanners typically havea 360° gantry rotation speed of about one sec-ond. With multislice CT, gantry rotation speeds of0.4 or 0.5 s; twice as fast as that of single-sectionhelical CT (see g-force).

gantry tilt (ct) The angle between the verticalplane, and the fan beam assembly containingthe x-ray tube and the detector array. The tilt ofthe CT gantry with respect to the patient canbe altered typically up to 30°; the scan plane isrotated around the x-axis such that the axis ofrotation and the direction of table feed are notorthogonal.

GARP (mri) Globally optimized alternating phaserectangular pulse.

Photon energy (keV)662

Compton edge

Backscatter

Photopeak

FWHM

Rel

ativ

e in

tens

ity

137Cesium

gas ionization detector (ct) A detector design forCT scanners, employing vessels filled with noblegas under high pressure (xenon). For multisliceCT machines, gas detectors are not suitable,especially in terms of overall spatial resolution.Their major limitation is the low quantum detec-tion efficiency of �50%, caused primarily by lowx-ray absorption in the xenon gas and by theabsorption of x-rays in the thick container hous-ing the pressurized gas (see Geiger counter, pro-

portional counter, ionization chamber).gas laws (phys) The variation of pressure, volume

and temperature on a gas, Boyle’s law (RobertBoyle 1627–91, Irish physicist) together withCharles’ law (JA Charles,1746–1823; Frenchphysicist) form basic relationships in physics andalso feature in the propagation of sound throughair. Boyle’s law states that pressure is inverselyproportional to volume at constant temperatureand Charles’ law states that volume and temper-ature are proportional at constant pressure.Summarizing these two statements gives:

PV/T � constant.

The gas laws play an important academic rolein the derivation of the SI scale for temperature(see equations of state).

gas multiplication (amplification) (phys) Aproperty of the Geiger counter where at reducedgas pressure, electrons released from the ioniz-ing event, can gain sufficient velocity under theinfluence of a high electrical field, to cause sec-ondary ionization by colliding with other gasatoms. This enables a large electrical signal tobe generated without electronic amplification.

Gastrografin® (cm) Commercial (Bracco) preparationof 66% meglumine diatrizoate and 10% sodiumdiatrizoate for the examination of the gastro-intestinal tract. Oral or rectal administration.


Meglumine 18.5 at 20°, 1940 370diatrizoate 8.9 at 37°

Gastromiro® (cm) Preparation containing612.4 mg mL�1 of iopamidol (Bracco) for inves-tigating the gastrointestinal tract.

Gastroview® (cm) Commercial preparation ofmeglumine diatrizoate 66% and 10% sodium diatrrozoate; gastrointestinal tract, osmolality2000 mOsm/kg iodine content 367 mg I mL�1

(Mallinckodt/Tyco Healthcare Inc).


gated acquisition–generator (radionuclide)

G

gated acquisition (xray) Image data acquisitionunder the control of a gating signal either elec-trocardiogram (ECG) or respiration. Linking phys-iological timing (cardiac, respiration) to dataacquisition in order to either freeze motion orcollect a series of temporal images (cardiac cycle:end diastole to end systole). Synchronization ofimaging with a phase of the cardiac or respira-tory cycles. A variety of means for detectingthese cycles can be used, such as the ECG,peripheral pulse, chest motion, etc. The synchro-nization can be prospective or retrospective.

gateway (comp) A network station used to inter-connect two or more dissimilar networks ordevices: may perform protocol conversion.

GATOR CIST (mri) Respiratory gated imaging.gauss (phys) G unit of magnetic flux in the

c.g.s. system (1 Tesl � 10 000 G) and 1 G cm�1 �

0.01 T m�1 (10 mT m�1).gaussian curve (math) See Gaussian distribution.gaussian distribution (stats) This describes the dis-

tribution of random events and is represented by:

where d is the width of the distribution meas-ured as the full width of the curve at half itsmaximum (FWHM). The gaussian distributiondescribes both energy and spatial resolution of adetector system, and approximates the spread offrequencies (bandwidth) of an RF or ultrasoundpulse. Other gaussian distributions occur indosimetry (see normal curve).

gaussian noise (image) Noise distributed in anormal (Gaussian) pattern. In such a distribution,approximately 65% of all points fall within onestandard deviation(s) of the mean.

1.0

0.5

0.0

0

Gaussian

y e x d

� � 2/

GBP (mri) Global bolus plot.GDI (image) Graphical device interface, the native

graphical language of Windows. A GDI-compliantprinter will print exactly what is displayed on theWindows screen without having to transpose itinto a printer language.

GE/GFE (mri) Gradient echo or gradient field echo,general sequence, Hitachi (see FFE, GRE, MPGR,GRECO, FE, PFI, Turbo-FLASH, TFF, SMASH, SHORT,STAGE).

Geiger, Hans W (1882–1945) German physicistwho developed a form of gas detector for count-ing alpha particles. Geiger and Marsden per-formed the original experiment with alphaparticles and gold foil that led Rutherford to pro-pose his model of the atom. He also demonstratedthat two alpha particles are emitted from uraniumduring decay. With Nuttall, he demonstrated therelationship between alpha particle range andecay constant known as the Geiger–Nuttall rule.He became professor at Kiel University where hedeveloped his particle counter with WaltherMüller to give the Geiger–Müller counter.

Geiger counter (phys, nmed) More correctly,Geiger–Müller counter used as an inexpensivecontamination radiation monitor. A higher volt-age supply encourages the ionization events(ions and electrons) to multiply and give a largesignal output; this multiplying effect prevents itbeing photon energy sensitive. A dampedGeiger counter is often calibrated in terms ofevents per second or dose rate (μSv hr�1) (seegas multiplication, ion chamber).

gelatin (film) The suspending medium for silverhalides in the emulsion.

gene (clin) A locality on the chromosome carryingspecific genetic information in the form of DNA.

generator (xray) The electrical system which pro-vides the high voltage (typically up to 150 kV)and low voltage (typically 8–12 V) which drivesthe x-ray tube. Present day generators are allhigh frequency designs with multiple feedbackcircuits controlling voltage, output and timing(see high frequency generator).

generator (radionuclide) (nmed) A device forgenerating clinically useful daughter productsfrom a long-lived parent, i.e. 99Mo/99mTc and81Rb/81mKr. Two types of generator exist com-mercially. Those showing:

● transient equilibrium, where the parent has a verymuch longer half-life than the daughter product;

G


generator (radionuclide)–geometric mean

● secular equilibrium where the parent has only asomewhat longer half-life than the daughterproduct (see individual generator types99Mo/99mTc, Rb/Kr, etc.).

generator (shielding) (nmed) Radiation levelsfrom a medium-size technetium generatorwould be an unshielded core of 18.5 GBq(500 mCi) generator 6 mSv h�1 at 0.5 m. For7.5 μSv h�1, this would require 6.8 cm Pb (HVLfor 99Mo, 740 keV γ 7 mm Pb). A typical shieldedcore with 5.0 cm Pb, additional lead shielding4.5 cm Pb, total lead shielding 9.5 cm Pb. Thetotal shielding supplied by the manufacturertypically exceeds the minimum levels.

genetically significant dose (GSD) (dose) Thedose that, if given to every member of a popu-lation, should produce the same hereditarydamage as the actual doses received by thegonads of an individual receiving radiation. It iscalculated as:

where Nxy is the number of patients in agegroups x undergoing patient examination y; Pxy

is child expectancy for persons in age group xundergoing examination y; Dxy is the averagegonad dose for patients in age group x under-going examination y; Nx is the number of peo-ple of age group x in the population; Px is thechild expectancy for age group x. The sub-scripts m and f denote male and female sub-sets. The GSD value has steadily declined overthe years, some values are:

Sweden 0.46 mSvGermany 0.41 mSvUSA 0.20 mSvJapan 0.17 mSvUK 0.12 mSv

Natural radiation delivers a GSD of about 1 mSv.Medical values are primarily an indicator of thecare with which the reproductive organs areprotected in medical procedures and theamount of radiography of pregnant women andchildren that is done in the country.

genetic effects (nmed) Radiation effects inducedin the offspring of irradiated persons (or ani-mals), if conception occurs after exposure (seesomatic effects).

genome (clin) Basic haploid set of chromosomes.

Σ

Σ

[ ] [ ]N P D N P D

N Pxy xy xy m xy xy xy f

x x

�

genotype (clin) Genetic constitution of an organism.

geometric dose efficiency (ct) The quantity ofradiation striking the detector as a fraction ofthe total radiation dose leaving the patient. Inpractice, the geometrical efficiency is less than100% mainly due to post-patient collimationand due to inactive areas (dead space) betweenadjacent detector channels (the septa); espe-cially with multi-row detectors, the geometricalefficiency is an important parameter of the sys-tem (see dose efficiency).

geometric efficiency (Geff) (ct) A measure of thebest dose utilization in the z-axis, as the ratio ofthe imaged slice section profile (SSP) relative tothe z-axis slice dose profile (SDP). High geometricefficiency is a requirement for multi-slice detec-

tors. Dead spaces are as small as possibleAntiscatter collimators should be as thin as possi-ble. Geometric efficiency should be 100%, butit is often less, especially for narrow beam col-limations where post-patient collimation maybe necessary to bring the imaged slice thick-ness closer to the chosen value. Values of80–90% are typical. In arrays with a finer sep-aration of detector elements in the z direction, areduction in geometric efficiency is seen. Thegeometric efficiency (Geff) of the detector arraycan be calculated by comparing the FWHM ofthe SDP with the SSP:

If the SSP and SDP dimensions are identical, asin the case of sequential CT, the geometric effi-ciency is 100%. For single slice and multislicespiral CT, the geometric efficiency can fall as lowas 60%. As the number of slices increases, theexcess dose not contributing to the image(‘wasted dose’) becomes less. The value for aquad slice scanner having 1.25 mm slice thick-ness is about 64%, whereas an eight-slicescanner with the same slice width gives a fig-ure closer to 76%.

geometric mean (stats) The average value of aset of n numbers expressed as the nth root oftheir product. Thus the geometric mean of nnumbers x1, x2, . . . xn is:

(see mean, average).

x x xnn

1 2� �… .

Geff � �FWHM (SSP)FWHM (SDP)

100%.


geometric mean–GIF

G

Geometric meanIf the annual percentage change in patient workloadfor a particular examination for 5 years is: 15.8, 8.3,13.4, 18.0, 11.9%, then the overall scaled effect is:

1.158 � 1.083 � 1.134 � 1.180 � 1.119 � 1.878.

So the average patient workload increases by

1 878 1 134 13 4 1 134 13 45 . . . %. . . %.� � or or

germinal tissue (dose) Tissue associated with thegonads in which sperm or ova are developing.

GFE (mri) Gradient field echo, Hitachi. Term forany GRE sequence.

GFEC (mri) Gradient field echo with contrast,Hitachi. Enhanced intensity; rewinding ofphase-encoding and no intentional spoiling(see GRASS, FGR, FISP, FAST, F-SHORT, SSFP).

g-force (units) Both centripetal and centrifugalforce use this value, F � mv2/r, where m ismass, v velocity and r the radius of rotation.geometrical resolution (ct) The ability to display

small structures of high contrast (line pair grat-ing). The geometrical resolution can be quanti-tatively described by means of the limitingspatial frequency (as line pairs), the point spread

function, the edge spread function, the line spread

function and the modulation transfer function.geometrical resolution limit (ct) Resolution limit

for high contrast, expressed as the minimumsize of an object detail that can be resolved (seeline pairs).

geometric unsharpness (xray) See unsharpness

(geometric).geometry (detector) (math) Relating to detector

surfaces 2π describes a flat surface, e.g. agamma camera which has a 180° acceptanceangle; a 4π detector describes a volume devicewith a 360° or spherical acceptance, e.g. dosecalibrator.

germ cell (clin) Pertaining to the gametes: sperm,ovum or a cell from which they originate.

germanium (Ge) (elem) Non metallic semicon-ductor.

Atomic number (Z) 32Relative atomic mass (Ar) 72.59Density (ρ) kg/m3 5400Melting point (K) 1210.5K-edge (keV) 11.1Relevance to radiology: high resolution volumesemiconductor detectors as hyper-pure germanium(HpGe).

68Germanium (nmed) As a long-lived calibrationsource for PET scanners:

Production (cyclotron)

Decay scheme (β�) 68Ge 68Ge (electron capture) →68Ga (see 68Ga decay scheme)

Half-life 270 daysHalf value layer 4.1 mm Pb (511 keV γ)

or Ga p n Ge3169

32682( , )

3066

32682Zn n Ge( , )α

G-forceThe forces on an x-ray tube and HF generator of 150 kg having a rotation radius of 0.6 m and rotation time of 0.5 s. The velocity is therefore2π �0.6 �2 �7.54 m s�1 and the force (F) is

150 7 540 6

142132�

�.

..N

ghosting (mri) During periodic movement (respira-tion), some phase-encoding steps are acquiredduring inspiration, and others during expiration.This misregistration gives a displaced falseimage of the body region. The distance betweenghosting images determined by movementperiod and relaxation time, TR.

Gibbs’ artefact (image) Truncation artefact due toGibbs’ phenomenon.

Gibbs’ phenomenon (image) Ripples in the cal-culated value that occur near a discontinuitywhen reconstructing a mathematical functionfrom a limited portion of its Fourier transform. InMR imaging, it is visible as linear artefacts par-allel to sharp edges in the object, particularlywith the use of zero filling. CT images showpoor delineation and black spaces betweenbone and soft tissue (i.e. the skull) (see truncation

artefact).

GIF (comp) Graphics interchange format. A rastergraphics format developed to handle 8-bit (256)colour with high compression ratios. Combines

Gibbs phenomenon

Truncation

G


GIF–GMN

palette-based indexing and LZW compression.Contains a maximum of 256 colours taking fulladvantage of the ability to store images withfewer colours at lower bit-depths. Since GIFrestricts the number of colours, the typical flat-coloured GIF image is more suitable for LZWcompression. Apart from its minimized file size,GIF offers a number of other advantages.Interlacing means all even scan lines are storedfirst and then odd rows are split into three sets;the image appears in four passes with a‘Venetian blind’ effect, so a viewer can graspthe essence of the image after only 50% of thedata has downloaded. The 256-colour limit andLZW compression result in both poor imagequality and large files. JPEG was developed bythe Joint Photographic Experts Group to solvethis problem.

gigabyte (GB) (comp) 1024 megabytes.gigabit Ethernet (comp) an extension of the 10

and 100 Mbps (megabits per second) ethernetstandards that describes 1000 Mbps transmis-sion speeds. Originally required fibreoptics, butthe specification now supports these speedsover copper cable.

GINSEST (mri) Generalized interferography usingspin echoes and stimulated echoes.

glare (x-ray) See veiling glare.glass crown (mat)

Density (ρ) kg m�3 2600Melting point (K) 1400 K

glass flint (mat)

Density (ρ) kg m�3 4200Melting point (K) 1500 K

glass (lead) (xray) Density 4800 kg m�3 Pbcontent 48%; Ba content 15%.

Approximate Pb equivalent Weight thickness (mm) (mm Pb) (kg m�2)

6.5 1.5 358.5 2.1 45

10 2.5 5013 3.3 65

Global bolus plot (mri) Global time–density curve(GBP).

global maximum (us) The greatest value of aquantity evaluated over all times, over all loca-tions, and over all operating conditions for anygiven operating mode.

global shim (mri) Fat saturation, EPI, or spec-troscopy require high magnetic field homo-geneity where shim coils are used to optimizehomogeneity.

Glofil-125® (nmed) Commercial preparation ofsodium 125I Iothalamate (QOL Medical Inc).

glow curve (dose) The important properties of apractical thermoluminescent detector are thephoton energy response and allowing lightemission at about 200°C forming a glow-curvein or near the blue region of the spectrum. Thelight output is proportional to radiation exposure.

gluceptate/gluconate (nmed) As labelled 99mTc-gluceptate (99mTc-D-glycero-D-glucoheptonate asthe calcium salt (calcium glucoheptonate/gluconate) used for scintigraphy of kidney func-tion and cerebral perfusion. 99mTc-gluconate alsotends to accumulate in intracranial lesions. Renalretention is greater in the cortex than the medulla.

Generic name 99mTc-gluceptateGluceptate Draximage®

(Mallinckrodt)Glucoheptonate TechneScan®Gluconate Amerscan®Imaging category Renal function and

brain perfusion

gluconate (nmed) See gluceptate.GMC (mri) Gradient moment compensation.

Instrumentarium FC, flow compensation, GEPhilips. Reduction of motion-induced phaseshifts during TE (see GMR, GMN, FLOW-COMP,CFAST, MAST, FLAG, FC, STILL, SMART, GR).

GMN (mri) Gradient moment nulling GE. Reductionof motion-induced phase shifts during TE

50 100 150 200

Temperature (°C)

TLD Glow-curveLi

ght o

utpu

t (re

lativ

e)

250 300 350


GMN–gradient echo (GRE)

G

(see GMR, FLOW-COMP, CFAST, MAST, FLAG, GMC,FC, STILL, SMART, GR).

GMR (mri) Gradient motion rephasing, Siemens.Reduction of motion-induced phase shifts dur-ing TE (see GMN, FLOW-COMP, CFAST, MAST,FLAG, GMC, FC, STILL, SMART, GR).

Golay coil (mri) Term commonly used for a partic-ular kind of gradient coil. Commonly used tocreate magnetic field gradients perpendicularto the main magnetic field.

gold (Au) (elem)

Atomic number (Z) 79Relative atomic mass (Ar) 196.97Density (ρ) kg/m3 19 300Melting point (K) 1336.1Specific heat capacity, J kg�1K�1 128Thermal conductivity, W m�1K�1 317K-edge (keV) 80.7

195mAu (nmed) Daughter product of 195mHg/generator. Used for vascular imaging giving asteady-state picture of circulation.

Production (cyclotron)Decay scheme (i.t.) 195mAu 195mHg T½ 41.6 h

(γ keV) → 195mAuEluent Sodium thiosulphate/

sodium nitrateHalf-life 30.5 sHalf value layer mm Pb

198Au (nmed) Used for intracavity therapy.

Decay scheme (β�) 198Au 198Au (β � 1.37 MeV) → 198PtHalf-life 2.7 daysDecay mode β� 0.961 MeVDecay constant 0.256 year�1

Photons (abundance) 412 keV

goodness of fit (stats) How well a particular set ofdata fits a given relationship or distribution.The goodness of fit may be measured by usinga chi-squared test.

GPF (comp) General protection fault. A Windowswarning that a protected part of RAM is beingaccessed. Caused by a program trying to accessan area of memory being used either by anotherprogram or the operating system.

GPRS (comp) General packet radio services.Provides packet-switched data radio technologyfor GSM networks. Being always on, GPRS con-nections offer mobile users network availabilitycomparable to that of a corporate network.

GPU (comp) Graphics processing unit. Graphicscards with GPUs handle a great deal of the

79197

80195Au p,3n Hgm( )

resource-intensive functions that are tradition-ally handled by the main processor. Image pro-cessing is an example.

GR or GRE (mri) Gradient rephasing, Hitachi.Reduction of motion-induced phase shifts dur-ing TE (see GMR, GMN, FLOW-COMP, CFAST,MAST, FLAG, GMC, FC, STILL, SMART).

gradient (mri) The magnitude and rate of changein space of a magnetic field strength seen inmagnetic gradients of an MRI machine.

gradient coils (mri) These supply magnetic gradi-ents which define the strength and direction ofthe magnetic field in the field of view. Gives alinear change in the magnetic field in a specificdirection and determines the spatial orientationand resolution of the image. Magnetic gradientsare defined by rise time, duty cycle, gradient linearity,gradient strength and slew rate. Gradient coils oper-ate in pairs in the x-, y- and z-axis, having iden-tical properties, but opposite polarities. Each coilincreases the magnetic field by a specific frac-tion; the opposing coil reduces the field by thesame amount. This provides the linear gradient.

gradient echo (GRE) (or gradient recalled echo orfield echo) (mri) The formation of an echo by theapplication of a single excitation pulse followedby switching of the gradient polarity; echo sig-nal generated by a magnetic gradient fieldreversal instead of the 180° pulse in spin echo).The method chosen for fast imaging, but doesnot refocus field inhomogeneity. The gradientecho is generally adjusted to be coincident withthe RF spin echo. When the RF and gradientechoes are not coincident, the time of the gradi-ent echo is denoted TE and the difference intime between the echoes is TD, while TER refersto the time of the RF spin echo.The method ischosen for fast imaging but does not refocusfield inhomogeneity. Gradient echo images aresensitive to static field perturbations.

FEG frequencyencoding gradient

RF pulse

Echo

�

G


gradient field reversal–GRASS

gradient field reversal (mri) Switching gradientpolarity.

gradient magnetic field (mri) See magnetic field

gradient.gradient moment nulling (mri) Adjustment to

zero at the time TE of the net moments of theamplitude of the waveform of the magnetic fieldgradients with time. The aim is to minimize thephase shifts acquired by the transverse magne-tization of excited nuclei moving along the gra-dients, including the effect of refocusing RFpulses, and reduces image artefacts due tomotion.

gradient motion (mri) See flow compensation,rephasing.

gradient pulse (mri) The magnetic field gradientbriefly switched. This is used in gradient echo

pulse sequences.gradient recalled echo (mri) See gradient echo.gradient strength (mri) Magnitude and rate of

change in space of magnetic field strengthseen in magnetic gradients of an MRI system.Unit is mT m�1 (milliT per metre). Typically,20–35 mT m�1 for a 1.5 tesla machine and45 mT m�1 for a 3 T machine (see slew rate).

gradient swap (mri) Exchange of phase-encodingand readout directions in the image. Flow andmotion artefacts are rotated 90°. Prevents arte-facts.

grain (film) The film emulsion is composed of arandom distribution of silver halide crystals ingelatine. Light or x-ray photons expose this ran-dom distribution. Although the photons are ran-domly distributed (even in a structured image),they encounter a much greater grain randomic-ity leading to quantum error. On development,the silver halide is transformed into silverspecks or grains which give the structural prop-erty. Low photon images yield ‘grainy’ imageswith a high quantum error or mottle. Fine-grainemulsion is typically slower and has a largerlatitude or dynamic range than coarser grainedemulsion. Film grain size limits the enlargementor magnification of a film image. High developertemperatures increase image graininess.

gram (phys) Fundamental unit of mass in thec.g.s. system. In SI units, the kilogram (1000 g)is the base unit.

graph (math) A data plot depicting the relationshipbetween certain sets of numbers or quantitiesby means of a series of dots, lines, etc. Whenempirical data are plotted, the points are not

necessarily joined; a best fit may be drawnthrough them. A Cartesian graph has two uni-form scales on perpendicular axes x and y. Thex-axis is usually for the independent variable,the y-axis for the dependent variable. Linear andlogarithmic scales can be used. Polar graphs arecentred on a particular point; the size of the vari-able depends on its direction; they are used forsound or RF intensity over distance. Normal dis-tributions of statistical data can be plotted onprobability graph paper.

graphics adapter/card (comp) An expansionboard that converts images created by the cen-tral processing unit (CPU) into video signalsrequired by the monitor. It determines the max-imum resolution, refresh rate and colour depth.The VGA card is the basic graphics adapter. Theon-board graphics memory must be capable ofhandling specified resolution, etc. For a resolu-tion of 1024 �768 and a colour depth of 32bits this requires a minimum of:

or 4 MByte installed. Typical RAM installed is16–32 MByte, which will cater for 2000 �1500resolution 32-bit colour, a refresh rate ofbetween 75 and 100 Hz and bandwidths of250 MHz. Often incorporates a graphics accel-erator (AGP) (see AGP, VGA, SVGA, XVGA).

graphite (chem) An allotrope of carbon used asan alternative to tungsten as a backing materialfor x-ray tube anodes. It increases their rating(loadability) without increasing the anodeweight; offers a large increase in surface areaand mass.

Graphite Tungsten

Melting point 3500°C 3650Density 2300 kg m�3 19 320Thermal conductivity 5.0 W m�1K�1 1.78Specific heat capacity 710 J kg�1K�1

GRASE (mri) Gradient and spin echo hybrid. Fastimaging technique using multiple gradient andspin echoes. Both gradient-echo and spin-echotechniques are combined to acquire multiplelines in k-space during measurement followinga single spin-echo excitation.

GRASS (mri) Gradient recalled acquisition in thesteady state (GE Medical Systems). Enhancedintensity, rewinding of phase-encoding and no

1024 768 328 1024

3072� �

�� kBytes


GRASS–grid selectivity (Σ)

G

intentional spoiling (see FGR, FISP, FAST, GFEC, F-

SHORT, SSFP, FLASH).grating lobes (us) Side lobes produced by a

multi-element transducer. Grating lobes.Additional weaker beams of sound travellingout in directions different from the primarybeam as a result of the multi-element structureof transducer arrays.

gravity (g) (units) The standard value9.8062 m s�2. This is 9.78 m s�2 at the equatorand 9.832 m s�2 at the north pole (see g-force).

Gray LH (1905–65) British physicist who devel-oped and rationalized radiation protection. TheSI unit for absorbed dose is the gray (Gy).

gray (Gy) (dose) An SI unit of radiation absorbeddose equal to 1 joule of absorbed energy per kgof absorber. Measure of absorbed dose 1 J kg�1. Itreplaces the rad; 100 rad �1 Gy; 1 rad �10 mGyor 1 cGy (see sievert).

grey level (image) The value associated with apixel in a digital image, representing the bright-ness in the original scene at the point repre-sented by that pixel. It is inversely proportionalto the degree of blackening at the correspon-ding point (a low degree of film optical density

corresponds to a high value of grey level).grey scale (image) The number of steps between

extremes of white and black used in image for-mation. A typical computer display is repre-sented by a pixel depth of 8 bits representing28 or 256 different grey levels. A good qualityphotographic print can reproduce 20–30 greylevels, the human eye can discern about 35and a photographic negative transparencyabout 40–50. A computer video display unit(VDU) typically registers 10–15 levels (see colour

scale, transfer function).GRE (mri) Gradient-recalled echo or gradient

echo, general sequence, GE, Siemens. Allowsfaster imaging than spin-echo. Gives T2* infor-mation (see FFE, MPGR, GRECO, FE, PFI, GE, Turbo-

FLASH, TFF, SMASH, SHORT, STAGE).GREAT (mri) Ghost reduction by equalized acqui-

sition triplets.GREC (mri) Gradient field echo with contrast.GRECO (mri) Gradient-recalled echo, general

sequence, Resonex (see FFE, GRE, MPGR, FE, PFI,GE, Turbo-FLASH, TFF, SMASH, SHORT, STAGE).

grid (xray) A device which is positioned close tothe entrance surface of an image receptor toreduce the quantity of scattered radiationreaching the receptor (see anti-scatter grid).

grid control (xray) An x-ray tube whose cathodeassembly receives a separate negative voltagewhich when applied switches off the beam.This is a method of achieving very fast (�1 ms)beam switching with very little inertia. Can alsodynamically alter the focal size or, if the nega-tive charge is large enough, switch off the elec-tron beam entirely. The grid controlled x-raytube is used in cine-fluorography, digital sub-traction angiography (DSA) units and computedtomography (CT) where rapid pulses of x-raysare required.

grid factor (xray) Also known as the grid expo-sure factor or Bucky factor. It is calculated as:

Using the same kVp, if the two exposures givingthe same film density are 30 mAs (with) and10 mAs (without), then the grid factor would be3. The grid factor changes with kVp, thicknessof lead septa and the type of interspace mat-erial. Mammography grids have the lowest fac-tors (2–3), while high kV (chest radiography)have the highest factors (4–6).

grid line density (xray) In an anti-scatter grid, thisis measured as the number of lead strips per cm.This is calculated as 10/(d �D), where d and Dare thickness in mm, of the lead septa and inter-space material, respectively. A superfine gridused for chest radiography (d �0.025 mm andD �0.05 mm) would have 133 lines per cm. Astandard grid (d �0.05 mm, D �0.1 mm) wouldhave a density of 66 lines per cm.

grid ratio (xray) Ratio of grid height h to inter-space distance D as h/D. For three grid typesPb6/28, Pb12/44 and Pb12/100, completespecifications are:

L/mm Ratio Factor Selectivity

28 6 4.1 4.544 12 5.2 6.6100 12 6.2 8.4

grid selectivity (Σ) (xray) This parameter isinfluenced by both grid ratio and lead septathickness, so considers all three measurementsof spacer thickness, lead septa height andthickness. It measures transmitted scatter Ts asa percentage of the primary beam Tp reachingthe imaging surface: Σ � Tp/Ts. A perfect anti-scatter grid would stop all scatter and pass all

Exposure with gridExposure without grid

.

G


grid selectivity (Σ)–gyromagnetic ratio

the primary beam so Σ � �. If 20% of x-rayscatter is transmitted (Σ� 5), this would seriouslydegrade image quality. Improved grids wouldtherefore have higher values than this (Σ� 6to 8).

grid tagging (mri) See tagging.GROPE (mri) Generalized compensation for reso-

nance offset and pulse length errors.ground state (phys) The lowest energy state of a

system (molecule, atom, nucleus).group classification (nmed) A radionuclide toxi-

city classification ranging from group 1 (mosttoxic) to group 4 (least toxic). Group classifica-tion determines limits for safe disposal.

GSD (dose) See genetically significant dose.GSM (comp) Global support for mobiles. Currently,

GSM systems operate at 800, 900, 1800 or1900 MHz. GSM standards bodies have beendefining data networking technologies, such asGPRS, to build on GSM. Three non-compatiblemobile wireless protocols are being developedfor the wireless mobile market. They are GSM,TDMA and CDMA.

GSO (rad) See gadolinium oxy-orthosilicate.GUI (comp) Graphical user interface. A system that

simplifies selecting computer commands byenabling the user to point to symbols or illus-trations (called icons) on the computer screenwith a mouse. The basic Windows and Linuxscreen format.

gyromagnetic ratio (mri) The precessional fre-quency f of the nucleus can be calculated as:

where μ is the proton magnetic moment, L theproton spin angular momentum and Bo the

fBLo�

μπ2

magnetic field strength in Tesla. It can be sim-plified where μ and 2πL are treated as con-stants being fixed for any particular nucleus (inthis case a hydrogen proton). Together theydescribe the gyromagnetic ratio γ as:

The gyromagnetic ratio for the hydrogen protonis calculated in the example below. Simplifyingthe above equation by substituting γ yieldsωL �γ Bo relating the precession frequency tomagnetic field strength (Bo) and is the Larmor

equation. ωL is the Larmor frequency, where ωrepresents the angular frequency of precession.

γμπ

�2 L

.

The gyromagnetic ratio γ for hydrogenProton magnetic moment: 1.41031 � 10�26 J T�1

The proton spin angular momentum is:

0.527 � 10�34 J s�1 so

or 42.58 MHz T�1.

A 200 Hz variation due to magnet inhomogeneity,would give a variation of: 0.0002/42.582252 or4.7 ppm.

γμπ π

� ��

� ��

�

�21 41 10

2 0 527 1042582252

26

34L.

.Hz

Clinical magnet strengthsThe precession frequency of a 1H (proton) for aHigh 3 T f � 42.58 � 3.0Medium 1.5 T f � 42.58 � 1.5Low 0.3 T f � 42.58 � 0.3 12.77 MHz


Ho–half-life (effective)

H

HHo (mri) See magnetic field intensity.H1 (mri) See magnetic field intensity.H and D curve (film) F Hurter (H) and VC Driffield

(D) pioneers in the evaluation of film emulsionperformance and speed. Their work led to theproduction of the modern characteristic curve.The ASA/DIN/BS/ISO are now the internation-ally quoted speed ratings for film.

haematocrit (Ht) (clin) Separating the blood cellsfrom the plasma, by centrifugation, so that thevolume percentage of cells (erythrocytes) can beestimated. The normal value varies with age,being 56.6% at birth, 35.2% at 1 year, 39.6% at12 years and about 43% for an adult (male46%; female 40%). Also called the packed cellvolume (PCV).

haemodynamics (clin) The study of blood circula-tion. Achieved by either following x-ray contrastbolus transit (digital subtration angiography(DSA) or computed tomography (CT)), nuclearmedicine 99 mTc bolus transit (in vivo red bloodcell (RBC) labelling, diethylene triamine pen-taacetic acid (DTPA)), ultrasound (Doppler) ormagnetic resonance imaging (MRI).

haemopoiesis (clin) The formation of red bloodcells. There is a clinically significant depression foracute radiation exposure at 500 and 400mGy y�1

over many years.hafnium (Hf) (elem)

Atomic number (Z) 72Relative atomic mass (Ar) 178.49Density (ρ) kg/m3 133 00Melting point (K) 2423K-edge (keV) 65.3

Hagen–Poiseuille law (us) This describes therelationship between volume flow of a liquid ofknown viscosity to diameter and length of ablood vessel The vascular resistance (R) is:

where η is fluid viscosity, r and l are vesselradius and viscosity, respectively. The volumeflow (Q) per unit time is given as:

Qp rl

�π

η� �Δ 4

8

Rr

l�

�

� �1

8

4πη

where Δp is the pressure difference betweenthe ends of the blood vessel (perfusion pres-sure). Simplified to:

where P is the perfusion pressure gradient (seeblood flow).

Hahn echo (mri) The formation of a spin-echoafter applying two or more RF pulses.

Hahn, Otto (1879–1968) German physicist, whotrained under William Ramsay and ErnestRutherford in London and Montreal. At the KaiserWilhelm Institute, he worked with Lise Meitneron the irradiation of uranium and thorium withneutrons, which (with Fritz Strassmann) led tothe discovery of nuclear fission. Hahn wasinvolved with the discovery of several radio-elements. He received the Nobel prize for chem-istry in 1944.

Hahn (von Hahn) filter (di) See Hanning filter.halation (film) Commonly applied to intensifying

screens where the incident radiation is fullyabsorbed, but the light is emitted at all anglescausing radiographic unsharpness or penumbraformation and loss of resolution.

Half-Fourier matrix (mri) The MRI raw data matrixhas a specific symmetry, which suggests sam-pling of only half the matrix is sufficient. Theother half can be symmetrically reconstructed;mathematically, the matrices are conjugatedcomplexes. Unavoidable phase errors due tominor magnetic field inhomogeneity require aphase correction, therefore a little more than halfof the phase-encoding steps are acquired.Measurement time is reduced by 50%.

half (partial) Fourier (mri) Method for acquiringimage data using approximately half the usualnumber of phase-encoding steps.

half-life (nmed) See half-life (physical).half-life (biological) (nmed) Time required for

clearing one half the amount of a substancefrom a biological system. The time required forthe mass of a drug substance to be reduced toexactly 50% of its original value due solely tobiological elimination. For each drug there is asingle organ biological half-life and a wholebody biological half-life. They are almost neveridentical.

half-life (effective) (nmed) The time required forthe radioactivity level of a living thing to be

QPR

�

reduced to exactly 50% of its initial value as aresult of both biological elimination andradioactive decay. The sum of the biologicaland physical half-lives (see effective half-life).

half-life (physical) (nmed) The time required forthe radioactivity level of a source to decay toexactly 50% of its original value. The half-life ofa radionuclide, the energy type and energy ofemission are all identifying characteristics. Thecommon formula used for half-life determina-tion uses the exponential function e is:

At � Ao � e�λt

where Ao is the original activity and At is thevalue representing new activity at time t, influ-enced by λ, the decay constant, which is thequotient:

λ λ

λ

� � �

�

0 693 0 5

2

12

12

12

. .

.

/ since and

ln

t e t

t

If the thickness x absorbs half the radiation, thehalf-value-layer (HVL) is defined as:

Since e�0.693�0.5, then μ*HVL �0.693 and so

The influence on the incident beam of a certainnumber of half-value layers can be calculatedfrom 1/2n, where n is the number of half-valuelayers; plotted below for intensity versusabsorber HVL.

Accurate determination of HVL can be calcu-lated from the formula:

where yo is the incident radiation intensitymeasured without absorber, y1 the intensitymeasured after x1mm filtration added, and y2 the intensity measured after the x2mm filtra-tion is added. The radiation intensity can begiven in mR or mGy. The HVL defines the x-raybeam quality being the thickness of aluminiumwhich reduces the x-ray beam intensity byhalf. This should be at least 2.5 mm aluminiumat 80 kVp.

HVLx

yy

xy

y

yy

o o�

��

� ��

12

21

2

1

2 2ln ln

ln

⎡

⎣⎢⎢

⎤

⎦⎥⎥

⎡

⎣⎢⎢

⎤

⎦⎥⎥

⎡

⎣⎢⎢

⎤

⎦⎥⎥

100

10

1

0 1 2 3 4

Number of HVL’s

Inte

nsity

rem

aini

ng (

%)

5 6 7 80.1

μ �0 693.HVL

II

ex

o

HVL� � �0 5. μ*

Isotope decayThe half-life for 99 mTc is 6 h. If the original activity Ao

was 800 MBq. Then the activity after 1 day whereAt � Ao � e�λt and after time t (24 h) with λ, (decayconstant) for 99 mTc as 0.693/6.0 �0.1155 h, then:

A et � �

� �

�

� �800800 0 0625350

0 1155 24.

. MBq

(see decay constant).half-scan interpolation (ct) See 180° interpolation.halftone (image) Since inkjet printers only have

the four process colours, they produce othershades by laying down patterns of primarycolour dots, varying the pattern and ratio ofeach colour; this is half-toning.

half-value layer (HVL) (phys) The thickness ofan absorber that reduces the intensity of aphoton beam to 50% of its initial value; thefraction of photons (x- or gamma radiation)removed from a beam per unit thickness ofabsorber (m�1). The linear attenuation coefficient μfor a particular tissue at a particular energy is ameasure of the ability of the tissue to removephotons from a photon beam. From the generalexponential relationship:

Ix � Io � e�μx.

H


half-life (effective)–half-value layer (HVL)


half-value layer (HVL)–harmonic imaging

H half-wave rectification (elec) A rectifier circuitwhere only alternate half-waves of either asingle or three-phase a.c. supply are effectivein forming the d.c. supply.

Hall effect (elec) An effect occurring when a con-ductor is placed in a magnetic field and orien-tated so that the field is at right angles to thecurrent direction. There is a small potential dif-ference (the Hall voltage) set up across the con-ductor. Semiconductor Hall effect generators areused for calibrating magnetic field strengths.

Hamming filter (image) A filter formed by alteringthe relative proportions of the d.c. and cosinecomponent in the Hanning filter, so improving thesidelobe levels (see filters (signal), Bartlett filter).

handshake (comp) The exchange or recognitionof a coded signal between electronic (computer)devices or peripherals. Modems use this prior tolinking and transmitting data.

Hanning filter (image) A signal filter with a cosineshape having a main spectral lobe similar to atriangular window but with a d.c. level that

1.0

0.5

0.0

0.0 0.5 1.0

Hn

Hm

B

R

Frequency (relative)

Filt

er m

agni

tude

makes all the sampled values positive. Used inmagnetic resonance imaging (MRI) to reducetruncation artefacts. (nmed) Also a common filterused in nuclear medicine which reduces higherspatial frequencies, so losing fine detail. Thegraph compares Hanning (Hn), Hamming (Hm),Butterfield (B) and simple ramp (R) (see filters (sig-

nal), Bartlett filter, Hamming filter).haploid (clin) Relates to single-stranded DNA

cells: bacteria and gametes.hard copy (di) Recording the image on film or

paper (see film formatter).hard disk drive (HDD) (comp) The major bulk

magnetic storage device for small and largecomputers. Non-volatile data storage usingmultiple stacked platters/disks made from lightaluminium alloy or glass and coated with a thinlayer of magnetic cobalt-ferrous alloy. Large-scale, reliable storage led to the introduction ofconfigurations such as the redundant array ofindependent disks (RAID), network attachedstorage (NAS) systems and storage area network(SAN) systems that provide efficient and reliableaccess to large volumes of data. Common stor-age capacity ranges from 100 Gb to 1 TB(1000 GB) and above; these have data transferrates of 1 Gbit s�1 or higher.

hard radiation (xray) See beam hardening.harmonic imaging (us) As the transmitted ultra-

sound signal propagates within tissues or isscattered by contrast medium microbubbles,the echo signals will show non-linear effectsand have more than just fundamental frequencies.They will also exhibit multiples of the transmitfrequency (second, third, etc., harmonic frequen-

cies). These are utilized in harmonic imaging,where fundamental and harmonic echo signalsare separated either by:

● Second-harmonic or narrow band harmonicimaging separates the strongest second har-monic from the fundamental components usinga high-pass filter. There is some degree of fre-quency overlap.

● Wide-band harmonic imaging applies phaseinversion to the transmit pulse and uses thetransducer’s full bandwidth. There is a smallerdegree of frequency overlap allowing a widertransmit frequency spectrum. Narrow-bandharmonic imaging (1) requires a long transmitpulse and the suppression of the echo funda-mental frequency by using a high pass filter.

Half value layer, mammographyA mammography unit gives the following readingsfor aluminium:Measurement with no absorber �718 (yo)Measurement with 0.1 mm aluminium �583 (y1)Measurement with 0.5 mm aluminium �275 (y2)

HVL �

� �� 0 1 0 5

2 275718

2 583718

275583

. .ln ln

ln

⎡

⎣⎢⎢

⎤

⎦⎥⎥

⎡

⎣⎢⎢

⎤

⎦⎥⎥

⎡

⎣⎣⎢⎢

⎤

⎦⎥⎥

HVL ��

��

0 02665 0 24240 7514

0 358. .

.. .mm

H


harmonic imaging–heat storage

Wide-band harmonic imaging (2) uses twoshort phase-inverted transmit pulses and thesuppression of uneven echo frequencies bysignal addition. The side lobes seen in conven-tional imaging are reduced. The narrowerwidth of the second harmonic main lobe givesbetter lateral resolution, but degrades axial reso-

lution. Wide-band harmonic imaging preservesaxial resolution. The images appear sharperand exhibit higher contrast resolution and lessnoise (see contrast medium (ultrasound)).

HASTE (mri) Half-Fourier acquisition single shotturbo spin-echo (Siemens). A turbo spin-echotechnique with one very long echo train that usesk-space reordering and half-Fourier reconstruction tocontrol the contrast and maximize resolution in theT2-weighted image. All of the echoes are recorded inone TR; the effective TE is �100msec. Half of k-space is filled on each acquisition in an inter-leaving lines fashion. Equivalent pulse sequencesare SS-FSE (GE Medical Systems, Philips), FSE-ADA(Hitachi) and FASE (Toshiba).

havar foil (nmed) Commonly used target windowfor medical cyclotrons designed for pressurizedwater target for 18F production. An alloy of Co,Cr, Fe, Ni, Mn, W and Mo. Since this is exposedto a high flux of protons and secondary neu-trons, when replaced the foil must be treated asradioactive waste and treated accordingly.

Hayes command set (comp) A standard set ofcoded instructions for controlling basic modemfunctions, such as dialling and hanging up,devised by the modem manufacturer Hayes.

HDP (nmed) See oxidronate.HD-TV (di) High definition television. Video scan

line density greater than 512/625 domesticstandard. European standards are now 2459lines giving an image matrix of 2590 � 2048,non-interlaced (progressive scan) with a 54 cmdiagonal and a refresh frequency of 72 Hz (hor-izontal frequency 150.2 kHz). Special closedcircuit video exceeds 3000 lines (see video).

heat (phys) This is now strictly defined as thetransfer of energy from a body at high tempera-ture to one at a lower temperature due to thetemperature difference. This process can be byconduction, convection and radiation; all radiationwavelengths emitted are considered as heat(ultraviolet, visible, infrared, etc.), since they allinject energy into the system. The SI unit of heatis the joule (J) (see energy, work, heat units (HU)).

heat capacity (phys) The heat required (in joules)to raise a body’s temperature by 1 K or 1°Cmeasured as J K�1 (or J °C�1). It is the character-istic of the material independent of its size orunit mass (unlike specific heat capacity). Heatcapacities of common materials used for anodematerials, with mass 2 kg are:

Anode material (2 kg) Heat capacity (J K�1)

Tungsten 272Titanium 1046Zirconium 560Molybdenum 492TZM 692

Heat capacity is simply specific heat capacity �

mass; So for 2 kg tungsten, the heat capacity is136 �2 or 272 J K�1.

heat loading (xray) The amount of heat energydeposited in the anode during an x-ray exposure.The loading depends on peak kV, waveform, tubecurrent, exposure time and rate of exposures perunit time. Heat loading is assessed by referenceto a rating chart (see heat storage, rating).

heat loss (xray) The removal of heat from thefocal spot of an x-ray tube involves conduction,convection and radiation. All three maintain anacceptable anode cooling rate which affects theloadability of the x-ray tube.

heat storage (xray) Heat storage of an x-ray tubeis the product of anode mass, specific heat andtemperature rise. If the maximum heat capacityof the anode is 150 kHU in the diagram thenthis may be exceeded if the rate of heat pro-duction is high (A in HU s�1) and must bereduced to B (see rating).

200

150

100

50

00 1 2 3

Minutes

Hea

t uni

ts (

khU

)

Anode heating

Anode cooling

4 5

A

B


heat units (HU)–Hepatolite®

H

heat units (HU) (xray) A measure of heat storagefor an x-ray tube. The heat capacity of an anodeis sometimes expressed in heat units HU forthree-phase and single-phase supply, calcu-lated as the product of kV, mA and secondsHU �V �mAs. For a three-phase supply:HU �1.35 �kV �mA �sec. The 35% increaseis due to the increased efficiency of the wave-form; for constant current generators (high fre-quency generators) this is 1.4. Heat units werecommon when single phase and three-phaseelectrical generators supplied x-ray sets, sincethe high voltage electrical supply had a ripple

component which produced a cyclic variation inx-ray (and hence heat) production. Contemporaryx-ray equipment now use high frequency genera-

tors and are virtually constant current so the rip-ple component is negligible. The heat unit hasbeen superseded by measuring heat capacity injoules (J) and heat loss in joules per second orwatts (W).Conversion: HU �0.71 � joules (J);J � 1.41 � HU (see x-ray tube rating).

heating (ultrasonic) (us) The heating of tissue(including bone) due to ultrasound absorption(see ultrasound (safety)).

heel effect (xray) The diminishing intensity acrossthe x-ray beam toward the anode; the non-uniform distribution of air kerma rate in an x-raybeam in a direction parallel to the cathode-anode axis. The x-rays produced by bremsstrahli-

ung in the target pass through the tungstentarget material and are attenuated. The thick-ness of tungsten and therefore x-ray attenuation,is greater on the anode side of the beam, so theintensity is reduced asymmetrically. As the tar-

get surface wears, the heel effect becomes morepronounced. Beam uniformity changes withanode angle. Collimating the useful field of viewreduces the heel effect, but does not eliminateit. The heel-effect decreases with increase inanode angle: 7°, 12° and 20°, but increasing theangle adversely affects image resolution andthermal rating (loadability) (see field size).

helical/spiral CT (ct) A particular technique ofscanning in which there is continuous rotation ofthe x-ray tube coupled with continuous lineartranslation of the patient through the gantryaperture in order to achieve volumetric dataacquisition. Also known as spiral or volume CT.

helical scan (acquisition) (ct) Continuous rota-tion of a fan beam assembly while the scan bedmoves incrementally; also known as spiral scan.

helium (He) (chem) An inert/noble gas belongingto group 18 of the periodic table. The elementhas the lowest boiling point of all substancesand can be solidified only under pressure.

Atomic number 2Relative atomic mass (atomic weight) 4.0026Density 0.166 kg m�3

Melting point 0.95 K at 26 atmBoiling point 4.21 K4He �100%3He 0.00013%Relevance to radiology: As a cryogen for MRI andcyclotron superconductor magnets. Negative heliumions 3helium and 4helium are used for cyclotron beamproduction.

3Helium (nmed) Stable isotope of heliumwith fractional abundance of 0.00013%. Usedas a negative ion beam 3He� carrying threeelectrons, stripped to yield a 2p,1n cyclotron

beam for radionuclide production (see hydrogen,deuterium).

4Helium (nmed) The most stable isotope ofhelium with approximately 100% abundance. Itis the decay product of the alpha particle. Used asa negative ion beam 4He� carrying three elec-trons, stripped to yield a 2p,2n (alpha) cyclotron

beam for radionuclide production (see hydrogen,deuterium).

Helmholz coil (mri) A pair of identical current-carrying coils used to create a uniform mag-netic field.

henry (H) (unit) A measure of inductance. Theinductance has a value of one henry, if a currentchanging at the rate of one ampere per secondinduces an electromotive force of one volt.

H � V (A s�1)�1 � V s A�1 � Wb A�1.

Magnetic permeability is measured as H m�1.The inductance of a closed loop that gives amagnetic flux of 1 weber for each ampere.

hepatic angiography (clin) Imaging the vascular-ity of liver, spleen and pancreas: intra-arterialDSA is being effectively replaced by contrastenhanced magnetic resonance angiography(MRA).

hepatic arteriography (clin) Opacification andvisualization of the hepatic artery using an iod-inated contrast medium (see TIPS, transarterial

chemo-embolization (TACE)).Hepatolite® (nmed) A CIS commercial product of

99 mTc disofenin (DISIDA).

(42 He)

(32 He)

H


hereditary–hippuran

hereditary (dose) Affecting future generations(see genetically significant dose).

Hertz, Heinrich R (1857–94) German physicist.Confirmed Maxwell’s predictions of electromag-netic transfer by experimental discovery of‘Hertzian waves’ or radio waves; further devel-oped by Marconi.

hertz (Hz) (phys) The SI unit for frequency ascycles per second.

Very low frequency (VLF) �1–10 Hz AtmosphericLow frequency 50–100 Hz A.C. powerMedium frequency 20 kHz SoundHigh frequency 1 MHz AM radioVery high frequency (VHF) 100 MHz FM radioUltra high frequency (UHF) 1 GHz Mobile (cell)

phonesInfra-red, x-rays 1014 HzGamma rays �1023 Hz

HESPA filter (nmed) High efficiency sub-micronparticulate air filter. Recommended for clean airinstallations and conforming with Hosch Class100 and the US Federal Standard 209B (0.003%).

Hevesy von, George Charles (1885–1966)Hungarian physicist. Studied in Freiburg, Zurichand Manchester where he worked withRutherford. His work with radioactive tracers tostudy chemical processes in biology started theidea of nuclear medicine. Awarded the Nobelprize for chemistry in 1943.

Hexabrix® (cm) Preparation of ionic dimer of ioxaglic

acid, introduced by (Mallinckrodt Inc.). Consists ofmeglumine ioxaglate 39.3% and sodiumioxaglate 19.6%.

Compound Viscosity Osmolality Iodine(cP) (mOsm/kg) (mg I mL�1)

Meglumine 15.7 at 20°, 600 320ioxaglate, 7.5 at 37°sodiumioxaglate

hf (hv) (phys) Depicting the energy E of a singlephoton: E � hf, where h is Planck’s constant andf frequency. Substituting c/λ for f thenE � hc/λ. The product hc is 12.4, so the rela-tionship between energy and wavelength isE � 12.4/λ.

HFGR (mri) Inversion-recovery fast GRASS rapidgradient-echo imaging techniquesT1-weightedcontrast GE (see FLASH, SPGR, FSPGR, RE Spoiled,3D-ME-RAGE, T1-FEE, STAGE-T1W).

hibernating myocardium (clin) See myocardium.HIDA (nmed) A dimethyl-substituted analogue of

imino-diacetic acid (IDA); complexed with 99 mTc,it is used for hepatobiliary imaging. Chemicalform: N-[N -(2,6-dimethylphenyl-carbamoyl-methyl] imidodiacetic acid. Hepatic iminodiaceticacid. Used for functional hepatobilliary imagingwhen labelled with 99 mTc as 99 mTc-lidofenin(HIDA) or 99 mTc–EHIDA. This family also includesEHIDA, DISIDA (Hepatolite®, Technescan®).

HFI (mri) Half Fourier imaging. Sampling only partof k-space and using phase conjugate symmetryto fill in the rest of the image plane.

high contrast resolution (image) See spatial

resolution.high frequency generator (xray) The most

recent design for all x-ray tube supplies whichtranslate (invert) the 50/60 Hz a.c. single orthree-phase supply into a 5–20 kHz supplywhich then feeds the a.c. circuitry. There is asignificant decrease in transformer size withincreasing frequency of supply. The cross-section, A, of the transformer and its number ofturns, n, are related to output voltage, V, andfrequency, f: A � V/(n�f ). The high frequencycircuit is controlled by microprocessor to stabi-lize kV and exposure.

high-osmolarity (cm) A solution having a higherosmolar concentration when compared to bloodplasma or intracellular fluid; cells shrink as aconsequence. A solution that has a higherosmolality than blood and body fluids, i.e.higher than 300 mOsm/kg water, is hypertonic.Commonly describes contrast media with a sig-nificantly higher ion concentration than bodyfluids (between five and seven times, termedhigh-osmolar contrast media (HOCM)). Thesecontrast media may show adverse patient reac-tions than contrast media having low osmolar-ity (LOCM) (see isotonic saline).

high pass filter (di) A filter designed to reject lowfrequencies, while retaining high frequencies.Seen in edge enhancement filters or kernels (seefilters (spatial)).

High performance liquid chromatography(HPLC) (nmed) Formerly known as high pres-sure liquid chromatography. Used for the ulti-mate purification of radiopharmaceuticals andfor their analysis. HPLC can separate all impuri-ties in one analysis and can give chemical, aswell as radiochemical, purity information.

hippuran (nmed) See iodohippurate.


histogram–hormesis

H

histogram (math) A chart which displays groupeddata in which the area of each bar is propor-tional to the occurrence or frequency that it represents.

histamine release (cm) The release of histaminefrom mast cells; a major cause of allergic reac-tions (see chemotoxicity).

HL7® (comp) Health Level Seven, Inc., a ‘not-for-profit’ volunteer organization centred at AnnArbor, MI, USA. The term describes a seven-layer International Organization of Standards(ISO) communication model. HL7 allows theinterchange of non-imaging data betweenelectronic patient administration systems,practice management systems, laboratoryinformation systems, dietary, pharmacy andbilling systems, as well as electronic medicalrecord (EMR) and electronic health record (EHR)systems. Each layer has a function. Layers 1–4deal with communication; comprising physical,data link, network and transport communica-tion layers. Layers 5–7 deal with functions likesession, presentation and application. Level 7 isregarded as the application level dealing withthe data definition to be exchanged and sup-ports various functions, such as securitychecks, operator identification, availabilitychecks, exchange mechanism negotiations anddata exchange structuring.

HMDP (nmed) Hydroxy-methylene-diphosphate.A radiopharmaceutical which when reconsti-tuted with 99 mTc forms a bone imaging agent.Rapid uptake enables earlier imaging than with MDP.

HMPAO (nmed) Hexa-methyl-propylene amineoxime (exametazine): a 99 mTc radio-pharmaceuticalused for brain perfusion imaging and white celllabelling (Ceretec®, Amersham/GE Healthcare).The labelled compound readily passes the intactblood–brain barrier and is retained in brain tissue.The uptake closely matches regional cerebralblood flow (rCBF).

holmium (Ho) (elem) A metallic element some-times used as a K-edge filter.


home page (comp) The main page of a websiteand the first screen that a visitor sees displayed

when connecting to that site; usually has linksto other pages, both within that site and toother sites (see URL).

homogeneity (image) (ct) Image quality parameter that describes the degree withwhich a test object made from a homogeneousmaterial (water phantom) is displayed with uniform mean value of the CT number at vary-ing positions within the image (central andperipheral).

homogeneity (magnet) (mri) This refers to theuniformity of a magnetic field with no objectpresent. A magnetic field is considered homo-geneous when it has the same field strengthacross the entire field; measured in parts permillion (ppm) of main field. Homogeneity of themain magnetic field defines the quality of themain magnet over a large field of view; a rela-tive measure independent of field strength thatuses as reference the Larmor frequency for water42.5759 MHz T�1. A 200 Hz variation wouldgive an inhomogeneity of:

Superconducting magnets have typically �0.03ppmfor a 10-cm spherical volume and �0.5ppm for40 cm. Permanent and resistive magnets have typicalinhomogeneities of 40ppm. Homogeneity isimportant for spectral fat saturation, a large meas-urement field, echo planar imaging and MR spec-troscopy.

homomorphic filter (image) Usually applied toan image to increase edge detection andreduce overall brightness. It achieves the sepa-ration of illumination and reflectance compo-nents, which roughly corresponds to slow orabrupt image variations, respectively.

homospoil (mri) Of a magnetic field gradient toeffectively eliminate residual transverse mag-netization by producing a strong positiondependence of phase within a resolution ele-ment. Also called ‘spoiler pulse’.

homospoil pulse (mri) See spoiler gradient pulse.hormesis (dose) Hormesis has been defined as ‘the

stimulating effect of small doses of substanceswhich in larger doses are inhibitory.’ The meaninghas been modified referring not only to a stimula-tory effect, but also to a beneficial effect. Hormesisnow contains a value judgement whereby a low

20042 5759 10

4 7 10 4 766

.. .

�� ppm.

H


hormesis–HTML

dose of radiation is considered beneficial to anorganism.■ Reference: BEIR VII, 2006.

Hotelling observer (stats) Demonstrates maxi-mum discrimination ability, in terms of the SNR

figure of merit, among all observers that arelimited to performing only linear operations onthe data. When the data are normally distrib-uted, including the unequal variance case, thisobserver also demonstrates maximum discrimi-nation as specified by the area under the ROC

curve. When the data are not normally distrib-uted, the SNR figure of merit may no longer bepredictive of the area under the ROC curve. Itwill then be necessary to obtain that area totest for optimality of observers.

hot-pluggable (comp) The ability to remove orchange circuit boards or connectors from aworking computer without interruption ofpower supply or computer service (see USB).

hot spot imaging (nmed) Small regions of aradioactive source observed in scintigraphywhere the activity of the source is significantlygreater than its surroundings. If the region ofthe source is smaller than the resolution of thegamma camera system, the image of the hotspot will appear in size equal to the gammacamera resolution element.

Hounsfield, Sir Godfrey Newbold (1919–2004)British engineer. Inventor of computed axialtomography using x-rays which led to the com-puted axial tomography (CAT) scanner. Awardeda Nobel prize in 1981 (see Cormack).

Hounsfield unit (HU) (ct) The comparative atten-uation value that represents the mean x-rayattenuation associated with each elementalarea (voxel) of the CT image; the Hounsfield unitexpresses the relative deviation of the meas-ured linear attenuation coefficient from that ofpure water, multiplied by 1000; the unit of x-ray absorption used in computed tomographyrelating linear attenuation coefficient μunknown

to μwater at a fixed kV as:

Water is taken as reference zero; air has thelowest negative value at about �1000 and bonehas the highest value at about �3000. Sincevalues are scaled to water, the Housfield unit isindependent of beam energy (voltage).

HU 1000 unknown water

water� �

�μ μμ

CT number calculation

80 kV 100 kV 150 kV

μmuscle 0.1892 0.1760 0.1550μwater 0.1835 0.1707 0.1504

At keV:

At keV:

80 10000 1892 0 1835

0 183531

100 10000 1

��

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�

. ..

. 7760 0 17070 1707

31

150 10000 1550 0 1504

0 15043

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hour (h) (phys) Not an SI unit, but can form com-pound units with SI units, e.g. kWh.

housing (xray) The x-ray tube is enclosed in asealed housing. Heat loss: circulating air is suffi-cient to cool mobile and mammography units(fan-assisted cooling can halve tube coolingtime), but circulating oil is necessary within theenclosure for cooling conventional and DSAequipment. The total heat capacity of the tubeenclosure is largely dependent on the volumeof oil it contains.

HSA (nmed) Soluble human serum albumin usedfor blood pool imaging. Largely replaced by99 mTc labelled red blood cells.

HT (dose) See equivalent dose.HTML (comp) Hypertext markup language, the

authoring language of the Internet, used to cre-ate web pages. Browsers interpret the codes to

3000

1000Compact boneSpongy

bone

Water

Hou

nsfie

ld u

nits

Fat

Air

Lungs

Breast

AdrenalsPancreas

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Blood

Kidney100

50

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200

100

50

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�50

�100

�200

�1000

HU


HTML–hydrophilic/hydrophilicity

H

give the text structure and formatting (such asbold, blue, or italic) (see hypertext).

HTTP (comp) Hypertext transfer protocol. A commonsystem used to request and send HTML docu-ments on the world wide web, defining howinformation is formatted and transmitted. It is thefirst portion of all URL addresses on the worldwide web (e.g. http://www.hospital.com).

HTTPS (comp) Hypertext transfer protocol secure.Often used in within-company Internet sites.Passwords are required to gain access.

HU (xray, ct) See CT number (Hounsfield units), heat unit.hub (comp) The central device in a star-configured

network; useful for centralized management, theability to isolate nodes from disruption, andextending the distance of LAN coverage. A deviceused on a star network to connect all work-stations together.

hue (image) The colour’s position along the colourspectrum (see saturation).

Huygen’s principle (us) Every point or elementof a multi-element transducer acts as a sourceof spherical wavelets producing a combinedwavefront. The wavelets advance with speedand frequency of the primary wave.

hv (phys) See hf .hybrid magnet (mri) Magnet system employing

both current carrying coils and permanentlymagnetized material to generate the magneticfield.

hybrid spectroscopy (mri) Combing single volumespectroscopy (SVS) with chemical shift imaging(CSI). The CSI measurement is performed over aselectively excited volume of interest. Throughvolume selection, areas with strong distortingsignals (e.g. fat) are not stimulated and thereforedo not contribute signal to the spectra.

hybrid subtraction (xray) This combines theadvantages of both dual energy subtractionand temporal filtering to remove interfering tis-sue (bone) and vessel movement.

HyCoSy (clin) Hysterosalpingographic contrastsonography. Ultrasound technique usingappropriate ultrasound contrast medium fordemonstrating uterus and ovaries, and tubalpatency.

hydrogen (elem) Consists of three isotopes:1Hydrogen (proton and electron); 2hydrogen ordeuterium (proton, neutron and electron);3hHydrogen or tritium (proton 2 neutrons andelectron).

Atomic number (Z) 1Relative atomic mass (Ar) 1.00Density (ρ) kg/m3 0.089Melting point (K) 14.01

2Hydrogen (nmed) See deuterium.3Hydrogen (nmed) See tritium.

Nuclide 3HProduction (cyclotron) 6Li(n,α) 3HHalf-life 12.3 yearsDecay mode β�

hydrogen bond (chem) An electrostatic bondingthat occurs in molecules that have hydrogenatoms linked to electronegative atoms (F, N, O).Hydrogen bonds impart significant effects on acompound’s physical property causing liquidwater to be liquid at room temperature withhigher than expected boiling point. In watermolecules (H–O–H), the oxygen atom attractsthe electrons in the O–H bonds, and since thehydrogen atom has no inner shell of electronswhich shield the nucleus, there is an electro-static interaction between the hydrogen protonand a lone pair of electrons on the electroneg-ative atom (oxygen). It is of great importance inbiological compounds, occurring betweenbases in DNA and between the C�O and N–Hgroups in proteins. Strengths of hydrogenbonds are about one tenth of a normal covalentbond (see contrast media).

hydrophilic/hydrophilicity (clin) Having an affin-ity for or associating with water; the preference(solubility) of a contrast medium for aqueous sol-vents; a property of polar radicals or ions. Non-

ionic contrast media do not dissociate and theirwater-solubility is generally achieved by severalhydrophilic hydroxyl groups. Several non-ionicmonomeric contrast media are available that havehydrophilic hydroxyl groups attached to allthree side chains; a feature contributing to their reduced toxicity. The multiple hydrophilic side groups of second-generation non-ionicmonomers protect the inner hydrophobic benzenering from interaction, thereby reducing theirchemotoxicity. Relative hydrophobic/hydrophilicproperties of the molecule is its partition coefficient;a low partition coefficient is advantageous sincehigh hydrophilicity contributes to low protein

binding. Non-ionic contrast media are sufficiently

http://www.hospital.com

H


hydrophilic/hydrophilicity–hysteresis

hydrophilic to only make small differences in thepartition coefficient (see lipophilicity).

hydrophobic (clin) Lacking an affinity for watermolecules; a property of apolar radicals.

hydrophone (us) An instrument for detectingsounds under water. Used for measuring thepower output from an ultrasound transducer.

Hypaque® (cm) A commercial preparation(Amersham/GE Healthcare Inc) of ionic diatrizoate-meglumine and diatrizoate sodium salt. Supplied in50, 60 and 76% solution concentrations.

Compound Viscosity Osmolality Iodine (cP) (mOsm/kg) (mg I/mL)

Diatrizoate- 3.43 at 20°, 1550 300meglumine 2.43 at 37°50%

Diatrizoate- 6.16 at 20°, 1415 282meglumine 4.10 at 37°60%

Diatrizoate- 9 at 37° 2016 370meglumine76%

Hypaque-Cysto® (cm) A commercial preparation(Amersham/GE Healthcare Inc) of diatrizoate-meglumine consisting of 30% solution (w/v) ofmeglumine salt of diatrizoic acid. Formulationfor retrograde cysto-urethrography.


Diatrizoate- 1.94 at 25°, 633 141meglumine 1.42 at 37°salt

hyperechoic (us) Having relatively strong echoes.hyperlinks (comp) Embedded text in web pages

that allow users to jump from one document toanother related document, regardless of whereit is stored on the Internet. By selecting the textor image with a mouse, the computer jumps toand displays the linked site (see HTML, hypertext).

hyperosmolar/hyperosmolality/hyperosmolar-ity (clin) An increase in the osmotic concentrationof a solution measured as osmoles of solute perkilogram (osmolarity (Osmol)/osmolality (Osm)) or litreof solution (osmolarity). Ionic contrast media disso-ciate in solution into anionic and cationic compo-nents and are markedly hyperosmolar. Non-ionic

contrast media do not dissociate in solution andare less hyperosmolar or isosmolar with blood. Theionic contrast media for intravascular use arehyperosmolar, having an osmolality seven to

eight times that of blood plasma. This hyperosmo-lality is partly responsible for several subjectiveand objective adverse effects, i.e. pain, endothe-lial damage, thrombosis and thrombophlebitis,disturbance of the blood–brain barrier, bradycar-dia in cardioangiography and increased pressurein the pulmonary circulation (see contrast medium

(radiographic)).hypertext (comp) A method of presenting informa-

tion that allows the user to jump between placesin a document by clicking on a highlighted wordor an icon, rather than being forced to navigateit in a linear fashion. Both the Help files and webpages make extensive use of this technique (see HTML).

hyperthyroidism (clin) A condition in which anoveractive thyroid gland produces excessivethyroid hormone, leading to a characteristicclinical picture. Since the thyroid uses iodine tomake its hormone (thyroxine or T-4), radio-active iodine in small dosages can be used toimage the thyroid and in large dosages to treatit (reduce its function).

hypertonic (cm) See high osmolarity.hyper-threading technology (comp) In order to

improve performance in early computers,threading was enabled in the software bysplitting instructions into multiple streams sothat multiple processors could act upon them.Hyper-threading technology (HT technology) isnow hardware built into the CPU and providesthe ability to run multiple parallel threads oneach processor, resulting in more efficient useof processor resources, higher processingthroughput, and improved performance onmultithreaded software. Hyper-threading tech-nology improves the performance of existingsoftware in multitasking server environments.Many applications are already multithreadedand will automatically benefit from this tech-nology, but performance will vary dependingon the specific hardware and software (see duo

core processors).hypo (film) Sodium thiosulphite.hypoechoic (us) Having relatively weak echoes.hypo-osmolar (cm) See low osmolarity.hysteresis (phys) Phenomenon seen in electro-

magnetic behaviour of materials in which a lagoccurs between application or removal of amagnetic force (examples can be found in MRIand magnetic recording).


hysteresis loss–hysterosalpingography

H

hysteresis loss (mri) Dissipation of energy thatoccurs when a magnetic material is subjectedto magnetic field change. Important factor withgradient field switching.

hysterosalpingography (clin) Injection of con-trast material into the uterus and uterine tubesand imaging the uterus and fallopian tubesthereafter. Also known as salpingography.

I


i–ideal filter

Ii (math) The complex variable representing .

Sometimes represented by j or q (see complex

numbers).I (units) Induction. (phys) See spin quantum number.IAEA (dose) International Atomic Energy

Authority. Established by the United Nations in1957 to advise member countries on the appli-cation of nuclear power and on security mat-ters. Headquarters in Vienna.

ibitumomab (nmed) 90Y-labelled antibody fornon-Hodgkin’s lymphoma therapy.

ICRP (dose) International Commission onRadiological Protection. Established in 1928,associated with the International Commissionon Radiation Units and Measurements, TheWorld Health Organization and the InternationalAtomic Energy Agency. The ICRP only concernsitself with ionizing radiation and with the pro-tection of man. The ICRP recommendations havehelped to provide a consistent basis for nationaland regional regulatory standards; the recom-mendations carry no regulatory powers, butform the basis of most national legislation. Themost recent (September 2008) ICRP reports ofinterest to medical applications are:

ICRP publication No.

106 Radiation dose to patients fromradiopharmaceuticals. ISBN-13: 978-0-7020-3450-3, ISBN-10: 0-7020-3450-9, 2009

105 Radiological protection in medicine, ISBN-13:978-0-7020-3102-1, ISBN-10: 0-7020-3102-X,2008

103 Recommendations of the ICRP: user’s edition.ISBN-13: 978-0-7020-3063-5, ISBN-10: 0-7020-3063-5, 2007

102 Managing patient dose in multi-detectorcomputed tomography (MDCT). ISBN-13: 978-0-7020-3047-5, ISBN-10: 0-7020-3047-3, 2007

95 Doses to infants from ingestion of radionuclidesin mother’s milk. ISBN-13: 978-0-08-044627-1,ISBN-10: 0-08-044627-2, 2004

93 Managing patient dose in digital radiology.ISBN-13: 978-0-08-044469-7, ISBN-10: 0-08-044469-5, 2004

92 Relative biological effectiveness (RBE), qualityfactor (Q), and radiation weighting factor (wR).ISBN-13: 978-0-08-044311-9, ISBN-10: 0-08-044311-7, 2004

90 Biological effects after prenatal irradiation(embryo and foetus. ISBN-13: 978-0-08-044265-5, ISBN-10: 0-08-044265-X, 2004

88 ICRP Supporting Guidance 2: Radiation and your patient: a guide for medical practitioners.

�1

ISBN-13: 978-0-08-044211-2, ISBN-10: 0-08-044211-0, 2003

87 Managing patient dose in computedtomography. ISBN-13: 978-0-08-044083-5,ISBN-10: 0-08-044083-5, 2002

84 Pregnancy and medical radiation. ISBN-13: 978-0-08-043901-3, ISBN-10: 0-08-043901-2, 2001

73 Radiological protection and safety in medicine.ISBN-13: 978-0-08-042738-6, ISBN-10: 0-08-042738-3, 1997

60 1990 Recommendations of the InternationalCommission on Radiological Protection, ISBN-13: 978-0-08-041144-6, ISBN-10: 0-08-041144-4, 1991.

ICRU (International Commission on RadiationUnits and Measurement) (dose) Formed in1925 for developing internationally acceptablerecommendations regarding quantities and unitsof radiation and radioactivity and makes recom-mendations for radiation protection. Cooperateswith the ICRP. Selected ICRU reports:

● Receiver operating characteristic (ROC) analysisin medical imaging (report 79) 2008

● Prescribing, recording and reporting proton-beam therapy (report 78) 2007

● Measurement quality assurance for ionizingradiation dosimetry (report 76) 2006

● Patient dosimetry for x rays used in medicalImaging (report 74) 2005

● Image quality in chest radiography (report 70)2003

● Tissues substitutes, phantoms and computationmodelling in medical ultrasound (report 61)1999

● Fundamental quantities and units for ionizingradiation (report 60) 1998

● Medical imaging – the assessment of imagequality (report 54) 1996

● Quantities and units in radiation protectiondosimetry (report 51) 1993

● Tissue substitutes in radiation dosimetry andmeasurement (report 44) 1989

● Modulation transfer function of screen-filmsystems (report 41) 1986.

IDA (nmed) See HIDA.IDE (comp) Integrated drive electronics. A control

system which allows a computer and a device tocommunicate. Now replaced by EIDE (enhancedIDE) which offers improved performance.

ideal filter (image) An ideal low-pass filter has arectangular transfer function. Commonly usedfilters are radially symmetrical about the origin


ideal filter–image acquisition time

I

(3D plot), so they can be specified by a cross-section extending from the origin in a 2D graph.On the graph Do is the cut-off frequency; allfrequencies inside a circle of radius Do arepassed with zero attenuation, whereas all fre-quencies outside Do are stopped. The idealhigh-pass filter has a transfer function thatattenuates low frequencies without disturbinghigh frequencies (see Butterworth, Hanning,Hamming, Shepp-Logan).

ideal observer (stats) The ideal observer is theBayesian decision-maker who minimizes the‘cost’ or ‘risk’ when determining a decisionstrategy for a given task.

IEEE (comp) The Institute of Electrical andElectronics Engineers, a body that defines stan-dards and specifications: IEEE 802 Standardsgovern networking.

IEEE 1394 (comp) An external bus that transfersdata from one computer to another at up to400 Mbps. Thanks to its high speed, IEEE 1394is the standard for multimedia and will clashhead on with USB 2.0 should it establish afoothold in the market. At present IEEE 1394 isin a dominant position, mainly thanks to man-ufacturers including it in their computerdevices, complementing their digital camerasand camcorders. IEEE 1394 is known asFireWire at Apple and i-link at Sony, where ithas been adopted as the standard across itspopular Valo notebook range. Although SCSI(small computer systems interface) has pro-vided an invaluable medium-term solution forusers who need extra speed; these ports alsohave serious disadvantages. Having installed aSCSI card, the user has to attach a thick,

Frequency

Sig

nal s

tren

gth

0.0

0.5

1.0

Low

Do Do

High

unwieldy parallel cable linking the PC to theperipheral. In addition, unlike the smaller andmore robust USB 2.0, SCSi cable ports are eas-ily damaged.

IHE (comp) Integrating the healthcare enterprise.Developed by the Radiological Society of NorthAmerica (RSNA) and Healthcare InformationManagement Systems Society (HIMSS). Thereare ambiguities and conflicting interpretationsbetween standards like DICOM and HL7. IHEcoordinates the use of these established stan-dards and answers issues that remain unre-solved in DICOM/HL7 operations.

iliocavography (clin) See pelvic venography.illuminance (phys) The total luminous flux

received by a unit area of surface expressed inlux (lumen m�2). Non-SI units are illuminance

(lumen ft�2) or phot (lumen cm�2). One lux � 104

phots. Illuminance is analogous to irradiance, butilluminance refers only to visible light weightedto the non-linear luminous efficiency of thehuman eye (see radiance).

image (image) A 2D representation of a volumedistribution either as a landscape or object. A volume object in diagnostic imaging can beinterrogated by x-rays, gamma rays, ultrasoundor radio frequencies to give an image distribu-tion in x and y dimensions (height and width).Signal strength is represented as a change inimage density either as a grey or colour scale.This can be analogue (film) or digital (computermatrix). A computer matrix has a third dimen-sion given by the pixel depth which determinesthe dynamic range (latitude) of the image (seetomography).

image (latent) (image) Light reacting with a pho-tographic emulsion causes some silver ions tobe converted into silver atoms. Developing thisemulsion causes further silver atoms to form inthe vicinity of these original atoms, giving alatent or hidden image which can be revealedby fixing with a chemical that removes theremaining unaffected emulsion silver salts.

image acquisition time (image) Time required tocarry out an imaging procedure. The number ofencoding signals required for reconstruction.Applicable to ultrasound and nuclear medicine.(mri) Time required to carry out a magnetic res-onance (MR) imaging procedure comprisingonly the data acquisition time. The total imageacquisition time will be equal to the product of the repetition time. The time required

I


image acquisition time–image intensifier

T is:T � TR � Lines � Nex, where TR is the rep-etition time, Lines the number of image lines(phase encoding steps) and Nex the data aver-aging. For a TR of 0.5 s, an image matrix of 512and �2 averaging the image acquisition wouldbe 512 seconds or 8.5 minutes. The additionalimage reconstruction time will also be impor-tant to determine how quickly the image canbe viewed. In comparing sequential planeimaging and volume imaging techniques, theequivalent image acquisition time per slicemust be considered, as well as the actual imageacquisition time.

image archiving (image) Recording a raw image

matrix on to bulk storage by compressing theinformation and so achieving the maximum uti-lization of the data medium capacity; there areloss-less compression and lossy compression meth-ods for image archiving.

image contrast (xray) See contrast. (mri) Thestrength of the image intensity in adjacentregions of the image is compared, or objectcontrast, where the relative values of a param-eter affecting the image (such as spin densityor relax-at ion time) in corresponding adjacentregions of the object are compared. There aremore object parameters affecting the image inmagnetic resonance imaging (MRI) and theirrelative contributions are dependent on theimaging technique used.

image depth (us) Maximum penetration that willyield image information. Decreases with increas-ing frequency and with decreasing transducersize or aperture (Fresnel or near zone shortened).The useful image depth is also dependent onpulse repetition frequency. Proportional to75/PRF (pulse repetition frequency). The dis-played distance from transducer face deter-mined by the pulse repetition period (PRP) andtransducer frequency. Calculated from the speed of sound in soft tissue (1500 m s�1) andallowing for a return path (so halving this dis-tance), then Dmax � 150 000 � 0.5 � PRP or75 000 � PRP cm. For a PRP of 200 μs this would give an imaging depth of 15 cm.Alternatively, Dmax can be calculated as75 000/PRF. Similarly, for soft tissue imaging:PRF � 1500/2 � Dmax. Higher frequency trans-ducers have higher PRP values so give shallowerimage depths. Since sound attenuationincreases with frequency, this is the majordetermining factor for image depth. The graph

plots the relationship between PRP and imagingdepth for a speed of sound of 1500 m s�1; trans-ducer frequencies used at various PRP values aresuperimposed on the graph; higher frequencytransducers give improved image resolution andowing to faster PRP are more able to give real-time displays and follow pulsatile (cardiac)movement.

(see dispersion, axial resolution, lateral resolution).image fusion (image) Combination of two or

more registered images, with the objective ofproducing a single image of additional diag-nostic information.

image intensifier (xray) An electronic method forincreasing the light intensity from a fluorescentscreen. The input fluorescent phosphor is typi-cally CsI:Na which has several advantages:

● good absorption for x-ray energies above theK-edges of Cs (33 keV) and iodine (36 keV);

● spectral matching with SbCs3 photocathodematerial;

● vapour deposition of CsI:Na produces mono-clinic crystalline structure (150 μm thick) which channels light quanta and prevents lightdiffusion.

The photocathode is in optical contact with theinput phosphor transforming light quanta intoan electron flux density distribution. The elec-trons are accelerated on to an output phosphorscreen which is generally ZnCdS:Ag, a fewmicrometres thick on a glass base. A thin alu-minium skin on the phosphor acts as the anodeand also prevents transmission of light back tothe photocathode. Large field image intensifiers

50

4

8

12

16

20

100

10 MHz

7.5 MHz

5 MHz

3.5 MHz

2.5 MHz

150

Pulse repetition period (ms)

Imag

e de

pth

(cm

)

200 250


image intensifier–image intensifier (speed)

I

are typically metal sheathed with a titaniuminput window up to 40 cm diameter and 2.5 cmoutput window coupled to either a video cam-era tube or CCD. X-rays are converted to lightby a caesium iodide scintillator; the light isthen converted to electrons by a photocathode.The electrons are accelerated by a high voltage(typically 30 kV) on to a phosphor applied to theoutput window. Minification and electronicgain intensify the original x-ray photon flux togive a visible image (see Coltman, fluoroscopy,conversion efficiency).

image intensifier (contrast ratio) (xray) Thedegree of contrast loss measured using a leaddisc covering the central area of the imageintensifier face. The lead disc should be at least2 mm thick. Contrast ratio is then:

The contrast loss is then defined as:

Typical values are 20 to 25:1.image intensifier (conversion efficiency) (xray)

A measure of efficiency for image intensifiers, asthe ratio of output phosphor luminance (cd m�2)to input dose (μGy�1s�1). Other methods use microCoulombs (cd m�2 μC kg�1s�1 or usemillirads (cd m�2mR�1s�1). Conversion factormultipliers:

Given Required (multiplier)

μGy�1 μC kg�1 mR�1

μGy�1 1.0 34.0 8.7μC kg�1 0.03 1.0 0.256mR�1 0.115 3.9 1.0

For a 40 cm (16 in) diameter image intensifier typical values are: 1280 cd m�2 μC kg�1s�1;327 cd m�2mR�1s�1; and 38 cd m�2 μGy�1s�2

image intensifier (low frequency drop, LFD)(image) The modulation transfer function (MTF)for an image intensifier shows a rapid dropfrom 1.0 to about 0.8. The percentage drop isthe LFD and represents a fall of 8 to 13% in res-olution, responsible for the degradation of allspatial frequencies. Two main factors areresponsible: (1) x-ray scatter in the input win-dow and phosphor; (2) light diffusion at theoutput window. The first factor can be reduced

100%.

Contrast ratio

Conversion factor without lead discConversion factor with llead disc

.

by using titanium input windows and CsI phos-phor; the second can be reduced by using afibreoptic plate between output window andvideo/CCD device.

image intensifier (resolution) (xray) Spatial reso-lution is often conveniently measured in terms ofline pairs per millimetre (Lp mm�1) by using a linepair test pattern consisting of an etched tungstenor tantalum grating, measuring high contrast fineobject detail in the output image. Overall resolu-tion of the image intensifier improves withincreased dose rate at the image intensifier face,but low contrast visibility is the limiting factor.Dose rate at the face is restricted to within0.15–0.2 μGy s�1 and 3.5–4.0 Lp mm�1 is cur-rently the best performance value. Typical per-formance figures for an image intensifier havethree zoom values 29, 22 and 16 cm, respec-tively; resolution is measured in Lp mm�1:

Operating mode Normal Zoom 1 Zoom 2

Entrance field size 29 cm 22 cm 16 cmLimiting resolutionCentre 4.4 5.0 5.693% radius 3.8 4.6 5.2

Contrast ratioLarge area 22:1 25:1 30:1Small detail 13:1 15:1 17:1

image intensifier (speed) (image) This is thereciprocal of the mean kerma in air at the inputface of the image intensifier (μGy21). For opti-mum performance under automatic control, val-ues should not exceed 1.0 μGy s21. For singlefilm and serial fluorography, it should be within

0

0.00

0.25

0.50

0.75

MTF

1.00

1 2

Resolution Lp mm�1

3

CharacteristicModulation

LFD

4

I


image intensifier (speed)–image reconstruction

the range 0.43–1.74 μGy per frame and within0.087–0.348 μGy per frame for cinefluorography.

image lag (image) Temporal response time,affecting dynamic events when viewed as aseries of images (cine). A common artefact influoroscopy when the video camera has a poorresponse time compared to the speed of thephysiological event.

image matrix (image, ct) The 2D arrangement ofvalues (i.e. CT numbers) at discrete positions; thesingle elements stored are voxels. The displayimage matrix are called pixels (see matrix).

image noise (ct) Noise contributions to the finalimage. Caused by many factors, notably sam-pling error, electronic noise, mechanical misplace-

ment (see noise).image orientation (mri) A recommended stan-

dard orientation for the presentation of nuclearmagnetic resonance (NMR) images is: (1) trans-verse: patient’s right on the left side of theimage, anterior or ventral on top; (2) coronal:patient’s right to left side of image, superior orhead to the top; (3) sagittal: patient’s head tothe top, anterior to the left side of image. R (right), L (left), P (posterior), A (anterior), and ifnecessary S (superior) should be shown on thescreen and the hardcopies, as appropriate.

image plate (image) Filmless recording using eitherphosphor material or selenium. The thermolumi-nescent material used for the phosphor imageplate is commonly a complex barium fluorohalideBaFX:Eu2� where X is the halide atom Cl, Br or I.Under x-ray exposure, the europium ion changesfrom divalent to trivalent state. On stimulationwith laser light of a particular wavelength, thisvalency state is reversed releasing a UV (ultra-violet) luminescence (photostimulated lumines-cence (PSL)) whose intensity is proportional to theoriginal x-ray intensity. This is captured by a pho-tomultiplier whose signal forms a digital image.

image processing (image) A method applied todigital image matrices using precise filters or

X-ray beam

Protective layer

Reflective layer

Conductive layer

Support layer

Phosphor layer150 to 400 μm

selecting thresholds within the image data toaccentuate certain information or accentuatecertain details (edge enhancement) or reduce noise(smoothing, unsharp masking, histogram equalization).

image quality (image) Described by the maximum/minimum range of contrast and resolution reg-istered by the image and its noise content (seesignal to noise, DQE, NEQ, contrast detail diagram,MTF, Wiener spectrum).

image reconstruction (ct) Calculation of the CTimage from the projection data. Attenuation coef-

ficients that have been collected as single-dimensional values must now be displayed as a2D image. The data matrix of these coefficients is obtained from the scanning pattern of the fan beam and is commonly represented in a512 � 512 format. The total absorption figure isknown: this is the ray sum. The separate matrixvalues are found by mathematical reconstruc-tion. The individual values in the matrix can becalculated by using either: iterative reconstruc-tion technique or backprojection (or its deriva-tives). The calculations are performed in adedicated array processor in order to providealmost instantaneous image display.

image reconstruction algorithm (ct) A methodused to reconstruct images from the measuredattenuation profiles (as CT numbers) usingalgebraic (iterative) reconstruction or filteredbackprojection reconstruction.

image reconstruction (back projection) (ct) Atightly collimated x-ray beam is used and itstotal absorption provides the ray-sum signal foreach matrix row, which is stored in the arrayprocessor image memory. This is shown in thediagram (a) for 0° 45° and 90° rotated scanpositions (a). Other projections (b) provide thecomplete data set. The central high uptakefrom the original is now distinguishable, but ithas a star-burst interference pattern. Earlyattempts at this form of reconstruction usedphotographic methods and the final imagescontained star artefacts (there is only one starartefact shown here matching the single highdata point in the original image). The artefactscan be removed by just accepting the high val-ues, but this is not a satisfactory solution.

image reconstruction (filtered backprojection)(ct) The star interference may be removed. A mask signal with negative going edges acts asa high-pass filter removing the remaining back-projected low frequency interference pattern.


image reconstruction–impedance Z

I

image reconstruction (iterative) (ct) This usesan exact mathematical solution for reconstruct-ing the image slice from the attenuation data.This was the original method for image recon-struction used by Godfrey Hounsfield (Britishengineer, 1919–2004) in the first CT machine.Its disadvantages are that it takes a consider-able amount of computer time and is slow. Italso suffers from rounding errors (0.95 � 1.0,etc.) which give imprecise CT values and all thedata must be collected before reconstructioncan begin.

image reconstruction algorithm (ct) Methodused to reconstruct images from the measuredattenuation profiles (as CT numbers) usingalgebraic (iterative) reconstruction or filteredbackprojection reconstruction.

image reconstruction time (ct) The additionaltime taken to display the image after acquisi-tion and axial reconstruction protocol.Important in CT and MRI, since reconstructionof the images can occupy a significant propor-tion of the delay. (image) Additional time after

Signal

BACKPROJECTION

X-ray beam

0°

45°

90°

1

FILTERED BACKPROJECTION

DISPLAYED IMAGE

3

2

�1 �1�1

�1

�1�1

8

�1

�1

Filter

FILTERMASK

(a)

(b)

(c)

completion of image data acquisition before thedisplay of the first image. Dedicated arrayprocessor, CPU speed and available memoryinfluence this.

image resolution (image) See resolution element.image segmentation (image) Partition of an

image into disjoint regions, each of which isuniform with respect to a certain characteristic(such as brightness or texture), but such that nounion of adjacent regions is uniform.

image set reconstruction time (image) Totaladditional time after completion of data acquisi-tion until all images of a set (e.g. multiple planeor volume imaging) are available for display.

image transform (image) Mapping of image datainto image data. An image transform generatesone or several resultant images out of one orseveral given images. It is generally consideredto be a process of analysis, breaking the imagedown into its elemental components (basicimages) and providing further information thatis not readily available in the raw image data.

imaginary signal (math) Out-of-phase compo-nent of the signal from a quadrature detector.

imaging depth (image) Maximum penetrationthat will yield image information. Proportionalto 75/PRF.

imaging volume (image) See volume of investigation.Imagopaque® (cm) Commercial form of iopentol

introduced by Nycomed in 1991.


Iopentol 12.0 at 25° 680 350

imciromab (nmed) See Myoscint®.Imeron® (cm) Commercial form of iomeprol, intro-

duced by Bracco in 1994.

Compound Viscosity Osmolality Iodine(cP) (mOsm/kg) (mg I/mL)

Iomeprol 1.4–12.6 300–726 150–400

impedance Z (phys) A varying a.c. power sourceconnected to an electrical network sees animpedance: Z � A � iB, where A is the ‘real’part of the impedance representing the resis-tive component which is energy dissipating. B is the ‘imaginary’ part of the impedance, madeup mainly of capacitance or inductance. Bothare loss-less energy storage elements and canretrieve energy stored at some previous time

I


impedance Z–111Indium

without loss, so they produce phase shift. AnRF signal is affected by both the capacitanceand inductance of a circuit, as well as its resist-ance; this total opposition to the signal is theimpedance. The matching of the impedancesbetween circuits is essential to prevent signalloss. (us) Defined as material density � soundpropagation speed. Typical range 1.3 � 106 to1.7 � 106 rayls (see input impedance).

impedance matching (phys) Adjusting the elec-trical impedance of two circuits that are beingconnected so that they are equal, so preventingsignal loss or ringing.

impulse (phys) A very short duration electrical sig-nal which can be represented by a Dirac function.An important property of ultrasound and MRI-transmitted pulses. (elec) A brief excursion of elec-tric voltage from its normal value, usually zero.

impulse response function (di) The response, inthe time domain, of a circuit (amplifier) or filterto a precisely known input and measuring theresulting output function. The total responseexpressed as a weighting function is theimpulse response function, uniquely definingthe system. Usually seen as a decaying wave-form in ultrasound or MRI pulses (see frequency

response function).incandescence (xray) The emission of visible

radiation from a material at high temperature. Itis applied to illumination (e.g. viewboxes), x-rayanodes when bombarded by an electron beam.

incidence (angle of) (us) The angle between theincident ray and the reflected ray. Anglebetween incident sound direction and a lineperpendicular to the boundary of the medium.

incidence (incidence rate) (dose) The rate ofoccurrence of a disease within a specifiedperiod of time, often expressed as a number ofcases with a disease per 100 000 individualsper year (or per 10 000 person-years).

incident (radiation) (phys) The primary radiationstriking a surface. Either electromagnetic orsound.

incidence (screening) (clin) The number ofnewly diagnosed cases (cancer) in a definedpopulation within a defined period. Usuallyexpressed as an incident rate of x per 1000 forsmall populations or x per 100 000 per year forlarge populations. An incidence of 8 per 1000means eight new cases of disease per year per1000 of the defined population (see incident

screen, interval disease).

incident (radiation) (phys) The primary radiationstriking a surface. Either electromagnetic or sound.

incident screen (screening) (clin) The screeninginvestigation carried out after the first screen,which detects the incident disease.

incoherent spins (phys) A state of a set of spinsin which the ensemble of spins in a voxel areuniformly distributed with phases between 0 and 2p reducing the transverse magnetiza-tion in a voxel to essentially zero.

independent event (stats) Events which have noinfluence on each other. Independent variableis a random variable.

independent variable (stats) (or explanatoryvariable) The independent variable appears onthe right hand side of the equation.

indirect splenoportography (clin) See splenopor-

tography.Indichlor® (nmed) 111Indium chloride preparation

from Amersham/GE Healthcare.indium (In) (elem)

Atomic number (Z) 49Relative atomic mass (Ar) 114.8Density (ρ) kg/m3 7310Melting point (K) 429.8K-edge (keV) 27.9Relevance to radiology: As a low melting point alloy forlubricating anode bearing in x-ray tubes.

111Indium (nmed) T½ as a scintigraphic label forradiopharmaceuticals (listed below)


Decay scheme (e.c.) 111In 111In (γ 171, 245 keV) →111Cd stable

Gamma ray constant 8.4 � 10�2 mSv h�1

GBq�1at 1 mHalf-life 2.8 daysDecay constant 0.2475 day�1

Annual limit on intake 150 MBq (4.0 mCi)Half value layer 0.23 mm Pb, 51 mm waterUses for labelling biological agents (somatostatin,antibodies and white cells). 111In-DTPA used for imagingthe cerebrospinal fluid (CSF) pathway. 113 mIn generatoralso used for general imaging (now uncommon).

Radiation attenuation for lead

Thickness (mm Pb) Coefficient of attenuation

0.23 0.52.03 10�1

5.13 10�2

8.34 10�3

11.20 10�4

or Ag( He, n) In47109 4

491112

48111

49111Cd(p,n) In


111Indium–inflow technique

I

Physical decay: 111Indium


0 1.0003 0.9706 0.94012 0.88524 0.78136 0.69048 0.610

111Indium DTPA (nmed) See 111Indium pentetate.111Indium chloride (nmed) Used as a label for a

variety of radiopharmaceuticals including mono-clonal antibodies and peptides (see OncoScint®

and Octreoscan®).111In-oxyquinoline (nmed) 111In-oxine, a diag-

nostic radiopharmaceutical (Amersham/GEHealthcare) for radiolabelling autologous leuco-cytes (white blood cells). The presence of redblood cells or plasma will lead to reduced leu-cocyte labelling efficiency. The transferrin inplasma competes for 111indium-oxyquinoline.

111Indium pentetate (nmed) As 111In DTPA, aradiopharmaceutical for cyeternography CSF.

111Indium satumomab pendetide (nmed) SeeOncoScint®.

111Indium tropolone (nmed) A neutral, lipid-soluble metal complex of 111In and tropoloneused as a tracer for white blood cell and plateletlabelling. Unlike oxine, which is soluble in ethylalcohol, tropolone is soluble in isotonic saline.

113mIndium (nmed) Generator-derived nuclide forbrain, liver and blood pool imaging.

ProductionDecay scheme (i.t.) 113mIn

Eluent 0.05 M HClGamma ray 4.6 � 10�2mSv hr�1GBq�1at 1 mconstant

Half-life 1.7 hoursHalf value layer (mm Pb)

individual dose (dose) (ICRP60) A distinctionbetween the collective dose of a selected pop-ulation and the dose to an individual in thatpopulation.

inductance (phys) Measure of the magnetic cou-pling between two current-carrying loops of anRF circuit. A property of a conductor resultingfrom the magnetic field induced when a currentflows. The SI unit of self and mutual inductance

113 12

113

113

118Sn days In ( 393 keV)

In stable

mT →

→

γ

50112

50113 113Sn (n, ) Sn Inmγ →

is the henry (H) which is the inductance of aclosed loop that gives a magnetic flux of 1 Wb A�1.

induced genomic instability (dose) The induc-tion of an altered cellular state characterized bya persistent increase over many generations inthe spontaneous rate of mutation or othergenome-related changes.

induction B (phys) See magnetic induction.inductor (phys) A device (e.g. a coil) possessing

inductance.in-elastic scattering (phys) See Compton scattering.inert gases (noble gases/rare gases) (chem)

A group of gaseous elements forming group 18 (previously group 0) in the periodic table.The relative abundance of these gases in airtogether with the major constituents is as follows:

Gas Symbol Z Abundance Density (%) (kg m�3)

Air – 7.78 100 1.293(eff)

Nitrogen N2 7 78 1.25Oxygen O2 8 28 1.429Carbon CO2 – 0.04 1.97dioxideHelium He 2 – 0.166Neon Ne 10 0.0018 0.839Argon Ar 18 0.93 1.66Krypton Kr 36 0.0001 3.5Xenon Xe 54 0.0009 5.5Radon Rn 86 – 9.73

inertia (phys) Any mass when in motion tends toresist being retarded. In the case of a rotatinganode, it can be shown that its tendency topreserve its state of motion (its inertia) is deter-mined by its mass and its radius. The inertia I ofa disc of uniform density having mass M andradius R is given by from which it isseen that the inertia can be increased byincreasing either the mass or the diameter ofthe disc.

inferior mesenteric arteriography (clin)Arteriography of the inferior mesenteric arteryusually demonstrating gastrointestinal bleed-ing or mesenteric ischaemia, and sometimestumours of the large bowel (see superior mesen-

teric arteriography).inferior venocavography (clin) Angiography of

the inferior vena cava (IVC), using a commonfemoral vein approach.

inflow technique (mri) See time of flight angiogra-

phy (TOF).

I MR� 12

2

I


inflow amplification–integration/differentiation

inflow amplification (mri) Slowly moving bloodperpendicular to the slice returns a strongersignal than the surrounding tissue. If a boluswithin a selected slice is excited using a 90°pulse, the spins fully recover within a shortrepetition time (TR). They retain their satura-tion, but the signal is weaker than that from a long TR in relationship to T1. Conversely,spins outside the slice are fully magnetized andspins flowing out of the slice are replaced by fresh inflowing spins. The overall effect isthat vascular magnetization within the slice decreases.

information density (nmed) The count density in units of counts/cm2 of gamma cameraimages.

information technology (IT) (comp) A widerange of techniques used to handle and distrib-ute information. Incorporating computing andall forms of electronic communication.

infrared radiation (phys) A section of the elec-tromagnetic spectrum capable of producingheat. Situated between wavelengths 0.7 μm to1 mm. It has subregions IRB and IRC.

inherent filtration (xray) See filtration.inhomogeneity (mri) Degree of lack of homo-

geneity; for example, the fractional deviation ofthe local magnetic field from the average valueof the field.

input impedance (elec) The impedance pre-sented by an electronic circuit at its input.Charge amplifiers connected to the output of aphotomultiplier have a very high input imped-ance, RF-coil amplifiers (MRI) have a low inputimpedance (see impedance).

instantaneous dose rate (dose) The dose rateaveraged over 1 minute at a particular location.

insufflation (peri-renal) (clin) Obsolete proce-dure which injects air or CO2 around the kidneys in order to visualize the adrenal glands.

insulator (phys) A material having a wide forbid-den zone between electron orbits and conduc-tion zone, thus preventing orbital electronsacting as electrical carriers. The permeability indi-cates the degree to which the medium canresist the flow of electric current compared tofree space. The value εr varies from unity (for avacuum) to over 4000 for some composite mate-rials. Gases including water vapour have rela-tive permittivity values marginally greater than1. Plastics have values between 2 and 3.5, glassbetween 5 and 10 and pure water is 80.4.

intake (i) (dose) Activity that enters the bodythrough the respiratory tract or gastrointestinaltract from the environment.

integer (math) A whole number.integral (math) The area underneath a curve

describing a function.integral dose (dose) The total dose received by a

cell culture, tissue or population over a periodof time.

integral uniformity (nmed) See uniformity (integral).integrated panorama array (IPA) (mri) An inte-

grated panorama array (Siemens) acceleratesset-up time and increases patient throughputby employing up to 4, 8 or 16 independentarray coil systems linked simultaneously.

integrated imaging (diagnostic) (stats)Combining two or more diagnostic tests, whichutilize different imaging functions, improves theoverall sensitivity and specificity for detecting dis-ease. For tests x and y reporting two sensitivitiesSenx and Seny and two specificities Specx andSpecy, then for these two test conditions:

1 If x and/or y are positive or x and y are nega-tive, integrated sensitivity is:

Senx � ((100 � Senx) � Seny)/100%

Integrated selectivity is: (Specx � Specy)/100%.

2 If x and y are positive or x and/or y are nega-tive, the integrated sensitivity is:

(Senx � Seny)/100%

Integrated selectivity: Specx � ((100 � Specx) �

Specy)/100%.If the following percentage diagnostic accura-cies have been reported:

Sensitivity (%) Selectivity (%)

Nuclear medicine 80 60Computed 90 90

tomography

For condition (1), sensitivity becomes 98% andspecificity 54%; for condition (2), sensitivitybecomes 72% and specificity 96%. High sensitiv-ity excludes disease and is most valuable whenscreening. High selectivity confirms that diseaseis present and prevents unnecessary treatment.

integration/differentiation (math) Integration isthe inverse of differentiation; it is the area undera curve. The integral of a function f (x) withrespect to x is written as . The symbol

being the old English for S, representing sum.∫f x dx( ).∫


integration/differentiation–interactive real time

I

The definite integral f (x) defined by an interval a to b is:

The infinite integral applies if one or both of thelimits tends to infinity. The integral dose orexposure over an organ or surface can be rep-resented as a gaussian distribution whichserves to demonstrate the integration process.Differentiation is concerned with studying therate of change of a function with respect to itsvariable (e.g. time); integration is the converse.■ Reference: Gonzalez and Wintz, 1987.

intensification factor (xray) Product of minifica-tion gain and electronic gain.

intensifying screen (image) Thin sheets of plas-tic impregnated with phosphor which convertsx-radiation into UV/visible light. Early phos-phors used calcium tungstate; current phos-phors are complexes of lanthanum andgadolinium (rare earth phosphors); other phos-phors which emit exclusively in the UV spec-trum are complexes of yttrium, tungsten andtantalum. Either a single or double screen isused in the film cassette applied directly to thefilm emulsion (single- or dual-sided film). A largeincrease in quantum efficiency enables the film tobe exposed with a significantly lower patient

radiation dose. Intensifying screens are gradedaccording to speed (sensitivity). Conventional or‘par’ speed is 100. Detail screens with a thinnerphosphor layer have speeds below par. Fastscreens which offer very low dose investiga-tions (e.g. paediatrics, lumbar spine) havespeeds up to 400.

intensity (phys) The power carried by a wave oroscillation divided by the area (W m�2) (see pho-

ton fluence). (us) See ultrasound (intensity).intensity, instantaneous (i) (us) The instanta-

neous ultrasound intensity transmitted in thedirection of acoustic wave propagation, per unit

Protective layer 5 to 10 μm

Phosphor 100–500 μm

Reflective layer 20 μmPlastic base 200 μm

f x dxb

a

( ).∫

area normal to this direction, at the point con-sidered. It is given by: i � P2/2ρc, where P isthe instantaneous acoustic pressure; ρ is the den-sity of the medium; and c is the speed of soundin the medium. The unit is Watt per square-centimetre, W cm�2.

intensity (maximum) Im (us) The intensity aver-aged over the time of the half cycle (halfperiod). The unit is W cm�2.

intensity modulation (image) The variation ofsignal intensity (video signal) in a light imageon a video screen (CRT (cathode ray tube) or flatfield: LCD (liquid crystal display) or TFT (thinfilm transistor)) (see video display, video signal).

intensity profile (us) Acoustic intensity acrossthe ultrasound transducer has an approxi-mately gaussian distribution having a spatialpeak intensity (SP) and spatial average inten-sity (SA) (see intensity).

intensity reflection coefficient (IRC) (us)Defined as reflected intensity/incident inten-sity. Both units measured in W cm�2.

intensity transmission coefficient (ITC) (us)Defined as transmitted intensity/incidentintensity. Approximately equal to 1 � IRC (inten-

sity reflection coefficient), both units measured in W cm�2.

intensity reflection coefficient. Reflectedintensity divided by incident.

intensity, temporal average (ITA) (us) The timeaverage for the intensity at a point in spacefrom the transducer. For non-autoscan systems,the average is taken over one or more pulserepetition periods. For autoscan systems, theintensity is averaged over one or more scan rep-etition periods for a specified operating mode.For autoscan modes, the average includes con-tributions from adjacent lines that overlap thepoint of measurement. For combined modes, theaverage includes overlapping lines, from allconstituent discrete operating mode signals.Measured in mW cm�2.

intensity, temporal peak ITP (us) The peak valueof the intensity at the point considered. The unitis Watt per square-centimetre, W cm�2.

intensity transmission coefficient. Transmittedintensity divided by incident intensity; thefraction of incident intensity transmitted intothe second medium.

interactive real time (mri) Changing the meas-urement parameters in real time by operatorcontrol.

I


interactive shim–Internet

interactive shim (mri) Manual tuning of the shimcoils to improve magnetic field homogeneity.Shim currents are set individually for a selectedpulse sequence.

interface (comp) The interconnection (eitherhardware or software) that allows a device, aprogram or a person to interact. Hardwareinterfaces are the cables that connect thedevice to its power source and to other devices.Software interfaces allow the program to com-municate with other programs (such as theoperating system), and user interfaces allowthe user to communicate with the program (e.g.via mouse, menu commands, icons, voice com-mands, etc.).

interference (phys) The interaction of anunwanted structured signal (not noise) with thewanted signal. Examples would be interferingsignals from adjacent slices in MRI whichdegrade the information contained within thetargeted slice. When waveforms having thesame frequency but slightly different phasecombine, they interfere with each other asshown in the figure. Waveforms in-phase(coherent) show constructive interference andproduce a resultant of greater amplitude (Cn).Waveforms out of phase (incoherent) showdestructive interference (Ds). Increasing phasedifferences give greater degrees of destructiveinterference until a 180° phase difference causethe signal to disappear giving a zero resultant.

(see beat frequency).interlacing (image) A method for reducing the

bandwidth necessary to give a video display.Fields are interlaced to give each frame. Non-interlaced (progressive scan) displays requirehigher bandwidth, but have better resolution.

interleaved image acquisition (mri) Combinedcollection of data for two or more separate

Cn

Ds

images; a subset of k-space samples for thesecond image is acquired immediately after thefirst image. This avoids misregistration betweenthe two images and allows for accurate subtrac-tion of the two images.

interleaved k-space coverage (mri) The sequen-tial collection of raw data from multiple excita-tions such that each excitation samplesmultiple lines or curvilinear paths in k-space.

interleaved slices (mri) See slice sequence.internal conversion (nmed) A nucleus in an

excited state may interact with K or L shellelectrons and transfer excess energy to theseelectrons. This internal conversion complimentselectron capture. Internal conversion is a compet-ing process with gamma ray emission and aninternal conversion coefficient. The number ofparticles emitted per transition, or the fractionalabundance, takes part in MIRD calculations.Since internal conversion creates a vacancy inan electron orbit of the atom characteristic x-rays

and Auger electrons will be emitted which con-tribute to the total tissue radiation dose.

internal conversion coefficient (nmed) Describesthe proportion of internal conversion events in agamma decay process which blocks gammaemission as:

This ratio can range from zero (all transitionsresult in gamma emission) to infinity (all transi-tions are internally converted: no gamma emis-sion). It is estimated that 10.4% of all 99mTcnuclear transformations involve internal conver-sions by the K, L and to a lesser extent M shellelectrons. About 40% of nuclear transformationsinvolve internal conversion in the decay of 81mKr.

internal focus (us). A focus produced by a curvedtransducer element.

internal reflection (phys) This occurs when thereflection angle for the light source is �90° (seefibreoptics).

International Commission on Radiation Unitsand Measurement (dose) See ICRU.

International Commission on RadiologicalProtection (dose) See ICRP.

Internet (comp) A global network able to link upa large number of computers (millions). A com-puter joins the Internet via a modem or broad-band router and adopts the standards and

Internal conversion processGamma emission

.


Internet–intrathecal

I

software set-up that allows it to take advan-tage of the services (e-mail, web, FTP file trans-fers) (see FTP, TCP/IP, e-mail, worldwide web).

interpolation (math) Estimation of a value of avariable between two known values. A mathe-matical method of averaging or smoothingimages that are being displayed on a largernumber of pixels than that for which they wereoriginally reconstructed.

interpolation (nearest neighbour) (image) Thesimplest interpolation method basically makingthe pixels bigger. The greyscale or colour of anintroduced pixel in the new image is the colour ofthe nearest pixel of the original image. Mostimage viewing and editing software use this typeof interpolation for enlarging a digital image forcloser examination; this does not change thegreyscale/colour information of the image anddoes not introduce aliasing. It will increase thevisibility of jaggies or pixelation.

interpolation (bicubic) (image) Producessmoother edges than bilinear interpolation. A new pixel is a bicubic function using 16 pixelsin the nearest 4 � 4 neighbourhood of the pixelin the original image. This is the method mostcommonly used by image editing software,printer drivers and many digital cameras forresampling images.

interpolation (bilinear) (image) This determinesthe value of a new pixel based on a weightedaverage of the four pixels in the nearest 2 � 2neighbourhood of the pixel in the originalimage. The averaging has an anti-aliasingeffect and therefore produces relatively smoothedges with hardly any jaggies/pixelation.

interpolation (fractal) (image) Mainly useful forextreme enlargements; it retains the shape offeatures more accurately with cleaner, sharperedges and fewer halos and less blurring aroundthe edges than would bicubic interpolation. Anexample is Genuine Fractals Pro from TheAltamira Group.

interpolation algorithms (ct) To reconstruct anaxial image from a helical data set, single-slicescanners have commonly used 180° linearinterpolation algorithms. The z-sensitivity pro-file (imaged slice width) for the helical scan,with a pitchx � 1, is similar to that of an axialimage. On multi-slice scanners, the use of a180° linear interpolator, in conjunction withpitchx � 1, results in wider z-sensitivity pro-files, and increased artefacts, in comparison to

single-slice systems. Helical interpolation algo-rithms for multi-slice CT are different thanthose for single-section CT, but in general donot cause any increase in image artefacts com-pared with the algorithms for single-sectionhelical CT and are generally much less notice-able in multi-slice CT because most scanning isperformed at lower pitch values than was prac-tical with single-section systems.

interpulse times (mri) Times between successiveRF pulses used in pulse sequences. Particularlyimportant are the inversion time (TI) in inversionrecovery and the time between 90° pulse andthe subsequent 180° pulse to produce a spinecho, which will be approximately one half thespin echo time (TE). The time between repetitionsof pulse sequences is the repetition time (TR).

interscan delay time (ISD) (ct) Limitations tocollecting data as sequential axial slices particu-larly when small lesions are being imagedsince these can be missed if they are locatedbetween adjacent slices. The sequential sliceprotocol, although giving complete disc-likesections, includes delays between slices sincethe sequential process consists of:

● fan beam assembly accelerated to scan speed;● x-ray tube pulsed and data collected;● fan beam assembly halts and returns to its

home position;● table indexed to next longitudinal position

while x-ray tube cools.

This series of events constitutes the interscandelay time and adds a significant time to theclinical study causing problems if the patientmust hold their breath or patient movement ispresent.

inter-slice distance (ct) The distance betweenthe adjacent nominal margins of consecutiveslices in serial CT scanning. It is dependentupon the couch increment between slices.

intervention (dose) (ICRP60) Those activities whichsubtract from a person’s overall radiation expo-sure. Controls applied to the source are usuallythe most effective interventional methods, e.g.improved quantum efficiency of detection (imag-ing) and heavier shielding (see practices (ICRP)).

intra-arterial (clin) Relating to substances (con-trast media) injected or deposited into an artery.

intracellular (clin) Within the cell.intrathecal (clin) Within either subarachnoid or

subdural space.

I


intrathecal contrast media–intrinsic quantum efficiency

intrathecal contrast media (cm) Water-solublecontrast media can enter the brain by diffusion.Penetration of brain tissue by contrast mediaafter intrathecal injection has been observed byboth ionic and non-ionic contrast material,causing a variety of changes and reactions dueto chemotoxic effects. Unlike the blood–brainbarrier, there appears to be nothing similarbetween the cerebral spinal fluid and extracel-lular fluid of the brain.■ Reference: Dawson and Clauss, 1999.

intravenous (clin) Relating to substances (con-trast media) injected or deposited into a vein.

intravenous cholangiography (clin) Where intra-venously injected, contrast medium is excretedinto the bile by hepatocytes. The contrastmedium concentrates in the gall bladder and isexcreted in the faeces and urine unchanged.There is strong binding to plasma proteins.

intravenous cholecystography (clin) Slow injec-tion of contrast medium over a period of 1 hour.Replaced in some instances by endoscopic ret-rograde cholangiography (ERC) or percutaneoustranshepatic cholangiography (PTC).

intravoxel incoherent motion (mri) Diffusion andperfusion both act to reduce the signal observedin vivo; the two effects are difficult to separateand the term ‘intravoxel incoherent motion’ isused to describe their combined effect.

intravoxel phase dispersion (mri) Signal loss dueto spread of spin phases within a voxel (usuallybecause of motion or susceptibility effects).

intrinsic efficiency (image) The quantum effi-ciency of a system (imaging detector) when allexternal devices (collimation) have been removed.

intrinsic resolution (image) The primary resolu-tion of the gamma camera crystal without itscollimator. This is usually measured by using aslit source placed directly on the crystal faceand a line spread function obtained.

interval cancer (screening) (clin) The incidenceof disease between screening intervals andafter a negative test result. Results are false neg-

ative if a cancer becomes known within 1 yearof a screening result. After that time it becomesan interval cancer (see incidence (screening)).

interventional level (dose) (ICRP60) A value ofequivalent or effective dose at which interven-tion measures should be considered.

intranet (comp) An internal network of computers(RIS or HIS) controlled by a network administratorwho decides access privileges and controls the

software. A network maintained by a companythat is only available to its staff or other author-ized users and looks like a private Internet.

intra-osseus venography (clin) Intra-osseusvenography visualizes the pelvic veins or lowerlimb veins where the contrast medium is intro-duced via a cannula directly inserted into the medullary space of the greater femoraltrochanter or lateral malleolus.

intravascular space (clin) Within the vessel(blood or lymphatic).

intravenous angiocardiography (clin) Usuallyperformed on children or young adults whencatheter procedures are unsuitable.

intravenous cholangiography (clin) Radiographyof the bile ducts opacified by hepatic secretion ofintravenously injected contrast medium.

intravenous cholecystangiography (clin)Demonstrates the extrahepatic biliary tree andgall-bladder (see Biligrafin).

intravenous pyelography (IVP) (clin) See intra-

venous urography (IVU).intravenous urography (IVU) (clin) Radiography

of kidneys, ureters and bladder following injec-tion of contrast medium into a perpheral vein(also called excretory urography) (see nephroto-mography, zonography).

intravoxel incoherent motion (mri) Diffusion andperfusion both act to reduce the signal observedin vivo; the two effects are difficult to separateand the term ‘intravoxel incoherent motion’ isused to describe their combined effect.

intravoxel phase dispersion (mri) Signal lossdue to spread of spin phases within a voxel(usually because of motion or susceptibilityeffects).

intrinsic filtration (ct) Filtration of an x-ray beam bythe component parts of the x-ray housing (x-raytube envelope glass or exit window); the intrinsicbeam filtration is also caused by the anode itself,the oil used for cooling (see added filtration).

intrinsic quantum efficiency (ct) Absorption of anx-ray photon by the scintillator host lattice gener-ates numerous free electrons and holes in theconduction and valence bands, respectively. Oncethe electron-hole pairs are captured and bound toa selected site, the recombination energy of thesepairs is transferred to that site where radiative(luminescence) energy is emitted. Ideally, there is afast recombination of electron-hole pairs at theactivator sites (luminescent centres). In this way, auseful scintillator will produce several thousand


intrinsic quantum efficiency–in vivo

I

visible photons from the absorption of one x-rayphoton.

intrinsic resolution (nmed) The primary resolu-tion of the gamma camera crystal without itscollimator. This is usually measured by using aslit source placed directly on the crystal faceand a line spread function obtained.

intrinsic semiconductor (elec) A pure semiconduc-tor where hole and electron densities are practi-cally equal. Used for the manufacture of detectors;the so-called hyperpure germanium (HpGe).

invalid page fault (comp) Caused when a page ofdata or program code in the swap file cannot, forvarious reasons, be loaded into the main memory.

invariant (image) The response of a system is the same at every position of the x axis (seeconvolution).

invasive probe (us) An ultrasound probe that isintended to contact tissue other than intact skinor the surface of the eye. These include transvagi-nal, transoesophageal, transrectal, transurethral,intravascular and intraoperative probes.

inverse Fourier transform (math) Form of theFourier transform that reverses the process, e.g.if the Fourier transform is used to analyze afunction of time into its equivalent frequencycomponents, the inverse Fourier transform willsynthesize that function of time from these fre-quency components.

inverse image transform (di) Process of synthe-sis, reassembling the original image from itscomponents via summation.

inverse square law (phys, dose) The reduction ofradiation intensity from an isotropic point source(Id) at a distance d will vary as Id/d2. Variations onthis basic formula can be used for calculatingintensities or doses at different distances.

the reciprocal of the other, x 1/y, so x isinversely proportional to y. Examples would beresolution and sensitivity in nuclear imagingand distance and dose from a point source: theinverse square law.

inversion (mri) The magnetization vector orientatedopposite to the magnetic field produced by 180°pulse or gradient switching. A nonequilibriumstate where the macroscopic magnetization vector isoriented opposed to the magnetic field; producedby adiabatic fast passage or 180° RF pulses.

inversion recovery (mri) A pulse sequencewhere the nuclear magnetization is invertedprior to a spin-echo pulse sequence by an ini-tial 180° pulse. The time between the 180°pulse and the SE sequence is the inversiontime TI. The signal is dependent primarily onT1 (the image is T1 weighted). Imaging time isrelatively long due to long TR.

inversion recovery sequence (mri) A pulsesequence where the nuclear magnetization isinverted prior to a spin-echo pulse sequence.The time between the 180° pulse and the SEsequence is the inversion time TI; an initial180° pulse to invert the magnetization followedby a spin-echo or gradient-echo sequence.Representation of data from image acquisitionas a two-dimensional matrix of points. Thecoordinates of each point represent a uniquecombination of frequency and phase corre-sponding to the time-integral of the frequency-and phase-encoding gradients, respectively.

inversion-recovery-spin-echo (IRSE) (mri)Form of inversion-recovery imaging in whichthe signal is detected as a spin echo. For YEshort compared to the 72 relaxation time, therewill be only a small effect of T2 differences onimage intensities; for longer TEs, the effect ofT2 may be significant.

inversion time (TI) (mri) See inversion recovery.inversion transfer (mri) See saturation transfer.inverter (phys) Any circuit that converts d.c. to

a.c. A common component in high frequency x-ray generators.

in vitro (xray) Literally ‘in glass’, referring to apreparation outside the body, e.g. in vitro labellingof red blood cells; the radionuclide labelling iscarried out under sterile conditions in the labora-tory and then the preparation reinjected.

in vivo (xray) Literally ‘in life’, referring to a pro-cedure that occurs within the body, e.g. in vivolabelling of red cells when the radionuclide is

Inverse square law

1 The radiation dose rate at 30 cm from a pointsource is 7.5 μSv hr�1. If the known dose 7.5 μSv isId at distance d, then dose rate Idn at 80 cm (dn) is:

2 The distance that reduces 7.5 μSv h�1 at 10 cm to2.5 μSv h�1 is:

dI d

Ind

dn

22

55��

� cm.

II d

ddn

d

n

��

� �2

211� μSv h .

inversely proportional (math) Describes twonumbers, one of which is in a constant ratio to

I


in vivo–129Iodine

post-injected after a previous injection pre-pares the cells for labelling (pyrophosphate).

I/O port (comp) A connector on the back of a com-puter for accepting input or output data from anexternal device.

iobenguane (nmed) m-Iodobenzylguanidine, aBristol-Myers-Squibb preparation for MIBG.

iobitridol (cm) For angiography, venography,urography, arthrography, angiocardiographyand hysterosalpingography (Guerbet).


Iobitridol 4–10 585–915 250–350

iocetamic acid (clin) An x-ray contrast medium;generic name for Cholebrine.

iodamide compounds (clin) Ionic monomer x-raycontrast material. A water-soluble, monomeric,ionic x-ray contrast medium based on tri-iodinated benzoic acid (see Uromiro).

iodecol (cm) Non-ionic dimer with low toxicity,but insufficiently water soluble for clinicalapplications.

iodine (I) (elem)

Atomic number (Z) 53Relative atomic mass (Ar) 126.9Density (ρ) kg/m3 4940Melting point (K) 386.6K-edge (keV) 33.1Relevance to radiology: Complex compounds of iodineare used as x-ray contrast for vascular studies.

123Iodine (nmed) Used for Na123I in capsules forthyroid function and morphology (scintigraphy).An important radionuclide for imaging; ideal forimaging using labelled compounds (hippuran,MIBG), thyroid tests and labelling fatty acids,monoclonal antibodies and proteins.


Decay scheme (e.c.)123I

Gamma ray constant 4.4 � 10�2 mSv h�1GBq�1

at 1 mPhotons (abundance) 27 keV (0.86)

159 keV (0.834)529 keV (0.014)

Half value layer 0.4 mm Pb, 46 mm water

123 12123

13 159

1 2 1013

I h( keV)

Te years

T

→ . �

52122

53123

54124

55123

123 123

Te(d,n) I

or Xe(p,2n) Cs

Xe I(prefer→ → rred)

(European: I(p,5n) Xe)127 123

125Iodine (nmed) As a label for in vitro radio-immunoassay tests and sometimes as a thyroidtherapy agent.

Production (reactor)

Decay scheme 125I (27 keV x-ray) :125Te stable(e.c.) 125I

Gamma ray 3.4 � 10�2mSv h�1GBq�1 at 1 mconstant

Annual limit on 1.5 MBq (0.04 mCi)intake

Photons 35 (0.07)(abundance) x-rays 27–32 (1.40)

Half value layer 0.02 mm Pb

Physical decay for 125iodine


1 0.9892 0.9775 0.94410 0.89115 0.84120 0.79425 0.75030 0.70840 0.63150 0.56260 0.501

Shielding for 125iodine

Lead thickness (mm) Attenuation factor

0.017 0.50.058 10�1

0.12 10�2

0.2 10�3

0.28 10�4

125Iodine (iothalamate) (nmed) Sodium iothala-mate having been iodinated with 125iodine; forthe determination of the glomerular filtrationrate (GFR) (see Glofil®).

125Iodine (albumin) (nmed) Radioactive iodi-nated serum albumin (RISA) (see Isojex®).

129Iodine (nmed) As a substitute calibrationsource for 125iodine.

ProductionDecay scheme (first stage) 129I

Photons 30–35 keV (0.69)(abundance) 40 keV (0.075)

Half-life 1.57 � 107yDecay constant 4.414E–8 y�1

Gamma ray 1.7 � 10�2 mSv h�1GBq�1 at 1 mconstant

Half value layer 0.02 mm Pb

129 12

129

1 57 107 38I years ( KeV)

Xe stable

T . �

→

nn

nnX(x,x) Y

54124

54125 125 125Xe (n, ) Xe Xe Iγ → →


131Iodine–iodixanol

I

131Iodine (nmed) Thyroid function tests. Therapyfor thyrotoxicosis and thyroid cancer

Production (reactor)Decay scheme

(first stage) 131I

Photons 80 (0.026)(abundance) 284 (0.061)

364 (0.812)637 (0.073)723 (0.018)

Decay constant 0.086194 day�1

Gamma ray 5.7 � 10�2m Svh�1GBq�1

constant at 1 mHalf value layer 2.4 mm Pb, 63 mm water

Physical decay for 131iodine


0 1.0001 0.9172 0.8423 0.7724 0.7085 0.6506 0.5967 0.5478 0.5029 0.46010 0.42211 0.38712 0.35513 0.32614 0.299

Shielding for 131iodine

Lead thickness (mm) Attenuation factor

2.40 0.58.90 10�1

16.0 10�2

25.5 10�3

37.0 10�4

iodine content (contrast medium) (cm) Viscosity

and osmolality of a contrast medium are relatedto the iodine content or concentration of thecontrast medium solution, referred to as itsstrength in milligrams of iodine per millilitre(mg I mL�1). The opacifying power of the con-trast medium solution increases with itsstrength along with osmolality and viscosity;the physiological tolerance declines. Thismakes it necessary to have several differentstrengths (in mg I mL�1) for each contrastmedium preparation.

131 12

131

8 0 364I days ( , keV)

Xe stable

T . β γ�

→

92235

52131 131U (n, ) Te If →

Contrast Iodine content Viscosity agent (mg mL�1) (mPa.s)

20°C 37°C

Iotrolan 300 17.8 9.5Diatrizoate 306 – 5.0Ioxaglate 320 14.6 9.5Iodixanol 320 24.6 12.2Iopentol 350 21.9 11.2Iohexol 350 21.9 11.2Iopromide 350 20.1 9.5Iopamidol 370 18.7 9.8

iodine ratio (contrast medium) (cm) Iodine con-tent is a measure of radiopacity; this is measuredas a ratio of iodine atoms to particles or ions. Theionic-monomer contrast material dissociates intotwo ions: anion and cation. The anion carriesthree atoms of iodine between the two ions, sothe ratio of iodine atoms to ions is 3:2 or 1.5:1(ratio 1.5). The ionic dimer carries six iodine atomsfor the two ions, so has an iodine ratio of 6:2 or3:1 (ratio 3). The nonionic-monomer does not dis-sociate so has an iodine ratio of 3:1 (ratio 3).Their osmolalities are about half that of ionicmonomer, or 2.5–3 times the osmolality of bloodat the highest available concentrations. The non-

ionic dimer has a ratio of 6:1 (ratio 6). Non-ionicdimeric contrast media have two linked iodi-nated benzene rings. These molecules have sixiodine atoms per particle (‘ratio 6’).

iodine release (contrast medium) (cm)Elemental iodine is normally not a degradationproduct of contrast media. The release ofcolourless iodide ion occurs in both non-ionicand ionic CM. This is catalyzed by heavy metalions, pH value, light exposure and increasedtemperature.■ Reference: Dawson and Clauss, 1999.

iodipamide (clin) The group of ionic x-ray con-trast media that contain the meglumine salt ofiodipamide. A large molecule contrast mediumappearing commercially as Biligrafin andEndografin.

iodixanol (cm) Non-ionic dimer for cerebralangiography, cardiac and peripheral arteriogra-phy, urography, venography, ERCP, hystero-salpingography and gastrointestinal studies.

Iodine (mg/mL) Osmolality Viscosity

150–320 290 12.2

(see Visipaque®).

I


iodohippurate–ion beam (cyclotron)

iodohippurate (nmed) A compound which is com-pletely excreted by the renal tubules and com-pletely extracted on its first pass through therenal capillary system. 123I-ortho-iodohippurateis used in scintigraphy/renography for the accurateestimation of renal function.

Iodotope® (nmed) 131Iodine-labelled sodiumiodide (Bracco) for thyroid uptake and therapy.

iodoxamic acid (clin) Generic name for Endomirabil.iofetamine/iofetaminal (nmed) An ampheta-

mine analogue, rapidly taken up by the lungsand from there redistributed primarily to thebrain and liver. Labelled with 123I, it is used asa brain imaging agent in the localization andevaluation of certain kinds of stroke.

ioflupane (nmed) Radio-iodinated cocaine ana-logue (123I-FP-CIT or 123I-β-CIT-FP). Chemicalform: N-w-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)-nortropane. As 123I-ioflupaneused for detecting loss of functional dopaminer-gic neuron terminals in the striatum of patientswith clinically uncertain Parkinsonian syn-dromes helping to differentiate essential tremorfrom Parkinsonian syndromes related to idio-pathic Parkinson’s disease (PD), multiple systematrophy (MSA), progressive supranuclear palsy(PSP). It is claimed that the binding of ioflupanereflects the number of dopaminergic neurons inthe substantia nigra (see DaTSCAN®).

ioglycamide compounds (cm) Aliphatic acidsalts as meglumine salt of ioglycamic acid forcholecystographic contrast media. Has pharma-cokinetic action similar to iodipamide group ofcompounds. Preferentially excreted into the bileand a small proportion enters enterohepaticreabsorbtion (see Biligram®, ipodate group).

iohexol (cm) Low osmolar 880, non-ionicmonomer contrast media and tri-iodinated sub-stituted ring compound do not dissociate insolution, so that hypertonicity is avoided. Sidechains have been altered to make moleculeshighly hydrophilic to increase solubility with-out dissociation. Increased ratio of iodine perosmotic particle (3:2 for ionic) versus (3:1 fornon-ionic) osmolality halved (non-ionic).

Agent Viscosity Osmolality Iodine (cP) (mOsm/kg) (mg I mL�1)

Iohexol 11.2 862 350

(see Omnipaque®).

iomeprol (cm) Non-ionic monomer for angiogra-phy and CT introduced by Bracco in 1995; con-taining up to 400 mg I mL�1

Agent Viscosity Osmolality Iodine (cP) (mOsm/kg) (mg I mL�1)

Iomeprol 1.4–12.6 300–726 150–400

(see Imeron®, Iomeron®).Iomeron® (cm) Commercial form of iomeprol

(Bracco) produced in iodine concentrations of150, 200, 250, 300, 350 and 400 mg I mL�1

(30.62–81.65%)


Iiomeprol 2.0–27.5 300–726 150–400at 20°C,

1.4–12.6at 37°C

ion (phys) An electrically charged particle formedwhen one or more electrons is lost by an atomeither by ionizing radiation hf:

H2O � hf → H� � OH—

or when an ionic compound is dissolved in water:

(NaCl → Na� � Cl—).

ion beam (cyclotron) (nmed) Most hospitalcyclotrons accelerate negative hydrogen ions(H�) since negative ions have a higher extrac-tion efficiency. A filament in the ion sourceassembly puts a negative charge on the hydro-gen ion by the addition of two electrons. Theseare stripped to protons by a carbon stripper foilprior to bombarding the target. H� → p � e�.Larger cyclotron accelerate both negative andpositive ions.

Ion source Name

H� (p) Negative hydrogen ion, as proton source2H� (D) Deuteron3H� (t) Triton3He� Helium ion4He� Alpha (α)

At the extraction port, carbon stripper foilsremove electrons from the negative ions leav-ing positively charged ions (a H� proton or a D�

deuteron). A single or dual beam can beobtained.


ion dose (I)–ionic monomer

I

ion dose (I) (dose) The dose delivered by ionizingradiation. It is the quotient of dQ and dm,where dQ is the electrical charge of the ions ofone sign produced by radiation in air having avolume of dV; dma � ρa � dV is the mass of airwith density ρa so that I � dQ/dma. Ion dose ismeasured in Coulombs per kilogram (C kg�1),replacing the roentgen (see dosimetry, cavity ion

dose, standard ion dose).ion pair (phys) A pair of negative and positive

charged ions caused by ionizing radiation. Ionpairs are formed in gas detectors which givethe signal.

ion source (cyclotron) (phys) A source ofcharged particles in a particle accelerator.Hydrogen or helium gas is ionized by means ofan electron beam or electric discharge, yieldingpositive and negative ions The latter are eithersimple electrons or added electrons to a neutralatom. Conversion of negative ions to the posi-tive state (protons) is achieved in a gas or thinmetal foil strippers. In radio frequency (20 MHz)ion sources, a plasma is produced in a gas con-tained in a glass vessel. Some common ionsused in particle accelerators are: 1H�, 2H, 3H,D�, 3He��, 4He��) (see cyclotron).

ion toxicity (cm) A surplus or deficit of variousions in the solution.

ionic (clin) Ionic compounds will dissociate intoanions and cations when dissolved in water.Contrast media which are chemically salts oforganic compounds containing iodine. Ioniccontrast media will dissociate into two particles,one anion and one cation when dissolved inwater. The anions are iodinated benzoic acid.They might occur as single benzoic rings(monomeric contrast media) like diatrizoate,metrizoate or iothalamate, or as two connectedrings (dimeric contrast media) like iocarmate orioxaglate. The cations are either metals: Na�,Ca��, Mg�� or organic cations like meglu-mine (methylglucosamine). Ionic monomericcontrast media will have a high osmolarity ofgreater than seven times the osmolarity of bloodat the highest clinical concentrations. Ionicdimeric contrast media have a lower osmolarity,but most often higher chemotoxicity.

ionic contrast medium (cm) These are salts ofweak organic acids containing iodine, watersoluble that will split into an anion and cation

(ions or particles) when dissolved. These agentsare based on the six-carbon benzoic acid ring

rather than the earlier five-carbon pyridinering. This structure is able to carry three atomsof iodine and is therefore more radiopaque. Twosubstituted benzoic acids came into commonusage: diatrizoate and its predecessor, iothala-mate. These fully substituted benzoic acidsinclude three iodine molecules per unit and arecompletely dissociated in solution. The anioncomponent is the iodinated benzene ring,where one of the side chains is a weak organicacid (benzoic acid). They are either single rings;monomeric contrast media, such as diatrizoate,metrizoic acid/metrizoate compounds or iothalamate,or double-linked rings as dimeric contrast media,such as ioxaglate or iocarmate. In summary:

● ionic monomer: one tri-iodobenzene ring (metri-zoate, Isopaque, diatrizoate, Hypaque);

● ionic dimer with two tri-iodobenzene rings(ioxaglate, Hexabrix).

Cations (not radiopaque) are either inorganicsuch as sodium, calcium or magnesium, ororganic such as meglumine. Current formula-tions have meglumine, sodium, or a mixture ofboth depending on the formulation of the man-ufacturer. Ionic contrast media have high osmolal-

ity five to ten times that of normal serum, whichis associated with unwanted side effects.

ionic dimer (clin) These have intermediate vis-cosity, intermediate i.v. tolerance.

Examples: ioxaglate, iodine atoms to particlesratio: 6:2 or 3:1, osmolar concentration to bloodat 300 mg: �2.

Generic Trade Iodine Viscosity Osmolality

name name ratio (cP) (mOsm/

kg H2O)

Ioxaglate Hexabrix 3:1 12 at 20°C, 600

6 at 37°C

ionic monomer (clin) These x-ray contrast mat-erials have the lowest viscosity, the highestosmotoxicity and the lowest i.v. tolerance. (cm)Hypertonic contrast medium between 1500 and

I

I

I

(R)R

COO�

Ionic dimer

I

I

I

R

COO�

I


ionic monomer–Ionising Radiation

2000 mOsm/kg water (blood, 300 mOsm/kg).The general structure being:

Generic Trade Iodine Viscosity Osmolality

name name ratio (cP) (mOsm/

kg H2O)

Iothalamate Conray 1.5:1 5–9 at 20°C, 1500–1600

3–5 at 37°C

Diatrizoate Vasoray

Metrizoate Isopaque

Amidotrizoate Urografin

Ioxithalamate Angiografin

Gastrografin

Telebrix

ionization (phys) Loss of an orbital electron froman atom forming a charged ion. The productionof negative or positive ions. A neutral atom ormolecule acquires or loses an electric charge.Certain compounds (electrolytes) ionize in solution: NaCl → Na� � Cl2 forming cationsand anions, respectively.

ionization chamber (phys) An enclosed volumeof gas (air, nitrogen, argon, xenon), the enclo-sure forming one electrode and the other elec-trode centrally placed. A high voltage is applied(typically 200–400 volts) and the small ioniza-tion currents collected by a charge amplifier.These detectors are integration devices and ifair filled are tissue equivalent. The thickness of

Geiger

Proportional

Ionization

Applied voltage

Pul

se h

eigh

t

I

I

I

RR

COO�

Ionic monomer

the enclosure can be increased to encouragesecondary electron formation (see gas

ionization chamber (CT), Geiger counter, propor-

tional counter).ionization potential (phys) The minimum energy

necessary to remove an orbital electron from anatom causing ionization.

ionizing radiation (phys) Electromagnetic radia-tion (x- or gamma rays) or particulate radiation(alpha particles, beta particles, electrons,positrons, protons, neutrons and heavy parti-cles) capable of producing ions by direct or sec-ondary processes in passage through matter(see radiation (ionizing)).

Ionising Radiations Regulations (1985)IRR85 (dose) (UK) ICRP 26 recommendationswere published as directives 80/836, then84/467 EURATOM Basic Safety Standardsadopted by the UK as the Ionising RadiationRegulations 1985 (IRR85). ICRP60 and subse-quent directive 96/29/EURATOM revised thebasic safety standards and were adopted bythe UK as Ionising Radiations Regulations (1999)

IRR99 which replaced this IRR85 legislation (seeIonising Radiation (Protection of Persons undergoing

Medical Examination or Treatment) 1988).Ionising Radiations Regulations (1999) IRR99

(dose) (UK) ICRP60 and subsequent directive96/29/EURATOM revised the basic safety stan-dards and were adopted by the UK as IRR99,replacing Ionising Radiations Regulations (1985) IRR85.They relate principally to the protection of work-ers and the public, but also address the equip-ment aspects of patient protection. The essentiallegal requirements arising out of this legislationare: authorization, notification, prior risk assess-ment, restriction of exposure, maintenance andexamination of controls, contingency plans, radi-ation protection adviser, information, dose limits,instruction, training, designated areas, local rules,classified persons, duties of manufacturers, QAprogrammes.

Ionising Radiation (Medical Exposure)Regulations (2000) IR(ME)R2000 (dose) (UK)These relate to patient protection as set out inEuropean Council Directive 97/43/Euratom ofJune 30, 1997 ‘The Medical Exposures Directive’,derived from ICRP60, replacing IonisingRadiation (Protection of Patients UndergoingMedical Examination or Treatment (POPUMET)Regulations 1988. The essential legal require-ments dealt with in these regulations are: duties


Ionising Radiation–Iothalamate (125I)

I

of employers, duties of the practitioner, operatorand referrer, justification of medical exposures,optimization, clinical audit, expert advice, equip-ment and training.

Ionising Radiation (Protection of PersonsUndergoing Medical Examination orTreatment) Regulations (POPUMET) (1988)(dose) (UK) ICRP 26 recommendations were pub-lished as 84/466/EURATOM (Patient Protection)and adopted by the UK as these regulations.Directive 97/43/EURATOM ‘The Medical ExposuresDirective’, derived from ICRP60 replaced this 1988Act with Ionising Radiation (Medical Exposure)

Regulations (2000) IR(ME)R2000.Ionising Radiations Incident Database (IRID)

(dose) (UK) In 1996, the National RadiologicalProtection Board (NRPB), the Health and SafetyExecutive (HSE) and the Environment Agency(EA) jointly established the Ionising RadiationsIncident Database (IRID) extending back to1974 in the UK. The objectives of the databaseare to act as a reference for radiation incidents,primarily in the non-nuclear sector and provideanalyses of data that help in assessing priori-ties in resource allocation.

ion toxicity (cm) A surplus or deficit of variousions in the solution.

Iopamidol® (cm) Introduced in 1981 by Bracco,available as monomeric, low osmolar, water-soluble, non-ionic contrast medium, tri-iodinated substituted ring compound. Does notdissociate in solution so that hypertonicity isavoided. Side chains have been altered to makemolecules highly hydrophilic to increase solubil-ity without dissociation. An aqueous solution(Gastromiro®) for oral administration is used forgastrointestinal diagnosis.

Compound Viscosity Osmolality Iodine (mg/mL)

Iopamidol 2.0–9.4 413–796 200–370

(see Iopamiron®, Iopamiro®, Solutrast®,Niopam®, Isovue®).

Iopamiro® (cm) Preparation of the non-ionicmonomer iopamidol introduced by Bracco in 1981.

Compound Viscosity Osmolality Iodine(cP) mOsm/kg (mg I mL�1)

Iopamidol 2.0–9.4 @ 413–796 150–370(306.2– 37°C755.3 mg)

(see Iopamiron®).

Iopamiron® (cm) A preparation of the non-ionicmonomer iopamidol introduced by Bracco asIopamiro® in 1981 having 370 mg I mL�1 andan osmolarity of 796 (see solutrast, Niopam,Isovue).

iopanoic acid (clin) Generic name for Telepaque.iopentol (clin) A non-ionic monomer for vascular

use produced by Nycomed/Amersham (GEHealthcare)

Compound Viscosity Osmolality Iodine (cP) mOsm/kg (mg I mL�1)

Iopentol 12.0 680 350

(see Imagopaque®).iopodate compounds (clin) Ionic x-ray contrast

media comprising sodium and calcium ipodatemanufactured by Schering AG for oral adminis-tration as Biloptin.

iopromide (cm) Non-ionic monomeric x-ray con-trast material for angiography, digital subtrac-tion angiography (DSA), intravenous urogram(IVU), venography, hysterosalpingography, fis-tulography and general CT applications.


Iopromide 9.5 at 20°C; 610 37062.3% 4.6 at 37°C

(see Ultravist).iothalamic acid/iothalamate compounds (cm)

Conventional high osmolar monomeric ionic con-

trast medium developed in the 1960s; salts oftri-iodinated benzoic acid (sodium) as the anioniothalamate is the radiopaque portion, but boththe anion and cation are osmotically active sowill be hypertonic to plasma.


Na-iothalamate 2.7 at 37°C 1843 325

(see Conray®, osmolality).Iothalamate (125I) (nmed) Sodium iothalamate

having been iodinated with 125iodine; for thedetermination of the glomerular filtration rate(GFR).

Generic name IothalamateCommercial names Glofil®

Non-imaging category Glomerular filtration (GFR)

I


iotrolan/iotrol–59Iron

iotrolan/iotrol (cm) Hydrophilic non-ionic dimericaqueous preparation isotonic with CSF.


Iotrolan 9.1 at 37°C 360 300

(see Isovist®).iotroxate (meglumine) (cm) For intravenous

cholegraphy and cholecystography (seeBiliscopin®).

ioversol (cm) A non-ionic x-ray agent for cerebralangiography produced by Mallinckrodt as Optiray.


Ioversol 5.0 at 37°C 780 339–741

ioxaglate/ioxaglic acid (cm) Ionic dimer pro-duced as Hexabrix®


Ioxaglate 6.2 at 37°C 560 300

ioxilan (cm) Intra-arterial (cerebral, cardiac andaortography), intravenous (CT head and body,excretory urography)


Ioxilan 5.1–8.1 585–695 300–350

(see Oxilan®).ioxithalamate compounds (clin) Ionic monomer

x-ray contrast material (see Telebrix®).IP (comp) Internet protocol, an addressing system

of TCP/IP that governs packet forwarding.IP (Internet protocol) address (comp) An

Internet protocol address is a unique set ofnumbers used to locate another computer on anetwork. The format of an IP address is a 32-bitstring of four numbers separated by periods.Each number can be from 0 to 255. Within aclosed network IP addresses may be assignedat random, but IP addresses of web serversmust be registered to avoid duplicates.

IPA (mri) See integrated panorama array.iPAT (mri) Integrated parallel acquisition techniques.

Siemens’ procedure for reducing acquisition time.IPP (mri) Integrated panoramic positioning.ipodate group (cm) Sodium and calcium ipodate

absorbed after oral administration and excreted

in the bile. These compounds undergo entero-hepatic re-absorption (see cholegraphic contrast

agents, ioglycamide group, Biligrafin®, Biloptin®).IPX (comp) Internet packet exchange, a NetWare

protocol that provides connectionless commu-nications between devices on a network. (mri)See inversion recovery.

IRC (comp) Inter relay chat; using the Internet likea CB (citizens’ band) radio.

IR-EPI (mri) Inversion recovery echo planar imaging.IRFGR (mri) Inversion recovery fast gradient

recalled acquisition in the steady state.iridium (Ir) (elem)

Atomic number (Z) 77Relative atomic mass (Ar) 192.2Density (ρ) kg/m3 22 420Melting point (K) 2716K-edge (keV) 76.1Relevance to radiology: 192Ir used as a short half-life (74 days) therapy nuclide. 191 mIr has been used as agenerator-derived nuclide for cardiology.

iris (xray) A circular diaphragm placed behind alens system for adjusting its aperture andimproving the image depth.

iron (Fe) (elem)

Atomic number (Z) 26Relative atomic mass (Ar) 55.84Density (ρ) kg/m3 7870Melting point (K) 1808Specific heat capacity, J kg�1K�1 449Thermal conductivity, W m�1K�1 80.2K-edge (keV) 7.1Relevance to radiology: As a general x-ray shieldingmaterial or as shielding for low background laboratories(old stock).

55Iron

Nuclear data 55FeHalf-life 2.7 yearsDecay mode e.c. 100% x-raysDecay constant 0.2566 years�1

Photons (abundance) 5.9–6.5 keV

59Iron

ProductionHalf life 44 daysDecay mode β � 1.562 MeVDecay constant 0.01575 years�1

Photons (abundance) 1.0 MeV (0.56)1.2 MeV (0.43)

Uses in radiology: Both nuclides are used for tracerstudies in iron metabolism.


iron-binding–isobaric transition

I

iron-binding (nmed) Forming protein complexes,particularly concerned with trapping 67Gallium.Gallium is believed to localize in inflammatorylesions by diffusing across ‘leaky’ capillariesinto the extracellular space and binding to pro-teins that are in relatively high concentrationsin inflammatory lesions. Other iron binding pro-teins are:

● Transferrin: a primary transport protein for 67Gawithin the circulatory system;

● Lactoferrin: stored within specific leucocytegranules and is released by the leucocytes atsites of inflammation;

● Ferritin: an intracellular protein, which medi-ates uptake of iron/gallium within bacteria;

● Siderophores: low molecular weight com-pounds produced by bacteria that also mediateuptake of iron/gallium within bacteria.

IRQ (comp) Interrupt request signals are used bydevices to interrupt the processor in order togain priority. For example, a keyboard generatesan interrupt signal indicating that an event hasoccurred when a key is pressed that requires anaction. Each device must have its own IRQ orconflicts will occur.

IRR99 (rad) See Ionising Radiations Regulations (1999)

IRR99.IR(ME)R2000 (rad) See Ionising Radiation (Medical

Exposure) Regulations (2000) IR(ME)R2000.IRSE (mri) See inversion-recovery-spin-echo.irradiance (phys) The flux per unit area received

by a surface element or radiant flux falling onan area of surface divided by the area. The unitis W m�2. It describes the irradiance level on asurface and is used in hazard and safety analy-sis (see flux).

irradiation (phys) Exposure of a material (or tis-sue) to radiation, either non-ionizing (UV) or ion-izing (x-rays, gamma rays, alpha/beta particles,neutrons etc.).

ISA (comp) Industry standard architecture. The orig-inal PC set up which allows extras to be added toa system by connecting plug-in adapter cardsinto slots on the computer motherboard.

ischaemia (clin) Insufficient blood supply.Myocardial ischaemia being insufficient bloodsupply to the myocardium, usually caused byreduced diameter of the coronary arteries, ulti-mately causing myocardial infarction (myocardium

(stunned), hibernating myocardium).

ISDN (comp) Integrated services digital network. Atelecommunications standard for sending digi-tized voice, video and data signals over theexisting public switched telephone network. Itincreases the amount of information that can betransmitted over regular phone lines, support-ing speeds of up to 64 Kbps. Routers and othercommunication devices connect to ISDN throughstandardized interfaces. Two channels combineto give 128 kbits per second data transfer or onechannel giving a 64 kbps Internet access, whileusing the other channel for voice analogue (seeADSL, modem).

ISIS (mri) Image selected in vivo spectroscopy.Single voxel localization achieved without form-ing an echo like STEAM and PRESS. A localizationscheme consisting of eight scans with differentcombinations of three inversion pulses andreceiver phase for each scan. Summation in thecorrect manner causes all the signals outside ofthe desired volume to cancel. Widely used forphosphorus 31P spectroscopy.

ISO (International Organization forStandardization) A large, worldwide developerand publisher of international standards. A net-work of the national standards institutes of 157countries, one member per country, with a cen-tral secretariat in Geneva, Switzerland, that coor-dinates the system. ISO is a non-governmentalorganization that forms a bridge between thepublic and private sectors. Many of its memberinstitutes are part of the governmental structureof their countries, or are mandated by their gov-ernment. Other members are in the private sec-tor, having been set up by national partnershipsof industry associations. ISO enables a consen-sus to be reached on solutions that meet boththe requirements of business and the broaderneeds of society. ISO 9000, 14000, 22000 and27000 are international management standards.ISO 9000:2005 describes fundamentals of qual-ity management systems, which form the sub-ject of the ISO 9000 family, and defines relatedterms.

isobars (nmed) Nuclides having a constant mass,but varying proton and neutron number. Anexample series would be:

isobaric transition (phys) A nuclear decayprocess where the atomic mass is unchanged,

80201

121 81201

120 82201

119Hg : TI : Pb

I


isobaric transition–isotope

where neutron: proton or proton : neutron. Betadecay is an example:

isocentre (ct) Centre of gantry. (ct) Intersection ofthe axis of rotation with the scan plane.

isodiapheres (phys) Series of radionuclides wherethe difference between the number of neutronsand protons is the same. For example, losing anα-particle, the uranium series are isodiaphereswhere each stage emits an alpha particle 2α2:

(see isotopes, isotones, isodiapheres, isobars).isodose (dose) A series of contours drawn over a

target area depicting areas expected to receivethe same dose rate. Used to plot radiation treat-ment plans.

isomeric states (phys) States of a nucleus havingdifferent energies and observable half-lives.

isomeric transition (nmed) The decay of ametastable state yielding a single gamma photononly (pure gamma emitter). Isomers would be99mTc and 99Tc. These excited states usually lastfor extremely short times (picoseconds), but otherscan last for relatively long periods (many secondsor even hours) and are called metastable states.These can be considered as transition processes(see internal conversion, generator, internal conversion).

isomers (nmed) An atomic nucleus having thesame atomic number and mass as another orothers, but a different energy state. Two atomsof the same element in which the numbers ofprotons and neutrons are the same, and theatomic mass is the same, but the nuclearenergy levels are different (e.g. 99mTc and 99Tc).

Isojex® (nmed) 125I-RISA (human serum albumin,HSA) for plasma volume determination(Mallinckrodt Inc.).

isolated array coils (mri) Array coils, the signalswhose elements are kept separate until afterprocessing.

isolating transformer (xray) A transformer withequal primary and secondary windings used forisolating equipment from the a.c. mains supply.The electrical safety of the equipment isimproved since there is no leakage path to earth.

isomeric states (phys) States of a nucleus havingdifferent energies and observable half-lives.

.... 90230

140 88226

138 86222

136

84218

134 82214

1

Th Ra Rn

Ra Ra

→ →

→ → 332 ....

53131

54131

1122

1022

I Xe ( gammas)

Na Ne (paired 511 ke

→

→

� ��

� �

β

β VV gammas)

isometric venography (intra-osseous spinalvenography) (clin) Visualizing the long saphe-nous vein for evaluating its suitability as a graftin arterial reconstruction surgery; achieved bycontrast medium injection into a foot vein in thesupine patient during isometric tension of thelimb muscles.

Isopaque® (cm) An ionic monomer used as a radio-graphic contrast agent; generic name metrizoic

acid/metrizoate compoundsate.


Na-metrizoate – 2100 370

(see Triosil®).isotonic saline (clin) Also called physiological

saline. A solution of sodium chloride (9.448 g) in1000 g of water, giving approximately 0.9%sodium chloride which possesses the sameosmotic pressure as intracellular fluid:290 mOsm/L. Plasma has an osmolality of285 mOsm/kg water.

isotones (nmed) Nuclides having different protonnumbers and different atomic mass, but thesame number of neutrons. Two examples arethe series:

isotonic (isotonicity) (clin) A fluid having the sameosmolality or osmolarity as another fluid or sur-rounding tissue. Used synonymously with iso-osmolar. All ionic and non-ionic monomericcontrast media are hypertonic compared to blood,ionic contrast media having osmolarities up to �7of blood, non-ionic monomers up to �3 times.Non-ionic dimers are much less hypertonic andare supplied as isotonic with blood in lower iodineconcentrations. One of the non-ionic dimers,iodixanol, is actually isotonic with blood in all con-centrations. This gives a very low osmotoxicity.

isotope (phys) Nuclides with the same protonnumber, but different neutron numbers. Thethree stable isotopes of oxygen are

The corresponding radioactive isotopes wouldbe (T ½ 1.2 m) and (T ½1/2 2 m). Thethree isotopes of hydrogen are known ashydrogen , deuterium and tritium

. Various isotopes have different nuclear13

2H( )12

1H( )11

0H( )

815

7O814

6O

816

8 9 818

10O : O : O817 .

1430

1631

16 1632

16 3888

3989

5090Si P S and Sr : Y Zr15 16 40 50: : :


isotope–isowatt

I

magnetic moments and hence, have quite differ-ent resonant frequencies (see isotones, isodia-

pheres, isobars).isotopic abundance (phys) The number of atoms

of a particular isotope in a mixture of the iso-topes of an element expressed as a fraction ofall the atoms of the element.

isotropic (phys) Independent of direction, havingproperties which are uniform and do not varywith direction. A point source of activity isisotropic, emitting in all directions over aspherical volume.

isotropic motion (mri) Motion which is uniformin all 3D dimensions. This is generally used inreference to molecular diffusion or rotationwhich gives rise to relaxation of the spin sys-tem through dipole–dipole interactions (see dif-

fusion imaging).isotropic sampling (ct) A coordination system

giving equal dimensions in the x-, y- and z-axes. Images can be created in any plane withthe same spatial resolution as the original sec-tions. The x,y axis represents the normal trans-versal CT view of the body with the x-axis in alateral direction and the y-axis in ananterior–posterior direction. The perpendicularto the x,y axis oriented z-axis is parallel to thelongitudinal axis of the body and the systemaxis. An isotropic data set is achieved by usingthe small focal spot with thin section collima-tion (0.5 mm), giving a longitudinal resolution(z-axis) nearly identical to the in-plane (x-, y-axes) resolution. Images (coronal, sagittal andaxial) can be reconstructed from one multi-sliceacquisition and will have the same spatial res-olution as sections from the original acquisition.

X,Y

Z

X,Y

Z

Isovist® (cm) Commercial (Schering) form ofiotrolan, a non-ionic dimer


Iotrolan 6.8 at 20°C, 270 240512.59 3.9 at 37°Cmg mL�1

Isovue® (cm) Commercial (Bracco) preparation ofiopamidol.


Isovue 3.3 at 20°C, 413 200M200 41% 2.0 at 37°CIsovue 8.8 at 20°C, 616 300M300 61% 4.7 at 37°CIsovue 20.9 at 20°C, 796 370M370 76% 9.4 at 37°C

isowatt (xray) Describes the characteristic curvefor tube voltage and current in fluoroscopywhere at a certain threshold current the tubekilovoltage is steadily reduced maintaining thesame power in watts (isowatt). Automatic con-trol based on this curve is not suitable for imag-ing so an ‘anti-isowatt’ control is availablewhere tube voltage and current are increasedover a restricted range giving optimum imagecontrast and brightness for all object sizes. Thegraph plots kilovoltage against tube currentcontrol and shows the maximum permittedlevel (450 W in this case) which forms the‘isowatt’ limit for the particular x-ray tube.

The linear functions plotted in the graph differaccording to the emphasis placed on mA or kV;slope (1) predominantly alters mA, while

3.

2.

1.

110

100

90

80

70

60

50

40

kVp

0 2 4 6 8 10 12

Tube current (mA)

450W limit

I


isowatt–IVP

(2) alters mA and kV with equal measure; slope(3) rapidly alters kVp to a maximum of 110 kVp.During isowatt/anti-isowatt operation, thevalue of tube voltage and tube current relatedirectly to the transparency of the examinedobject since dose rate at the input screen of theimage intensifier is kept at an optimum fixedvalue (typically 0.175 μGy/s).

ISP (comp) Internet service provider. A commercialconcern controlling connection to the Internet.

iteration (math) A repeated procedure whichallows better and better approximation to the

true answer. Iterative reconstruction is some-times used in tomographic imaging.

iterative reconstruction (ct) This uses an exactmathematical solution for reconstructing raysums. This was the original method for imagereconstruction. Its disadvantages are that ittakes a long time and suffers from roundingerrors. All data must be collected before recon-struction can begin.

IUP (clin) Intravenous urography.IVIM (mri) Intravoxel incoherent motion.IVP (xray) Intravenous pylorogram.


j–junction diode

J

Jj (math) One of the symbols used for representing

the complex variable (see i, complex numbers).J-coupling (mri) See spin–spin coupling.J-modulation (mri) Changes in the relative phase

of the component lines of a multiplet (seespin–spin coupling) caused by differential phaseaccumulations, dependent on the particularacquisition parameters employed. For example,in multiple spin echo sequences the resultingmodulation of the net intensity of the multipletcan affect the apparent 72 s in a mannerdependent on the choice of interpulse delaysemployed to observe the echo (see spin–spin

coupling).jaggies (image) See pixilation.Java® (comp) An object-orientated program lan-

guage that enables interactive elements towork across operating systems making it anideal language for web page design. Javaallows programmers to create small programsor applications (applets) to enhance web sites.

Javascript®/ECMA script (comp) A scripting lan-guage, similar to Java, which allows web pro-grammers to create dynamic content, such asinteractive games, or search engines on theirweb sites.

jaz drive (comp) Transportable disks capable ofstoring 1-Gbytes with access times of 17.5 msand transfer rates of 5.4 M-bytes s�1 (see zip disk).

JCAHO (USA) Joint Commission on theAccreditation of Healthcare Organizations.Establishes dosimetry standards.

jet effect (mri) Signal loss where spin dephasingoccurs with complex flow patterns like turbu-lence. The degree of signal loss depends on theflow patterns and pulse sequence used; impor-tant when quantifying vascular stenosis.

Joliot-Curie Irène (1897–1956) and Frédéric(1900–58) continued the work of Marie andPierre Curie observing the penetrating radiationejected from paraffin wax (later identified asneutrons by Chadwick). They produced the firstartificial radioisotope by bombarding alu-minium with alpha particles to produce phos-phorus. They were jointly awarded the NobelPrize for chemistry in 1935.

Joule, James Prescott (1818–89) British physi-cist. Investigated the heating effect of electrical

current on conductors. The mechanical equiva-lent of heat. His ideas were recast in terms ofthe principle of the conversion of energy. Gavehis name to the SI unit.

joule (phys) A measure of the quantity of energy,work or heat. The SI unit of work 1 J � 1 N m�2;1 eV � 1.602 � 10�19 J and 1 Gy � 1 J kg�1.The derived units are:

Measure Joule equivalent

Energy density J m�3

Specific energy J kg�1

Absorbed dose (Gy) J kg�1

Equivalent dose (Sv) J kg�1

Watt (W) J s�1

Heat capacity J K�1

Coulomb (C) J V�1

Volt (V) J C�1

Specific heat capacity J kg�1 K�1

Measure Joule equivalent

107 ergs 1 J1 W s 1 J1 kW h 3.6 MJ6.24 �1018eV 1 J6.24 �1015keV6.24 �1012MeV1 eV 1.602 � 10�19 J1 keV 1.602 � 10�16 J1 MeV 1.602 � 10�13 J

JPEG (di) Joint Photographic Experts Group.Compression algorithm and also populargraphics format (JPG). Usually referring to agraphic image format defined by this groupthat has become an alternative to GIF for com-pact images. All JPEG images can use 16.7 mil-lion colours at a higher compression rate that GIF.

jumper (comp) A method for altering a hardwiredconnection on the motherboard by plugging asmall wire connector across two pegs.

junction diode (phys) The n-type and p-typesemiconductors are combined to form the p–njunction diode. The junction diode exhibits dif-ferent properties when connected to differentpolarity (�, �). Before current supply (battery)is connected, a small depletion layer existsbetween the p- and n-boundary. Electronsmigrate from the n-type a small way into thep-type layer. Holes from the p-type alsomigrate into the n-type material. If the positive

junction diode–justification (ICRP)

pole of the battery is connected to the p-typeand the negative pole to the n-type then a cur-rent flows; the diode is then forward biased.Reversing the polarity causes a wide depletionlayer to be formed and no current is able toflow; the diode is reverse biased.

justification (ICRP) (dose) A control measure torestrict exposure from practices. It implies thatthe detriment from exposure should be justifiedby the benefit resulting from the practice andthus preventing frivolous applications involvingradiation (see practices (ICRP)).

J



K

K56Flex–Kaiser window

KK56FIex (comp) A proprietary protocol developed

by Lucent Technologies and Rockwell specify-ing a 56 Kbits/sec download speed and33.6 Kbits/sec upload. Employs pulse codemodulation technology similar to ISDN andrequires the line on the host side to be digitallyterminated. K56Flex is incompatible with X2.

K6, K6-2, K6-III (comp) Central processing unitsmanufactured by AMD Corporation.

k bits/s (kbps) (comp) Abbreviation for kilobitsper second. Used for measuring serial datatransfer in modems or other serial devices.1 kb s�1 is exactly 1024 bytes per second. A megabyte (Mb) is 1024 kb or 1 048 576 bytes(see baud rate).

K-edge (filters) (xray) The abrupt increase inphoton absorption when the incident photonenergy equals the K-orbit binding energy.Metals with higher K-edge values (20–30 keV)are useful filters in radiology since they prefer-entially remove higher energy photons, unlikealuminium and copper filters that remove lowenergy photons. K-edge filters are commonlyfound in mammography where they reducepatient radiation dose by removing energiesabove 28 keV which only play a minor role inimage formation for this examination. CommonK-edge metals used for x-ray beam filtrationare shown in the table from Z � 42 onwards.The K-edge for tin is useful since this allowsthis metal to be used as lightweight shieldingin protective aprons.

Element Z K-shell energy (keV)

Carbon 6 0.28Nitrogen 7 0.4Oxygen 8 0.5Phosphorus 15 2.1Sulphur 16 2.5Calcium 20 4.0Molybdenum 42 20.0Rhodium 45 23.2Palladium 46 24.3Tin 50 29.2Barium 56 37.4Iodine 53 33.2Samarium 62 46.8Erbium 68 57.5Tungsten 74 69.5Lead 82 88.0Uranium 92 115.6

(see filters (radiation)).

k-space (mri) A convenient way for describingdata sampling process in MRI. The dataacquired for MR image reconstruction generallycorrespond to samples of k-space; they repre-sent values of the Fourier transform of the imageat a particular set of locations in k-space; a twodimensional map with most of the signal con-centrated at the centre (low frequencies) andhigh frequency component on the periphery.The low frequency components of central k-space contain most signal intensity (con-trast); the high frequency components in theperiphery contain edge information (resolution).The Fourier transformation of k-space yieldsthe spin density function. k-space and imagespace are inversely related through the Fouriertransform (see spatial frequency, truncation error).

k-space filling (mri) The matrix position and orderof data in two-dimensional spatial frequencyspace (k-space); the Fourier transform of thismatrix gives the MR image. Conventional MRpulse sequences (spin-echo and gradient-echoimaging) fill a single line of k-space with eachdata measurement. A different phase encodingstep fills out another parallel line of k-space. Thefull set of measurements completes a Cartesiangrid of points in k-space. Other options for k-space filling include radial filling (back-projec-tion imaging) or spiral filling (spiral imaging).

k-space trajectory (mri) The path traced in thespatial-frequency domain during data collectionas determined by the applied field gradients.

Kaiser window (di) The filter windows such asHanning and Hamming have fixed shapes givinga trade-off between width of main spectrallobe and side-lobe levels. The Kaiser window

Taper

Filter parameters

Ripple

Transition width

Kaiser window–kerma (K)

allows this trade-off to be adjusted based onthe allowable ripple, controlling taper and side-lobes, and transition width (see filters (spatial)).

kappa statistics (coefficient) (stats) Correctedindex of agreement between two quantities.The value takes unity where there is perfectagreement. Commonly used to judge diagnostic

accuracy. The assessment of a series of imagesfor the presence of a subtle abnormalityimproves if more than one imaging procedure isused (e.g. mixing plane film radiograph, MRIand nuclear medicine) or more than one clini-cian views the images. In the latter case it isessential to establish that the clinicians con-cerned are looking for the same features. Thekappa measure of agreement (κ) is used for thispurpose; its value ranges from 0 (no agreement)to 1.0 (perfect agreement). The ROC analysis

would answer the question ‘is there animprovement between imaging techniques?’;the kappa statistic would answer the question‘is there an agreement between investigators?’

Pos (A). Neg (A). Totals

Pos (B). 150 (a) 10 (b) 160 (a � b)Neg (B). 20 (c) 20 (d) 40 (c � d)Totals 170 30 200

(a � c) (b � d) n

kBytes s�1 (kBps) (comp) Kilobytes per second. A measure of data transfer. 1 kBps is exactly1024 bytes per second. One megabyte (MB) persecond is 1024 kB or 1 048 576 bytes per second (see byte).

kelvin (K) (phys) The SI unit of temperature. kelvinK (phys) the SI unit of temperature starting atabsolute zero where T � 0 K. Degrees Celsius (°C)have identical intervals to degrees kelvin.

K °C °F

Absolute zero 0 �273.15 �459.67Freezing point of water 273.15 0 32Boiling point of water 373.15 100 212

Kelvin, William, Thomson (1824–1907)British scientist, proposed the absolute (Kelvin)scale of thermodynamic temperature adoptedas an SI unit.

kerma (K) (dose) This term has replaced exposure

in radiation dosimetry and is an acronym forKinetic Energy Released per unit Mass. It is ameasure of the removal of energy from thephoton beam by ionization creating secondaryelectrons in the absorber. At diagnostic ener-gies (20–200 keV) the photon energy is lost asa two-stage process:

1 Photon radiation transfers energy to electronsthrough photoelectric and Compton scattering.

2 The electron energy transfered to the absorbervia atomic excitations and ionization.

The absorber material must be specified (air,water, tissue, etc.). When the absorbing mediumis air, the term air kerma or kerma in air is used(Kair). For x- and gamma radiation, kerma can becalculated from the mass energy absorption coeffi-

cient of the material (μen/ρ) and the photon energy

fluence as: K � Ψ(μen/ρ). The SI units for photon

energy fluence are J m�2, and for the mass energy

absorption coefficient are m2kg�1. Therefore theproduct, kerma, has units of joule per kilogram(J kg�1) and is given the special name gray (Gy).For diagnostic energies (20–200 keV), kerma inair (Kair) and absorbed dose in air (Dair) can betreated as equal. The energy to form one ion pairin air is 34 J C�1 (34 eV per ion pair) for diagnos-tic radiation energies and over a wide range ofbiological materials. Since 1 Gy � 1J kg�1 thenDose(air) (Gy) � 34 E (C kg�1). Converting a dosein air to dose in an absorber (patient) for thesame incident photon flux, the energy absorbedper unit mass of absorber depends only on the

Kappa statisticsA special selection of 200 mammograms containingknown lesions of varying significance are given to tworadiologists (A and B) for assessment (positive � ornegative �). The results are:The observed measure of agreement (Io) and theexpected measure (Ie) are now computed as:

For the data given above, Io � 0.85 and Ie � 0.71.The kappa statistic (κ) is computed as:

This signifies a fair consistency of assessmentbetween the two radiologists, but would requireimprovement if breast cancer screening were contem-plated (see diagnostic accuracy).

κ ��

��

l ll

o e

e10 48.

Ia c a b b d c d

ne �

� � � � � � �( ) ( ) ( ) ( )2

Ia d

no ��( )

K


■ Reference: Armitage and Berry, 1996.


K

kerma (K)–kilo-electron volt keV

mass energy absorption coefficient of the medium. Atdiagnostic energies the mass energy transfer coeffi-

cient and mass energy absorption coefficient areinter-changeable (see f-factor).

kernel (ct, image) A discrete filter for imagereconstruction. The mask used for producing aLaplace filter in a two-dimensional image con-volution. The kernel contains weighting factorsthat will alter the image system in the fre-quency domain. Each pixel of the filtered imageis calculated as the sum of the pixel of the orig-inal image at the same position and the sur-rounding points encompassed by the kernel(3�3) in this example:

Smoothing kernel

0.1 0.1 0.10.1 0.1 0.10.1 0.1 0.1

Edge enhancement kernel

�1 �1 �1�1 �8 �1�1 �1 �1

Thus the kernel is shifted uniformly over eachpixel in the original image matrix. Kernel sizeand values decide filter behaviour (see filter

(spatial)).Kety–Schmidt principle (clin) A technique for

reliable measurement of cerebral blood flow inman. The subject breathes an inert gas and therate at which the gas comes to an equilibriumbetween blood and tissue is determined byserial measurements of gas concentration inarterial and venous samples. From curves ofarterial and venous concentration over theperiod of equilibrium the blood flow per unitmass of tissue can be calculated from:

(Ct � S � Cv) where Ca is the arterial, Cv thevenous and Ct the tissue concentration of gas attime t. The partition coefficient between bloodand tissue is S. Radioactive gases 133Xe and85Kr are used with external detection.■ Reference: Veall and Vetter, 1958

keV (xray) Thousands of electron volts. Used as aprecise measure of x-ray photon energy.

QC

C Ca v dt

t��

t

( )0∫

Kevlar® (chem) A stiff chain polymer plastic,which is both strong and lightweight. It is usedas an alternative to carbon fibre in the manufac-ture of x-ray couches and as a spacer materialin anti-scatter grids.

kilobytes (kB) (unit) The terms kilo- and mega-do not strictly work out to 1000 and 1 millionrespectively:

● 210 � 1024 bytes � 1 kilobyte (kB)● 220 � 1048 kB � 1 megabyte (MB)● 230 � 1073 megabytes � 1 gigabyte (GB)

The term kilobyte has become confusing as aunit for computer storage since it can be either1024 bytes or 1000 bytes (103), depending oncontext. This name has been discontinued asan SI standard. It has been recommended thatthe quantity of 1024 bytes is now called akibibyte (KiB), the term kilobyte reserved for1000 (103) bytes. Similarly the term megabyte(MB) describes strictly 10002 bytes andmebibyte (MiB) to describe 10242 bytes. TheInternational Electrotechnical Commission (IEC)approved the names and symbols for prefixesfor binary multiples for use in the fields of dataprocessing and data transmission. The prefixesare as follows:

Factor Name Symbol Origin Derivation

210 kibi Ki kilobinary: kilo:(210)1 (103)1

220 mebi Mi megabinary: 2 mega:(210) (103)2

230 gibi Gi gigabinary: giga: (210)3 (103)3

240 tebi Ti terabinary: tera: (210)4 (103)4

250 pebi Pi petabinary: peta: (210)5 (103)5

260 exbi Ei exabinary: exa:(210)6 (103)6

Examples of the revised nomenclature andcomparisons are:

One kibibit 1 Kibit � 210 bit � 1024 bitOne kilobit 1 kbit � 103bit � 1000 bitOne mebibyte 1 MiB � 220 B � 1 048 576 BOne megabyte 1 MB � 106 B � 1 000 000 BOne gibibyte 1 GiB � 230 B � 1 073 741 824 BOne gigabyte 1 GB � 109B � 1 000 000 000 B

kilo-electron volt keV (phys) Thousands of elec-tron volts. Used as a precise measure of x-rayphoton energy.

kilogram kg–kVp

kilogram kg (phys) The mass of a body is thequantity of matter it contains. The basic SI unitis the kilogram 1000 grams for all m.k.s. systems including SI. 1 gram is 10�3kg and 1 milligram is 10�6kg. Some derived units are:

● kg � m�3 for density● kg � m�1 � s�2 or pascal for pressure● kg � m � s�2 or Newton for force (see weight).

kilowatt hour (phys) Used to describe electricalwork 1 kWh � 1 kW � 3600 s � 3.6 � 106J.

kinematic viscosity (phys) The ratio of viscosityto density. measured as m2 � s �1.

kinetic energy (Ek) (phys) Energy associated withmovement so that: E � ½ mv2. Increases withmass m and velocity v. If an object is brought torest in time t the final velocity is zero as:

F � ma or momentum change s �1 � mv/t.

Work done (kinetic energy) is:

Ek � F � s � (mv/t) � (vt/2)

cancelling t to give: Ek � ½ mv2. If the velocitymoved by the object diminishes uniformly overdistance d, in time t, from v to zero, then:

d � average velocity � time � vt/2.

The forms of kinetic energy include:

● Translational where the entire object moves.● Vibrational where small masses move back and

forth. Simple gas molecules, having one atom(hydrogen, helium, oxygen, etc.) only havetranslational kinetic energy. More complex gasmolecules (H2O, NH4) may have rotational andvibrational kinetic energy.

● Rotational given by object spin.

krypton (Kr) (elem)

Atomic number (Z) 36Relative atomic mass (Ar) 83.8Density (ρ) kg/m3 3.49Melting point (K) 116.5K-edge (keV) 14.3

81 mKrypton (nmed) A lung ventilation agentcapable of steady state scintigraphy. The gener-ator demonstrates secular equilibrium.

Production → 81 mKrDecay scheme (i.t.) 81 mKr81Rb T½ 4.7 hr (–)

→ 81 mKr T½ 13.3 s (i.t. γ 190 keV) → 81KrT½ v.long

Photons (abundance) 190 keV (1.000)Eluent Air or 5% dextroseDecay constant 0.05210 s�1

Half Life 13 secondsHalf value layer 0.6 mm Pb 50 mm water

kVeff (xray) See effective energy.kVeqv (xray) See equivalent energy.kVp (xray) The peak photon energy of an x-ray

beam.

3682Kr(d,3n) Rb37

81

Electron kinetic energyComputer display screen:Velocity of electron beam with kinetic energy of 30 kV(typical applied display tube voltage).

1 eV � 1.602 � 10�19 J so 30 keV � 4.906 � 10�15 J

Electron rest mass: 9.109 � 10�31 kgSince E � ½ mv2 then v � � 1.02 � 108m s �1.The electron is travelling at 1/3 the speed of light.

X-ray tube:Applied voltage 100 keVElectron beam velocity: 1.87 � 108 ms�1 or almosttwo-thirds the speed of light.

2E m/

K



labelled compound–Larmor frequency

L

Llabelled compound (nmed) A compound in which

one or more of the molecules is replaced by aradioactive isotope (75Se-seleno-methionine) or aradiopharmaceutical to which a radionuclide isattached (99 mTc-DTPA). The radiolabel must notchange the chemical nature or geometry of theoriginal compound and so alter its original physiological property.

labelling (blood cells) (nmed) See blood cells

(labelling).lambert (L) (phys) A non-SI unit of luminance

where 1 lambert � (104/π) candela m�2 or 1/πcandela cm�2.

Lambert–Beer law (ct) See x-ray attenuation.Lambert’s Law (phys) This states that equal

paths in the same absorbing medium absorbequal fractions of radiation that enters them.Originally conceived for light absorption butapplies to all radiation (x and gamma) as well asultrasound. Expressed as:

I � Io � e�kx

where k is the absorption coefficient and x thepath length (see Beer’s Law, attenuation).

laminar flow (phys) A non-turbulent flow patternin which the gas or liquid moves in layers (seeReynold’s number).

laminar flow cabinet (nmed) An enclosed orcontained workstation for preparing and dis-pensing radiopharmaceuticals. The cabinetshould conform to a recognized standard of airpurity and flow pattern (e.g. Class I and IIBritish Standard BS5295, German TUV DIN12950). The air is required to be cleaner thanthe room environment and maintained at posi-tive pressure. Negative pressure cabinets arealso used (see HESPA filter).

LAN (comp) See local area network (see WAN).Langmuir, Irving D (1881–1957) American

physical chemist. Studied electron emissionand temperature relationships of a heatedtungsten filament in a vacuum, recognizingthat electron emission depends only on thetemperature of the filament. From this workCoolidge developed his version of the x-raytube. In 1937 he proposed a design for an elec-tronic image intensifier. A practical version wasmade by Coltman in 1948.

lanthanum (La) (elem)

Atomic number (Z) 57Relative atomic mass (Ar) 138.9Density (ρ) kg/m3 6150Melting point (K) 1190K-edge (keV) 38.9Relevance to radiology: Its complex compounds areused for intensifying screens, emitting blue light.

Laplace transform (math) A method for solving dif-ferential equations by transforming them into analgebraic equation so considerably simplifyingtheir solution (see integration/differentiation).

Laplace equation (sound) (math) Relates to thespeed of sound c in a gas to the density ρ pres-sure P and heat capacities h of a gas as

LAPM (comp) Link access protocol for modems.One of the two protocols specified by V.42 usedfor detection and correction of errors on a com-munications link between two modems.

large area (macro) transfer factor (image) The fac-tor describing the scaling of information betweenthe input and output of an imaging system. It isconcerned with changes in the signal over a largearea, c.f., spatial detail transfer characteristic.

Larmor, Sir Joseph (1857–1942) Irish physi-cist, demonstrated the precessive motion of thenuclear orbit in a magnetic field.

Larmor equation (mri) This describes the fre-quency of precession of the nuclear magnetic

moment being proportional to the magnetic field as:

where μ is the proton magnetic moment, L theproton spin angular momentum and Bo the mag-netic field strength (tesla). Since μ and 2πL arefixed for any particular nucleus, they candescribe a gyromagnetic ratio where γ � μ/2πL.Simplifying the first equation by sustituing γgives the precessional frequency:

ωL � γBo

which is the Lamor equation relating preces-sional frequency ωL to magnetic field strength.

fBLO�

μπ2

hertz.

c � ( ).hP/ρ

Gyromagnetic ratio for hydrogenProton magnetic moment: 1.41031 � 10–26 J T�1

Proton spin angular momentum: 0.527 � 10–34 J s�1

Hence γ � μ/2πL � 42 582 252 Hz

Larmor frequency (mri) The frequency at whichmagnetic resonance can be excited (fo or wo);

given by the Larmor equation. For H� the Larmorfrequency is 42.58 MHz T�1; the frequency atwhich the nuclear spins precess about themain field. Depends on the nucleus type andstrength of the magnetic field applied. By vary-ing the magnetic field across the body, a mag-netic field gradient can be used to encodeposition due to variation in Larmor frequency.

for optical transmission and reading laser-disksand CD-ROMs (see fibre optics, CD-ROM, DVD).

laser diode (elec) This is a semiconductor lightemitting diode (LED) operating at high current.The p–n regions are manufactured from gal-lium arsenide and gallium aluminium arsenide.Stimulated emission produced by making thesemiconductor faces optically flat. Spectralspread is much less than an LED, typically1–2 nm, giving low dispersion in fibre optics.

laser imager (image) See film formatter.laser printer (image) A tightly focussed laser

beam can be used for imaging by:

● Exposing a charged selenium plate (paperprinter)

● Reading an image phosphor plate● Exposing a photographic film as in wet laser

film formatters.● Selectively removing a carbon or silver film

from a plastic surface to produce an image (thedry laser process).

The resolution of a laser printer is measured indot pitch; typical values are 300, 600 and1200 dpi (see image plate, laser imager).

last image hold (LIH) (xray) Storing and display-ing the most recent image from (commonly) afluoroscopy study. The image can be held digi-tally (computer image matrix) or on video disc.The patient is not exposed to further radiationduring this procedure so is a significant dosereduction facility.

late somatic effects (dose) Radiation effects occur-ring a considerable time (years) after exposure toradiation; these effects include mutagenesis, terato-

genesis and carcinogenesis.latency/latent period (dose) The period between

exposure to ionizing radiation and the appear-ance of radiation effects (i.e. cancer). The medianperiod is 8 years for leukaemia and two to threetimes longer for solid tumours (e.g. breast, lung).There is a minimum latency period of approxi-mately 2 years for myeloid leukaemia.

latent heat (phys) The amount of heat per unitmass that is added to or removed from a substance undergoing a change of state, i.e. fromsolid to liquid or liquid to gas. The heat requiredto change the ice to water is the hidden or latentheat. The latent heat of fusion; a change of state from solid to liquid. The latent heat ofvapourization; a change of state from liquid to

1H

31P

Larm

or fr

eque

ncy

(MH

z)

Magnetic field strength (tesla)

150

0

50

100

43210

(see gyromagnetic ratio, precession).laryngography (clin) Radiography of the larynx

after coating mucosal surfaces with contrastmedium.

laser (phys) Light Amplification by stimulatedemission of radiation. A strong monochromaticcoherent light source can be produced by caus-ing many atoms to make transitions from oneexcited state to another so that all the lightwaves are in phase (coherent). Lasers produce:

● An intense coherent beam● Monochromatic● Virtually noise free● Highly directional beam● Can be tightly focussed

Many materials can be made to lase ruby, carbondioxide, helium/neon. These are high powerdevices commonly found in laser film formatters.A material such as a ruby or gas mixture (helium–neon) is optically pumped continuously or inbursts to excite atoms from their ground state to ametastable state from where electrons fall back to their ground state emitting coherent mono-chromatic light. Semiconductor laser diodes are lowpower devices producing red/infrared light used

L


Larmor frequency–latent heat


latent heat–LD50

L

vapour. Latent heat is the energy necessary toovercome the forces of attraction between themolecules of a substance. The latent heat ofvapourization (liquid to gas) is therefore greaterthan the latent heat of fusion (solid to liquid).The latent heat of vapourization for helium is21 kJ kg�1, this is a cooling process so requiresenergy when changing from gas to a liquid.Units are (J kg�1). The table gives latent heat offusion and vaporization for some common sub-stances in MJ kg�1.

Substance Lf (fusion) Lv (vapourization)

Alcohol – 1.1Water 0.335 2.260Aluminium 0.400 12.3Copper 0.205 4.80Iron 0.275 6.29Tungsten 0.192 4.35Lead 0.023 0.87Helium – 0.021

(see specific heat, specific heat capacity).lateral (us) Perpendicular to the direction of

sound travel.lateral resolution (us) Minimum reflector separa-

tion perpendicular to the sound path that isrequired to produce separate echoes; equal tothe beam width in the scan plane. Also calledazimuthal, angular or transverse resolution(see axial resolution, ultrasound (resolution)).

latitude (film) (film) This refers to the dynamic range

of exposures (LogE values) which can be given toa film over which density continues to increase ata useful rate – that is, the maximum luminancerange that can be differentially reproduced. Thefilm latitude should be broad enough to registerall tissue types of interest (bone and soft tissue inthe chest radiograph) but should give high con-trast between similar tissue types (mammogram).

Contrast and latitude are therefore balanced; thegreater the film gamma the smaller the latitude(see dynamic range, film speed, film contrast, film

gamma).lattice (mri) A property seen by nuclear magnetic

resonance in solids. The magnetic and thermalenvironment with which nuclei exchangeenergy in longitudinal relaxation.

Lauterbur, Paul C. (1929–2007) Americanchemist who developed nuclear magnetic reso-nance in 1973 as an imaging technique usinggradient fields. Awarded a Nobel Prize for thiswork in 2003.

Lawrence, Ernest Orlando (1901–1958)American physicist/engineer, who was the firstdirector of Berkeley Radiation Laboratory in 1936(now Lawrence Berkeley Radiation laboratory).Constructed the first cyclotron particle acceleratorfrom ideas developed in 1929 with M. StanleyLivingston Edlefsen and others, extending the lin-ear accelerator techniques of Cockcroft and Walton.Awarded the Nobel Prize for physics in 1939.

LD50 (radiation) (dose) The lethal dose where 50%of the population die within a standard time,usually 30 days. Large mammals (including man)have an LD50 between 2.5 and 3 Gy, whereassmaller mammals have LD50s of approximatelydouble this value. Intermediate symptoms areapparent 1 hour after partial exposure of about0.5 Gy (500 mGy). These are prodromal reactionsand include nausea, vomiting and diarrhoea ifthe gut wall has been exposed. The prodromalreactions are followed by a latent period beforethe main phase of the damage becomes apparent(see dose–mortality curve).

LD50 (contrast media) (cm) The amount of contrastmedium (usually expressed in g iodine per kg) thatwill kill 50% of a group of treated animals (mice).The LD50 value is inversely related to toxicity.Iodine contrast media are normally well toleratedand the LD50 value is correspondingly high.

CM LD50

Ionic Meglumine iothalamate 6Ioxaglate 10

Non-ionic Metrizamide 15Iopamidol 20Iohexol 25

MRI

Non-ionic contrast agents have higher LD50

due to low chemotoxicity. LD50 data publishedfor various non-ionic contrast media relate to

Range

Opt

ical

den

sity

Log relative exposure

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.03.53.02.52.01.51.00.50.0

L


LD50–Lenard von, Philipp Anton (1862–1947)

concentrations greater than 21 g iodine/kg (mice),exceeding the maximal anticipated clinical dosein humans by an order of magnitude (�10).

lead (Pb) (elem)

Atomic number (Z) 82Relative atomic mass (Ar) 207.1Density (ρ) kg/m3 11340Melting point (K) 600.4Specific heat capacity J kg�1 K�1 129Thermal conductivity W m�1 K�1 35.3K-edge (keV) 88.0Relevance to radiology: used as a radiation shieldingmaterial although its K-edge is awkwardly placed in thediagnostic range (see lead (shielding)).

210Lead (210Pb)

Half life 22yDecay mode β�3.7 MeVDecay constant 0.0315y�1

Photons (abundance) 17 keV (0.21)46 keV (0.04)

Freshly mined lead contains sufficient 210Pb ascontaminant to cause problems when used forshielding in low background equipment or laboratories.

lead equivalent (dose) A standard method for esti-mating the effectiveness of shielding materials(e.g. concrete, glass, brick, etc.) and protectiveclothing (e.g. aprons, gloves). Lead equivalent isnormally quoted in mm Pb-equivalent.

lead glass (xray) The density of lead-glass variesbetween 4360 and 6200 kg m�3. For this high-est density the following measurements are:

Thickness (mm) Weight Pb-equivalent (kg m�2) (mm Pb)

4 18 1.06 28 1.58 37 2.0

(see Clear-Pb®, glass (lead)).lead (shielding) (xray) For a density of

11 350 kg m�3 the following weights are givenfor lead sheet:

Thickness (mm) Code Weight (kg m�2)

0.5 – 5.41.0 – 10.761.32 3 14.61.5 – 17.201.80 4 19.52.0 – 21.502.24 5 24.42.65 6 29.33.15 7 34.23.55 8 39.1

The lead sheet must be firmly fixed to a rigidstructure (e.g. wood or concrete) or sandwichedbetween wood laminate (plywood). The halfvalue layer is affected by its K-edge at 88 keV.

Hal

f val

ue la

yer

(mm

)

Photon energy (keV)

1

0.1

0.01

10 100

lead time (screening) (stats) The period betweenthe time of detection by screening and the date itwould have been diagnosed if not screened (seelength time bias, screening (population)).

lead time bias (screening) (stats) Since survival ismeasured from the time of diagnosis thenscreening brings this time forward. Survival timeis therefore lengthened even if the point of even-tual death is not altered. This is the lead time bias(see length time bias, screening (population)).

lead zirconate titanate (LZT) (us) A ceramicpiezoelectric material.

leakage radiation (xray) Measured from an x-raytube housing with collimators (diaphragms)closed. Leakage measured at a distance of 50 mmover an area not exceeding 100 cm2. It should beexpressed as mGy hr�1 at 1 m and should notexceed 1 mGy. USA maximum leakage is 100 mR(1 mGy); the acceptable level being 50 mR(0.5 mGy).

least significant figure/bit (math) The rightmostfigure in a number (decimal or binary) (seesignificant figure).

least squares, method of (stats) A way of esti-mating the best fit straight line for a plotted setof values and as a criterion for best fit.

LED (elec) Light emitting diode. A forward biasedgallium arsenide semiconductor light source.Recombination of electron hole pairs causeslight to be emitted (see laser diode).

Lenard von, Philipp Anton (1862–1947)Czech/German physicist. Demonstrated themagnetic deflection of cathode rays and elec-trostatic properties and on this work suggested


Lenard von, Philipp Anton (1862–1947)–limiting spatial frequency

L

that atoms contain both positive and negativeunits. Nobel Prize for physics, 1905.

length biased sampling (stats) The bias in ascreening or sampling protocol where somemembers are more likely to be selected thanothers (see length time bias, screening (population)).

length time bias (screening) (stats) There is atendency for screening to detect preferentiallycancers that have a long asymptomatic phase.The extended duration of this phase impliesslower growth and therefore good prognosis.More invasive cancers become symptomaticearlier and may be detected as interval cancers(see lead time bias, screening (population)).

Lenoscint® (nmed) Preparation of 99mTc-MDP(Bristol–Myers–Squibb).

lens (us) Curved material that focuses the soundbeam in an ultrasound transducer.

LET (dose) See linear energy transfer.leukaemia (dose) Forms of cancer due to the multi-

plication of malignant bone marrow cells.Leukaemias are differentiated depending on theirmarrow cell origin. The four main types identifiedinclude acute and chronic versions of myeloidand lymphatic forms. Acute lymphoid occursmost commonly in young children. Leukaemia isthe first manifestation of radiation damage to apopulation and shows a linear dose response char-acteristic. In-utero exposure to radiation canincrease the risk of childhood leukaemia by 40%.■ Hall and Brenner, 2008

Levovist® (cm) Commercial (Schering) preparationfor ultrasound imaging consisting of galactose/palmitic acid granules containing microbubblesof gas.

Lexidronam® (nmed) Generic name for153Samarium used as a palliative treatment forbone pain (see Quadramet® samarium).

lidofenin (nmed) See HIDA.life span study (dose) Refers to the long-term cohort

study of health effects on the Japanese atomicbomb survivors in Hiroshima and Nagasaki.

lifetime risk estimates (dose) Several estimatesare used for calculating the risk over a lifetimethat an individual will develop or die from aspecific disease caused by an exposure:

● Excess lifetime risk (ELR), the differencebetween the proportion of people who developor die from the disease in an exposed popula-tion and the corresponding proportion in a similar control population without exposure;

● Risk of exposure-induced death (REID) defined asthe difference in a cause-specific death rate forexposed and unexposed control populations of agiven gender and age at exposure, as an addi-tional cause of death introduced into a population,

● Loss of life expectancy (LLE) which describes thedecrease in life expectancy due to the exposure.

● Lifetime attributable risk (LAR) an approxima-tion of the REID and describes excess deaths (ordisease) over a follow-up period with popula-tion background rates determined from controlunexposed individuals.

ligand (chem) An ion or molecule that donates apair of electrons to a metal at or ion in forminga complex compound. A double bond is formedbetween the metal and the ligand. Certain lig-ands are seen in chelating agents.

light (phys) Normally concerning the visiblewavelengths of the electromagnetic spectrumfrom 390 to 740 nm, appreciated by the visualsystem as a series of colours. The absorption oflight by various materials (air, water, glass, etc.)depends on the wavelength.

light exposure (phys) A measure of the totalamount of light energy incident on a surfaceper unit area, measured in lux (see luminance,illuminance).

light guide (phys) A transparent material whichconducts light onto a detecting system. A fibre

optic cable as a transmission system is a lightguide. Transparent plastic is used as a lightguide for scintillation detectors; between thefluorescing substance and the light detector(photomultiplier or photodiode). Light guidesarea also found in the gamma camera.

likelihood ratio (L) (stats) Ratio of the probabilitythat the data set (image data, g) would occur froma specified hypothesis H1 to the probability that itwould occur from the alternative hypothesis, H2.

limiting diameter (image) the diameter of thesmallest holes in a hole pattern test object whichstill appear separated in an image with givenscan and reconstruction parameters (see geomet-

rical resolution limit, low contrast resolution limit).limiting spatial frequency (ct) The spatial fre-

quency at which the modulation transfer func-tion falls off to a certain minimum contrast levelwhich can be distinguished. The minimum contrast is not commonly defined, but is oftentaken as 2% of the modulation transfer function(see limiting diameter, geometrical resolution limit).

L


limiting spatial frequency–line spread function (LSF)

limiting value (qc) A value of a parameter which, ifexceeded, indicates that corrective action isrequired, although the equipment may continueto be used clinically (used in CEC 1993). Limitingvalues for dose or air kerma are derived differ-ently from reference values, i.e. reference ESD isbased on third quartile values derived during sur-veys whereas limiting values are derived fromresults which can be achieved in practice.

limulus test (nmed) Derived from the horseshoecrab; Limulus polyphemus. Hemolymph (amoebo-cyte), the circulating blood cell of the horseshoecrab, is extremely sensitive to bacterial endotox-ins and endotoxin-like substances which inducecoagulation of the haemolymph lystae. Thisestablished a Limulus test for detection of endo-toxin in radiopharmaceutical products.

linac (phys) A linear accelerator for charged particles(electrons) using an RF signal aligned co-axiallywith the chamber. The electron beam bombards atransmission target to give high-voltage (4 MV) x-rays with high efficiency. Energy gain in cur-rent linacs are 7 MeV m�1 for electrons and1.5 MeV m�1 for protons. Linacs can also producenarrow spectrum x-rays which have been usedfor phase contrast mammographic imaging.

line density (xray) See grid line density.line focus principle (xray) The formation of a

symmetrical (square) effective focal spot froman angled real focal spot on the anode surfaceachieved by choosing an angle that projectsthe same length as the real focal spot width.The example uses the geometry in the diagram.

line imaging (image) See sequential line imaging.line of response (LOR) (nmed) Identified in PET

when an event is detected in coincidence, theline passing through the site of annihilation.

line pairs (image) Resolution measurement using a millimetre grating of paired light and dark lines. Line pair spacings are available up to20 Lp mm�1 for testing radiographic imagingsystems. The resolution limit is reached whentwo lines are just resolvable spaced 1/x mmapart, representing x Lpmm�1. A 1 mm lineseparation would represent 1 Lp mm�1 and0.1 mm separation 10 Lpmm�1. The eye can resolve about 30 to 35 Lp mm�1. Nuclearmedicine measures resolution as Lp per centimetre.

A A

Real FS

Effective FS

CB

�

The line focus principleFrom the diagram the electron beam of width A-A

strikes the anode target area. Distance BC is deter-mined by sin θ � AC. If the real focal spot (AC) is2 mm, the effective or apparent focal spot is then0.2588 � 2 or 0.5 mm square. Doubling the real focalspot size (4 mm) will give a 1 mm effective focal spot.

10 Lp mm�1

1 cycle � 1LP

1 mm

line response function (LRF) (ct) See line spread

function.line scanning (image) See sequential line imaging.line shape (mri) Distribution of the relative

strength of resonance as a function of fre-quency which establishes a particular spectraltine. Common line shapes are Lorenizian andGaussian.

line spectral (mri) See spectral line.line spread function (LSF) (image) Obtained by

imaging a slit source rather than a point, asused in the point spread function. If the imagingsystem is shift invariant, the shape of the linespread function perpendicular to the slit musthave practical dimensions similar to the pointsource. The radiant exposure distribution in theimage of an infinitely narrow and infinitelylong slit (line source) of unit radiant energy perunit length (ICRU report, 1986) (see point spread

function).


line spread function (LSF)–linear array

L

usually shown. If the rate of change of velocity isdecreasing, the body is retarding (or decelerat-ing), and this is negative acceleration �a.

linear accelerator (phys) See linac.linear array (ct) A solid state symmetrical x-ray

detector where all the detectors have identicalsizes. Unlike the adaptive array design, the lineararray configuration is easily developed to pro-vide more than four slices per rotation. Thedimension of the z-axis is enlarged by increas-ing the width of the detector, increasing the z-dimension to give 4, 16 and 64 slice machineswhich allows a much wider coverage of theanatomy for each rotation. A four slice lineararray is shown. Currently 16, 64, 128 and 256slice linear arrays are available.

Responseprofiles

10 �m

Detector surface

line standard (di) The standard number of videolines in a display, i.e. 525, 625, 1024, 1249,etc. (see HD-TV).

line width (mri) Spread in frequency of a reso-nance line in an MR spectrum. A commonmeasure of the line width is full-width at half-maximum (F WHM).

linear absorption coefficient (μen) (phys) Partof the linear attenuation coefficient attributa-ble to absorption (photoelectric effect) some-times given the symbol μτ. The unit is thereciprocal meter m�1. The total attenuation ofthe photon beam (μtot) is the sum of linearabsorption coefficient μen (photoelectric) andlinear scatter coefficient μscat (Compton scatter)events, so: μtot � μen � μscat.

Attenuation (�)

Absorption (�en)

Line

ar c

oeffi

cien

t (m

�1 )

Photon energy (keV)

100

101

102

103

10010

linear acceleration (phys) When the velocity of a moving mass is changing the mass isaccelerating. The rate at which the velocity of amass is increasing is its acceleration, measuredas the change of velocity (x/t) per unit time texpressed as ‘per second, per second’:

When the rate of change of velocity is increasing,the acceleration is �a, although the � sign is not

acceleration m s s� � �xt

1 1

(us) Array made of rectangular elementsarranged in a line whose elements are pulsedin a sequence to give a rectilinear shapedimage or curvilinear for a curved face.

32 mm z-axis

4 � 0.5 mm4 � 1.0 mm4 � 5.0 mm4 � 8.0 mm

2 � 10.0 mm

Transducer body

Aperture

Transducer body

Scan line

Image depth

Field of view

L


linear attenuation coefficient (μ)–linear momentum (I)

linear attenuation coefficient (μ) (phys) The frac-tional reduction in intensity per unit thickness ofmaterial as an x-ray beam passes through anabsorber. For a polychromatic beam, the effectivelinear attenuation coefficient depends on theeffective energy of the beam, and the densityand atomic number (composition) of the material.For narrow beam geometry and a monoenergeticphoton beam incident on the detector (Io) with noabsorber in the beam, the rate at which photonenergy crosses a unit cross section perpendicularto the beam direction is measured as the photon

fluence Φ � N/A m�2, or photon flux is � � Φ/t m�2 s�1. As absorbers are introduced, the inten-sity now incident on the detector (Ix) will decreaseaccording to an exponential law

Ix � Io � e�μx

where absorber thickness is x and the symbol μis the linear attenuation coefficient, a functionof the photon energy and the atomic number ofthe absorber. Rearranging the above equation:

●

●

The logarithmic difference between the transmit-ted photons is directly proportional to the differ-ence in thickness x. The fractional decrease inintensity is constant, relating to attenuation ofwave or beam of particles along the medium’spath attributable to all processes (absorption andscattering). The unit is the reciprocal meter m�1.The value of μ decreases with increasing photonenergy. Since μ has greater differences at highkeV they are used preferentially for computingHounsfield units. The total attenuation of the pho-ton beam (μtot) at diagnostic x-ray energies, is thesum of photoelectric absorption (the absorption coeffi-

cient μen) and Compton scatter events (the scatter

coefficient μscat) so: μtot � μen � μscat.

ln ln and ln( ) ( )I I xII

xx oo

x� � �μ μ

II

eII

eII

eII

ex

o

x o

x

x x

o

x o

x

x� � � �� μ μ μ μ; ; ; ;

Attenuation coefficient for aluminiumThe value obtained for Io is 718 and for Ix is 516 for 1 cmthickness at 200 keV. The attenuation coefficient for 1 cm aluminium is then ln � 0.33 cm�1 or33.0 m�1.

IoI x

⎛

⎝⎜⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟⎟

Linear attenuation Coefficient for aluminiumkeV μ (m�1)

50 99.3100 45.9200 33.0

(see linear absorption coefficient, mass attenuation

coefficient).

linear dose response (dose) A dose responsemodel that expresses the risk of an effect (cancer) being proportional to dose.

linear energy transfer LET (dose) Energy trans-ferred to material absorber by charged particleor photon throughout its path. A measure of thedensity of ionising events along a radiationpath as keV μm�1. Used for estimating radiationweighting factor (wR).

● Gamma and x-rays � 1,● α radiation � 20● Beta radiation between 1 and 1.5.

(see weighting factor (radiation)).linear momentum (I) (phys) Mass � velocity the

SI unit of measurement is therefore the kilo-gram multiplied by the meter per second: kg m�1

s�1. The product of the mass (m) and the velocity (v) of the body so: I � mv. Since force ismomentum change per second (mass � veloc-ity change) and velocity change per second isacceleration a then momentum change (ΔI) isalso described by F � ma. The conservation ofmomentum is fundamental when consideringelastic and inelastic collisions between objects(radiation and orbital electrons).

Muscle

Bone

Fat

Line

ar a

ttenu

atio

n co

effic

ient

m�

1 (�

)

Photon energy (keV)

1000

100

10

10010


linear momentum (I)–linear sequenced array

L

linear scatter coefficient (σ) (phys) The Compton

scattering process results in beam attenuationand in energy loss. The linear scattering coeffi-cient σ decreases steadily with increasingenergy: high energy radiation is less scatteredthan lower energy radiation: Total � σs�σa. Notonly the amount of energy transferred to therecoil electron increases with increasing incidentphoton energy, but an increasing proportion of the total beam energy is taken by the recoilelectron. Consequently, at higher energies theenergy of the scattered photon is a smaller frac-tion of the total. A 30 keV photon scattered at a90° angle retains 97% of its energy; a 100 keVretains 91%; a 1 MeV retains 67% and a 10 MeVretains only 9%.

linear sequenced array (us) An ultrasoundtransducer array operated by applying voltage

Photon

Electron

Total

Sca

tter

coef

ficie

nts

(rel

ativ

e)

Photon energy (keV)

100

10

1

100010010

When a force has set a body in motion, thebody is stopped only by the application of otherforces (such as friction, braking, magnetic force,etc.).

Linear parameters

Distance � (m)Velocity v (m s�1)Acceleration a (m s�2)Mass mForce F � maMomentum p � mvWork W � FsPower P � FvKinetic energy ½ mv2

linear-non-threshold hypothesis (dose) A doseresponse model suggesting that, in the lowdose range, radiation doses above zero willincrease the risk of excess cancer and/or heritable disease proportionately.

linear phased array (us) Linear array operatedby applying voltage pulses to all elements, butwith small time differences (phasing) to directultrasound pulses out in various directions.

linear-quadratic dose-response (dose) A doseresponse model that suggests the risk of aneffect (e.g. disease, death or abnormality) is the sum of a two component effect, one proportional to dose (linear term) and the otherproportional to the square of dose (quadraticterm).

linear relationship (stats) A relationshipbetween two variables (e.g. dose and effect)which gives rise to a straight line on aCartesian graph, conforming to the formulay � mx�c; m being the gradient and c theintercept with the y-axis.

1 2 3 4 5

Element width

Line width

3�4group

Radiation dosimetry: Nuclear recoil238U nuclear recoil due to alpha particle emission. Thenuclear mass mn due to 92 protons and 146 neutronsis 3.97 � 10�25 kg. Alpha particle mass mα (2protons: 2 neutrons) is 6.68 � 10�27, the alpha particle velocity (vα) is measured as 6.25 � 107 m s�1.The original nucleus, before α emission, is at rest solinear momentum is zero (vn and vα � 0).Conservation of momentum requiresmn � vn�mα � vα � 0.

The nuclear recoil velocity (vn) therefore is:

�(ma /mn) � va

��1.05 � 106ms�1 (recoil has minimum velocity)

L


linear sequenced array–lithotripsy

pulses to groups of elements sequentially. Thelinear array is formed from a large number ofindividual transducer elements arranged ingroups. During each transmission/receive cyclea transducer group from 16 to 32 elements isactive.

linear shift-invariant system (image) An imag-ing system which is both linear, i.e. the magni-tude of the output signal is a linear function ofthat of the input signal, and shift invariant, i.e.shifting the position of the input signal resultsonly in a displacement of the output by thesame amount (or a simple scaling if magnifica-tion is allowed).

linear tomography (xray) Longitudinal sectionalinformation obtained by moving the x-ray tubeand film in synchrony about a fulcrum or axis.The fulcrum defines the sectional plane. The sec-tional images are degraded by blurring causedby interfering absorption on either side of theplane of interest (see tomography, computed tomog-

raphy). Linear tomography has suffered a sharpdemise with the widespread availability of com-

puted tomography but is being reintroduced usingdigital imaging (see direct radiography).

linear velocity (v) (phys) The velocity of a movingpoint, or of a body is the rate of its displacementor the rate at which it changes its position, in agiven direction. The rate of displacement of thebody is the distance it moves during each unit oftime. If it moves through s m in t seconds thenv � s/t m s�1. Velocity is a vector quantity havingboth magnitude and direction, but when thedirection is constant (a straight line) and the bodycovers equal distances in equal times, the veloc-ity is uniform and is measured by the displace-ment per unit time.

Units of linear velocity

SI m s�1

CGS cm s�1 1 cm s�1 � 1 � 10�2 m s�1

Other miles hr�1 1 m.p.h. � 0.44 m s�1

linearly polarized coil (LP coil) (mri) Designedto excite or detect spins using one RF transmitand/or receive channel. The magnetic field haspredominately a single direction.

Linux (comp) A UNIX® based, open-source (freelyavailable) operating system originally devel-oped by Linus Torvalds. Linux is suitable formany platforms, both PCs and Macintoshes.

lipiodol (clin) The first practical x-ray contrastmedium introduced for mylography by Sicardin 1921. Because of induced pulmonary andperipheral fat micro-emboli oily x-ray contrastmedia have been discontinued.

lipophilic agents (cm) See lipophilicity.lipophilicity (clin) Refers to its preference for fat-like

(lipid) organic solvents such as n-butanol. Foundto correlate approximately with toxicity of ioniccontrast media; readily cross the blood–brain barrier

(BBB) during their first pass through the cerebralvasculature. On the other hand, water-solublecontrast media such as non-ionic iodinated con-trast used in CT, and water-soluble paramagneticmedia used for MRI, are not lipophilic, have a highaffinity for plasma water, and do not pass the nor-mal BBB. Certain radiopharmaceuticals (HMPAO,IMP, HIPDM) are lipophilic compounds crossing theblood–brain barrier; conversion occurs within thebrain tissue into a hydrophilic agent which trapsthe compound. Properties other than lipophilicity(e.g. hydrogen bonding) can also be responsible forinteractions with biological molecules and membranes.

liquifier (mri) System for reliquification of cryo-genic gases: if closely matched with a super-conducting magnet, zero net cryogen boil-offcan be achieved.

LIS (mri) Lanthanide-induced shift. A techniqueinvolving the substitution of a paramagnetic lan-thanide ion for the calcium ion in a selected pro-tein which results in lanthanide-induced shiftsand broadening in the 1H NMR spectrum of theprotein. These shifts are sensitive monitors of theprecise geometrical orientation of each protonnucleus relative to the lanthanide ion.

lithium (Li) (elem)

Atomic number (Z) 3Relative atomic mass (Ar) 6.94Density (ρ) kg/m3 534Melting point (K) 452

lithotripsy (xray) A non-invasive procedurewhich uses high power ultrasound to fracture

Photon time of flightThe distance travelled by a 0.511 MeV gamma photonafter 1 nanosecond (10�9s). All electromagnetic radia-tion has a velocity of approximately 3.0 � 108m s�1.

Distance travelled � (3.0 � 108) � 10�9 � 0.3 m.

linearity (ct) In CT, the extent to which the CTnumber of a given material is exactly propor-tional to its density (in HU unit).


lithotripsy–logarithm

L

kidney stones in situ (see extracorporal shock

lithotripsy).litre (L) (phys) The SI unit of volume. Up to 1964 the

definition was the volume occupied by a mass of1 kg of pure water at maximum density (�4°C)under normal atmospheric pressure (101 325 Pa).1 litre then equalled 1.000028 dm3. This waschanged in 1964; the SI unit of m3 is used forderiving the litre as equal to 1 dm3 � 10�3m3.For practical purposes the millilitre (L�3; ml) andcm3 are equivalent.

liver (biliary contrast media) See cholegraphic

contrast agents.LMR (mri) See localized magnetic resonance.LNT (dose) See linear no threshold.

loadability (phys) The thermal loadability of an x-ray tube for short, medium and long exposures isdetermined by the anode’s heat capacity and heatloss (cooling rate). A high loadability/high-outputtube has two distinguishing characteristics:

● An anode disc with a large diameter, providinggreater heat radiation and greater heat storage

● A high conduction through larger surface areasleeve bearings

Short term loadability is a measure for very shortexposure time of 0.1 s or less, and is determinedby the size of the region which is directly bom-barded by the electrons. The focal track of arotating-anode tube is exposed to direct bom-bardment by electrons, and therefore representsthe region with the highest thermal load. Longterm loadability; determined by the anode cool-ing rate; achieved by rapidly restoring the heatcapacity of the anode by providing suitable cool-ing (heat loss) for the anode.

Loadix® (xray) A device attached to the rearenvelope of an x-ray tube used by Siemens tomonitor the temperature of the anode and pro-tect it from excessive heat damage.

loading (mri) See coil loading.local (comp) Typically refers to devices attached

to the user’s workstation, as opposed to remotedevices that are accessed through a server.

local area network (LAN) (comp) A small networkthat is generally confined to a single office orbuilding. Workstations and computers that aretied together in a specific work area in the samegeneral location. Peer-to-peer networks are atype of LAN architecture. Each workstation dou-bles as a server in this network, providing theability to share peripherals and resources with

any other workstation. Peer-to-peer resourcesharing can be effective on small networks, butsecurity and reliability issues are important.

local bus (comp) Also known as the system bus.An internal bus line between CPU, memory andother peripheral devices.

local coil (mri) Anatomically specific coils used foreach region of the body to be examined (surfacecoils). The signal-to-noise ratio is significantlyimproved but over a smaller measurement field(see integrated panorama array).

local rules (dose) A list of recommendations andregulations drawn up by a hospital to serve alllocalities (radiology, surgery, laboratory, etc.)that use radiation sources. The local rules sup-plement, emphasize and clarify the nationallegislation, identifying the various categories ofcontrolled, supervised and high radiation areas,together with procedures for decontamination.Estimations of risk to the foetus from radiationexposure in-utero should be included for refer-ence as well as recommended radiographicprotocols for low dose examinations.

localization techniques (mri) Means of selectinga restricted region from which the signal isreceived. These can include the use of surfacecoils, with or without magnetic field gradients.Generally used to produce a spectrum from thedesired region.

localized magnetic resonance (LMR) (mri) A par-ticular technique for obtaining NMR spectra, forexample of phosphorus, from a limited region bycreating a sensitive volume with inhomogeneousapplied gradient magnetic fields, which may beenhanced with the use of surface coils.

localized MIP (mri) Localized maximum intensityprojection. This improves image quality and sig-nificantly reduces reconstruction time. A partialdata volume is collected containing the vessel ofinterest.

LocalTalk (comp) Apple Computer’s proprietaryLAN, based on the AppleTalk architecture.

location encoding (mri) Defining position andorientation of a slice using the frequency andphase-encoding gradients. The signal origin isencrypted in the MR signal and reconstructedwith the image.

lock (mri) See field lock.logarithm (math) The power to which a fixed

number, the base, must be raised to obtain agiven number. Abbreviated to log for base10

and ln for basee (see exponential).

L


logarithmic subtraction–luminance

logarithmic subtraction (xr) See digital subtraction

angiography (DSA).logic circuit (elec) The component part of a digi-

tal circuit which acts as a gate, steering dataaccording to a logical sequence. The gatesoperate according to Boolean logic giving AND,NAND, OR, NOR operations.

logical gradients (mri) Each of the three physicalgradients has one task: slice selection, frequencyencoding and phase encoding. For oblique slices,the logical gradients are a mix of the physicalgradients.

longitudinal magnetization (Mz) (mri) Themacroscopic magnetization aligned along theZ-axis. After the RF pulse, Mz returns to equilib-rium Mo with a characteristic time constant T1.

Mz (t ) � Mo(1 � e�t/T1)

(see transverse magnetization).longitudinal relaxation (mri) Return of Mz to Mo

after excitation. Requires an exchange of energybetween the proton spin and the molecular lat-tice. Measured by time constant T1 (see magnetic

vector).longitudinal relaxation time (mri) T1 constant.longitudinal spatial resolution (ct) See spatial

resolution.longitudinal wave (us) Compression wave in

parallel to wave direction; the same directionas the direction of travel. The most commonexample is a sound wave where compressionsand rarefactions move along with the speed ofthe wave-form each particle vibrating about amean position transferring energy to the nextparticle (see transverse wave).

look-up table (LUT) (di) A series of stored valuesin memory which is used for mathematicaltransformations (log or non-linear image pro-cessing) on incoming signals. LUTs also controldisplayed grey scale.

Lorentzian line (mri) Typical line shape in anNMR spectrum, with a central peak (central res-onance frequency) having long tails; propor-tional to:

where f is frequency and fo is the frequency ofthe peak. The Lorentzian function is the Fouriertransform of a decaying exponential.

LOTA technique (mri) Long term averaging. Dataaveraging for reducing motion artefacts.

11

22 2[( ) ]T ( )� �f fo

LP (mri) Linear prediction.LP (mri) Linear polarization.low contrast resolution (image) A measure of the

ability to discriminate between structures withslightly differing attenuation properties (CT num-ber). It depends on the stochastic noise and isusually expressed as the minimum detectable sizeof detail discernable in the image, for a fixed per-centage difference in contrast relative to the adja-cent background. (ct) The geometrical resolution forlow-contrast details within the object; a measureof the ability to discriminate between structureswith slightly differing attenuation properties (CT

number). In addition to the modulation transfer func-

tion of the CT system, quantum noise and othersources of noise inherent to the scanner deter-mine the limit for separation of small low-contrastdetails within the image (see image noise, contrast

detail diagram, contrast noise ratio).low frequency drop (LFD) (xray) See image inten-

sifier (low frequency drop).low osmolarity (cm) A solution of salts, sugars,

proteins, acids, etc. with a moderate osmolality orconcentration. Describes a contrast medium witha slightly higher concentration than body fluids(two to three times). Such contrast media causeless pain and heat when injected compared tohigh-osmolar contrast media (see high osmolarity).

low-pass filter (di) A filter designed to reject fre-quencies above a certain value while retainingthe lower frequencies. They are used in smooth-

ing filters or kernels which reduce the influenceof noise in the signal (see ideal filter).

LS (mri) Line scanning.LSR (mri) Lanthanide shift reagentLSO (rad) See Lutetium oxy-orthosilicate.Lucite®/Perspex® (phys) See PMMA

lumen (lm) (phys) The SI unit of luminous flux,which is the amount of light passing through aunit area at a unit distance per second. Onelumen is also defined as the light falling on a unitarea at unit distance with a luminous intensity of 1 candela. One lumen of visible light at 555 nmwavelength dissipates an energy of 1.47 � 10�3

watts. Lamp output is measured in 103 lumensand efficiency as lumens W�1. For a theoretical100% efficiency, a white lamp would give220 lm W�1. In practice, a tungsten lamp gives15 lm W�1 and fluorescent tubes approximately50 lm W–1 (see luminance).

luminance (phys) A measure of the light comingfrom a surface. It is the photometric brightness


luminance–luminous intensity

L

measured in candela per unit area (SI-unit cdm�2; also known as a nit). The non-SI unit isthe lambert.

Quantity SI unit Abbr.

Luminous flux lumen (cd � sr) lmLuminous energy lumen second lm � sIlluminance lux (lm m�2) lxLuminous intensity candela (lm sr�1) cdLuminance candela per square metre cd m�2

Source Luminance

Film viewing box 2000 cd m�2 or 0.2 cd cm�2

Video (TV) screenDisplay monitor 300 cd m�2 to 600 cd m�2

Computer colour display 100 cd m2

HD-TV 1.0 cd m�2 (Lmin) and amaximum of 263 cd m�2

(Lmax) giving a contrast ofLmax/Lmin or 263

■ Reference: Assessment of Display Performance.

Task Group 18 AAPM 2002.

(see radiance).luminescence (phys) A general property of some

inorganic crystals and plastics involving electrontransition between valency and conduction bands.Gamma, x-radiation or particulate radiation(alpha, beta, electrons) interacts with the phos-phor or scintillator producing a light eventwhose intensity is proportional to the photonenergy deposited. Light production can beinstantaneous or delayed and the duration of thelight signal can be measured in nano-seconds(10�9s) or tenths of seconds (10�1s). Propertiesof a good luminescent scintillator are:

● transparency to emitted light● large light output● high photon absorption for γ-and x-rays● available in large sizes.

The term luminescence covers three majorphosphor types that are commonly found inradiology and used for both imaging and radi-ation dose measurement such as counters orstorage devices. These are:

● phosphorescence● fluorescence● thermoluminescence.

Summary of luminescent detectors:

Scintillator Application

FluorescenceNaI:Tl Scintillator for NMCsI:Tl Scintillator for CT

Summary of luminescent detectors (Contd.):

Scintillator Application

CaWO4 Intensifying screenGd2O2S:Tb Intensifying screen

PhosphorescenceZnS complex (P4) Video monitorZnCdS:Ag (P20) Image intensifier

ThermoluminescenceLiF DosimeterBaF(X) X � F, I or Br Image plate

(see fluorescence, phosphorescence, thermolumines-

cence).

luminescent efficiency (phys, ct ) The ratio ofthe total energy of visible emission to total x-ray energy. After irradiation the intensity of thevisible emission quickly decreases with time.Once the luminescent intensity drops to a fewpercent of the luminescent light, further decayproceeds much slower (afterglow), caused bythe trapping of electrons and/or holes in crystalimperfections. The intrinsic luminescent effi-ciency should be �5% (signifying efficientrecombination of electron-hole pairs at theactivator sites) so producing a good signal.Strongest peak emission should be at wave-lengths between 500 and 900 nm where silicon

photodetectors have highest sensitivity. Optical

transparency at the peak emitting wave-lengthshould be high to permit good optical light col-

lection efficiency at the photodetector.luminosity (phys) The arbitrary brightness of a

colour on a scale from black to white (see hue,saturation).

luminous flux (phys) Luminous intensity inte-grated over a solid angle. The unit is the lumen.

luminous intensity (phys) The quantity of lightemitted per second by a point source in unitsolid angle. The unit is the candela (cd).

Electronshells

Conduction band

Valency band

Forbidden zoneTrap

Electron Hole

L


lutetium oxy-orthosilicate (LSO)–lymphography

lutetium oxy-orthosilicate (LSO) (rad) Ceriumactivated lutetium oxyorthosilicate (Lu2OSiO4: Ceor LSO),LSO offers the best combination of properties for PET imaging. LSO has a highereffective Z (number of protons per atom) and den-sity compared to BGO which results in a higherdetection efficiency. It has a short decay constantfor improved coincidence timing (decay constant40 ns), and higher light output (compared toBGO). The crystal is rugged and non-hygroscopicand lends itself to precise machining.

lux (lx) (phys) A measure of illuminance as alumen per square metre lm m�2 � m�2cd sr.

lutetium (Lu) (elem)

Atomic number (Z) 71Relative atomic mass (Ar) 174.9Density (ρ) kg/m3 9800Melting point (K) 1925K-edge (keV) 63.3Relevance to radiology: lutetium yttrium aluminiumperovskite Lu(Y)AP fast decay scintillator for PET.

177Lutetium (nmed) xxβ–497 keV, γ 208 keV T1/26.65 days. Short range beta 1–3 mm.

lymphocyte (dose) A white blood cell formed in the lymphatic tissue and in normal adultscomprising approximately 22 to 28% of the cir-culating leukocytes (see blood cells (labelled).

lymphangiography (clin) Imaging the lymphaticsystem (lymphatics and lymph nodes) by intro-ducing CM into the lymph vessels.

lymphography (clin) Assessment of the regionallymphatics, lymph nodes and the thoracic duct,using an oily based contrast medium. Also givescontrast visualization of peripheral lymphatics inthe extremities, the trunk, face and neck regions.Non-invasive methods using ultrasound, CT, MRIand radionuclides have largely replaced lym-phography as a routine procedure. Direct (lym-phangiography) is used in a few cases andinvolves direct puncture of lymph vessels andsubsequent use of oily contrast media. Indirectuses a water based contrast medium infused intotoes, fingers or backs of feet and hands. Dependson the CM rapidly diffusing through the lymphatic wall.


�o–magnet (resistive)

M

Mμo (unit) The symbol for permeability of free space

having a value 4π �10�7 N/(ampere-turn)2.Mo (mri) Equilibrium value directed along the static

magnetic field lines. Proportional to spin density.Mxy (mri) See transverse magnetization.Mz (mri) See longitudinal magnetization.M mode (us) Mode of operation in which the dis-

play presents a spot brightening for each echovoltage delivered from the receiver, producing atwo-dimensional recording of reflector position(motion) versus time.

MAA (nmed) Degradable macro-aggregates ofhuman serum albumin. A radiopharmaceuticallabelled with 99mTc and used for either lungscanning, venography or tracing the origins ofartero-venous malformations.

Generic name 99mTc-MAACommercial names Pulmonite®–CISMacrotec® (Bracco)Technescan® MAAAlbumoscint®

Draximage®

Amersham® MAANumbers of particles/vial 8 �4 �106

Imaging category lung perfusion

Aggregates of albumin ranging from approxi-mately 10 to 60 μ m (mean size 40 μ m with 90%�90 μ m); these lodge in the capillaries and pre-capillary arterioles.

Mackensie Davidson, James (1856–1919) pio-neer British radiologist, president of the RoentgenSociety, later to become British Institute ofRadiology.

macro (comp) A script that operates a series ofcommands to perform a function. It is set up toautomate repetitive tasks.

macroaggregates (nmed) See MAA.macroscopic magnetization vector (mri) Net

magnetic moment per unit volume of a sample,considered as the integrated effect of all theindividual microscopic nuclear magnetic moments(see magnetic moment, magnetic vector).

MAG3® (nmed) Benzoyl-mercapto-acetyl-triglycine, a kit for the preparation of 99mTcmertiatide, a renal diagnostic radiopharmaceu-tical manufactured by Mallinckrodt for labellingwith 99mTc as Technescan – MAG3. High renalclearance primarily through proximal renal

tubule, small amount of GFR and distal tubulesecretion. It resembles the excretion pattern ofortho-iodohippurate.

magnesium (Mg) (elem)

Atomic number (Z) 12Relative atomic mass (Ar) 24.3Density (ρ) kg/m3 1741Melting point (K) 924Specific heat capacity J kg�1 K�1 1020Thermal conductivity W m�1 K�1 156K-edge (keV) 1.3

magnet (phys) Ferrous material (commonly iron,cobalt or nickel) exhibiting permanent or elec-tromagnetic behaviour.

magnet (homogeneity) (mri) Refers to the uni-formity of a magnetic field with no object pres-ent. Homogeneity is quoted in parts per million(ppm) a relative measure independent of fieldstrength that uses, as reference, the Larmorfrequency for water 42.5759 MHz T�1. A 200 Hzvariation would give an inhomogeneity of:

200/(42.5759 � 106) � 4.7 � 10�6 � 4.7 ppm

Permanent and resistive magnets have typicalinhomogeneities of 40 ppm and superconduct-ing magnets of 1–3 ppm.

magnet (permanent) (mri) Certain ferromagneticalloys form a permanent magnetic field andneed no power to maintain their field strengthbut will gradually lose their original magneticstrength over a period of several years.Permanent magnets have small fringe fields socan be operated in a small area. They are heavy(about 20 tonnes) and have field strength limi-tations of about 0.3 T but some are muchsmaller. Their open field aspect allows easypatient interventional studies; however, theycannot be switched off during emergencies andtheir magnet homogeneity is poor.

magnet (resistive) (mri) Operate using the basicelectromagnetic principles. Consist of a collec-tion of coils through which a large current ispassed. They have a high electrical consump-tion and create considerable heat that must beremoved by cooling. They have a practicalupper limit of 0.3 T. They have larger fringefields than the permanent magnet and theirweight is between 5 and 10 tonnes. They canbe switched off during an emergency so theyare popular as interventional machines. Magnet

homogeneity is fair.

M


magnet stability–magnetic fringe field

magnet stability (mri) Temporal stability of themagnetic field. Factors that affect stability are:field decay of superconducting magnets in per-sistent mode, aging of permanent magnet mate-rial, temperature dependence of permanentmagnet material, and temporal stability of magnet power supplies.

magnet (superconducting) (mri) These are a spe-cial type of electromagnet using windings madefrom alloys which become superconducting atliquid helium temperatures (4 K). An extremelyhigh, stable magnetic field can be produced from1 to 4 T although smaller magnets with fieldstrengths of 9 T have been used for neonatalstudies. The large field strengths allow spectro-scopy and fast gradient switching pulse sequences(EPI) that are not available with other magnetdesigns. They have very high magnet homogene-

ity, which enables small volume acquisition andfat saturation routines.

magnetic dipole moment (μ) (mri) The ability ofa current loop to produce a magnetic field atdistances from the loop (signal coil). μ is themagnetic moment which is a vector quantity.The SI unit is the ampere-metre2 (A m2), webermetres or joules per tesla (J T�1). An electriccurrent loop or the effective current of a spin-ning nucleon can create a dipole equivalent tothe north and south magnetic poles separatedby a finite distance on a magnet (see magnetic

moment).magnetic disk (comp) See hard disk drive.magnetic field (B) (mri) Other names are mag-

field, magnetic flux density, magnetic inductionfield and magnetic induction. A vector quantitydescribing the magnetic flux per unit area of amagnetic field at right angles to the magneticforce. The region surrounding a magnet (or cur-rent carrying conductor) where a small magnetin such a region experiences a torque that tendsto align it in a given direction. The magneticfield produces a magnetizing force on a bodywithin it. The net magnetic effect from an exter-nally applied magnetic field and the resultingmagnetization. B is proportional to H, the mag-

netic field intensity (B �μH) The fundamentalquantity for describing a magnetic field. SI unittesla or weber m�2 (Wb m�2) or newton perampere-meter (N �A �m�1) (see magnetic field

intensity, magnet homogeneity, safety limits (MRI)).magnetic field gradient (mri) The rate of change

of a component of the magnetic field with

position. Gradients in each of three directions:∂Bz/∂z, ∂Bz/∂x, ∂Bz/∂y, a magnetic field whichchanges in strength in a certain given directionproducing positional information. Such fieldsare used in NMR imaging with selective excita-tion to select a region for imaging and also toencode the location of NMR signals receivedfrom the object being imaged. The SI unit is teslam�1, (0.01 T m�1 �1 gauss cm�1). If the gradi-ent coil imposes a change in the magnetic fieldof 0.01 T over 1 m then the magnetic field gra-dient is 10 mT m�1 (positive or negative). Typicalvalues are 20 to 35 mT m�1 for a 1.5 T systemand 45 mT m�1 for a 3.0 T system (see slew rate).

magnetic field intensity (H) (mri) or magneticfield strength. The alternative measurement toB where H �B/μ where μ is the permeability ofthe medium. The SI unit is ampere per metre (A m�1) (see magnetic field).

magnetic field strength (H) (mri) See magnetic

field intensity (H).magnetic flux (Φ) (mri) A measure quantifying

the magnetic field strength or magnetic field inten-

sity (H); the magnetic flux density is another termfor the magnetic field. The flux dΦ through aregion of area dA perpendicular to B is given bydΦ �BdA. So the magnetic flux is the productof a magnetic field through a defined area,expressed as T m2. The magnetic flux through asurface is proportional to the number of mag-netic field lines that pass through the surface.The SI unit is the weber (Wb) or volt-secondwhere: 1 Wb (V �s) �1 T m2.

magnetic flux density (B) (mri) See magnetic field.magnetic forces (mri) Forces resulting from the

interaction of magnetic fields. Pulsed magneticfield gradients can interact with the main mag-netic field to produce acoustic noise. Inducedmagnetization reacts with the gradient of themagnetic field to produce an attraction towardthe strongest area of the field. There is also atorque or twisting force on objects, for example,intracranial aneurysm clip tends to align alongthe magnet’s field lines. The torque increaseswith field strength while the attraction increaseswith field gradient. The magnetic saturation ofthe object attraction is roughly proportional toobject mass. Motion of conducting objects inmagnetic fields can induce eddy currents thatcan have the effect of opposing the motion.

magnetic fringe field (mri) Region surrounding amagnet. The earth’s magnetic fringe field is


magnetic fringe field–magnetic resonance spectroscopy (MRS)

M

typically 0.05–0.1 mT. Due to the physicalproperties of magnetic fields they form closedfield lines. Depending on the magnet construc-tion, the returning flux will penetrate largeopen spaces (unshielded magnets) or will beconfined largely by a massive iron cage orthrough secondary coils (shielded magnets).

magnetic gradients (mri) See gradient.magnetic induction (B) (mri) See magnetic field.magnetic intensity (B) (mri) See magnetic field.magnetic moment (m) (mri) The ratio between

the maximum torque exerted on a magnet, current-carrying coil, or moving charge in amagnetic field and its strength. The maximumtorque is given when the axis of the magnet isperpendicular to the field. The magnetic momentis a measure of the net magnetic properties ofan object or particle. A nucleus with an intrinsicspin will have an associated magnetic dipole

moment, so that it will interact with a magnetic

field (resembling a tiny bar magnet). The SI unitis Wb m�1.

magnetic permeability (μ) (phys) See permeability.magnetic resonance (MR) (mri) Resonance phe-

nomenon resulting in the absorption and/oremission of electromagnetic energy by nucleior electrons in a static magnetic field. afterexcitation by a suitable RF magnetic field. Thepeak resonance frequency is proportional tothe magnetic field, and is given by the Larmorequation. Only unpaired electrons or nuclei witha non-zero spin exhibit magnetic resonance.

magnetic resonance angiography (MRA) (mri)Allowing vascular structures to be seen withoutusing contrast material. Produces images ofblood vessels, for example with flow effects orrelaxation time differences. Blood flow can bequantified and flow directions can be deter-mined by using specific techniques and pulsesequences. Some common approaches use thewashout of saturated spins from a region byblood flow to increase the relative intensity ofblood vessels within images.

magnetic resonance cholangiopancreatico-graphy (MRCP) (mri) Magnetic resonance imag-ing technique in which a two-dimensional (2D)multislice or three-dimensional (3D) RARE pulsesequence using partial Fourier techniques or a3D gradient echo pulse sequence with heavyT2 weighting is used to acquire data from theliver and the pancreas. The T2 weighting givesessentially all signals from the liver; other solid

parenchyma is suppressed and only fluid-filledstructures, (gall bladder, bile and pancreaticducts) contribute important signal intensity.

magnetic resonance signal (mri) The signalemitted by a sample in a strong magnetic fieldof a nuclear magnetic resonance system. A radio-frequency (1–100 MHz) signal is imposed atright angles to this main field and as the mag-netic field or RF signal is altered there is a pointwhere the RF signal is strongly absorbed. Thisresonance (detected by an inductor or antenna),produces a precise signal which is the mag-netic resonance signal for the sample (typicallyhydrogen protons but also 31Phosphorus) (seefree induction decay).

magnetic resonance spectroscopy (MRS) (mri)Concerns itself with the free induction decaysignal which carries information about thechemical nature of the material. The local mag-netic field varies slightly within the compoundmolecule due to chemical bonds, size of atomsand position of the atom relative to others. Thehydrogen atom in water will experience a dif-ferent local magnetic field to a hydrogen atomin fat leading to a chemical shifts caused byslight variations in Larmor frequency (measuredin parts per million (ppm) of the main magneticfield). The degree of chemical shift is character-istic of the chemical bonding. Spectroscopy isused for demonstrating the functional chem-istry of compounds other than hydrogen.Phosphorus spectroscopy shows metabolicpathways and fluorine spectroscopy is used forfollowing the fate of chemotherapeutic agents.All magnetic resonance spectroscopy requiresfield strengths higher than 1 T in order to pro-vide resolvable peaks and also exceptionalmain magnetic field homogeneity (see CHESS).

ATPPD

PI

5 10 15 20 25 ppm

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magnetic resonance venography–magnetic vector

magnetic resonance venography (clin) Veins maybe imaged and the flow rate determined directly,without the use of paramagnetic contrast media.

magnetic shielding (mri) Confining the region ofstrong magnetic field surrounding a magnet:most commonly the use of material with highpermeability (passive shielding) or by employ-ing secondary counteracting coils outside of theprimary coils (active shielding). The high perme-ability material can be employed in the form ofa cover or yoke, or an opposing coil immedi-ately surrounding the magnet. Shielding isaccomplished by forcing the magnetic returnflux through more confined areas or structures,not by absorbing it. Soft iron (mu-metal) is acommon material used for shielding againstmagnetic interference and can be placedaround sensitive equipment (image intensifiers,photomultiplier tubes, etc.), since it can concen-trate quite weak magnetic fields reducing theexternal magnetic flux and so protecting deviceswhich are surrounded by the material.

magnetic shielding (mu-metal) (phys) Soft iron(mu-metal) is a common material used for shield-ing against magnetic interference and can beplaced around sensitive equipment (image inten-sifiers, photomultiplier tubes, etc.), since it canconcentrate quite weak magnetic fields reducingthe external magnetic flux and so protectingdevices which are surrounded by the material.

magnetic strength (phys) This is dependent on thepole strength m1 and m2 at a distance d. Theseattract or repel each other with a force F so that:

where K is a constant, the force obeys theinverse square law. Intensity of the magneticfield is measured as:

where p is the permeability of the medium (largefor ferromagnetic materials; p for air �1). Themagnetic flux would be p times as great if the coils are wound over a magnetic core material(soft-iron); this is exploited in transformer design.

magnetic susceptibility (mri) An applied mag-netic field induces magnetization of a material.The susceptibility measures the ability of a sub-stance to become magnetized. A dimensionless

Fm mp d

��

�1 2

2

F Km m

d� �

�1 22

unit whose value depends on the properties ofthe atoms and molecules of the material (seepermeability).

magnetic tape (comp) See storage (bulk).magnetic units (units) These are analogous to

electrostatic units.

Quantity and symbol SI unit

Magnetic dipole moment A m2 or J T�1

Magnetic field strength (H)MagnetizationMagnetic field intensity H (A m�1)Magnetic flux (Φ) weber (Wb) T m2

Magnetic fieldMagnetic flux density (B)Magnetic induction tesla (T) Wb m�2

Magnetic moment henry (Wb m)Magnetic constant (μo)Permeability of free space henry m�1

Inductance Wb A�1

Reluctance ampere turn perweber (A Wb�1)

magnetic vector (mri) When a sample is exposed toan external magnetic field a net alignment of theindividual magnetic moments (protons) will resultin a preferred direction along the external field.The nuclei that make up this alignment contributeto a resultant magnetization vector (M). There is alongitudinal magnetization vector (ML) and trans-verse magnetization vector (Mxy). A transversecomponent of the magnetization vector can beproduced by applying a transverse magnetic field(90° RF pulse or other flip angle) which will move Maway from the magnetic field; as the strength ofthe RF pulse increases the longitudinal compo-nent ML of the magnetization vector decreases. Mwill assume a position between M zero and Mxy.

M(t)Mz

Mxy

�z

�z

y

x


magnetization–mask filter

M

magnetization (mri) See magnetic field strength.magnetization transfer (mri) The change in

magnetization within a multi-component spinsystem when one of the component peaks isselectively perturbed. This is observed as achange in relative signal intensities. One of themost common forms of perturbation in imagingis selective saturation. For example, this phe-nomenon can be exploited as part of an imag-ing sequence to produce image contrast basedon differential amounts of magnetization trans-fer, magnetization transfer contrast (MTC).

magnetization transfer contrast (MTC) (mri)Production of change in relative signal intensitiesby magnetization transfer. The signal from spe-cific tissue regions such as the brain parenchymais reduced and the signal from fluid components(blood and CSF) is retained, so enhancing theseareas. More closely coupled states will show agreater resulting intensity change.

magneto-optical disk (comp) Removable opticaldisk which is re-writeable. Capacity 2.6 G-byte.Data transfer rates 3.4 M-bytes read, 1.7 M-bytes write. Average seek time 25 ms.

Magnevist® (cm) Commercial (Schering/BerlexLabs. Inc) preparation of ionic gadopentetatedimeglumine for MR imaging.

Compound Concentration Viscosity cP Osmolalitymg mL�1 mOsm/kg

Gd-DTPA 469 4.9 @ 20°2.9 @ 37° 1960

magnification (xray) Image enlargement re-displaying a section of the image to full displaydimensions. No improvement in intrinsic reso-lution (see geometric unsharpness).

magnitude contrast angiography (MCA) (mri)Displaying a slow blood flow with good resolu-tion across a large volume. Two data volumesare measured: the flow rephased image showsbright flow and the flow dephased imageshows dark flow; tissue in unchanged in bothdata volumes. After subtraction the remainingimage represents flowing blood.

magnitude image (mri) The normal image dis-play, where the gray scale of a pixel corre-sponds to the magnitude of the MR signal atthat location (see phase image).

main frame (comp) A large central computer hav-ing giga-bytes of memory and terabytes of diskstorage.

Mallard JR (1927–) British medical physicistwho pioneered developments in medical imag-ing, particularly nuclear medicine and magneticresonance imaging.

mammography (xray) A low kV x-ray imagingtechnique for imaging breast tissue. Since photoelectric events predominate in the image formation, small soft tissue differences can bedistinguished.

man-Sievert (dose) (ICRP60) Dosimetric quanti-ties relating to exposed groups take intoaccount the number of people exposed to thesource. These quantities are the collective equiv-

alent dose ST which relates to a specified tissueor organ exposed (e.g. lungs in miners) and thecollective effective dose S (e.g. equivalent wholebody dose for miners). The unit of these collec-tive quantities is the man-Sievert obtainedfrom the product of the individual dose andpopulation number. The patient populationman-Sievert is used as the reference for reduc-ing clinical dose rates (optimization).

manganese (Mn) (elem)

Atomic number (Z) 25Relative atomic mass (Ar) 54.9Density (ρ) kg/m3 7440Melting point (K) 1517K-edge (keV) 6.5Relevance to radiology: as a paramagnetic contrastagent in MRI as Mn-DTPA, also as manganesedipyridoxyl-diphosphate (Mn-DPDP) as a para-magneticcontrast agent for hepato-biliary imaging.

MAP shim (mri) Multi-angle projection. An earlyshim procedure that tunes the shim currents toa large region. The shim program can also beapplied to the entire measurement field.

Marinelli formula (nmed) An early attempt atcalculating internal dosimetry estimating organand whole body radiation dose from beta, non-penetrating radiation Dβ and gamma Dγ pene-trating radiation. It uses a rough geometricfactor g allowing for simple variations in organshape (liver, lungs, kidney etc.) (see MIRD for-

mula).mAs product (xray) The product of tube current

and exposure time. Since one milliampere is1 mC s�1 then mC s�1�seconds representscoulombs and 1 mAs �1 mC (see exposure).

mask (dsa) An early image containing tissuedetail which is subtracted from later images.

mask filter (di) See kernel.

M


mass–matching network

mass (phys) The SI base unit is the kilogram (kg).The only base unit which is a multiple of asmaller unit, the gram.

● 1 kg �1000 gm.● 1 metric ton (tonne) �103kg.● 1 pound �0.453 kg (see kilogram).

mass absorption coefficient (phys) The equiva-lent of the linear absorption coefficient. It is thefraction of energy contained in the beam (gammaor x-radiation) which is absorbed per unit massof the medium through which it passes whenunit area is irradiated.

mass attenuation coefficient (μ/ρ) (phys) Thequotient of linear attenuation coefficient andmaterial density μ/ρ. The total mass attenuationcoefficient is the fraction of radiation removedfrom the beam (gamma or x-radiation) of unitcross sectional area in a medium of unit mass. Thelinear attenuation coefficient μ has units of m�1; themass attenuation coefficient μ/ρ has units of m2

kg�1. The mass attenuation coefficient is inde-pendent of the state of the absorber (gas, liquid,solid). The table shows values for the same keV.

State μ ρ μ/ρ

Ice 0.196 0.917 0.214Water 0.214 1 0.214Steam 1.2 �10�4 5.9 �10�4 0.214

Coefficient (at 150 keV)Muscle 0.0150 1.00 0.015Fat 0.0135 0.91 0.015Bone 0.0277 1.85 0.015

mass energy absorption coefficient (μen/ρ)(xray) charged particles (electrons) from photo-electric and scattering travel through theabsorber and create bremsstrahlung. The masstransfer coefficient is related as:

where μen is the energy absorption coefficient, μtris the energy transfer coefficient, g is the averagefraction of kinetic energy of the secondarycharged particles (electrons) produced in photo-electric and Compton interactions, subsequentlylost in radiative processes (bremsstrahlung). Inpractice, since g is very nearly zero at diagnos-tic energies then the mass energy absorptioncoefficient is equal to the mass energy transfer

coefficient (μen/ρ �μtr/ρ). The table shows valuesfor water.

μρ

μρ

en tr� ( )1� g

keV μ (m�1) μen μ /ρ (m2 kg�1) μen/ρ(m2 kg�1)

20 80.9600 55.03 8.096e–02 5.503e–0240 26.8300 69.47 2.683e–02 6.947e–0360 20.5900 31.90 2.059e–02 3.190e–03100 17.0700 25.46 1.707e–02 2.546e–03150 15.0500 27.64 1.505e–02 2.764e–03

mass energy transfer coefficient (μtr/ρ) (xray)This represents the energy transferred from theincident photons (X- or γ-radiation) to chargedparticles in the absorber. The mass energytransfer coefficient can be seen as the escapeof secondary photon interactions at the initialphoton/atom interaction site. For diagnostic x-ray energies (photon energies below 200 kV)the definition is: μtr/ρ � (fpe � fscat), where μtr isthe energy transfer coefficient, ρ the density, fpe

and fscat represent the average fractions of thephoton energy that is transferred to the kineticenergy of the charged particles (electrons) fromthe photoelectric (characteristic x-rays) and theCompton scattered photon. For diagnostic ener-gies the contribution from pair and triplet pro-duction is ignored (see mass energy absorption

coefficient).mass number (phys) See atomic mass number.MAST (mri) Motion artefact suppression tech-

nique. A pulse sequence for reducing motioninduced phase shifts during TE (Picker MedicalInc.) (see GMR, GMN, FLOW-COMP, CFAST, FLAG,GMC, FC, STILL, SMART, GR).

matched sample (stats) A sample in which thesame attribute, or variable, is measured twiceunder different circumstances or two samplesin which the members are clearly paired.

matching (elec) See impedance matching.matching layer (us) An additional material to the

front of an ultrasound transducer for improvingthe inevitable mismatch between transducerface and the tissue. A layer ¼λ thick is chosenwith an acoustic impedance as the geometricmean between transducer and tissue.

matching network (mri) An arrangement of reac-tive elements (inductors and capacitors) used totransform an input impedance of a given valueto an output impedance of a second value. Suchcircuits are used in interfacing high impedanceRF coils to low impedance (usually 50 or75 ohms) transmission lines that feed RFenergy to the coil or send the NMR signal toNMR preamplifier.


math co-processor–MDP

M

math co-processor (comp) Mostly incorporatedinto the CPU but a companion chip is some-times included with the CPU that carries outarithmetic functions. Program speeds increaseconsiderably when using image processing orgraphics.

mathematical erosion (image) Mathematicalmorphology process that acts in a neighbour-hood as a local minimum filter.

matrix (image) A two-dimensional x, y array,commonly stored in computer memory. Eachlocation in the array can represent:

● a pixel containing a grey-level value for display;● a voxel as an absorption coefficient or equiva-

lent (CT number) representing a tissue volume(slice width dependent);

● ordered phase/frequency information for subse-quent transfer into spatial domain for display.

An image matrix can be square or any rectangu-lar pattern (DSA is typically 2048 �2048, Imageplate 1760 �2140). (ct) 2D arrangement of CTnumbers; used synonymously with image matrix.

matrix algebra (math) Many problems of numericalanalysis can be reduced to the problem of solvinglinear systems of equations. Ordinary and partialdifferential equations by finite difference meth-ods, eigen value problems, least squares fitting ofdata and polynomial approximation can conve-niently use matrix notation and matrix algebraicprocedures. Since any kind of matrix can be givena unique symbol it means that this symbol repre-sents a whole array or matrix of numbers. Thesenumbers may be manipulated as single entitiesare in ordinary algebra.

matrix element (ct) See voxel, picture element, pixel.matrix of magnitudes of wavelet coefficients

(image) Matrix of the magnitudes of the horizon-tal and vertical wavelet coefficient matrices.Magnitudes of wavelet coefficients representlocal intensity variations, corresponding to edges.

matrix size (mri) The size of the raw data matrixwhich influences both the measurement time,resolution and image signal-to-noise ratio.

maximum intensity projection (MIP) (image)MIP allows the selection of bright pixels in 2Dslices or in 3D volume, displaying maximumpixel value along a given path. Only high signalintensity pixels are projected into the finalimage and does not convey depth relationships.It is threshold independent. It is most usefulwhen displaying rotating 3D anatomy. It lacks

discrimination between tissue types (vascular/non-vascular) where blood is represented aswhite on black background. Generally, several3D reconstructions are made at regular angularincrements around a given axis. These can thenbe viewed in sequence to give more informa-tion, for example about vessel origins:

● displays maximum pixel value along given path;● does not convey depth relationships;● threshold-independent;● gives attenuation information;● depicts calcification;● editing of bone necessary (see minimum intensity

projection (mIP)).

Maxwell coil (mri) A particular gradient coil, commonly used to create gradient magneticfields along the direction of the main magneticfield.

Mayneord, William Valentine (1902–1988)British physicist who was a pioneer developerof nuclear medicine imaging building a rectilin-ear scanner in 1958. Also produced early workon ultrasound in medicine and carcinogeneticeffect of hydrocarbons.

MBS–MRA (mri) Minimum basis set magnetic resonance angiography.

mCi (nmed) milliCurie. The non-SI measure forradioactivity. 1 mCi �3.7 �107 Bq or 37 MBq(see curie).

MD-76® (cm) Commercial preparation of meglu-mine and sodium diatrizoate (66%:10%).

Compound Viscosity Osmolality Iodine (cP) mOsm/kg mg I/mL

Meglumine/Na 16.4 @ 25° 1551 370diatrizoate 10.5 @ 37°

MDI (medium dependent interface) (comp) Thepredefined physical layer interface for 10 MbpsEthernet.

MDP (nmed) Disodium dihydrogen methylenediphosphonate used for bone scintigraphy,labelled with 99mTc. Taken up in three phases:vascular, enzymatic and bone surface (hydroxy-appatite) accretion.

Generic name 99mTc- medronate (MDP)

Commercial names Osteolite® – CISTechneScan® MDPMDP Draximage®

Imaging category bone

M


MDP–mean time between failures (MTBF)

Approximately 50% of the injected dose isretained by the skeleton.

mean (stats) The average usually the arithmeticmean:

where x1 . . . xn are n separate numbers and w arethe appropriate weightings (see geometric mean).

mean absorbed dose (dose) In a tissue T ororgan DT: the absorbed dose DT, averaged overthe tissue or organ T (ICRP 100 (2006a)) whichis given by:

where εT is the mean total energy imparted toa tissue or organ T and mT is the mass of thattissue or organ.

mean down time (MDT) (stats) The average timethat a machine is broken or out of service, includ-ing the repair time, corrective and preventivemaintenance and any delays due to organization.MDT includes all delays involved and mean time to

recovery/repair (MTTR) concerns only repair time.mean error (stats) See standard errors.mean glandular dose (dose) This is difficult to

measure directly so is calculated from tabulatedconversion constants appropriate for themachine HVL. Glandular tissues receive varyingdoses depending on:

● depth from the skin entrance site;● beam quality (kv and filtration);● breast thickness;● breast consistency;● optical density of the image.

In general, the MGD to the standard breast shouldnot exceed 5 mGy. Current values are typically3 mGy with more modern sets giving 1.5 mGy.

DT �εT

Tm

Weighted mean:w x w x w x w x

w w w wn n

n

1 1 2 2 3 3

1 2 3

� � �

� � �

……

Harmonic mean: 1

1 1 1 1 1

1 2 3n x x x xn� � …

⎛

⎝⎜⎜⎜⎜

⎞

⎠⎟⎟⎟⎟

Geometric mean: x x x xnn

1 2 3� � �…

Arithmetic mean: x x x x

nn1 2 3� � �…

A standard 50:50 breast phantom with a 0.5 cmadipose layer is used for the measurements.■ Reference: Dance, 1990.

An estimated measure of the radiation dose tothe total organ (breast) at a stated beam qual-ity, this is difficult to measure directly so is cal-culated from tabulated conversion constantsappropriate for the machine HVL. Glandular tissues receive varying doses depending on:

● depth from the skin entrance site;● beam quality (kv and filtration);● breast thickness;● breast consistency;● optical density of the image.

In general, the MGD to the standard breast shouldnot exceed 5 mGy. Current values are typically3 mGy with more modern sets giving 1.5 mGy. Astandard 50:50 breast phantom with a 0.5 cmadipose layer is used for the measurements.

mean life (nmed) See average life time.mean time between failures (MTBF) (stats) The

mean or average time between failures of asystem (computer or radiology imaging sys-tem), and is typically applied to the working lifeof a machine excluding the start and end of thebathtub curve. The average time between failingand being repaired is the mean down time(MDT) which includes the mean time to repair(MTTR). The MTBF is calculated as the sum ofthe recorded downtimes minus the uptimesdivided by the number of failures:

MTBF(downtime uptime)number of failures

��Σ

.

5 cm tissue thickness

Mea

n gl

andu

lar

dose

(m

Sv)

3

2

1

26 28 30 32 34

kVeff


mean time to recovery/repair (MTTR)–meglumine

M

mean time to recovery/repair (MTTR) (stats) Theaverage/mean time that a machine will take torecover from a non-fatal breakdown. This wouldinclude fuse replacement, circuit board replace-ment and mechanical part replacement/repair.The MTTR would be agreed as part of a mainte-nance contract. Computer systems (HIS/RIS/PACS)should have an MTTR of zero when back-to-backsystems (built in redundancy) have secondarydevices (computer systems) that can take overthe instant the primary one fails. The MTTR canbe measured from the repair efficiency (see RAID).

measurement field (mri) The spherical volume,or field of view, in the centre of the magneticfield where the field has a defined homogene-ity and least distortion.

measurement geometry (ct) See scanner geometry.measurement matrix (mri) Raw data matrix stored

as voxels, not to be confused with the imagematrix.

measurement time (mri) See image acquisition time.mebrofenin (nmed) The generic name of imin-

odiacetic acid (HIDA) derivative as a hepatobil-iary agent (see Choletec®, CholeCis®).

mechanical index (MI) (us) An indicator of non-thermal mechanism activity; equal to the peakrarefaction pressure divided by the square rootof the centre frequency of the pulse bandwidth.The spatial-peak value of the peak rarefaction

pressure, derated by 0.3 dB/cm-MHz at eachpoint along the beam axis, divided by the squareroot of the centre frequency, that is:

where p�0.3 is the peak rarefactional pressure inmegapascals (MPa) derated by 0.3 dB/cm-MHz tothe point on the beam axis where the pulse inten-

sity integral (PII0.3) is maximum; and fc is the centre

frequency in megahertz. The mechanical indexhas no units.

MI exceeds 0.3 Minor damage to neonatal lung orintestine

MI exceeds 0.7 Risk of cavitation

■ Reference: AIUM/NEMA, 1998; AIUM, 2000.

mechanical transducer (us) A transducer thatscans the beam by moving the element(s) or abeam reflector with a motor drive. Replaced byelectronic switching.

MIp

fc� �0 3.

median (stats) The middle value in a distribution.Thus if there are n values the median is thatranked (n�1)/2. For example 13, 11, 10, 9, 15, 12,8 has a median value 11 (see normal distribution).

MEDIC (mri) Multi-echo data image. Multipleechoes are acquired and combined giving ahigher SNR per time period and fewer artefacts.

medical exposure (dose) The intentional irradia-tion of a person (patient) either externally orinternally, for the purpose of their own medicaltreatment or diagnosis or as the subject ofmedical research. It does not include the inci-dental exposure of others. ICRP 26 (1977) esti-mates that 1 man-Sievert can cause either0.012 cases of cancer or 0.01 hereditary disor-ders. The population dose from medical x-raysis between 520 and 1300 μSv per person peryear. The genetically significant dose (GSD) is113 μSv. For 1 million people the populationdose is estimated as 520–1300 man-Sv, whichcould cause between 6¼ and 15½ cancers in apopulation of this size (See exposure).

(The) Medicines (Administration of RadioactiveSubstances) Amendment Regulations 1995(dose) In the UK these regulations amend the Medicines (Administration of RadioactiveSubstances) Regulations 1978, prohibiting theadministration of radioactive medicinal productsexcept by doctors or dentists. Prior authorizationis required and certain controls implemented.Published by the UK Stationery Office Ltd. ISBN01 10533607■ Reference: Medicines (Administration of Radioactive

Substances) Amendment Regulations, 1995.

medronate (nmed) A 99mTc-MDP preparation(Amersham/GE Healthcare).

mega-byte (M-byte) (comp) Approximately one mil-lion bytes or exactly 1024 kilo-bytes. A typicalcomputer memory would be 16 to 128 M-bytes.

megahertz (phys) One million hertz, MHz.meglumine (cm) A common non-radiopaque

cation component associated with many ionicx-ray contrast material. the full chemical name is N-methyl-D-glucamine; the empirical formula is: CH3NHCH2(CHOH)4CH2OH with amolecular weight of 195.22. Meglumine is anorganic cation chosen because of its lowerpharmacological activity than that of thesodium ion (and therefore better tolerance), butbeing a much larger ion, meglumine produces amedium of much higher viscosity. Meglumine isalso diuretic. Meglumine sometimes shares the

M


meglumine–metre

cation component of the contrast medium withsodium. Meglumine is used combined withradiopaque compounds such as diatrizoatemeglumine and iodipamide meglumine. It alsoacts as an excipient which is an inert substanceadded to ensure the shelf-life of the product(contrast medium) that can be long enough tobe active until internal use. Excipients also support the active ingredients in vivo.

meglumine diatrizoate (clin) Iodine containingx-ray contrast material simultaneously synthe-sized by Schering and Sterling Winthrop in1954 (see Urografin®, Angiografin®, Gastrografin®,Angiovist®).

meglumine iodipamide (cm) See Biligrafin®,Endografin®.

meglumine iodoxamate (cm) See Endobil®.meglumine ioglycamide (cm) See Biligram®.meglumine ioxaglate (cm) See Hexabrix®.meglumine iotroxate (cm) See Biliscopin®.Meitner, Lise (1878–1968) Austrian physicist.

Together with Otto Hahn in the Kaiser WilhelmInstitute, Berlin, set up a laboratory for nuclearphysics. Co-discoverer of protactinium. Fled toSweden in 1938. Made the original suggestionthat Hahn’s results with uranium were due tonuclear fission. Retired to England in 1960.

members of the public (dose) Individuals of thepopulation, excluding exposed or designatedworkers or students during their working hours.Nurses and administrative staff in hospitalsshould be treated as members of the public alongwith patients’ visitors (see public (exposure levels)).

memory (comp) The main temporary central stor-age for information, including applications anddocuments. Computer memory is measured inmegabytes or gigabytes. More energy efficientDDR2 memory uses up to 30% less power thanDDR1 and up to 58% less power than FBDIMM.

Mendelian disease (dose) An heritable diseaseattributed to a single gene mutation (see multi-

factorial disease).mercury (Hg) (elem)


195mMercury (nmed) Parent of 195mHg/195mAugenerator


Decay scheme (i.t.) 195mAu 195mHg (γ keV) → 195mAu

Half life 41.6 hours

197Mercury (nmed) Early radionuclide for brainimaging, replacing 203Hg. Now discontinued.

Decay scheme 197Hg→ 197Hg (ec 268 keV) → 197AuHalf life 1.5 days

203Mercury (nmed) Early radionuclide for brainimaging. Now discontinued.

Decay scheme 203Hg 203Hg (β–279 keV) → 203TlHalf life 46 days

Mertiatide (nmed) A carboxylated, diamido-,disulphur- compound as betiatide (N-[N-[N-[(benzoylthio) acetyl] glycyl]glycyl] glycine).;labelled with technetium 99mTc it forms 99mTc-mertiatide (disodium[N-[N-[N-(mercaptoacetyl)glycyl]glycyl] glycinato (2-) -N,N ,N�,S ]oxotech-netate (2-)) and is used in functional and anatom-ical renal imaging (see MAG3).

meson (phys) A nuclear family consisting of thepion, kaon and eta particles.

MESS (mri) Multiple echo single shot.metabolic trapping (nmed) The uptake of certain

compounds (e.g. 18F-fluorodeoxyglucose) bytumours because of a higher metabolic rate forsugars than normal tissue. There is uptake, butlittle or no metabolism, causing trapping of thelabelled sugar in the tumour.

metabolites (PET) (nmed) Compounds associatedwith energy transfer in cells (glucose, lipids).18FDG with a specific activity of 0.1–1.0 mCi permole can image metabolic sites holding con-cen-trations of 10�5 to 10�6 mole per gram or0.2 to 1.0 mg per gram tissue (see receptor site

(PET), neuro-transmitters (PET)).metastable state (phys, nmed) An excited

nuclear state existing after α or β decay lastingfor seconds, minutes, hours or days. Sometimescalled isomeric state. Indicated as 99Tcm or99mTc. An excited state of an atom with a meas-urable half-life (e.g. 99mTc).

methionine (nmed) See selenomethionine.metre (unit) The base SI unit of length. Fractions

used in radiology are:

● 10�1 decimetres (dm); used for defining volume

(litre);

79197

80195Au p,3n Hgm( )


metre–micro-spheres

M

● 10�2 centimetres (cm); a convenient distancemeasurement (radiation output);

● 10�3 millimetres (mm); image resolution as line

pairs per mm;● 10�6 micrometers (μ m); image resolution (micro-

calcifications);● 10�9 manometers (nm); used for visible light

wavelength.

A multiple is the kilometre 103. Convertingbetween metres and non-SI units:

● Angstrom (Å) 10�10m;● 1 inch, 2.54 �10�2m;● 1 yard, 0.914 m;● 1 foot, 0.304 m;● 1 mile, 1.6 �103m or 1.6 km (see area, volume).

metrizamide (cm) Developed by Torsten Almén.the first iodinated, water-soluble, non-ionic

monomer contrast medium developed in the1970s. Low osmolar. The molecule is highlyhydrophilic, increasing solubility without dissocia-tion and has an increased iodine ratio. Althoughthis agent was inconvenient and expensive forroutine intravascular applications, it was usedextensively for lumbar myelography because ofits favourable neurotoxic profile (see Amipaque®,diatrizoate, iothalamate).

metrizoic acid/metrizoate compounds (cm) Ioniccontrast media monomeric salts of tri-iodinatedbenzoic acid with substituted side-chains atpositions 3 and 5; iodine atoms at 2,4 and 6;cation at position 1. Both the anion and cation areosmotically active, therefore the solution will behypertonic to plasma (see adverse reactions, contrast

media (hypertonicity), contrast media (toxicity)).MeV (phys, nmed) Million electron volts. Some

gamma radiation used in radiology is measuredin MeV (60Cobalt has two gamma energies of1.173 and 1.333 MeV). All alpha radiation ismeasured in MeV.

mFISP (mri) Mirrored FISP.Mgrad (image) See film sensitometry.MHz (phys) Abbreviation for megahertz.MI (us) See mechanical index.MIBG/mIBG (nmed) Meta-iodo-benzyl-guanidine.

(Iobenguane® CIS/Shering). An analogue of nor-epinephrine with affinity for the sympatheticnervous system and related tumours; MIBG,labelled with 123I is used for the diagnostic imag-ing of neuroendocrine tumours, disorders of theadrenal medulla and neuroendocrine tumours

such as phaechromocytoma. 131I-iobenguane isused for local radiation therapy in the treatmentof carcinoid syndrome, pheochromocytoma, andneuroblastoma. Scintigraphy as labelled com-pound 123I-MIBG of sympathetic nervous system.Also used in therapy as 131I-MIBG.

MIBI (nmed) Methoxy-isobutyl-isonitrile. Labelledwith 99mTc and incorporated into mitochondria-rich sites (cardiac muscle, vertebral muscle, sometumours).

micro-dosimetry (dose) The absorbed dose levelsat the cellular level (chromosomes, cell mem-brane, etc.). It is defined as the measurement orcalculation of energy deposition by ionising radi-ation in volumes of the order of 1 μm or less.■ Reference: Kliauga et al., 1996)

Micro-dosimetry parameters

Quantity Definition Unit

Energy Total energy keVimparted ∈ deposited within

micro-volumeLineal Quotient of energy keV μ m�1

energy y imparted by single event and mean chord length of volume (equiv. LET)

Lineal energy The probability distribution f(y) function of y

Frequency mean The expectation keV μ m�1

lineal energy yF value of f(y)Dose probability The fraction of

density d(y) absorbed dose delivered to the volume

Dose mean lineal The expression keV μ m�1

energy yD value of d(y)

Microlite® (nmed) A kit for 99mTc labelled albumincolloid used for RES imaging.

Micopaque® (cm) Barium sulphate suspension(Guerbet).

microprocessor (comp) A complete central pro-cessing unit (CPU) contained on a single siliconchip.

micro-spheres (nmed) A fixed size albumin orlatex spheres labelled with radionuclide. Usedfor vascular flow studies or estimating the perfu-sion rate of selected tissues. The albumin iscommonly labelled with 99mTc for imaging thevascular distribution, and after a short whilebecomes disassociated in vivo. The latex spheresare inert and insoluble and become permanentlytrapped in the selected organ.

M


micturating cystogram–mission critical

micturating cystogram (clin) Voiding cystogram(see urethrography).

MII (comp) Medium independent interface. Thepredefined physical layer interface for 100BASE-T.

millicurie (mCi) (nmed) The non-SI measure forradioactivity. Since 1 mCi �3.7 �107 Bq, then1 mCi �37 MBq (see curie).

MiniDisk® (comp) A magnetic-optical digitalrecording/playback disk produced by Sony.Used almost exclusively for audio recording,giving CD quality and 74 minute recordingtime. Track numbers can be marked for rapidlocation. Not used for computer bulk storage(see ZIP® drive).

minification gain (xray) Ratio of input to outputscreen area of an image intensifier.

minimum intensity projection (mIP) (image) Isemployed where blood is represented as blackon a white background (see maximum intensity

projection).MIP (image) Maximum intensity projections.mips (comp) Millions of instructions per second.Miraluma® (nmed) See Cardiolite®.MIRD (dose) Medical Internal Radiation Dose

Committee, Society of Nuclear Medicine whichdeals with internal radiation dosimetry.Publishes data on decay schemes for tracersused in nuclear medicine, tables for dose com-putation and reports on dosimetric analysis forspecific radiopharmaceuticals.

MIRD formula (dose) An improved internal doseestimation with better organ geometry factorand the addition of contributing organ activity.An approach to internal dosimetry proposed bythe Medical Internal Radiation Dose (MIRD)Committee in the USA. The MIRD technique:

● Groups both penetrating and non-penetratingradiations together.

● Considers biological and physical data regard-ing distribution and resident time

● The irradiation of adjacent organs by the targetorgan.

Dose calculations can be solved for:

● radio-tracers that accumulate mostly in oneorgan (liver colloid, lung perfusion);

● tracers that are distributed in a number oforgans (vascular, hepatobiliary system);

● time varying distribution (bolus studies);● dose variations to selected organs (the bladder

depending on voiding frequency).

MIRD schema (dose) A series of MIRD equationsused for estimating the absorbed dose to the tis-sue of interest, taking into consideration themany kind of radiations from the radionuclide aswell as the distribution of the radionuclide whichmay be taken up in a number of different tissuesadjacent or associated with the tissue of interest.The MIRD units used in the dose calculation are:

Physical MIRD Unit Symbolquantity symbol

Activity A becquerel BqCumulated Ã becquerel Bq s

activity secondAbsorbed D gray

doseMean dose S

per unit cumulated

activityMean energy Δ

emitted per nucleartransition

Residence time τ second sEnergy per E joule J

particleelectron volt eV

Particle per N 1/becquerel (Bq s) �1

transition secondAbsorbed υ

fractionSpecific Φ 1/kilogram kg�1

absorbedfraction

Mass of organ M kilogram kgPhysical T

half-lifeEffective (Tj)eff second s

half-timePhysical decay λ

constantBiological λj 1/second s�1

decay

■ Reference: Loevinger, 1991.

mirror image (us) An artificial grey-scale, colour-flow or Doppler signal appearing on the oppo-site side (from the real structure or flow) of astrong reflector.

mission critical (comp) A term which emphasizesthe importance of a continuous service, such asa hospital or radiology information system,which is expected to give 24 hour, 7 days aweek reliability. Normally applied when specify-ing a control computer system (file server) whose

Gy kgBq s

gray-kilogrambecquerel-sec.

GyBq s

graybecquerel-sec.


mission critical–modulation transfer function (MTF)

M

reliability must approach 100% achieved byhaving dual processor design with hot-plugable

capability (see down-time).m.k.s. (unit) Metric measurement of metre, kilogram,

second used in SI units.M mode (us) Mode of operation in which the dis-

play presents a spot brightening for each echovoltage delivered from the receiver, producing atwo-dimensional recording of reflector position(motion) versus time.

MNP (comp) Microcom networking protocol. A setof protocols designed to improve communica-tions between modems but superseded by LAPM.MNP 4 error correction and MNP 5 compressionare used as fallbacks if a remote modem doesnot support LAPM or V.42 bis.

Mo (mri) Equilibrium value directed along thestatic magnetic field lines. Proportional to spindensity.

mode (stats) The value of the variable whichoccurs most frequently; the value of a distribu-tion peak (see normal distribution). (us) One ofthe following system operations: A-mode, M-mode, static B-mode, real-time B-mode, CWDoppler, pulse Doppler, static flow mapping,real-time flow mapping, or any other singledisplay format for presenting clinical informa-tion. Note Under this definition, the Food andDrug Administration (FDA) considers amplitudeDoppler to be a mode.

modem (comp) MOdulator/DEModulator. A devicewhich allows a workstation or computer sys-tem to communicate and exchange informationwith other modem equipped computers usingtelephone lines. A device that connects twocomputers together over a telephone or cableline by converting the computer’s data into anaudio signal (see ISDN, ADSL).

moderator (phys) Neutron absorbing material(graphite or heavy water) lying between the fuelrods in the core of a nuclear reactor to slowdown and reduce the energetic neutronsreleased during fission to thermal (slow) neutronenergies. This increases the probability of fur-ther nuclear fission events. Typical moderatorsare graphite or heavy water (D2O) (see hydrogen).

modulation (elec) Superimposition of an ampli-tude, frequency, phase or pulse train onto acarrier wave of high frequency. These variousforms of modulation are seen in ultrasound,MRI and computer network communications(see PACS, LAN, WAN). (ct) A periodic variation of

a signal in the spatial or temporal domain; themodulation is assumed to be a sinusoidal (mod-

ulation transfer function) or rectangular (square

wave modulation transfer function).modulation (amplitude: AM) (elec) A high fre-

quency carrier wave (sound or RF) whoseamplitude varies at the signal frequency. Thedepth of modulation is measured by comparingthe amplitude of the carrier to the signal. It istypically seen in a video signal where the veryhigh frequency carrier is modulated in sympa-thy with the signal information.

modulation (frequency: FM) (elec) The fre-quency of the carrier wave is varied with theamplitude and polarity of the input signal. Theamplitude of the carrier wave is unchanged. FMgives a higher signal to noise ratio than AM.

modulation (phase) (elec) Used in RF communica-tions where the phase of the carrier frequency isvaried by an amount proportional to the instanta-neous amplitude of the input modulating signal.

modulation (pulse code: PCM) (elec) A train ofpulses is used as the carrier wave, one or moreof the signal parameters (amplitude) modifyingthe pulse train. A carrier wave can also be usedwhich is modulated (FM) in a pulsatile pattern.

modulation transfer function (MTF) (image) Acomplete description of the display resolution.A varying frequency signal (supplied by a linepair grating or line spread function) is suppliedto the imaging system. As the frequency of theinput signal increases, the ability of the record-ing (imaging) device begins to fail. The ratio ofthe output signal amplitude to the input signalamplitude at each frequency is the MTF; a per-fect system would register a straight line graphwhere the MTF �1.0. It is measured in practiceby taking individual readings across a linespread function. The formula used in the calcu-lation involves a Fourier analysis of theresponse to reveal each frequency component:

where L(xj,z) is m individual line spread valuesat Δx sampling interval; v is the frequency incycles cm�1 computed. The MTF graph com-pares two imaging surfaces on linear scales.

L x z vx i vx

L x z

j j jj

m

jj

m

( , ) (cos sin )

( , )

� � ��

�

2 21

1

π π∑

∑

M


modulation transfer function (MTF)–99Mo/mTc generator

modulus (image) Magnitude.modulus of elasticity E (phys) A measurement of

material stiffness represented by stress/strain.The inverse is compressibility or 1/E.

mole (phys) The SI unit (mol) for the amount of asubstance that contains as many elementaryentities as there are atoms in 12 g (0.012 kg) ofcarbon as 12C. The former name was the gram-atom. The elementary entities may be atoms,electrons, molecules, ions etc., but must be spec-ified. Can also be defined as a millimole (mmol):

1 mol � molecular weight in grams

NaCl has a molecular weight of 58.45, so58.45 g in 1 L of water is a molar solution.Isotonic (normal) saline is 300 mmol and con-tains 9.448 g NaCl per litre of water.The number of elementary particles is a con-stant known as Avogadro’s constant and is6.022 �1023mol�1; atoms per mole. The atomsper unit mass � N/A, where A is atomic mass.Since Z (atomic number) also represents thenumber of electrons, then electron densitye �N � [Z/A]. The lighter elements have agreater electron density (see osmolarity).

molecular imaging (image) Changes in size andstructure are detected by anatomic imaging.Molecular imaging characterizes and monitorsbiological processes at the cellular and molecularlevels. Molecular imaging techniques includepositron emission tomography, planar nuclearmedicine (NM) and single photon emissioncomputed tomography (SPECT), optical fluores-

cence, optical bioluminescence, magnetic resonance

spectroscopy, molecular MRI (mMRI), and func-tional MRI (fMRI). Molecular imaging positron

Direct (Se)

Indirect (GdOS)

Spatial resolution (Lp mm�1)

0

1.0

0.8

0.6

0.4

0.2

0.01 2 3 4 5

MT

F

emission tomography provides functional infor-mation in patients following intravenous admin-istration of tracers that are incorporated intovarious biochemical and cellular processes (seephysiologic imaging).

molecular weight (unit) See relative molecular mass.molybdenum (Mo) (elem)

Atomic number (Z) 42

Relative atomic mass (Ar) 95.94Density (ρ) kg/m3 10 200Melting point (K) 2880Specific heat capacity J kg�1 K�1 250Thermal conductivity W m�1 K�1 138K-edge (keV) 20.0Relevance to radiology: as a support for the anode disk(axle) since it has relatively low thermal activity. As a K-edge filter material in mammography. Relevantnuclides.

99Molybdenum (nmed) The parent radionuclidefor the 99Mo/99mTc generator.

Production

Decay scheme 99Mo (β–, γ 740 keV) : 99mTc(beta decay) 99Mo T1/2 6.02 h

Gamma ray 4.1 �10�2mSv hr�1 GBq�1

constant @ 1 mHalf life 2.76 daysHalf value layer mm Pb

99Mo/mTc generator (nmed) The most commonlyused radionuclide generator system found in allscintigraphic nuclear medicine departments.The production of the radionuclide Technetium-99 m is based on isomeric transition (i.t.) by thedecay of its parent radionuclide:

99Mo (T½ 2.76 d) (β–, γ 740 keV)→ 99mTc (T½ 6.02 h) (i.t. 140 keV)→ 99Tc(β �2.1 ��105 y)→ 99Ru (stable)

The construction consists of glass or plastic column containing alumina (aluminium oxide,Al2O3) and the 99Mo is loaded onto the columnas 99MoO4

2� (molybdate ion). The eluent is nor-mal saline. Since 99Mo is in the 99MoO4

2� formwith a 6� oxidation state. 99Mo decays into 99mTcas 99mTcO4� (the pertechnetate ion), having a7� oxidation state. The 6� oxidation state of Moprobably provides a firm binding to the alu-mina, whereas the 7� oxidation state of 99mTcprovides a weaker binding and can be removedwith 0.9% saline, the molybdate remainingbound to the column. The common QC testsinclude: breakthrough (Mo- and aluminium. The

92235

4299

50135U n Mo Sn( , ) ( )f �


99Mo/mTc generator–motion (harmonic)

M

pH of the eluate should be in the range of 4.5–7.5. Radiochemical purity of the eluatecan be analysed by chromatography. Sterility/apyrogenicity and freedom from particlesshould be checked (see quality control (nuclear

medicine), limulus test).momentum (phys) Mass � velocity. The SI unit of

measurement is therefore the kilogram multi-plied by the metre per second: kg m�1 s�1.

momentum (angular) (phys) See angular momen-

tum, angular displacement.momentum (linear) I (phys) See linear momentum.moment of inertia (phys) For a rigid body mov-

ing about a central fixed axis. Resembles linearmotion with moment of inertia replacing mass,angular velocity replacing linear momentum.Moment of inertia is important to consider withx-ray anode design, keeping the surface arealarge but the mass (moment of inertia) low byusing lightweight materials (graphite).

monitor (dose) To determine the level of ionizingradiation and radioactive contamination in agiven region. Also a device used for this purpose.

Moniz, Egas (1874–1955) Portuguese radiologistfirst to use thorium dioxide suspension as con-trast medium for carotid arteriography in 1929.Nobel Prize for Medicine and Biology 1949.

monochromatic beam (nmed) All the gamma-ray energy is the same (e.g. 99mTc is monochro-matic because essentially all the photons havean energy of 140 keV).

monochromatic radiation (phys) Electromagneticradiation having a single wavelength. Laserradiation is monochromatic; some phosphorsalso emit very nearly single wavelength light socan be termed monochromatic. (ct) x-ray beamwith all its photons having one and the sameenergy or in practice a very narrow energy range(see polychromatic radiation).

monoclonal (clin) Describes a monoclonal antibody.monomer (contrast medium) (cm) Contrast

media consisting of one benzoic ring with threeiodine atoms. Monomeric contrast media can beionic (Iodine ratio 1.5) or non-ionic (Iodine ratio 3).All have osmolalities higher than blood/CSF,ionic CM 5–7 times as high, and non-ionic CM 2–3 times. Iodine-complex contrast mediaare built typically on a benzene ring structurewhere up to three iodine atoms can be chemi-cally attached; this is the basis for monomericcontrast media like diatrizoate, metrizoate oriothalamate (see non-ionic monomer, dimer).

monomeric (contrast media) (cm) Contrastmaterial consisting of one benzoic ring withthree iodine atoms. Monomeric contrast mediacan be ionic (‘Ratio 1.5 contrast media’) or non-ionic (‘Ratio 3 contrast media’).

Monte Carlo analysis (stats) A technique forobtaining an approximate solution to certainmathematical and physical problems, charac-teristically involving the replacement of a prob-ability distribution by sample values, usuallyperformed using a computer. A method of find-ing the probability distribution of all the possi-ble outcomes of a complex process by simulationwhere a theoretical analysis is not possible.The probabilities of various outcomes are esti-mated by repeated computer simulations toform a probability pattern.

morbidity (clin) The incidence of non-fatal illness.Disease of any type or the risk of such illness (e.g.number of illnesses per 1000 appendectomies).

mortality (nmed) Death or the risk of death, i.e.number of deaths per 1000 procedures. Thenumber within a chosen population (e.g. womenbetween the ages 50–84) who die within adefined period. Rates per 1000 or 100 000 arecommonly quoted or a simple percentage.

mosaic images (mri) Where 16 to 64 EPI imagesare combined into one mosaic image. Thisincreases the clarity of BOLD display. Motionartefact is reduced since random or involuntarymovement (breathing, heartbeat, blood flow,eye movement, swallowing) and patient move-ment are obscured and appears as ghosting orsmearing in the images in phase-encodingdirection only.

motion artefact (ct) Artefact caused by move-ments of the object during data acquisition; theinconsistencies of the measured projections notonly result in unsharpness, as in conventionalradiography, but can also cause long-rangeartefacts in the CT image.

motion compensation (mri) Modifying the fieldgradients used in a pulse sequence such thatflow and acceleration do not induce any additional phase effects.

motion (harmonic) (phys) When the force actingon a system is directly proportional to its dis-placement x from a fixed point, then the varia-tion of x with time follows a sine relationship:x �a sin ω t, where a is the amplitude of thewaveform or the greatest displacement fromthe equilibrium position. The constant ω equals

M


motion (harmonic)–MP-RAGE

2π f, where f is the frequency of oscillationmeasured in cycles per second or Herz (Hz). Theperiod T of the waveform which is a measure ofthe time to undergo one complete cycle is 1/f soω �2π/T. When the waveform completes acycle it moves forward a distance λ, the wave-length. In one second when f vibrations occur,the wave moves forward a distance f λ. Hencethe velocity c of the waves, which is the dis-tance a peak moves in one second is c � f λ.

motion (rotational) (phys) A force is beingapplied to a disc or wheel so as to producerotary motion, variation of the force will usuallycause a proportional variation in the speed ofrotation and therefore of the kinetic energy ofthe rotating part. Any mass when in motiontends to resist being retarded. In the case of arotating anode it can be shown that its ten-dency to preserve its state of motion (its inertia)is determined by its mass and its radius. Theinertia I of a disc of uniform density havingmass M and radius R is given by:

Inertia can be increased by increasing eitherthe mass or the diameter of the disc.

I MR� 12

2

1.0

0.5

0.0

�0.5

�1.0

Cycle(wavelength)

3002001000

Degrees (time)

Am

plitu

de

moving average filter (di) A technique for reduc-ing rapid temporal fluctuations (particularlyimage data in DSA). Each average or smoothedvalue is computed from the average of preced-ing raw data values. It is non-recursive sincethe output relies solely on input data.

moving slit scan (xray) A technique for moving aline source of radiation over an object so thatthe whole of the object’s surface is scanned bythe radiation (See AMBER).

movement unsharpness (image) See unsharpness

(movement).MP3 (comp) MPEG audio layer 3. A standard for

compressing and storing audio sequences in verysmall files while maintaining high-quality sound.MP3 encoders (or rippers) use a mathematicalmodel of the human ear and removing frequen-cies that would not normally be detected.

MPEG (comp) Motion Picture Experts Group. A com-pression format for video enabling high qualitypicture sequences to be stored. A committee thatdefines the standards for digital video and audiocompression techniques. MPEG-i compressionallows video CDs to be played on PCs at 3O fps(frames per second) while MPEG-2 is used fordigital TV broadcasts and OVO movies.

MPGR (mri) Multiplanar gradient recalled, generalsequence, GE (see FFE, GRE, GRECO, FE, PFI, GE,Turbo FLASH, TFF, SMASH, SHORT, STAGE).

MPPS (mri) Modality performed procedure step.MPR (mri) See multiplanar reconstruction.MP-RAGE (mri) Magnetization prepared rapid

gradient echo. A pulse sequence developed bySiemens for T1-weighted contrast. A rapid gradient-echo acquisition preceded by a

ADC

Moving weightedaveraging

Dold

Dnew

Sum

D

Memory

Video signal

Rotational motion: CT assemblyCentripetal/centrifugal forces on a CT x-ray tube andsupport of mass 100 kg and 0.6 m radius revolving at0.5 s or 2 rev per second.Force � ma �m(v2/r)Velocity � 2πr �2 metres per second � 7.54 m s�1

Centripetal or centrifugal force is (100 � (7.54)2)/0.6 �

9466 N

MOTSA (mri) Multiple overlapping thin slabacquisition.


MP-RAGE–multiple line-scan imaging (MLSI)

M

magnetization preparation (MP) period thatensures proper image contrast.

MR signal (mri) Radiofrequency electromagneticsignal produced by the precession of the trans-verse magnetization of the nuclear spins.

MRA (mri) See magnetic resonance angiography.mrad (dose) A fraction of a rad as 10�3 rad. The

non-SI measurement of absorbed dose.MRCP (mri) Magnetic resonance cholangio-

pancreatiography. This relies on the high con-trast between the fluid-filled ductal structures,yielding high T2 signal intensities and the surrounding tissues, yielding lower T2 signalstrengths. Usually implemented with a singleshot RARE sequence.

mrem (millirem) (dose) The sum of the productsof the absorbed dose in mrad and a quality fac-tor: mrem � (dose1 �QF ) � (dose2 �QF ).

MRS (mri) See magnetic resonance spectroscopy.MSAD (ct) See multiple scan average dose.MS-DOS (comp) An early operating system devel-

oped by Microsoft Corporation (Microsoft DiscOperating System).

MS-EPI (mri) Multi-shot echo planar imagingMTBF (stats) See mean time between failures.MTC (mri) See magnetization transfer contrast.MTSA (mri) Multiple thin slab acquisition.MUGA (nmed) Multi-gated-acquisition using the

ECG for gating heart images according to theirposition in the cardiac cycle.

multi-core (comp) In April 2005, Intel announceda Pentium based duo-core processor, providingadvantages for multitasking jobs and improve thethroughput of multithreaded applications. A dual-core processor consists of two complete executioncores in one physical processor both running at the same frequency. Both cores share the same packaging and the same interface with the chipset/memory. Dual-core designs supporthyper-threading technology and can process foursoftware threads simultaneously. Currently, quad

core processors have become available (Intel®Xenon® 5300 series). Processors with DDR2memory are designed to offer a seamless upgradepath from dual-core to quad-core (see memory).

multidose (nmed) Refers to a vial, etc. that containsenough of a radiopharmaceutical whereby dosesfor more than one patient can be taken from it.

multi-echo sequences (mri) Differentiating the T1and T2 contributions in an image can beachieved by using a 180° pulse train after theinitial spin echo sequence. The successive 180°

echoes have approximately the same T1 contentbut different T2 content. Multi-echo sequencescan be run with multi-slice routines.

multifactorial disease (dose) Diseases that areattributable to multiple genetic and environ-mental factors.

Multihance® (cm) Commercial (Bracco) preparationof gadobenic acid as the dimeglumine salt (Gd-BOPTA). MRI ionic paramagnetic contrast agent.

Compound Concentration Viscosity Osmolality mg mL�1 (cP) mOsm/kg

Dimeglumine 529 9.2 @ 20° 1970gadobenate

(Gd-BOPTA) 5.3 @ 37°

multimedia (comp) The use of sound and graphics.The PC to be supplied with sound card, speakers,multi-speed CD-ROM drive and equipped withmemory and disk space for sound and image files.

multipath (us) Relating to paths to and from areflector that are not the same.

multiphase study (nmed) A dynamic study usinga mixture of frame timings.

multiplanar reconstruction (MPR) (mri) Theability to display not only axial images but alsosagittal, coronal and selected oblique viewsfrom a data set. A new images or any orienta-tion can be reconstructed as a post processingtechnique based on a 3D or continuous multi-slice measurement.

multiple echo imaging (mri) Spin echo imagingusing spin echoes acquired as a train. Typicallya separate image is produced from each echo ofthe train. Includes rapid acquisition with relax-ation enhancement (RARE) techniques (fast-spin echo (FSE) or turbo-spin echo (TSE)) wheremore than one echo is acquired per excitationpulse. Carr–Purcell (CP) sequences and Carr–Purcell–Meiboom–Gill (CPMG) sequences areexamples of multiple-echo imaging techniqueswhere distinct images are constructed fromsignal echoes acquired at a different TE values,yielding different T2 weighting to each imageset. Echo-train techniques speed image acqui-sition by applying a different phase-encodingto each echo but combining echoes with differ-ent T2 weightings into a single image set.

multiple line-scan imaging (MLSI) (mri)Variations of sequential line imaging techniquesthat can be used if selective excitation methodsthat do not affect adjacent lines are employed.

M


multiple line-scan imaging (MLSI)–multi-slice machine

Adjacent lines are imaged while waiting forrelaxation of the first line toward equilibrium,which may result in decreased image acquisitiontime. A different type of MLSI uses simultaneousexcitation of two or more lines with differentphase encoding followed by suitable decoding.

multiple quantum coherence (mri) Excitation byan RF pulse creates a transition or ‘coherence’between different energy levels. Transitionsare only allowable between states of the spinsystem differing in quantum number by oneunit ‘single-quantum coherence’, but multipleRF pulses can act in cascade and produce mul-tiple-quantum coherence. Only single quantumcoherence produces a directly observable sig-nal, however, requiring indirect observation ofmultiple-quantum frequencies.

multiple reflection (us) Several reflections pro-duced by a pulse encountering a pair of reflec-tors; reverberation.

multiple scan average dose (MSAD) (ct) TheMSAD is the average dose across the centralslice from a series of N slices (each of thicknessT) when there is a constant increment Ibetween successive slices:

where DN,I(z) is the multiple scan dose profilealong a line parallel to the axis of rotation (z).For a sufficient number of slices such that thefirst and the last in the series do not contributeany significant dose over the width of the central slice:

SDP

MSAD

z-axis position

Rad

iatio

n do

se (

rela

tive)

MSADTI

CTDI� * ( )mGy

MSADI

D z dzN I��

�10 5

0 5

,.

.( ) ( )∫ mGy

multiple sensitive point (mri) Sequential lineimaging technique utilizing two orthogonaloscillating magnetic field gradients, an SFP

pulse sequence. and signal averaging to isolatethe NMR spectrometer sensitivity to a desiredline in the body.

multiple slice imaging (mri) Sequential planeimaging used with selective excitation techniquesthat do not affect adjacent slices. Adjacent slicesare imaged while waiting for relaxation of thefirst slice toward equilibrium. Reduces imagingtime for a slice set.

multiple spin echo (mri) A pulse sequence leadingto the production of multiple spin echoes afteran initial excitation (see multiple echo imaging).

multislice (ct) See adaptive and linear array.multiplet (mri) A pattern of multiple resonances

(spectral lines) observed when the initially sin-gle Larmor frequency of a given nucleus is splitby interactions with neighbouring spinsthrough the scalar or spin–spin interaction. Themagnitude of this interaction is independent ofthe applied magnetic field; referred to as J, thespin–spin coupling constant.

multiple tuned coil (mri) RF coil designed tooperate at more than one resonance frequency.so that NMR of more than one kind of nucleuscan be observed with the’ same coil.

multiply tuned coil (mri) RF coil designed tooperate at more than one resonance frequency,so that NMR of more than one kind of nucleuscan be observed with the same coil.

multi-row detector (ct) Detector array with cur-rently between 4 and 256 independent detec-tor rows. The multi-slice machine has its focalspot-to-isocenter and focal spot-to-detectordistances shortened in order to cover patientanatomy; the number of detector elementsalong the detector arc can then be increased.The multi-slice detector is divided into manyelements along the z-axis (see multi-slice system).

multi-slice artefacts (ct) These include partial volume

artefact, spiral artefacts, cone-beam artefacts, rod arte-

facts. Spiral interpolation artefacts, especially instructures that change rapidly in the z-axis, arean important issue for multi-slice scanners.

multi-slice imaging (mri) A variation of sequen-tial imaging. The recovery period of the firstexcited slice is used to measure additionalslices. The slices are interleaved.

multi-slice machine (ct) CT scanner capable ofmeasuring more than one slice simultaneously,


multi-slice machine–myocardium (hibernating)

M

based on multi-row detectors. Multislice CTscanners (MSCT) which acquired two transaxial

slices simultaneously, began in 1992 using twoparallel banks of detectors. A variant is the elec-

tron-beam scanner also with a dual set of detec-tors. Multi-row detector CT scanning was madeavailable in 1998 using solid detectors andsimultaneously imaging four slices in eachrotation of the x-ray source. Multi-slice CT hel-ical scanners are all third-generation (fan beamvariation) systems mostly with low voltage sliprings. Faster rotation sub-second times (0.5 s)reduce exam time while producing image qual-ity similar to that of single slice scanners.Multi-slice helical CT performs differently fromsingle-slice helical scanners, with respect topatient dose, pitch, image artefacts and its methodfor image reconstruction. Fundamental advan-tages of MSCT include:

● Shorter acquisition times with improved temporal

resolution (less motion artefacts);● retrospective creation of thinner or thicker sec-

tions from the same raw data;● improved 3D rendering with diminished helical

artefacts;● increased volume coverage per unit time;● high axial resolution; examination can be per-

formed with thinner sections, leading to higherspatial resolution along the longitudinal axis;

● Intravenously administered iodinated contrast

material can be delivered at a faster rate,increasing contrast enhancement in the images.

These factors improve the spatial, temporal andcontrast resolution of the images, significantlyincreasing the diagnostic accuracy.

multistage tumorigenesis (dose) A progressiveacquisition of cellular properties that can leadto the development of tumour from a single(target) cell.

multitasking (comp) Running more than one pro-gram or doing more than one job simultaneouslyon the same computer or connected group ofcomputers; the ability to run two or more pro-grams from one computer at the same time, con-trolled by the operating system. The number ofprograms that can be effectively multitasked inthis way depends on several factors: the amountof main memory available, CPU speed, disk capac-ity as well as the efficiency of the multitaskingprogram itself (see pre-emptive multitasking).

multithreading (comp) A feature which allowsan appropriately designed operating system torun several tasks concurrently.

multi-venc sequence (velocity encoding) (mri)Phase contrast angiography. This sequence isnot equally sensitive to various flow velocitiesand used for acquiring wide variations in flowvelocity commonly in the peripheral arteries.

mu-metal (phys) A trade name for a nickel-basedferromagnetic alloy used for shielding magnet-ically susceptible devices (image intensifiers, photo-

multipliers). Also a ferromagnetic alloy used formagnetic shielding and transformer cores. Hasa very high relative permeability. Consists of78% Ni, 17% Fe, 5% Cu and some Cr and Mo(see magnetic permeability).

mutagenesis (nmed) Induction of a change ingenetic material by radiation or any otheragent; this could be either a somatic or agenetic effect, depending on whether bodycells or germ cells are affected.

mutation (dose) A genetic change that can betransmitted to offspring as an inheritable diver-gence from the parent.

mutation component (MC) (dose) A measure ofthe relative change in disease frequency perunit relative change in mutation rate as ameasure of responsiveness. MC values differ fordifferent classes of heritable disease.

mutation rates (dose) The frequency, per gameteof mutations, in a given species per unit time.

Mxy (mri) See transverse magnetization.myelography (clin) Radiological investigation of

the central nervous system: spinal canal, sub-arachnoid space. Contrast medium is injectedinto the sub-arachnoid space, replacing andgradually mixing with the cerebral spinal fluid.

Mylar® (chem) A polyester film used in the man-ufacture of recording tapes.

myocardium (clin) The muscle composing theheart and some of the pulmonary vessels.

myocardium (hibernating) (clin) Hibernatingmyocardium occurs when coronary artery dis-ease has chronically caused a significantreduction in myocardial perfusion. Contractionceases through insufficient energy productionbut the tissue remains viable. Metabolism ismaintained by a complete change from fattyacid to glycolytic metabolism. Removal of thestenotic lesion re-establishes normal myocar-dial function.

M


myocardium (stunned)–Mz

myocardium (stunned) (clin) Non-infarctedmyocardium fails to regain contractility followingremoval of the stenotic lesion. The perfusionmay be normal but the ischaemic damage hascaused functional changes. Over a period oftime the function may return to normal.

myelography (clin) Imaging the spinal chord andsubarachnoid space by injecting CM via the cis-teral or lumbar route. Either negative CM

(gas/air) or oil based iodine compounds may beused. (clin) Radiography of the spinal cord andnerve roots using x-ray contrast medium intospinal subarachnoid space.

Myoscint® (nmed) 111Indium imciromab pentetatemonoclonal for imaging myocardial necrosis.

Myoview® (nmed) A version of tetrofosmin (GEHealthcare) for labelling with 99mTc. Myoview®.

Mz (mri) See longitudinal magnetization.


NanoCis®–neon (Ne)

N

NNanoCis® (nmed) CIS/Schering preparation of

colloidal rhenium sulphide (nanocolloid).nanocolloid (nmed) Very small particle colloid

with a particle size distribution 95% less than80 nm; used for imaging reticulo-endothelialsystem. Typically a preparation of 99 mTc-anti-mony trisulphide (see colloid).

narrow beam (rad) A collimated radiation beam (x-ray) preventing scatter events reaching anydetector (see scatter coincidence events). The half

value layer is less than broad beam measurements.(us) A beam focused laterally (see collimation).

National Council on Radiation Protection andMeasurements (NCRP) A United States ofAmerica agency that seeks to formulate and widely disseminate information, guidanceand recommendations on radiation protection andmeasurements which represent the consensus ofleading scientific experts. The Council monitorsareas in which the development and publicationof NCRP materials can make an important contri-bution to the public interest. Recent reports are:

Report Titlenumber

157 Radiation Protection in Educational Institutions149 A Guide to Mammography and Other Breast

Imaging Procedures (2004)148 Radiation Protection in Veterinary Medicine

(2004)147 Structural Shielding Design for Medical x-Ray

Imaging Facilities (2004)145 Radiation Protection in Dentistry (2003)140 Exposure Criteria for Medical Diagnostic

Ultrasound; II. Criteria Based on All KnownMechanisms (2002)

The information supersedes the recommenda-tions that address such facilities in NCRP Report No. 49, Structural Shielding Design andEvaluation for Medical Use of X Rays andGamma Rays of Energies Up to 10 MeV, whichwas issued in September 1976.

native image (mri) A contrast study without theuse of contrast agent as in BOLD imaging.

navigator echo (mri) Additional spin or gradientechoes used for detecting changes in objectposition in a measurement volume, or otherchanges. Can be used for interventional proce-dures or respiratory gating.

NCRP See National Council on Radiation Protection

and Measurements.

natural background radiation (nmed) Radiationoriginating from natural sources such as cosmicradiation, naturally radioactive minerals andgases in the earth and naturally radioactiveelements in the body (14 C, 40 K), typically con-tributes a dose of 1–3 mGy per year in theUnited States and Europe.

near field/near zone (us) See Fresnel zone.negative contrast medium (cm) See contrast

medium (gaseous).negative predictive accuracy (diagnostic) See

predictive accuracy (negative).negligible individual dose (NID) (dose) The

NRCP Report 91 defines this as a risk levelbelow which efforts to reduce exposure are notconsidered important. NCRP Report 116 furtherrecommends that an annual effective dose of0.01 mSv may be considered a negligible indi-vidual dose for a source or practice. ICRP has notdefined this quantity.

neighbourhood (image) A set of pixels locatednear a given pixel.

neighbourhood operation (image) An imageprocessing operation that assigns a grey levelto each output pixel on the basis of the greylevel of pixels located in the neighbourhood ofthe corresponding input pixel.

NEMA (National Electrical Manufacturer’sAssociation) The trade association for theelectrical manufacturing industry. Founded in1926 and headquartered near Washington, DC,its approximately 450 member companiesmanufacture products used in the generation,transmission and distribution, control and end-use of electricity. These products are used inutility, medical imaging, industrial, commercial,institutional and residential applications.

neodymium (Nd) (elem)

Atomic number (Z) 60Relative atomic mass (Ar) 144.24Density (ρ) kg/m3 6960Melting point (K) 1297K-edge (keV) 43.5Relevance to radiology: used as a dopant in variousphospors.

neon (Ne) (elem)

Atomic number (Z) 10Relative atomic mass (Ar) 20.18Density (ρ) kg/m3 0.839Melting point (K) 24.5Relevance to Radiology: used as a gas mixture in laserrecording and printers.

N


neoplastic transformation–neutrino

neoplastic transformation (dose) See cell

modification.

Neoscan® (nmed) A 67 Ga-citrate preparation(Amersham/GE Healthcare).

Neospect® (nmed) A 99mTc imaging agent whichbinds to somatostatin receptors on malignanttumours; produced by (Amersham/GE Healthcare).The active substance is depreotide (as trifluoroac-etate), a synthetic peptide. When reconstitutedwith pertechnetate, 99mTc-depreotide is formedwhich binds with high affinity to somatostatinreceptors, which are expressed in both small celland non-small cell lung carcinoma.

Neotect® (nmed) 99 mTc-depreotide for somato-statin receptor bearing pulmonary neoplasms(GE Healthcare).

Nephroscint® (nmed) A preparation for 99 mTc-DMSA (Bristol Myers Squibb).

nephrosography (clin) Radiography of the kidneyby injecting contrast medium through nephros-

tomy tube and opacifying the renal pelvis.nephrostomy (clin) Establishing a connection

between pelvis of the kidney through its cortexto the exterior of the body. Opening betweenpelvis of the kidney through its cortex to theexterior of the abdomen under fluoroscopiccontrol. Percutaneous drainage by collectingsystem via catheter inserted through skin ofthe flank, under fluoroscopic viewing.

nephrotoxicity (contrast medium) (cm) Distur-bances of the kidney function. Iodinated contrastmaterials are nephrotoxic. The acute deteriorationof renal function is sometimes seen followingexposure to iodine contrast material, calledcontrast-induced nephropathy (CIN). Ionic contrastmedia can significantly reduce the glomerular fil-tration and affect the tubular function. Non-ionicand isosmolar contrast media have a very low riskof causing loss of kidney function. The effect onrenal vascular supply is biphasic: first there is amild vasodilatation, followed by a more prolongedvasoconstriction. Renal ischemia and breakdownof some renal basement membrane junctions isfrequent. Studies have suggested that non-ionicagents have less nephrotoxicity.■ Reference: Dawson and Clauss, 1994.

NEQ (image) See Noise equivalent quanta.net optical density (film) Optical density exclud-

ing base and fog.network (comp) Several computers linked together

with their output devices shared (printers, film-formatters etc.). The three most common wiring

types for networks are twisted pair, fibre optic andcoaxial. Local area (LAN) and wide area (WAN) net-work designs are available (see LAN, WAN, NOS).

network operating system (NOS) (comp)Software that manages the resources of a net-work, typically provides file sharing, e-mail,print services, security measures, etc.

Neurolite® (nmed) Produced by (Bristol–MyersSquibb) N N -ethylenedi-L-cysteinato 3- oxo[99 mTc V] diethyl ester dihydrochloride, Bicisate(ECD) a radio-pharmaceutical labelled with 99 mTcdemonstrates cerebral perfusion which crossesthe blood–brain barrier by passive diffusion andexhibits uptake and retention in the normal brain.

neurotoxicity (contrast medium) (cm) Theblood–brain barrier can be compromised (dam-aged) by hyper-osmolar contrast material. This islessened with low osmolar agents. Where theblood–brain barrier is normally incomplete(choroid plexus, stalk of the pituitary etc), contactwith iodine contrast materials can cause centralnervous system reactions. Central nervous systemneoplasms, infections and infarctions can alsoincrease leakage across the blood–brain barrier.Neurotoxicity is related to both concentration andtype of ions present in the contrast material.Charged ions can inhibit normal neuro-transmission. Consequently non-ionic agentshave minimal electrical effects. Neurologic sideeffects include seizures, cortical blindness, paresisand encephalopathy. The carboxyl ion present inionic contrast media is associated with high neu-rotoxicity in the subarachnoid space. Low neuro-toxicity is particularly important with intravascularinjections of contrast media if damage to theblood–brain-barrier is suspected. Neurotoxicity ismost critical with myelography. Non-ionic con-trast media tend to have a low neurotoxicity■ Reference: Dawson and Clauss, 1994.

neurotransmitters (PET) (nmed) Substanceslabelled with positron emitters resembling thoseproduced by synaptic systems in parasympa-thetic and sympathetic nerve pathways (adrena-line, nor-adrenaline etc.). A specific activity of 102

to 103mCi (�5–50 GBq) per mole, it has beenestimated that tissue concentrations of 10�8 to10�9 mole per gram (0.2–2.0 μg per gram) can beimaged (see receptor site (PET), metabolites (PET)).

neutrino (phys) A neutral elementary particle(fermion) that only takes part in weak interactions.The rest mass is zero and so moves at the speedof light. A neutrino transfers mass, energy and


neutrino–noise

N

momentum. It is produced during beta decay andits arbitrary mass energy transfer causes the betaparticle to exhibit a range of energies in the formof a continuous spectrum. The particle responsibleis now considered to be an anti-neutrino (seepositron).

neutron (phys) A neutral particle within thenucleus. A member of the baryon family havingzero charge with a rest mass and energy(1.674 � 10�27kg and 939.550 MeV) slightlygreater than the proton and a mean lifetime of932 s (15 min).

neutron capture (phys) Slow and thermal neutronreaction of the form (n,γ) indicating gamma pho-ton emission. A common method for preparingclinical nuclides, i.e. 60Co, 125I. Boron and cad-mium have a very large cross section for neutroncapture. Boron neutron capture is used as a tech-nique for increasing tumour neutron dose byusing boron-labelled, tumour-seeking agents.Cadmium, in the form of control rods, regulatesthe power output in nuclear reactors.

NeutroSpec® (nmed) A variety of fanolesomab, amurine IgM monoclonal antibody for labellingwith 99 mTc. Manufactured by Mallinckrodt Inc.

newton (N) (phys) The SI unit of force: F � maprovides a mass of 1 kg an acceleration of 1 ms�2 (see force, mass, weight).

NEX (mri) A term used to represent the number ofsignals averaged for each phase encoding step(see NSA).

NIC (comp) A network interface card. A PCI or ISAadapter card stalled in a PC in order to allow Itto connect it to a network. Some motherboardshave an integrated network chip (see adapter).

nickel (Ni) (elem)

Atomic number (Z) 28Relative atomic mass (Ar) 58.71Density (ρ) kg/m3 8900Melting point (K) 1726Specific heat capacity J kg�1 K�1 444Thermal conductivity W m�1 K�1 90.7K-edge (keV) 8.3Relevance to radiology: in the manufacture of thecathode assembly in x-ray tubes.

niobium (Nb) (elem)

Atomic number (Z) 41Relative atomic mass (Ar) 92.90Density (ρ) kg/m3 8570Melting point (K) 2741K-edge (keV) 18.9Relevance to radiology: sometimes used as a K-edge filter.

Niopam® (cm) Preparation of iopamidol, a non-ionic monomer radiographic contrast agentintroduced by Bracco in 1981.

nit (phys) Equivalent to candela per m2 (cd m�2).nitrogen (N) (elem)

Atomic number (Z) 7Relative atomic mass (Ar) 14.01Density (ρ) kg/m3 1.165Melting point (K) 63.3Relevance to radiology: used as a refrigerant forcryogen protection surrounding the helium cryostat.

13Nitrogen (nmed) a positron emitter used in PETscintigraphy.

Production

Decay scheme 13N T½ 10 m (β�, 2γ(β�) 13 N 511 keV) → 13C stableDecay constant 0.0693 min�1

Uses in radiology: cyclotron produced nuclide forPET imaging.

NMR signal (mri) Electromagnetic signal in theradiofrequency range produced by the preces-

sion of the transverse magnetization of the spins.The rotation of the transverse magnetizationinduces a voltage in a coil, which is amplifiedand demodulated by the receiver; the signalmay refer only to this induced voltage.

noble gases (elem) See inert gases.node (comp) Each of the individual computers or

other devices on the network.NOE (mri) See nuclear Overhauser effect.nofetumomab merpentan (nmed) A fragment of

a monoclonal antibody that when tagged with99 mTc as 99 mTc-nofetumomab merpentan, candetect a protein found on the surface of mostsmall cell lung cancers. determine the extent ofdisease in patients diagnosed with small celllung cancer (SCLC). Distributed under the tradename Verluma® by the Dupont Merck Phar-maceutical Company, Billerica,

noise (phys) Any undesired signal whether elec-trical, RF or sound. Thermal noise in electricalcircuits is due to the interchange between amaterial (resistive or semiconductor) and itssurroundings. Thermal noise can be affected byambient temperature and can be reduced bycooling. Impulse noise (switching transients) is aninstantaneous disturbance. Both wideband andimpulse noise can be reduced by signal filtering.

612

713C(d,n) N

N


noise–noise figure

(image) Noise in an image is considered to berandom events characterized by a probabilitydensity function (PDF), the most common noisePDFs being Gaussian noise, Rayleigh noise, uniformand impulse noise. (ct) The point-to-point variationin image density that does not contain usefulinformation. The magnitude of noise is indicatedby the percentage standard deviation of the CTnumbers within a region of interest in the imageof a uniform substance (generally water), relativeto the difference in CT numbers between waterand air. For a given CT system, noise is inverselyproportional to resolution. Generally, image noiseis proportional to when mAs is doubled

the noise decreases by . Factors affecting noise and low contrast detectabilitydepend on:

● photon flux reaching the detector: influenced bykVp, filtration, mAs and patient size;

● system noise: mechanical or electrical noisewithin the CT system;

● detector efficiency;● reconstruction algorithm;● slice thickness;● x-ray tube age.

Data acquisition using multi-slice systems coversmore patient length per rotation, so for extended-length studies x-ray tube current can beincreased compared to single-slice machines.The higher current reduces image noise andimproves image quality, which is critical forthin-section extended-length studies, but atthe expense of increased patient CT dose.

noise correlation (ct) The relationship betweentwo quantities that carry statistical noise; for CTthe reconstructed images show a noise correla-tion; the pixel noise at one position within theimage is not independent from that at otherpositions, since each measured attenuationvalue contributes randomly with every pixel inthe reconstructed image (see space invariant etc.).

noise equivalent power (NEP) (image) The RMSvalue of a modulated radiant power source inci-dent on a detector surface that will give an RMSsignal equal to the detector RMS noise voltage.The detectability D � 1/NEP. A detector generatesa signal in the absence of any radiative flux; thisis the dark current. If the dark current signal ran-domly fluctuates as σd then:

NEP � σd/R

1 2 0 707/ ( . )�

1/ mAs

where R is the responsivity of the detectordefined as the ratio between incident flux andresulting signal. It is essentially the minimumradiative flux that can be measured with a par-ticular detector for a specified frequency band(see Wiener spectrum).

noise equivalent quanta (NEQ) (image) A com-prehensive measure of image quality involvingthe intensity transfer function (characteristiccurve) (G), the modulation transfer function(MTF) and the noise power spectrum (W):

It is a measure of the number of quanta whichform the final image; the theoretical minimumphoton density required for a visible image.With photon noise limited images, the effectivenumber of input quanta (NEQ) per unit area willgive the same signal to noise ratio (SNR) in anideal imaging system, as the actual quantarequired in a real imaging system. The relation-ship to the total number of photons at thedetector face defines the DQE. The equivalentinput exposure deduced from the measurednoise and signal transfer characteristics for aphotographic or radiographic system is:

where γ is the slope of the curve of the densityvs. log exposure, MTF the modulation transferfunction and WΔD the noise power spectrum atthe operating point of interest. Analogous quan-tities for other modalities are found throughoutthe image assessment literature with the largearea factor K in the place of γ(1og10e). They areoften loosely referred to as NEQ because theyplay the same role as NEQ in the ideal-observersignal-to-noise ratio.

■ Reference: Dobbins JT 3rd, 1995.

noise figure (mri) A measure of the noise per-formance of an amplifier or chain of amplifierssuch as an NMR receiver. In NMR systems thepreamplifier should have a very low noise fig-ure to prevent significant degradation of thesignal-to-noise ratio of the NMR signal. Noisefigure is a ratio in dBs, and is given by:

20 log[Vo/Vi(G)]

where Vi is the input thermal noise voltage, Vo

is the amplifier output noise level and G is the

NEQ ve MTF v

W vD( )

(log ) ( )( )

=γ2

102 2

�

NEQG MTF

W�

�2 2


noise figure–non-parametric test

N

voltage gain of the amplifier, when the inputand output impedances of the amplifier areequal.

noise power spectrum (image) An alternativename for the Wiener spectrum.

noise structure (ct) Non-regular patterns visiblein a reconstructed CT image, caused by noisecorrelation.

nominal (tomographic) slice thickness (ct) Theslice thickness selected and indicated at thecontrol panel of the CT scanner.

nominal risk coefficient (dose) A gender andage (at exposure) averaged lifetime risk esti-mates for a representative population.

nominal slice thickness (ct) The slice thicknessselected and indicated at the control panel ofthe CT scanner.

non-autoscan (non-autoscanning) (us) The emis-sion of ultrasonic pulses in a single direction,where scanning in more than one directionwould require moving the transducer manually.

non-cancer disease (dose) A disease other thancancer such as cardiovascular, cataract, fetalmalformation.

non-circular orbits (nmed) Since humans in crosssection are essentially ellipsoid, the distancebetween camera face and subject will vary witha circular camera orbit. Resolution deteriorateswith distance and resulting tomographic imageswill show unsharpness. Non-circular orbits canbe achieved by either moving the detector in anelliptical path or by moving the table towardsand away from the camera face while the cameraitself traces a circular orbit. With the moving tabletechnique image reconstruction is simpler thanwith elliptical orbiting of the camera head.

non-ionic (clin) Compounds that will not dissociateor that dissociate minimally when solved inwater. Non-ionic contrast media will not dissoci-ate when dissolved in water. The number of ionsin solution per iodine atom is therefore lowerthan for ionic contrast media. Non-ionic mono-meric contrast media consist of one benzoic ringwith three iodine atoms (‘Ratio 3 contrast media’).These will have osmolalities of about half that ofionic monomeric contrast media, typically 2.5–3times the osmolality of blood at the high estiodine concentrations. Non-ionic dimeric con-trast media have two iodinated benzoic ringsconnected by a nondissociating bridge. Non-ionicdimeric contrast material have six iodine atomsper ion (‘Ratio 6 contrast media’).

non-ionic dimer (cm) Non-ionic dimers (e.g.iodixanol) combine dimerization with nonionic-ity to achieve isosmolality.

Generic Trade Iodine Viscosity Osmolality name name ratio cP mOsm/kg

H2O

Iodixanol Visipaque 6:1 25 @ 20° 300Iotrolan Isovist 10 @ 37°

■ Reference: Dawson and Clauss, 1994.

(see anaphylactic/idiosyncratic reactions, blood–brain

barrier, distribution coefficient, excretion/elimination half-

life, osmolality/osmolarity, pH value, pharmacokinetics,protein binding, tissue-specific, toxicity, viscosity).

non-ionic monomer (cm) The elimination ofcharge and the masking of the hydrophobiciodinated core by multiple hydrophilic substi-tuted groups gave monomeric nonionic agentswhich provided a marked reduction in osmolal-itv and a reduction in chemotoxicity as com-pared with their ionic predecessors.

Generic Trade Iodine Viscosity OsmolalityName Name Ratio cP mOsm/kg

H2O

Iohexol Omnipaque 3:1 11 @ 20° 500–700Iopamidol Iopamiro 6 @ 37°Iopromide UltravistIoversol OptirayIopentolIobitridol Xenetix

non-ionizing radiation (clin) See radiation (non-

ionizing).non-parametric test (stats) Tests of significance

making no assumptions about population distri-bution. Tests involve ranking. Examples would

R

I

Non ionic monomer

II

R

R

I

I

I

(R)R

R

Nonionic dimer

I

I

I

R

R

N


non-parametric test–NOS

be: Kendall and Spearman’s rank correlation,Mann-Whitney U-test. Anova may be carried outon ranks of data (see non-parametric test).

non-prewhitening matched filter (NPWMF)(image) The NPWMF is a sub-optimal observerin that, while using all known informationregarding the signal parameters perfectly, isunable to undo any correlations in the data.The NPWMF observer uses a template matchedto the expected difference image to form a teststatistic, regardless of the sources of variabilityin the data.

nonquadratic shape (xray) Much larger in radialthan in azimuthal direction (see focal spot size).

non-recursive filter (di) A signal processing tech-nique where the output signal from the filterdepends only on present and previous inputs.Inherently stable as a filter design. Since non-recursive filters have a finite impulse responsethey can be made symmetrical producing linearphase characteristics and no phase distortion.They are used for removing reconstruction arte-facts in axial tomography (see side lobe, Hanning fil-

ter, Hamming filter, moving average).non-selective pulse (mri) Transmitted pulses

that affect all of the tissue within the coil. Usedwith other sequences to select and defineslices (3D) or at a frequency removed from theresonant frequency (MTC).

non-specular reflection (sound) (phys) A roughsurface will give non-specular sound reflection.

non-stochastic (dose) See deterministic.non-uniformity (differential) (nmed) The per-

centage maximum difference between two adja-cent pixels in an image matrix is the differentialnon-uniformity and is defined by NEMA as thehigh and low values obtained from image matrixpixel values measured over a range of five pixelsin all rows and columns. percentage maximumdifference between two adjacent pixels. Typicalvalue ��1.5%. Companion measured to integralnon-uniformity (see uniformity).

non-uniformity (integral) (nmed) Measured as thepercentage difference between the maximumand minimum pixel counts in a sampling area(UFOV or CFOV). The formulas used in calculatingintegral and differential uniformities, where Ui(�)and Ui(�) are the measured extreme values fornon-uniformity. Cmax and Cmin are the maximumand minimum pixel counts and Cx is the meanpixel count within the sampled area of an imagematrix. Typical value ��2.0%. The NEMA (1980)

definition for integral non-uniformity is definedbelow. Both equations give the same result.

Integralnon-uniformity

Integralnon-uniformity (NEMA)

Differentialnon-uniformity

(see uniformity).normal distribution (stats) A continuous distri-

bution of a random variable with its mean,median and mode equal. The shape of the normal curve is defined as:

where N is the number of observations, σ is thestandard deviation and x is the magnitude of Nthe sample value. The normal curves, in thediagram above, show two standard deviationsfor the same sample size. In a normal distribu-tion approximately two-thirds of the distribu-tion lies less than 1 SD from the mean (34%each side) and 95% lies less than 2 SD.

(see standard deviation, percentile, quartile).normalization filter (mri) Signal intensity equal-

ization by using surface coils. The greater sig-nal intensity from locations close to the coil isreduced while signal intensity is increased inlocations further from the coil.

NOS (comp) Network operating system. Softwaredesigned to run on a server that controls access

Standard deviation

Freq

uenc

y

Mean value

1σ 66%

2σ 96%

3σ 99.8%

�4 �3 �2 �1 0 1 2 3 4

yN

e x� � �

σ πσ

2

2 22( )/

high low pixelhigh low pixel

�

�

C CC C

max min

max min

�

�

UC C

Cix

x

( )( )

%min� ��

� 100

UC C

Cix

x

( )( )

%max� ��

� 100


NOS–nuclear reactions

N

from client PCs to services such as email orprinter sharing. Windows NT Server and NovellNetWare are the two most common NOSs.

Noyce, Robert Norton (1927–1990) Americanphysicist and engineer, was co-founder ofFairchild Semi-conductors where he developedthe first integrated circuits. He also co-foundedIntel.

NRC (USA) (rad) Nuclear Regulatory Commission.NRPB (UK) (rad) National Radiological Protection

Board.NSA (mri) Number of signals averaged together,

so reducing noise component before determin-ing each position-encoded signal to be used inimage reconstruction.

NTFS (comp) NTFile System. An advanced form offile coding used for hard disks. It allows file namesof 255 characters. Replaces FAT file systems.

NTSC (image) National Television SystemsCommittee. Television transmission standarddeveloped in the USA offering 525 lines at 60 fps.

nuclear fission (phys) Splitting of a transuranicelement to give two or more lighter element frag-ments. A heavy nucleus splits into two approxi-mately equal halves. Uranium and plutonium arethe common fissionable elements in a nuclearreactor providing commercial quantities ofnumerous lighter element radionuclides: 131I,133Xe, 125I, 99Mo:

nuclear fusion (phys) If two light nuclei are com-bined to form a nucleus of Z, � 56 energy isreleased. The benefits over fission are that thelight nuclei are plentiful and the end productsare usually stable light nuclei. The reactionwhich has been chosen for power production isthe deuterium-tritium or D-T reaction: 2H � 3H →4H � n (Q � 17.6 MeV). A disadvantage is thatmost of the energy is given to the neutron fromwhich it is not easy to extract.

nuclear magnetic moment (phys) The magneticmoment of an atomic nucleus that arises from thespin of the protons and neutrons. It is mainly amagnetic dipole moment. The nuclear magneticmoment varies from isotope to isotope of an ele-ment. It can only be zero if the numbers of pro-tons and of neutrons are both even. Oddnumbered nucleons are detected in NMR.

237

99

131

133

U n

Mo

I

Xe

�

�

�

�

Isotope Natural Γ (MHz) Signal abundance (%) intensity

1H (Proton) 99.98 42.58 1.000019Fluorine 100 40.05 0.830023Sodium 100 11.26 0.093031Phosphorus 100 17.24 0.066017Oxygen 0.037 5.77 0.029013Carbon 1.11 10.71 0.016035Chlorine 75.5 4.17 0.008415Nitrogen 0.37 4.30 0.001039Potassium 93.1 1.99 0.0005

nuclear magnetic resonance (NMR) (mri) A hydro-gen proton can align its magnetic moment eitherparallel or antiparallel in a strong static magneticfield. A pulse of electromagnetic radio frequencyof exactly the resonant frequency (equal to thedifference between these levels) will excite theminto a higher energy state. When the protonsrelax back to their ground state, they each emitenergy of the same resonant frequency which canbe detected. NMR spectroscopy depends on thephenomenon that the electrons in a moleculeshield the protons to some extent from the mainmagnetic field, depending on the chemical con-struction. This effect changes the Larmor frequency

very slightly so giving chemical shifts. Fourieranalysis translates the frequency differences intopeaks which can identify different compounds.

nuclear Overhauser effect (NOE) (mri) A changein the steady state magnetization of a particu-lar nucleus due to irradiation of a neighbouringnucleus with which it is coupled by means of aspin–spin coupling interaction. This interactionmust be the primary relaxation mechanism ofthese nuclei. Such an effect can occur duringdecoupling and must be taken into account foraccurate intensity determinations during suchprocedures.

nuclear reactions (elem) A reaction where thereis a change in the atomic nucleus, resulting in the formation of a different nuclide. The reac-tion may be a spontaneous natural one (ura-nium disintegration), obtained using a nuclearreactor (99Mo fission product) or by a particleaccelerator (cyclotron derived 18F).

Nuclear reactorAn overall reaction in a nuclear reactor can bedescribed as:

where Q describes the overall energy requirements of thereaction. This is conveniently shortened to 32S(n, p) 32P.

1632

01

1532

11S P� �n p Q→ +

N


nuclear reactions–Nyquist limit

mass number A; the element is completelydefined by the atomic and mass numbers:

nuclide (phys) This defines a specific position inthe periodic table of elements, described byproton and neutron number. A specific nuclearspecies identified by the form where A isthe atomic mass, Z the proton number and Nthe neutron number. Nuclides can be stable orunstable naturally occurring (40K, 238U) or manmade (99 mTc, 131I).

null hypothesis (stats) Usually based upon theassumption that nothing special is differentbetween two samples. A statistical test chal-lenges this assumption. If the significance ofthe difference (probability) is sufficiently largethen the null hypothesis is rejected.

number of measurements (ct) The total numberof attenuation values measured during theacquisition of the raw data for a single slice.

nutation (mri) A displacement of the axis of a spin-ning body away from the simple cone-shapedfigure which would be traced by the axis duringprecession. In the rotating frame of reference, thenutation caused by an RF pulse appears as a sim-ple precession, although the motion is more com-plex in the stationary frame of reference.

nylon (material) Hard plastic used as a tissuesubstitute in dosimetry.

Density (ρ) kg/m3 1150 kg/m3

Melting point (K) 470 K

Nyquist frequency (image) An analogue signalcontaining components up to some maximumfrequency f Hz may be completely representedby regular sampling provided the sampling rateis at least 2f samples per second where f is theNyquist frequency. This corresponds to twosamples per period of the highest frequencypresent. The sampling interval is then 1/2f (seealiasing, Shannon equations).

Nyquist limit (image) Frequency of a signal beyondwhich aliasing will occur in the sampling process.This frequency is equal to one half the samplingrate. (us) The Doppler shift frequency abovewhich aliasing occurs; one half the pulse repeti-tion frequency.

ZA

NX

92238

4399

614

13U Tc, C, Hm,

Nuclear fission

235U (n,f) 99Mo

235U (n,f) 131I

92235

143 01

53131

78 39102

63 012U I Y� � �n n→

92235

143 01

4299

57 50134

84 012U Mo Sn� � �n n→

nuclear reactor (phys) A critical assembly of fis-sionable material (235U, 238U, 239Pu) and modera-tor (carbon or heavy water). The nuclear reactor isused for thermal energy production and also as asource of neutrons for radionuclide production byeither neutron irradiation of a prepared sample orfrom products of nuclear fission.

nuclear spin (mri) An intrinsic property of certainnuclei that gives them an associated character-istic angular momentum and magneticmoment. Shown by nuclei with an odd numberof neutrons and protons. For imaging, mostlyonly hydrogen protons are used. For MR spec-troscopy, other nuclei are used, such as phos-phorous, fluorine and carbon (see sensitivity).

nuclear spin quantum number (mri) See spin

quantum number.nucleon (phys) General term to describe either

the proton or neutron.nucleus (phys) Central mass within an atom con-

sisting in its simplest form, protons and neu-trons (collectively called nucleons). The numberof protons determines the atomic number Z.The variation in neutrons gives various isotopesof the same element. The total nucleons is the

CyclotronUsing accelerated charged particles in a cyclotron(protons, deuterium nuclei or alpha particles) givereactions of the form:

proton reactions

11B (p,n) 11C

deuteron reactions

82K (d,3n) 81Rb

alpha reactions

14N (α,p) 17O714

7 24

2 817

9 11N O� �α → p

( )24

2α

3682

46 12

1 3781

44 013K Rb� �d n→

( )12

1d

511

6 11

611

5 01B C� �p n→

11p0


Object–Oldendorf technique

O

Oobject (image) In pattern recognition, a pixel or

connected set of pixels usually correspondingto a physical object in the scene represented bythe image.

objective contrast (film) Optical density.oblique incidence (us) Sound direction not per-

pendicular to transducer boundaries.oblique slice (imaging) Obtained by rotating

an orthogonal slice (sagittal, coronal or axial)about a coordinate axis in the slice plane (seeorthogonal slice).

occupancy factor (shld) The occupancy factor isestimated from the fraction of an 8-hour day or2000-hour year for which a particular area maybe occupied by a single individual. It is esti-mated as a ‘worst case’ for the fraction of timespent by the single person who is in the areathe longest. The critical groups for shieldingpurposes are not patients or patients’ visitors,but radiology staff both clinical and non-clinical(secretaries, porters, etc). The constraint level of 3/10 of the maximum limit areas whereexposure can be greater than 6 mSv per yearshould be controlled; design factors andassumptions on occupancy should reflect thislimit. A lower limit of 0.5 mSv per annum or anoccupancy factor of 0.05 (5%) is sometimes rec-ommended. An assessment should use the0.3 mSv per annum constraint value for publicplaces, including the surrounding area/rooms.The table suggests some working values foroccupancy and use factors. These values do notrepresent fixed rules but are suggestions.

T: Occupancy

1.0 Control areas and offices0.25 Corridors and wards0.06 Toilets and outside areas

(see primary barrier).occupational dose (dose) The dose received by

an individual in a restricted area, or in thecourse of employment in which the individ-ual’s duties necessarily involve exposure toradiation (medical doses involving diagnosis ortreatment of the exposed individual areexcluded).

occupational dose limits (dose) There is mostlyagreement between ICRP 60 (1991a) and NCRPreport 116 (1993).

Based on stochastic 50 mSv annual effective effects dose limit and either

100 mSv over 5 years or 10 mSv � age cumulative effective dose limit

Annual limits on intake (ALI)

Annual reference levels of intake (ARLI)

occupationally exposed (dose) Exposed to radi-ation in connection with occupational duties,e.g. nuclear medicine technologists, nurses,physicians, etc.

OCR (comp) Optical character recognition.Software that analyses scanned images andtranslates the letters and numbers it recognizesinto text for use in other applications such asword processors.

Octreoscan® (nmed) A kit for the preparation of111Indium-pentetreotide (Mallinckrodt). A diag-nostic radiopharmaceutical. An agent for thescintigraphic localization of primary andmetastatic neuroendocrine tumours bearingsomatostatin receptors.

Octreotide® (nmed) Analogue of somatostatinproduced for nuclear medicine imaging byMallinckrodt Inc., which binds to receptor sitesexpressed by a range of neoplasms. Commonlylabelled with 111Indium for imaging.

oersted (Oe) (unit) The c.g.s. unit of magneticfield (H) where 1 Oe � 79.6 A m�1.

off-centre slice (mri) Moving the centre of a slicegroup from the centre of the magnetic fieldwithin the slice plane.

off resonance (mri) When the Larmor frequencyof a spin isochromat is different from that of theexciting RF field.

ohm W (phys) The measure of electrical resistancewhen 1 A flows at a potential difference of 1 V.

oil (mineral) (material)

Density (ρ) kg/m3 760–870Melting point (K) –Boiling point (K) 533–603Specific heat capacity J kg�1 K�1 2130Thermal conductivity W m�1 K�1 0.150Relevance to radiology: x-ray tube coolant

Oldendorf technique (clin) An injection techniquefor obtaining a tight bolus when following organflow (time activity curve). After positioning anantecubital butterfly needle, a sphygmo-manometer cuff applied to the subject’s arm is

20

501

mSv

BqE( )�

20

501

mSv

BqE( )�

O


Oldendorf technique–optical transfer function (OTF)

inflated slightly above arterial pressure. The cuffis quickly released and, at the same time, thesmall volume bolus injected. The action of reactive hyperaemia in the arm ensures a rapidtransit of the intact bolus.

OLE (comp) Object linking and embedding. Allowsfiles or data created by one application to belinked or embedded in another.

outlier (stats) Observation which is far removedfrom the others in a set.

Omnipaque® (cm) Commercial preparation ofnon-ionic iohexol introduced by Nycomed(Amersham/GE Healthcare) in 1982.


Iohexol 39% 2.81 @ 20° 411 1802.05 @ 37°

Iohexol 52% 4.33 @ 20° 504 2403.08 @ 37°

Iohexol 65% 10.35 @ 20° 709 3006.77 @ 37°

Iohexol 76% 18.50 @ 20° 862 35011.15 @ 37°

Omniscan® (cm) Generic name gadodiamide:gadolinium DTPA-bis methylamide (Gd-DTPABMA). A non-ionic neutral analog of Gd-DTPA.Manufactured by GE- Healthcare for MRI.

Compound Concentration Viscosity Osmolality mg mL�1 (cP) mOsm/kg

Gadodiamide 287 2.0 @ 20° 789(GdDTPA-BMA) 1.4 @ 37°

OncoScint® (nmed) 111Indium-labelled pentetrotideused for imaging colorectal and ovarian metasta-tic disease. (Cytogen) (see satumomab).

open source (comp) Computer programs whoseoriginal source code was revealed to the generalpublic so that they will be developed openly.Software licensed as open source can be freelychanged or adapted to new uses. Programmersmay redistribute and modify the code.

operating condition (us) Any one combination ofthe possible particular output control settings fora mode.

operating frequency (us) Preferred optimum fre-quency of operation of a transducer, i.e. maxi-mum efficiency.

operating system (comp) A set of computerinstructions automatically loaded into themachine at start up (boot-up) from hard disk, foroperation when it is turned on, performing allthe basic or housekeeping instructions, i.e. disk

transfer of program material, erasing or relocat-ing data, etc. It sets up a filing system to store filesand sets the display information on the monitordisplay. Most operating systems are DOS. Well-known operating systems include UNIX®, Linux®,Macintosh® and Windows®.

operative cholangiography (clin) Demonstrationof the biliary tree during surgery by direct injection of the CM into the gall bladder, cysticduct or common bile duct.

optical density (OD) (film) The logarithm of theratio of the intensity of the incident light on afilm to the light intensity transmitted by the film.The logarithm of the ratio of the reference inten-sity of perpendicularly incident light (Io) on a filmto the light intensity (I) transmitted by the film:

OD � log10Io/I

Optical density differences are always measuredin a line perpendicular to the tube axis to avoidinfluences by the heel-effect (see characteristic curve).

optical disk drive (comp) Larger version of theread/write CD having capacities of typically2.6 G-bytes. Data write transfer 1.7 M-bytess�1 and data read transfer of 3.4 M-bytes s�1

with an average seek time of 25 ms.optical sensitisers (film) Dyes added to film

emulsion to increase spectrum response.optically stimulated luminescence (OSL) (phys)

This technology was pioneered in 1992 in con-junction with Battelle Northwest NationalLaboratory and Oklahoma State University usingcrystalline aluminum oxide crystals (Al2O3), as thedetector material, The first OSL dosimeter wasfirst distributed in 1996. The radiation level expe-rienced by this material is measured by stimulat-ing the Al2O3 material with green light from eithera laser or light emitting diode source. The result-ing blue light emitted from the Al2O3 is propor-tional to the amount of radiation exposure.Optically stimulated luminescence is a signifi-cantly faster process than thermoluminescence

readout which requires heating. The light expo-sure can be controlled with very high precisionand, as a result, the accuracy of dosimetric meas-urements is greatly improved. Unlike TLDs, thegreen light OSL allows multiple readouts and canbe used to reconfirm reported radiation doses.Measuring capabilities down to 10 μGy (1 mrad)up to exposures as high as 100 Gy (10 000 rad).

optical transfer function (OTF) (image) Thetwo-dimensional Fourier transform of the point


optical transfer function (OTF)–osmolarity (Osmol)/osmolality (Osm)

O

spread function. The values of the OTF on a linein a specified direction through the origin of thespatial frequency domain are given by the one-dimensional Fourier transform of the corre-sponding line spread function. The OTF isgenerally a complex function and can be sepa-rated into the MTF and the phase transfer function.

Optimark® (cm) Non-ionic MRI contrast agent(Mallinckrodt/Tyco Healthcare Inc) a prepara-tion of gadoversetamide.

Compound Viscosity (cP) Osmolality mOsm/kg

Gadoversetamide 3.1 @ 20° 11102.0 @ 37°

optimization (dose) (ICRP60) Maintains the prin-ciple that radiation exposure should be as lowas reasonably achievable (ALARA) taking socialand economical considerations into account(see practices (ICRP)).

Optiray® (cm) Commercial (Mallinckrodt/TycoHealthcare Inc) preparation of ioversol intro-duced in 1988, non-ionic monomer. Producedas Optiray 160, 300, 320 and 350 dependingon concentration of iodine.


Ioversol 51% 4.6 @ 20° 502 2403.0 @ 37°

Ioversol 63.6% 5.5 @ 37° 645 300Ioversol 68% 9.9 @ 20° 720 320

5.8 @ 37°Ioversol 74% 14.3 @ 20° 792 350

9.0 @ 37°

Optison® (cm) Echogenic ultrasound contrastagent produced by Amershassm/GE Healthcare.Consists of microspheres of human serum albu-min entrapping the gas perflutren (empiricalformula C3F8). Mean diameter 3.0–4.5 μm; amaximum diameter of 32.0 μm with more than95% less than 10 μm (see Sonovue).

Oracle (comp) A large database and applicationsoftware vendor. Oracle 8 is a database man-agement system.

oral cholangiography (clin) Demonstratingextra-hepatic biliary tree and gall-bladder.

oral cholecystography (clin) Using water solublecholeographic contrast agent orally and imagestaken 10–12 hours after ingestion. Its role hasbeen diminished by high quality ultrasoundimaging, CT, endoscopic retrograde chole-dochography ERC and MR cholangiopan-creaticography.

order of magnitude (math) An increase ordecrease by a factor of 10.

ordinate (math) The y-axis of a graph.orientation (mri) The three basic orthogonal slice

orientations are: transverse (m), sagittal (S) andcoronal (C). The basic anatomical directions are:right (CR) to left (L), posterior (P) to anterior (A),and feet (F) to head (H). Considered as positivedirections. In the R/L and P/A, directions can bespecified relative to the axis of the magnet; theF/H location can be specified relative to a convenient patient structure.

orthochromatic film (film) Film emulsion sensi-tive to blue/green light. Matched to gadoliniumphosphors.

orthogonal slice (mri) Slices oriented perpendi-cular to each other. The basic orientations are:sagittal, coronal and transverse (axial).

Osborne, E.D. American physician who firstobserved and described x-ray opaque iodinecompounds in 1923.

oscillation (phys) In the form of sound or radio-fre-quency, waveforms play a most important part inradiology. The interactions of sound waves andradiowaves show the same general behaviour:

● harmonic motion;● signal decay;● signal resonance;● interference between signals having different

frequency and phase. The waveform character-istics of oscillating signals can be analysedusing a Fourier transform.

oscillator gate (us) The electronics of a pulsedDoppler system that converts the continuousvoltage of the oscillator to a pulsed voltage.

OSI (comp) Open system inter-connection. A LANcommunication model developed by ISO.

osmium Os (elem)

Atomic number (Z) 76Relative atomic mass (Ar) 190.2Density (ρ) kg/m3 22 480Melting point (K) 3300K-edge (keV) 73.8Relevance to radiology: rarely used as a K-edge filter

osmolarity (Osmol)/osmolality (Osm) (chem)The osmole (osm) is a unit used in biology andmedicine and is a measure of the total numberof particles (ions) dissolved in water.

● The osmolarity depends on the number of mol-ecules dissolved in a litre of solution expressedin milliOsmol L�1 (mOsmol L�1).

O


osmolarity (Osmol)/osmolality (Osm)–osmotic pressure/potential

● The osmolality is the number of molecules perkilogram of water usually expressed inmilliOsm kg�1 (mOsm kg�1).

Except under very specific circumstances, theseterms can be treated as the same. The osmole

(osm) is a unit frequently used in biology andmedicine. Ionic contrast media form anions and acations in solution, whereas non-ionic contrastmedia have a solution of intact molecules; thiseffects the iodine ratio. Hyperosmolality is animportant contributing factor to toxicity. Cationsin solution account for 50% of the overallosmolality of ionic contrast materials. Osmoticpressure is one source of adverse reactions toradiographic contrast materials (osmotoxicity).Other clinically significant effects that can beattributed to the osmolality problem includedamage to the blood–brain barrier, renal dam-age and disturbance of electrolyte balance.

Substance Osmolality (mOsm)

Sea water 1000Isotonic saline 290Blood plasma 285–290Ionic monomer 1530–1843Ionic dimer 580Non-ionic monomer 610–645Non-ionic dimer 290–320

Osmalality of blood and cerebrospinal fluid isapproximately 290mOsm/kg H2O. Almost all cur-rent ionic and non-ionic monomeric and ionicdimeric contrast material have osmolalities inexcess of this figure, although some are very muchhigher than others (see hypertonic, isotonic saline).

osmole (osm) (cm) The amount of a solute thatwhen dissolved in water gives a solution of thesame osmotic pressure as that expected fromone mole of an ideal non-ionized solute. Theosmole (osm) is a unit frequently used in biologyand medicine. It can be freely replaced by themole, representing the mass of 6.023 � 1023

osmotically active particles (Avogadro’s constant)in an aqueous solution and is therefore a molerelated to the solution phase. It differs from themole only when it relates to the undissociatedsolute. The total osmotic concentration or osmo-larity of a solution is usually estimated bymeasuring the vapour pressure or the freezingpoint depression of the solution.

osmotic pressure/osmotic potential (clin) Thepressure exerted by the flow of water througha semi-permeable membrane separating twosolutions with different concentrations of

solute. The osmotic pressure of physiologicalsolutions and X-ray/MRI contrast materialmostly control their toxicity. Using a version ofthe van’t Hoff equation, the osmotic pressure pcan be estimated for a given solution:

p � φiRTm

where i is the van’t Hoff factor. The van’t Hofffactor is normally 1 for each ion in solution so fora simple ionic solution (NaCl) i � 2, however ran-dom ion pairing reduces this; φ is the osmoticcoefficient; a correction factor to adjust for ran-dom ion pairing; m is the molarity moles/litre(see mole); R is the gas constant which in thecase of solutions is represented by solute mole-cules dispersed in solvent; this is 0.08206 L atmmol�1 K; T is the temperature in kelvin, commonlyused values are 273.15 K (0°C), 293.15 K (20°C)and 310.15 K (37°C). Values for i and φ forsolutes of physiological interest:

Solution Mol. wt. i φ

NaCl 58.5 2 0.93Glucose 180.0 1 1.01

The product φ im is called the osmolar concen-tration with units osmoles per litre.

Osmotic pressureThe osmotic pressure of 155 mM solution of sodiumchloride (normal physiological saline) at 37°C; fromthe formula: p � φ iRTm and the table values above:

p � 0.93 � 2 � (0.08206 � 310.15)� 0.155 � 7.3 atm

The osmolarity of this solution:

φim � 0.93 � 2 � 0.155 � 0.288 osmol L�1

� 288 mOsm.

The osmolarity of blood plasma is between 285 and300 mOsm L�1.

Measurement of osmotic pressure of a solution (con-trast medium) is more conveniently estimated from thedepression of freezing point. The relation that describesthis for water and a solute is:

ΔTf � 1.86φic

where ΔTf is the freezing point depression in °C. Theeffective osmotic concentration (osmoles) is φic �

ΔTf/1.86. Red blood cells are commonly used to testosmolarity of contrast media. At a concentration NaClof 155 mM (isotonic with 310 particles or ions) the vol-ume of the cells is the same as that of plasma. At molarsolutions greater than this the cells shrink (hypertonic)and conversely cells expand in hypotonic solutions andcan burst, releasing haemoglobin at 1.4 times theiroriginal volume.

■ Reference: Berne and Levy, 1999.


osmotoxicity–oxidation reduction

O

osmotoxicity (cm) Iodine contrast media have varying degrees of osmolality (see osmolarity

(Osmol)/osmolality (Osm)), since the iodinated andnegatively charged ions (diatrizote, iothalamate,metrizoate) are associated with non-iodinatedpositively charged ions (sodium ions, meglumineions). A hyper-osmolar agent causes suddenshifts of fluid from the intracellular compartmentto the extracellular compartment. This hypertonic-ity causes fluid loss from blood, endothelial cellsand other tissues, causing consequent damageand patient reactions. A source of hyper-osmolal-ity and therefore osmotoxicity, in ionic contrastmedia is that cations in solution account for 50%of the overall osmolality of iodine contrast mate-rial. Since they do not effectively attenuate x-rayphotons they yield no diagnostic information.

osmotoxicity ratio (OTR) (cm) Osmotoxicity poten-tial may be expressed with the osmotoxicity ratio(OTR). An isotonic solution having an OTR of 1.0.and typical low osmolar contrast media having anOTR of �2.0; ionic high osmolar contrast mediawill have an OTR of about 5–7. Osmotoxicity isdirectly responsible for a number of clinicallyimportant effects, including sensations of heat oreven pain. Other clinically significant effectsinclude damage to the blood–brain barrier, renalimpairment and disturbance or electrolyte bal-ance (see chemotoxicity, toxicity, neurotoxicity).

OsteoCis (nmed) Preparation by CIS/Schering ofoxidronate as 99 mTc-HDP.

OsteoScan® (nmed) (Mallinckrodt Tyco) Version ofoxidronate.

Osterkamp formula (xray) Describes the x-raytube loadability L, where:

where P is the power input, A is the focal spotarea, t is the load time, λ is the thermal conduc-tivity, c is specific heat and ρ is the density (seex-ray tube (loadability)).

ounce (phys) An imperial measure of weight; thereare 16 avoirdupois ounces to the pound. Thecommon (avoirdupois) ounce is 2.834952 �

10�2 kg or approximately 28.35 g. There are 12troy and apothecary ounces to the pound andboth are approximately equivalent to 31.103 g.

outlier (stats) Observation which is far removedfrom the others in a set.

output control settings (us) The settings of the controls affecting the acoustic output of an

L2

� �PA

tcπλρ

ultrasound instrument. Such controls wouldinclude but are not limited to the power outputcontrol, the focal zone control and the imagingrange control.

Output display standard (us) The standard forreal-time display of thermal and mechanicalacoustic output indices on diagnostic ultrasoundequipment.■ Reference: AIUM/NEMA, 1996.

over diagnosis (clin) The tendency for screeningto identify borderline abnormalities as diseaseeven though they make lack true significance.

over framing (image) See exact framing.oversampling (frequency and phase) (mri)

Frequency oversampling: doubling the samplingpoints in the frequency encoding direction withoutextending the measurement time. Phase over-sampling: data acquisition beyond the FOV inphase-encoding direction. Increases SNR. Measure-ment times are increased but prevents aliasingartefacts.

overscan (ct) Acquisition of more than the full360° range during a single slice scan; the addi-tional data range of approximately 10 to 40° isused to minimize inconsistencies which canoccur between data acquired at the start and atthe end of the scan, e.g. by averaging.

oxidation state (99 mTc) (nmed) This is related tothe valency state of an ion which influenceschemical combination. Technetium has sevenoxidation states:

Tc[I] In certain organic complexes (MIBI)Tc[II] Not importantTc[III] May be the active form of HIDATc[IV] The most common reduced state in labelled

kit preparationTc[V] Modified reduction in [V]-DMSATc[VI] Not importantTc[VII] The most stable oxidation state. Eluted

material as pertechnetate[VII]

oxidation number (chem) See oxidation state.oxidation reduction (chem) Previously described

as a reaction with oxygen, the converse beingreduction: the loss of oxygen. A more generalidea of oxidation and reduction was developedin which oxidation was loss of electrons andreduction was gain of electrons. This latter defi-nition embraced those reactions that did notinvolve oxygen. An oxidation number consists ofa sign, which indicates an increase (negative) ordecrease (positive). An integer which gives thenumber of electrons involved. The oxidation

O


oxidation reduction–15Oxygen

number is used in naming inorganic compoundssuch as technetium (Tc(IV to VII). Compoundsthat readily undergo reduction are oxidizingagents and those that undergo oxidation arereducing agents (see oxidation state (99 mTc)).

oxidronate (nmed) Oxidronate sodium forms anunknown complex with 99 mTc to form 99 mTc-HDP,a diagnostic skeletal agent for demonstratingabnormal osteogenesis. Rapid blood clearance.

Oxilan® (cm) Commercial preparation of ioxilanintroduced by Guerbet in 1995.


Oxilan 300 9.4 @ 20° 585 3005.1 @ 37°

Oxilan 350 16.3 @ 20° 695 3508.1 @ 37°

oxiquinoline (nmed) Oxine; see 111In oxyquino-line (see tropolone).

oxygen O (elem)

Atomic number (Z) 8Relative atomic mass (Ar) 16.0Density (ρ) kg/m3 1.33Melting point (K) 54.7

Relevant isotopes15Oxygen (nmed) Positron emitting radionuclide


Decay scheme (β�) 15O 15O (β�, 2γ 511 keV)→ 15N stable

Half life 2 minutesHalf value layer 4.1 mmPb (511 keV γ)Uses Nuclide for PET imaging.

714

815N(d,n) O


pacemaker effect–paradigm

P

Ppacemaker effect (mri) Implanted electronic

devices are susceptible to magnetic fields.Static magnetic field, RF fields and pulsed gradients can induce voltages in circuits.Transcutaneous control and/or adjustment ofpacing rate frequently uses reed switches acti-vated by an external magnet to open/close theswitch. Others use rotation of an external mag-net. The fringe field around an MR magnet canactivate these switches or controls. Areas withfields higher than 0.5 mT (5 G) commonly haverestricted access and/or are posted as being arisk to persons with pacemakers.

packet (comp) Data and associated information,including source address and destinationaddress, formatted for transmitting from onenode to another. A unit of data, which is typicallya part of a file, that has been prepared for trans-mission across a network. When a large block ofdata is to be sent over a network, it is broken intoseveral packets, transmitted and then reassem-bled at the other end. Packets often includechecksum codes to detect transmission errors.The exact coding is determined by the protocoland network architecture being used. A typicalcoding of information that includes a header(containing information like address destination)and, in most cases, user data (see token ring).

packet analyser (comp) A network diagnostictool that connects into a LAN and analyses itstraffic, capable of capturing a packet, examin-ing it and breaking it down into its componentparts of destination, origin, protocol, data, etc.

packing factor (ct) This influences the patientdosimetry for multislice CT, the packing factor(p) allows distribution of radiation densityevenly over the volume of investigation when theslices are not contiguous. For a series of N slices,each of thickness T, and with a couch increment

I such that the total scan length is L then thepacking factor p is:

where p � 1 for contiguous slices; p �1 for over-lapping slices; p � 1 for gaps between slices.

PACS (comp) Picture archiving and communica-tion systems. The central design of a filmlessdigital image radiology department, enabling

pTN

I N TTNL

��

�( 1)

electronic picture archiving on bulk digital stor-age media (magnetic and optical disks or tape)and networking image workstations (local areaand wide area networks). Teleradiology andteleconferencing have also been introduced tothe available procedures.

pair production (phys) The formation of a positronand electron from the collision with a nucleus ofa photon whose energy exceeds 2 mc2 (equals1.638 � 10\�13J or 1.022 MeV).

Mass–Energy equivalenceTotal rest mass electron/positron � 9.1 � 10�31 � 2 or1.82 � 10�30 and c2 � 9 � 1016ms�1 then E �

mc2 � 1.63 � 10�13 J or 1.022 MeV (1 J � 6.24 �

1018 eV). This energy is emitted as two opposed0.511 MeV gamma photons. Conversely, a gammaphoton with E �1.022 MeV in a nuclear field mayundergo conversion into a positron and electron sincem � E/c2 yielding an electron/positron rest masses of9.1 � 10�31kg each.

PAL (image) Phase alternation line. A televisiontransmission standard developed in Europe asthe alternative to the colour NTSC signal. It provides 625 lines at 50 fps.

palladium (Pd) (elem) Relevant details:

Atomic number (Z) 46Atomic mass (Ar) 106.4Density (ρ) kg/m3 12000Melting point (K) 1825K-edge (keV) 24.3Relevance to radiology: a K-edge filter metal inmammography

109Palladium (nmed) Beta emitting nuclide, usedas an antibody therapeutic agent.

Decay scheme (β�) 109Pd 109Pd (β�) → 109Ag stableHalf life 13.4 hours

pancreatic arteriography (clin) Pancreatic arteri-ography intravenous contrast medium adminis-tration used for enhancing difference betweenabnormal (tumour) and the normal parenchyma.

papillotomy (clin) An incision into the major duodenal papilla associated with biliary stent-ing and endoscopic retrograde choledocho-pancreatography.

parabolic flow (phys) Laminar flow with a para-bolic profile.

paradigm (mri) Planned sequence for functionalmeasurement (i.e. BOLD imaging), where, forinstance, 10 baseline images, 10 active images,2 ignored images are collected.

P


parallel beam–partial Fourier imaging

parallel beam (xray) A parallel beam of x-rays isapproximated when the FFD is large (�2 m).

parallel hole collimator (nmed) A multiple-holecollimator with holes that are parallel to oneanother and perpendicular to the plane of thecamera crystal. The common design for a gamma

camera collimator. Produces a ‘same-size’ imagewithout distortion at depth. High resolution andhigh sensitivity versions exist (see converging col-

limator, diverging collimator).parallel imaging (mri) Use of multiple receiver

coils collecting different portions of the imagein physical space simultaneously, or differentdata points in k-space, which are then used toreconstruct collected images. Increases datacollection and so decreases total imaging time,with some loss in signal-to-noise and longerpost-acquisition reconstruction times. Parallelimaging examples include vendor-specificmethods such as sensitivity-encoding (SENSE,mSENSE), simultaneous acquisition of spatialharmonics (SMASH), generalized auto-calibratingpartially parallel acquisition (GRAPPA), integratedparallel acquisition techniques (iPAT).

parallel port (comp) A connection to the computerfor input/output that can transfer a completebyte of data at a time. A typical parallel devicewould be a printer or film formatter. Transfersone byte of data at a time at speeds up to100 KBps (kilobytes per second).

parallel saturation (mri) Saturating areas parallelto the slice plane but outside the slice of inter-est, so blood flowing to the measurement areaproduces almost no signal at the beginning ofthe measurement, thus eliminating the intra-luminal vascular signal and preventing ghost-ing. This presaturation process can beperformed on both sides of the slice.

paralyzable dead-time (nmed) A characteristic ofa scintillation detector system in which its count-ing efficiency decreases and eventually falls tozero beyond the point at which it has achieved amaximum counting rate (see dead time).

paramagnetic (mri) A substance with a small butpositive magnetic permeability. The addition of asmall amount of paramagnetic substance maygreatly reduce the relaxation times of water.Typical paramagnetic substances usually possessunpaired electrons in their outer orbital shells,particularly atoms or ions of transition elements (i.e.gadolinium, manganese), rare earth elements (lan-

thanides), some metals and some molecules,

including molecular oxygen and free radicals.They are considered promising as paramagnetic

contrast agents MR imaging (see diamagnetic).paramagnetic contrast agents (cm) Are water sol-

uble, consisting of a metal ion and a chelating agent

(i.e. DTPA); they are presented for intravenousinjection. The best paramagnetic metal ions arethose with an unpaired electron that exist in theirouter orbital shell. This is seen in the elements ofthe lanthanide and transitional metal series of theperiodic table. Metal ions from these seriesinclude gadolinium (Gd3�) and manganese(Mn2�). Gadolinium is the best since it has sevenunpaired electrons, which allows it to demon-strate the largest paramagnetic moment, and isthe most sufficient in exchanging the relaxation ofhydrogen protons of the water molecule. TheseMRI contrast agents contain magnetic centres thatcreate magnetic fields many orders of magnitudehigher than those corresponding to water pro-tons. They have their strongest effect on the T1,by increasing T1 signal ntensity in tissues wherethey have accumulated.

parameter (math) A quantity to which a value canbe given in expressing performance or for use ina calculation. Examples would be exposure/dose,optical density and acoustic impedance. Thesequantities would take values that depended on circumstances so would not be fixed.Physiological measurements of flow, respiratoryrate, gas washout are also parameters.

parametric (functional) image (image) An imagedisplay where the colour or grey scale no longerrepresents count or signal density but its scalerepresents a varying parameter such as bloodflow (colour Doppler) or metabolic activity (PET).

parametric test (stats) Pertaining to exact meas-urement (count density, weights, temperatureetc.). Tests of significance that assume a normalpopulation distribution and involve parametersof mean, standard deviation and covariance.Examples are t-Test, correlation coefficient, F-Test. Non-parametric measurements arescores or ranks. Anova can be parametric ornon-parametric (see non-parametric tests).

partial Fourier imaging (mri) An image recon-struction method where incomplete phaseencoding data (as few as half the total) are usedfor generating the entire image. Reconstructionof an image from an MR data set comprising anasymmetric sampling of k-space. A completeimage can be reconstructed since k-space is


partial Fourier imaging–partition/distribution coefficient

P

symmetrical: an opposing value at a mirrorimage location in the matrix can be computedknowing the single value; conjugate symmetry.This method can be used for either shorteningimage acquisition time, by reducing the numberof phase encoding steps required, or to shortenthe echo time (TE) by moving the echo off centrein the acquisition window. In either case the signal-to-noise ratio is reduced (see half Fourier).

partial saturation (PS) (mri) Repeated RF pulsesapplied in time periods shorter than T1. Giving adecreased signal amplitude but deliveringimages with increased contrast between regionswith different relaxation times. Signals due tovariations in the interpulse time, TR, can be usedfor calculating the regional T1. Partial saturation

is sometimes referred to as saturation recovery

but should specify a particular case of partialsaturation where recovery after each excitation effectively takes place from true saturation.

partial saturation spin echo (PSSE) (mri) Partialsaturation in which the signal is detected as aspin echo. Even though a spin echo is used,there will not necessarily be a significant contri-bution of the T2 relaxation time to image con-trast, unless the echo time TE is of the order ofor longer than T2.

partial scan reconstruction (ct) Image reconstruc-tion algorithm utilizing data from a projectionangle range less than 360°; the minimum datarange being 180°. Partial scan reconstructionenables shorter scan times so increasing tempo-ral resolution for fluoroscopic CT or cardiac CT.

partial volume artefact (ct) Artefact caused bysharp material inhomogeneities (bone/soft tissue)within the beam. The mean value of the incidentintensity within the detector element is notequivalent to an average of the linear attenuation

coefficient itself, which is a source of nonlinearerrors and thereby inconsistencies for attenua-tion measurements along different directions;partial volume artefacts occur within the slice andincrease with slice thickness. Since spiral CTincreases the width of the slice profile, the partialvolume effect will also increase. (mri) Loss of con-trast between neighbouring tissue types in animage due to insufficient resolution so that morethan one tissue type occupies the same voxel (or pixel). Unsharp images are also caused by averaging different tissue types in the voxel froman MRI data set. (nmed) An object containing ahigh radioactive concentration that is smaller

than the volume that can be resolved. The objectappears larger on the display (e.g. point source,phaeochromocytoma, small bone nidus). Thisimpairs activity– volume quantitation.

particle (phys) A charged entity rather than partof the electromagnetic spectrum. Positive andnegative beta events are particles, also alpha‘radiation’, neutrons and protons. Ionization ingas causes particle formation: electrons andions (H� H�).

particle acceleration (phys) Charged particlescan be accelerated over distance by applyinghigh voltages. This technique first developedby Cockroft and Walton is the basis of all linacs

and cyclotrons.particle fluence, Φ (dose) The quotient of dN by

da, where dN is the number of particles incidenton a small sphere of cross-sectional area da:

particle motion (us) Back-and-forth movementof particles in a medium (gas, liquid, solid) as asound wave travels through.

particle velocity (phys) Typically 3.5 cm s�1 dis-placement to and fro from the rest position.

partition (comp) A segment of memory or storagememory, most commonly describing a section ofa hard drive. A portion of a hard disk that theoperating system treats as if it were an entirelydifferent drive. Separating a disk into severalpartitions can speed up data transfer. When for-matting a hard drive, the number of partitionscan be assigned. The computer will then recog-nize each partition as a separate drive, and willshow up as different drives under most operat-ing systems, such as C:, D:, E: etc.

partitions (mri) 3D imaging application wherewhole volumes are excited, instead of individ-ual slices. A 3D slab comprises multiple parti-tions in sequence without gaps. The number ofpartitions corresponds to the number of slicesfor a 2D measurement.

partition/distribution coefficient (cm) Since n-butanol does not mix with water, it is possibleto assess the relative degree of lipophilicity of acontrast medium by adding equal parts of waterand n-butanol to a small sample. The solvents areallowed to separate into two layers and the ratioof contrast medium dissolved in each layer ismeasured; this is termed the partition coefficient,which is therefore high for compounds of high

Φ �dN

da

P


partition/distribution coefficient–PC board

passive shielding (mri) Magnetic shielding throughthe use of high permeability material (seemagnetic shielding, self shielding and room shielding).

passive shimming (mri) Shimming by adjustingthe position of suitable pieces of ferromagneticmetal within or around the main magnet of anNMR system.

passive transport (nmed) Passage of an agent(radiopharmaceutical) across a cell membraneby by diffusion and not involving any energydependent metabolic process (99 mTc diffusionacross the blood–brain barrier) (see active transport).

patient radiation dose (ct) Factors influencing doseinclude beam energy, filtration, collimation (slicethickness), slice number and slice spacing (MSAD),desired image quality (noise) and scan pitch.

pattern (image) A vector of features, expressing ameaningful regularity characterizing membersof a class, which can be measured and used toclassify objects.

pattern classification (image) Process of catego-rizing input data into identifiable classes viathe extraction of significant features from abackground of irrelevant detail.

pattern recognition (image) The detection,measurement and classification of objects in animage by automatic or semi-automatic means.

Pauli, Wolfgang (1900–1958) Theoreticalphysicist born in Austria but became a Swisscitizen in 1928. Proposed a fourth spin quantumnumber where electrons could have �½ (up) or �½ (down) spin. Pauli’s exclusion principlestates that no two electrons can have exactlythe same state. Pauli suggested in 1931 thatthe energy expended during beta decay could beshared by a massless particle (later discoveredto be the neutrino) so preserving energy conser-vation; this idea was further developed by Fermi.Pauli was awarded the Nobel Prize in 1945.

PBP (mri) Percentage of baseline at peak.PC (comp) Personal computer. Generally refers to

computers running Windows® with a Pentium®

processor. The competing design is the AppleMacintosh® which has a different architectureand operating system. Recent Intel® Macintoshcomputers can now run PC software includingWindows®. (mri) Phase contrast signal varia-tions in flow caused by phase changes; an MRAmethod exploiting this phenomenon.

PCA (mri) Phase contrast angiography.PC board (comp) Printed circuit board. A board

printed or etched with a circuit and processors.

lipophilicity and low for compounds of lowerlipophilicity (high hydrophilicity). A measure ofthe relative hydrophobicity/hydrophilicity of acompound. The relative hydrophilicity of a con-trast medium is estimated from its distributionbetween a solvent which is not miscible withwater (e.g. n-butanol) and an aqueous buffer (e.g.water or saline) at varying pH. For example, apartition (or distribution) coefficient of 0.2 meansthat the compound is distributed with 20% in thesolvent and 80% in the aqueous solution.

partition thickness (mri) The effective slice thick-ness of individual partitions in a 3D slab; the slabthickness divided by the number of partitions.

parts per million (ppm) (mri) The range of chemical

shifts in magnetic resonance spectroscopy differs foreach type of nucleus and is very small comparedto the main frequency (expressed in MHz); chem-ical shift is given in hertz and this varies with themain magnet field intensity so comparison usingfrequency alone is difficult between twomachines having slightly different magnetic fieldstrengths. Measurement using parts per millionor ppm is independent of field strength. Althoughthe term parts per million in this application is nota measure of chemical concentration, it is relatedsince the chemical shift can be changed bychemical dilution. Chemical shift tables givechemical shift ranges for various compounds.

Signal reference measurementThe frequency difference Δf between water andtetramethylsilane (a reference compound used as astandard frequency) for two field strengths: 1. 5 T and2 0 T are:

For 1.5 T (where ωo is 63 MHz), Δf � 334 HzFor 2.0 T (where ωo is 84 MHz), Δf � 445 Hz

The chemical shift (ppm) is:For 1.5 T: 334/63 � 106 � 5.3 � 10�6 � 5.3 ppmFor 2.0 T: 445/84 � 106 � 5.3 ppm.

The chemical shift between water and lipids iftypically 3 and 3. 5 ppm.

pascal (Pa) (phys) The SI unit for pressure meas-ured in N m�2 and 1 mmHg � 1.33 � 102Pa.Atmospheric pressure is 100 kPa equivalent to1 bar. 1 millibar is 100 Pa.

Pasche, Otto Swiss radiologist who introducedmoving slit exposures in 1903.

pass-band (image) The band of frequencies thatis transmitted through a filter with maximumstrength (see filtering (signal)).


PC board–Pentium™

P

Power supplies, information storage devices, canbe attached. This usually forms the motherboard.

PCI (comp) Peripheral component interconnect.This is a standard bus design for computermotherboards and expansion slots that cantransfer 32 or 64 bits of data at one time. Itsexpansion slots are compatible with ISA cards.

PCL (comp) Printer control language. A languagedeveloped by Hewlett Packard to control itsLaserJet printers and supported by virtually allprinter manufacturers. It consists of commandscalled ‘escape sequences’ that programs use totell the printer about the number of copies to print, the resolution and page formatting;current release being PCL 6.

PCM (comp) Pulse code modulation. Audio–stereoquality audio that allows the user to record ineither 12-bit or 16-bit audio. In 12-bit, two setsof stereo audio tracks are available, useful fordubbing onto an existing recording. In 16-bit,only one set of stereo audio tracks is availablefor recording (see modulation (pulse code)).

PDA (comp) Personal digital assistant. A hand-heldcomputer that can store daily appointments,phone numbers, addresses and other importantinformation. Most PDAs link to a desktop or lap-top computer to download or upload information.

PDF (comp) Portable document format. A formatpresented by Adobe Acrobat® that allows doc-uments to be shared over a variety of operatingsystems. Documents can contain words andpictures and be formatted to have electroniclinks to other parts of the document or to placeson the web.

peak rarefactional pressure/peak negativepressure (us) Maximum of the modulus of thenegative instantaneous acoustic pressure in anacoustic field during an acoustic repetitionperiod. The unit is the megapascal, MPa.

PE (mri) See Phase encoding.PEAR (mri) Phase-encoded artefact reduction

Philips. Respiratory ordered phase encoding(see RESCOMP, RSPE, FREEZE).

peer to peer (comp) A network design linkingcomputers together without a central server.This design is only efficient linking small num-bers of computers. Each user can decide whatresources on their workstation they wish toshare. This design is only efficient for linkingsmall numbers of computers.

pelvic aortography (clin) Visualization of thepelvic structures and organs. Transfemoral

percutaneous catheterization is the methodused, advancing the catheter just proximal tothe aortic bifurcation.

pelvic venography (clin) (Iliocavography).Angiography of the iliac veins and inferior venacava, usually performed by puncturing one orboth femoral veins and injecting contrast mate-rial through an angiographic cannula or a shortcatheter with sideholes. The main indicationsare thromboembolic disease, usually in connec-tion with deep venous thrombosis (DVT), andevaluation of venous external compression orinvasion by tumour masses.

penetration depth (us) Ultrasound attenuationincreases with increasing frequency. Soft tissueattenuation is approximately 0.5 dB/cm/MHz.High frequencies have less penetration thanlow frequencies. After the transmission pulseall echo signals are received before a secondpulse is transmitted (PRF). Frame rate (fR), num-ber of focuses (fN) and lines per frame (fL) allinfluence penetration depth. Propagation speedfor soft tissue is approximately 15 � 104cm s�1

(7.7 � 104 being half this) then the relation-ship: penetration � fN � fL � fR leq;7.7 � 104.

(see pulse repetition frequency (PRF)).PentaCis® (nmed) Preparation of DTPA for 99 mTc

labelling (CIS/Schering).pentetreotide (nmed) A DTPA conjugate of

octreotide as an analogue of human somato-statin. 111In pentetreotide (Octreoscan®) for thescintigraphic localization of primary and metasta-tic neuroendocrine tumours bearing somatostatinreceptor sites.

Pentium™ (comp) One of Intel® family of micro-processors; introduced in 1993: a fast processorwith a 32-bit bus. The Pentium MMX (multimedia)

11.0

10.0

Axi

al r

esol

utio

n (m

m)

Imag

e de

pth

for

3dB

loss

cm

�1

100.0

10

Transducer frequency (MHz)

0.1

1.0

P


Pentium™–percutaneous abdominal aortography

uses extra instructions and a 32k-bit cache. ThePentium-Pro (discontinued) has a 256k-bit cachedesigned for full 32-bit operating systems likeWindows NT (since discontinued). Pentium II cur-rently is the standard model CPU with clockspeeds exceeding 400 MHz incorporating allMMX functions. The evolution of this processor is:

● Pentium® I (1993) 60 MHz, bus speeds of66 MHz and a 64 kB cache;

● Pentium Pro® (1995) with speeds in the 166 to 266 MHz range. has a 256 k-bit cachedesigned for full 32-bit operating systems likeWindows NT;

● Pentium® II (1997) series which began at 233to 450 MHz with bus speeds of 100 MHz and anL2 cache of 512 kB;

● Pentium Xeon® (1998) a revised XEON versionwas made in 1999 in Pentium III configurationwith speeds in the 800� MHz range.

● Pentium® III series (1999), from 450 MHz to�1 GHz speed, 133 MHz bus with a 512 kB to2 MB L2 cache with full speed capability;

● Pentium® IV (2000) 1.5 GHz. The P4 has a 2.2 to3.6 Ghz version. A P4 Xeon® has been pro-duced. Intel® Xeon® processor family uses hyper

threading (HT) technology;● The Pentium Celeron® was designed to lower

the overall cost of computers that did not needultra high performance. The Celeron® has aslower bus, smaller cache and less efficient(slower) decision-making path.

The Pentium series CPUs were designed to runWindows but will run ot her operating systemssuch as Linux (see multicore).

penumbra (xray) A point source projects a sharpshadow (umbra) onto a screen. The diagramdemonstrates that with a line source (focalspot) then image unsharpness causes shadow-ing; the penumbra. The size of the penumbradepends on the focal spot size f and its positionrelative to the object and image plane. Theparameters i and o in the diagram determinethe penumbra dimension p (unsharpness) sothat:

Umbra and penumbra effects are produced bythe x-ray tube fine and broad focal spots.

pi f

O�

�

percentage collective dose (dose) See collective

dose.percentile (stats) A location parameter for the nor-

mal curve. Given a set of observations x1, x2,x3, . . . xn the pth percentile P is a value such thatp% of the observations are less than P and(100�p)% are greater than P. The 10th percentileis designated P10, the median is P50. If a set ofreadings for the radiation dose (in mGy) are:

Minimum: 0.6325th percentile: 1.03Mean: 1.2475th percentile: 1.47Maximum: 2.60

then for the 25th percentile 25% of the read-ings are below 1.03 mGy and 75% are above.For the 75th percentile 75% of the readings arebelow 1.47 mGy and 25% above.

percutaneous abdominal aortography (clin)Catheteriztion is the preferred procedure for

Sam

ple

freq

uenc

y

0 25 50 75 100

Percentile group

Penumbra

Umbra

Non-pointsource

f

oi


percutaneous abdominal aortography–peripheral venography (arm, leg)

P

aortography. The entry sites available aretransfemoral, transaxillary, translumbar andtransjugular approach, depending on the typeof study. Percutaneous transfemoral catheteri-zation is most commonly employed.

percutaneous cholangiography (clin)Radiographic examination of the bile ducts afterintroduction of contrast media using a needlethrough the skin inferior to the right costal mar-gin, inserting it into the liver or gall bladder.

percutaneous nephrostomy (clin) Drainage bycollecting system via catheter inserted throughthe skin of the flank, under fluoroscopic viewing.

percutaneous splenoportography (PSP) (clin)Radiography of the splenic and portal veins bydirect needle puncture of the spleen (directsplenoportography). This has an advantage overindirect splenoportography since portal pres-sures can be measured through the intra-spleniccatheter or needle. These techniques have beenreplaced with the advances in non-invasivetechniques (Doppler ultrasound and magneticresonance angiography (MRA)).

percutaneous transhepatic cholangiography(PTCH) (clin) Radiographic examination inject-ing contrast medium percutaneously through aneedle placed into the intrahepatic bile ducts;contrast medium opacification of the biliary ductsystem via direct puncture of a biliary duct. Forevaluating biliary obstruction followed by sub-sequent transhepatic biliary drainage and forstent placement in the palliative managementof obstructive jaundice. PTC has mostly beenreplaced by the less invasive endoscopic retro-grade choledochography ERC or magnetic reso-nance cholangiopancreaticography (MRCP).

percutaneous transluminal angioplasty (PTA)(clin) A general term describing procedures usedfor the percutaneous approach treating the nar-rowing and occlusion of arteries; enlarging astenosed vessel region by balloon inflation andwithdrawing through the stenotic lesion. Mayinvolve placement of intravascular stent.

percutaneous transluminal (coronary) angio-plasty PT(C)A (clin) Opening the stenosis ofnarrow coronary vessel segment by means of aballoon catheter. In addition to the dilation ofthe stenosis, a stent may be inserted.

perfusion imaging (mri) A percentage of base-line at peak image can be reconstructed for theslice. The grey scale displays the signal change

relative to a basic image prior to contrast agentadministration.

perfusion-weighted imaging (mri) Acquisitionmethods that highlight blood moving througharteries, veins and capillaries.

period (T) (math) The period of a waveform is thetime for a complete wavelength which is theinverse of the frequency. Period � 1/f. For a 50 Hzfrequency this is 20 ms; for a 60 Hz frequency theperiod is 16.6 ms. (us) Time per cycle. Range 0.1to 0.5 μs. T � 1/f (see sine wave).

periodic table (phys) When the elements aregrouped in ascending order according to atomic

number; members of a particular family occur atregular intervals obeying a periodic law. The fam-ilies generally run vertically downwards. Thesefamilies fall into alkali metals (e.g. sodium potas-sium), halogens (e.g. chlorine, iodine bromine),inert gases (e.g. helium, argon, radon). The ele-ments in the middle of the table are classified astransition elements. The rare earth elements areusually separated from the main body of thetable as the lanthanoids, as are the uraniumseries as actinoids. Metallic elements tend towardthe left-hand side and non-metals on the right-hand side.

peripheral angiography (clin) Visualization ofthe limb vessels (extremities) using iodine con-

trast material. Gd-enhanced MRI is replacingconventional x-ray angiography for imagingperipheral vascular disease. MR angiography ofthe peripheral vascular system has specialrequirements: Arterial flow is often pulsating:Large volumes are measured and images mustclearly distinguish between arteries and veins.3D gradient echo protocols with contrast agentare used most frequently.

peripheral aortography (clin) Investigation ofthe peripheral vascular disease of the lowerlimbs, visualization of the abdominal aorta, iliacarteries and arteries of both legs.

peripheral arteriography (clin) Lower extremityarteriography visualizing the peripheral arterialcirculation in arterial occlusive disease,aneurysms, trauma, vascular malformation andoccasionally for musculoskeletal tumours in thelegs. The peripheral (arm, leg) imaging of thesubclavian artery or femoral artery and itsbranches requires very high spatial resolution inorder to visualize collateral and tumour vessels.

peripheral venography (arm, leg) (clin) Forupper extremity venography, the contrast

P


peripheral venography (arm, leg)–PET detectors

medium examination investigates venousdrainage of the arm and their communicatorswith the superior vena cava. For ascendingvenography; visualization of the lower limbveins from the level of the foot to the lowervena cava using contrast media. Only veinsdraining blood mixed with contrast are visual-ized so the deep femoral and the internal iliacveins are sometimes not seen (see isometric

venography, intra-osseus venography).permanent magnet (mri) See magnet (permanent).permeability (magnetic) (μ) (phys, mri) Tendency

of a substance to concentrate a magnetic field.Absolute permeability (μ) is the ratio of magnetic

flux density (B in tesla) in a material to the exter-nal magnetic field strength (H in A m�1) induc-ing it, so μ � B/H. The SI unit is henry per metreHm�1, equivalent to NA�2 and TmA�1 so the‘permeability of free space’ or the magnetic con-stant μo is determined from the definition of theampere (A) to be exactly:

● 4π � 10�7 H m�1; or● 1.256637 � 10�6 H m�1;●

μo/4π � 1 � 10�7 H m�1 exactly.

Then B � μo � H (tesla) which has a magnitudedependent on the medium. The relative perme-ability (μr) is the ratio μ/μo and has no units.Permeability can also be expressed as magneticsusceptibility χm where χm � μr � 1; this isdimensionless. For most substances μr has aconstant value that does not vary with fieldstrength. For a vacuum and air μo � μr approxi-mately. If μr is less than unity (�1.0) the mate-rial is diamagnetic; if μr exceeds unity (�1.0) it isparamagnetic. Ferromagnetic materials have highpermeabilities, which are not constant but varywith the field strength and the conditionsunder which it is measured must be stated.Some radiologically important materials are:

Material μr χm Property

Vacuum/air 1.00000Water 0.99999Bismuth 0.999985 �1.5 � 10�5 diamagneticSodium 0.9999976 �2.4 � 10�6 diamagneticOxygen 1.0000021 �2.1 � 10�6 paramagneticPlatinum 1.0003 �3.0 � 10�4 paramagneticMn�2 1.001435 �1.44 � 10�3 paramagneticGd3� 1.0028 �2.8 � 10�3 paramagneticIron 5500 ferromagneticNickel 600 ferromagneticMu-metal 80 000 ferromagnetic

permeability of free space, μo (phys) The mag-netic constant having a value of 4π � 10�7

H m�1 or 1.256637 � 10�6N A�2.permendur (phys) A ferromagnetic alloy used for

magnetic shielding and transformer cores. Hasa very high relative permeability Consists of50% Fe; 50% Co (see magnetic permeability).

perpendicular incidence (us) Sound wave perpendicular to a media boundary.

personal dose equivalent (Hpd) (dose ICRU)The dose equivalent in soft tissue below a spec-ified point on the body at depth d. For weaklypenetrating radiation a depth of 0.07 mm for theskin and 3 mm for the eye are employed; forstrongly penetrating radiation a depth of 10 mmis employed.

Perspex™/Lucite™ (PMMA) (material) See PMMA.pertechnetate (nmed) The 99 mTcO4 anion as eluted

from the 99Mo/99 mTc generator.PET (nmed) See Positron emission tomography.PET detectors (nmed) The detection of 511 KeV

photons for PET scanners require:

● high coincidence photopeak efficiency (41%);● good timing resolution (approximately 3 nsec);● good energy resolution (approximately 13%

FWHM); and● fast scintillation decay constant (better than

300 nsec).

The commonly used scintillation materials are:bismuth germinate (BiGeO4 or BGO), cerium acti-vated lutetium oxyorthosilicate (Lu2OSiO4:Ce orLSO), cerium activated gadolinium oxyorthosili-cate (Gd2OSiO4:Ce or GSO) and thallium activatedsodium iodide (NaI:Tl)

Property Characteristic Desired value

Density and Defines detection Higheffective efficiency of detector atomic number and scanner sensitivity

Decay time Defines detector dead Lowtime and randoms rejection

Relative light Impacts spatial and output (%) energy resolution High

Energy Influences scatter Lowresolution (%) rejection

Nonhygroscopic Simplifies manufacturing, Yesimproves reliability

Rugged ness Reduces service costs Yes

Bismuth germanate (BGO) and lutetium orthosilicate (LSO) are currently the preferred


PET detectors–phantom

P

scintillation-detector materials in PET scannersbecause their higher density and atomic num-ber make them more sensitive than NaI(Tl) tohigher energy 511-keV gamma photons. Theattenuation length for 511-keV photons in bothBGO and LSO crystals is approximately 10 mm.The scintillation light yield of BGO (14%) is sig-nificantly less than that of NaI(Tl), but is suffi-cient for high-energy photon detection; LSO hasan improved light yield (75%); both are non-hygroscopic, permitting the material to be cut intosmall sections and packed tightly into elementarrays without a metal hermetic seal.

PET (scanning detector) (nmed) Current PETscanners have large fields of view, built inattenuation correction and computerized wholebody techniques that allow images of the entirebody to be acquired in approximately 30 min-utes. The specification for a current PET scannerwould be:

Item Scanner A

No. detector rings 32Total detectors 10 000 to 20 000Detector ring diameter 800–1020 mmAxial FOV 108 mmImage planes 31–90Sensitivity 3000–20 000 cps/kBq/cm3

The detector system itself would have a typicalspecification of:

Detector ring Specification

No. of rings 32Block detectors 288PMT per block 4PMT total 1152Segments 8 � 8Size (mm) 4.05 � 4.39 � 30Coincidence time resolution 6 ns

petabyte (comp) A measure of memory or storagecapacity and is approximately a thousand tera-bytes (1015 bytes). Common storage capacitiesare giga-byte (1 GB � 109 bytes), tera-byte(1 TB � 1012 bytes) and peta-byte (1 PB � 1015

bytes) (see byte).petaflop (comp) A theoretical measure of a

computer’s speed and can be expressed as athousand-trillion (1015) floating-point opera-tions per second (see flop).

Pfahler, George E. (1874–1957) Americanradiotherapist who first filtered x-ray beams in1905 with shoe leather.

PFI (mri) Partial flip imaging, general sequenceToshiba (see FFE, GRE, MPGR, GRECO, FE, GE,Turbo-FLASH, TFF, SMASH, SHORT, STAGE).

pH scale (chemistry) A measurement of the con-centration of hydrogen ions in a solution, byusing a logarithmic scale for expressing the acid-ity or alkalinity of a solution simply expressed as

pH � �log10H�

where H� denotes the activity of H� ions insolution; the pH measurement is dimension-less. pH means ‘potential of hydrogen’. The pHscale was first introduced by the Swedishchemist Sørensen in 1909.

Substance pH

HCl (0.01N) 2.0Gastric juice 2.5Urine 6.2Saliva 6.5Water 7.0Bile 7.15NaHCO3 (0.1N) 8.4

PHA (nmed) See pulse height analyser.phagocytosis (nmed) Physical trapping of col-

loidal particles by Kupffer cells in the reticu-loendothelial system. The main process fortrapping radiopharmaceuticals targeted at thereticuloendothelial system (liver, marrow).

phantom (qc) Test object, often a PMMA-blockwith various embedded measuring devices. A configuration of absorber materials used foreither monitoring resolution and contrast or (withdefined absorption properties) an estimation ofsurface and depth dose. (us) A tissue-equivalent

device that has some characteristics that arerepresentative of tissues (e.g. scattering orattenuation properties). The most popular is theAmerican Institute of Ultrasound in Medicine(AIUM) standard phantoms. (xray) Carefully con-structed item with known dimensions (length,diameter) and material characteristics (tissueequivalent plastic, etc.). Usually a fluid-filledcontainer with built-in plastic structures of var-ious sizes and shapes. Phantoms are used totest the system and quality features of imagingsystems and sometimes either radiation dose orSAR values. (ct) Object usually made from tissue

P


phantom–phase elimination

equivalent material (PMMA), simulating the geo-metrical body shape. There are also physicalphantoms for testing high and low contrastdetection purposes (bar or line test pattern) (seehigh contrast, low contrast).

phantom, anthromorphic (dose) A standardphantom used by the MIRD committee for cal-culations of the absorbed fraction.

pharmacokinetics (contrast medium) (clin)Features influencing pharamacokinetics are:high hydrophilicity, low plasma binding, renal excre-tion, elimination T½ of 1½ to 2 hours, minimalextra-renal elimination, distribution/eliminationlinear or proportional to dose, zero biotransfor-mation, zero enterohepatic circulation, minimalblood–brain barrier or placental transport, minimalcentral absorption.

phase (phys) The measure of the differencebetween two oscillating systems with respect totime. Phase is measured as an angle, each oscil-lation (one wavelength) being 360° or 2π radi-ans. Two systems are in phase when they are atthe same stage of oscillation. The second oscil-lation is 90° out of phase when it starts at the π /2 or 90° point on the x-axis with reference tothe first oscillation.

phase angle (phys) The fraction of a cycle passedby an oscillating system (sine wave) given asan angle. The formula for a sine-wave is: A sin(ωot � ϕ), where A is the amplitude, ωo theangular frequency and ϕ the phase angle. For acomplete cycle ϕ is 360° or 2π radians; a halfcycle is 180° or π radians.

(see phase difference).phase contrast angiography (mri) The blood con-

trast in the image is proportional to the local blood

1.0

0.5

0.0

Am

plitu

de

�0.5

�1.0

0 90 180

Phase angle (degrees)

270 360

velocity. Method for displaying vascular angiogra-phy in terms of velocity. The phase change of thespins in flowing blood distinguishes the bloodfrom stationary tissue where only flowing spinscontribute to the signal. This technique can alsobe the basis for flow measurements.

phase contrast imaging (xray) Developed formammography where tissue differences aredisplayed according to the slight differences inphase between tissue types imparted on amonochromatic x-ray source.

phase correction (mri) Either (i) corrective pro-cessing of the spectrum so that spectral lines atdifferent frequencies all have the absorption-made phase or (ii) in imaging, adjustment of thesignal indifferent parts of the image to have aconsistent phase.

phase cycling (mri) Techniques of signal excita-tion in which the phases of the exciting or refo-cusing RF pulses are systematically varied andthe resulting signals are then suitably com-bined in order to reduce or eliminate certainartefacts.

phase difference (phys) The graph shows twosine-waves with the same frequency but witha 90° (π/2) phase difference in time. Phase differences are measured by reference to astandard waveform.

phase elimination (mri) Fat and water protonshave only slightly different resonant frequency,resulting in phase cycling; fat and water spinsswitch in and out of phase after the applicationof an RF pulse. The strength of the oscillationdepends on the relative proportion of fat andwater protons in the tissue. Occurs primarilywith gradient echo sequences, where the signal

1.0

0.5

0.0

Am

plitu

de

�0.5

�1.0

0 90 180

Phase angle (degrees)

270 360


phase elimination–phased array coils

P

intensity of a voxel containing fat and wateroscillates with an increasing echo time.

phase encoding (mri) Encoding the distribution ofsources of MR signals along a direction in spacewith different phases by applying a pulsed mag-netic field gradient along that direction prior todetection of the signal. The amplitude of thephase-encoding gradient changes incrementallyfrom excitation to excitation. For this reason,each row of raw data has different phase infor-mation. The pulsed magnetic field gradientchanges frequency for a short time so that afterthis pulse the nuclei resume their original fre-quency but now with phase differences. It isnecessary to acquire a set of signals with a suit-able set of different phase-encoding gradientpulses in order to reconstruct the distribution ofthe sources along the encoded direction. Phase-encoding steps are required to fully scan theslice, the number depending on the matrix size.The applied Fourier transform can allocate the various phases to the respective rows.

phase encoding order (mri) The timing in whichthe phase encoding gradient pulses areapplied. The order can be sequential, centric,reverse centric, random, etc.

phase gradient vectors (mri) Matrix of thephases of the matrix of gradient vectors.

phase image (mri) Phase images can be reconstructed from the raw data measured. Inthe magnitude image, the grey scale of thepixel corresponds to the MR signal magnitudeat that location. In the phase image, each pixelgrey scale represents the respective phasingbetween �180° and �180°. Stationary spinshave the same phasing, moving spins have dif-fering phasing depending on blood velocity.

phase oversampling (mri) See oversampling.phase quadrature (phys) Two signals differing

by a quarter cycle.phase reordering (mri) Phase reordering Hitachi.

Respiratory ordered phase encoding (seeRESCOMP, RSPE, PEAR, FREEZE).

phase shift (mri) Loss of phase coherence in pre-cessing spins (signal reduction). In most situa-tions, vascular spins move at variable velocities;faster flowing spins undergo a stronger phaseshift than slower flowing spins.

phase sensitive detector (mri) A method fordetecting and distinguishing the phase of asignal by reference to the phase of a referencesignal (see quadrature detector).

phase transfer function (PTF) (mri) The phase ofthe optical transfer function, represents the phaseshift, expressed as a function of spatial frequency,of an output corresponding to a sinusoidal input.

phased array (us) An array that steers andfocuses the beam electronically (with shorttime delays). A transducer where the elementsare pulsed together using signal delays to steerthe beam in a sector scan.

The principle of the phased array is shown inthe figure above. The transmit pulses areapplied to each element via a delay whichgives a swivelled (angled) wave-front. Thedegree of swivel (�45°) requires very narrowelements of approximately ½λ dimensions. Theeffective aperture A is A � cos ϕ where A is thetotal aperture or array length without theswivel. During receive mode, the same pulsedelays are used. The phased array can producedirectional and focused beams giving electronicdynamic focusing in conjunction with beamsteering. Unlike the linear sequenced array, allthe elements are pulsed at the same time dur-ing transmission and reception and the phasedarray image formation is achieved by usingpolar co-ordinates and not rectilinear.

phased array coils (mri) Multiple linked surfacecoils served by separate amplifiers. A typicaldesign consisting of six coils which can be usedseparately or combined for 3D data collection.

A

�

P


phased array coils–phosphorescence

A large field of view can be obtained with highsignal to noise ratio.

phased linear array (us) Linear array with phasedfocusing added; linear array with phased steer-ing of pulses to produce a parallelogram-shapeddisplay.

phasors (math) These represent voltage strength Vin phase angles Φ as as a circular diagramwhere Φ is measured in radians. The fixedlength of the phasor either represents maximumvoltage or the root mean square voltage. Themathematics of phasors resembles that of vectors.

phlebography (clin) Radiography of the veinusing contrast medium (see venography).

phosphates (chem) Salts based on phosphorus(V)oxoacids and in particular salts of phosphoricacid. A large number of polymeric phosphatesexist, containing P-O-P bridges. Linear polyphos-phates, cyclic polyphosphates and cross-linkedpolyphosphates or ultraphosphates are known.These are used in bone seeking radiopharmaceu-ticals and bone therapy agents (see MDP, HMDP,pyrophosphate (PYP)).

Phosphocol® (nmed) Chromic 32phosphate(Mallinckrodt Inc) (see 32Phosphorus).

phosphor (image) A compound exhibiting lumines-cence (fluorescence, phosphorescence and thermo-

luminesence). Fluorescent phosphors are used forintensifying screens and scintillation detectors.The common types are:

Phosphor Application

CaWO4 Early intensifying screenGd2O2:Tb Terbium doped gadolinium intensifying

screenLa2OBr:Tm Thulium doped lanthanum intensifying

screenY2O2S:Tb Terbium doped non-rare earth

intensifying screenYTaO4 Ultraviolet light intensifying screen

(DuPont Ultravison®)

V∠ φ

Masteroscillator

1 2 3 4 5

Masteroscillator

1 2 3 4 5

Masteroscillator

1 2 3 4 5

Phosphorescent phosphors are most often seen asoutput screens in image intensifiers and displaymonitors (generally of the form ZnCdS:Ag). Specialphosphors having matched light output havebeen developed P45 and P4. Thermoluminescentphosphors are used in radiology as dosimeters oras image-plate detectors:

Phosphor Application

LiF Tissue equivalent personal TLDCaF2:Mn Environmental, high dose TLDCaSO4:Dy Environmental TLDBaFCl:Eu Image plate phosphorBaFBr:Eu Image plate phosphor

phosphorescence (phys) The term phosphores-cence is often incorrectly considered synony-mous with luminescence. If the luminescencecontinues after the radiation causing it hasstopped, then it is known as phosphorescence.Philipp Lenard first advanced a theory for itsaction. A phosphor material emits light whichpersists after the exciting source is removed,unlike fluorescence where de-excitation of theelectrons is almost instantaneous. The material isexcited to a metastable state from which a tran-sition to the initial state is forbidden. Emissioncan occur when thermal energy raises the elec-tron to a state from which it can de-excite sophosphorescence is temperature dependent.Light output can continue for some time afterstimulation since electrons trapped in the forbid-den zone are periodically excited into the conduction band. A summary of events is:

● The phosphor has empty traps before stimulation.● After stimulation electrons from the valency

band are ejected into the conduction band.● Conduction band electrons fall into traps in the

forbidden zone.● Intrinsic energy lifts some trapped electrons

into the conduction band.● Electrons fall into valency band emitting broad

continuous light spectrum.

Its value is found in CRT and flat-panel displaysand film intensifying screens, however phospho-rescence produces an unwanted afterglow inthese applications. Impurities or doping of thephosphor material plays an important part byserving as activators or coactivators of the phos-phorescent phenomenon (see energy bands, fluo-

rescence, thermo-luminescence).


phosphorus (P)–photoelectric effect

P

phosphorus (P) (elem)

Atomic number (Z) 15Relative atomic mass (Ar) 30.97Density (ρ) kg/m3 2200(r)

1800(y)Melting point (K) 317.2K-edge (keV) 2.14

32Phosphorus (nmed) Therapy agent for poly-cythaemia rubra vera and also used for bonepain therapy.

Production

Decay scheme (β�) 32P 32P T½ 14.3 d (β� meanenergy 694.9 keV) → 32S stable

Decay constant 0.04846 d�1


1 0.9535 0.78510 0.61615 0.48320 0.37930 0.233

Uses in radiology: 32P is a pure beta therapyisotope (see Phosphocol®).

Phosphotec®(nmed) A kit for the preparation of99 mTc-pyrophosphate (PYP) (Bracco).

phot (phys) A non-SI measure for illuminance

equivalent to 1 lumen cm�2.photocathode (xray) Directly applied to the glass

envelope window of the photomultiplier tube orplaced immediately behind, and in contact with,the very thin (500 μm) input phosphor of theimage intensifier and transforms the intensity dis-tribution of the x-ray image into an electron fluxdensity. The photocathode of the image intensi-fier is in optical contact with the input phosphortransforming light quanta into an electron fluxdensity distribution. The electrons are acceler-ated onto an output phosphor screen. The pho-tocathode is a complex antimony/caesium(cesium) compound SbCs3. The electron flux fromthe large photocathode area is focused onto amuch smaller output screen area; luminous flux isnot increased but luminance is, so providing mini-

fication gain. Quantum efficiency varies betweenthe various transformation stages of an imageintensifier; the photocathode demonstrates oneof the best quantum efficiencies.

1632

1532S(n,p) P

photoconductive detectors (rad) Used as an x-raydetector; a common example being amorphousselenium (α-Se). Unlike crystalline seleniumwhich is a semiconductor, amorphous seleniumhas a very high electrical resistance but on expo-sure to light or ionizing radiation it becomes a photoconductor. Sensitivity depends on the x-ray absorption efficiency of the selenium layer,the surface charge neutralized per unit energyabsorbed. Efficiency is quite high at low photonenergies due to the density (4810 kg m�3) andthickness of the plate and having a Z � 34,however, efficiency falls off rapidly as the pho-ton energy increases. The K-edge and fluores-cent yield of selenium (12.66 keV and 0.56,respectively) are fairly low and the energyabsorption efficiency does not show largechanges in efficiency associated with the K-edge of materials of higher atomic number.

photoconductor (phys) The forbidden energyband in some semiconductors is relatively small,consequently only a small amount of energycan move valence electrons into the conductionband. In photoconductors such as selenium andsilicon this energy can be supplied by incidentelectromagnetic radiation (light, heat x-rays,etc.). Electrons are then elevated into the con-duction band and an electric current can flowunder the influence of an applied voltage.

Material Band gap (eV)

Silicon 1.11Germanium 0.67Cadmium telluride 1.58Selenium 1.3–1.9

(see luminescence, direct radiography).photo-diode (elec) Light energy falling on the

depletion layer of a reversed biased p-n junc-tion produces electron hole pairs. Visible light of550 nm has a photon energy of 2.3 eV; greaterthan the ionization energy of silicon of 1.1 eV.

photoelectric effect (phys) The complete absorp-tion of a photon by interaction with a bound elec-

tron (K or L-shell). The photon interacts with theelectron and gives it its entire energy, the pho-ton disappearing in the process. This only occursif the photon energy can overcome the binding

energy Eb of the electron. The kinetic energy ofthe electron equals E1–Eb, the vacancy in the Kor L shell is filled by an electron cascade yieldingcharacteristic fluorescent x-rays; Auger electrons can

P


photoelectric effect–photon exposure

also be ejected in this process. The photoelectriceffect involves bound electrons since it cannotgive all its energy to a free electron. Photoelectricabsorption involves the atom as a whole, theatom undergoing recoil simultaneously. Thephotoelectron is important in radiation detectionand dose (see Compton scatter, linear absorption

coefficient, mass absorption coefficient, K-edge).

photoelectron (phys) Electron ejected from K orL-shell as the result of a photoelectric effect.

photometric quantity (phys) An indication oflight levels spectrally weighted by the standardphotometric visibility curve, peaking at 550 nm.Photometric quantities such as luminance(cd m�2) and illuminance (lux) are hybrid quan-tities defined by the visible light spectrum(non-linear eye response).

photomultiplier tube (phys) A light sensitivedevice for amplifying very small light signals. Itis a vacuum tube device with a photocathode andaccelerating dynodes steering the acceleratingelectrons onto a positively charged collector. Theoutput signal represents a �106-fold amplifica-tion of the photon fluence.

photon (phys) A quantum of electro-magnetic radia-

tion having zero rest mass is called a photon (aterm first used by G.N. Lewis in 1926); it pos-sesses energy related as E � hf where h isPlanck’s constant and f is the frequency. Theenergy can be expressed as joule seconds whereh � 6.62 � 10�34Js. The relationship derivedby Einstein is E � hf � mc2 which becomesmc �E/c or hf/c where mc is the photon momen-tum (related to frequency). Momentum and wave-

length are related as mc � h/λ (de Broglie); energyand wavelength are related as E � hc/λ.

Recoil orphoto electron

EB

E1

Bound election

Incident photon

KL

Substituting the constants for h and c yields1.986 � 10�25 J; since 1 keV is equivalent to1.6 � 10�16J then hc � 1.2412 keV. The con-version formula E � 1.2412/λ or λ � 1.2412/Eallowing wavelength (nm) and energy (keV)conversion, e.g. an 80 keV photon will have awavelength of 0.0155 nm.

photon energy (phys) Measured in electron voltswhere 1 eV is equivalent to 1.6 � 10�19 J. Visiblewavelengths have an eV of 2 to 3, x-rays from25 to 150 keV and gamma radiation overlaps x-radiation and extends to beyond 1 GeV.

Photon fluence and photon fluxA red laser (wavelength 760 nm) has an output of1 mW (1.0 � 10�3 W) and a beam diameter of 1 mm.Radiation frequency (f � c/λ) is 3.95 � 1014 Hz.

The energy of each photon (E � hf), where h isPlanck’s constant, is:

6.626 � 10�34(J Hz�1) � 3.95 � 1014 (Hz) �2.62 �

10�19 J

Photon fluence:

Photon flux: (for a 1 mm diameter laser):

Laser energy: photon flux � photon energy� 4.86 � 1021 � 2.62 � 10�19 J� 1273.3 W (1.27 kW m�2)

(area mphotons s

m0 786 10

3 82 10

0 786 10

4 86 10

6 215 1

6 2.

.

.

.

� ��

�

� �

��

�

221 1 2photons s m� �

1 0 10

2 62 103 82 10

3

1915 1.

..

�

��

�

�� W

J photons s

photon exposure (xray) This is measured aseither photon fluence (photons cm2) or photonflux (photons cm2 s�1). The number of x-rayphotons from an exposure can be calculatedknowing kV, efficiency of x-ray production andmAs. From this basic calculation the photon flu-

ence and photon flux can be calculated.

Standard radiographFor a standard radiograph exposure of 60 kV(0.06 MeV)at 100 mA at 0.05 s (5 mAs), the photonfluence for 5 mAs � 5 � 10�3 coulombs where theelectron charge is 1.6 � 10�19C.

Then the number of electrons is:

5 � 10�3/1.6 � 10�19 � 3.3 � 1016.

Allowing x-ray production of 0.5% efficiency then1.5 � 1014 x-ray photons will be available.


photon fluence (Φ)–phytate

P

photon fluence (Φ) (xray) A measure of photonintensity per unit area. For N photons of energyE incident on a surface area A for time t then:

Φ � � �NA

photons cm or m2 2

For a poly-energetic beam the proportion ofeach energy per unit time (Ei) gives the energyflux density which is the sum of all the differ-ent energy components:

Energy flux density depends on the anodematerial, tube current and applied kilovoltageso that as already seen in ψ Z � I � E2,where Z is the atomic number, I the tube cur-rent and E the applied kilovoltage. A change intube kilovoltage has a much greater effect onintensity than a change in tube current. Fromthe above formula, increasing the kV by 10 kVfrom 60 to 70 has the same effect on theenergy flux density as increasing the tube current by roughly �1.5.

photopeak (nmed) The part of the gamma spec-trum which identifies the photoelectric event.This represents the peak gamma energy. Thepeak in an energy spectrum (e.g. scintillationdetector) corresponding to complete photoelec-tric absorption.

photopenia (image) Areas on an image that havedecreased density due to either lack of radioac-tivity or x-ray photons. Examples would be livermetastases and breast micro-calcifications.

photopic (clin) Visual response associated withcones are the principal receptors. The illumina-tion intensity is high in order to record colourinformation (see scotopic).

photostimulated luminescence (phys) Lightemitted by phosphor plate when stimulated byinfrared laser (see image plate).

physical half-life (nmed) See half-life (physical).physiologic imaging (image) Detects blood flow

within tumours (see molecular imaging).PhytaCis® (nmed) Commercial preparation of phy-

tate for labelling with 99 mTc.phytate (nmed) A radiopharmaceutical as sodium

phytate labelled with 99 mTc for imaging liver,spleen and bone marrow (RES).

ψ φ� � � �( )MeV cm sEi2 1∑

photon fluence (energy) Ψ (xray) Photon energy(E) deposited per square centimetre. For amono-energetic beam this is expressed as

or ΦE which is measured in joules asJ m�2 or more conveniently as J cm�2.Ψ = NE

A

X-ray photon fluenceFrom the photon number 1.5 � 1014 (calculated fromphoton exposure) the x-ray photon fluence over achosen 1500 cm2 area is:

Φ � ��

�

� � �

NA

1 5 10

1 5 10

1 0 10

14

3 2

11 2

.

.

.

photons

cm

photons cm

photon flux (φ) (xray) The photon fluence per unittime t is:

φ � � � � � �NA t t�

Φ photons cm s or m s2 1 2 1

X-ray photon energy fluenceFrom the photon fluence of 1.0 � 1011 photons cm�2

then for a single energy photon beam the photonenergy fluence is:

φ � ��

� � � �

Φt

1 0 100 5

2 0 10

11

12 2 1

..

. photons cm s

X-ray photon fluxFrom the standard radiograph giving 1.5 � 1014 x-rayphotons the x-ray photon fluence of 1.0 � 1011 pho-tons cm�2 the photon flux is:

φ � � � ��

� � � �

Φ Φt t

φ1 0 10

0 05

2 0 10

11

12 2 1

..

. photons cm s

photon flux (energy) ψ (xray) For a mono-ener-getic beam this is simply photon flux or Φ E.NE

A t�

X-ray photon energy fluxFrom the standard radiograph having an x-ray photonflux of 2.0 � 1012 photons cm�2 s�1 the photonenergy flux is:

φE � 2.0 � 1012 � 0.06 MeV cm�1 s�1

ψ � 1.2 � 1011MeV cm�2 s�1

P


Picker, Harvey (1915–2008)–pitch

Picker, Harvey (1915–2008) American physi-cist, inventor and businessman. Produced firstcommercially available machines for radiother-apy and nuclear imaging.

pie chart (stats) A 360° display of data; typicallythe entire 360° representing 100% and sec-tions or slices representing the individual per-centages. The example shows a 3D pie chart fordiagnostic imaging collective dose. The impor-tance of each can be seen at a glance.

piezoelectric (phys) The property of certain crys-tals that causes them to oscillate when sub-jected to electrical pressure (voltage). Thepiezoelectric effect shown by a material whosedimensions change with electric charge.Mechanical deformation produces an electricalcharge.

piezoelectric effect/piezoelectricity (us) Shownby a material whose dimensions change withelectric charge. Mechanical deformation pro-duces an electrical charge.

pigment inks (image) While conventional inksare essentially oil-based dyes, pigment inksconsist of tiny chunks of solid pigment sus-pended in a liquid solution. According to theirproponents, such as HP which uses them for itsinkjet range, pigment inks offer richer, deepercolours and have less of a tendency to run,bleed or feather.

pile-up (phys) Pulse pile up in a detector due todetector or ADC dead-time; an inability toprocess fast count rates so information is lost.

pincushion distortion (xray) Display distortion(see barrel distortion).

PIN diode (phys) A semiconductor device havingan intrinsic layer interposed between the p- and n-type layers. Its RF resistance varies inproportion to an applied DC bias current. PINdiodes are used in RF coils for rapid switchingbetween transmit and receive modes and forcoil decoupling.

Other 6%

Angio. 14%

Abdomen 1% Head 1%

CT 48%

GI Tract10% Skeleton 10%

Chest

10%

COLLECTIVE DOSE 2003

pinhole collimator (nmed) A single-hole colli-mator used to magnify images of a patient’sthyroid gland.

PIOPED (nmed) Prospective investigation of pul-monary embolism diagnosis. A set of criteria forjudging the probability of pulmonary embolismfrom the results of nuclear medicine ventilationperfusion (V/Q) images, including normal, low,intermediate and high probability. Other finercategories also exist. In general:

Ventilation Perfusion Chest x-ray Probabilityfor PE

Normal Normal Normal V. lowNormal Small Normal Low

defectDefect Medium Abnormal Fair � also

defect COADNormal Large Abnormal High

defect

PIOPED was endorsed as a method for judgingnuclear medicine lung scans by the AmericanMedical Association in 1990.

pitch (ct) Ratio of table feed (T) per 360° rotationand the total slice collimation (slice width d).

Manufacturers use a variety of pitch definitions;however the generally accepted definition ofpitch should be in agreement with IEC 60601regulations for computed tomography. MultisliceCT systems use two different definitions of pitch.The terminology pitchz (x-ray beam width or z-dimension) and pitchd (detector width) differ-entiates between the two definitions. Pitchz isdetermined by x-ray collimation which definesthe number of active detectors in a multislicemachine, while pitchd will depend on the individual detector dimensions:

so: pitchd � pitchz � number of slices per rotation

pitchtable travel per rotation

single slice detector aperd �tture

pitchtable travel per rotationx-ray beam collimationz �

Single detector

pitchtable travel per rotation

slice width�


pitch–plaster (gypsum)

P

■ Reference: WA Kalender, 2000.

pitch factor (ct) The ratio of the patient couchtravel in horizontal direction per rotation of thex-ray tube divided by the product of the num-ber of tomographic sections produced by a sin-gle rotation of the x-ray tube times the nominaltomographic slice thickness (see couch/travel incre-

ment). This ratio of table feed to total slice widthis generally termed the pitch factor:

For a four or eight-slice CT design with a nominalslice width of 1 mm and a table feed of 4–8 mmper rotation respectively, gives a pitch factor of 1.0. The generally accepted definition of pitchaccording to the equation is in agreement withthe IEC 60601 regulations for com-puted tomography. Choice of pitch factor ismostly determined by clinical requirementsdepending on examination time and a given scanrange.

pixel (image, ct) The abbreviation for represent-ing the individual picture element/cell of atwo-dimentional digital image. Pixel size isdetermined by the field of view area and thenumber of elements in the display matrix. Pixeldepth determines the gray/grey or color/colourscale. In CT the pixel represents the sampled(windowed) voxel within the scanned slice.

pixel noise (ct) Statistical variation in CT numberreconstructed for a single pixel within theimage matrix due to projection noise (variationin voxel position) within the measured attenua-tion profiles. The level of pixel noise dependson the geometrical resolution and the convolu-tion kernel: pixel noise increases with increas-ing resolution.

pixel size (mri) See spatial resolution.pixels per inch (ppi) (image) See dot pitch.pixilation (image) The matrix pixels become visi-

ble on a low resolution digital image.

pitch factortable feed

number of slices slice width�

�

4 slice

Tz z

d

8 slice placentography (clin) Radiography of placentafollowing intrauterine injection of contrastmedium. via a catheter placed proximal to theaortic bifurcation. Entirely replaced by ultra-sound techniques.

planar imaging (nmed, mri) Acquiring the vol-ume activity image of an organ either as astatic or dynamic sequence.

planar spin imaging (mri) One particular tech-nique of planar imaging that creates an MRimage of a plane from one excitation sequenceby selectively exciting a grid of points withinthe plane and then applying a magnetic fieldgradient so that each point has a differentLarmor frequency. Fourier transformation of theFID can be used to separate the signals fromeach selected point and create the image.

Planck’s constant (h) (phys) A constant whichallows the energy of the photon in terms ofeither joule seconds (6.626205 � 10�34J � s(J Hz�1)) or eV seconds (4.136 � 10�15eV � s(eV Hz�1)) to be calculated from its frequency.The rationalized Planck constant or Dirac con-stant symbol

–h � h/2π is 1.054589 � 10�34

J � s (see Compton scattering).plasma proteins (clin) A mixture of dissolved pro-

teins in blood plasma which carry out a numberof different functions. The most common plasmaprotein is albumin, and its main function is totransport other substances, particularly wasteproducts and foreign substances (drugs andsome contrast media) to the liver for destructionand/or to the kidneys for elimination. Contrastmedia with a high degree of binding to plasmaproteins (and albumin in particular) is seen as asign of chemotoxicity; the higher the degree ofprotein binding, the more likely a substancemay cause adverse reactions.

plasma volume (plasmacrit) (clin) See blood

volume.plaster (barium) (shld) Barium plaster having a

typical density of 3200 kg m�3. The measure-ments are:

Thickness Weight Pb-equivalent(mm) (kg m�2) (mm Pb)

10 32 1.021 64 2.0

plaster (gypsum) (shld) Hydrated calcium sul-phate/sulphate (CaSO4.H2O), having a typicaldensity of 840 kgm�3 plaster board thickness

P


plaster (gypsum)–point spread function (PSF)

includes the thick paper covering. Typicalequivalents are:

Thickness Weight Pb-equivalent (mm) kg m�2 (mm Pb)

9.5 6.5 0.512.5 8.515.0 10.017.0 14 1.038.0 28 2.0

plastic (acrylic-lead) (dose) See Clear-Pb, shielding

(glass).

Thickness Pb equivalent Weight (mm) (mm-Pb) (kg m�2)

8 0.3 1212 0.5 1918 0.8 2922 1.1 3535 1.5 5646 2.2 7470 3.0 112

platform (comp) The operating system, such asUNIX®, Macintosh®, Windows®, on which acomputer is based.

platinum (Pt) (elem)

Atomic number (Z) 78Relative atomic mass (Ar) 195.09Density (ρ) kg/m3 21450Melting point (K) 2042Specific heat capacity J kg�1 K�1 130Thermal conductivity W m�1 K�1 71.6K-edge (keV) 78.3Relevance to radiology: occasionally used as a K-edgefilter. Used in QC equipment to give narrow slits forfocal spot cameras.

plug flow (us) Fluid portions traveling with thesame flow speed and direction.

Plumbicon (elec) A type of vacuum video camerawith higher sensitivity than the basic vidiconand low lag time.

plutonium (Pu) (elem)

Atomic number (Z) 94Relative atomic mass (Ar) 244Density (ρ) kg/m3 19740Melting point (K) 914K-edge (keV) –Relevance to radiology: Contaminant in reactorproduced generators

239Plutonium

Nuclear data Emission

Half life 2.4 � 104yDecay mode αDecay constant 2.88E–5y�1

Photons γ (very weak)

Seen sometimes as a contaminant in reactorprepared radionuclides.

plymax (radpro) A shielding material consisting ofwood layers (plywood) which acts as a supportfor sheet lead of various thicknesses. Useful forpartitions or doors in radiology and nuclearmedicine departments.

PMMA (dose) The synthetic material polymethyl-methacrylate, a plastic material used for manufac-turing tissue equivalent absorbers. Also known asLucite, Perspex or Plexiglas.

Effective atomic number (Zeff) 6.56Density (ρ) kg/m3 1190Melting point (K) 350

PMT (rad) See photomultiplier tube.Pochin, Edward Sir (1910–1990) Clinician

who investigated thyroid disease and its radia-tion dosimetry. Chairman of ICRP 1962 andUNSCEAR.

point imaging (mri) See sequential point imaging.point scanning (mri) See sequential point imaging.point source (image) An infinitesimal point emit-

ting light, gamma or x-radiation used for assess-ing the resolution of an imaging system as a pointspread function. The practical limitations associ-ated with a point source concern its dimensions,which should be two to three times smaller thanthe system resolution. For conventional x-rayimaging this requires a 10 micrometre hole drilledin platinum. A 100 μm hole is usually sufficientfor CT measurements and a 2 mm hole in thicklead sheet for gamma camera measurements.Long imaging times are required to accumulatesufficient photons to give a dense spot.

point spread function (PSF) (image) The countprofile through a displayed point source, obtainedfrom an infinitesimal point object or point source:

PSF(x, y) � δ(x)δ(y)

where δ (x) and δ (y) are delta (Dirac) functions inthe x and y planes for a shift invariant system(ICRU, 1986). (ct) The image of a point source or


point spread function (PSF)–population dose

P

circular object with infinitely small diameter (incomparison to the machine resolution) orientedorthogonal to the image plane. The PSF of a CTscanner can be measured using a thin wire(tungsten) aligned parallel to the z-axis; thewire’s diameter is smaller than the expectedFWHM of the PSF (see line spread function, modu-

lation transfer function (MTF)).

poise (phys) Unit of viscosity where 1 poise �

0.1 N.s.m�2 � 0.1 Pa.s.Poiseuille flow (phys) See flow.Poiseuille’s Law (phys) Describing the depend-

ence of volume flow rate on pressure, vessellength and vessel radius together with fluidviscosity (J.L. Poiseuille 1799–1869 Frenchphysiologist) (see Hagen–Poiseuille’s Law).

polar co-ordinates (us) The positional geometryused by a phased array transducer producing asector scan.

polarization transfer (mri) Polarization transferfrom hyperpolarized gas to 1H, 13C, and otheratomic nuclei, holds promise for sensitivityenhancement of solution-state NMR and lowsignal to noise imaging using hyperpolarized,supercritical xenon. Detection of Overhauserenhancement of solute proton magnetizationby factors of 3 to 7 confirms the intimate con-tact between 129Xe and 1H.■ Reference: Leawoods et al., 2000..

pole piece (mri) High permeability material usedto shape the uniformity of the useful volume ofa magnet, especially a permanent magnet.

polycarbonate (material) A thermoplastic poly-mer with high density and impact resistancemaking it useful for engineering construction.

Density (ρ) kg/m3 1200–1220Melting point (K) 540

�y

�y

�x

�x

polychromatic radiation (ct) X-ray beam with abroad spectral distribution of the photon energy(see Bremsstrahlung, monochromatic radiation).

polyester (chem) A synthetic polymer made fromterephthalic acid and glycol. Can be manufacturedin sheet form (used as a film base) or as filamentswhich can be woven as Terylene® or Dacron®.

polyethylene (material) A thermoplastic polymerwith tissue equivalence so useful for tissuephantom construction. Low melting point.

Density (ρ) kg/m3 920 (high density) 955Melting point (K) 410

polymethylmethacrylate (PMMA) (chem) SeePMMA.

polynomial functions (image) The standard equa-tion for a straight line is y � a � bx, a linear poly-nomial (first degree) containing two constants aand b. The constant b multiplied by a variable x isa coefficient. This basic formula can be elaboratedby adding further coefficients: y � a � bx � cx2

describes a quadratic (second degree), y �a �

bx � cx2 � dx3 describes a cubic (third degree)and so on (quartic, quintic etc.). The series arepolynomial functions and are used for closeapproximation curve fitting to analogue signals.

polypropylene (material) A thermoplastic poly-mer with good resistance to fatigue (bending).

Density (ρ) kg/m3 900Melting point (K) 450

polystyrene (material) A thermoplastic polymerwith good transparency.

Effective atomic number (Zeff) 5.74Density (ρ) kg/m3 1050Melting point (K) 510

polyvinylidene fluoride (us) A piezoelectricthin-film material.

POMP (mri) Phase-offset multi-planar RF multiplex-ing technique (GE Medical) to acquire multipleslices simultaneously.

pooled analysis (dose) Epidemiology study, originaldata from several sources analysed in parallel.

PoP (web) Point of presence. Location of the near-est node for an ISP. Used for connection to theInternet.

population dose (dose) The whole body expo-sure to a population group is measured as the

P


population dose–positron (labelled compounds)

collective effective dose. A community of 40 000people receiving 2 mSv and 20 000 receiving4 mSv both receive a collective effective dose of80 man-sieverts (man-Sv), which on presentestimates would cause one cancer.

POPUMET (dose) See Ionising Radiation (Protection of

Persons Undergoing Medical Examination or Treatment)

Regulations.positive predictive accuracy (stats) See predic-

tive accuracy.positron (phys) β�, the anti-particle to β� or nega-

tron (electron). The two mutually annihilate pro-ducing 180° opposed 0.511MeV gamma photons.Proton decay in the nucleus yields the positron:

A continuous β� spectrum is produced sincethere is a simultaneous emission of a neutrino

(υ). This compares with a β� decay with simul-taneous emission of an anti-neutrino (–ν):

137

136 0 511N C ( 2 keV ’s→ →� �� β β γ.

ZA

ZAX Y e v→ � �� 1 1

000

11

0 01

1p n e→ � �

(see electron capture).positron emission tomography (PET) (nmed) An

imaging technique using radio-isotopes thatundergo positron decay and emit positrons (pos-itively charged electrons), whose annihilationphotons (opposed 511 keV gamma photons) areimaged in coincidence to form tomographic viewsof the body. The gamma photons are detected byeither a modified gamma camera or dedicatedPET scanner. Because of the short half-life ofthe nuclides, they are produced on site. Somepositron generators are available (see radionuclides

(positron), radionuclides (generator produced)).positron (radionuclides) (nmed) See radionu-

clides (positron).positron energy (nmed) The mean distance trav-

elled by a positron before anhillation with a freeelectron is an important factor which deter-mines the positional resolution in tomography(see event location (PET)).

positron (labelled compounds) (nmed) Someapplications for clinically active agents labelledwith positron emitters are given in the tablewith estimated patient dose:

Positron emitter Labelled compounds Dose(HE) mSv/40MBq Clinical use

11Carbon 11CO 11CO211CN-11CH3I H11CHO 0.05–0.2 Dopamine sites. Opiate receptor

sites. Amino acid metabolismCellular proliferationMethionine and receptor binding agents

13Nitrogen 13N-N213NH3 0.002–0.05 Blood flow

Myocardial perfusion15Oxygen 15O-O2 H2

15O 0.02–0.2 Metabolic ActivityC15O C15O2 Cerebral blood flow

18Fluorine 18F-F2 H18F 0.5 Glucose metabolism18F- 18FDG Dopamine storage

Cellular proliferation

Compound Application

[15O]–O2 Cerebral oxygen extraction and metabolism[15O]–CO2 Cerebral blood volume[15O]–CO2 Myocardial blood volume[15O]–H2O, [11C]-n-butanol Cerebral blood flow[15O]–H2O, [13NH3], [82Rb] Myocardial blood flow [11C]–glucose, [18F]-FDG Cerebral glucose metabolism [11C]–palmitate, [11C]-acetate Myocardial metabolism[18F]–FDG Myocardial glucose metabolism [18F]–FDG Tumour glucose metabolism[18F]–spiperone, [18F]-N-methyl-spiperone Dopamine receptor binding[18F]–16α-fluoro-17β-oestradiol Oestrogen receptor binding[68Ga]–citrate Plasma volume


positron range–power (ultrasonic)

P

positron range (nmed) See event localization (PET)

POST (comp) Power-on self test. The first processa PC runs when it is switched on which checksthat memory, processor, graphics, etc., are allfunctioning.

post-injection (nmed) Injecting a further aliquotof radionuclide. Example would be 201Thalliumrest study after exercise (or vice versa) and99 mTc in-vivo labelling of red cells using an ear-lier injection of cold (unlabelled) pyrophosphate.

posterior probability (stats) The situation wherenew data are added to a set of observations.The updated probability of the disease or eventafter new information has been received;where event A occurs if it is already known thatindependent event B has occurred (see prior

probability, Bayes’ theorem).■ Reference: Armitage P, Matthews JNS, Berry G, 2001.

PostScript (comp) A printer control language devel-oped by Adobe Systems that describes each pageas a collection of geometric shapes, rather thantelling the printer where to place each dot of inkor toner. It is more suited to graphics printingthan PCL as it can produce better output qualityat higher print resolutions. While PostScript isgenerally used by laser printers, it is also com-mon in high-end inkjet models.

potassium (K) (elem)


40Potassium

Half life 1.3 � 109yDecay mode (mixed) β�β�

Decay constant 5.33E–10 y�1

Photon 1.461 MeV

Natural isotope of potassium, comprising0.0118% abundance. A measure for body com-position when using a whole body counter.Delivers an average effective dose of 0.3 mSvper year (see specific activity).

potassium chlorate (clin, dose) In purified formused as a suppressant of thyroid activity pre-venting the organification of iodine. Protects thethyroid from accidental ingestion of radioactiveiodine.

potential difference (PD) (units) The unit is thevolt (V) where 1 V � 1 J C�1. 1 V � 1 WA �1.

potential energy (phys) The energy of a body dueto its condition or state. Forms of potentialenergy are electrical energy, chemical energy andnuclear energy. The amount of potential energy ismeasured by the work the system performs untilthe energy source reaches its ground state.

potential exposure (dose) Radiation exposurethat is unlikely but may result from an accidentat a source or owing to an event or sequence ofevents including equipment failure and operat-ing error and spills.

potential recoverability correction factor(PRCF) (dose) Factors that consider the proba-bility of different germ line mutations will show differing degrees of recoverability in theoffspring, such as differing capacities for completion of embryonic foetal development.

Potter H.E. (1880–1964) American radiologistwho in 1917 introduced a focused grid thatwas moved transversely. The Potter–Bucky gridwas marketed in 1921 (see Bucky).

pound (phys) An imperial measure of weight. Thecommon (avoirdupois) pound is equivalent to0.45359237kg exactly, or approximately 453.6 g.The troy and apothecary pounds are bothapproximately equivalent to 373.236 g.

power (phys) The rate of doing work or the timetaken to do an amount of work (SI unit is the watt(W) and 1 J � 1 W s). This is defined as the rate ofdoing work (spending energy) per second:

It is measured in joules per second, the SI unitbeing watts W where 1 W � 1 J s�1 and 1 jouleis 1 watt-second. Electrical energy is measuredin joules and electrical power in watts.

Units of power

S.I. units1 W s�1 1 Jwatt (W) 1 W � 1 Js�1 � 1 Nms�1 � 1 m2 kg s�1

1 Wh 3600 JkW hour 103 W � (3.6 � 103s) � 3.6 � 106 Jc.g.s. unitsCGS erg s�1 1 W � 1 � 107 erg s�1

power (ultrasonic) (us) The amount of ultrasoundenergy emitted by the transducer per unit time.A quantity describing the rate at which acoustic

Power=work donetime taken

P


power (ultrasonic)–predictive accuracy (negative)

energy travels per unit time in the direction ofpropagation. Unless stated otherwise, all refer-ences to power measurements in this standardwill be to temporal-average values; work done perunit time Js�1. Symbol: Wo. Units: Watts, W

power density (phys) The power per unit crosssectional area in an electromagnetic field. Theunit is watts per square metre: Wm�2

power Doppler (us) Colour-flow display in whichcolour scale is assigned according to thestrength (amplitude, power, intensity, energy) ofthe Doppler-shifted echoes (see Doppler (power)).

power law (math) A function giving a straight-line when plotted on log–log axes.

PowerPC® (comp) A family of RISC chips which is acollaboration between IBM, Apple and Motorola.Used on all Apple Macintosh computers andmany IBM workstations. Apple has currentlyinstalled Intel® processors.

power rating (phys) A measure of power dissipa-tion in an electrical machine or component.

power spectral density (image) Spectral decom-position can be obtained from auto-correlationfor random signals without harmonic compo-nents. A Fourier series analysis will give peakamplitudes in the frequency domain. The powerspectral density of the signal in the presence ofnoise containing several strong periodic compo-nents such as chromatography or magnetic resonance spectroscopy.

power spectrum (image) See cepstrum,autocorrelation.

ppi (pixels per inch) (image) See dot pitch.ppm (mri) See parts per million.PPP (comp) Point to point protocol; communica-

tions program.practices (ICRP) (dose) Activities that cause an

increase in a person’s overall exposure to radi-ation by introducing new sources, pathways,etc. These would include medical and occupa-tional practices. These practices are controlledby justification, optimzation and dose limits (seeintervention (ICRP)).

praseodymium (Pr) (elem)

Atomic number (Z) 59Relative atomic mass (Ar) 140.91Density (ρ) kg/m3 6800Melting point (K) 1208K-edge (keV) 41.9Relevance to radiology: sometimes used as a dopant inphosphors.

preamplifier (elec) An analogue circuit, con-nected to the input signal source, whose mainfunction is to establish the correct input imped-ance or fixed gain before signal processing. Adevice that amplifies very low-level signals. Apreamplifier is generally placed close to its sig-nal source and has a very low noise figure as itis the principal determinant of electronic noisewithin the system.

precession (mri) The rotation axis of a spinningbody about another line intersecting it. Themovement of a spinning body which traces outa conical shape. The magnetic moment of a pro-ton with spin will precess at an angle to themagnetic field precessing at the Larmor frequency.

precession frequency (mri) See Larmor frequency.precision (math) The variation (usually relative

standard deviation) in observed values. (math)A measure of incremental size.

z-axis

Precession Angular momentum

Gyroscope(proton)

x, y plane

PrecisionA kVp meter suitable for mammography should havea precision of �0.1 kV. The display has three digits,which cover readings from 25 to 40 kV. Since there isan uncertainty of 10% in the least significant digit,this instrument cannot guarantee a precision of0.1 kV so a four digit kVp meter is chosen.

Dependent on the number of significant figuresand decimal places. A value of 13.6428 has sixsignificant figures and is accurate to four deci-mal places; its least significant figure is eight,giving a rounded value of 13.643. A radiationmeter able to give a maximum reading of19.99 mGy is more precise than one giving amaximum of 999 mGy although not necessarilyas useful if readings above 20 mGy are required(see truncation).predictive accuracy (negative) (stats) The negative diagnostic predictive accuracy is ameasure of the likelihood of a patient with a


predictive accuracy (negative)–prevalence

P

negative test actually being normal or not having the disease. It is expressed as:

From the results of the diagnostic accuracy data:(300 � 56950)/60000 � 95.4%.

predictive accuracy (positive) (stats) (diagnos-tic) This is a measure of the likelihood of apatient with a positive test actually having thedisease. It is expressed as:

From the results of diagnostic accuracy:300/(300 � 2700) � 10%

pre-emphasis (mri) Means of compensating forthe non-ideal response of a system such as themagnetic field gradient system by modifyingthe input function.

pre-emptive multitasking (comp) A feature ofthe hardware and operating system that allowsthe CPU to share processing time with all therunning programs. Gives the appearance thatall programs are running simultaneously asopposed to round-robin multitasking wheneach program can control the CPU for as long asit needs it (see multitasking).

pre-heating (xray) A preparatory step whenmaking an x-ray exposure. The filament is pre-heated by a low current prior to the full operat-ing current. This extends filament life.

Premac® lead acrylic (shld) Transparent substi-tute for lead glass (Wardray Premise (UK) plc). Alead loaded acrylic copolymer resin can beused in the production of glove boxes, win-dows, mammography screens, laboratory ware,cabinets and bench shields.

PRESAT (mri) Pre-saturation, Picker Medical Inc.,Siemens, Toshiba motion artefact reductiontechniques. spatial pre-saturation to reduce MRsignal intensity in specific locations (see SAT,REST, EFAST, SATURATION).

pre-saturation (mri) Frequency-selective pre-satu-ration (fat saturation, water saturation), pre-satu-ration with inversion pulses (dark fluid imaging).Regional pre-saturation can be used to reducethe signal from undesired tissue which minimizesartefacts caused by movement of the chest. Anadditional saturation pulse is applied at thebeginning of the pulse sequence to saturate thespins within the saturation slice. The saturated

PPATP

TP FP�

��100%

NPATN

TN FN%�

��100

region produces almost no signal and appearsblack in the image (see PRESAT, saturation).

PRESS (mri) Point resolved spectroscopy. Similarapplication to STEAM but uses a 90° sectionselect pulse followed by two mutually orthogo-nal 180° section select refocusing pulses. LikeSTEAM, only spins that have experienced allthree RF pulses can form a spin echo so signalsinside the slice but outside the delineated voxelare suppressed with spoiler gradients. Usefulfor long TEs and has a larger signal to noiseratio (see ISIS).

pressure (phys) The weight of material pressingon its surface or by collisions of atoms or mole-cules of gas within a container (i.e. gas radiationdetectors), it acts in all directions. The SI unit isthe pascal (after B. Pascal 1623–62: Frenchmathematician) which is related to the kelvin as1 pascal (Pa) � 1 N m�2. Atmospheric pressureis approximately 1.01 � 105 Pa or approximately105Nm�2. The non-SI unit mmHg extensivelyused in medicine is retained by agreement andis related: 1 mmHg � 1.33 � 102Pa. Standardatmospheric pressure previously given as760 mmHg is now taken as 105 Pa. Pressure P isdefined as the force per unit area:

Hence, pressure is not the same as force andthe result is measured in newtons per squaremetre (Nm�2). This is true for incompressiblesubstances solids and liquids.

1 millibar 102Pa1 atm �105Pa1 mm Hg 1333.32 Pa � 1.00 torrStandard pressure 1 atm or 760 torr1 lb in�2 (1 psi) 6.89 � 103 Pa

Conversion

mmHg to KPa divide by 7.5cm H2O to mmHg divide by 1.34cm H2O to KPa divide by 10

(us) See acoustic pressure.prevalence (stats) The total number in a popula-

tion with the disease. The prevalence of a dis-ease A gives the unconditional probability ofthe disease as P(A) or:

number of incidences of the diseasetotal population at the time

PF

A� �total force on surface

area of surface Nm 2

P


prevalence–program/programming language

For example, if there are 330 000 incidences ofbreast cancer in a population of 3 millionwomen, then the prevalence is 11%.

prevalent screen (clin) The first screening inves-tigation which will detect all the prevalent dis-ease which would have accumulated up to thattime.

pre-whitening matched filter (PWMF) (mri)Observers which whiten, i.e. remove spatialcorrelations (or colour), from image noise beforeanalysing the image. The ideal observer acts asa pre-whitening matched filter.

PRF (us) Pulse repetition frequency.PRFT (mri) Partially relaxed Fourier transform.primary barrier (shld) Protection from the

primary x-ray beam must be incorporated intoany part of the ceiling, floor, walls and controlarea that the primary beam can be directedtoward. The information necessary for the calculation of the primary barrier:

● yearly dose limit under constraint (P);● workload in mA min per week (W);● maximum likely kilovoltage;● occupancy factor (T);● use factor (U).

The allowable transmission B for maintainingradiation levels:

primary radiation (shld) Measured in the colli-mated x-ray beam.

Primovist® (cm) Commercial (Schering) preparationof disodium gadoxetic acid (181.43 mg mL�1

gadoteridol) for MR imaging.print server (comp) An application-specific com-

puter that manages printers and requests forprint services; allows multiple users to share anetwork printer.

print spooler (comp) A software application, typ-ically installed on a LAN server, that managesmultiple print requests.

print through (film) See halation, separation.prior probability (stats) The situation where the

probability of an event A occurs with respect toanother independent event B; the outcome of Bis not known at the time (see posterior probability,Bayes’ Theorem).

B ��

� � �

P dW U T

2

52

probability (stats) A measure of the relative fre-quency or likelihood of a disease (see posterior

probability).probability density function (stats) For continuous

variables the theoretical probability distributionor probability density function. This is repre-sented by a continuous curve; the y-axis repre-senting the density for a given value and thex-axis the frequency. The probability distributiongraph is a smooth curve with the area under thecurve scaled to 1.0; this is the probability densityfunction f(x). The area of the probability for thesegment a and b is selected in the graph.

■ Reference: Sokal and Rohlf, 1995 (see noise (image)).

probe (nm) A single detector (typically a scintilla-tion detector) used for detecting in-vivo dis-persed radionuclide in a selected organ (thyroid,stomach, kidney) using tight collimation in orderto reduce background interference.

product (math) The result of multiplying twonumbers or quantities.

profile of CT numbers (ct) Representation of theCT numbers of the pixels along a specifieddirection in a CT image.

progenitor cell (dose) Undifferentiated cell capable of limited proliferation.

program/programming language (comp) Aseries of instructions written by a programmeraccording to a given set of rules or conventions(‘syntax’). High-level programming languagesare independent of the device on which theapplication (or program) will eventually run;low-level languages are specific to each pro-gram or platform. Programming language

Frequency

Pro

babi

lity

dens

ity fu

nctio

n

0a b


program/programming language–protocol

P

instructions are converted into language spe-cific to a particular machine or operating sys-tem (‘machine language’) so that the computercan interpret and carry out the instructions.Some common programming languages areBASIC, C, C++, dBASE, FORTRAN.

progressive saturation (mri) See saturation

recovery.progressive scanning (imaging) A non-interlaced

video display giving higher definition and lessflicker (see interlacing).

Prohance® (cm) Commercial preparation of non-ionic gadoteridol (Bracco Diagnostics) for MRimaging.

Compound Concentration Viscosity Osmolalitymg mL�1 cP mOsm/kg

Gadoteridol 279.3 2.0 @ 20° 630Gd-HP-DOTA 1.3 @ 37°

projected dose (dose) The expected radiation if aspecified countermeasure, set of countermea-sures or zero countermeasures are taken.

projection (ct) Synonymous with the 1D or 2Dattenuation profile.

projection angle (ct) Angular position at whichthe x-ray source is located when measuring anattenuation profile.

projection noise (ct) Noise within the ray projec-tions measured in a CT axial scan. The mainsources for projection noise are quantum noise(varying photon flux) and electronic noisecaused by the A/D conversion; the imagereconstruction algorithm also contributes to thenoise figure.

projection profile (mri) Spectrum of NMR signalwhose frequency components are broadenedby a magnetic field gradient. In the simplestcase (negligible line width. no relaxationeffects, and no effects of prior gradients), it cor-responds to a 1D projection of the spin densityalong the direction of the gradient: in this formit is used in projection-reconstruction imaging.

projection-reconstruction imaging (mri)Imaging technique in which a set of projectionbody profiles are obtained by using suitablecorresponding sets of magnetic field gradients.Images are then reconstructed using techniquessimilar to conventional computed tomography(e.g. filtered back projection). It can be used forvolume imaging or with plane selection tech-niques. for sequential plane imaging.

propagation period (us) Equal to reciprocal frequency as 1/f in microseconds μs.

propagation velocity (us) Speed of sound dis-placement through the medium. Typical value1540 m s�1 for soft tissue.

Tissue Propagation velocity (m s�1)

Water 1492Brain 1530Blood 1570Liver 1549Bone 4080

proportional counter (phys) An ionization chamber

where the applied voltage is kept stable atapproximately 1000 V. The secondary electronevents are then proportional to the primaryevents. The dead time is of the order of microsec-onds and so the counter can be operated inpulse mode. Proportional counters are used asaccurate radiation dose monitors.

protection quantities (dose) ICRP dose quantitiesdeveloped for radiological protection that allowquantification of exposure to ionizing radiationfrom both whole and partial body external irra-diation and from intakes of radionuclides.

prospective synchronization (mri) See synchro-

nization (prospective).ProstaScint® (nmed) A kit for the preparation of

111Indium-Capromab Pendetide (Cytogen Corp)a murine monoclonal antibody, 7E11-C5.3,conjugated to glycyl-tyrosyl-(N,diethylene-triamine-penta-acetic acid)-lysine hydrochloride(GYK-DTPA-HCl). An antibody directed against aglycoprotein expressed by prostate epitheliumknown as prostate specific membrane antigen(PSMA) (see capromab pendetide).

protein-binding (cm) Cholegraphic contrast mate-rial are excreted and concentrated in the bile,rather than eliminated by the kidneys, due totheir very high protein-binding. The cholegraphic

contrast agents, therefore being ionic, have ahigher chemotoxicity than urographic contrastmaterial. A low partition coefficient is an advan-tage in contrast media since a high hydrophilicity

contributes to low protein binding. Gallium-67citrate (a radiopharmaceutical agent) binds toplasma proteins and then localizes in tissues, par-ticularly the liver, spleen, bone marrow andskeleton.

protocol (comp) Any defined set of procedures,conventions or methods that, when adhered to,

P


protocol–pulse

allow two devices to inter-operate; used toimplement LAN services.

proton (phys) A nuclear particle having a chargeequal and opposite to an electron, its rest massis 1.672 � 10�27kg and rest energy 938.2 MeV(slightly smaller than the neutron).

proton density (mri) Number of hydrogen pro-tons per unit of volume (spin density).

proton density image (mri) In a proton density-weighted MR image, contrast is affected prima-rily by the proton density of the tissue to bedisplayed.

proxy server (comp) A server that sits betweenthe browser and a web server, the proxy serverintercepts all requests and checks if therequested web page is already stored on thehard disk. Proxy servers speed up Internetaccess for large numbers of users, and can alsobe used by companies to filter out requests forunsuitable web pages.

PSIF (mri) Time inverted (mirrored) FISP steady-state GRE sequence with sampling of theSP/STE component used by Siemens producingstrong T2-weighted contrast in a short meas-urement time. Commonly used for imaging ofcerebrospinal fluid (see SSFP, DE-FGR, CE-FAST,True-FISP, ROAST, T2-FEE, E-SHORT, STERE).

pseudo-gating (mri) Obtained with a TR corre-sponding to the R-R interval in the cardiaccycle. Prevents flow artefacts (assuming a stable heart rate).

PS (mri) See partial saturation.PSSE (mri) See partial saturation spin echo.PTCA (clin) Percutaneous transluminal coronary

angioplasty. Dilatation of narrow vessel seg-ment by using balloon catheters. By positioninga dilatation balloon at the narrow vessel seg-ment, inflation of the balloon will dilate thevessel lumen. The damage to the vessel wallduring dilatation will depend on the degree ofatherosclerosis.

public area (shld) Areas immediately outside theradiology zones (controlled area, supervisedarea), where dose constraint levels of 0.3 mSvy�1 prevail (ICRP60, NCRP 116) (see public dose

limits).public (exposure levels) (dose) Individuals of the

population, excluding exposed or designatedworkers or students during their working hours.Nurses and administrative staff in hospitalsshould be treated as members of the public,along with patients’ visitors.

public dose limits (dose) There are differencesbetween ICRP and NRCP as:

ICRP60 (1991a) NCRP 116 (1993)

Stochastic Annual average 1 mSv annual effects over 5 years effective dose

not to exceed limit for 1 mSv continuous

exposure or 5 mSv for infrequentexposure

Deterministic 15 mSv annual 50 mSv annual effects equivalent equivalent

dose limit to dose to lens, lens of eye; skin and 50 mSv to skin extremitiesand extremities

Embryo/foetus 2 mSv equivalent 0.5 mSv dose on equivalent dose declared per month on pregnancy declared

pregnancy.Negligible – 0.01 mSv annual

individual effective dose per sourcedose (NID)

PUGWASH (dose) Series of conferences held todiscuss the implications of nuclear develop-ments on the world population. First held inPugwash, Nova Scotia 1957.

PulmoCis® (nmed) Preparation of MAA by CIS/Schering.

pulmonary angiography (clin) Angiographydemonstrating the pulmonary arteries andveins by using a catheter passed into the trunkof the pulmonary artery. Generally, a femoralapproach is used, but alternatively the internaljugular vein or the median cubital vein may beapproached.

pulsatile flow (us) Flow pattern that acceleratesand decelerates with each cardiac cycle.

pulsatility index (us) Description of the relation-ship between peak-systolic and end-diastolicflow speeds or Doppler shifts.

pulse (elec) The simple pulse is a single electrical,RF, sound or fluid/gas disturbance, increasingfrom zero to a maximum value in a short timeperiod. Undamped pulses have a large over-shoot, ringing and undershoot. Criticallydamped pulses are designed to reduce thesefactors. A short burst of RF or sound energyconsisting of two to three wavelengths used in


pulse–pulse intensity integral (PII)

P

MRI or ultrasound. An x-ray pulse is a veryshort exposure (�1 ms) from a grid controlled x-ray tube (see cathode cup).

pulse 90° (π/2 pulse) (mri) An RF pulse whichrotates Mz (macroscopic magnetization vector)into the transverse plane Mxy. RF pulse designedto rotate the 90° in space as referred to therotating frame of reference, usually about anaxis at right angles to the main magnetic field. Ifthe spins are initially aligned with the magneticfield, this pulse will produce transverse magne-tization and an FID signal.

pulse 180° (π pulse) (mri) RF pulse which invertsthe magnetization vector as �Mz. RF pulsedesigned to rotate the macroscopic magnetiza-tion vector 180° in space as referred to the rotat-ing frame of reference, usually about an axis atright angles to the main magnetic field. If thespins are initially aligned with the magneticfield, this pulse will produce inversion.

pulse average intensity (IPA) (us) Average inten-sity over repetition period. The ratio of the pulse

intensity integral (energy fluence per pulse) to thepulse duration as:

The unit is Watt per square-centimetre, W cm�2.Typical values are given under ultrasound (intensity).

pulse bandwidth (mri) The transmitter band-width or the range of frequencies in the excita-tion pulse and the gradient strength defines theslice thickness. Narrow RF bandwidths corre-spond to thinner slices. The receiver bandwidthdefines the pixel size, defined as: Npf/ts whereNpf is the number of phase or frequency encoding

ITA

PA �duty cycle

Overshoot

Pulse width

0

steps and ts is the sampling time. Also sincenoise content increases with bandwidth:

(us) The bandwidth of an ultrasound pulse, thenumber of frequencies in the pulse, is specifiedbetween �6 dB points. Bandwidth is inverselyrelated to spatial pulse length. Fractional band-width is bandwidth divided by operating frequency. The quality factor is:

● Operating frequency/bandwidth; or● 1/fractional bandwidth (see pulse).

pulse duration (period) (PD) (us) The time for2–3 wavelengths, a typical pulse duration is0.5–3 μs.

1.25 times the interval between the time whenthe time integral of intensity in an acoustic pulseat a point reaches 10% and when it reaches90% of the pulse intensity integral.

Pulse duration (PD) � n/f micro-seconds (μs) (seespatial pulse length).

pulse, gradient (mri) See gradient pulse.pulse height analyser (nmed) An electronic cir-

cuit that can threshold the lower and upperlimits of the photopeak energy and accept signals just within these limits.

pulse, gradient (mri) See gradient pulse.pulse intensity integral (PII) (us) The time integral

of instantaneous intensity, for any specific point

Pulse Geometry

SPL (mm)PD (μs)

Am

plitu

de

Time

PDcycles period or cycles

frequency�

�

Signal to noisebandwidth

�1

P


pulse intensity integral (PII)–pulse sequences

and pulse, integrated over the time in which theenvelope of acoustic pressure or hydrophone sig-nal for the specific pulse is nonzero. It is equal tothe energy fluence per pulse. For a transducerassembly operating in a non-autoscanning mode, itis equal to the product of temporal-average inten-sity and pulse repetition period. Unit: J cm�2.

pulse length (mri) Time duration of a pulse. For anRF pulse near the Larmor frequency, the longerthe pulse length, the greater the angle of rota-tion of the macroscopic magnetization vector;also the narrower the equivalent range of fre-quencies in the pulse will be (narrower thebandwidth). (us) See pulse duration

pulse NMR (mri) NMR techniques that use RFpulses and Fourier transformation of the NMRsignal have largely replaced the older continu-ous wave techniques.

pulse pile up (nmed) A cause of acquired dataloss due to the slow response of pulse handlingelectronics at high count rates. Mainly associ-ated with the ADC and its sample and hold cir-cuit. A form of pulse pile up is seen in nucleardetectors when their dead time prevents eachsingle event from being distinguished.

pulse power (us) Each ultrasound pulse transferspower measured in watts (or mW). The energyemitted per unit time or the average transmit-ted power (A): A � pulse energy � PRF, wherePRF is pulse repetition frequency. The total energyis this value multiplied by the transducer ontime (time of study) (see duty cycle).

pulse programmer (mri) Part of the spectrometer orinterface that controls the timing, duration, phaseand amplitude of the pulses (RF or gradient).

pulse repetition frequency (PRF) (us) For apulsed waveform, the number of pulses gener-ated per second. Determines the distance ordepth from where the echoes are collected. A chain of timed transmission pulses deliveredto the transducer. PRF � 1/PRP which is typi-cally 2–10 kHz. A lower PRF increases image

depth (see table below). The timing betweenthe transmission pulses, the pulse repetitionperiod (PRP), is critical for each transducer typesince this determines the rate of image forma-tion or frame rate. If a 1 μsec transmission pulseis followed by a waiting period of 200 μs(receive period) this will give a pulse repetitionfrequency as:

106 � 200 � 5000 Hz(5 kHz)

Typical PRF values range from 2 to 10 kHz fordiagnostic imaging. Pulse duration is typically1 μs compared to a pulse repetition period of200 μs. The pulse repetition period (PRP) ismatched to the image depth or scan line length,otherwise echo pulses may clash with trans-mission pulses if the PRP is too short.

PRF (kHz) PRP (μs) Depth (cm)

5 200 15.48 125 9.6

12.5 80 6.015 66 5.020 50 3.8

pulse repetition period (PRP) (us) Time to repeatultrasound pulse; the time from one pulse to thenext. PRP � 1/PRF which has a typical range0.1 to 0.5 ms. Pulse repitition period (PRP), thetime between pulses or waiting time for echocollection typically 200 μs.

pulse, RF (mri) See RF pulse.pulse sequences (mri) Set of RF pulses and/or

gradient magnetic field changes and time peri-ods between these pulses, synchronized withmagnetic field gradients and NMR signal recep-tion to produce NMR images. Recommendedshorthand of interpulse times used to generatea particular image is to list the repetition time(TR), echo time (TE); if using inversion-recovery,the inversion time (TI) is given in milliseconds.A 2500/30/1000 sequence would indicate aninversion-recovery pulse sequence with TR of2500 ms, TE of 30 ms and TI of 1000 ms; with a

PRP(ms)

PD(μs)SPL

(mm)


pulse sequences–PZT

P

multiple spin echo sequence (e.g. CPMG) thenumber of spin echoes should be stated.

pulse triggering (mri) Pulse triggering suppressesmotion and flow artefacts as a result of pulsat-ing blood and fluid. The pulse wave obtainedwith a finger sensor is used as the trigger.

pulse wavelength (us) This is derived as c/f mm.pulse width (us) The pulse period or pulse dura-

tion measured in microseconds.pulsed Doppler (us) A Doppler device that uses

pulsed-wave ultrasound.pulsed mode (us) Mode of operation in which

pulsed ultrasound is used.pulsed ultrasound (us) Ultrasound produced in

pulsed form by applying electric pulses or volt-ages of a few cycles to the transducer.

pulsed wave (us) A wave consisting of a series ofpulses, each containing a few cycles of ultra-sound; not continuous.

purity (radiochemical) (nmed) See high perform-

ance liquid chromatograph.PVDF (us) Polyvinylidene fluoride, a piezoelectric

thin-film material.PW (us) Pulsed wave.pyelography (clin) Radiography of the kidneys,

ureters and often the bladder by either injectingcontrast medium intravenously or via a ureteralor nephrostomy catheter. Also performed percu-taneously. Also known as pelvi-ureterography,pyelo-ureterography or uretero-pyelography.

Retrograde pyelography (urography) is the radi-ographic examination of the renal pelvis andureter by means of contrast medium counterflowinjection using a ureteric catheter (see urography

(retrograde)).pyrogens (nmed) Substances that induce a

febrile reaction (see limulus test).pyrophosphate (PYP) (nmed) As sodium pyrophos-

phate; its structure is condensed phosphate rad-icals forming chains of -P-O-P- units. A boneseeking radiopharmaceutical labelled with 99 mTc.A radiopharmaceutical previously used for skele-tal imaging but now used for labelling red bloodcells, in vivo or in vitro. 99 mTc-PYP uptake is afunction of blood flow to the bone and the boneefficiency in extracting the complex. The complexalso reacts with mitochondrial calcium com-pounds within infarcted myocardial cells.

Generic name 99 mTc - pyrophosphate

Commercial names Phosphotec® – (Bracco)Pyrolite® (CIS)Technescan® PYPAmersham PYP®

Imaging category In-vivo / in-vitro RBCBlood pool labelling

(see MDP, HMDP).PyTest® (nmed) See 14Carbon (labelled urea).PZT (us) See lead zirconate titanate.


QCSI–quality control (radiopharmaceuticals)

QQCSI (mri) Quantitative chemical shift imaging.Q-factor (phys) See quality factor.QMRI (mri) Quantitative magnetic resonance

imaging.quad core processor (comp) Quad core chips fea-

ture two separate duo core processors enabling itto perform four separate tasks simultaneouslyinstead of two. A typical specification would be4 � 2.93 GHz duo cores with a total of 2 � 4 MBL2 cache per 65 nm core. Multi-threading pro-grammes can be operated efficiency. Typically,each duo core has a cache memory of 4 MB. Thequad-core chip commonly features a heatspreader which diffuses the CPU’s heat over alarger surface area.

Quadramet (nmed) Commercial preparation of153Samarium (Schering Inc.) (see samarium).

quadrapole moment (phys, mri) Measure of thenon-spherical distribution of electrical chargepossessed by nuclei with a nuclear spinnuclear spin greater than 1/2. The resultinginteraction with electric field gradients in themolecule can lead to a shortening of relaxationtimes and a broadening of spectral lines.

quadrature (mri) Two periodic quantities (sine-wave) having the same frequency are in quad-rature when they differ in phase by 90°.

quadrature coil (mri) A coil that produces an RFfield with circular polarization by providing RFfeed points that are out of phase by 90°. Whenused as a transmitter coil, a factor of two powerreduction over a linear coil results: as a receiveran increase in SNR of up to a factor of canbe achieved.

quadrature demodulator (mri) An electronic cir-cuit part of the quadrature detector, sensitive tophase and frequency differences which analy-ses the signal from coils having quadrature orcircularly polarized features.

quadrature detector (mri) See quadrature phase

detector.quadrature phase detector (mri) A phase sensi-

tive detector or demodulator (also called circu-larly polarized detector), either an analogue ordigital electronic circuit, that detects the pro-portion of a signal in phase with a referencesignal and 90° out of phase with the reference.By joining a pair of coils at 90° and drivingthem during the transmit cycle through apower divider and phase shifter, a rotating field

2

can be produced that only requires half the RFpower. Single phase detection uses only onereference frequency. A phase sensitive quadra-

ture (circularly polarized) detector uses two ref-erence signals in quadrature, and since twosignals are being obtained the SNR is improvedby or 1.4. Patient motion will reduce thisbenefit. Both positive and negative phase dif-ferences can be detected. Any poor adjustmentof the phase shift will give a ghost image arte-fact; this can be eliminated by adjusting phaseand gain of the receiver minimizing the quad-rature peak in an off-resonance signal.

quality (xray) See beam quality.quality assurance QA (dose) A protocol set up to

serve purchase specifications, commissioningand quality control of diagnostic imagingequipment together with ancillary equipment(dosimeters, dose calibrators and contaminationmonitors). Defined by WHO as ‘All those plannedand systematic actions necessary to provideadequate confidence that a structure, system orcomponent will perform satisfactorily in service’(ISO 62 15-1980). Satisfactory performance inservice implies the optimum quality of the entirediagnostic process (the consistent production ofadequate diagnostic information with minimumexposure of both patients and personnel). (ct)See acceptance tests and quality control.

quality control (qc) Defined by WHO as ‘The setof operations (programming, coordinating, car-rying out) intended to maintain or to improveequipment (ISO 3534-1977). As applied to adiagnostic procedure, it covers monitoring,evaluation and maintenance at optimum levelsof all characteristics of performance that can bedefined, measured and controlled’. (xray) Afterpurchase and commissioning, the equipmentperformance is monitored so that it remainswithin original specifications and radiation safetylevels are maintained. This is carried out on adaily (e.g. film processors, gamma camera uni-formity), weekly, monthly (e.g. personal dosime-try, resolution/contrast, film cassettes) or yearly(e.g. focal spot, beam quality) basis. A standardof precision and accuracy for the QC instrumenta-tion is required for testing reproducibility. Anerror of �5% and precision of 0.1 kV is neces-sary for mammography kVp meters.

quality control (radiopharmaceuticals) (nmed)The study of radiochemical purity (RCP) definedas the fraction of the radionuclide which is in

2


quality control (radiopharmaceuticals)–quality factor Q

the correct chemical form. The major analyticaltechniques routinely used are:

● Chromatography (thin layer and paper (TLC/PC));● Chromatography (high performance liquid (HPLC));● Electrophoresis.

These procedures are designed to separateprecursors, impurities and decomposition prod-ucts as well as to identify the desired product.Simple chromatographic techniques employ gelpaper and either alcohol or acetone as the sol-vent. The solvent front advances along a stripof absorbent gel (thin layer chromatographystrips): an activity profile is taken through theactive areas on the strip. The quality controlresult gives a measure of:

● bound 99 mTc;● free 99 mTc (TcO4);● hydrolyzed 99 mTc (TcO2).

quality factor Q or wR (dose) See weighting factor

(radiation), weighting factor (tissue).quality factor Q (phys) This applies to any reso-

nant circuit and is a measure of the quality of aresonating system (ultrasound transducers andMRI coils) indicating the sensitivity of the system at resonance. It is defined as:

where fR is the frequency giving maximumresponse, which is the resonant frequency; f1 andf2 the frequencies either side of resonance (fR)where the response falls to 0.707 of maximum( � 0.5). The ratio of the resonance fre-quency fR, to the band-width f2–f1 is the Q fac-tor of the transducer. The Q factor describes thesharpness of the frequency response curve:The undamped curve shows a narrow fre-quency range and a distinct peak at the reso-nance frequency. Alternatively, the dampedcurve has a frequency response that is broaderand not so sharply peaked at the resonancefrequency. Q is inversely related (1/Q) to selectiv-

ity, the range of frequencies over which thesystem will show resonance and the ability ofthe system to select a given frequency withinnarrow limits. The quality factor affects the SNRsince both transmitted and detected signal inultrasound in MRI increases proportionally to Q

2

Q ff f

R��2 1

whereas the noise increases as . The Q of acoil in MRI will be affected by external influ-ences (patient volume and tissue type). Twosystems are plotted in the graph exampleshowing high and low Q values. A high Q ultra-sound transducer produces a pure single frequency with a long duration (Doppler wave-form); a low Q transducer producing a shortduration mixed frequency pulse.

(mri) Applies to the coil(s) and determines theoverall efficiency. Inversely related to the frac-tion of the energy in an oscillating system andalso inversely related to the range of frequencyover which the system will exhibit resonance. Itaffects the signal-to-noise ratio, because thedetected signal increases proportionally to Qwhile the noise is proportional to . The Q ofa coil will depend on the circumstances underwhich it is measured; ‘unloaded’ (no patient) or‘loaded’ (patient). (us) Describes the sharpnessof the frequency response curve. The curve forthe undamped transducer displays a narrowfrequency range and shows a distinct peak at the resonance frequency fR. Alternatively, in the damped transducer, due to increasedabsorption the frequency response is broaderand not so sharply peaked at the resonancefrequency. Points f1 and f2 on the undamped(sharp) curve represent frequencies on eitherside of the resonance frequency where the response has diminished to half (�3 dB).The ratio of the resonance frequency fR, to

Q

Frequency

Tran

sduc

er r

espo

nse

f1

fR

f2

A

B

Q

Q


quality factor Q–quasi-ideal observer

the bandwidth f2–f1 is the Q factor of the transducer.

quantile (stats) General name for the values of avariable which divide its distribution into equalgroups (see quartile, normal curve).

quantitative computed tomography (QCT) (ct)The use of CT images and the corresponding CT numbers for quantitative characterization of organs or tissues. QCT is most widely used in relation to the determination of bone mineral content and treatment planning inradiotherapy.

quantization noise (image) The uncertaintyintroduced into a digital image matrix by allo-cating random photon events on a surface (film)into the regular pixel pattern of a digital imagedegrades image information; this is the noiseassociated with quantization of the image sig-nal. It is reduced by employing high bit numberanalogue to digital converters along with finermatrices but this then introduces quantum noise

(see digitization noise).quantum efficiency (phys) The efficiency of

converting photons to electrons (photocathode)or vice versa (scintillation detector). The detec-tor response to electromagnetic radiation. Since the photon carries energy h f and thequantum of electric charge is e (the elementarycharge) then quantum efficiency η is the ratioof induced elementary charge Ne and the num-ber of incident photons (the fluence) Φ asη � Ne/Φ.

quantum mechanics (phys) Newton’s laws can-not be used for describing events on the atomicscale. The English polymath James Young(1773–1829) perhaps sparked off the conceptof wave/particle duality with his experimentsusing slit light sources. The quantum theorygrew later from observations made by Planckand Einstein on photon events, developed fur-ther by the work of Schrödinger and Dirac(wave mechanics), then Born and Heisenberg(matrix mechanics).

quantum mottle (film, image) Random noiseexhibited by a display system (see quantum

noise).quantum noise (stats) Determined by the number

of quanta or photons responsible in forming theimage; also referred to as quantum mottle.Poisson statistics are obeyed so the standarddeviation of the photon density N follows .N

quantum sink (phys) The point loss of signaldata. The quantum sink in an image intensifieris the photon detector surface (caesium iodide).Every absorbed x-ray quantum generates sev-eral thousand light quanta of which only asmall fraction reach the photocathode leadingto the emission of photoelectrons with a quantum yield of a few percent.

quartile (stats) The value of a variable belowwhich three-quarters (first or upper quartile) orone-quarter (3rd or lower quartile) of a distribu-tion lie. The median divides a distribution intwo halves. The three quartiles cut the distribu-tion at 25, 50 and 75% at points dividing thedistribution into 1st, 2nd, 3rd and 4th quartersby area. The second quartile is the median (seepercentile).

quasi-ideal observer (stats) Model observerswhose performance is lower than that of the

103 104 105 106 107

Photon flux (cm2 s�1)

Sta

ge

X-ray Input

10% Window Loss

65% Absorbed

108 109 1010 1011

Light Photons from CsI

Electrons

Light Photons at Output Screen

Quantum noiseIf each unit area of surface receives:

100 photons, then � 10, so there will be 10%variation or quantum noise.

1000 photons, then � 30, so there will be3% variation or quantum noise.

1 � 106 photons then � 106 � 1000, so therewill be 0.1% variation or quantum noise. (ct) Noisecontribution due to the random processes and thestatistical nature of x-ray generation, attenuation anddetection; Since efficient image detectors are used inlow dose (exposure) imaging these are prone to showquantum noise (see projection noise).

1

1000

100


quasi-ideal observer–quotient

Q

ideal observer, e.g. Hotelling observer and theNPWMF, but whose performance may be meas-urable in circumstances where the use of theideal observer is not appropriate.

quasi-threshold dose Dq (dose) An extrapolationpoint on the exponential portion of a multi-targetdose-response survival curve (see survival curve).

Quartiles

Sam

ple

freq

uenc

y

Q1

0

1

Q2 Q3 Q4

quench (mri) Sudden loss of superconductivity bya local temperature increase in the magnet. Thecryogen used for superconductivity (liquidhelium) evaporates rapidly and the magneticfield strength is reduced. As the magnetbecomes resistive, heat will be released thatcan result in rapid evaporation of liquid heliumin the cryostat. This may present a hazard if notproperly planned for.

quenching (phys) A form of signal loss in the gasGeiger detector.

QUEST (mri) Quick echo split imaging technique.FSE variant using multiple unequally spaced RFpulses and collecting STEs and SEs

QuickTime® (comp) Audio–visual software thatallows cine-delivery via the Internet and e-mail.QuickTime mages are viewed on a monitor.

Quimby, Edith Hinkley (1891–1982) Americanmedical physicist. Pioneer in nuclear medicineand exact measurement of radiation dose.

quotient (math) The result of the division of onenumber or quantity by another. A ratio of twonumbers or quantities to be divided.


R1–radiation (non-ionizing)

RR1 (mri) Longitudinal relaxation rate equal to

reciprocal of relaxation time (R1 � 1/T1).R2 (mri) Longitudinal relaxation rate equal to

reiprocal of relaxation time (R2 � 1/T2).RACE (mri) Real time acquisition and velo-

city evaluation. A single dimension velocitymeasurement.

rad (dose) The non-SI unit of dose superseded bythe gray (Gy):

● 1 rad � 100 ergs g�1;● 1 rad � 10 mGy � 1 cGy;● 100 rads � 1 Gy;● 1 mrad � 10 μGy;● 1 Gy � 1 J kg�1.

(see kerma, sievert).radian (unit) A supplementary SI unit for the

plane angle (rad). The angle which as the cen-tral area of a circle radius 1 m cuts an arc 1 mout of the circumference. 1 rad � �57.295°. Itis used for describing the 2π geometric effi-ciency of a flat detector surface (see degree,geometry, steradian).

radiance (phys) The flux per unit projected areaper unit solid angle leaving a source or surface.Radiance is the combined (integral) of the spec-tral radiances from a surface. It is a measure ofthe power given by an emitting or reflectingsurface detected by an optical system (eye)looking at the surface from the solid angle(steradian) subtended by the eye. Radiance andluminance are both used for describing the‘brightness’ of a source. Radiance has the SIunits W sr�1 m�2 or m Wsr�1 cm�2 This meas-ure is used for hazard and safety analysis.

Quantity SI unit Abbr.

Radiant flux Watt WIrradiance Watt per W � m�2

square metreRadiant intensity Watt per W � sr�1

steradianRadiance Watt per W � sr�1 � m�2

steradian per square metre

Source Radiance

Fluorescent lamp 2.5 mW cm�2 sr�1

1 kW tungsten/halogen lamp 58 W cm�2 sr�1

radiant density (phys) The radiant energy perunit volume of the radiation field. The unit is J m�3 (see photon energy).

radiant energy fluence rate (phys) The radiantpower incident on a small sphere divided bythe cross sectional area of the sphere. The unitis Wm�2 (see photon energy fluence).

radiant energy Q (phys) The total energy in aradiation field or the total energy delivered bysuch a radiation field. The unit is the joule (J)(see photon energy).

radiant exitance (phys) The flux per unit arealeaving the surface of a source of radiation. Theunit is Wm�2 (see photon flux).

radiant exposure (phys) Energy per unit areareceived on a surface. The unit is J m�2 (seephoton exposure).

radiant intensity (phys) The flux per unit solidangle emitted by a source in a given direction.The unit is W sr�1 (see photon energy).

radiant power (phys) (radiant flux). The rate atwhich radiant energy is transferred from oneregion to another by the radiation field. Theunit is the watt (W).

radiating cross-sectional area (S) (us) The areaof the surface at and parallel to the face of theactive transducer element(s) and consisting ofall points where the acoustic pressure isgreater than �12 dB of the maximum acousticpressure in that surface. The area of the activeelement(s) of the transducer assembly may betaken as an approximation for the radiatingcross-sectional area. The unit is cm2.

radiation (phys) The emission of energy aswaves or particles in the case of electromag-netic radiation or as pressure changes in amedium (air) in the case of sound radiation.

radiation (ionizing) (dose) Radiation having suf-ficiently high energy (greater than 13.6 eV) whichcauses ionization of the absorbing material (e.g.air, water, soft tissue, etc.). It can be electromag-netic (photons) or particulate (alpha, beta, neu-trons, etc.). Ionization is defined as the formationof free radicals and does not apply to simple dis-sociation of molecules into ions seen in othernon-radiation events (e.g. NaCl → Na� and Cl�).When ionization occurs in air, an average of34 eV is dissipated (probably less in liquids andsolids).

radiation (non-ionizing) (phys) This describesthat part of the electromagnetic spectrum withenergies less than x- and gamma radiation.

R


R

radiation (non-ionizing)–radioactive concentration

The threshold energy is taken as 13.6 eV. Itincludes ultraviolet light, visible light, infraredand radio waves. Electromechanical radiation(ultrasound) is sometimes considered. Since noionization is caused in their interactions, theterms associated with ionizing radiation (spe-cific ionization, LET, etc.) do not apply, howevernon-ionizing radiation undergoes attenuationand absorption and the rate at which energy isdeposited is described as the specific absorption

rate (SAR).radiation (thermal) (phys) The transfer of heat

by conduction and convection requires a mate-rial medium, either solid, liquid or gas, for itstransport but heat can be transmitted through avacuum by radiation (infrared). The radiationfrom an x-ray tube anode depends on thenature of its surface, temperature T and surfacearea A. So that intensity I of radiation emittedby a body: I AT4. So a doubling of tempera-ture increases the heat intensity by 24 or �16.Conversely, a reduction of surface temperaturefrom 1000 to 500°C reduces heat radiated to6% of the original.

radiation area (USA) (dose) Defined (USA) as anarea accessible to staff where radiation levelscould approach 50 μSvh�1 at 30 cm (50 μSv or5 mrem), either from the source itself or anysurface that the radiation penetrates. A highradiation area is where radiation levels to staffcould approach 1 mSvh�1 at 30 cm.

radiation chemistry (nmed) The study of thechemical effects of radiation on matter.

radiation detriment (dose) Assumed to be a stochastic effect. The detriment (e.g. radiationinduced cancer) is never certain to occur buthas a certain probability of occurrence propor-tional to radiation exposure. It is defined byICRP to have several factors, including incidence of radiation-related cancer or hered-itary defects, lethality of these conditions, qual-ity of life and years of life lost due to theseconditions.

radiation dose (acute) (dose) Doses greater than40 Gy cause vascular system damage and cere-bral oedema leading to death within 48 hours.Doses of 10–40 Gy cause less severe vasculardamage with fluid and electrolyte loss throughthe intestinal wall, causing death within 5–10days. Doses of 2–10 Gy cause bone marrowdamage leading to infection; death occurs within

4 to 5 weeks. LD50 is approximately 3–3.5 Gy tothe bone marrow.

radiation dose (late effects) (dose) Mostly secondary effects from vascular damage.Significant increases in leukaemia, thyroid,lung and breast tumours are mainly seen inpopulations exposed to doses greater than 1 Gy(see erythema).

radiation energy (phys) See photon.radiation exposure (mAs) (xray) See exposure,

mAs product, workload, radiographic exposure.radiation output (xray) The air kerma measured

free-in-air (without backscatter) per unit oftube loading at a specified distance from the x-ray tube focus and at stated radiographicexposure factors. Free air exposure 1 m fromthe focal spot at 80 kVp with 2.5 mm total filtration. Expected values from currentmachines are:

● 30–45 μGy (3.0–4.5 mR) per mAs for singlephase;

● 50–100 μGy (5–10 mR) per mAs for constantpotential.

At 30 kVp using a 0.3 mm focal spot at 65 cm, atypical value for digital mammography could be100 μGymAs�1.

radiation protection adviser (RPA) (dose) A qual-ified (postgraduate) and experienced physicist(typically 5 years) appointed in accordancewith national ionizing radiation regulations toadvise an employer on compliance with thenational regulations and on radiation safetymatters (equipment QC).

radiation protection survey (nmed) Evaluationof the radiation hazards incidental to the pro-duction, use or presence of radioactive materi-als or other source of radiation. Such evaluationincludes measurement of the dose rates ofradiation being emitted from the material.

radiation quality (xray) A measure of the pene-trating power of an x-ray beam, usually char-acterized by a statement of the tube potentialand the half-value layer (see kerma).

radiation weighting factor (dose) (ICRP60) Seeweighting factor (radiation).

radioactive concentration (nmed) The ratio ofactivity of a radionuclide to the total mass ofthe material or volume of solution. Usuallyexpressed in Bq g�1 or Bq cm�3.

R


radioactive (transport)–radionuclides (cyclotron produced)

radioactive (transport) (nmed) Certain condi-tions have been defined by the InternationalAtomic Energy Authority (IAEA) for the packag-ing and transport of radioactive material.Transport indices are indicated on the threetypes of package label:

Category label Surface dose Transport μSv hr�1 index

I White: Low level 5 0II Yellow: Moderate level 5–500 �1III Yellow: High level 500–2000 1–10

radioactive waste (nmed) Radioactive waste fordisposal may be identified as:

● decayed sealed sources;● spent radionuclide generators (99 mTc, 81 mKr,

185 mAu etc.);● laboratory solutions of low activity;● low activity liquid washings from vials;● liquid scintillants immiscible with water;● biologically contaminated solid waste, i.e.

syringes, vials;● radioactive gases.

Classification No control Controlled

GROUP 1 Not used Not used

GROUP 2 5 � 104 Bq 1 � 107Bq125I, 131I (1.4 μCi) (270 μCi)

(USA 1.0 μCi) (USA10.0 μCi)

GROUP 3 201Tl, 5 � 105 � Bq 5 � 106Bq32P, 67Ga, 51Cr, (14 μCi) (140 μCi)111In,57Co, 58Co, 99Mo (USA10.0 μCi) (USA 100 μCi)

GROUP 4 5 � 106 Bq 5 � 107 Bq99 mTc, 133Xe (140 μCi) (1.4 mCi)

(USA 100 μCi) (USA 1 mCi)

radioactivity (unit) See bequerel (Bq).

radiochemical (nmed) A radioactive chemicalsuitable for in vitro studies, but unsuitable foradministration to humans.

radiochemistry (nmed) The study and produc-tion of radionuclides and compounds.

radiofrequency (RF) pulse (mri) Oscillatingmagnetic field (B) typically of relatively shortduration, produced by an RF coil. In MR imag-ing, RF pulses are applied to excite a selectedslice of tissue.

radiofrequency radiation (RF) (phys) The sec-tion of the electromagnetic spectrum used fortelecommunication. The range of frequencies

between 3 kHz and 300 GHz. The subdivisionsare: very low frequency, low frequency,medium frequency, high frequency, very highfrequency, ultra high frequency, super highand extremely high frequency.

radiographic exposure (xray) Product of tubecurrent and exposure time mA � s � mAs.

radioimmunoassay (RIA) (nmed) An in vitro test inwhich very small quantities of certain substancesin blood, urine, etc. can be measured by usingspecific antibodies or other agents which havebeen labelled with radioactive tracers. Since thepatient does not receive the radioactive material,there is no patient radiation exposure involved.

radioisotope (nmed) An unstable atom havingthe same atomic number but a different number of neutrons in the nucleus than thecomparable stable element.

radiometric quantity (phys) A set of light meas-urement quantities used to describe the bright-ness of a source (radiance) and irradiance levelson a surface.

radionuclide (nmed) An unstable nuclide.radionuclides (cyclotron produced) (nmed)

Reaction mechanisms and reaction dynamicsfollow

At � NFα(1 � e�λt).

At is the number of activated atoms at time t perunit volume, N the number of target atoms perunit volume, F the cyclotron beam flux, and α theactivation cross section for the specific neutronenergy with λ the decay constant of the productnuclide. The irradiation time is approximatelythree to four half-lives of the product. The parti-cle ion beam is commonly derived from a lightgas. Cyclotron produced radionuclides have aproton excess. Decay is by positron emission orelectron capture. Since the nuclides tend to be rel-atively neutron deficient, in order to gain a sta-ble neutron : proton ratio they decay by creatinga neutron from a proton by one of two reactions:

Positron emission (β�) by proton decay:

p → n � e� � neutrino

Orbital electron capture by a nuclear proton:

p � e� → n � anti-neutrino

The most common reaction for the smallcyclotron using a proton beam is (p, n), one


radionuclides (cyclotron produced)–radionuclide production

R

neutron is lost. Typical small current (p, n) reac-tions are 15O, 13N and 18F (see radionuclides

(positron)). The commonly produced radionuclidesfrom small and medium sized cyclotrons are:

Target Reaction Nuclide Beam energy MeV

14N (d, n) 15O 318O (p, n) 18F 1012C (d, n) 13N 1010B (d, n) 11C 1068Zn (p, 2n) 67Ga 20–30203Tl (p, 3n) 201Tl 20–30112Cd (p, 2n) 111In 20–3082Kr (d, 3n) 81Rb (81 mKr) 20–30124Xe (p, pn) 123Xe . . . 123I 20–30127I (d, 2n) 127Xe 20–30

If the target is bombarded with energic deuteronsthen two neutrons are emitted in a (d, 2n) reac-tion. With higher energy deuterons (d, 3n) reac-tions can be achieved. Typical large currentreactions. Many more reactions are theoreticallypossible but cost plays an important part.

radionuclides (generator produced) (nmed)Radioactive decay can lead to the formation of anintermediate or metastable state, which decays byisomeric transition usually yielding a pure gammaor positron emitting radionuclide. Metastablestates can exist for periods of seconds (81 mKr and185 mAu) to hours (99 mTc and 113 mIn). Metastableproducts form useful imaging nuclides if theirparent isotope has a sufficiently long half-life toallow for generator construction and shipment.As the parent decays the activity of the daughterrises. The carrier free daughter is then eluted fromthe generator by passing a solvent over the col-umn. Clinically useful generators showing parent(pt) and daughter (dt) properties are:

pt : dt Production Pt T½ Dt T½ γ-Energy

81Rb → 81 mKr Cyclotron 4.6 h 13 s 190 keV99Mo → 99 mTc Reactor 66 h 6.0 h 140 keV113Sn → 113 mIn Reactor 115 d 99 m 390 keV62Zn → 62Cu Cyclotron 9.3 h 9.7 m 511 keV (β�)68Ge → 68Ga Cyclotron 270 d 68 m 511 keV (β�)82Sr → 82Rb Cyclotron 25 d 1.3 m 511 keV (β�)178W → 178Ta Cyclotron 21 d 9.3 m 93 keV191Os → 191 mIr Reactor 15 d 5 s 130 keV195 mHg → 195 mAu Cyclotron 41 h 30 s 190 keV

(see Bateman equation).radionuclides (positron) (nmed) The most com-

mon positron radionuclides and their propertiesand typical applications are:

Nuclide T½ (minutes) β� yield (%) β� energy(MeV)

Cyclotron produced15O 2.0 99.9 0.73513N 9.9 99.8 0.49111C 20.4 99.8 0.38518F 109.8 96.9 0.242Generator produced68Ga 68.1 89 0.74082Rb 76.4 95 1.40962Cu 9.7 97 1.280122I 3.6 77 1.087

Nuclide Application

11C Metabolic and pharmacological studies13N Metabolic and pharmacological studies15O Blood flow, blood volume and metabolic

studies18F Labelled glucose analogues for regional

cerebral blood flow and myocardialmetabolism

68Ga Transmission source, tumour imaging82Rb Myocardial perfusion62Cu Myocardial perfusion122I Thyroid metastases

(see positron (labelled compounds), radionuclides

(generator produced).radionuclide production (nmed) Three methods

are available for producing nuclear medicineradionuclides:

● Bombardment of stable elements with chargedbeams (cyclotron).

● Irradiation of stable elements with neutrons ina nuclear reactor.

● Generator production.

The first two procedures mentioned above obeythe same basic equation for the rate of isotopeproduction:

A � C(1 � e�λt)

where A is the amount of activity and C is asaturation constant, since it is the maximumamount of activity that can be produced for thegiven conditions; t is the irradiation time and λthe decay coefficient for the nuclide. At a cer-tain time, during irradiation, a production limitis reached: the number of atoms being pro-duced is balanced by those decaying. Thispoint is the saturation limit. The factor C for reactor irradiation is calculated as:

Cw

� �� 1 6 10 8.

φ σAtomic weight

R


radionuclide production–radium (Ra)

where w is the sample weight in grams, φ isthe neutron flux and σ is the cross-section forthe nuclear reaction in barns.

radionuclides (reactor) (nmed) Some commonlyproduced radionuclides from neutron irradia-tion using the (n,γ) reaction or as fission

products are:

Radionuclide Use

99Molybdenum 99Mo/99 mTc generator125Iodine Radio-immunoassay and thyroid therapy131Iodine Thyroid therapy and some scintigraphy32Phosphorus Therapy133Xenon Ventilation imaging51Chromium Red blood cell labelling60Cobalt Therapy and calibration57Cobalt Reference

radionuclide therapy (nmed) Radiation therapyfrom a radiopharmaceutical given directly tothe patient (e.g. 131 J for hyperthyroidism).

radiopharmaceutical (nmed) A radiolabelledcompound administered for the purpose ofdiagnosis or therapy. Conforming to a recog-nized standard of clinical purity. These com-pounds do not elicit physiologic effects on thebody because of the carrier-free nature of theradionuclide and the inherent safety of mostradiopharmaceuticals. A radiopharmaceuticalcontains two parts, the radionuclide and thepharmaceutical, e.g. 99 mTc DTPA.

radiopharmacy (nmed) The design require-ments for laboratories where radionuclides areto be dispensed for clinical use should complywith national legislation. The degrees of usageare categorized into low, medium and highlevels.

1.00

0.75

0.50

0.25

0.00

0 1 2 3 4 5

Irradiation half lives

Per

cent

of s

atur

atio

n ac

tivity

Radiotoxicity Low Medium High

Group 2 89Sr, �500 kBq 500 kBq to 500 MBq to 125I, 131I 500 MBq 5 GBq

Group 3 51Cr, 57Co, �5 MBq 5 Bq to 5 GBq 5 to 500 GBq99Mo, 111In

Group 4 99 mTc �500 MBq 500 MBq to 500 GBq to 500 GBq 50 TBq

The radiopharmacy must comply with con-trolled/supervised area restrictions of access.■ Reference: Frier et al., 1988.

radio-resistive (dose) Tissues that have a higherresistance to radiation damage than others due to their relatively low rate of cell division.ICRP60 places vascular and connective tissueas intermediate and CNS the slowest torespond.

radiosensitive (dose) Tissues that have a higher sensitivity to radiation damage thanothers. It is essentially the processes of celldivision that are radiosensitive and the cellpopulations with the higher rate of cell divi-sion show the earlier response to radiation.ICRP60 places gonads, bone marrow andintestine high on the scale. Breast tissue wasranked high in the ICRP26 report but has beendowngraded since both male and female tis-sue is ranked together in ICRP60 (see weighting

factor (tissue wT)).radio waves (phys) Radio waves are a form of

electromagnetic radiation produced by oscil-lating electrons in an inductor (coil). Thewaves are transferred to an aerial which atlow frequencies can be a short length of wirebut at higher frequencies a system of conduc-tors is necessary to shape the transmittedbeam or increase the ability to detect radio-waves of a certain frequency band. Very highfrequency (VHF) radio waves (20–80 MHz) arewithin the RF spectrum of magnetic reso-nance imaging signals. At these frequenciesit is essential to maintain impedance match-ing otherwise the signal strength will bereduced.

radium (Ra) (elem)

Atomic number (Z) 88Relative atomic mass (Ar) 226Density (ρ) kg/m3 5000Melting point (K) 970K-edge (keV) 103.9Relevance to radiology: abandoned as a therapy agentin favour of other nuclides.


226Radium–RAID

R

226Radium

Half life 1600yDecay mode alphaDecay constant 0.000433 y�1

Photons/particles α 4.5–7.6 MeVβ� 0.42–3.2 MeVγ 0.29–1.7 MeV

radon (Rn) (elem)

Atomic number (Z) 86Relative atomic mass (Ar) 222Density (ρ) kg/m3 9.73Melting point (K) 202K-edge (keV) 98.4

Of the 20 known isotopes only two are signifi-cant in radiation protection:

●220Rn (thoron) a decay product of 232Th;

●222Rn (radon) a decay product of 238U.

The uranium series starts with 238U and endswith stable 206Pb. The mass number 238 isdivisible by 4 with a remainder of 2 so thisseries is known as the ‘4n�2’ series, where nis an integer between 51 and 59. Similarly,220Rn starts with 232Th and ends with stable208Pb; this is the ‘4n series’.

220Rn

Half life 55.6 sDecay mode alpha

A short lived radionuclide decaying to astatineand polonium each giving an alpha emission:

222Radon

Half life 3.8 dDecay mode ComplexDecay constant 0.1823 d�1

Photons/particles Complex emissions startingwith 238U

The 222Rn decays further in the lung tissue to218polonium and 214lead, each giving an alphaemission:

92238

84218

82214

82206

U ( ) Po( )

Pb( ) Pb

→ →

→ →

. . .

. . .

86222Rn α α

α sstable.

90232

85218

84216

82208

Th ( ) At( )

Po( ) Pb

→ →

→ →

Rn86220 α α

α . . . staable.

Of the 20 known isotopes only three occur innature: 222Rn (radon: T½ 3.8 d) a decay productof 238U; 220Rn (thoron: T½ 54 s) a decay productof 232Th and 219Rn (actinon: T½ 3.9 s) a decayproduct of 235U. 222Rn is an important contami-nant of buildings, giving higher radiation expo-sure to some hospital staff than x-rays.Polonium-218 and 214, as radon daughters,deliver an alpha dose to lung tissue. FromICRP65 the annual effective dose of 10 mSv canbe maintained by applying the following constraint levels:

Situation Constraint (Bq m�3)

Domestic dwellings 600Workplaces 1500

An action level is set below these figures, typi-cally 200 and 400 Bq m�3 for domestic andworkplaces respectively (see alpha decay).

radon concentration (dose) The indoor con-centration depends on the composition of theunderlying rock formation. The mean radon con-centration from an NRPB survey was 20.5 Bq m�3

implying a mean effective dose of 1 mSv y�1.Typical values in Bq m�3 from some localities are:

Switzerland 150UK (Cornwall) 110Ireland 80USA 60Germany 50Austria 15Japan 10

radon dose (dose) The mean radon concentrationfrom an NRPB survey was 20.5 Bq m�3 imply-ing a mean effective dose of 1 mSv y�1. In highradon concentrations (Cornwall) effective doselevels exceed 50 mSv in 6% of houses and ayearly effective dose rate of 20 mSv is common.Action levels recommended by most nationalinstitutions is 200 Bq m�3 with schools havinga lower action level of 150 Bq m�3.

Radon transform (ct) In 1917 an Austrian mathe-matician working on gravitational theory showedtheoretically that an object can be reconstructedfrom the infinite set of all its projections. The firstreconstructed images using a type of Radontransform were obtained in radioastronomy.

RAGE (mri) Gradient echo.RAID (comp) Redundant array of inexpensive/

independent disks. A method of spreadinginformation across several disks set up to act as

R


RAID–rapid-excitation MRI

a unit, using two different techniques, storesinformation across several disks or with diskmirroring: simultaneously storing a copy of infor-mation on another disk so that the informationcan be recovered if the main disk crashes. Speedof access and reliability are improved. Levels ofaccess are available from immediate (most recentcases) to less immediate (cases older than acertain time period: e.g. weeks, months).

RAM (comp) Random access memory where datacan be written to and read from. One of two basictypes of memory. Portions of programs are storedin RAM when the program is launched so that theprogram will run faster. Only portions of RAM willbe accessed by the computer at any given time.Also called memory. Stored data are lost whenpower is removed (computer switched off).

RAMBUS (comp) A form of RAM which processesinstructions 16 bits at a time, rather than 5 bits,making the process twice as fast as standardRAM.

RAM disk (comp) A simulated disk drive createdand maintained by a special driver that storesdata in the main memory. Very fast access fordisk intensive application programs since theyoperate at memory speed. The data stored arelost when power is removed.

RAM-FAST (mri) Rapidly acquired magnetizationprepared Fourier acquired steady state.Reduced acquisition matrix FASTTurbo-FLASH-like sequence (Picker Medical Inc.).

ramping (mri) Changing the strength of the magnetic field of a magnet.

ramp time (mri) Time required for a change in themagnetic field strength, shown in the graph.Usually measured in mT min�1: depends on con-struction of the magnet and design of the mag-net power supply. Typical values are from 1 to10 mT min�1 but for dast sequences such as EPI,rise times of 0.2 ms are required (see slew rate).

ON

Ramp timeOFF

Time

Gra

dien

t str

engt

h

random events (nmed) For coincident countingin PET, simple probability provides that givensingles rates S1 and S2 for a pair of detectorsand a coincidence window of width 2τ thenevents will be found in coincidence due to random occurrence with a rate:

R � 2τ * S1 * S2

These coincidences occur at random within thecoincidence window. Since the detector’s ‘singles’rate is proportional to the imaged activity, thenthe ‘random’ rate is proportional to the square ofthe activity. This contrasts with the ‘true’ coinci-dent events, which are only proportional to theactivity. Random coincident events become a lim-iting factor at higher activities and may limit theactivity which may usefully by imaged. As thereal coincidence count rate increases, the propor-tion of randoms also increases, eventually becom-ing unacceptable; this is particularly seriouswhen the detector efficiency is low.

random error (stats) These errors may vary in anon-reproducible way but can be treated sta-tistically by using probability methods.

random labelling (nmed) Red blood cell (RBC)labelling using either 51Cr as chromate or 99 mTclabelling pyrophosphate either being incorpo-rated into the globin portion of haemoglobin byrandomly aged cells.

range (math) The absolute or relative differenceof minimum and maximum values of measuredquantities.

range ambiguity (us) The artefact producedwhen echoes are placed too close to the trans-ducer because a second pulse was emittedbefore they were received.

range equation (us) The relationship betweenround-trip pulse-travel time, propagationspeed, and distance to a reflector.

range gating (us) A selection of the depth fromwhich echoes are accepted based on echoarrival time.

rapid-excitation MRI (mri) An approach forspeeding up the MRI data acquisition processby repeating the excitation RF pulses in timesshort compared to T1, using small flip anglesand gradient echo refocusing. When TR isequal to or shorter than T2, the repeated RFpulses will tend to refocus transverse magneti-zation remaining from prior excitations, settingup a condition of steady state free precessionand a dependence of signal strength (and


rapid-excitation MRI–ratio (iodine)

R

image contrast) on both T1 and T2 which canbe modified commonly by either:

● Spoiling the tendency to build up a steady stateby reducing coherence between excitations;

● By variation of the phase or timing of consecu-tive RF pulses or of the strength of spoiler gradient pulses. Thus increasing the relativedependence of signal strength on TI; or

● Acquire the signal when it is refocusing immedi-ately prior to the next RF pulse, thus increasingthe relative dependence of signal strength on T2.

RARE (mri) Rapid acquisition with relaxationenhancement. The original fast spin echo acquir-ing multiple RF echoes with different phaseencoding steps. A very rapid scan technique thatconsists of a train of individually phase-encodedspin-echoes and gave rise to TurboSE. Dataacquired from the image acquisition, organizedas a two-dimensional matrix with points alongthe horizontal axis representing individual fre-quency-encoding samples and points along thevertical axis representing individual phase-encoding lines. Shows bright fat; prone to ghostsand edge artefacts. High specific absorption rate.

rare earth elements (lanthanides/lanthanoids)(elem) A series of 15 elements in the periodictable from atomic number 58 (cerium) to 71(lutetium); to include:

Z Element Use

57 lanthanum La Phosphor; Blue lightintensifying screen

58 cerium Ce Phosphor dopant59 praseodymium Pr Laser dopant60 neodymium Nd Laser dopant61 promethium Pm –62 samarium Sm Permanent magnet alloys63 europium Eu Phosphor doping; image

plate dopant64 gadolinium Gd Green/yellow light

intensifying screen;paramagnetic MRIcontrast agent

65 terbium Tb Phosphor doping;intensifying screendopant

66 dysprosium Dy TLD dopant67 holmium Ho K-edge filter68 erbium Er K-edge filter69 thulium Tm Phosphor/intensify screen

dopant70 ytterbium Yb –71 lutetium Lu Phosphor

(see phosphor).

rarefaction (us) Region of low density and pres-sure in a compressional wave.

rarefaction pressure (us) The amplitude of anegative instantaneous ultrasonic pressure inan ultrasound beam.

rasterize (image) Breaking down an image intoindividual scan lines. Most graphics monitorsand printers are raster based.

Räth, Curt German chemist who first patented apyridine derived intravenous urographic x-raycontrast agent in 1927 (Uroselectan).Developed its clinical application with Binz.

rating (xray) The energy rating for x-ray anodesrange from 250 kJ to 3.5 MJ. Heat dissipation isgiven in watts (W) or joules per second. Thetable shows some heat ratings for typical x-raytubes, anode heat capacity and anode heat dis-sipation are shown in heat units, joules perminute and watts.

Use Anode heat Anode heat Anode capacity dissipation diameter

Conventional 300 kHU 60 kHU/min 80 mm210 kJ 44.4 kJ/min

740 WCT 6.3 MHU 840 kHU/min 120 mm

4.7 MJ 621.6 kJ/min10.3 kW

Fluoroscopy 300 kHU 908 kHU/min 200 mm210 kJ 672 kJ/min

11.2 kW

(see cooling curve).

ratio (iodine) (cm) One of the physical descriptionsof an iodine contrast agent measuring its abilityto attenuate x-rays and its tendency for inducingside-effects (osmotoxicity). Measured as a ratio of

100 kV

90 kV

80 kV

0.01

800

700

600

500

400

300

200

100

00.1 1 10

Time (s)

X-r

ay tu

be c

urre

nt (

mA

)

Overload

Acceptable

R


ratio (iodine)–receiver gate

the number of iodine atoms per volume contrastmedium by the number of particles (contrastmedium ions or contrast medium molecules) pervolume contrast medium solution.

rational number (math) Any number that can bewritten as x/y where x and y express the ratioof two integers. Examples of rational numbersare 0.5 (1/2) 1.5 (3/2) and 0.33333. . . (1/3).Compare an irrational number π or e where thenumber cannot be represented as a ratio.

RAW (image) These are image files that are not yetprocessed. Normally the RAW image file will beprocessed before conversion into RGB, TIFF orJPEG file format for storage or hardcopy. There isno standard RAW format and they can be similaror radically different between manufacturers.

raw data (ct) The values of x-ray detectorresponse from all views and rays within a scan.These data are convolved with the convolutionfilter and undergo back projection to produce aCT image.

raw data pre-processing (ct) A processing stepthat is applied to the measured attenuationprofiles in order to correct for the beam harden-ing. Variation in detector sensitivity and dis-tances between detector channels, along withother sources of measurement errors. Theseessential corrections must be performed beforeimage reconstruction (see data post-processing).

ray (ct) The collimated narrow beam of x-raysfrom the tube window incident on a singledetector within a detector array, giving rise tothe detector signal. Each view or projection iscomposed of numerous rays (see ray sum).

ray sum (ct) The total absorption figure for a rowof values constituting a projected line across theobject. The ray sums then undergo back projec-

tion in order to find the separate matrix values.rayl (us) Unit of acoustic impedance. The m.k.s. (SI)

value measured as N � s � m�3 (kg m�2 s�1);the c.g.s. value is dyne � s � cm�3 or 10 N �

s � m�3 (g cm�2 s�1).Rayleigh noise (image) A distribution describing

the magnitude of the noise amplitude followinga Gaussian distribution. The mean value of thisdistribution is roughly 1.25 σ, where σ is thestandard deviation of the original Gaussian dis-tribution (see probability density distribution).

reactor (nmed) See nuclear reactor.reactor (nuclear) (nmed) A critical assembly of

fissionable material (235U, 238U, 239Pu) and mod-erator (carbon or heavy water). The nuclear

reactor is used for thermal energy productionand also as a source of neutrons for radionu-clide production by either neutron irradiation ofa prepared sample or from products of nuclear fission.

readout delay (mri) See TE.real focal spot (xray) The rectangular area on the

anode, bombarded by the electron beam. Thearea and angle of the real focal spot determinesthe effective focal spot. Single filaments are usedin x-ray tubes having a single anode target;dual sized focal spots can be obtained by alter-ing the electron beam size. Dual filaments, oper-ated in parallel on a single focal spot, are usedin mammography tubes to overcome the space

charge limitations; these are operated as the sin-gle filament (above) to obtain dual focal spotsizes. Separate dual focal spots are also used(fluoroscopy) but operated independently andfocused on separate differently angled targetsto give dual focal spots. Focal spots can bemoved to slightly different positions on the tar-get surface by control coils which surround thex-ray tube; used in some CT machines as ‘fly-ing’ focal spots to increase image resolution (seefocal spot (effective), filament, line focus principle).

real signal (mri) In-phase component of signaldetected with a quadrature detector.

real-time (us) Imaging with a rapid-frame-sequence display.

real-time display (us) Employing a sufficientframe rate that the display appears to imagemoving structures or a changing scan planecontinuously.

receiver (mri) Portion of the MR apparatus thatdetects and amplifies RF signals picked up bythe receiving coil includes a preamplifier,amplifier and demodulator.

receiver bandwidth (mri) Describes the fre-quency content of the pixel in MRI and dependson the number of phase/frequency encodingsteps N and signal sampling time t whichdecides matrix dimensions:

Bandwidth � N/t

receiver coil (mri) A coil or antenna which picksup the MR signal.

receiver dead time (mri) Time after exciting. RFpulse during which FID is not detectable due tosaturation of receiver electronics.

receiver gate (us) A device that allows only echoesfrom a selected depth (arrival time) to pass.


receiver operating characteristic (ROC)–reference cassette

R

receiver operating characteristic (ROC) (image)A method for analysing data that takes note ofthe operator’s skill or bias. It is a plot of the con-ditional probability of deciding that an observeddata set (e.g. image) was generated by a speci-fied state (e.g. that a specified disease was pres-ent) when that state was in fact present (truepositive; TP) versus the conditional probability of deciding that the data were generated by thespecified state when, in fact, it was absent (falsepositive; FP). This is equivalent to a plot of the‘sensitivity’ of a diagnostic test versus oneminus the ‘specificity’ of the test. Observersreport findings with a range of responses fromdefinitely abnormal through various grades ofequivocal findings to definitely normal. Thesensitivity and specificity of an imaging systemencompassing this range can be described visu-ally by applying ROC analysis. Different pointson the ROC curve (i.e. different compromisesbetween TP and FP or between ‘sensitivity’ and‘specificity’) are achieved by adopting differentsettings of the critical value of the decision vari-able that distinguishes ‘negative’ decisions from‘positive’ ones, i.e. the decision criterion.

■ Reference: Metz CE, 1986 (see contrast detail

diagram).

receptor binding (nmed) A mechanism by whichthe radiopharmaceutical binds to receptor siteson tumours (e.g. imaging of a wide variety oftumours containing somatostatin receptor sitesusing mm OctreoScan, a somatostatin analogue).

Improvement

Random choice

1.0

1.0

0.8

0.8

0.6

0.6

0.40.2

0.4

0.2

0.0

0.0

Tru

e po

sitiv

e fr

actio

n

False positive fraction

0.00.20.40.60.81.0

Specificity

receptor site (PET) (nmed) Regions in the centralnervous system that are associated with druglocalization (morphine, DOPA, etc.). Achieving aspecific activity of 105 to 107 mCi (4 to 40 TBq)per mole, it has been estimated that tissue con-centrations can be imaged having 10�11 to10�13 mole per gram or 0.04 to 4.0 ng per gramof material. (see neurotransmitters (PET), metabo-lites (PET)).

recessive (dose) In genetics, a trait that does notmanifest itself in the presence of traits that aredominant to it.

recoil electron (phys, nm, dose) The path takenby the free electron after a Compton interaction(see linear scatter coefficient).

reconstruction algorithm (ct) Mathematical pro-cedure used to convert raw data into an image.Different algorithms are used to emphasize,enhance or improve certain aspects of the data.

reconstruction increment (ct) The spacing ofimages reconstructed from spiral CT data setsalong the z-axis.

reconstruction matrix (ct) The array of rows andcolumns of pixels in the reconstructed image.

rectifier/rectification (phys) Conversion from analternating current (reversing) to a direct current(one-way) supply. AC rectification is achievedby using one-way current devices (diodes)which, in early machines used thermionic emis-sion but in present day equipment use semi-conductors. These rectifiers only allow passageof current in one direction so their output has asingle polarity. A half wave rectifier uses a sin-gle diode and is 50% efficient since it does notutilize the negative half of the AC waveform. A full-wave rectifier uses four diodes and is100% effective in utilizing all the AC power.

rectilinear co-ordinates Position geometry usedby a linear sequenced array.

recursive filter (di) The output from a recursivedigital filter depends on one or more previousoutput values as well as on inputs; it involvesfeedback (see averaging filter).

reduced matrix (mri) Measurement time is savedby not acquiring the high spatial frequencies(high resolution) raw data lines. Rows that arenot measured are filled with zeroes prior to theimage calculation (zero filling); corresponding toan interpolation in the phase-encoding direction,so a square image is still displayed on screen.

reference cassette (qc) The identified cassettethat is used for the QC tests.

R


reference compound–refractive index

reference compound (mri) Used as a standard ref-erence spectral line when defining chemicalshifts for a given nucleus. For 1H the reference istetramethylsilane (TMS) and for 31P it is phosphoricacid. For some biological applications, water andPCr have been used as secondary references forhydrogen or phosphorus spectroscopy, respec-tively. The reference compound can be in a cap-sule outside the subject (external) or can be inthe subject (internal): internal references aregenerally preferable where possible.

reference date (generator) (nmed) Radio-nuclide generators are sized according to theiractivity on a specified reference or calibrationday. This is always quoted with the generatorspecification. From the calibration graph, activ-ity levels at elution are known and a specificgenerator reference activity on a specified daycan be chosen (e.g. 11, 15, 30 GBq on Mondayor Thursday) to suit requirements over theworking week.

reference exposure (qc) The exposure of thephantom to provide an image at the referenceoptical density.

reference image (mri) Post-processing templatefor defining reconstruction methods, i.e. MIP or MPR.

reference individual (dose) An idealized humanwith characteristics defined by the ICRP for thepurpose of radiological protection.

reference man (nmed) A model with the anatom-ical and physiological characteristics of anadult male as defined in ICRP Publication 23(ICRP Publication 23, 1975) (see standard man).

16

14

12

10

8

6

4

2

00 1 2 3 4 5 6 7

7GBq (200mCi) Generator

Time (days)

Act

ivity

(G

Bq)

Delivery date

Reference date

reference optical density (qc) The optical den-sity of 1.0 OD, base and fog excluded, measuredin the reference point.

reference person (dose) A person with theanatomical and physiological characteristicsdefined in the report of the ICRP Task Group onReference Man (Publication 89; ICRP Publication89, 2001).

reference phantom (qc) A phantom similar to thestandard phantom, but of a different statedthickness.

reference point (mamm) A measurement positionin the plane occupied by the entrance surface ofa 45 mm thick phantom, 60 mm perpendicular tothe chest wall edge of the table and centeredlaterally.

reference value (dose) The value of a quantityobtained for patients which may be used as aguide to the acceptability of a result (used in CEC1990). In the 1995 version of the Quality CriteriaDocument it is stated that the reference valuecan be taken as a ceiling from which progressshould be pursued to lower dose values in linewith the ALARA principle. This objective is alsostated to be in line with the recommendation inparagraph 180 of ICRP Publication 60 that con-sideration be given to the use of ‘dose con-straints or investigation levels’ for application insome common diagnostic procedures. Referencevalues may be specified to a greater degree ofprecision than that which would be chosen toreflect the certainty with which the value isknown, in order to avoid the accumulation ofrounding errors in a calculation.

reflection (non-specular) (us) Reflection ofultrasound from a rough surface.

reflection (specular) (us) Reflection of ultrasoundfrom a smooth surface (see angle of reflection).

reflection angle (us) Angle between thereflected sound direction and a line perpendi-cular to the media boundary.

reflector (us) Medium boundary that produces areflection; reflecting surface.

reflector speed (Doppler) (us) See Doppler (reflec-

tor speed).refocusing (mri) See spin echo.refraction (us) The change in the direction of a

wave front when passing from one medium toanother (see Snell’s Law).

refractive index (us) The product of the sine ofangle of incidence to the sine of the angle ofrefraction (see Snell’s Law).


refresh rate–remote boot

R

refresh rate (comp) An indication of how fast thegraphics card will refresh the computer video dis-play; the rate at which video monitors/displaysare refreshed to prevent image fading. Measuredin Hz. Typical values are from 56 to over 100 Hz.A minimum of 75 Hz is required to minimize display flicker. Rates above 80 Hz are preferable.

refrigeration (refrigerator) (mri) System foractively cooling structures in a superconductingmagnet. If only cryoshields are cooled (two-stagerefrigerator), no liquid nitrogen will be neededand He boil-off will be reduced. If, additionally,the superconducting coil support is activelycooled (three-stage refrigeration) the helium con-sumption can be essentially reduced to zero.

region of interest (ROl) (ct) A circular, rectangu-lar region, operator selected, identifying anarea of measurement of of anatomical interest.Based on the ROI local statistical informationcan be calculated from the CT numbers. (nmed)Used for identifying whole organs (kidneys) orspecific regions (lung transit studies). Havingselected a single or series of ROIs, a total countover these regions for all the frames collectedin the study is made. The results are then pre-sented as a time/activity curve which is therenogram in the case of kidney studies.

registered images (image) Two or more imagesof the same scene that have been positionedwith respect to one another so that the objectsin the scene occupy the same positions.

regression (stats) A test which calculates thebest fit for a straight line through a set of pointson a graph when y is the dependent variableand x the independent (see least squares).

rejection (us) Elimination of small-amplitudevoltage pulses.

relative atomic mass (r.a.m.) (phys) Replacesatomic weight. The relative mass of an atomdepends on the isotope mix. For gadoliniumwith eight stable isotopes the average value is157.25; for aluminium with only a single stableisotope it is exactly 27.

relative biological effectiveness (RBE) (dose)The relationships between different ionizingradiations to produce the same biological effectdepends on the subjective measurement orranking of the precise effect itself (LD50, DNAdamage, cell transformation, etc.). An attempt togive an accurate scaling factor to the radiationwas the RBE. It compares the absorbed doses ofdifferent ionizing radiations required to give the

same biological damage. The RBE is now calledthe radiation weighting factor (wR) in ICRP60 (seeweighting factor (radiation)).

relative density (unit) Relative mass density (seerelative volumic mass).

relative life lost (dose) Ratio of the proportion ofobserved years of life lost in an exposed popu-lation dying of disease to the correspondingproportion in a similar control population with-out the exposure.

relative molecular mass (r.m.m.) (unit)Formally called molecular weight. The ratio ofthe mass per molecule of an element or com-pound to one twelfth of the mass of one atomof 12 C. For naturally occurring forms of thematerial, usually involving a mixture of iso-topes, the ratio applies to the average mass permolecule (see relative atomic mass (r.a.m.)).

relative permeability (phys) See permeability.relative risk (stats) See risk (relative).relative survival (dose) Ratio of the proportion of

cancer patients who survive for a specifiednumber of years (typically 5 years) followingdiagnosis to the proportion in a control set ofcancer-free individuals.

relative volumic mass (unit) (also relative den-sity, relative mass density) The dimensionlessratio of the mass of a volume of an object.

relaxation frequencies (us, mri) Frequency ofmaximum absorption in a medium.

relaxation rates (mri) Reciprocals of relaxationtimes T1 and T2 (R1 � 1/T1; R2 � 1/T2). Thereis often a linear relation between concentrationof MR contrast agents and the resulting changein relaxation rate.

relaxation times (mri) After excitation by the RFpulse, net nuclear spins M will tend to return totheir equilibrium state where transverse magneti-

zation Mxy is zero and longitudinal magnetization

Mz is maximum and in the direction of the mag-netic field orientation. The transverse magneti-zation relaxation decays toward zero (timeconstant T2) and longitudinal relaxation returns toequilibrium with time constant T1.

rem (dose) (roentgen equivalent man) The non-SIdose equivalent. The absorbed dose in rads multi-plied by the radiation quality factor. The SI unit isthe sievert where 100 rem � 1 Sv; 100 mrem �1 mSv or 1 mrem � 10 μSv.

remote boot (comp) A firmware-based programin a network adapter that asks to have theworkstation’s operating system downloaded

R


remote boot–resistance

from a boot server on the network; used bynetworked diskless workstations.

remote execution (comp) The ability to run pro-grams on remote systems; exporting time con-suming processes to other systems frees up thelocal workstation. An example would be animage array processor.

renal (contrast medium) (cm) Intravenous urog-raphy using non-ionic dimer iodixanol, non-ionic monomer iohexol.

renal insufflation (clin) Obsolete techniqueusing air or carbon dioxide to give negativecontrast for visualizing the adrenal glands.

Reno-60® (cm) Commercial preparation (Bracco)of ionic salt meglumine diatrizoate.


Meglumine 6.4 @ 20° 1404 282diatrizoate 4.3 @ 37°60%

Renocal® (cm) (Bracco) A mixture of meglumine dia-trizoate and sodium diatrizoate; 66% and 10%.


Meglumine 15.0 @ 20° 1870 370diatrizoate 9.1 @ 37°

RenoCis® (nm) Commercial preparation(Schering) of DMSA.

Reno-DIP® (cm) Commercial preparation (Bracco)of ionic salt meglumine diatrizoate.


Meglumine 2.0 @ 20° 607 141diatrizoate 1.5 @ 37°

Renografin® (cm) Commercial preparation (Bracco)of ionic salt and sodium/meglumine diatrizoate,52%:8%.


Diatrizoate- 6.2 @ 20° 1450 292Na-meglumine 4.2 @ 37°52%

renogram (nmed) An example of a dynamic studyusing multi-phase acquisition where (forinstance) the vascular phase is collected as

30 frames at 0.5 s, GFR phase as 30 at 1 s andexcretion phase as 30 at 20 s.

repeater (comp) A device that regenerates andamplifies signals to create long-distance networks.

repetition time (TR) (mri) The time between two excitation pulses. In the case of the spinecho sequence it is the time between the 90°pulses. Within the TR interval, signals may beacquired with one or more echo times, or one ormore phase-encodings (depending on themeasurement technique). TR is one of themeasurement parameters that determines con-trast, largely influencing T1 contrast (see echo

time, T1 weighting).rephasing (mri) Reversal from dephasing; the

spins go back into phase. Achieved through a 180° pulse that creates a spin echo, or a gradient pulse in the opposite direction.

rephasing gradient (mri) Magnetic field gradientapplied for a short period after a selective exci-tation pulse opposing the direction of theselective excitation gradient; a magnetic fieldgradient pulse applied to reverse the spatialvariation of phase of transverse magnetizationcaused by a dephasing gradient. The gradientreversal rephases the spins forming a gradientecho. The result of the gradient reversal is arephasing of the spins (which would havebecome out of phase with each other along thedirection of the selection gradient).

reproducibility (math) or consistency. The relia-bility expressed as accuracy of measurement.Several readings are taken at timed intervalsfrom a constant source and the percentageerror between the readings calculated. Theresults are expressed as a percentage error fora single machine or if a number of machinesare being assessed the error range is given aspercentiles (25th, mean, 75th). (qc) Indicatesthe reliability of a measuring method or testedequipment. The results under identical condi-tions should be constant (see precision).

RESCOMP (mri) Respiratory compensation GE.Respiratory ordered phase encoding (see RSPE,PEAR, FREEZE).

residual dose (dose) Applied to chronic exposure.The dose expected to be incurred in the futureafter intervention has been terminated (or adecision has been taken not to intervene).

resistance, φ (phys) Defined by the proportional-ity between the voltage across a conductor andthe current flowing in it: V � IR. The unit of


resistance–resolution (temporal)

R

resistance is the ohm φ. One ohm maintains acurrent of one amp at one volt so that: R � V/I.Variations of this basic formula are: I � V/R andV � IR. If a is the cross sectional area of a con-ductor then providing the length L is constantits resistance is proportional to cross-sectionalarea. So that:

If the diameter of the wire is doubled (increas-ing the area a) then resistance decreases by ¼.Also it determines the power, P, consumed asheat in the conductor, P � IV. In NMR, magnetsand resistance of the windings limit the currentthey carry and thereby the strength of the Bfield they can produce.

resistance (flow) (us) Pressure difference dividedby volume flow rate for steady flow.

resistive magnet (mri) See magnet (resistive).resolution (ct) See geometrical resolution, high con-

trast resolution, low-contrast resolution.resolution (axial) (us) See ultrasound (resolution).resolution (display) (comp) See flat panel display,

line pairs.resolution element (mri) The smallest spatially

resolved image element. It may be isotropic (thesame x, y and z dimension), or anisotropic wherex and y dimensions are different; the slice thick-ness z does not match one or other of the x, ydimensions. The resolution element may belarger than the pixel or voxel and is dependent onsampling theory.

resolution (energy) (phys) The FWHM dimensionof a photopeak, the energy spread at this levelexpressed as a percentage of the peak energy.Typical value for a single NaI(Tl) detector wouldbe 8% for most current gamma cameras(140 keV gamma). (us) The FWHM dimension ofa photopeak, the energy spread at this levelexpressed as a percentage of the peak energy.Typical value for a single NaI(Tl) detector wouldbe 8%. A gamma camera value would be 11%.

resolution (extrinsic) (nmed) This is a measureof the camera system resolution with the colli-mator in place, frequently referred to as thesystem resolution Rs, this combines the effectsof intrinsic resolution (Ri), selected collimatorresolution (Rc), image magnification (M), so that:

R R R MS C i� �2 2( / )

RL

a 2

Measured in mm. Typical high resolution collima-

tor gives 5 mm FWHM.resolution (intrinsic) (nmed) The resolving

power of the gamma camera system withoutany collimation is obtained by exposing theuncovered crystal to a point or line source ofactivity. In practice, the crystal is covered by a lead-sheet in which fine points or a thin sin-gle line has been cut. Intrinsic resolution isquoted with count rate; typical values are 5 mmat 75 k cps and 5.7 mm at 150 k cps.

resolution (lateral) (us) Resolving power acrossthe beam. Depends on focal dimensions of the beam influenced by aperture size and electronic focusing.

the minimum distance required by a display toresolve two point sources.

Frequency Image depth Axial res. Lateral res.

2.0 30 0.7 3.03.5 17 0.4 1.75.0 12 0.3 1.27.5 8 0.2 0.8

10.0 6 0.15 0.6

resolution (spatial) (image) See spatial resolution.resolution (system) Rs (nmed) This combines the

effects of intrinsic resolution (Ri), selected colli-mator resolution (Rc), image magnification (M),so that:

resolution (temporal) (math) The ability to sep-arate to events in time. This depends on image

R R R MS C i� �2 2( / )

Lateral resolution Raλ

Side view Endview

Electronicfocus (lateral)

Acousticlens focus

R


resolution (temporal)–restricted area

matrix size and storage speed for each imageframe.

resolution (visible) (image) Maximum visual res-olution is approximately 30 Lp mm�1 but at dis-tance this is reduced to 15 Lp mm�1. The acuityof scotopic vision is highest approximately 20°from the fovea, where the density of rods isgreatest; for photopic vision it is highest on thefovea where the cones are most dense. Photopic

acuity is always greater than scotopic. Thehuman eye is most sensitive to variation in theintensity of objects with a spatial frequency ofbetween 6 and 10 Lp mm�1.

resolution element (mri) The smallest spatiallyresolved image element. It may be isotropic (thesame x, y and z dimension), or anisotropicwhere x and y dimensions are different, or theslice thickness z does not match one or other ofthe x, y dimensions. The resolution elementmay be larger than the pixel or voxel and isdependent on sampling theory.

resonance (phys) The frequency of the undampedwaveform or the system’s natural frequency.When the input or forcing frequency is equal tothe natural frequency of the system then reso-nance occurs producing the greatest power out-put. Of practical use in radiology where anoscillating system (e.g. ultrasound transducercrystal or tuned MRI inductive circuit) has thesame frequency as that of the driving force(electrical pulse). Considerable energy isabsorbed at resonant frequency from the sys-tem supplying the external force. The two reso-nance signals in the graph show a strongundamped resonant signal together with aslightly damped signal of lower amplitude.

�10 �5 0 5 10

resonant frequency (phys) The frequency atwhich resonance will occur. Determined bymechanical size in ultrasound, by conductanceand inductance components of an electroniccircuit or given by the Larmor equation in NMR.

Resovist® (cm) Ferrous based MRI agent (Schering)containing 28 mg iron as ferucarbotran, a super-paramagnetic iron oxide in the form of nanopar-ticles coated with carboxydextran.

respiratory gating (mri) Synchronization ofpatient’s breathing with data acquisition.Technique for reducing respiratory artefacts. A respiratory signal acquired with suitable sen-sors or MR methods (navigator echo) is used asthe trigger signal.

respiratory ordering of phase encoding (mri)Respiratory synchronization that acquiresimage data at regular times independent of therespiratory cycle, but chooses the sequence ofphase encoding data acquisition so as to mini-mize the respiratory motion-induced artefactsin the resulting image. For example, choosingthe sequence of phase encoding such thatadjacent samples in the final full data set haveminimal differences in respiratory phase willminimize the spacing of ghost artefacts in thefinal image.

RE spoiled (mri) FAST RE spoiled Fourier-acquired steady-state technique rapid gradi-ent-echo imaging techniques T1-weightedcontrast (Picker Medical Inc.) (see FLASH, SPGR,FSPGR, HFGR, 3D-ME-RAGE, T1-FEE, STAGE-T1W).

REST (mri) Regional saturation technique Philips,motion artefact reduction techniques, spatialpre-saturation to reduce MR signal intensity inspecific locations (see SAT, PRE-SAT, PRESAT,EFAST, SATURATION).

responsivity (R) (image) This is a measure of thesensitivity of a detector to radiation and is the ratiobetween the radiative flux φ incident on the detec-tor area and the resulting signal s, so that s � Rφ.A good detector shows a constant responsivity, R,over a wide flux range so that the signal output isproportional to the input flux.

reproducibility (qc) Indicates the reliability of ameasuring method or tested equipment. Theresults under identical conditions should beconstant.

restricted area (dose) A limited access areawhere exposure to individuals may occur. Thelimits are stated in national and local radiationsafety rules.


retrograde–RIMM

R

retrograde (clin) Counter or backflow.retrograde angiography (clin) Imaging sections

of blood vessels lying upstream of the injectionor catheterization point and introducing contrastmedium in a direction counter to the blood flow.

retrograde pyelography (clin) or cystoscopicurography. See retrograde urography.

retrograde urography (clin) Radiography of theurinary tract following injection of contrastmedium directly into the bladder, ureter or renalpelvis.

retrospective gating/synchronization (mri)Simultaneous acquisition of untriggered dataand the ECG signal. The ECG signal is used dur-ing subsequent post-processing to assign theimages to the correct phase in the cardiaccycle. Can also be used for pulsatile flow (seesynchronization, retrospective).

Reynold’s number (Re) (us) A dimensionlessvalue used as an indication of laminar flow.Mean fluid velocity (v), vessel radius (r), density(ρ) and viscosity (η).

This formula indicates whether flow will belaminar or turbulent. Typically, values above1200 indicate turbulent flow.

Reynolds number (critical) The Reynolds num-ber above which turbulence occurs.

RF antenna (mri) See RF coils.RF coils (mri) These act as antennas which transmit

RF pulses and/or receive MRI signals. Transmittercoils should excite the protons/nuclei in the vol-ume of interest. Receiver coils should receive theMRI signal with a high Q. The signal strengthdepends on the excited volume measured in thecoil and its distance to the measurement object.Noise depends primarily on the coil size.

RF (radiofrequency) pulse (mri) An impulsecomprising 2–3 wavelengths of radio-frequency delivered by an RF transmitter. If theRF pulse is at or near the Larmor frequency itwill rotate the macroscopic magnetization vec-tor into the rotating frame of reference (transverseplane). The amount of rotation depends on thestrength and duration of the pulse. A 90° pulse(π/2) and 180° pulse (π) are used together withintermediate flip angles.

RF-FAST (mri) RF spoiled Fourier acquired steadystate.

Re �r ⋅ ⋅ν ρ

η

RF shield (mri) Efficient shielding against radio-frequency interference from:

● computers and their display units;● radio-pagers;● radiotransmitters;● television transmitters.

Harmonic frequencies from these sources may be a problem even though their primarytransmit frequency is above the bandwidth ofinterest.

RF spoiling (mri) Preventing a steady state pre-cession occurring in rapid excitation sequenc-ing by using varying phase or timing of the RFpulses.

rhenium (Re) (elem)

Atomic number (Z) 75Relative atomic mass (Ar) 186.2Density (ρ) kg/m3 20 500Melting point (K) 3450Specific heat capacity J kg�1 K�1 137Thermal conductivity W m�1 K�1 47.9K-edge (keV) 71.6Relevance to radiology: used as an alloy with tungstenfor x-ray anodes.

186Rhenium

Decay scheme 186Re (β� 1.0 MeV, γ137 kev) →(β�) 186Re 186W stable

Half life 3.7 daysDecay mode β� 1.07 MeVDecay constant 0.1872 d�1

Emission β� 1070 keV (max) γ 137 keVUses: Rhenium [186Re] sulphide used as a therapy/palliative agent in cases of synovectomy.

rhodium (Rh) (elem)

Atomic number (Z) 45Relative atomic mass (Ar) 102.91Density (ρ) kg/m3 12 440Melting point (K) 2230Specific heat capacity J kg�1 K�1 242Thermal conductivity W m�1 K�1 150K-edge (keV) 23.2Relevance to radiology: as a target and K-edge filtermaterial for mammography

RICE (mri) Rapid imaging using composite echo,the FSE/RARE technique used by Toshiba.

RIMM (comp) RAMBUS inline memory module.Also known as RDRAM, this is the latest type ofmemory that has the potential to run at up to800 MHz as opposed to SDRAM’s 100 MHz.Speed is also increased by processing instruc-tions 16 bits at a time.

R


ring artefacts–rod artefacts

ring artefacts (ct) Circular artefacts, usuallyfound in third-generation scanners, caused byfaulty calibration or a defect in detector func-tion. (nmed) Or bullseye artefact A concentricring artefact in SPECT of alternating high- andlow-count densities that results from a gammacamera with inadequate field uniformity.

ring detector (ct) Fan beam scanner (4th genera-tion) with detector elements positioned on a fullcircle and only the x-ray tube rotating aroundthe object.

ring-down artefact (us) An artefact resultingfrom a continuous stream of sound emanatingfrom an anatomic site.

ring topology (comp) A network cabling configu-ration in which each system is connected in aseries, forming a closed loop.

ripple (xray) A superimposed AC waveform (typi-cally 50 or 60 cycles) on the rectified DC powersupply. Seen as maximum interference in sin-gle phase half-wave rectified supplies and at aminimum (�1%) in constant potential or highfrequency supplies.

RISA (nmed) Radio-iodated human serum albu-min as 125I-RISA for plasma volume estimation.Available as Isojex®.

RISC (comp) Reduced instruction set computer. A type of CPU which can reduce to a minimum thenumber of instructions that are processed simul-taneously which increases the CPU efficiency.

rise rate (mri) See slew rate.rise time (mri) The time required for the gradient

field to rise from zero to the maximum value.risk (dose) The probability of injury, harm or dam-

age from a hazard; usually radiation.risk (assessment) (dose) The risk of cancer per

unit dose is calculated by comparing an irradi-ated population group with an identical unex-posed group then:

Risk factorincreased cancer incidence

population dose�

�

The population size studied increases as thesquare of the reduction in dose. A 5 mSv expo-sure study would require 5 � 108 people, how-ever, the response is assumed to be linear so5 mSv will give a risk of 1 : 4000 (see risk (nomi-

nal), relative risk).risk (nominal) (dose) Since there is still uncer-

tainty in the risk estimates given in ICRP60, theprobability of a fatal radiation induced cancer istermed the nominal risk coefficient. The nomi-nal risk for an adult is:

4 � 10�2 Sv�1 (1:20) or 1:25000 mSv�1

For the whole population (of all ages):

5 � 10�2 Sv�1 (1:20) or 1:20000 mSv�1

Support for these risk factors comes from theNational Registry for Radiation Workers (UK)which covers 95 000 occupationally exposedworkers (see risk (assessment), relative risk).

risk factor (dose) The probability of cancer orhereditary damage per unit equivalent dose oreffective dose. The risk factor is measured asper Sievert, Sv� 1 (see risk (nominal), relative risk).

risk (relative) (dose) Cancer risk may relate tothe spontaneous risk or natural incidence ofcancer in a given population. The absolute riskmodel where the additional risk associatedwith radiation exposure is independent of thespontaneous/natural risk, contrasted with therelative risk model where the additional risk isproportional or related to the spontaneous risk.

RMS (math) See root mean square.road mapping (xray) Subtraction of a contrast

filled reference image continuously from thedisplay to reveal catheter placement (see digital

subtraction angiography (DSA)).ROAST (mri) Resonant offset averaging in the

steady state. Steady-state free precession com-monly used for imaging of cerebrospinal fluid,Siemens (see: SSFP, DE-FGR, CE-FAST, True-FISP,PSIF, T2-FEE, E-SHORT, STERE).

Roberts, John Eric (1907–1998) British med-ical physicist. Pioneer of medical dosimetry.Founder member of the British HospitalPhysicists’ Association.

ROC (di) See receiver operating characteristic.rod artefacts (ct) If a spiral scan of a cylinder or

rod is made, angulated with respect to the scanplane, while the table holding the rod is movedduring acquisition the rod in the scan planechanges. Every subsequent projection locates

Risk assessmentIf 500 000 people are exposed to 500 mSv over theirlifetime and show a increased cancer incidence of5000 then the risk is:

This cancer increase of 1 in 50 is due to radiation at this level. There is an expected natural cancer incidence of 1:7 of this population.

5000500000 0 5

0 02 2 1012

2 1

��

.. Sv


rod artefacts–rotating frame zeugmatography

R

the rod at a different position. Without tablemovement, i.e. in a conventional scan, theimage would show an ellipse. With table move-ment, the variation in the registered position ofthe rod, together with the interpolationscheme, gives a distortion of the ellipse. Theserod artefacts are especially seen in the liver inthe area of the ribs.

ROI (di) Region of interest. A defined area, eitherby the operator or automatically, of a region ona digital image for subsequent detailed analy-sis (time/activity curve etc.).

Rollins, William Herbert (1852–1929) Americanclinician/dentist who first advocated the use ofcollimating diaphragms and shielded x-ray tubehousings in 1898. Introduced the concept of radi-ation protection for diagnostic radiology. He alsointroduced the idea of double intensifyingscreens to increase sensitivity. He suggested apulsed supply to the x-ray tube in order to reducedose during fluoroscopy and also selective beamfiltration before Pfahler published the idea.

ROM (comp) Read only memory. A part of memorycontaining a permanent program sequence (typ-ically in the form of special chips) for essentialstart-up procedures (boot-up and disk read rou-tines). ROM contains only permanent informationput there by the manufacturer. Information inROM cannot be altered, nor can the memory bedynamically allocated by the computer or itsoperator. These data are not lost when the com-puter is switched off.

Röntgen/Roentgen, Wilhelm, Konrad(1845–1923) German physicist who in 1895discovered a form of electromagnetic radiationhe called x-rays. Awarded the first Nobel prizein physics in 1901.

röntgen/roentgen (R) (dose) The non-SI unit ofexposure mainly applied to x-rays defined as2.58 � 10�4 coulombs kg�1 of air. Replaced bythe gray (Gy). The energy absorbed in tissuefrom 1 R is 0.0095 J kg�1; equivalent to approx-imately 0.87 rad or 8.7 mGy; 1 mR � 8.7 μGy.

room shielding (mri) See magnetic shielding. (dose)See shielding.

root mean square (RMS) (image) The value ofalternating current or voltage which would dis-sipate heat at the same rate in a given resistancewhich for direct current is given by: W � I2 �

R � t. For an AC waveform the electrical energyis not converted into heat energy at a constantrate. The sine wave for a mains supply shows

maximum energy when the current is maximumand zero when the current is zero. Then a meanvalue is derived called the root mean square(RMS) value. The RMS value of an AC powersource, restating the above definition, is simplythat value of DC which produces the same poweror heating effect. For any sinusoidal waveformthe RMS value is:

Alternating supply (mains a.c.) is quoted as anr.m.s. value, so 220 V a.c. has a peak voltage of311 V and 115 V a.c. a peak of 162 V. Thequoted mains supply is always given as itsRMS value, the peak voltages are obtained bymultiplying by (or 1.414).

RMS value Peak value

115 162 (USA)220 311 (Europe)240 339 (UK)

ROPE (mri) Respiratory ordered phase encoding.Respiratory compensation technique duringMRI data collection.

rotating anode (xray) An x-ray tube having a cir-cular rotating anode usually angled at theperiphery. The periphery acts as the x-ray target,so the position of the focal spot on the targetsurface changes continuously (see target (x-ray)).Since the target area is bombarded by the elec-

tron beam during a small time window the tubeloading can be significantly higher than that forstationary targets.

rotating frame of reference (mri) A frame of ref-erence (with corresponding coordinate sys-tems) that is rotating about the axis of the staticmagnetic field Bo (with respect to a stationaryframe of reference) at a frequency equal to thatof the applied RF magnetic field B1 (RF pulse).Although B1 is a rotating vector, it appears sta-tionary in the rotating frame, leading to simplermathematical formulations.

rotating frame zeugmatography (mri) Techniqueof MR imaging that uses a gradient of the RFexcitation field (to give a corresponding variationof the flip angle along the gradient as a means ofencoding the spatial location of spins in thedirection of the RF field gradient) in conjunctionwith a static magnetic field gradient (to give

2

peak value2

R


rotating frame zeugmatography–82Rubidium

spatial encoding in an orthogonal direction). Itcan be considered to be a form of Fourier trans-form imaging.

rotation time (ct) This is defined as the timetaken for the tube/detector system to rotate360° around the patient. This can be selectedbetween 0.8, 1.0, 1.5, 2.0 and 3.0 seconds inmodern scanners. If rotation time is decreased,spiral coverage/body length, increases and viceversa. With shorter rotation times, thinnerslices, for the same volume, can be acquired inthe same amount of time. Selecting a 100 mmanatomical range using a 10 mm slice, pitch 1(10 mm feed/rotation); and a rotation time of1 s, will take a scan time of 10 s. Decreasing therotation time to 0.8 s will reduce this to 8 s tocover the same range. Shorter rotation timesare chosen with thinner slice collimation, sincethe same range can be covered in the sametime; however, x-ray tube current may need tobe increased to maintain the same mAs andmaintain image quality (noise).

rotor (xray) An integral part of the anode stemmaking up the induction motor. Made from cop-per it is attached to the anode by a molybde-num stem. This revolves about a central axlewhich forms the positive electrode (�75 kV).The axle bearing consists of either ball-races,lubricated by a silver paste, or sleeve bearingsalso lubricated by a high temperature paste.Excessive heat transfer along the anode stem isrestricted since molybdenum has relatively poorheat conductivity. Additionally, sleeve bearingshave an oil circulation path which passes alongthe axle. Larger disc diameters require bettersupport and the anode stem in these tubes iscarried forward and supported by its own bearing, giving support both front and rear.

round-off error (di) Computational inaccuracyindependent of truncation error whereunwieldy numbers (e.g. π taken as 3.142 tofour significant figures or three decimal places)are restricted in size. The numbers will be inex-act even if the formula is precise (see truncation).

router (comp) A device used to link multiple LANstogether; connects two networks at the networklayer (Layer 3) of the OSI model. Operates like abridge but also can choose routes through a net-work. It examines packets of data from theInternet and sends them to their appropriatedestination. More effective when multiple pathsare available as it can read the destination

address of each packet of data and determinethe best path for it to take. In a packet-switchingnetwork, such as the Internet, a router is adevice which examines packets of data andsends them on to their appropriate destinations.

routine exposure (xray) The exposure of thephantom under the conditions that would nor-mally be used to produce a mammogram. It isused to determine image quality and dose underclinical conditions.

rows (mri) The phase-encoded portion of themeasurement matrix. Often also a row in thedisplayed image (see Columns).

RS (mri) Rapid scan.RS-232C (comp) Recommended standard 232C.

A standard specifying the connector, pin func-tions and voltages used to connect a DTE andDCE. V.42: An error detection and correctionstandard used by modems that specifies bothLAPM and MNP 4.

RSPE (mri) Respiratory-sorted phase encodingGE. Respiratory ordered phase encoding (seeRESCOMP, PEAR, FREEZE).

rubidium (Rb) (elem)


81Rubidium

Production (cyclotron)Half life 4.6 hDecay mode e.c. and β�

Decay constant 0.150 h�1

Photon (decays to 81 mKr) Multiple (190 keV)Uses: The parent for 81 mKr generators

82Rubidium (nmed) Generator derived positronnuclide. 82Sr/82Rb (as CardioGen-82®). For theelution of 82Rb chloride. After intravenousinjection clears from the blood and is extractedby the myocardium.

Production (generator) 82Sr (e.c.) → 82RbDecay scheme (β�) 82Rb 82Rb T½ 76s (β�

1.4 MeV, 2γ 511 kev)→ 82Kr stable

Eluent 0.9% (normal) salineDecay constant 0.00911 s�1

Half life 1.25 minutesHalf value layer 4.1 mm Pb (511 keV)


Rubratope®–Rutherford, Ernest (1871–1937)

R

Rubratope® (nmed) 57Cyanocobalamine prepara-tion for Schilling Test (Bracco).

RUFIS (mri) Rotating ultra-fast imaging sequence.run length encoding (comp) This algorithm is

based on the observation that image areaswhich contain little detail have neighbouringpixels with the same value, so these locationscan be stored as a pixel value and pixel range –a ‘run length’. Storage area for the image canbe considerably reduced since radiographscontain large areas of black background whichcan be omitted (see compression (image)).

ruthenium (Ru) (elem)


Rutherford, Ernest (1871–1937) New Zealandborn physicist who in 1895 started work at theCavendish Laboratory, Cambridge University,UK, where he discovered the three types ofuranium radiations, alpha, beta and gamma. In1898 he became professor of Physics at McGillUniversity, Montreal where he formulated thetheory of atomic disintegration. In 1907 hebecame Professor of Physics at Manchesterwhere, with Geiger and Marsden, he investi-gated the nucleus by bombarding gold foil withalpha particles and derived the orbital modelfor the atom. He also discovered that alphabombardments of nitrogen liberated hydrogennuclei. In 1919 he was appointed to theCavendish Laboratory, suceeding J.J. Thomson.He predicted the existence of the neutron laterdiscovered by Chadwick. He was awarded theNobel Prize for Chemistry in 1908.

S


S-value–sampling theorem

SS-value (dose) MIRD parameter describing the

mean dose per unit cumulated activity. Mostdose calculations using MIRD techniques usetabulated values of the S-value.

SAAV (mri) Simultaneous acquisition of artery andvein.

saddle coil (mri) RF coil design commonly usedwhen the static magnetic field is coaxial withthe axis of the coil along the long axis of thebody (e.g. superconducting magnets and mostresistive magnets) as opposed to solenoid orsurface coil.

safety (mri) Safety concerns in MR include mag-netic field strength, RF heating induced currentsdue to rapidly varying magnetic fields (dB/dt)effects on implanted devices such as pacemak-ers, magnetic torque effects on in dwelling metalsuch as clips and possible missile effect of mag-netic forces and acoustic noise (see SAR).

safety (ultrasound) (us) See ultrasound (safety).sagittal plane (ct) Anatomical plane oriented paral-

lel to the symmetry (median) plane of the humanbody, orthogonal to the transverse and the coro-nal plane; in the CT coordinate system the sagit-tal plane is usually oriented parallel to they/z-plane.

(see coronal plane).saline (clin) See isotonic saline.Saloman, Albert German surgeon who first radi-

ographed the breast in 1913.salpingography (clin) See hysterosalpinography.samarium (Sm) (elem)


153Samarium (nmed) Chelated with EDTMP (eth-ylene-diamine-tetramethylene-phosphonate)

(Anterior)Sagittal

(Posterior)

as a bone seeking agent. Beta emission pro-vides palliative properties and the gammaemission allows scintigraphy.

ProductionDecay scheme 153Sm T½ 46.8 hr (β� 803,(β�) 153Sm γ 103 keV) → 153Eu stableHalf life 46.8 hoursDecay constant 0.01474 h�1

Photons (abundance) β� 803 (0.2)700 (0.53)630 (0.26)

γ 70 (0.34)103 (0.77)

Generic name 153Sm-EDTMPCommercial names QuadrametNon-imaging category palliative treatment of bone

painUses: Palliative therapy for bony metastases.

sample size (stats) The population number to beincluded in a trial or investigation. The knownvariance, the population mean and the normaldeviate should be assessed.

sample volume (us) The anatomic region fromwhich pulsed-Doppler echoes are accepted.

sampling (elect) Conversion of the continuous(analogue) signal to a series of discrete (digital)values by measurement at a set of particulartimes: this utilizes the analogue to digital con-verter. If the rate of sampling is less than twicethe highest frequency in the signal, abasingwill occur. The duration of sampling determineshow small a difference of frequencies can beseparated.

sampling distance (ct) Distance between equi-distant points at which a continuous function issampled for measurement (see sampling fre-

quency).sampling frequency (ct) Expressed as the recip-

rocal value of the sampling distance; group ofparameters (detector spacing etc.) which deter-mine projection geometry and distance duringthe CT scan; the most important parameters are the distance between single detector chan-nels (sampling distance), detector quarter shift;effective width of the detector elements andthe angular increment between successive projections.

sampling theorem (math, ct) Defined by Shannon

sampling theory, stating that a continuous, band-limited function can be completely recovered

92235

62153

3080 3U(n, ) Sm( Zn n)f � �


sampling theorem–scan line

S

from a finite number of measured values, if thisfunction does not contain spatial frequenciesabove the Nyquist frequency (see sampling fre-

quency, aliasing).sampling time (mri) Duration over which the MR

signal is measured. Longer sampling time resultsin higher image signal-to-noise.

sampling window (comp) See acquisition window.SA (us) See spatial average intensity.SAPA (us) See spatial average/pulse average.SAR (mri) See specific absorption rate.SAT (mri) A regional saturation pulse sequence for

spatial presaturation to reduce the MR signalintensity in specific locations. A motion artefactreduction technique applied to blood flow imag-ing. Motion artefact reduction techniques. Spatialpresaturation to reduce MR signal intensity inspecific locations (see REST, PRE-SAT, PRESAT,EFAST, SATURATION).

SATA (us) See spatial average/temporal average.satumomab pendetide (nmed) A murine mono-

clonal antibody of the immunoglobulin subclass IgG 1, satumomab (MAb B72.3), local-izes or binds specifically to a tumour-associ-ated glycoprotein (TAG-72), a cell surfaceantigen expressed at high levels on nearly allcolorectal and ovarian adenocarcinomas (seeOncoScint®).

saturation (image) The purity of a colour’s hue ona scale from grey to primary colour. (mri) A non-equilibrium state where equal numbers of spinsare aligned against and with the main magn-etic field; zero net magnetization. Produced by repeated short interval RF pulses comparedto T1; repeat 90° RF pulses with short TR times (see SAT, REST, PRE-SAT, PRESAT, EFAST,SATURATION).

saturation constant (nmed) See radionuclide pro-

duction, saturation limit.saturation limit (nmed) At a certain time, during

irradiation, a production (saturation) limit isreached: the number of atoms being producedis balanced by those decaying.

saturation pulses (mri) A sequence of either 90° orgradient pulses so timed to produce saturation.Usually accompanied by a spoiler pulse. Used forreducing the signals from flowing blood by sat-urating regions upstream from region beingimaged. A sequence of RF or gradient pulsesdesigned to produce saturation, typically in aselected region or set of regions most often by

the use of selective excitation followed by aspoiler pulse.

saturation recovery (mri) A partial saturationpulse sequence where after an RF saturationpulse the timing allows some net magnetiza-tion to establish. A particular type of partialsaturation pulse sequence in which the pre-ceding pulses leave the spins in a state of sat-uration, so that recovery at the time of the nextpulse has taken place from an initial conditionof no magnetization. A method for generatingprimarily T1 dependent contrast through aseries of 90° excitation pulses. Immediatelyafter the first pulse, longitudinal magnetizationis zero because the tissue is saturated. Thenext 90° pulse is not applied until longitudinalmagnetization has recovered. The repetitiontime (TR) depends on the T1 constant of thetissue.

saturation slice (mri) Regional presaturation tosuppress undesired signals for specific areas,either within the slice or parallel to it.

saturation transfer (or inversion transfer)(mri) Nuclei retaining their magnetic orientationthrough a chemical reaction. When RF energyis given to the spins at the chemical shift fre-quency of the nuclei in one chemical state producing saturation or inversion; then anysubsequent chemical reactions transform thenuclei into another chemical state with a differ-ent chemical shift. The NMR spectrum canshow the effects of the saturation or inversion.Used to study reaction kinetics/chemical trans-formations of suitable molecules.

scalars (math) Quantities that are fully specifiedby a statement of purely size. Examples aremass, distance, speed, work and energy (seevectors).

scale (image) In wavelet analysis, a term mean-ing the same as scale in geographical maps,very large scales mean global views, while verysmall scales mean detailed views.

scan converter (elec) A device that stores imag-ing information in one format and reads it outfor display in another.

scan cross-sectional area (us) For auto-scan-ning systems the surface within the beamcross-sectional area during the scan. Measuredas centimetre squared, cm2.

scan line (us) A line produced on a display thatrepresents the echoes from a pulse. The signal

S


scan line–scatter coincidence events

line produced by a transducer element whoselength defines image depth.

scan projection radiograph (SPR) (ct) Genericname for the digital image obtained by linearlytranslating the patient through the gantry aper-ture during an x-ray exposure while the x-raytube remains stationary. The SPR has a similarappearance to a plain radiograph and is usedprimarily for localizing the required region ofscanning. Synonymous terms include radi-ographic mode and localizer image, together withthe proprietary names Pilot scan, Scanogram,Scanoscope, Scoutview, Surview and Topogram.

scandium (Sc) (elem)


scanhead (us) Transducer assembly.scanner (comp) An electronic device that uses

light-sensing equipment to scan paper or filmimages such as text, photos and illustrationsand translate the images into signals that thecomputer can then store, modify or distribute.

scanner (film) (image) A mechanical/optical devicefor digitizing film or paper records. It consists of anintense linear light source, which travels over thefilm or paper surface. Its mirrored reflected signalis superimposed on a linear CCD array havingmore than 2000 cells. For black and white imagesa single light source is employed; for colour infor-mation red, green, blue light sources are used.The resolution offered by current radiographyscanning devices is approximately 3 Lpmm�1 ona 35 � 43cm (14 � 17�) film. Laser film imagerscan achieve a pixel size of 0.08mm giving a reso-lution of 1/(2 � 0.08) � 6Lpmm�1 for this filmsize and 35 mm film scanners give a pixel size of0.0128 mm which delivers 38 Lp mm�1 for thissmall format (see charge coupled device, film for-

matter).scanner geometry (ct) Geometrical arrangement

of the x-ray tube and detector array withrespect to the axis of rotation.

scanogram (ct) See scan projection radiograph.scan plane (ct) The x/y-plane of the coordinate sys-

tem forming the axial image; the scan plane con-tains the central trajectory and is consequently

the plane of rotation of the CT gantry. The gantry

tilt alters the scan plane but in most cases thescan plane coincides with the transverse plane.

scan time (ct) The time interval between the begin-ning and the end of the acquisition of attenuationdata for a single exposure. For some CT scanners,this may be longer than the exposure time due tothe pulsing of x-ray emission.

SCART (comp) An audio/video connector used inconsumer equipment, especially in Europe. TheSCART connector’s 21 pins has two audio in andout channels, in and out video channels, RGB sig-nals, ground and some additional control signals.

scatter radiation (phys) A general term describ-ing both elastic and in-elastic (Compton) eventswithin an absorber. The primary photon isdeflected from its path and in the case ofCompton scattering loses energy to a free elec-tron. (ct) Radiation scattered from the main sec-tion (SSP) produced by interaction of theprimary beam with a material medium. Theinteraction can be characterized by a reductionin radiation energy and/or by a change in thedirection of the radiation. (xray) Radiation scat-tered from the main beam (x-ray or gamma)produced in the interaction of the original radi-ation with a material medium. The interactioncan be characterized by a reduction in radiationenergy and/or by a change in the direction ofthe radiation. (us) See ultrasound (scatter), linear

scatter coefficient, back-scatter.scatter coincidence events (nmed) A scatter

coincidence in positron emission tomography(PET) occurs when one or both γ-photons froman annihilation event are scattered but both aredetected within a single detector ring. Scattercoincidences are seen as true coincidence events,so the correction for accidental coincidencesdoes not compensate for them. Annihilationradiations may undergo scattering while pass-ing through the body tissue, and because of thehigh energy (511 keV), most of these scatteredradiations move in the forward direction with-out much loss of energy to be accepted withinthe energy window of 511 keV and thereforebe counted as true coincidences. The positioninformation of the event is lost. In practice,scattered events constitute a significant frac-tion of the events detected by the system(10–20% of total counts in a ‘2D system’ and40–60% of total counts in a ‘3D system’). PET


scatter coincidence events–sealed source

S

machines using septal collimator rings markedlyreduce scatter events from being collected, dueto improved geometric efficiency.

scattered photon (phys) The path taken by theincident photon after a Compton interaction(see Compton scatter).

scatterer (us) An object that scatters sound becauseof its small size or its surface roughness.

scattering (us) Redirection of sound in severaldirections upon encountering a particle suspen-sion or a rough surface (see specular reflection).

Schilling test (nmed) This, combined with a test for the intrinsic factor, indicates malabsorption ofvitamin B12. 58Co-labelled B12 given orally after afasting period. A flushing dose of non-radioactive(cold) vitamin B12 is then injected. A completeurine collection is made. A dual radionuclidemodification 58Co-labelled B12 and 57Co-labelledB12 attached to intrinsic factor (Dicopac Test,Amersham/ GE Healthcare Inc) distinguishes mal-absorption of B12 due to lack of intrinsic factorand that due to a lesion of small intestine.

scientific notation (math) A numerical form sim-ilar to floating point: a � 10n or aEn where a isa number 1 to 10 and n is a whole number asbefore, e.g. 5.64E–2 or 0.0564.

scintigraphy (nm) General term for imaging aradionuclide distribution usually applied togamma camera studies.

scintillation detector (phys) Radiation detectorsconsisting of inorganic or organic crystals havevalency/forbidden/conduction bands. A scintilla-tor coupled to either a photomultiplier or photo-

diode which gives an electrical signal whoseamplitude corresponds with the energy of theradiation (particulate or photon). Electromagneticradiation or particulate energy (electron beam)allows transition between bands. These form thebasis of luminescence which includes fluorescence,phosphorescence and thermoluminescence (seesemiconductor).

scintillator (phys) An inorganic crystal, organiccompound (plastic) or liquid capable of demon-strating fluorescence when bombarded by parti-cle or photon radiation. (ct) Substance thatemits visible light when exposed to radiation;used in the construction of solid state detectors(see ceramic detector, luminescence, phosphor).

scotopic (clin) Visual process associated withrods as the principle receptors and used partic-ularly with low light levels; colours cannot beidentified (see photopic).

ScoutView (ct) See scan projection radiograph (SPR).screen activators (phys) Mainly rare earth trace

elements added to phosphors to increase efficiency of emission.

screen pigment (film) Pigment added to intensi-fying screen to prevent light diffusion.

screening (population) (clin) Screening a popu-lation for hidden disease (e.g. tuberculosis,breast cancer, cervical cancer, prostate cancer,heart disease, etc.). Normally certain criteria areconsidered before organizing a screening programme:

● The disease is common enough in the population.● The disease has a serious early mortality.● Early detection and intervention will signifi-

cantly improve life expectancy or quality of life.● The aetiology of the disease is understood.● The screening interval can be maintained

shorter than cancer appearance.● The screening test is accurate and safe.● Proven, acceptable and effective treatment can

be readily organized (see lead time bias, predictive

accuracy).

SCSI (comp) Small computer systems interface. Amethod for connecting devices to a PC. A singleSCSI adapter can handle a mix of up to eightdifferent devices such as hard drives, CD-ROMsand scanners. The most recent Ultra3 SCSI sup-ports up to 16 devices (including the SCSIadapter card or chip) and has transfer rates ofl60 MBbps (megabytes per second).

SDRAM (comp) Synchronous DRAM. A type of mem-ory that synchronizes itself with the speed of theCPUs bus and can run at 100 MHz. It is abouttwice as fast as EDO RAM and the latest SDRAMcan run at 133 MHz on PCs that support thisspeed.

SE (mri) Spin echo pulse sequence.Seaborg, Glen T (1912–1999) American nuclear

physicist who synthesized plutonium and dis-covered the fissile nature of Uranium-235achieving the first chain reaction with Fermi inDecember 1942. He synthesized americium andcurium in 1944, then berkelium and californiumin 1950. His team later synthesized einsteinium,fermium and mendelevium. In 1951 Seaborgshared the Nobel Prize for Chemistry withMcMillan.

sealed source (nmed) Radioactive material perma-nently bonded or fixed within a capsule ormatrix designed to prevent release and dispersal

S


sealed source–selectivity

of the radioactive material under the most severeconditions likely to be encountered in normaluse and handling.

SECAM (image) Système électronique couleuravec mémoire. Television transmission stan-dard developed in France providing 625 linesat 50 fps.

second (phys) The SI unit of time defined as theduration of an exact number of periods corre-sponding to the transition between two hyper-fine states in 133Cs. The standard time periodscommonly used are:

Second Minute Hour Day

Seconds 1 60 3600 86 400Minutes 1 60 1 440Hours 1 24

secondary barrier (shld) Any surface receivingscatter or leakage radiation and not subjectedto potential exposure from the primary beam(see primary barrier).

secondary radiation (shld) or scattered radia-tion. Depends on the volume of the patient orobject being exposed to the primary beam.

secondary reconstruction (ct) Image recalcu-lated from a series of raw reconstructed, adja-cent or overlapping CT images; usually asecondary reconstruction is oriented orthogonalto the slice plane (see sagittal plane, coronal plane).

SE-CSI (mri) Spectroscopy hybrid procedurebased on spin echo.

section thickness (ct) The narrowest or focalpoint of a section or slice; thickness of thescanned tissue volume perpendicular to the scanplane; also called slice thickness. (us) The nar-rowest or focal point of a section or slice. sec-tion thickness. thickness of the scanned tissuevolume perpendicular to the scan plane; alsocalled slice thickness.

sector. (us) A geometric figure bounded by tworadii and the arc of a circle included betweenthem.

sector scan (us) Produced by a phased or annular

ultrasound transducer.secular equilibrium (nmed) A parent:daughter

decay series where decay constant λd � λp.The decay constant for 81Rb parent is 0.000042and 81mKr is 0.0533. There is almost an instantaneous growth of daughter activity andcontinuous elution is possible (steady state

studies 81 mKr).

(see transient equilibrium).segmented HASTE (mri) Variant of standard HASTE

technique where half the image information isacquired after the first excitation pulse, and halfafter the second. The acquired raw data are theninterleaved into a raw data matrix. Long repeti-tion time (TR) is selected which allows the spinsystem to recover between excitation pulses. As a consequence the length of the multi-echo pulsetrain halved. HASTE sequences may also bedivided into more than two segments.

segmented k-space data acquisition (mri) A setof k-space lines collected in a specified order butnot constituting a complete coverage of k-space.Several segmental acquisitions are needed forcomplete coverage of k-space.

SeHCAT® (nmed) Taura-23-selena-25-homocholicacid. A 75Se-labelled agent used for followingbile acid absorption (Nycomed-Amersham/GEHealthcare). A non-imaging technique; maximumadministered activity typically 400 kBq.

selection bias (clin) The tendency for peoplewho elect to be screened to differ from the gen-eral population (more health conscious, so tendto be fitter).

selective excitation (mri) A specific region of tissueexcited by controlling frequency bandwidth of anRF pulse while applying a magnetic field gradi-ent. Originally used to excite all but a desiredregion but more commonly used to select only a desired region, such as a plane, for excitation.

selective irradiation (mri) See selective excitation.selectivity (mri) The ability of an RF receiver to

distinguish between selected and unwantedsignals. Measured as FWHM of the pass band.Influenced by aerial (coil) design and placement.

0

0.0

0.2

0.4

0.6

0.8

1.0

20

81Rb

81mKr

40

Time (seconds)

Rel

ativ

e ac

tivity

(81

Rb/

81m

Kr)

60 80 100


selenium (Se)–sensitive point

S

selenium (Se) (elem)

Atomic number (Z) 34Relative atomic mass (Ar) 78.96Density (ρ) kg/m3 4810Melting point (K) 490K-edge (keV) 12.6Relevance to radiology: an imaging plate detector for x-rays.

75Selenium (nmed) Adrenal gland and pancreasimaging.

ProductionDecay scheme (e.c.) 75Se T½ 120 d (γ 136, 265, 75Se

280 keV) : 75As stableGamma ray constant 5.6 � 10�2mSv hr�1 MBq�1

@ 1 mHalf life 120 daysDecay constant 0.005784 d�1

Photons (abundance) 121 keV (0.171)136 keV (0.588)265 keV (0.590)280 keV (0.252)401 keV (0.115)

Uses: combined with nor-cholesterol to image theadrenals, combined with methionine to image thepancreas and with taurocholic acid for tracer studies ofbile acid loss.

75Se–homocholic acid taurine (nmed) See SeHCAT®.75Se–seleno-cholesterol (nmed) Has limited

role in adrenal imaging. Discontinued.75Se–selenomethionine (nmed) Early agent for

imaging hepatomas. Discontinued.75Se–selenomethyl-19-norcholesterol (nmed)

An imaging agent (Scintadren®) that has beenused for the adrenal cortex.

self-shielding (mri) Magnetic shielding byattaching a high permeability yoke to the mag-net (passive shielding) or by incorporatingadditional magnetic field-generating coilsdesigned to reduce the external field (activeshielding) (see magnetic shielding).

semiconductor (phys) A material (element or com-pound) whose conductivity can be influenced byadding impurities. The pure materials producerelatively few ‘free’ electrons at normal temper-atures. The valency and conduction bands in anintrinsic semiconductor are very close in energyso an appreciable number of electrons will haveenough thermal energy to enter the conductionband. These electrons (negative charge) and thecorresponding ‘holes’ left in the conduction

3474

3475Se (n, ) Seγ

band by absent electrons (positive charge) arefreely mobile. An intrinsic semiconductor suchas silicon or germanium can therefore conductelectric current under certain conditions.Semiconductors such as germanium, silicon,selenium and a variety of complex compoundsare manufactured in very pure crystalline formand have very small levels of impurities added(doping); these impurities produce defects in thecrystalline structures and function as localizedelectron donors just below the conduction bandso the energy required to lift an electron intothe conduction band is much smaller. Dopingcreates n and p type semiconductors. ‘n-type’where the n depicts conduction due to negativecharges (electrons) and ‘p-type’ when dopingleaves positively charged electron vacancies or‘holes’ in the valence bands, which are highlymobile and can conduct an electrical current.Semiconductors are used in signal and powerrectifying devices (diodes), RF and AF amplifyingdevices (transistors) and switching devices(thyristors, triacs, transistors). They can beassembled as integrated microcircuits. Semi-conductors as photoconductors can be used asradiation detectors and imaging surfaces (sele-nium, silicon). Hyperpure germanium (HpGe)and lithium drifted silicon is used as a cooleddetector giving excellent energy resolution, thelatter for lower photon energies.

semiconductor detector (ct) X-ray detectormade completely of a semi-conducting mate-rial; the incident radiation causes ionizationwithin the material which is directly trans-formed into an electrical (current or voltage)output signal (see solid state detector).

seminal vesiculography (clin) Seminal vesicleidentified by ultrasound them imaged usingnon-ionic contrast material.■ Reference: Jones et al., 1997.

sensitive plane (mri) Technique of selecting aplane (or sequential plane) imaging by using anoscillating magnetic field gradient and filteringout the corresponding time-dependent part ofthe NMR signal. The gradient used is at rightangles to the desired plane and the magnitudeof the oscillating magnetic field gradient isequal to zero only in the desired plane.

sensitive point (mri) Technique of selecting out apoint for sequential point imaging by applyingthree orthogonal oscillating magnetic field gradients such that the local magnetic field is

S


sensitive point–septal collimator rings

time-dependent everywhere except at thedesired point. and then filtering out the corre-sponding time-dependent portion of the NMRsignal.

sensitive volume (mri) The region of the tissue ororgan from which the NMR signal will prefer-entially be acquired because of strong mag-netic field inhomogeneity elsewhere. Achievedby altering the bandwidth of the RF pulse andapplying oscillating magnetic gradient fields.Effect can be enhanced by use of a shaped RFfield that is strongest in the sensitive region.

sensitivity (phys) The response of a detector todifferent photon energies. (stats) See sensitivity

(diagnostic). (us) Ability of an imaging system todetect weak echoes. (mri) Atomic nuclei musthave a nuclear spin; this excludes all atomicnuclei with an even number of protons andneutrons. Because the hydrogen isotopes arethe most sensitive, it is set as a reference to all other atomic nuclei and has the relative sensitivity of 1 (or 100%).

Nucleus Relative sensitivity

1H 1.019F 8.3 � 10�1

23Na 9.3 � 10�2

31P 6.6 � 10�2

13C 1.6 � 10�2

(nmed) See sensitivity (system).sensitivity (diagnostic) (stats) A measure of the

performance of a diagnostic test which detectsthe presence of disease in a selected populationgroup. Calculated from an analysis of diagnosticaccuracy, representing the proportion of peoplewith the disease that will be detected by theimaging method used. For a clinical test series(i.e. screening results), the results can be listedaccording to true positive (TP), false positive (FP):

Test result Expected� Test� Total

Positive test TP FP TP � FPNegative test FN TN FN � TNTotal TP � FN FP � TN TP � FP �

FN � TN

Sensitivity is measured as the ratio of true posi-tive (TP) to the sum of true positive and falsenegative (FN):

TP(TP FN)

%�

�100

For the results presented in diagnostic accuracythe sensitivity is 300/350 � 85%. Selectivity ismeasured as:

Overall accuracy is measured as:

(see Bayes’ theorem, predictive accuracy).sensitivity (system) (nmed) The response of a

gamma camera to a known activity A with a cho-sen collimator. The counts registered by the cam-era are noted using a 20% window C so C/A cpsMBq�1 is a measure of the sensitivity. Typicalvalues for 99 mTc are:

● H R collimator 85 to115 cps MBq�1;● General purpose 150 to 200 cps MBq�1.

Excessive count rates approaching the cameradead time should not be used. The conversionbetween counts per second per megabecquerel(MBq) and counts per minute per microcurie(μCi) where the conversion is:

cps MBq�1 � 2.22 � cpm μCi�1

cpm μCi�1 � 0.45 � cps MBq�1

(see resolution (system)).sensitivity analysis (dose) Aims to quantify how

the results from a model depend upon the different variables included in it.

sensitivity centres (film) (see emulsion ripening).sensitivity profile (ct) Relative response of a sys-

tem for CT as a function of position along a lineperpendicular to the tomographic plane.

sensitometer (film) A calibrated light source forexposing a stepped grey-scale.

separation (film) Loss of definition due to separa-tion of the intensifying screen and film surfaceby dirt or distortion.

sEPI (mri) spiral EPI.septa (ct) Thin metal plates separating adjacent

detectors in an array: the septa are orientedparallel to the incident x-rays and parallel tothe axis of fan beam rotation.

septal collimator rings (nmed) PET machinesusing these markedly reduce scatter eventsfrom being collected, due to improved geomet-ric efficiency. If an annihilation occurs within a

TP TN��

Grand total100%

TNTN FP�

�100%


septal collimator rings–Shepp–Logan filter

S

particular detector ring then, if either of thephotons scatters, it is most likely that the newtrajectory of the photon will cause it to miss thedetector ring, thereby preventing a scatter coin-

cidence event.sequence time (mri) or repetition time TR. The

period between repeating an identical pulsesequence.

sequential CT (ct) Conventional CT scanningtechnique in which each slice is measured at afixed z-position followed by an appropriatetransport of the patient in the z-direction.Sequential CT has been replaced by spiral CTinvolving multiple detector rows.

sequential line imaging (line scanning; lineimaging) (mri) Imaging techniques in which theimage is built up from successive lines throughthe object. In various schemes, the lines are iso-lated by oscillating magnetic field gradients orselective excitation, and then the NMR signalsfrom the selected line are encoded for position bydetecting the FID or spin echo in the presence ofa magnetic field gradient along the line: theFourier transform of the detected signal thenyields the distribution of emitted NMR signalalong the line.

sequential multislice (mri) Slices in the imagingarea under examination (ROI) are measuredsequentially. The slices desired are selectedusing suitable gradients (selective excitation).

sequential plane imaging (planar imaging)(mri) Imaging technique in which the image ofan object is built up from successive planes inthe object in various schemes, the planes areselected by oscillating magnetic field gradientsor selective excitation.

sequential point imaging (point scanning) (mri)Imaging techniques in which the image is builtfrom successive point positions in the object. Thepoints are isolated by oscillating magnetic fieldgradients (sensitive point) or shaped magneticfields.

serial port (comp) A connection to the computerthat transfers one bit of information at a timedata queues up ‘in line’. This is much slowerthan the parallel port but is more universalbetween computer types (see RS232, USB).

server (comp) A central computer in a networkresponsible for holding data and program filesand serving workstations that are clients.

sestamibi (nmed) A 99 mTc complex containingisonitrile ligands for a myocardial perfusion

agent. Commercial (DuPont Pharma: Bristol MyersSquibb) kit as Cardiolite® or Miraluma® for thepreparation of 99mTc-sestamibi. 99mTc-(MIBI)�6 (as(Sestamibi or 2-methoxy-isobutyl-isonitrile) Cu(I)tetrafluoroborate) (see Myoview, thallium).

SFP (mri) See steady state free precession.shadowing (us) Reduction in echo amplitude from

reflectors that lie behind a strongly reflecting orattenuating structure.

shallow dose equivalent (Hs) (dose) The exter-nal exposure to the skin is the dose equivalentat a tissue depth of 0.007 cm averaged over anarea of 1 cm2 (see deep dose equivalent).

Shannon, Claude (1916–2001) American elec-trical engineer and mathematician. Created theconcept of information theory influencing digi-tal communications and information storage.Developed a mathematical theory of communi-cation in 1948, deriving his Shannon equations.

Shannon equations (image) Transmission ofinformation and the theory of communication isdescribed by two equations presented byShannon in 1948. The first is I � �p log2p; whichdescribes the signal content (I) (in bits) requiredto transmit certain information; p being the sys-tem probability. A simple application would befor alphabetical messages requiring 26 charac-ters; 5 bits gives 25 or 32 possible characters. Analphanumeric message (36 characters) wouldrequire 6 bits. The second equation called theShannon Limit describes data transmission C inbits per second for a given bandwidth and signalto noise ratio S/N as C � W log2(1 � S/N) show-ing that a restricted bandwidth requires a lownoise transmission system (see Nyquist frequency).

■ Reference: Shannon, 1948.

shaped pulse (mri) See tailored pulse.shelf-life (nmed) A measure of radionuclide stor-

age, related to half-life. 123I has a poor shelf-life(T½ 13 hr) whereas 201Tl has a good shelf-life(T½ 3.0 d).

Shepp–Logan filter (image) CT, SPECT and PET

image reconstruction using simple back-projection

gives a blurred image. Low pass filtering isapplied to the back-projection data to obtain anaccurate representation of the original object.There are a number of low pass filters, the sim-plest being the ramp filter but this is very sensi-tive to noise. Other filters do not have thisdisadvantage, the most common being theShepp–Logan filter, which combines a sinc-

function with the ramp filter. The Shepp–Logan

S


Shepp–Logan filter–SI prefixes

filter gives a small amount of blurring, but ismuch less sensitive to noise.

shielding (mri) See magnetic shielding, cryo-shield-

ing, RF shielding, Faraday shield.shielded gradient coils (mri) Using secondary

active coils inductively coupled to the gradientcoils inside the magnet cryostat reducing thegradient fringe field. The shielding can beaccomplished by secondary actively drivencoils or by passive screens which are induc-tively coupled to the gradient coils. In bothcases eddy current interference will be reduced.

shielding (brick) (dose) See brick (shielding).shielding (concrete) (dose) See concrete (shielding).shielding (generator) (nmed) See generator

(shielding).shielding (lead) (dose) See lead (shielding).shielding (lead-glass) (dose) See lead glass.shielding (legislation) (shld) NCRP report 147.shielding (plaster barium) (dose) See plaster

(barium).shielding (plaster gypsum) (dose) See plaster

(gypsum).shielding (steel) (dose) See steel (shielding).shielding (syringe/vial) (nmed) See syringe/vial

shielding.shift invariant (img) A linear system is shift

invariant if the impulse response function (point

spread function) is the same for all input points(see space invariant).

shift reagents (mri) Paramagnetic compoundsdesigned to induce a shift in the resonance frequency of nuclei with which they interact.Many rare earths have been used as shiftreagents for positive metal ions such as sodiumand potassium.

00.0

0.2

0.4

0.6

0.8

1.0

2 4

Frequency

Butterworth

Shepp-Logan

Ramp

Am

plitu

de

6 8 10

shim (mri) The correction of magnetic field inho-mogeneities caused by the magnet itself, ferro-

magnetic objects, or the patient’s body. Passiveshim usually involves the introduction of smalliron pieces in the magnet. The patient-relatedfine shim is software-controlled and performedusing active shimming with coils (see global shim,interactive shim, local shim, 3D shim).

shim coils (mri) Coils carrying a relatively smallcurrent that are used to provide auxiliary magnetic fields in order to compensate forinhomogeneities in the main magnetic field ofan NMR system. Shim coils can be either resis-tive or superconducting and typically up to 18in number.

shimming (mri) Correction of magnet inhomo-geneity; ‘active’ using shim coils, ‘passive’using iron sheets. Process of maximizing thehomogeneity of the static magnetic field. Activeshimming techniques use current carrying coilswithin the magnet bore, while passive shim-ming places sheets of iron at various locationswithin the magnet.

shock-excited mode (us) Excitation of a trans-ducer by a brief driving voltage impulse.

SHORT (mri) Short repetition technique pulsesequence (see FFE, GRE, MPGR, GRECO, FE, PFI, GE,TFF, SMASH, STAGE).

shoulder (film) The non-linear region of saturatedexposure typically showing an optical density

greater than 3.0.sialography (clin) Radiography of salivary glands

by using contrast medium in the ducts. Imagingthe ducts and acini of a salivary gland. Usuallyfor the parotid or submandibular gland.

SI units (phys) (see Système International).SI prefixes (units)

Prefix Symbol

1015 peta P1012 tera T109 giga G106 mega M103 kilo k102 hector h101 deca da10�1 deci d10�2 centi c10�3 milli m10�6 micro μ10�9 nano n10�12 pico p10�15 femto f10�18 atto a


side lobe–signal to noise ratio (SNR)

S

side lobe (image) A product of non-recursive filterdesign. Used for low pass filtering or smoothing.Simple moving average filters can have unwantedside lobes which are sometimes �20% of themain lobe. (us) Minor sound beams travelling atan angle from the main beam given by a singleelement.

siemens (S) (phys) The reciprocal ohm (1/Ω orΩ�1) so 1 S � 0.1 Ω; previously called the mho.Adopted as an SI unit.

Material Siemens m�1

Conductor 109

Semiconductor 109 to 10�7

Insulator 10�15

Sievert, Rolf Maximilian (1896–1966)Swedish radiologist giving his name to the SIunit for equivalent dose.

sievert (Sv) (dose) The unit of effective dose andequivalent dose, 1 Sv � 1 J kg�1. Obtained as theproduct of absorbed dose and radiation weighting

factor: gray � (Q or wR) (see gray).sigma function (math) A curve approximating to

1/(1 � e�x) which simulates the film characteris-

tic curve and dose response from low to highexposure levels and dose/mortality curves.

sigma value (stats) See standard deviation.signal averaging (image) Multiple signals

acquired under identical conditions (RF fre-quency, gradient field strength) and averagedto reduce noise; combining signals from identi-cal acquisition procedures to reduce signalnoise Usually four repeat signals are averagedto reduce noise by �—N. (mri) Taking the average

0

0.0

0.5

1.0

10 20

Exposure (relative)

Res

pons

e

30 40

1

(1 � e�x)

of a signal’s parameter (amplitude, frequency,etc.) acquired under the same or similar condi-tions so as to suppress the effects of randomvariations or random artefacts. The number ofsignals averaged together can be abbreviatedto NSA (see signal to noise ratio).

signal decay (ct) The falling output signal from adetector or signal amplifier as a function of timeafter the incident x-ray flux or the electrical inputsignal has been switched off (see sodium iodide).

signal elimination (mri) Areas in the image withoutsignal (displays as black). Caused by: metalitems, susceptibility artefacts, flow and satura-tion effects. Flow voids can occur with fastflows when using spin echo sequences if thebolus flows out of the slice between the 90 and180° pulses. No spin echo is produced, andblood appears black in the image.

signal suppression (mri) Elimination or reduc-tion of a selected signal by applying a narrowband frequency-selective pre-pulse centred onthe resonant frequency of the signal. Can alsobe achieved by using an inversion recoverytechnique to null the signal as it recovers itslongitudinal magnetization.

signal to noise ratio (SNR) (phys) The displayedsignal divided by the standard deviation of signalnoise. In the diagram:

definitions of the SNR vary. In the context ofsignal detection theory, the SNR is generallyproportional to a ratio of:

● The magnitude of the difference between themean values of some quantity under two con-ditions that are to be distinguished; to

● a measure of the magnitude of statistical varia-tion in that difference.

The ratio of the strength of the signal for infor-mation content in the image to the noise level(the standard deviation of the signal).

D2

�

D1

SNRD D

��2 1σ

S


signal to noise ratio (SNR)–Sinerem®

Ways to improve SNR in radiology and particu-larly MRI include:

● increasing the number of averagings;● increasing the measurement volume (although

spatial resolution degrades);● using special coils and local coils;● smaller bandwidth;● shorter echo time;● thicker slices.

(see noise, DQE, NEQ).significance (math) A probability of rejecting the

null hypothesis. p � 0.05 borderline signifi-cance, p � 0.01 significant, p � 0.001 highlysignificant.

significant figures (math) The number of signifi-cant figures in a given number is the number ofdigits from the left ignoring leading zeroes butcounting final zeroes. It is the number of mean-ingful digits, e.g. 1.2345, 123.45, 0.0012345and 12345.0 all have five significant figures.The number 12345000 has at least five signif-icant figures but without more precision it is notknown whether more figures are significant(see least significant figure).

silicon (Si) (elem)

Atomic number (Z) 14Relative atomic mass (Ar) 28.09Density (ρ) kg/m3 2300Melting point (K) 1680K-edge (keV) 1.83Relevance to radiology: as a low energy detector with avery narrow energy resolution. The depletion layer isincreased by doping with lithium Si(Li).

silver (Ag) (elem)

Atomic number (Z) 47Relative atomic mass (Ar) 107.87Density (ρ) kg/m3 10 500Melting point (K) 1234Specific heat capacity J kg�1 K�1 232Thermal conductivity W m�1K�1 429K-edge (keV) 25.5Relevance to radiology: Silver halides used exclusively asphotosensitive compounds in film emulsions.

silver halides (image) Film emulsion componentsof silver iodide, bromide and chloride.

SIMM (comp) Single in-line memory module. Asmall circuit board holding a row of memorychips and has a 72-pin connector and uses a32 bit-wide bus. Forms the basis of the RAM

memory. As Pentium processors have a 64-bitbus, SIMM must be installed in pairs.

simple diffusion (nmed) A mechanism of local-ization in which a radiopharmaceutical diffusesacross a cell membrane and is absorbed by thebloodstream (e.g. inhaled 133Xe gas appears inthe peripheral circulation).

simultaneous volume imaging (mri) See volume

imaging.SIMUSIM (mri) Simultaneous multi-slice imaging.sinc function (di) The Fourier representation of a

square wave in the time domain:

This is a pulse shape with tails stretching to infin-ity. In practice, these undergo truncation whichgives an imperfect square wave. The sinc functionshapes the RF pulse in MRI; truncation errors (toomuch foreshortening) leads to slice overlap.

sinc interpolation (mri) Interpolating image databy zero filling the high spatial-frequency com-ponents of the raw data so that after Fouriertransformation the image matrix size has beenincreased.

sine wave (math) A continuous sinusoidal signaldescribed by the equation f(t) � A sin(ωt � φ)where A is the signal amplitude and ω is theangular frequency 2ωf, where f is the frequencyof the waveform and φ the phase angle in radi-ans or the amount the wave is time shifted (seeoscillation).

Sinerem® (cm) Guerbet MRI agent; the brandname for colloidal ferromagnetic iron oxide todetect metastatic disease in lymph nodes.

Sinc function

Fourier profile

sinsin

c x( )( x)x

�π

π


Single photon emission computed tomography (SPECT)–slew rate

S

Single photon emission computed tomography(SPECT) (nmed) Single or dual head rotatinggamma camera 180 or 360° around the patient ora set of fixed detectors in a 360° array. Axialtomographic images acquired and reconstruct-ing multiple slices according to the camera field of view. The gamma camera heads take aseries of images at equal angular spacing calledprojections during its rotational movement. 3Ddistribution of a radionuclide obtained by sam-pling with a rotating gamma camera (1, 2 or 3 heads) to obtain a series of ray sums(emission) and by filtered back-production recon-structing matrix as a voxel matrix depending on activity concentrations. Attenuation correc-tion is also applied. Coronal and sagittal slicescan be obtained from the axial information.SPECT images were obtained in the early1960s using separate scanning detectors (DavidKuhl, USA).

single-shot technique (mri) All image informa-tion acquired in a single excitation pulse.Magnetization of a fully relaxed spin system isused. Each of the subsequent echoes is given a different phase encoding relationship betweenthe intensity of signal and noise. Only slightlymore than half the raw data are acquired. Theimage is obtained using Half Fourier recon-struction. Single shot techniques include EPI,RARE and HASTE.

single volume spectroscopy (SVS) (mri) Mappingthe metabolic information from the VOl in a spec-trum. Single volume techniques are advanta-geous in case of pathological changes thatcannot spatially be limited to a few VOIs: localinhomogeneity in the magnetic field can be com-pensated to a large extent using a local volume-sensitive shim. Clinical spectroscopy currentlyuses single volume techniques based on spinechoes (SE) or stimulated echoes (STEAM).

Sinografin® (cm) Bracco version of megluminediatrizoate and meglumine iodipamide with aniodine content of 380 mg I mL�1 indicated forhysterosalpingography.

sinograms (ct,nmed) Constructed by combininginformation from several lines-of-response(LORs) and representing these as projectedpoints on a sine wave. In PET line-of-response,coincident events for all projection rays arecommonly sorted and stored this way, soorganizing the data so that it can be examinedmore readily. The LOR has a certain angular

inclination and a radial distance from the centralaxis so it is convenient to adopt a cylindricalcoordinate system for storing other tomographicimage data. The collection of many such viewscan be presented as a 2D plot or image; each isa one-dimensional profile of measured attenua-tion as a function of position, corresponding to a particular angle. The sinogram of a 2D slice isa collection representing the entire image; thephase and amplitude of the sine wave is uniqueto the source location in the tomographic planeand the intensity (amplitude) of the sine waveindicates source strength. Sinograms are trans-ferred to the array processor and used fortransaxial image reconstruction using filteredback projection or iterative reconstruction with astandard spatial filter (Shepp–Logan, Butterworth,Hann, Hamming, a simple ramp filter or a specificuser defined filter).

SIP (mri) Saturation inversion projection.skew (stats) A distribution where mean (Mn), mode

(Mo) and median (Me) do not coincide as they doin the normal distribution.

skin depth (mri) Time-dependent electromagneticfields are significantly attenuated by conduct-ing media (including the human body): the skindepth gives a measure of the average depth ofpenetration of the RF field. It may be a limitingfactor in MRI at very high frequencies (highmagnetic fields). The skin depth also affects theQ of the coils.

slew rate (mri) Gradient field increase by unittime:

Slew rate=gradient strength

rise time

Mo

Me

Mn

Skewed Distributions

PositiveNegative

Sample value

Sam

ple

freq

uenc

y (r

elat

ive)

S


slew rate–slice thickness

Measured in tesla m�1 s�1. The magnetic gradi-ent speed represented by the slope of the linearramp; also known as ramp time. Defined as themaximum gradient rise time to maximum ampli-tude. Influences minimum TR and TE values.Typical values 20 to 60 T m s�1 although thisfigure can reach 200 Tm s�1 for 3T machines(see magnetic field gradient).

slice (ct) Tomographic section (defined by posi-tion and thickness) of a test phantom or patientunder investigation during a single CT expo-sure in serial scanning.

slice distance (mri) The separation between thecentre planes of two sequential slices or 3Dslabs.

slice dose profile (SDP) (ct) Represents the doseas a function of the position along the z-axisresulting; due to x-ray scatter the slice doseprofile (SDP) is always broader than the slice sen-

sitivity profile (SSP) in spite of detector collimation.slice gaps (mri) The gap between the nearest

edges of two adjacent slices. Not to be con-fused with the slice distance.

slice orientation (mri) Orthogonal planes areavailable for use as the basic slice orientation:

● sagittal;● coronal;● transverse.

An oblique or double-oblique slice is obtainedby rotating the slice out of the basic orientation.

slice plane (ct) A plane oriented orthogonal to theaxis of rotation and located centrally within theslice of the object shown in the reconstructedand displayed image. Altered by the gantry tilt.

slice position (mri) The position of the slice to bemeasured within the area under examination.Graphical positioning of the slices/saturationslices to be measured in a basic image.

slice profile (mri) The spatial distribution of sen-sitivity of the imaging acquisition in the direc-tion perpendicular to the plane of the slice(z-axis). When the profile deviates appreciablyfrom rectangular, the slice thickness alone maynot provide an adequate description (see slice

sensitivity profile, SDP).slice selection (mri) For orthogonal slices, a mag-

netic field gradient is applied perpendicular tothe desired slice plane (slice-selection gradient).Oblique and double-oblique slices are excitedby simultaneously applying two or three gradient fields.

slice sensitivity profile (SSP) (ct) Relativeresponse of a system for CT as a function of posi-tion along a line perpendicular to the tomo-graphic plane. The slice sensitivity profile is animportant factor of a CT machine since it deter-mines the image quality. The steeper the profileslope the less interference from adjacent slicesthat would cause partial volume artefacts. The per-fect sensitivity profile would be rectangular andfor a point source of x-rays this could beachieved by simple collimation. In practice, wherethe focal spot has a finite size, geometrical unsharp-

ness causes penumbra effects and tight collima-tion at the detector entrance is necessary. Whenopposing beam shapes are super-imposed (seenin a full 360° data collection) the two divergingbeams give a slice section that departs from atrue rectangle. The middle of the slice is thickerthan the periphery; particularly noticeable forthin slices.

(see slice dose profile (SDP)).slice sequence (mri) Parameter for multi-slice

measurements, the excitation sequence can beselected as desired:

● ascending (1, 2, 3, . . ., n);● descending (n, n�1, . . ., 3, 2, 1);● interleaved (1, 3, 5, . . ., 2,4, 6, . . .);● or freely defined.

slice shift (mri) Distance between the centre of aslice group and the centre of the magnetic field(FOV) in slice-selection direction.

slice thickness (ct) Effective thickness of thetomographic section; the nominal slice thick-ness (width) is the full width at half maximumof the slice sensitivity profile. This is usually meant

FWHM

Slice sensitivityprofile

Slice doseprofile


slice thickness–Snapshot FLASH

S

by the term slice thickness. Selection of a particular section thickness causes:

● movement of the pre- and post-patient colli-mators; and

● selection of detector rows.

Activating or deactivating the detector elementscreates all available section thicknesses for thelinear detector design. For unequal-width adap-

tive detector designs, post patient collimation is notneeded for wider section thicknesses (5.0 and2.5 mm); however, narrower section thicknesses(1.0 and 0.5 mm) require precise post patient col-limation to cover portions of the detectors, whichare exposed to radiation in the penumbra. Anynominal slice thickness can be reconstructedafter acquisition providing that it is thicker thanthe original single detector configuration(scanned nominal slice thickness). The recon-struction of thicker slices than originally obtainedreduces the number of images to be viewed anddecreases image noise. (us) See focus (ultrasound).

slice width (ct) See slice thickness.SLIP/PPP (comp) Serial line interface protocol/point

to point protocol. These are standards for con-necting directly to the Internet rather than simplylogging on to it via a host computer.

slip-ring (ct) A method for uninterrupted powersupply and data collection. Single slice or multi-slice spiral acquisition is a continuously rotatingfan beam assembly. A conductive ring usuallycoated and lubricated copper, allows continuouscontact between the x-ray tube (for its electricalsupply) and detectors (for signal collection).Continuous data collection can then be obtainedwithout rewinding the fan-beam assembly. Sliprings operate at either:

● low voltages (200–300 volts), when the gener-ator and x-ray tube must rotate together (tankunit); or

● high voltages (up to 140 kV) supplying just thex-ray tube alone.

Both low and high voltage techniques have theiradvantages and disadvantages. Low-voltageconnectors require a lightweight high-voltagegenerator rotating with the fan beam (the tank

unit contains the high frequency transformersand control electronics); this adds additionalmass to the rotating fan assembly. Centrifugal

forces are therefore an important consideration inhelical scanners capable of subsecond rotation

speeds. High voltage designs locate the genera-tor externally away from the fan beam assembly,reducing weight but requiring substantial insulation for the slip rings.

slope (math) The straight-line portion of a curve(the film characteristic).

small bowel enema (enteroclysis) (clin)Radiographic examination of small intestine byretrograde filling from barium contrast filledlarge bowel; barium contrast examination ofthe small bowel after duodenal intubation.Distension of the small intestine by the contrastinfusion provides detail and characterization oflesions. The contrast medium examination maybe performed as a single-contrast or a double-contrast study.

SMART (comp) Self monitoring analysis and report-ing technology or a feature of EIDE (on mother-boards that support it) where the BIOS canreceive data about hard disk performance andwarn a user if it predicts a failure is likely tooccur. (mri) Simultaneous multi-slice acquisitionwith arterial-flow tagging (Shimadzu). Reductionof motion-induced phase shifts during TE (seeGMR, GMN, FLOW-COMP, CFAST, MAST, FLAG, GMC,FC, STILL, GR).

SMASH (mri) Short minimum-angle shot (subsec-ond imaging), general sequence, Shimadzu (seeFFE, GRE, MPGR, GRECO, FE, PFI, GE, TFF, SHORT,STAGE).

smearing artefact (mri) With non-periodicmovement (such as eye movement), the excitedspins may be at a different locations in the gra-dient field at the echo time, resulting in wrongphase-encoding. The object is smeared in thephase-encoding direction. These artefacts aremore discrete for periodic movements.

SMI (mri) Simultaneous multislice imaging.smoothing (di) See filtering (spatial).SMPTE (comp) Society for Motion Picture and TV

Engineers. Provides standard test patterns forradiology video display quality.

SMTP (comp) Simple mail transport protocol. Ahigh-level protocol for exchanging mail mes-sages on the Internet. An extension of this isthe Post Office Protocol (POP) to serve mail thatare not permanently available on the Internet(turned off overnight).

Snapshot FLASH (mri) A very rapid FLASH sequence(scan time 200–500ms) using a low flip angle(approximately 5°) to produce proton density-weighted scans. Contrast can be manipulated by

S


Snapshot FLASH–soft tissue (muscle)

suitable preparation of the magnetization prior tothe scan (see TurboFLASH).

Snell’s Law (us) Describes the extent of the rela-tionship between angles of incidence and refrac-tion. The angles of incidence θi, reflection θr andrefraction θt (of the transmitted beam) are relatedby the following laws for specular reflection:

● the directions of incident, refracted and reflectedbeams are all in one plane, which is normal tothe surface of the two media;

● for specular reflection: θi � θr;● considering the respective velocities in the

incident and refractive medium (ci and ct

respectively) then:

This last statement is Snell’s Law and the con-stant k is the index of refraction of medium Z2

with respect to medium Z1. When k � 1 there isan angle of incidence where sin θi � k then theabove equation gives θt � 1 or 90° and therefracted beam is parallel to the surface; θi isthen the critical angle (θλ) which depends onlyon the velocity of ultrasound in the two media Z1

and Z2. If θi � θλ or sin θi � k then sin θt � 1which is not possible so no refracted beam existsonly a reflected one. The total reflection factor isimportant to fibre optics (see angle of reflection,

sound (refraction)).

Snell, Willebrord (1580–1626) Dutch mathe-matician at Leyden University described reflection

ReflectedIncident

Refracted

Z1

Z1

�i �r

�t

sinsin

θθ

i

t

i

t

cc

k� �

and refraction of light in 1621 as Snell’s Law whichalso applies to ultrasound.

SNMP (comp) Simple network management proto-col; a standard for managing network devices,including adapters, switches, routers, serversand workstations.

SNR (image) See signal-to-noise ratio.Soddy, Frederick (1877–1956) British radio-

chemist who shared the discovery of atomic dis-integration with Rutherford and discovered withRamsay helium production from alpha particles.He is best known for the discovery of isotopes.Awarded Nobel Prize for chemistry in 1921.

sodium (Na) (elem)

Atomic number (Z) 11Relative atomic mass (Ar) 22.99Density (ρ) kg/m3 970Melting point (K) 371Specific heat capacity J kg�1 K�1 1230Thermal conductivity W m�1K�1 141K-edge (keV) 1.0

22Sodium (nmed) Used as a calibration source forpositron tomography.

ProductionDecay scheme (β�)22Na 22Na T½ 2.6 yr (β� 1.82 MeV,

2γ 511 kev) → 22Ne stableHalf life 2.6 yearsHalf value layer 4.1 mm PbUses A calibration source for

positron emission detectors.

sodium iodide (rad) Sodium iodide (thallium-doped NaI(Tl)). This detector material has a rel-atively low stopping power compared to otherdetectors, it demonstrates very good energyresolution (11%) and has excellent light yield.The better energy resolution permits lowerenergy thresholds approaching 435 keV (ratherthan 350 keV) to limit scattered events withoutreducing true events (the window has an upperthreshold of 665 keV). The coincidence timewindow is typically 8 ns which is an improve-ment on bismuth germinate (BGO) systems. Ashorter coincidence time window shouldimprove counting characteristics. The systemhas a long crystal decay time (230 ns) com-pared to lutetium oxy-orthosilicate (LSO) systems(40 ns), but this is shorter than the decay timefor BGO (300 ns).

soft tissue (muscle) (material)

1123

11222Na(n, n) Na


soft tissue (muscle)–sound (velocity)

S

Effective atomic number (Zeff) 7.64Density (ρ) kg/m3 1040

soft tissue thermal index (TIS) (us) A thermal

index for thermal exposure caused by an ultrasound beam heating soft tissue and boneinterfaces (see ultrasound (heating)).

software (comp) The set of instructions. or com-puter program, that controls the activities of thecomputer. Programs may be written in machinelanguage (sequences of numbers directly inter-pretable by the computer). assembly language, orhigher level languages such as BASIC, C or FOR-TRAN. The software includes overall supervising‘executive’ programs, data acquisition programs,data processing programs (including imagereconstruction) and display programs (see macros).

solenoid coil (mri) A coil of wire wound in the formof a long cylinder. When a current is passedthrough the coil. the magnetic field within thecoil is relatively uniform. Solenoid RF coils arecommonly used when the static magnetic field isperpendicular to the long axis of the body.

solid state detector (ct) A photon detector can bea pure semiconductor device, ceramic or crystalscintillator in combination with a light sensitivesemiconductor diode.

Solutrast® (clin) Generic name iopamidol. A non-ionic monomeric radiographic contrast mediummanufactured by Bracco.

solvent suppression (mri) See suppression.somatic effects (dose) Pertaining to all cells except

reproductive cells. Radiation effects induced inthe person irradiated (see genetic effects).

somatostatin analogues (nmed) Somatostatin isa neuropeptide exerting an inhibitory effect ongrowth hormone secretion. Naturally occurringsomatostatin has a short biological half-life. It isfound in endocrine cell neurons with the high-est density in the brain, peripheral neurones,endocrine pancreas, gastrointestinal tract and insmaller amounts in glands such as the thyroid.Synthetic derivatives of somatostatin analogueshave prolonged survival in the circulation.Octreotide® is used for imaging tumours express-ing somatostatin receptors,

SonoVue® (cm) Bracco ultrasound agent usingsulphur hexafluoride microbubbles.

sound (phys) Sound is a longitudinal wave madeup of areas in a material (air, tissue, water, etc.)where the density and pressure are higher(compression) or lower (rarefaction) than normal.

The amplitude of the wave corresponds todenser compression events. High frequency isrepresented by denser packing of the compres-sion and rarefaction events.

sound (conduction) (phys) See conduction (sound).sound (intensity) (phys) Measured as sound

power per unit area and proportional to pres-sure squared: I P2; measured as J s�1m�2 orW m�2. Normal conversation has an intensity of10�7 to 10�4 W m�2 and the threshold of hear-ing is 10�12 W m�2. The ultrasound unit is 10�3

Wm �2 (mWcm�2). Average intensity obeys theinverse square law from the point source. So theintensity of a spherical wave decreases as 1/r2.The amplitude A decreases with distance as 1/rsince:

(see ultrasound (intensity)).sound (reflection) (phys) Sound is reflected from

a smooth surface. Smooth is defined as anyunevenness in the surface is much less thanthe wavelength; this gives specular reflection.A rough surface will give non-specular reflection.

sound (refraction) (phys) The transmitted wavewill change direction depending on materialcomposition. The beam is refracted and is dueto the fact that the speed of sound is differentin the two materials. The frequency of thesound will not change but the wavelength will.This will be less in the material in which thewave travels more slowly. The angle betweenthe refracted wave and the normal is the angleof refraction. The sine of the incident angledivided by the sine of the angle of refraction isa constant. This constant is the refractive index:

(see Snell’s Law).sound (velocity) (phys) As gas density (ρ) is propor-

tional to pressure then sound velocity is inde-pendent of pressure changes. From Charles’ Lawat constant pressure V T; since V 1/ρ then1/ρ T. Therefore, at constant pressure, soundvelocity is proportional to �—T. A list of soundvelocities (m s�1) is shown where the velocity ofsound in air is approximately 330 m s�1 beingindependent of pressure but proportional to �—T asstated above.

refractive index �sinsin

θθ

i

t

A Arr2 11

2�

⎡

⎣⎢⎢

⎤

⎦⎥⎥

S


sound (velocity)–spatial-peak/pulse average, intensity (ISPPA)

Material Velocity

Air 330Helium 1000Water 1540Soft tissue 1540Bone 4080Aluminium 6400

space charge (xray) Accumulation of an electroncloud around a filament. More pronounced atlow kV.

space invariant (image) If the image of a point

source has the same functional form in all posi-tions of the x, y plane, space variant distribu-tion would consider all four spatial co-ordinates(x, y and �x,�y).

SPAMM (mri) Spatial modulation of magnetization.spatial average intensity (ISA) (us) Average

intensity over transducer area.spatial average/pulse average (ISAPA) (us) A

measure of ultrasound intensity at the face of thetransducer divided by the duty cycle, given inmW cm�2 derived as:

a measure of sound intensity given in mW cm�2

(see spatial peak/temporal average).spatial average/temporal average intensity

(ISATA) (us) For non-autoscanning systems, the temporal-average intensity averaged over the beam

cross-sectional area (may be approximated as theratio of ultrasonic power to the beam cross-sectional

area) as:

ITAaSATA � 2

II

SAPASATA�

duty cycle

50 100 150 200

Max

imum

cur

rent

(Lo

g sc

ale

rela

tive)

X-ray tube kV

For autoscanning ultrasound systems, the temporal-

average intensity averaged over the scan cross-

sectional area on a surface specified; it may betaken as the ratio of ultrasonic power to the scan

cross-sectional area or as the mean value of thatratio if it is not the same for each scan. It is great-est at the mechanical focal point of the transducer.For non-autoscanning systems, the temporal-average intensity averaged over the beam cross-sectional area may be taken as the ratio ofultrasonic power to the beam cross-sectionalarea. ISATA is frequently quoted by manufacturersand is the lowest intensity measurement. A com-plete family of intensity values can be derivedfrom ISATA. Typical values are given under ultrasound (intensity). Unit: mW cm�2.

spatial average/temporal peak (ISATP) (us) Thepeak intensity over the transducer face. Aninfrequently used measurement for ultrasoundtransducer intensity which combines spatialand temporal intensity. Unit: W cm�2.

spatial distortion (nmed) The misregistration ofan image event on the display so that its trueposition on the crystal is distorted. This is com-monly due to unmatched PM tubes or variationover the face of the PM tube itself.

spatial frequency (mri) A dimension of theFourier transform space (k-space), having unitsof inverse distance. Higher values of spatial fre-quencies correspond to finer detail in theimage. (ct) The reciprocal value of the wavelength of a periodic structure.

spatial intensity (us) See ultrasound (intensity).spatial linearity (nmed) Establishing the image

deviation from an ideal grid of line sources.spatial non-uniformity (nmed) Irregular response

to radiation across the input field of view.spatial-peak intensity (ISP) (us) Highest inten-

sity in the ultrasound beam.spatial-peak/pulse average, intensity (ISPPA)

(us) The value of the pulse average intensity at thepoint in the acoustic field where the pulse aver-

age intensity is a maximum or is a local maximumwithin a specified region. The measurement isderived from the duty cycle as:

This is the maximum power given by the fullultrasound pulse from the transducer. It is a goodindicator for cavitation and other mechanical

II

SPPASPTA�

duty cycle


spatial-peak/pulse average, intensity (ISPPA)–spatial pulse length (SPL)

S

bio-effects. Typical values are given under ultra-

sound (intensity).

Both ISPTA and ISPPA are required by FDA foracceptance and registration of new ultrasoundequipment. Unit: W cm�2.

spatial-peak/temporal-average, intensity(ISPTA) (us) The value of the temporal-average

intensity at the point in the acoustic field wherethe temporal-average intensity is a maximum,or is a local maximum within a specified region.This is the sound intensity as the time aver-aged power, commonly quoted in ultrasoundspecifications and relates peak power to thepulse width and the duty factor; a higher pulse

repetition frequency (PRF) will increase the ISPTA

value. It is derived as:

For a peak power of 1 W cm�2 and a duty factorof 0.1, the ISPTA value would be 100 mW cm�2;it can also be derived as:

where the pulse repetition period (PRP ) �1/PRF.Highest values of ISPTA are seen in pulsed Dopplerand CW Doppler and indicates heating effects. BothISPTA and ISPPA are required by FDA for acceptanceand registration of new ultrasound equipment.Typical values are given under ultrasound (inten-

sity). Unit: mW cm�2.

I IPRPSPTA SPPA� �

pulse length

I ISPSASPTA SATA� �

ISPTA

ISPPA

Time

spatial peak/temporal peak (ISPTP) (us) A meas-urement of maximum (peak) intensity withinthe ultrasound beam at any instant. It isobtained from the highest value of the pulseamplitude and is a good indicator for cavitationeffects. Measured as:

A useful indicator for cavitation effects. Spatialpeak/temporal peak pressure, measured inMPa, is similar to ISPTP but refers to the peakpressure in the beam at any instant.

spatial pulse length (SPL) (us) Length of a sin-gle pulse which is (wavelength � number ofcycles). Calculated as:

and is related to wavelength as:

SPL � λ � number of cycles (mm)

The pulses are damped to give 2–3 completecycles. Dimensions are typically between0.45 and 2.25 mm for a diagnostic imagingrange.

Frequency (MHz) SPL (mm) PD (μs)

2.5 1.8 1.25.0 0.9 0.67.5 0.6 0.4

10.0 0.45 0.3

SPL

A

λ

cycles in pulse propagation speedfrequency

�

II

SPTPSPTA�

duty cycle

S


spatial registration–specific absorption rate (SAR)

spatial registration (multi window) (nmed) Asource of activity whose nuclide has more thanone gamma energy imaged at all separateenergies. Using radionuclides with multipleenergies (67Ga 93, 185, 300 keV; 111In 171,245 keV) events should be collected using twoor three energy windows in order to avoid lossof sensitivity and resolution. A 67Ga source isusually placed at the centre of the UFOV andimages collected for each energy window.Maximum registration error is then determinedby combining all images.

spatial resolution (or high contrast resolution)(image) The displayed resolution described ashigh contrast (measured in line pairs) or lowcontrast (measured in grey level separation). (ct)The ability to resolve different objects in thedisplayed CT image, when the difference inattenuation between the objects and the back-ground is large compared to noise. Spatial res-olution in the scan plane is referred to as theaxial spatial resolution. Resolution in the coordi-nate perpendicular to the scan plane (the z-axis)referred to as the longitudinal spatial resolution.Axial spatial resolution is principally deter-mined by the distances between the x-raytube, the centre of rotation and the detector, aswell as by the width of the focus and thedetector elements, and the number of meas-urements made per rotation. These factors aredetermined by the construction of the scanner.Longitudinal spatial resolution is principallydetermined by the dimension of the conebeam angle and the selected protocol. In conventional CT, the longitudinal spatial resolution is entirely determined by the slicethickness.

spatially localized spectroscopy (mri) Processby which regions of tissue are selectively sam-pled to produce spectra from defined volumesin space. These methods may be employed tosample a single region in space (single voxelmethod) or multiple regions simultaneously(multi-voxel methods). The spatial selectivitycan be achieved by several methods includingsurface coils, surface coils in conjunction withRF gradient methods or RF pulses in combina-tion with switched magnetic field gradients(volume-selective excitation). An indirectmethod of achieving spatial selectivity is thedestruction of coherence of the magnetizationin regions that lie outside the region of interest.

A variety of spatial encoding schemes havebeen employed for multi-voxel localization (seechemical shift imaging).

spatial resolution (intrinsic) (nmed) Measuredby using a multiple line source. Values aregiven for both the central and useful field ofview (CFOV, UFOV). The line spread function inboth the x and y directions yield the FWHM forthe spatial resolution of the camera. Typicalvalues would be 3–4 mm. Full width tenthmaximum (FWTM) measures the degree of lightscatter within the detector.

spatial resolution (system) Rs (nmed) See resolu-

tion (extrinsic). This is a function of the particularcollimator used and the distance from the colli-mator face. A line source similar to the one usedfor the intrinsic measurements gives FWHMvalues for CFOV and UFOV so:

where Rc is the collimator resolution, M thematrix size and Ri the intrinsic resolution. If Rs

and Ri are obtained then a value for Rc (the col-limator alone) can be derived independent ofcamera performance.

SPDIF (comp) Sony/Phillips digital interface. Wasthe most common digital audio interfacebetween external digital audio recorders andthe computer. The advent of optical connectionhas meant most high-end sound cards includeboth type of connection.

specific absorbed fraction (dose) The fraction ofenergy emitted as a specified radiation type ina source tissue which is absorbed in 1 kg of atarget tissue.

specific absorption rate (SAR) (phys) A safetyreference for measuring energy deposited intissue by non-ionizing radiation. The rate atwhich energy (electromagnetic RF or sound) isabsorbed by a unit mass of tissue as watt perkilogram (W kg�1). (mri) RF energy absorbedper unit time. Time varying electromagneticfields deposit energy in tissues; mostly in theform of heat which is considered the primarymechanism of biological effect. As a measureof heat transfer it is an important value forestablishing safety thresholds. The SAR esti-mates RF heating effects which increase withfield strength, duty cycle, flip-angle squared,

R RRMs c

i� �22⎛

⎝⎜⎜⎜

⎞⎠⎟⎟⎟


specific absorption rate (SAR)–specific activity (weight)

S

coil type and patient geometry. Inhomogeneityof the RF fields leads to a local exposure wheremost of the power that is absorbed is appliedto one body region rather than the entire per-son (local SAR). Averaging over the whole bodyleads to the global SAR where safety thresh-olds avoid thermoregulation or cardiac stress.The SAR is proportional to the product of tissuegeometry (r as radius), field strength (B), flipangle (α), and duty cycle (D). Coil type alsoplays a part, as:

SAR r2 � B2 � α2 � D W kg�1

High field strengths (3T) impart far greaterenergy than low strength magnets and inver-sion pulses (180°) deposit four times moreenergy than 90° pulses. Current FDA guide-lines recommend SAR does not exceed0.4 W kg�1 whole body or 3.2 W kg�1 to thehead.

specific activity (unit) (nmed) The activity of aradionuclide contained in a sample, dividedby the mass of the sample. The unit isBq kg�1.

specific activity (concentration) (dose)Radiochemical purity of the 99 mTc eluate can becalculated from the decay constants of the participating radionuclides.

specific activity (weight) (nmed) The weight ofradionuclide in a given activity is calculatedfrom the formula: W � (M � N)/k, where M is theatomic mass; N the number of radioactive atomsper gram of material of activity A (in becquerels)so A � λN and N � A/λ. k is Avogadro’s constant.

specific activity (intrinsic) (nmed) Each radio-active species has an intrinsic specific activitywhich is the activity of a unit mass of the purematerial. It depends on the decay constant andthe nuclear mass of the nuclide.

Specific activity, dispensingAdult patient dose is 600 MBq.Vial reconstituted to give 10 doses (Total vial volume10 mL). Total vial activity is 600 � 10 � 6000 MBq or6 GBq); so 6 GBq is required in 10 mL for vial reconsti-tution. Eluted activity is 20 GBq in 10 mL giving 20/10or 2.0 GBq per mL. Volume of eluate required (usingGBq values) is:

So 3.0 mL of generator eluate is made up to a total vol-ume of 10 mL in a syringe to give a total vial activity of6 GBq and added to the vial contents. Therefore, 1 mLgives a single adult patient dose of approximately600 MBq.

Total vial activity (GBq)Eluate activity (GBq/ml)

mL6

2 03 0

..�

Specific activity, intrinsicThe specific activity of a 4 mg sample containing 60Co(T½ 5.26 y) is 14 � 106 dps or 14 MBq representing14/4 � 3.5 MBq mg�1. The intrinsic specific activity of 1 mg.

The decay rate � λN � 4.18 � 10�9 � N � 4.18 �

1010 s�1 or 41 800 MBq mg�1 or 41.8 GBq mg�1.The given 4 mg sample of 60Co is obviously a much-diluted source and is not carrier free.

Number of nuclei (N)Avogadro’s number

At. Wt.

nuclei p

�

� �1 109 eer mg.

specific activity (volume) (nmed) Using thebasic formula At � Ao � e�λt

Specific activity, volumeA solution of thallous (201Tl) chloride has an activity of200 MBq on day 1 (T½ 73 h). The volume required foran 80 MBq study on day 3 (48 h later) is:200 � e�λt � 127 MBq, where λ is 0.693/73.A volume of 1.25 mL will contain approximately 80 MBq.

Specific activity, concentrationThe decay chain is 99mTc (T½ 6h) → 99Tc (T½2.1 � 105y). The chemical concentration of 99Tc willincrease on storage (in generator and eluate).

where λ1 and λ2 are decay constants for 99Mo and99 mTc. If the time between elutions is �10 h, then thechemical concentration (mole fraction) of 99Tcexceeds 99 mTc.

Mole fraction( )

( ) ( )

t t

t��

� �

� �

�

0 86 1

2 1

1 2

1

. λλ λ

λ λ

λe e

e

specific activity (dispensing) (dose)Reconstituting radiopharmaceuticals to give atotal activity, sufficient for a fixed number ofpatients, so that a known volume (typically1 mL) contains the required activity. The exam-ple shows a high specific activity dose but lowspecific activity doses can be calculated usingthe same procedure. Volume dispensing,instead of dispensing a measured activity,reduces hand dose.

S


specific activity (weight)–spectral broadening

Specific heat capacity and melting points (°C) ofsome common materials:

Substance Specific heat Melting capacity (J kg�1K�1) point °C

Water 4200Oil 2130Aluminium 910 660Graphite 711 �3550Titanium 523 1660Copper 386 1083Zirconium 280 1852Molybdenum 246 2607Rhenium 138 3180Tungsten 136 3377Glass 67 1127

Water and oil for heating and cooling liquids arechosen since their capacity to both store andtransport heat is good. Water is a good reservoirfor excess heat or a good cooling mediumwhereas aluminium and copper take up heat rap-idly and since they are also good heat conductors,are able to conduct it away rapidly. The specificheat of gases varies according to pressure.

specificity (diagnostic) (stats) Calculated from ananalysis of diagnostic accuracy, representingthe proportion of the population without thedisease that will be shown by the test to be freeof disease. It is the ratio of true negative (TN) tothe sum of true negative and false positive (FP):

TN/(TN � FP) � 100%.

For the results presented in diagnostic accuracy,the sensitivity is 56950/59650 � 95.5%.

speckle (us) The granular appearance of imagesand spectral displays that is caused by theinterference of echoes from the distribution ofscatterers in tissue.

speckle reduction imaging (SRI) (us) A tech-nique introduced by GE Healthcare whichreduces speckle noise in ultrasound by classify-ing the image content by comparing neighbour-ing pixels for expected greyscale trend or sharpchanges; the latter most probably due to noise.

SPECT (nmed) See single photon emission tomography.spectral analysis (us) Separation of frequencies

in a Doppler signal for display as a Dopplerspectrum.

spectral broadening (us) The widening of theDoppler shift spectrum; that is, the increase ofthe range of Doppler shift frequencies presentthat occurs because of a broadened range of

Specific activity, weightThe weight of iodine in a 550 MBq 131Iodine therapydose. T½ 8 d (7 � 105s). λ � 0.693/T½ � 10 � 10�7.The number of atoms N in 550 MBq 131I is:

N � (550 � 106)/(10 � 10�7) � 5.5 � 1014

W � 1.2 � 10�7g or about 120 nanograms.

The activity from naturally occurring 40K in the humanbody (75 kg). Standard man contains 140 g of potassiumof which 0.0168 g is 40K (0.012% abundance). Since:

WM A

k

AW k

M

��

��

�

( / ) then

Bq

λ

λ4366

specific gamma ray constant Γ (nmed) This isan early measure of the exposure rate as equiv-

alent dose per unit activity at a certain distancefrom a point source. This has been replaced bythe dose rate constant according to ICRU33,although the two terms are not identical. The SIunit for the specific gamma ray constant isC kg�1s�1Bq�1 at 1 m. The practical measurebeing mSv h�1GBq�1 at 1 m or μSv h�1MBq�1 at1 m. The original values are calculated in non-SI units were R h�1Ci�1 at 1 m (known as Rhm,pronounced ‘rum’) or alternatively R hr�1mCi�1

at 1 cm. Converting from mSv h�1MBq�1 tomrem h�1 μCi�1 multiply by 3.7. Values forunshielded point sources in mSv h�1GBq�1 at1 m (SI) and R hr�1mCi�1 at 1 cm (non-SI) are:

Nuclide Gamma Dose Dose energy rate rate (keV) (SI) (non-SI)

99mTc 140 0.017 0.78131I 364 0.057 2.2111In 171, 245 0.084 3.21137Cs 662 0.087 3.360Co 1.173, 1.333 MeV 0.360 13.2

In order to convert from R/hr/mCi at 1 cm tomSv/hr/GBq at 1 m, multiply by 0.235.

specific heat (specific heat capacity) (phys)This is the heat capacity per kg of the sub-stance. Specific heat capacities are expressedin J kg�1K�1. The specific heat of a substance isthe heat required to increase the temperatureof 1 kg by 1 K (or °C�1). So 42 J of heat raises:

2 gm water by 5°C2 gm aluminium by 20°C2 gm copper by 50°C


spectral broadening–spin density

S

flow velocities encountered by the sound beam;this occurs for disturbed and turbulent flow.

spectral display (us) Visual display of a dopplerspectrum.

spectral editing (mri) Methods of selectivelyenhancing or suppressing the signal from a par-ticular molecular substance by using its spinproperties. typically through spin–spin coupling(see J modulation).

spectral line-particular (mri) Distinct frequencyor narrow band of frequencies at which reso-nance occurs corresponding to a particularchemical shift.

spectral maps (mri) Mapping a CSI spectral matrixto an anatomical image using magnetic resonance

spectroscopy. Regional changes in metabolitescan be superimposed on contours.

spectral width (mri) The overall width in hertzneeded to observe a particular NMR spectrum.This width is generally set using the Nyquistlimit, namely, that the temporal sampling ratemust be equal to twice the maximum spread infrequencies.

spectrometer (mri) The portions of the NMRapparatus that actually produce the NMR phe-nomenon and acquire the signals, including themagnet, the probe, the RE circuitry, the gradi-ent coils, etc. The spectrometer is controlled bythe computer via the interface under the direc-tion of the software.

spectroscopy (MRS) (mri) Magnetic resonancespectroscopy. Provides an estimation of cellularmetabolic chemistry. The signal peak intensityis proportional to the concentration of metabo-lites. MR spectroscopy can be an importantmethod for in-vitro and in-vivo examination oftissue and organs (see gamma spectrum, magnetic

resonance spectroscopy).spectrum (mri) The frequency plot of the MRS sig-

nal. The signal intensity is displayed as a func-tion of the chemical shift (as ppm). Nuclei withdifferent resonant frequencies appear as sepa-rate peaks in the spectrum (see MR spectroscopy).

spectrum (power) (us) Range of frequencies (seepower spectrum).

spectrum analyser (us) A device that derives afrequency spectrum from a complex signal.

specular reflection (us) Reflection from a large(relative to wavelength), flat, smooth boundary;also mirror reflection; reflection at a smooth bor-der where the surface unevenness is much lessthan the wavelength. For specular reflection the

angle of incidence is equal to the angle ofreflection. The angle of incidence θi between thedirection of motion of the incident wave and thenormal (perpendicular from the surface) is equalto the angle of reflection θr. The sound wavemay be partially reflected and the remainingwave front travelling through the new medium.The transmitted wave will change directiondepending on material composition, undergoingrefraction (see angle of reflection, Snell’s Law, non-

specular reflection).speed (film) See film sensitometry.speed error (us) A propagation speed that is

different from the assumed value.SPGR (mri) Spoiled gradient recalled rapid gradient-

echo imaging techniques; T1-weighted contrastGE Healthcare (see FLASH, FSPGR, HFGR, RE Spoiled,3D-ME-RAGE, T1-FEE, STAGE-T1W).

spherical aberration (phys) If a wide parallelbeam is incident on a lens, not all the rays arebrought to the same focus causing image dis-tortion. Restricting the light beam to the centreof the lens by an iris diaphragm reduces effect.A distortion sometimes seen in fluoroscopywith large iris apertures.

SPI (mri) Selective population inversion; popula-tion inversion of a selected region in MR spec-troscopy and imaging. By using as a drivingfunction a complex radiofrequency (r.f.) pulsemagnetization can be accurately inverted over avery sharply defined bandwidth, while outsidethat region, magnetization is returned to its initial position, and population is unaffected.■ Reference: Silver et al., 1984.

spin (mri) The intrinsic angular momentum of anelementary particle, or system of particles such asa nucleus, that is also responsible for the magnetic

moment. The spins of nuclei have characteristicfixed values. Pairs of neutrons and protons align tocancel out spins. Nuclei with an odd number ofneutrons and/or protons will have a net nonzerorotational component characterized by an integeror half integer quantum nuclear spin number.

spin density (mri) The number of resonant spinsin a set region or locality which determines thestrength of the NMR signal. The SI unit is molem3. For water this is 1.1 � 105 moles H per m3.One of the principal determinants of thestrength of the NMR signal from the region.True spin density is not imaged directly butmust be calculated from signals received withdifferent interpulse times.

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spin echo–spiral/helical acquisition

spin echo (mri) Reappearance of an NMR signalafter the FID has decayed as a result of the effec-tive reversal of the dephasing of the spins (refocus-ing) by techniques such as specific RF pulsesequences (e.g. Hahn echo, or pairs of magneticfield gradient pulses (gradient echo) applied intimes shorter than or on the order of T2). Gradientechoes will not refocus phase differences duechemical shifts or inhomogeneities of the magnetic field (unlike RF spin echoes).

spin echo imaging (mri) Any of many MR imagingtechniques in which the spin echo is used ratherthan the FID. Can be used to create images thatdepend strongly on T2 if TE has a value on theorder of or greater than T2 of the relevant imagedetails. Spin echo imaging does not directly pro-duce an image of T2 distribution. The spinechoes can be produced as a train of multipleechoes by using the CPMG pulse sequence.

spin echo sequence (mri) An RF pulse series hav-ing 90° followed by 180° gives the Carr–Purcellsequence, depends strongly on T2.

spin-lattice relaxation (mri) T1 relaxation time,also known as the longitudinal or thermal relax-ation time. Regrowth of T1 relaxation whichrequires net transfer of energy from the nuclearspin system to the compound lattice (crystallinelattice). T1 relaxation only occurs when a protonencounters another micro-magnetic field at ornear the Larmor frequency; it is a dipole–dipoleinteraction (see spin–spin relaxation).

spin number (nuclear) (mri) See spin quantum

number.spin quantum number (I) (mri) Property of all

nuclei related to the largest measurable com-ponent of the nuclear angular momentum. Non-zero values of nuclear angular momentum arequantized (fixed) as integral or half-integralmultiples of (h/2π) where h is Planck’s constant.The number of possible energy levels for agiven nucleus in a fixed magnetic field is equalto 2 I � 1. Similarly, an unpaired electron has aspin of ½ and two possible energy levels.

spin tagging (mri) Nuclei will retain their magneticorientation for a time on the order of T1 even inthe presence of motion. Thus, if the nuclei in agiven region have their spin orientation changed,the altered spins will serve as a tag to trace themotion for a time on the order of T1 of any fluidthat may have been in the tagged region.

spin warp imaging (mri) A form of Fourier trans-form imaging in which phase-encoding gradient

pulses are applied for a constant duration butwith varying amplitude. The spin warp method,as other Fourier imaging techniques, is relativelytolerant of non-uniformities (inhomogeneities) inthe magnetic fields.

spin–spin coupling (mri) Some spectral lines mayconsist of groups of lines or multiplets; multipletstructure of spectral lines are due to interactionbetween nuclei that split the NMR energy levelsand result in the observation of multiple allowedtransitions separated by an amount of energyrelated to J (the spin–spin coupling constant); theinteractions are due to spin–spin coupling.

spin–spin relaxation (mri) T2 relaxation time ortransverse relaxation time. May take place withor without energy dissipation and may occurwithout T1 relaxation. The numerical value ofT2 is always less or equal to T1. T2 relaxationis the result of loss of coherence between adjacent spins (see spin-lattice relaxation).

spinal angiography (clin) See vertebral angiography.spinal venography (clin) Rarely used procedure

but may be indicated when others (myelogra-phy, spinal arteriography) have failed. Can beused for observing obstruction of the drainageinto the epidural venous system.

SPIR (mri) Spectral presaturation with inversionrecovery (Philips). Spectral presaturation to reduceMR signal intensity of fat (see FATSAT, ChemSat).

spiral (ct) See helical/spiral.spiral artefact (ct) The interpolation algorithm is

meant to overcome artefacts due to data incon-sistency. This is only successful to some extent.The remaining effects can be divided in twogroups: cone beam artefacts and rod artefacts.

spiral/helical acquisition (ct) Continuous fanbeam rotation and simultaneous continuousobject translation (couch/bed movement) in the z-direction, contrasting with sequential CT scanwhere the bed is stationary during fan beam rota-tion. This enables continuous acquisition of CTimage data (single or multi-slice acquisition) usinga machine with slip-rings. Detector signals can becollected by slip-rings or optical means. The CTaxial sections have a helical geometry, which iscorrected during the reconstruction process. Veryfast subsecond scan times are obtained so thatbreath-hold techniques or bolus chasing can beperformed. Spiral acquisition also allows a com-plete data set to be built up and multi-sectionaldisplays, e.g. sagittal, coronal and oblique as wellas 3D displays for angiography (see pitch).


SPL–square wave modulation transfer function

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SPL (us) Spatial pulse length � cycles � wave-length.

splenic arteriography (clin) Selective splenicarteriography (or coeliaco-arteriography) per-formed preoperatively to ascertain vascularanatomy prior to slenectomy or post traumaticprocedures. Demonstration of the splenic arteryis mostly performed by indirect splenoportogra-

phy. The procedure has been largely replacedby CT and MRI techniques.

splenoportography (clin) Imaging splenic andportal veins. Radiography of portal vessel of theportal circulation by introducing contrast mate-rial into the spleen. Injection of iodine contrastmaterial into the spleen to demonstrate thesplenic vein, portal vein and oesophagealvarices; as direct splenoportography where por-tal pressure can be measured at the same timeand indirect splenoportography performed via asimultaneous selective contrast medium injec-tion into the splenic artery (coeliac trunk) andsuperior mesenteric artery (see percutaneous

splenoportography, PSP).spline fit (di) A smoothly joined piece-wise polyno-

mial. It resembles the position that a draftsman’sspline (thin flexible ruler) would occupy if it wereconstrained to pass through the points. Cubicspline interpolation is a useful method for fittingthe data points and smoothing discontinuities. Itis an improvement on polynomial interpolationwhen the number of data points increases.

spoiler (mri) Gradient pulse or RF pulse applied to eliminate transverse magnetization that persists after the readout period.

Spline fit to data

spoiler gradient (mri) A gradient of sufficientamplitude and/or duration to fully dephase asignal. Often placed symmetrically about refo-cusing pulses so that they have no effect on therefocused signal but eliminate any signal originating at the refocusing pulse.

spoiler gradient pulse (homospoil pulse) (mri)A reverse magnetic field gradient to eliminateresidual magnetization in the nucleus, removingremnants of transverse magnetization. Magneticfield gradient pulse applied to reduce/removetransverse magnetization by producing a rapidvariation of its phase along the direction of thegradient, used to remove the unwanted signalafter an imperfect 180° refocusing RF pulse, acorresponding compensating gradient pulsemay be applied prior to the refocusing RF pulsein order to avoid spoiling the desired transversemagnetization resulting from the initial excita-tion (see FLASH).

SPPA (us) See spatial peak/pulse average.SPT (mri) Selective population transfer. A pulse

sequence for determination of coupling in small-and medium-sized organic compounds. Themethod uses a combination of the double pulsedfield gradient spin-echo (DPFGSE) and the selec-tive population transfer (SPT) techniques and isshown to be useful in magnetic resonance spec-troscopy with many overlapped signals.■ Reference: Uzawa and Yoshida, 2004.

SPTA (us) See spatial peak/temporal average.SPTP (us) See spatial peak/temporal peak.spreadsheet (comp) Primarily accounting software

that allows calculation in a format that is similarto pages in a conventional ledger (see Excel).

SQL (comp) Structured query language. An IBMdeveloped language used in client/server net-works to enable microcomputers to accessdatabases. SQL is data and device independent.There are several competing versions.

square wave modulation transfer function(square wave MTF) (image) Image contrast of arectangular modulation as a function of the spa-tial frequency of the modulation of the objectwith constant contrast; the square wave modula-

tion transfer function can be obtained directly fromimaging a bar test pattern and can be used tocalculate the sinusoidal MTF; because the areabelow a half wave of a rectangular modulation islarger than for a sinusoidal modulation. Thesquare wave MTF yields better figures for spatialresolution than sinusoidal MTF; comparisons of

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square wave modulation transfer function–standard man

two MTFs is therefore valid only if they are of thesame kind.

SR (mri) See saturation recovery.SRAM (comp) Static RAM. This retains information

until the power is switched off. Is slower andmore expensive than DRAM.

SS (mri) Slice select gradient.SSD (image) Shaded surface display.SSFP (mri) Steady-state free precession (Shimadzu).

Enhanced intensity rewinding of phase-encod-ing and no intentional spoiling; method of NMRexcitation in which strings of RF pulses areapplied rapidly and repeatedly with inter-pulseintervals short compared to both TI and T2. Thestrength of the FID will depend on the timebetween pulses (TR), the TI of the tissue and theflip angle of the pulse. With the use of appropri-ate dephasing gradients, the signal can beobserved as a frequency-encoded gradient echoeither shortly before the RF pulse or after it (seeGRASS, FGR, FISP, FAST, GFEC, F-SHORT, DE-FGR, CE-

FAST, True-FISP, PSIF, ROAST, T2-FEE, E-SHORT, STERE).stability (ct) The maintenance over time of con-

stancy of CT numbers and uniformity.STAGE (mri) Small-tip-angle gradient echo

(Shimadzu) (see FFE, GRE, MPGR, GRECO, FE, PFI,GE, TFF, SMASH, SHORT).

STAGE:T1W (mri) Small-tip-angle gradient echo.T1-weighted rapid gradient-echo imagingtechniques, T1-weighted contrast (Shimadzu)(see FLASH, SPGR, FSPGR, HFGR, RE Spoiled, 3D-ME-

RAGE, T1-FEE).stainless steel (18Cr/8Ni) (material) High ten-

sile strength metal alloy with magnetic proper-ties consisting of cobalt, nickel and steel invarious ratios. In general use for syringe needlesand surgical instruments as well as machineconstruction.

Composition (18Cr/8Ni)Tensile strength 600Density (ρ) kg/m3 7930Melting point (K) 1800

standard breast (mamm) A model used for cal-culations of glandular dose consisting of a40 mm thick central region comprising a 50 : 50mixture by weight of adipose tissue and glan-dular tissue surrounded by a 5 mm thick super-ficial layer of adipose tissue. The standardbreast is semicircular with a radius �80 mmand has a total thickness of 50 mm.

standard deviation σ (stats) A measure of sampledispersion. Square root of the variance. The first σoccupies an area 34% of the normal distribution,the second σ occupies 13% and the third σ 3%.Calculating the standard deviation is confused bytwo different formulas; the first using the divisorn �1 and the second using the divisor n:

The formula using n �1 is the most common.standard error (stats) Also called the standard

error of the mean. Due to random samplingerror, chance variations cause the sample meanto be different from the population mean. Thestandard error σn is a measure of uncertainty ofthe mean figure, where:

σn n�

σ

σ ��

�

� �

11

1

2

2

nx x

nx x

Σ

Σ

( )

( )σ

Standard errorFrom a large collection of mammograms 100 are meas-ured for mean optical density (OD) which is 1.8 OD witha standard deviation of 0.5 OD. The degree of uncer-tainty is � 0.05. Two standard deviationswould be 2 � 0.05 � 0.1 OD. So the true result wouldsuggest that the population (complete collection) didnot vary more than 0.1 OD from the mean.

0 5 100. /

standard ion dose Js (dose) The ion dose pro-duced in air by photon irradiation under condi-tions of electron equilibrium. The ICRU definitionof exposure (see ion dose, cavity ion dose).

standard man (dose) The chemical composition ofa standard 75 kg man has been estimated to be:

Element Mass (kg) Percent

Oxygen 45.5 65.0Carbon 12.6 18.0Hydrogen 7.0 10.0Nitrogen 2.1 3.0Calcium 1.05 1.5Phosphorus 0.7 1.0Sulphur 0.175 0.25Potassium 0.140 0.2Sodium 0.105 0.15Chlorine 0.105 0.15Magnesium 0.035 0.05Iron 0.004 0.006Copper 0.0001 0.0001Iodine 0.00003 0.00004


standard man–steel (shielding)

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The proportion of organ mass is:

Organ Weight (kg)

Muscles 30.0Skeleton 7.0Marrow (red) 1.5Marrow (yellow) 1.5Blood 5.4Gut 2.0Fat 10.0Lungs 1.0Liver 1.7Kidneys 0.3Spleen 0.15Pancreas 70 gThyroid 20 gTestes 40 gHeart 0.3Lymph 0.7Brain 1.5Skin 6.1Bladder 0.15Other tissues 2184 g

These values are used for estimating organdose (see MIRD schema).

standard phantom (mamm) A PMMA phantom torepresent approximately the average breast(although not an exact tissue-substitute) so thatthe x-ray machine operates correctly underautomatic exposure control and the dosemeterreadings may be converted into dose to glandu-lar tissue. The thickness is 45 � 0.5 mm andthe remaining dimensions are either rectangu-lar �150 mm � 100 � mm or semicircular witha radius of �100 mm.

standard temperature/pressure (phys) (STP) Inorder to make comparisons (i.e. relative density)many measurements are made at standardtemperature and pressure (STP). The standardtemperature is 298.15 K (25°C) and the pres-sure is 105Pa (approximately 760 mmHg) (seetemperature).

stannous chloride (SnCl2) (nmed) A strongreducing agent used in most radiopharmaceu-tical labelling procedures involving 99mTc.Reducing Tc[VII] to Tc[IV] with the reaction:

Tc[VII] � Sn�� → Tc[IV] � Sn��

star network (comp) Each networked PC is con-nected to a central controller, or hub, with itsown piece of cable. Unlike bus networks, if acable fails it will only affect the attached PCrather than the entire network.

star topology (comp) A network cabling configu-ration that uses a central connection point(called a hub), through which all communica-tion must pass.

statistical efficiency (stats) When comparing dif-ferent statistical methods for computing thesame parameter, the method returning the low-est variance is judged to be the most efficient. Tocompare two analytical designs, the relativeefficiency of one design compared to the otheris a ratio (usually as a percentage) of the variances resulting from the two designs.

stator (xray) The external winding surroundingthe rotor section which completes the inductionmotor (see rotor).

STD bus (comp) A small rugged bus design, orig-inally 8 bits but has been increased to 16 and32 bits.

STE (mri) Stimulated echo. generic term steadystate develops for both transverse and longitu-dinal components of magnetization to refer tothis sequence when the SE/STE is sampled (GEMedical).

steady state (nmed) Maintaining a constantdelivery of tracer material (liquid or gas) inorder to represent the dynamic function of anorgan in a single image. The material must beprevented from recirculating so that back-ground activity is kept as near zero as practical.This is achieved by using nuclides with a veryshort half life. 81mKrypton (T½ 13 s) for steadystate lung studies and in aqueous solution forcardiac studies; 195mAu (T½ 30 s) for venousflow.

steady state free precession (SSFP) (mri) A tech-nique for excitation where strings of RF pulsesare applied with short TR times. The strength ofthe FID will depend on TR, the T1 of the tissueand the flip angle. Signal strength will alsodepend on the tissue T2. To avoid SSFP it may benecessary to use RF spoiling.

STEAM (mri) Stimulated echo acquisition mode. A single voxel MR spectroscopy pulse train con-sisting of three mutually orthogonal 90° section-select pulses. The sequence of choicefor short TE spectroscopy.

steel (shielding) (chem) Mild steel sheet with 1%carbon can be used for shielding having a den-sity of 7900 kg m�3. Old steel (manufactured priorto 1945) is not contaminated with tiny quantitiesof radionuclide (60Co) used in the quality controlprocesses of modern steel manufacture. This old

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steel (shielding)–Stokes, Sir George Gabriel

steel can be used in low background countingsystems:

Thickness Weight Pb-equivalent (mm) (kg m�2) (mm Pb)

11 87 1.023 182 2.0

(see stainless steel).stem cells (dose) Cells capable of self renewal

and of differentiation to produce all the cells ina particular lineage (white and red blood cells).Non-differentiated, pluripotent cell, capable ofunlimited cell division.

STEP (mri) Stimulated echo progressive imaging. TI-weighted variant of RARE.

step and shoot (nmed) As opposed to continuousrotation where the camera stops at each angu-lar projection and acquires data. The gammacamera indexes around the patient, (stops tocollect counts over a fixed time), then moves tothe next projection and collects count data. Itprogresses around the patient until a full 360°(or 180°) data set has been collected.

step response (di) A companion to the impulseresponse. The step response displays charac-teristic of the bandpass filter then settles to ad.c. value. Useful for characterizing a system.

steradian (unit) The solid angle of a cone (Ω)defined as the ratio of the area F of the surfaceof a sphere cut by a cone whose apex coincideswith the centre M of the sphere to the square ofthe radius. Ω � F/r2. Its value describes the effi-ciency of a 4π detector (dose calibrator) or theisotropic dispersion of radiation from a pointsource (see radian, geometry 4π).

STERE (mri) Steady-state technique with refocused free induction decay steady-statefree precession commonly used for imagingcerebrospinal fluid (Shimadzu) (see SSFP, DE-FGR,CE-FAST, True-FISP, PSIF, ROAST, T2-FEE, E-SHORT).

STERF (mri) Steady-state refocused free induc-tion decay (Shimadzu); resembling PSIF.

sterility (dose) A deterministic effect in the male.Temporary sterility is observed for a singleexposure of approximately 150 mGy. Permanentsterility for a dose between 3.5 and 6 Gy.

stiffness (us) Property of a medium; applied pres-sure divided by the fractional volume changeproduced by the pressure. The resistance of amaterial to compression. Sound propogation ina solid medium increases with stiffness.

STILL (mri) Flow/motion compensation Elscint.Reduction of motion-induced phase shifts dur-ing TE (see GMR, GMN, FLOW-COMP, CFAST,MAST, FLAG, GMC, FC, SMART, GR).

stimulated echo (mri) A form of spin echo producedby three-pulse RF sequences, consisting of twoRF pulses following an initial exciting RF pulse.The stimulated echo appears at a time after thethird pulse which equals the interval betweenthe first two pulses. Commonly produced with90° RF pulses others can produce a stimulatedecho (except 180°). The echo signal strength isassociated with T1 relaxation time since theexcitation is stored as longitudinal magnetiza-tion between the second and third RF pulses.The use of stimulated echoes with spatiallyselective excitation with orthogonal magneticfield gradients permits volume-selective excitation for spectroscopic localization.

STIR (mri) Short TI inversion recovery. A pulsesequence to eliminate the fat signal using dif-ferences in T1 (T-one). It uses a modified IRsequence, the fat signal (having a short T1) canbe maintained at the zero crossing point and sois not visible (see FATSAT, inversion recovery).

stochastic (stats) An entirely random process.There is no threshold, having a linear responseto the incidence of damage (cancer induction)for all radiation exposures and giving a normal

distribution.stochastic effect (dose) (ICRP60) A linear or

curvilinear response to low doses of radiationwhere the incidence of cellular transformationto radiation has no threshold. The probability ofwhich, rather than their severity, is a functionof radiation dose without threshold. Exampleswould be leukaemia, breast and colon cancer.Somatic or hereditary effects which may startfrom a single cell. Excess number of malignan-cies in a population has been observed above200 mSv with a probable increase at 50 mSv(see deterministic effect (tissue reactions)).

stokes (unit) A non-SI (c.g.s.) unit of kinematic viscosity.

Stokes, Sir George Gabriel (1819–1903) AnIrish physicist and mathematician. Work onlight spectroscopy, identified x-rays as electro-magnetic rays, formulated Stokes’ Law for thefrictional force moving in a viscous medium. Healso established a law describing that the radi-ation emitted in fluorescence is always greaterthan the input (exciting) radiation.


stop-band filter–structured noise artefact

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stop-band filter (image) The stop band filter thefrequencies should be suppressed with a max-imum amplitude of δ2. A signal filter whichrejects a narrow band of frequencies. Exampleswould be rejection of interference from mainssupply frequency. The width of the transition inthe diagram determines how fast the filterchanges. In the pass-band the transfer function

should be one within a tolerance of δ1 (seeKaiser window).

storage (bulk) (comp) Refers to storage devicesother than the computer main memory wherehigh speed access is not required. Usuallymagnetic disk, optical disk or magnetic tape.These can be fixed or removable devices.

Storage Capacity Access Data transfer device (M-bytes) time (byte s�1)

Hard disk 100 GB–1 TB 10–20 ms 2.4 MBpsFlash 4–64 GB 100–450 ms 10–60 MBps

(375 MBps)Optical 400–650 150 ms V. slowDVD 17 GB 150 ms V. slowDAT 4 GB Slowest SlowestRAM 120–300 15–100 ns 10–250 MBps

STP (comp) Shielded twisted pair. A thin-diameternetwork wire, wrapped with a metal sheath forextra protection against electrical interference.Most installations use a superior data-grade.Shielded twisted pair (STP) wiring is wrappedin an extra layer of sheathing for better shield-ing against electrical noise, offering high-speed transmission. Category 5 wire supports100 Mbps data over UTP and STP; Category 4wire supports 20 Mbps data over UTP and STP;and Category 3 wire supports 16 Mbps dataover UTP and STP.

STREAM (mri) Suppressed tissue with refresh-ment angiography method.

stress relieved anode (xray) Large anode discdiameters increase the heat capacity and alsothe area radiating heat, however, there is poten-tial mechanical damage due to localized expan-sion. This is prevented by cutting radial slots intothe anode; these are stress relieved anodes.

strontium (Sr) (elem)


82Strontium (nmed) The parent nuclide for gen-erating 82Rubidium, a positron emitter forpositron emission tomography (PET).

Production (cyclotron)Decay scheme (e.c.) 82Sr → 82Rb T½ 75 s

82Sr/ 82Rb Generator (β�) → 82Kr stableHalf life 25 daysHalf value layer 7 mm Pb


1 0.9735 0.87110 0.75815 0.66020 0.574

85Strontium (nmed) 514 keV T½ 64.7 days89Strontium (nmed)

ProductionDecay scheme (β�)89Sr 89Sr T½ 51d (β�583,

γ 909 keV) → 89Y stableDecay constant 0.01372 d�1

Photons pure β� 1.46 MeV

Generic name 89StrontiumCommercial names MetastronNon-imaging category Palliative treatment of bone pain

90Strontium (nmed)

Production 235U (n,f)90SrDecay scheme (β�)90Sr 90Sr (b2) → 90Y T½ 2.7d (b2)

90Zr stableDecay constant 0.02423 y�1

Photons Pure β� 546 keVUses: Palliative therapy of bony metastases.

structured noise artefact (ct) The main compo-nents of structured noise are due mostly to par-

tial volume artefact and beam hardening. Bothdisplay streaking artefacts which are seen inregions of high contrast when a sharp discon-tinuity in object density, (air-tissue, tissue-bone or metal-tissue boundaries). Structurednoise artefact will also arise from mechanicalmisalignment within the scanner and patientmovement; it is also seen when using high-density contrast media.

3888

3889Sr(n, ) Srγ

3680

38823Kr d, pn) Sr(

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stunned myocardium–sum peak

stunned myocardium (clin) See myocardium

(stunned).subject contrast (image) Factors influencing the

emerging x-ray beam and film exposure. The x-axis on the film characteristic.

subtraction (DSA) (image) The essential require-ment for DSA is that the contrast signal corre-sponds linearly with the concentration of thecontrast material. Direct subtraction will pro-duce an image dependant on overlying struc-tures (bone) so non-linear subtraction methodsare available:

● logarithmic;● hybrid/dual energy.

Logarithmic subtraction ensures an artery ofuniform diameter traversing regions of mixedtissue type of varying thickness, appears withuniform contrast. It relies on the Beer–Lambertlaw for radiation absorption. The logarithm ofthe signals is obtained prior to subtraction so:

where Io is incident fluence; It is transmittedintensity before contrast; Ic is transmitted inten-sity after contrast; μt is tissue attenuation coeffi-cient; xt is tissue thickness; μv is iodineattenuation coefficient; and xv is vessel thick-ness. Direct subtraction of the transmitted inten-sities Is is not independent of overlying tissuebut dependent on incident fluence Io thus:

However, subtracting the logarithms of the trans-mission:

Is � log(Ic) � log(It)��(μt � xt � μv � xv) � (� μt � xt) � μt � xt

The logarithmic subtraction does not retain sta-tionary anatomical information. The display isnot influenced by patient size or tissue type andonly shows thickness and attenuation coefficientof the contrast medium. Hybrid/dual energy:Artefacts caused by involuntary motion (gutperistalsis or cardiac motion) can be suppressedby dual energy subtraction since μ differences ofgas or soft tissue change little between energies.The decrease for bone is much greater betweenenergies (typically 60 and 110 kVp).

I I I I e es c t ox x xt t v v t t� � � �� ( ) � � � �μ μ μ

I I e I I et ox

c ox xt t t t v v� �� μ μ μand ( )

Bone Tissue Iodine

MaskML kVlow �� zeroMH kVhigh � � zero

Contrast image

CL kVlow ��

CH kVhigh � � ��

Subtracting low and high kV masks leaves apre-contrast mask: ML�MH → MB (bone image).During the contrast phase low and high kVimages are subtracted to yield:

CL � CH → C(B�V)

Finally, the dual energy mask is subtracted togive the iodine contrast vessels alone:

C(B�V) � MB → C(V)

Hybrid subtraction combines dual energy andtemporal subtraction to remove overlying bonetogether with movement artefacts.

succimer (DMSA) (nmed) See DMSA.Sulesomab (nmed) Monoclonal antibody

LeucoScan® (Immunomedics).sulphur/sulfur (S) (elem)


35Sulphur/sulfur (elem) As a tracer label for pro-teins in autoradiography.

Production (reactor) 34S (n,γ) 35SHalf life 87 dDecay mode β� 167 keVDecay constant 0.00796 d�1

Photons Pure beta decay

sulphur/sulfur colloid (nmed) See colloid.sulphur/sulfur hexafluoride (cm) Inert gas used

as an ultrasound contrast agent.sum peak (nmed) A photopeak-like feature of a

PMT pulse-height spectrum that corresponds tothe simultaneous photoelectric absorption in ascintillation detector crystal of two or more primary unscattered, gamma emissions of aradionuclide.


SUP/PPP–surface-shaded reconstruction (SSR)

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SUP/PPP (comp) Serial line interface protocol/point to point protocol. These are both standardsfor connecting directly to the Internet, as opposedto having to log on to it via a host computer.

superconductor (mri) A substance whose electri-cal resistance essentially disappears at temper-atures near absolute zero. A commonly usedsuperconductor in NMR imaging system mag-nets is niobium-titanium, embedded in a cop-per matrix to help protect the superconductorfrom quenching.

superconducting magnet (mri) See magnet (super-

conducting).super-paramagnetic (mri) T2 or T2* contrast

agents; have unpaired electrons in their outerorbital shell, which gives them magnetic sus-ceptibility. When these compounds are placedin an applied magnetic field, a positive mag-netic moment is induced resulting in an attrac-tive force. Originally ferromagnetic substanceswhich have a very small size and thus have losttheir permanent magnetism. Also known asbulk susceptibility agents.

super scan (nmed) Applied to bone scintiscanswhere there is a diffuse and uniform increasein 99mTc-bone agent (HMDP, MDP) throughoutthe skeleton. There is generally a lack of focalactivity.

superficial (us) (as used with the musculoskeletalapplication) Structures located at a depth of1.5 cm or less.

superior mesenteric arteriography (clin)Radiography of the superior mesenteric arterydemonstrating the vascular supply to the smallbowel and the right side of the large bowel.

superior vena cavography (clin) Angiography ofthe superior vena cava performed either byfemoral venous approach into the proximalsuperior vena cava or by contrast medium injec-tion into one or both median cubital arm veins.

supervised area (dose) (ICRP73) The control ofoccupational exposure in medicine can be simpli-fied by the designation of workplaces into con-

trolled areas and supervised areas. A supervisedarea is one where the working conditions arereviewed but special procedures are not normallyneeded and there is very little danger of expo-sure. Commonly defined in the department’slocal rules as an area where dose rates exceed1/10 but are less than 3/10 of any maximum.

suppression (mri) One of a number of techniquesdesigned to minimize the contribution of a

particular component of the object to thedetected signal. For example, commonly used tosuppress the strong signal from water in order todetect spectral Tine from other components.

surface area (body) (clin) The surface area of adisk is: 2πr � (h � r) where r is disk radius andh is disk height; its volume is πr2 h so surfacearea varies in proportion with volume. The surface area of a sphere is 4πr2 whereas itsvolume is 4/3πr3 so surface area and volumealters disproportionately. This is variation is incorporated into the formula for body sur-face area where W is body weight and Hheight: W 0.425 � H 0.725 � 0.0072 m2 (see body

surface area).surface coil (mri) A localized receiver coil that

does not surround the body. Placed close to thesurface over a region of interest having a selec-tivity for a volume approximately subtended bythe coil circumference and one radius deep.Used to restrict the region of the body con-tributing to the detected signal. Only the regionclose to the surface coil will contribute to thenoise, there will be an improvement in the signal-to-noise ratio for regions close to the coilcompared to the use of general receiver coilsthat surround the body. A surface coil used forlocalization measurements of chemical shiftspectra and blood flow studies.

surface rendered (image) or surfaced shadedreconstruction. A method of 3D image recon-struction that simulates reflected light/shade,so conveying depth relationships. Thresholddependent. Does not give attenuation informa-tion or depict calcification (see maximum intensity

projection).surface-shaded reconstruction (SSR) (image) or

surface rendered. A method of 3D image recon-struction that simulates reflected light/shade,so conveying depth relationships. Thresholddependent. Does not give attenuation informa-tion or depict calcification; the most commonlyused 3D images are surface-shaded reconstruc-tions (SSR) and maximum-intensity projection(MIP). These have differing features. Generally,several 3D reconstructions are made at regularangular increments around a given axis. Thesecan then be viewed in sequence to give moreinformation, for example about vessel origins:

● simulates reflected light/shade;● conveys depth relationships;

S


surface-shaded reconstruction (SSR)–syringe/vial shielding

● threshold dependent;● does not give attenuation;● does not depict calcification;● editing not always necessary.

surge protector (comp) A filter that protects acomputer from surge spikes and smoothes vari-ations in voltage (see UPS).

survey (nmed) (see radiation protection survey).Survey meter: a device for monitoring the doserate in an area.

survey meter (nmed) Ionization chamber used tosurvey the exposure rate in radiation areas.

survey radiograph (ct) See scan projection radiograph.survival curve See dose survival curve.susceptibility (mri) See magnetic susceptibility.susceptibility artefact (mri) At interfaces

between tissues of different magnetic suscepti-bility (e.g. tissue and air) the local magneticfield becomes inhomogeneous setting up alocal field gradient causing signal dephasing(signal loss) and spatial misregistration. FLASHand FISP sequences frequently show this in different muscle groups and air voids in temporal bone.

SVS procedure (mri) See Single volume spectroscopy

(SVS).swap file (comp) Also known as virtual memory.

A part of the hard disk is used exclusively ascache memory (Windows) to speed up transfersbetween memory and disk.

swept gain (us) See time gain compensation (TGC)

Swick, M. American physician who first pub-lished a series of intravenous urograms in 1929using iodides of pyridine for excretory urogra-phy, leading to the development of Hypaque,Renografin and Conray.

switch (comp) An intelligent hub that reads thedestination addresses of incoming data packetsand only sends them to the port where therecipient is physically attached.

switchable coil (mri) An RF array coil consistingof several separately resonant elements, anyone of which can be selected as the receiver coilat a particular time. Coils not in use are decou-pled. Applications of switchable coils includeimaging the whole spine without patient repo-sitioning (where the coil elements may collec-tively be known as a ladder coil), imaging ofbilateral structures, such as TMJ or orbit usingseparate coils. or imaging using a coil withselectable field-of-view.

switching (comp) The process by which packetsare received, stored and transmitted to theappropriate destination port.

synchronization (cardiac) (mri) Acquiringimages of particular phases of the cardiac cycle,through either retrospective or prospectivesynchronization. Also sometimes called cardiacgating.

synchronization (prospective) (mri) Controllingthe timing or sequence of image data acquisi-tion according to the phase of respiratory orcardiac cycles.

synchronization (retrospective) (mri) Sorting andpossibly adjusting image data acquired asyn-chronously with the cardiac or respiratory cycle,according to the phase of the cycle at which itwas acquired so as to reconstruct a set of imagescorresponding to different phases of the cycle.

synchronization (respiratory) (mri) The respira-tory phase can be used to control imagingeither by only acquiring the image data duringa particular portion of the respiratory cycle(which increases image acquisition time) or byadjusting the sequence of image data collectionaccording to the phase of the respiratory cyclein such a way as to minimize motion-inducedartefacts in the reconstructed image.

synchronous transmission (comp) Data trans-mission between stations in synchrony. Dataare transmitted in continuous stream (see asyn-

chronous transmission).synthetic aperture (us) Digital beam former tech-

nology used to improve spatial resolution bysynthetically enlarging the receiving aperture.Optimal lateral focus in echo signals originatingfrom deep regions is obtained by using the fullwidth of the transducer array. Synthetic aper-ture technology is applied to the total area ofthe transducer array which is divided into sub-apertures processed sequentially. Syntheticallyenlarged apertures increase transducer sensi-tivity and lateral resolution in deep bodyregions but at reduced frame rates.

syringe/vial shielding (nmed) A cylinder made oflead-containing glass absorbs radiation emittedfrom radioactive material in a syringe, therebyreducing the radiation dose to personnel. Leador tungsten syringe shields of approximately3 mm thickness reduce finger and body dosesby approximately �200 from syringe activities.The dose rate from an unshielded vial contain-ing 4 GBq (100 mCi) 99mTc would be 800 μSv h�1


syringe/vial shielding–system magnification

S

at 30 cm; the suggested shielding is based onan HVL of 0.25 mm Pb (140 keV). Maintaining asurface dose level of 7.5 μSv h�1 would require1.67 mm Pb. A typical vial lead-pot supplies 3 to 10 mm.

system characteristic curve (image) Curve thatrelates a system input value to the correspondingsystem output value (e.g. an optical density ver-sus exposure curve for a film detector, or a greylevel versus optical density curve for a digitizer).

system (comp) Anything that accepts an inputand produces an output in response.

system axis (ct) See axis of rotation.system resolution (nmed) See resolution (extrinsic).Système International (SI) (phys) The 10th and

14th General Conference on Weights andMeasures (1954, 1971) agreed seven independ-ent base units for:

Quantity Dimensions

Length metre (m)Mass kilogram (kg)Time second (s)Electric current intensity ampere (A)Thermodynamic temp kelvin (K)Amount of substance mole (mol)Luminous intensity candela (cd)

Derived SI units are formed by combination of powers of base units, e.g. the SI unit of energy asm2kg s�2 is the joule (J), the newton metre (Nm)or the volt ampere second (VAs). Supplementaryunits are the plane angle as the radian (rad) andthe solid angle as the steradian (sr).

Derived units (SI) Dimensions non-SI unit

Area m2 cm2

Volume m3 cm3

Force (newton N) kg m s�2 dynePressure (pascal P) N m�2 dyne cm�2

Density (ρ) kg m�3 g cm�3

Work/Energy (joule J) kg m2s�2 ErgSpeed (c) m s�1

Linear velocity m s�1 cm s�1

Angular velocity rad s�1

Acceleration m s�2 cm s�2

Momentum kg m s�1 g cm s�1

Angular momentum kg m2s�1 g cm2s�1

Moment of inertia kg m2 g cm2

Electric charge (coulomb C) A s

Systeme International SI (Contd.)

Derived units (SI) Dimensions non-SI unit

Electric conductance siemensElectric capacitance faradPower (watt W) J s�1 erg s�1

Voltage (volt V) J C�1

Resistance (ohm Ω) V A�1

Thermal conductivity W m�1 K�1

Frequency (hertz Hz) s�1 s�1

Radioactivity (bequerel Bq) s�1 curieLuminous flux (lumen lm) cd srIlluminance (lux) lm m�2

Magnetic flux (weber Wb) V sMagnetic flux density (tesla T) Wb m�2

Inductance (henry H) Wb � A�1

Radiation dose (gray (Gy)) J kg�1

Surface tension dyne cm�1 N m�1

Viscosity kg m�1s�1 poise

Supplementary units (dimensionless)

Plane angle (Radian) radSolid angle (Steradian) sr

system magnification (xray) Depends on focus-to-film distance (FFD) and object-to-film distance(OFD). Geometrical unsharpness Ug increases if thefocal spot remains the same. Dual focal spottubes overcome this (mammography tubes having 0.4 and 0.1 mm FS).

FS

FFD

OFD.1

OFD.2

FFD

FFD � OFD m �

Ug

T


TT-tube cholangiography (clin) Postoperative

investigation when a T-tube is left in situduring surgery to exclude presence of calculi inthe common bile duct.

T1 contrast (mri) Contrast of a T1-weightedimage depends primarily on the various T1time constants of the different tissue types.

T1 (mri) Longitudinal relaxation time. Returning toequilibrium after RF excitation. Also known asspin-lattice relaxation process. Measured by T1relaxation time. T1 time is influenced by magnetfield strength, spin mobility, paramagnetic agents.The standard indirect technique for measuring T1uses a series of 90° pulses. The variation of T1with magnet field.

Tissue T1 T2

0.5T 1.0T 1.5T

Fat 210 240 260 80Liver 350 420 500 40Kidney 430 590 690 58Muscle 550 730 870 45Heart 570 750 880 57White matter 500 680 780 90Grey matter 650 810 900 100CSF 1800 2160 2400 160

(see T2).T1 FAST (mri) Fourier acquired steady state

(T1-weighted).T1–FEE (mri) Contrast-enhanced fast field echo

rapid gradient-echo imaging techniques T1-weighted contrast (Philips) (see FLASH, SPGR,FSPGR, HFGR, RE spoiled, 3D-ME-RAGE, STAGE-T1W).

T1 TFE (mri) Contrast enhanced TFE (T1-weighted).T1 weighting (mri) Pulse sequences with short TR

(200–500 ms) and short TE (15–30 ms), oftenused to indicate an image where most of the con-trast between tissues or tissue states is due todifferences in tissue T1. A T1 contrast state isapproached by imaging with a TR short com-pared to the longest tissue T1 of interest and TEshort compared to tissue T2 (to reduce T2 contri-butions to image contrast). Due to the wide rangeof TI and T2, an image that is T1-weighted forsome tissues may not be so for others.

Grey scale Tissue

White FatLight grey Bone marrowDark grey CSFBlack Blood

Examples of T1-weighted pulse sequences are:FLASH, spoiled gradient recalled (SPGR) and RAGE.

T2 (mri) Spin–spin or transverse relaxation time,the characteristic time constant for loss of phasecoherence among spins oriented at an angle tothe static magnetic field due to interactionsbetween the spins, with resulting loss of trans-verse magnetization and NMR signal startingfrom a non-zero value of the magnetization. Inthe xy plane, the xy magnetization will decay sothat it loses 63% of its initial value in a time T2if relaxation is characterized by a simple singleexponential decay.

T2 constant (mri) Tissue-specific time constant.Describes the decay of transverse magnetiza-tion, taking into account the inhomogeneity instatic magnetic fields and the human body. Thecontrast of a T2-weighted image depends pri-marily on the various T2 time constants of thedifferent tissue types.

T2* (mri) The FID time constant influenced by lossof phase coherence among spins oriented at anangle to the static magnetic field. Influenced bya combination of magnetic field inhomogeneitiesand spin–spin relaxation with resultant morerapid loss in transverse magnetization and NMRsignal. NMR signals can usually still be recov-ered as a spin echo in times less than or on theorder of T2.

T2* constant (mri) Characteristic time constantalways less than T2.

T2-FFE (mri) Contrast-enhanced fast field echosteady-state free precession commonly usedfor imaging of cerebrospinal fluid (Philips) (seeSSFP, DE-FGR, CE-FAST, True-FISP, PSIF, ROAST,E-SHORT, STERE).

T2 PEDD (mri) T2 Proton electron dipole dipoleinteraction.

T2 PRE (mri) T2 Proton relaxation enhancement.T2 weighting (mri) Pulse sequences that have

long TR (2000–3000 ms) and long TE(100–200 ms), used to indicate an image wheremost of the contrast between tissues or tissuestates is due to differences in tissue T2. A T2contrast state is approached by imaging with aTR long compared to tissue T1 (to reduce T1contribution to image contrast) and a TEbetween the longest and shortest tissue T2s ofinterest. A TR greater than 3 times the longestT1 is required for the T1 effect to be less than5%. An image that is T2-weighted for some tissues may not be so for others.

T-tube cholangiography–T2 weighting


T

T2 weighting–target region

Grey scale Tissue

White CSFLight grey BrainDark grey FatBlack Blood

An example of a T2-weighted pulse sequenceis FISP.

T2*weighted (mri) The effective T2-weighting ina gradient-echo acquisition when magneticfield inhomogeneity effects are included. ShortTR, 2D sequence (TR 5–10 ms) with a contrastcontrolling inversion-pulse preceding the usual2D data collection, and a scan time on the orderof 1 second.

table feed/increment (ct) The direction of thetable travel between successive sequentialscans or during spiral scanning; the direction oftable feed coincides with the patient’s longitu-dinal direction, and with the system’s z-axis (ifno gantry tilt is applied). For a given examinationtime t (s) and a given scan range R (mm) thedesired table speed Ts � R/t (mm per second).This can be obtained if the selected slice widthS, pitch factor p, and the number of slices perrotation M are known:

The capability of the most modern scanners toacquire several slices simultaneously, where M is4 or greater, gives the advantage of being able toachieve thin slice volume date sets at high scanspeeds (bolus chasing and breath hold). A largertable increment gives faster acquisition, but leadsto a wider slice sensitivity profile (SSP) and conse-quently lowers resolution. The scan width speci-fied by the operator is approximately equal to fullwidth at half maximum of the SSP for one recon-structed image from the spiral volume data set.

TACE (clin) See transarterial chemoembolization.TAD (mri) Duration of acquisition window (time of

analogue to digital conversion).tagging (mri) Spin tagging. A grid of saturation

lines across the cardiac image in order to viewmyocardial motion. Stripe tagging: Parallelstripes for viewing myocardial motion in primary axis view or four-chamber view.

tailored excitation (mri) See selective excitation.tailored pulse (mri) Shaped pulse whose magni-

tude (and possibly phase) is varied with time in

Tp M S

s �

rotation time� �

a predetermined manner. Affects the frequencycomponents of an RE pulse in a manner deter-mined by the Fourier transform of the pulse.

tank unit (ct) Contains the high frequency trans-formers and control electronics small enough tobe located on the rotation gantry. It receives itslow voltage power supply (200–300 V) from theslip ring assembly. This construction adds con-siderable mass to the already heavy rotationalstage, which contains the high-power-tubehousing and related components (see centripetal

acceleration).tantalum (Ta) (elem)

Atomic number (Z) 73Relative atomic mass (Ar) 180.95Density (ρ) kg/m3 16 600Melting point (K) 3269Specific heat capacity J kg�1 K�1 140Thermal conductivity W m�1K�1 57.5K-edge (keV) 67.4Relevance to radiology: as a shielding material and inthe construction of collimators. Yttrium tantalate is usedas scintillants for intensifying screens.

178Tantalum

ProductionHalf life 9 minDecay mode Useful x-raysDecay constant 0.077 min�1

Photon x-rays55–65 keVUses: 178W generator (T½ 28 d) produced 178mTa hasbeen used for imaging the blood pool (low dosepaediatric imaging).

target (x-ray) (xray) The angled periphery on theanode that is bombarded by the electron beam.Since useful x-rays originate from 100 to500 μm of the metal surface depth; the targetthickness is reduced to approximately 1 mmthick. Dual targets are placed concentrically,each having a different angle.

target OD (mamm) The optical density (OD) at thereference point of a routine exposure, chosenby the local staff as the optimal value, typicallyin the range 1.3 to 1.8 OD, including base and fog.

target organ (dose) The organ receiving mostactivity. The thyroid is the target organ foriodine radionuclides and 99 mTc; for labelledcomplex phosphates the target organ is bone.

target region (dose) Region in which radiation isabsorbed. This may be an organ, a tissue, the

T


target region–teflon™

contents of the gastrointestinal tract or urinarybladder, or the surfaces of tissues as in theskeleton and the respiratory tract.

TCP/IP (comp) Transmission control protocol/Internet protocol. This is the standard govern-ing communication between all computers onthe Internet. TCP/IP works by sending packetsof information across multiple networks.

TD (mri) Difference in time of formation of RE spinecho (TER) and gradient echo (GRE).

TDMA (comp) Time division multiple access. A sec-ond generation European mobile cellular radiotechnology which has now been adopted insome 100 countries. TDMA operates by dividinga single radiofrequency into time slots allocat-ing these to multiple calls. A single transmissionfrequency can support multiple simultaneousdata channels. Three non-compatible mobilewireless protocols are GSM, TDMA and CDMA.

TE (mri) Time to echo; the time between middle of90° RF exciting pulse and middle of spin echoproduction. For multiple echoes TE1 and TE2etc. are used. When the RF spin echo and gra-dient echo are not coincident in time, TE refersto the time of the gradient spin echo.

TEeff (mri) See effective echo time.teboroxime (nmed) Generic name 99 mTc-

teboroxime for Cardiotec®, a myocardial perfu-sion agent.

Teceos® (nmed) CIS/Schering preparation of DPD;a bone agent for 99 mTc labelling.

Technegas® (nmed) A commercial radio-aerosolused for ventilation lung scintigraphy. Theaerosol is manufactured by vaporizing analiquot of 99 mTcO4 in a graphite crucible.

TechneScan-PYP® (nmed) A kit for the preparationof 99 mTc-pyrophosphate (Mallinckrodt); a diag-nostic skeletal imaging agent used to demon-strate areas of altered osteogenesis in adults andchildren. Also used for cardiac imaging.

technetium (Tc) (elem)

Atomic number (Z) 43Relative atomic mass (Ar) 99Density (ρ) kg/m3 11 400Melting point (K) 2500 KK-edge (keV) 21.0Relevance to radiology: exclusively as 99mTc metastableisotope.

99mTechnetium Decays by isomeric transition witha physical T½ of 6.02 hours. Widespread use inscintigraphy.

Production (generator) 99Mo/99 mTcDecay scheme (i.t.) 99 mTc 99 mTc (γ 140 keV) →99Tc

T½ 2.13 � 105yHalf life 6.02 hrDecay constant 0.11492 h�1

Photon (abundance) 18–21 keV (0.077)140.5 keV (0.879)

Gamma ray constant 1.7 � 10�2mSv hr�1 GBq�1

@1 mHalf value layer 0.25 mm Pb

45 mm H2OUses: generator produced 99mTc is the most commonnuclide for nuclear medicine imaging.

Radiation attenuation coefficient for 140 keV:

Shield thickness (mm Pb) Attenuation coefficient

0.17 0.50.8 10�1

1.6 10�2

2.5 10�3

3.3 10�4

Physical decay for 99mTc T½ 6.02 hr:

Hours Fraction Hours Fraction remaining remaining

0 1.000 8 0.3981 0.891 9 0.3552 0.794 10 0.3163 0.708 11 0.2824 0.631 12 0.2515 0.5626 0.5017 0.447

(see 99Mo/99mTc generator).99Technetium (nmed) The decay product of

99 mTc; also as a reactor fission product:

Production (fission)Decay scheme 99Tc T½ 2.13 � 105y(beta decay) 99Tc (β�292 keV) →99Ru stable.Decay constant 3.253 E�6y�1

Photon Very weak gamma β�293 keV

Technegas™ (nmed) A commercial radio-aerosolused for ventilation lung scintigraphy. Theaerosol is manufactured by vapourizing analiquot of 99 mTcO4 in a graphite crucible.

Teflon™ (PTFE) (mat) Product registered byDuPont. Polyterafluoroethylene. Extremely lowcoefficient of friction.

Density (ρ) kg/m3 2200Melting point (K) 600Relevance to radiology: General bearing surface orinsulator.

92235

4399

49134U (n, ) Tc ( In)f �


teleconferencing–temporal average intensity (ITA)

T

teleconferencing (comp) Communication byvision and sound on a LAN, WAN or Internetusing a compatible modem and video camera.Image compression (MPEG) allows almost realtime viewing of video movements.

Telebrix® (cm) Preparation of 66% meglumineioxithalamate (Guerbet). Ionic monomer.


Meglumine 5.2 @ 37° 1500 300ioxithalamate

Telepaque® (cm) Generic name iopanoic acid. Amonomeric hepatobiliary x-ray contrast mate-rial for oral use (see cholegraphic contrast agents).

tele-radiology (comp) Transmitting and receiv-ing diagnostic images (radiographs, MRI, ultra-sound etc.) between workstations that arenetworked by LAN or WAN. This is an importantpart of a PACS design and systems in thedesign should be DICOM compatible.

tellurium (Te) (elem)


temperature (phys) The base SI unit of thermo-dynamic temperature is the kelvin (K) definedas the fraction 1/273.16 of the triple point ofwater (where solid, liquid and gaseous phasesare in equilibrium). Absolute zero is 0K. The gas

laws play an important academic role in thederivation of the SI scale for temperature. Theincrease in volume per unit volume of gas at0°C per °C rise in temperature keeping pressureconstant forms a volume coefficient a:

This is a constant whose measurement indicatesthat a value of 3.6609 � 10�3 or 1/273.15°C�1

for all gases. Charles’ Law states that a given massof gas increases by 1/273.15 of its volume at0°C for every degree rise in temperature at con-stant pressure. The magnitude of one degreekelvin is identical with 1° Celsius. Standard temper-

ature is the ice point of water 0°C or 273.15 Kand 100°C � 373.15 K. Three temperature

Increase in volume from COriginal volume at C Temperat

00

°° � uure rise

scales have been used for measuring change inheat output:

● Celsius or centigrade: 0°C is melting ice and100°C is boiling water both at normal pressure105N m�2;

● Fahrenheit (now discontinued in Europe);● The kelvin is the SI unit and its derivation has

been given from the gas laws stated above. Zerodegrees Kelvin (0 K) is absolute zero (�273.15°C).

Temperature conversion from Fahrenheit Tf toCelsius Tc uses a polynomial function where:

Tf � 9/5(Tc � 32) and Tc � 5/9(Tf � 32)

Since Celsius and Kelvin scales have the samemagnitude, conversion is simply Tc � Tk �

273.15:

Physical state ¡F ¡C K

Absolute zero �460 �273.15 0Freezing point water 32 0 273.15Boiling point water 212 100 373.15

(see standard temperature/pressure).temporal average intensity (ITA) (us) Time

averaged intensity for the period that thetransducer is used:

duty factor � pulse average intensity

It is the intensity of the entire pulse train aver-aged over time. If the PRF of the transducer ishigh then ITA will be high and vice versa. If theultrasound is continuous (CW Doppler) thenpulse average � ITA. The time average of inten-

sity at a point in space. For non-autoscan sys-tems, the average is taken over one or morepulse repetition periods; intensity of the entirepulse train over a stated time (time of imageframe) as:

For autoscan systems, the intensity is averagedover one or more scan repetition periods for a spec-ified operating mode. For autoscan modes, theaverage includes contributions from adjacentlines that overlap the point of measurement. Forcombined modes the average includes overlappinglines, from all constituent discrete operating mode

signals. The unit is milliwatt per square-centime-ter, mW cm�2. Typical values are given underultrasound (intensity).

ITA �total power per frame

frame duration

T


temporal peak intensity (ITP)–201Thallium

temporal peak intensity (ITP) (us) The highestintensity of the pulse. The peak intensity withineach pulse of the ultrasound imaging system isthe temporal peak; the average intensity overeach pulse is the pulse average (PA). Measuredas Watt per square-centimeter, W cm�2

temporal resolution (image) Ability to distin-guish closely spaced events in time; improveswith increased frame rate.

tensor (math) An extension of vector quantitiesinto a matrix array of components (usually inorders of two or more dimensions in space);describes anisotropic distributions. A tensor oforder zero is a scalar; a tensor of order 1 is avector (see diffusion tensor imaging).■ Reference: Daintith and Nelson, 2003.

TER (mri) Time of formation of RE spin echo whenadjusted to be different from gradient spin echo.

terabytes (TB) (comp) A thousand gigabytes(1012 bytes), 1000 GB (see byte, kilobyte).

teraflop (comp) A measure of a computer’s speed.It can be expressed as a trillion floating-pointoperations per second (1012).

teratogenesis (nmed) The production of physicaldefects in offspring in utero.

terbium (Tb) (elem)

Atomic number (Z) 65Relative atomic mass (Ar) 158.92Density (ρ) kg/m3 8300Melting point (K) 1629K-edge (keV) 51.9Relevance to radiology: as a dopant for gadoliniumintensifying screens.

terboroxime (nmed) Myocardial imaging agent,marketed as Cardiotec® (Bracco Diagnostics) forlabelling with 99 mTc (see Cardiolite®, sestamibi).

Tesla, Nikola (1856–1943) Croatian/Americanphysicist and electrical engineer. Worked withboth Edison and Westinghouse. Developed ACelectricity as a power source obtaining patentson a poly-phase (3-phase) system and also pre-dicted wireless communication two years beforeMarconi.

tesla (T) (unit) A magnetic field of 1T will producea force of one newton (1 N) on each metre of con-ductor carrying 1 A at 90° to the field, so 1 T �

1 N m� 1 at 90° to the field. 10 mT m�1 � 1gauss cm�1, also 1 T � 1 Wb m�2.

Teslascan® (cm) (Nycomed Amersham/GE Health-care Inc). Preparation of manganese basedtrisodium salt of mangafodipir; the chelating

agent fodipir with manganese ion as a paramagnetic agent.

Compound Concentration Viscosity Osmolality (mg mL�1 (cP) (mOsm/kg)

Mangafodipir 37.9 0.8 @37° 298

test object (qc) A device without tissue-like prop-erties that is designed to measure some char-acteristic of an imaging system (see phantom).

test phantom (qc) Object of particular shape, sizeand structure (including standardized represen-tations of human form), used for the purposes ofcalibration and evaluation of performance of CTscanners (see CTDI, MIRD, body phantom).

tetramethylsilane (mri) A reference compound formeasuring chemical shifts. Tetramethylsilane isassigned a value of 0 parts per million (ppm); othercompounds have chemical shifts greater than 0.

tetrofosmin (nmed) Chemically [6,9,-bis(2-ethoxy-ethyl)-3,12-dioxa-6,9-diphosphatetradecane].Indicated for myocardial scintigraphy at rest andexercise. The radiopharmaceutical kit consists oftetrafosmin which is reconstituted with sodium(99 mTc) pertechnetate to yield 99mTc-tetrofosmin(Myoview®). Since it is concentrated in mito-chondria, it has been used for imaging othermitochondria-rich sites (e.g. tumours) (seeCardiotec®, Cardiolite®, terboroxime, sestamibi).

texture (image) In image processing, an attributerepresenting the amplitude and spatialarrangement of the local variation of grey levelin an image. It is a measure of image coarse-ness, smoothness and regularity.

TFE (mri) Turbo field echo, general sequence(Philips) (see FFE, GRE, MPGR, GRECO, FE, PFI, GE,SMASH, SHORT, STAGE).

TFT (elec) See thin film transistor.TGC (us) See time gain compensation (TGC).TGSE (mri) Turbo gradient spin echo.thallium (Tl) (elem)

Atomic number (Z) 81Relative atomic mass (Ar) 204.37Density (ρ) kg/m3 11 860Melting point (K) 576.6K-edge (keV) 85.5

201Thallium (nmed) As 201Thallous chloride formyocardial perfusion and parathyroid scintigra-phy. Accumulates in viable myocardium as apotassium analogue. Also localizes in parathyroid


201Thallium–thermal units

T

adenomas and normal skeletal muscle. Weakgamma emitter scintigraphy achieved with Hg x-rays.

ProductionDecay scheme 201Tl (γ135, 167 keV: 70–80 (e.c.) 201Tl x-rays) → 200Hg stableDecay constant 0.009480 h�1

Half life 72 hrPhotons 68–82 keV (0.93) x-rays(abundance) 135 keV (0.028) γ

167 keV (0.106) γGamma ray 1.2 � 10�2 mSv hr�1GBq�1 @ 1 mconstantHalf value layer 0.3 mm Pb

43 mm H2OUses: Behaves as a potassium analogue for musclechemistry and so is used as a myocardial imaging agent.


6 0.94018 0.8424 0.8036 0.7148 0.6372 0.5196 0.40

Pb(mm) Attenuation

0.006 0.50.15 10�1

0.98 10�2

2.1 10�3

3.3 10�4

thermal conductivity/heat loss (xray) Thespeed of heat measured in watts per metre perdegree kelvin (or Celsius) W m�1 K�1 (°C�1). It isthe characteristic of the material independentof size or shape.

Substance Specific heat Therm. cond. (J kg�1K�1) (W m�1K�1)

Water 4200 0.59Oil 2130 0.15Air 993 241Silver 235 427Aluminum 910 237Graphite 711 �130Titanium 523 23Copper 386 401Zirconium 280 22Molybdenum 246 140Rhenium 138 48Tungsten 136 178Glass 67 0.9–1.3

(see specific heat).

81203

82201

82201

812013 1T1(p, n) Pb( Pb T )→

thermal equilibrium (mri) A state in which allparts of a system are at the same effective tem-perature, in particular where the relative align-ment of the spins with the magnetic field isdetermined solely by the thermal energy of thesystem (in which case the relative numbers ofspins with different alignments will be givenby the Boltzmann distribution).

thermal index (TI) (us) An indicator of thermalmechanism activity (estimated temperature rise).A measure related to the potential for ultrasonicheating. The thermal index is given by the ratioof the ultrasonic power emitted by the trans-ducer to the ultrasonic power required to raisetissue temperature by 1°C for the exposure con-ditions. The average ultrasonic attenuation inthe model is assumed to be 0.3 dB cm�1MHz�1

along the beam axis. The thermal index has nounits and can be separated into:

● TIS where only soft tissue is exposed;● TIB where bone is exposed at depth;● TIC where bone is close to the surface (cranium).

For foetal exposure, TI should be limited to nomore than 0.7 and the tissue temperatureincrease maintained below 1.5°C.

TI Maximum exposure time (minutes)

0.7 601.0 301.5 152.0 42.5 1

■ Reference: WFUMB, 1998.

thermal neutrons (phys) Neutrons which havevelocities that are approximately the same asmatter in which they are diffusing – in thermalequilibrium usually at 20°C. Standard velocitiesare 2200 m s�1 with an energy of 0.025 eV.Common thermal neutron reactions are desig-nated n, gamma (n,γ) (see radionuclides (reactor)).

thermal units (units)

Heat measurement Units

Heat unit 1.4 J (1 J � HU � 0.7)1 W 1 J s�1

1 cal 4.186 J1 British thermal unit 1 Btu � 1055 J1 kWh 3.6 � 106 JTemperature kelvin K (0°C � 273.15 K)Heat capacity J K�1

Specific heat J kg�1K�1

Thermal conduction W m�1K�1

Latent heat J kg�1

T


thermionic emission–thin-film transistor (TFT)

thermionic emission (phys) When heat is appliedto a wire filament, electrons close to the surfacegain energy and leave the metal due to thermionicemission, forming a cloud. The concentration ofelectrons causes a negative space charge whichrepels further electron emission. Placing a posi-tive charged electrode (anode) above the filamentwill draw electrons so a current will flow.Electrons only flow from the negative cathode(filament) to the positive anode. As the filamenttemperature is increased, the current increasesnon-linearly reaching a saturation point orplateau. The saturation current level depends onthe applied voltage between cathode and anode.This device is non-linear and unidirectional (adiode), allowing current to pass from cathode toanode (not vice versa) which removes the nega-tive half of the alternating current; this is the rectification action.

thermoluminescence (phys) A variation of theluminescent process where the traps in the for-bidden zone are empty, as in phosphorescence(see luminescence). These traps are well belowthe conduction band so the electrons requireadded energy (heat) in order to enter the con-duction band and subsequently return to thevalency band and emit light which is propor-tional to earlier radiation dose. Thermo-luminescence differs from phosphorescence andfluorescence since the energy obtained from theradiation exposure is stored indefinitely withinthe crystal matrix and the output signal (light) isonly emitted when the trapped electrons aredislodged by infrared energy (heat or infraredlaser). Summarizing the events:

1 The phosphor has empty traps in the forbiddenzone at different energy levels and during x-ray interaction electrons are ejected from thevalency band into the conduction band.

2 These electrons then fall in to the empty trapswhere they can stay indefinitely.

3 Energy (heat) is required to eject the trappedelectrons once again into the conduction band.

4 This process can be stimulated by an electric field,by infrared light, or by simply warming crystal.

5 The electrons fall back into the valency bandemitting a broad light spectrum whose inten-sity is equivalent to the original radiation exposure in (1).

Thermoluminescent materials are used fordosimetry (TLD) and imaging.

thermoluminescent dosimeter (TLD) (nmed) Adosimeter containing a crystal-line solid formeasuring radiation dose, plus filters (absorbers)to help characterize the types of radiationencountered. (When heated, TLD crystals thathave been exposed to ionizing radiation give offlight proportional to the energy they receivedfrom the radiation). Common TLD materials are:

Material Application

LiF (Mg or Ti doped) Personal dosimeterTissue equivalent

CaSO4 (Dy or Tm doped) Sensitive environmentalmonitor. Not tissue equivalent.

Lithium borate (Li2B4O7:Mn) Personal dosimeterTissue equivalent

thermoluminescent material (phys) A sub-stance which having been irradiated releaseslight, when heated to a specific temperature, inproportion to the quantity of ionizing radiationabsorbed (see glowcurve).

thick Ethernet (comp) The original Ethernetcable specification, requiring an AUI connector;noise-resistant but expensive and difficult toinstall (see thin-net).

thimble dosimeter (dose) The design of this smallion-chamber compensates for the difficulties ofusing a large standard ion-chamber by provid-ing a solid medium surrounding a central elec-trode. By suitable choice of materials thethimble chamber mimics an air-equivalentdevice and is calibrated over the diagnosticenergy range. The thimble chamber is the basisfor much of the dosimetry in radiography andradiotherapy after the readings are correctedfor temperature/pressure and f-factors convertfor tissue type (see dosimetry).

thin client (comp) Client with small data process-ing capability. Most of the processing is per-formed by the server (see fat client, server/client).

thin-film transistor (TFT) (elec) An insulatedgate field effect transistor constructed as a flatdevice fixed to an insulating substrate ratherthan as part of an integrated circuit chip. Manydiscrete FETs are printed on the surface sinceTFTs, with their companion charge detector(capacitor), can occupy a very small area. Theyare used in colour flat field displays for work-stations and computers. Combined with an x-ray detector material (either fluorescence orphotoconductor) the TFT matrix acts as a position-sensitive device and forms the basis of direct


thin-film transistor TFT–TIC

T

radiology (DR) flat field x-ray detectors for pro-jection radiography, mammography and multi-slice CT. For a workstation/computer high qualityliquid crystal flat panel display the TFT matrixuses between one and four transistors per pixelto control illumination. Each transistor requiresvery little power and has a fast response.

thin layer/paper chromatography (TLC/PC)(nmed) Includes reverse phase thin layer chro-matography (RPTLC) and instant thin layerchromatography (ITLC), the main vehicles for determining radiochemical purity. Usuallyemploys mixtures of commonly available sol-vents and chemicals but can suffer from theinability of any one system to separate out allthe likely impurities. Practical quantificationhas made these techniques popular for theanalysis of radiopharmaceuticals.

thin-net (comp) (thin Ethernet) A CSMA/CD net-work based on thin coaxial cable (also calledthin Ethernet), based on the 10BASE-2 IEEEstandard. Thin Ethernet suffers from a few seri-ous disadvantages: (i) If a user inadvertentlydisconnected or damaged the cable at onenode, the whole network stops, (ii) if a user getsreassigned to a new location, the cable must bere-routed to the user’s new location.

Thompson, Silvanus P. (1851–1916) Britishphysicist and educationalist. First president ofRoentgen Society in 1897, later to becomeBritish Institute of Radiology. Awarded Fellow ofthe Royal Society (FRS) in 1891 (see Mackensie

Davidson).thoracic aortography (clin) Imaging the thoracic

aorta and its major branches using percutaneoustransfemoral catheterization (see aortography).

Thoravision™ (xray) A system developed byPhilips for digital chest radiography using aselenium detector on a moving drum. The elec-trostatic signal is retrieved by scanning elec-trodes. The field size 49 � 43 cm is representedby 2000 � 2000 pixels.

thorium (Th) (elem)

Atomic number (Z) 90Relative atomic mass (Ar) 232.04Density (ρ) kg/m3 11500Melting point (K) 2000K-edge (keV) 109.6Relevance to radiology: historically naturally occurring232Th as thorium oxide was used as a contrast agent(Thorotrast); since it is an alpha emitter it was quicklyreplaced with barium and iodine agents.

Thorotrast (cm) A thorium dioxide x-ray contrastmaterial proposed by Moniz in 1931. Seriousradiation hazard from alpha activity caused it tobe abandoned in the 1940–50s, but long-termretention can still be seen.

threshold contrast (image) The contrast thatproduces a just visible difference between twooptical densities.

threshold dose for tissue reactions (dose) Doseestimated to result in only 1% incidence of tissue reactions.

threshold value (film) (image) The lowest den-sity measurement above base � fog level (seecharacteristic curve, film gamma).

thresholding (image) One of the most importantapproaches to image segmentation. It is theprocess of producing a binary image from greyscale image by assigning each output pixel thevalue 1 if the grey level of the correspondinginput pixel is at, or above, the specified thresh-old, and the value 0 if the input pixel is belowthat level. Thresholding can be applied to aproperty other than grey level by first using anoperation that converts that property to greylevel.

THRIFT (mri) Throughput heightened rapidincrease flip T2 (Picker Medical Inc.); large flipangle SE technique

thulium™ (elem)

Atomic number (Z) 69Relative atomic mass (Ar) 168.93Density (ρ) kg/m3 9300Melting point (K) 1818K-edge (keV) 59.3Relevance to radiology: as a dopant for lanthanum andyttrium intensifying screens.

thyroid function (clin) By measuring a 24 houruptake, using 131iodine in capsule form andcomparing thyroid activity levels with a care-fully prepared reference phantom, an estimateof thyroid activity can be made. Euthyroid con-ditions show a 15–35% uptake; hypothyroid a0–10% and hyperthyroid conditions 40–80%.Non-toxic goitre (endemic) shows a 40–90%uptake.

TI (mri) Inversion time; inversion recovery, timebetween the inverting 180¡ RF pulse and thesubsequent exciting 90¡ pulse; detects amountof longitudinal magnetization.

TIB (us) Thermal index for bone.TIC (us) Thermal index for cranial bone.

T


TIFF–TIS_as

TIFF (comp) Tagged image file format. This is a tag-based file structure. Where a BMP file is built ona fixed header with fixed fields followed by thesequential data, a TIFF has a much more flexi-ble structure which is a simple 8-byte headerthat points to the position of the first image filedirectory (IFD) tag, which can be of any lengthand contain any number of other tags, thusenabling completely customized headers. BMP isan important format for fast and efficient screendisplay, but TIFF is a better standard for print-based work. TIF image data are not stored scanline by scan line, but can be broken into taggedstrips of multiple scan lines, convenient forhandling large print files, allowing for easybuffering and random access.

Tim (mri) A Siemens term for total imaging matrix.A selection of various organ specific coils.

time (phys) See second.time activity curve (nmed) A plot of increase or

decreased radioactivity through an organ (e.g.kidney) or tissue (e.g. lung) with time. A kidneytime activity curve is the renogram. The curve iscreated by collecting total count date from aseries of ROIs placed over the organ of interestin a dynamic series.

time (spatial) domain (di) A signal whose ampli-tude varies with time (e.g. a sine wave, ultra-sound echo or RF pulse) exists in the timedomain (see frequency domain, Fourier transform).

time of flight (TOF) (nmed) Improving PET resolu-tion by including time of detection for the coin-cidence gammas. TOF imaging systems havevery short resolving times and coincidencelocalization can be obtained to within a fraction of a nanosecond, permitting improvedsensitivity, signal to noise ratio. and improvingimage resolution. Commercial systems canresolve events less than 200 pico seconds apart(0.2 ns). (mri) When the local magnetization ofmoving tissue or fluid is selectively altered in a region, e.g. by selective excitation, it will carrythe altered magnetization with it when itmoves. This is the source of several flow effects.The flow of non-saturated fully relaxed bloodinto the slice generates a high signal, for time-of-flight angiography. Stationary spins are par-tially saturated and so give a lower signalintensity.

time-gain compensation TGC (us) Signal postprocessing amplification for correcting attenua-tion loss. The slope of the TGC curve can be

altered to compensate for both low and highattenuation tissues. The maximum gain limitsimage depth. The gain can also be alteredselectively at particular depths.

time series (TS) (mri) T2* images labelled withnumber and time position in the series.

time to peak map (TTP) (mri) Shows the regionaldistribution of the time needed to the minimumperfusion signal, either grey scale or colourcoded. It is generated for the TIR sequence.

TIR (mri) See turbo inversion recovery.TIRM sequence (mri) See turbo inversion recovery

magnitude (TIRM).tin (Sn) (elem)

Atomic number (Z) 50Relative atomic mass (Ar) 118.69Density (ρ) kg/m3 7300Melting point (K) 505.1Specific heat capacity J kg�1 K�1 228Thermal conductivity W m�1K�1 66.6K-edge (keV) 29.2Relevance to radiology: as a lightweight shieldingmaterial for protective aprons. The K-edge makes itmore effective than lead over diagnostic energies.

tip angle (mri) See flip angle.TIPS (clin) Transjugular intrahepatic portosys-

temic shunt. The non-surgical creation of aportosystemic shunt in portal hypertensionusing a transjugular stented channel betweenan intrahepatic vein and the portal vein.

TIS_as (us) The soft-tissue thermal index at surfacefor non-autoscanning mode as:

TIS_as o� �W fc1 1

210�

0 2 4 6 8 10

Gain dB cm�1

Rel

ativ

e si

gnal

gai

n (d

B)

Depth (cm)


TIS_as–TONE technique

T

where Wo1�1 is the bounded-square output power

in mW and fc is the centre frequency in mega-hertz. It has no units.

tissue characteristics (clin) The approximateatomic number and electron density, used inattenuation calculations. The electron density(volume) is the product of the density and elec-tron density (weight).

Material Z Density Electron density (g cm�3) (�1023cm instant thin

layer chromatography3)

PMMA 7.4 1.19 3.87Water 7.4 1.0 3.34Muscle 7.4 1.04 3.45Fat 6.1 0.91 3.08Bone 12.7 1.66 5.25Titanium 22 4.51 12.47

tissue equivalent (dose) Having the same sensi-tivity to photon energy as soft tissue. Applied todetectors which mimic closely the response oftissue to radiation so accurate tissue doses canbe calculated. Air and lithium fluoride are tissueequivalent detectors; film emulsion is not.

(us) Tissue equivalence having similar acoustic

impedance to soft tissue.tissue reactions (dose) Injury in populations of

cells. Characterized by a threshold dose, and anincrease in the severity of the reaction as thedose is increased further (see deterministic effect).

tissue synchronization imaging (TSI) (us)Analyses tissue velocity within the image forassessing delayed cardiac wall motion. Thedelay value produces a functional image.

tissue weighting factor wT (dose) (ICRP60) Seeweighting factor (tissue).

10�1

10�2

10�3

10 100

AirSoft tissue

LiF

Photon energy (keV)

Mas

s A

ttn. C

oef.

(m�

2 kg

�1 )

titanium (Ti) (elem)

Atomic number (Z) 22Relative atomic mass (Ar) 47.90Density (ρ) kg/m3 4540Melting point (K) 1948Specific heat capacity J kg�1 K�1 523Thermal conductivity W m�1 K�1 21.9K-edge (keV) 4.96Relevance to radiology: very low density input windowfor image intensifiers. Tensile strength �8 of aluminiumand equal to that of stainless steel but half the density.

TLA (clin) Translumbar abdominal aortography.TMR (mri) Topical magnetic resonance.TMS (mri) Tetramethylsilane reference compound

for hydrogen spectroscopy.TMZ (xray) See TZM as titanium, zirconium, molybde-

num alloy for anode construction.toe (film) (image) The non-linear minimum recorded

density on the film’s characteristic curve (see film

gamma).TOF (nmed, mri) See time of flight.token (ring) bus network (comp) A network trans-

mission method that requires a node to have con-trol of a ‘token’ before it can send messages; betterdistribution than CSMA/CD on busy networks, butmore complicated to implement (see bus, topology).

tolerance (contrast medium) (cm) Tolerance ofiodinated ionic contrast media (CM) for generalintravascular use, depends on two factors: osmo-lality (see osmolarity (Osmol)/osmolality (Osm)) andchemotoxicity, including ionic effects. Osmotoxicpotential may be expressed with the osmotoxicity

ratio (OTR). A balanced addition of sodium and cal-cium adjust the electrophysiology of cardiac cellsand improve contrast media tolerance in cardiacpatients, especially during angiocardiographicprocedures.

Tomocat® (cm) A suspension of barium sulphate(Mallinckrodt/Tyco Healthcare Inc), containing5% w/v BaSO4.

tomography (image) A technique for separatingsections from a volume distribution. Early x-raymachines used analogue-mechanical methodsinvolving swinging x-ray sources (linear tomog-

raphy). This was extended to nuclear medicine(H.O. Anger dual head Pho-Con). Digital tech-niques are used in computed tomography, sin-

gle photon emission computed tomography (SPECTor SPET) and positron emission tomography (PET).

TONE technique (mri) For time of flight angiogra-phy to minimize saturation effects for volumeblood flow.

T


topogram or scoutview–transfer function

topogram or scoutview (ct) See survey radiography.topology (network) (comp) Describing designs

available for networking computers. Bus topol-ogy, the physical layout of a network in whichall systems connect to a main cable; also knownas linear bus. Token ring, IBMs implementationof token passing, governed by the IEEE 802.5standard, second most popular network topol-ogy after Ethernet. Star network, is a centralizeddesign with the appearance of a star, the centralcomponent being typically a file server. A highercost system since each workstation requiresdirect cabling to the server. More reliable since acable break will only affect a single workstation.

torque (phys) A vector quantity given by the productof the force and the position vector where theforce is applied for a rotating body. The effectiveforce on a rotating body turning the body about itscentre or pivot point. The torque exerted by a forceis also known as the moment of the force. Angular

momentum may be changed by applying a torqueto the rotating body. An applied torque mayincrease or decrease the rotation motion, or it maychange the direction of the rotation axis. For rotat-ing objects, torque (τ) is equal to the time rate ofchange of angular momentum. When an objectpossesses both angular momentum and a mag-

netic dipole moment is placed in a uniform externalmagnetic field, the resulting motion can be com-plex. A torque will be produced which will causea rotation motion at a constant angular frequency.This is referred to as the ‘precession frequency’.

tositumomab (nmed) A treatment for non-Hodgkinslymphoma. Commercial name Bexxar®.

TOSS (mri) Total suppression of sidebands.total effective dose equivalent (TEDE) (dose)

The sum of the deep dose equivalent (externalexposures) and committed effective dose equivalent

(internal exposures).toxicity (contrast medium) (clin) The total toxicity

of a contrast medium is the sum contribution ofthe chemotoxicity, the osmotoxicity and the ion tox-icity. CM agents are excreted mainly through thekidneys, and the use of high volumes and con-centrations in several diagnostic proceduresmakes this a target organ for the evaluation ofCM toxicity in man. There is a higher biocompat-ibility of non-ionic dimers in terms of osmotoxic-ity, chemotoxicity, organ specific toxicity thanthe non-ionic monomers, suggesting specificindications for use of this contrast material inhigh risk patients.

TPPI (mri) Time-proportional phase incrementation.TR (mri) Repetition time. The period of time

between the beginning of a pulse sequence andthe beginning of the succeeding (essentiallyidentical) pulse sequence (see sequence time).

TReff (mri) See effective repetition time.trace image (mri) Contrast in diffusion images is

generated by the direction of the diffusion tensor (see diffusion tensor imaging).

tracer (nmed) A radiopharmaceutical used totrace a physiological or biochemical processwithout affecting it.

tracer kinetics (nm, mri) The fundamental variablesassociated with the tracer indicators: intravascu-lar (proteins), inert hydrophilic (DTPA), inertlipophilic (xenon), active transport (MAG3), recep-tor target (monoclonal antibodies), blood cells(platelets). Parameters that can be measured aremean transit time, distribution volume, clearance,extraction fraction, blood flow, permeability.■ Reference: Peters, 1998.

transarterial chemoembolization (TACE) (clin)Embolization of liver tumours, haemobilia, arte-riovenous malformations and fistulae of theliver, and demonstration of mass lesions.

trans-axial (image) A body sectional image per-pendicular to the body axis. The most commondisplay format for CT and MRI.

transducer (us) A device that converts energyfrom one form to another (see piezoelectricity).

transducer array (us) Ultrasound transducerassembly containing several transducer ele-ments; transducer element(s) with dampingand matching materials assembled in a case.

transducer assembly (us) The transducer array,transducer housing (probe), and associatedelectronic circuitry.

transducer element (us) A single piezoelectricsensor in a transducer assembly.

transfer function (image) The part of a digital filter

design concerned with transferring information inthe pass-band. Information on the transfer func-tion characteristics of a filter is available permittingthe conversion into discrete forms so that digitalfilters can be designed. For a look-up table (LUT),the manipulation of the grey scale can be achievedin terms of a transfer function, relating brightnessvalues to each pixel. If g(x,y) is the image formed bythe convolution of an image f (x,y ) and an operatorh(x,y ) then in the frequency domain:

G(u, v) � H (u, v) � F (u, v)


transfer function–transient equilibrium

T

where H(u,v) is the transfer function.transfer rate (comp) The speed with which data

can be transferred between a storage medium(disk) and the CPU; measured in MBytes s�1

(MBps). A hard disk is typically 2–4 MBps andRAM is 10–250 MBps.

transform (math) These are essentially rules forconverting a function of a variable as, for exam-ple, f (t) (time) into a function of another variableas F (w) (frequency w). The inverse transform willrecover f (t) from F (w). The transformed functionusually has different properties that either sim-plify the handling of the original function orreveal different properties.

transform domain filtering (image) Modificationof weighting coefficients (transform coeffi-cients) prior to reconstruction of the image viainverse transform.

transformer (phys) A device consisting of twocoils, a primary and secondary wound on a com-mon soft iron core (in the case of low frequencypower supplies 50 or 60 Hz) or on sintered ferrite cores for high frequency transformers(�1000 Hz). If Ep and Es are the voltages at theprimary (np) and secondary (ns) windings:

The parameters np and ns are the number ofturns. The ratio ns/np is the turns ratio. A stepup transformer has a turns ratio �1 and a stepdown transformer �1. The current in a trans-former changes inversely with the voltage; theenergy in the transformer obeys the conserva-tion of energy, thus the energy in the second-ary, at any instant, equals the power suppliedto the primary. If Is and Ip and the secondaryand primary currents then:

A step-up transformer with a turns ratio 60:1voltage increases, the currents are steppeddown in the ratio 1:60. A step-down trans-former will increase current in the secondarywinding but at a reduced voltage. There is aloss of energy in the transformer due to eddy

currents which are induced in the iron core bythe changing magnetic flux in the windings;these losses can be reduced if the core is lami-nated. Compact high voltage transformers can

I E I EII

E

E

n

ns s p ps

p

p

s

p

s� � � � �and

EE

nn

s

p

s

p�

be designed by increasing the AC supply fre-quency since the induced secondary voltage Es

is influenced by the cross section area of thetransformer core A and the number of turns nand the frequency f where:

Es � (A � n) � f

High frequency transformerUsing the basic formula is Es � (A � n) � fFor an x-ray tube supply of 100 keV and a relativeoverall size (A � n) for 50 Hz transformer being 2000.

2000 � 50 Hz � 100 keV

1666 � 60 Hz � 100 keV

1000 � 100 Hz � 100 keV

20 � 5 Hz � 100 keV

10 � 10 Hz � 100 keV

Operating a generator frequency of 10 kHz, reducestransformer size by �200.

(see tank unit, slip ring).transhepatic cholangiography (clin) Imaging

biliary tree either presurgery (percutaneous tran-

shepatic cholangiography) or during surgery byinjecting contrast medium directly into the liver.

transient equilibrium (nmed) A parent:daughterdecay series where λd � λp. The parent 99Modecay constant is 0.0103 and for 99 mTc is0.1155. The daughter has a much longer half-life than in secular equilibrium and requires a muchlonger build up time. A condition in which iso-topic equilibrium has been reached and the ratioof half-life of parent to half-life of daughter issmall (approximately 10:1).(see generator, secular equilibrium).

806040200

Rel

ativ

e ac

tivity

(99

Mo

and

99m

Tc)

1

0.5

0.1

99mTc 99Mo

Time (hours)

T


translation rotation system–transmission scan

translation rotation system (ct) An early designof CT scanner that performs a translation of thedetector and the x-ray tube parallel to the scanplane in order to linearly sample the attenua-tion profiles; each translation is followed by asmall increment around the axis of rotationwhen the next translational movement is carried out. Now replaced by the constantlyrotation fan beam.

translumbar abdominal aortography (clin)TLA direct injection of iodine-based contrastmedium into the abdominal aorta through thelumbar region and imaging the abdominalaorta with its branches. Employed in cases ofperipheral vascular disease.

transmission (image) (image) Transmission imag-ing is the basis of CT and plane film radiography.It is also part of the attenuation correction mecha-nism in SPECT where an external radionuclidesource is used. Attenuation varies according togamma energy (see graph). Transmission scansare used for assessing true attenuation. Hardwareand software are available for this purpose. Acommon arrangement uses a 123Gd source posi-tioned between patient and gamma camera.123Gd emits dual gamma energies of 97 and104keV. Simultaneous collection of 104keV from123Gd and 140keV from 99mTc (patient) is madeduring the SPECT study using separate energywindows; calculation for real time attenuation ismade from 104keV absorption.

transmission (mono-energetic) (shld) Thebroad beam and narrow beam transmission of agamma source through absorbing material isthe shielded air kerma rate/unshielded air kerma

0 2 4 6 8 10 12 14 16

Radius of water phantom (cm)

140 kVp

100 kVp

Rel

ativ

e tr

ansm

issi

on

1.0

0.8

0.6

0.4

0.2

0.0

rate at a defined distance. It varies with photonenergy and absorber thickness (shieldingmaterial or tissue). For a mono-energetic source

transmission obeys the exponential law (figurebelow). Transmission is inversely proportionalto absorption (see transmittance).

transmission (x-ray; polyenergetic) (shld) Thebroad beam transmission of an x-ray beamthrough absorbing material (shielding) is theshielded air kerma rate/unshielded air kerma rateat a defined distance. It varies with photon energyand absorber thickness (shielding material or tis-sue). Since an x-ray source is polychromatic thetransmission varies as the shielding material non-uniformly filters lower photon energies (see image

contrast, transmittance).

transmission scan (nmed) The attenuation due tothe patient is determined by using a transmissionacquisition, which is then used to generate

100

10�1

10�2

10�3

10�4

10�5

10�6

125 kVp50 kVp

0 1 2 3 4 5

Lead thickness (mm)

Bro

ad b

eam

tran

smis

sion

Broad beam

Narrow beam

1.0

0.8

0.6

0.4

0.2

0.00 1 2 3 4

Tran

smis

sion

Absorber thickness


transmission scan–true coincident events

T

transaxial maps which resemble CT images andprovide sufficient information to define the var-ious density inside the thorax (lungs and heart).These maps, along with the emission data, areused to correct for attenuation correction usingan iterative reconstruction method.

transmission angle (us) Angle between thetransmitted sound direction and a line perpen-dicular to the media boundary.

transmit/receive (T/R) coil (mri) An RF coil thatacts as both a transmitter producing the B1 exci-tation field, and as a receiver of the NMR signal.Such a coil requires a fast switching circuit tochange between the two modes. A body coil istypically a T/R coil, but smaller volume T/R coils(head/ extremities) are often used at high field asa means of reducing RE power absorption.

transmittance (transmission coefficient) (film)The ratio of light intensity transmitted throughthe film image to the incident light. The recip-rocal of transmittance is optical density.

transport index (nmed) An indication of doserate on the surface of a package used to trans-port radionuclides. The numerical equivalent ofthe maximum dose rate in air at 1 m from theexternal surface of the package transport cate-gories currently used:

Transport Maximum radiation Labelindex level at any point on the

external surface

0 �5 μSv h�1 (0.5 mrem h�1). WHITE-I.�0 but �1 �5 μSv h�1 (0.5 mrem h�1 YELLOW-II.

�0.5 mSv h�1 (50 mrem h�1)�1 but �10 �0.5 mSv h�1 (50 mrem h�1) YELLOW-III.

�2 mSv h�1 (200 mrem h�1)�10 �2 mSv h�1 YELLOW-III

transverse magnetization (Mxy) (mri) The compo-nent of the macroscopic magnetization vector inthe x,y plane; oriented perpendicular to theapplied magnetic field. The precession of transversemagnetization induces a decaying signal due tothe loss of phase coherence between precessingspins. After RF excitation, Mxy decays to zero attime constant T2 (ideal) or T2* (real).

transverse plane (ct) Anatomical plane orthogo-nal to the body’s longitudinal (z) axis; in mostcases identical with the scan plane.

transverse relaxation (mri) Loss of transversemagnetization from a non-zero value in the Mxy

plane. Described by the T2 time constant.transverse relaxation time (mri) See T2.

transverse wave (phys) Where displacement of the transmitting medium is perpendicular to thedirection of propagation. Electromagnetic radiation

is an example where electric and magnetic fieldsvary sinusoidally at right angles to each other butin the direction of propagation. Sound is trans-mitted as a longitudinal wave. Transverse wavesare propagated by vibrations perpendicular to thedirection of the wave travel.

traveling saturation slice (mri) Slice positioningwhere a presaturation pulse can be applied toone side of the slice to reduce the signal inten-sity of spins (typically blood) that are about toflow into the side of the slice. Arteries andveins can be displayed selectively, since theflows are often in the opposite direction. Theslices are measured sequentially (slice by slice).The presaturation pulse retains its position rel-ative to the slice.

trend (stats) The movement of a variable over aperiod of time. An erratic pattern may obscure thegeneral trend but this can be detected by using a moving average. Regression equations are usedfor computing trends and confidence limits canbe set for detecting abnormal movements (poten-tial malfunction). Trend calculations are appliedto film processor performance and QC.

trigger delay (TD) (mri) ECG triggering. Inter-val between the trigger and release of themeasurement.

Triosil® (cm) Sodium-metrizoate preparation (Glaxo)(see metrizoic acid/metrizoate compounds).

Compound Osmolality Iodine (mg I/mL)(mOsm/kg)

Metrizoate-Na 2150 370

tritium 3H (nmed) The nucleus comprising 1 pro-ton and 2 neutrons. An unstable isotope ofhydrogen decaying by β� emission to 3He witha half life of 12.26 years (see hydrogen, tritium).

triton (nmed) Nucleus of tritium atom 3H� used inthe cyclotron ion beam.

tropolone (nmed) A chelating agent used for blood

cell labelling with a radionuclide.true black (image) Black produced by a separate

black ink rather than a mixture of cyan,magenta and yellow.

true coincident events (nmed) A true PET coincidence is the simultaneous interaction of emissions resulting from a single nucleartransformation. PET relies on the coincident

T


true coincident events–TSR

detection of photons in two detectors. Pulsesare considered to be coincident if they occur intwo detectors within a specified resolving timeτ of each other. Because of this finite resolvingtime, there is the possibility of two independ-ent pulses occurring by chance so as to pro-duce a random coincidence. The resolving timeτ for the coincidence window in the mostrecent PET scanners is typically 6 ns with acoincidence time resolution of �3 ns within thewindow (see scatter coincidence).

true FISP (mri) Fast imaging with steady stateprecession having heavy T2 weighting(Siemens), commonly used for imaging cere-brospinal fluid. Gradient echo sequence thatprovides the highest signal of all steady statesequences. Contrast is a function of T1/T2; withshort TR and short TE, the T1 portion remainsconstant and the images are primarily T2weighted. FISP and PSIF signals are generatedsimultaneously. As the signals are superim-posed, true FISP is sensitive to inhomogeneityin the magnetic field. Images may contain inter-ference stripes, since the periodic oscillationsoccur parallel to transitions in tissue the arte-facts are bands of alternating high and low sig-nal intensity (see SSFP, DE-FGR, CE-FAST, PSIF,ROAST, T2-FEE, E-SHORT, STERE).

true positive fraction (TPF) (stats) The condi-tional probability of deciding that an observeddata set (image) was generated by a specifiedstate (that a specified disease was present)when the disease state was in fact present. TPFis equivalent to the ‘sensitivity’ index oftenused in the medical literature to indicate theability of a diagnostic test to detect diseasewhen it is present (see receiver operating charac-

teristic (ROC), predictability).truncation (image) Due to practical restrictions

the discrete Fourier transform which representsa finite approximation of a signal must be repre-sented by an infinite integral (Fourier transform).As a consequence, the Fourier series will notconverge smoothly and a truncation error willresult (signal or image distortion/artefacts); atruncation error results due to an inexact orforeshortened formula. An impulse in the fre-quency domain implies an infinitely long timewindow, so a compromise is required betweentime and frequency domain performance bytruncating the window. Smoothing filters aredesigned to minimize these errors.

truncation error (image, ct) Insufficient terms in aFourier transform leading to incomplete conver-gence. A problem in tomography with restrictedbeam size where a fan beam (nuclear medicineor CT) misses certain anatomy in its rotation sodata sets are incomplete. Truncation artefactsare seen in axial tomography (CT or SPECT) whencertain parts of the anatomy lie outside the fan

beam at certain projections (arms included in athorax CT) or inconsistent anatomy in magnifi-cation views (cardiac SPECT). In the diagram theshaded sections are not covered by this fanbeam angle on certain positions of the beamrotation, causing incomplete data sets whichlead to truncation errors.

TSE (mri) Turbo-spin echo; FSE/RARE technique(Siemens/Philips).

TSI (us) See tissue synchronization imaging.TSR (mri) Total saturation recovery.

Truncation error (CT)

Error


tube-current exposure-time product (mAs)–twisted pair

T

tube-current exposure-time product (mAs)(xray) See mAs.

tube current modulation (ct) During the helicalscan, thicker denser regions of the patientrequire higher tube current (increased x-ray flu-

ence), thinner or less dense regions use a lowertube current. Alternatively the tube current canbe modulated during the course of a single tuberotation used when the body has cross sectionsthat are not circular (shoulders and pelvis),where the lateral projection through the patientattenuates more than the anterior–posterior pro-jection. Exposure to the patient can be varied byup to 80% in the course of a 0.5 s rotation. Thishas a dose advantages and the added advan-tage of making the image quality more constantfor the individual slices in each series.

tube loading (xray) The tube-current exposure-time product (mAs) that applies during a partic-ular exposure.

tube potential (xray) The potential difference (kilo-volt, kv) applied across the anode and cathode ofthe x-ray tube during a radiographic exposure.

tungsten (W) (elem)

Atomic number (Z) 74Relative atomic mass (Ar) 183.85Density (ρ) kg/m3 19320Melting point (K) 3650Specific heat capacity J kg�1 K�1 130Thermal conductivity W m�1K�1 174K-edge (keV) 69.5Relevance to radiology: Major anode material for x-raytubes (see rhenium, tantalum).

tungsten (shielding) (xray) A syringe shield con-structed from 1.5 mm tungsten will reduce radia-tion exposure from 99 mTc (140 keV) by up to 94%(see syringe/vial shielding). The shielding is lighterand more resistant to abrasion than lead.

tuning (mri) Process of adjusting the resonant fre-quency of the RF circuit, to the desired value ofthe Larmor frequency. More generally, theprocess of adjusting the components of thespectrometer for optimal performance, includ-ing matching impedances.

turbo angiography (mri) Fast 3D angiographytechniques; increases speed by a factor of 2using zero filling (interpolation techniques inslice-selection direction) short TR and TE.

turbo factor (mri) Time saved using a turbo-spin

echo sequence compared to a conventional spinecho sequence.

turbo-FLASH (mri) Fast long angle shot. Fast gra-dient switching, using a spoiler gradient pulse,with subsecond image acquisition; flip angles6–15°. Gradient reversal replaces 180° rephas-ing pulse. Allows breath holding which makesabdominal imaging possible (see FFE, GRE, MPGR,GRECO, FE, PFI, GE, TFF, SMASH, SHORT, STAGE).

turbo gradient spin echo (turbo GSE) (mri)Additional gradient echoes are generated beforeand after each spin echo. Spin echoes are allo-cated to the raw data matrix centrally to givepure T2 contrast. Fat is darker, more sensitive tosusceptibility effects than turbo-SE.

turbo inversion recovery (TurboIR, TIR) (mri) ATurboSE sequence with long TI for fluid sup-pression. The turboIR sequence gives a trueinversion recovery display.

turbo-spin echo (turbo-SE) (mri) Fast spin echoimaging technique with an acquisition time of100–300 s using flip angles 90 and 180°. A spin-echo technique that acquires more than oneindividually phase-encoded echo per TR period.With turbo spin-echo more than one TR periodis used to fill all of the image k-space. The turbofactor increases speed, and is usually used toimprove resolution.

turbo-SHORT (mri) Turbo version of SHORT ver-sion of Turbo-FLASH-like sequence (Elscint).

turbulent flow (turbulence) (phys) A state of fluidor gas in which there is disturbed or non-regularmotion; random, chaotic, multidirectional flow ofa fluid with mixing between layers; flow that isnot laminar (see Reynold’s number).

TWAIN (comp) (technology without an interestingname; reputedly). A program interface betweenscanner and graphics software allowing imagesto be scanned and acquired directly into thecomputer program. Twain drivers enable ascanner to operate from and acquire the imagedirectly to an image editing application placedin Word documents.

twisted pair (comp) A popular and low-cost LANcabling method, also commonly used for tele-phone wiring. This type of cabling features twoor more pairs of copper wires, each pair inter-twined then wrapped to minimize electricalinterference. Twisted pair wiring comes in twomajor varieties: UTP and STP, although twistedpair cabling supports less bandwidth than coaxor fibre optic cable but it is cheaper. Wiringtypes are assigned category numbers based ontheir transmission capacity.

T


two dimensional Fourier transform imaging (2DFT)–TZM

two-dimensional Fourier transform imaging(2DFT) (mri) A form of sequential plane imagingusing Fourier transform imaging.

two dimensional NMR (mri) Form of NMR spec-troscopy in which an additional dimension isadded to the conventional chemical shiftdimension by allowing varying amounts of dif-ferent interactions between spin systems (suchas NOE, spin–spin coupling or exchange).

typical value (math) The common or expectedvalue found in the majority of facilities.

Usually obtained by analysing the measure-ments from a large selection of patients orequipment and finding the mean value from ahistogram display. A common method forassessing patient dose and x-ray equipmentperformance.

TZM (xray) An alloy of titanium, zirconium, molyb-denum used as a foundation for x-ray tubeanode construction. This is then covered with alayer of tungsten and then the tungsten/rhe-nium target (Trinodex®, Philips).


U

UUART (comp) The universal asynchronous

receiver/transmitter chip which controls thetransfer of data over a serial port (e.g. modems).

uberschwinger artefact (ct) Edge enhancementcreating large density differences betweeninterfaces. Commonly seen with metal prosthe-ses giving an increased opacity parallel to theinterface so simulating loosening of a prothesis.Also seen as false images of infection or pneu-mothorax.

UDMA (comp) Ultra DMA. EIDE controllers linkingthe hard drive to a computer; supporting trans-fer rates of 33 MBps (megabytes per second)and 66 MBps; these are UDMA33 and UDMA66.

UFOV (nm, rad) See useful field of view.UHF (units) Ultra high frequency radio. Those fre-

quencies circa 1 GHz. Television is commonly470–512 and 512 to 806 MHz.

Name Frequency Applications

Ultra high 300 MHz to Television, mobilefrequency (UHF) 3 GHz (cell) telephones,

wireless networking, microwave ovens

Super high 3 to 30 GHzs Wireless networking,frequency (SHF) radar, atellite links.

ultrasound (us) Sound having a frequencygreater than 20 kHz. Clinical ultrasound has aspectrum typically 2–10 MHz although experi-mental probes have exceeded this range.

ultrasound (bioeffects) (us) Biologically sensi-tive areas are:

● first trimester embryo;● foetal skeleton (heating);● neonatal transcranial Doppler;● eye;● intracavitary studies.

Biological effects have not currently beenobserved in vivo at spatial peak/temporal average

intensity ISPTA:

ISPTA �100 mW cm�2 for �500 s (unfocused)ISPTA �1 W cm�2 for �50 s (focused)

Damage to biological systems is divided into:

● Chromosome damage: sister chromatid exchangehas been seen in vitro with single strand DNA

UART–ultrasound (duty factor)

breaks. An ISPTA of 94 W cm�2 at 8 MHz CW gaveconsistent breaks with ultrasound cavitation.

● Foetal damage: prolonged ultrasound investiga-tions could have side effects on embryogenesisand on later prenatal and postnatal develop-ment. Exposure should be minimized whenusing pulsed Doppler studies on the foetus; thiscan induce ultrasound heating in foetal bone.

The AIUM (1982) reported no significant biologi-cal effects observed using intensity levels below100 mW cm�2. In 1993 the FDA increased theISPTA limit from 94 mW cm�2 to 720 mW cm�2

(see ultrasound (heating), ultrasound (cavitation)).ultrasound (cavitation) (us) This is gas bubble

growth and is related to acoustic pressure p, inten-sity I and acoustic impedance; related as: I � p2/z.At high intensities used in therapy, cavitation hasbeen demonstrated but it has not been reportedat diagnostic intensities.

ultrasound (contrast agents) (cm) Consist ofmicrobubbles filled with air or gases. Theseconsist of thin shells composed of albumin,lipid or polymer confining a gas (usually nitro-gen or a perfluorocarbon). Current generationsof microbubbles have diameters from 1 to 5 μm.Among the blood pool agents, transpulmonaryultrasound contrast agents offer higher diag-nostic potential compared to agents that can-not pass the pulmonary capillary bed afterintravenous injection. In addition to their vas-cular phase they can exhibit a tissue- or organ-specificity. Current contrast agents are:

Gas Stabilizing Commercial shell Product

TranspulmonaryPerfluoropropane Phospholipids Definity®

Dodecafluoropentane Surfactant Echogen®

Octafluoropropane Albumin Optison®

Air Albumin Quantison®

Perfluorohexane Surfactant Imavist®

Sulphur hexafluoride Phospholipids Sonovue®

Transpulmonary/organ specificAir Palmitic acid Levovist®

Air Cyanoacrylate Sonavist®

Perfluorocarbon Surfactant Sonazoid®

ultrasound (duty factor) (us) A pulsed beamconsists of an ‘on period’ (mark) and an ‘offperiod’ (space). The mark (m): space (s) ratio:m/(m � s) � 100% is the duty factor. This is an important ratio when computing the safetylimits (see duty cycle).

U


ultrasound (heating)–ultrasound intensity (temporal)

ultrasound (heating) (us) This is caused by theconversion of ultrasonic energy to heat in thetissue. The intensity I of ultrasound energy.

ultrasound intensity (continuous CW) (us) Wherethe duty factor equals 1 and the pulse average(PA) and temporal average (TA) are the same.

ultrasound intensity (pulsed) (us) Where thevalues for the pulse average intensity (IPA) and tem-

poral average intensity (ITA) are influenced by theduty factor. There are six composite intensitiesthat combine spatial and temporal values. Thecommonly used intensity measurements forultrasound are ISPPA (the maximum pulse inten-sity) and ISPTA (the peak power given by thecomplete pulse) the magnitude of the intensityvalues follows the sequence:

Spatial peak temporal peak ISPTP

Spatial peak pulse average ISPPA

Spatial peak temporal average ISPTA

Spatial average/temporal peak ISATP

Spatial average/pulse average ISAPA

Spatial average/temporal average ISATA

The absolute values of the intensity measure-ments are influenced by focal dimensions.Transducers with tight focus in the near field willproduce higher ISPTA values than those that arefocused in the far field. Typical values for thecommonly applied intensity measurements are:

Intensity Value

ISPTA 720 mW cm�2

ISPPA 0–700 W cm�2

ISATA 410 mW cm�2

ISAPA 500 mW cm�2

TP

PA

TA

ISPTA

ISPPA

Time

ultrasound intensity (spatial) (us) Spatial peak (SP)and spatial average (SA) intensities. The ultrasonic

intensity transmitted in the direction of acousticwave propagation, per unit area at the point con-sidered normal to this direction. For measurementpurposes, this point is restricted to where theacoustic pressure and particle velocity are in phase,i.e. the far field or the area near the focal surface.Measured as the energy flux crossing unit area inunit time or the rate of delivery of ultrasoundenergy per unit area of tissue as W m�2 ormW cm�2 where 1W m�2 � 0.1mW cm�2. Thespatial peak (SP) is the beam intensity measuredon the central axis. The spatial average (SA) is themean intensity averaged over the beam crosssectional area.

Intensity parameter Unit (typical value)

ISP �10 mW cm�2 (unfocused)up to 1 W cm�2 (focused)

ultrasound intensity (temporal) (us) Temporalpeak intensity (ITP) and temporal average

103102101100mW cm�2

Imag

ing

Dop

pler

ISPTA

Pulsed

Linear

Phased

Mechanical

CW/Obst

Cardiac/PV

SP

SA


U

intensity (ITA); the latter is the intensity of theentire pulse train averaged over time as:

which gives the average intensity of the pulse train. In the figure, PA is the pulse average

intensity IPA (see duty cycle).ultrasound (resolution) (us) Two axes of resolu-

tion affect the ultrasound image (i) lateral (ii)axial. A further aspect of resolution, that of theslice width, is fixed by the radial curvature ofthe transducer face. Approximate resolutionvalues (mm) are given for typical transducerfrequencies (MHz):

Frequency Axial Lateral

2.0 0.7 3.03.5 0.4 1.75.0 0.3 1.27.5 0.2 0.8

10.0 0.15 0.613.0 0.11815.0 0.102

The axial resolution is parallel to the ultrasoundbeam; the ability to separate detail with depth.Can be estimated from the spatial pulse lengthas SPL/2 or from the pulse geometry as:

ultrasound (safety) (us) The graph plots theboundary of safe intensity levels for time spentin the investigation.

104103102101100100

101

102

103

104

105

I SP

TA (

mW

cm

�2 )

Study time (S)

Potentially damagingregion

Safe region

Newequipment

0 77. � cycles in pulsefrequency

total power per frameframe duration

ultrasound intensity (temporal)–ultrasound (values)

The FDA intensity limits for the heart (Ht), periph-eral vascular (PV), ophthalmology (Op) andobstetrics (Ob) are:

Ht PV Op Ob

W cm�2

ISPTP 310 310 50 310ISPPA 190 190 28 190

mW cm�2

ISPTA 430 720 17 94ISATA 430 720 17 94

From the AIUM/NEMA 1998 report:

Ophthalmic: Thermal index �1.0; mechanical index �0.23 and derated ISPTA0.3 �50mW cm�2.

Foetal heart: ISATA: �20 mW cm�2 for CW andISAPA: �20 mW cm�2 for pulsed devices.

■ Reference: AIUM/NEMA, 1998.

ultrasound (scatter) (us) Caused when the beamstrikes irregularities similar in size to the ultra-sound wavelength. Scatter also occurs when thebeam traverses particulate matter (blood cells).Scattered echoes form a cone about the reflectionaxis whose angle depends on the wavelength ofthe ultrasound. For smaller wavelengths (higherfrequencies) the scatter angle is wider. Intensityof scatter decreases as (frequency)4. Scatter isresponsible for speckle which is scattered, ultra-sound from different sites interfere or add constructively (see acoustic absorption, reflection).

ultrasound (values) (us) The term ‘ultrasound’ isapplied to sound waves from 20 kHz to above100 MHz. Medical ultrasound operates at fre-quencies from 1 to 20 MHz. Tissue detail andpenetration are compromised outside this range.

Frequency (MHz) 2.0 3.5 5.0 7.5 10.0

Wavelength (mm) 0.77 0.44 0.31 0.21 0.15

Application according to frequency (MHz)

1 2 3 4 5 7.5 10 15 20Therapy

Abdomen/obstetrics/cardiacSuperficial/thyroid

Opthalmic

Tissue Density Speed rayl Absorption (kg m�3) (c) m s�1 (Z) kg.m.s. dB MHz cm

Muscle 1040 1568 1.63 2Fat 970 1470 1.42 0.5Bone 1700 3600 6.12 4.1

U


ultrasound venography–UNSCEAR

ultrasound venography (clin) Ultrasound imagingmay be used for the non-invasive examination ofperipheral as well as central veins and their sur-rounding tissues. A valuable non-ionizing radia-tion procedure using either grey-scale real-timeimaging or by combining the image with pulsedDoppler for indicating vascular velocity. Duplexscanning combines the two-dimensional grey-scale image with the flow velocity signal superim-posed in colour; sound effects are also available.

UltraSPARC® (comp) Computer architecture intro-duced by Sun that are 64 bit CPUs that run 32 bitSPARC software. The UltraSPARC® T1 processorgives you up to eight processing cores with fourthreads per core. The UltraSPARC® T2 processorhas up to 8 cores per chip and up to 64 threadsper processor with dual 10Gbit Ethernet andPCI-E integrated onto the chip.

Ultratag® (nmed) A commercial stannous chloride

preparation for labelling red blood cells.UltraVent® (nmed) A radioaerosol delivery sys-

tem (Mallinckrodt Inc.).Ultravist® (cm) Commercial (Schering) prepara-

tion of non-ionic iopromide introduced in 1985.Supplied in iodine concentrations of 150, 240,300, 350, 370 mg I/mL�1.


Iopromide @ 20° 9.0 530 @ 37° 350

umbra (phys) A point source of light projects asharp shadow onto a screen. When this ceasesto be a point source the image unsharpnesscauses shadowing, the degree of which dependson the focal spot f and its position relative to theobject and image plane. The parameters i and odetermine the penumbra dimension p (unsharp-ness) so that:

Umbra and penumbra effects are shown by x-rays since they are produced by fine and broadfocal spots and mimic the light effectsdescribed here.

uniformity (ct) Consistency of the CT numbers inthe image of a homogeneous material acrossthe scan field.

uniformity (differential) (nmed) The intrinsicdifferential non-uniformity of sensitivity Ud for

pi f

o�

�

a gamma camera is Ud � ΔC/M � 100% whereΔC is the maximum difference in countsbetween two adjacent pixels and M is thelarger of the two counts. Typical current values,for non-uniformity in central regions, are�2.5% and for useful field of view �2.8% (seeintegral uniformity, uniformity (intrinsic)).

uniformity (integral) (nmed) Measured as the per-centage difference between the maximum andminimum pixel counts in a chosen samling area.Typical values �2.5% CFOV, �4.5% UFOV for pla-nar acquisition. This must be improved for SPECTimaging (�1% CFOV) (see uniformity (intrinsic)).

uniformity (intrinsic) (nmed) The gamma camera

resolution, without collimator. This is best per-formed by using a point source 2–3 m from theuncollimated camera face. Two measurementsof uniformity are calculated:

Integral uniformity where maximum and mini-mum values over a 64 � 64 matrix are used inthe equation:

Differential uniformity is measured over a limiteddistance in the matrix (5 pixels) collecting thelargest positive and negative deviations fromthe mean:

uninstaller (comp) This small program usuallylogs the installation of a program package sothat all files can be removed when the programis erased from the system.

unit dosage (nmed) A precalibrated single dosageof a radiopharmaceutical in an individual con-tainer, intended for use in only one patient.

UNIX (comp) A multiuser, multitasking operating

system produced by AT&T. Has become standardprotocol for TCP/IP and SMTP and forms thebackbone for many specialized operating systems

(see Linux).UNSCEAR (dose) United Nations Scientific

Committee on the Effects of Atomic Radiation.Some recent reports:

● Sources and Effects of Ionizing Radiation.Publication E.77.ix.1.

● Sources and Effects of Ionizing Radiation.● Sources Effects and Risks of Ionizing Radiation.● Genetic and Somatic Effects of Ionizing Radiation.

� ��

� � �100

[( ) ( )][( ( )]

max minmax) min

� ��

�100

[ ][ ]max minmax min


U

unsharpness (geometric) Ug (image) The penum-

bra (Ug) or blurring of an edge or point sourcedue to geometrical factors of the imaging sys-tem (typically x-ray unit) which considers: focalspot size (FS), object to focus distance (OFD) andfocus to film distance (FFD), sometimes calledsource to image distance (SID), such that:

(see system magnification).

unsharpness (movement) Um (image) Displaypenumbra due to movement. Cardiac movementin chest radiography must have an exposuretime of 0.01 s in order to maintain a 1 Lp mm�1

at cardiac edges. In the diagram where h is theedge height and d the degree of movementthen visible contrast is lost as d increasesalthough h is unchanged.

Movement

FS

FFD

OFD

FS � OFD

FFD � OFD Ug �

Ug

Ug ��

�

FS OFDFFD OFD

unsharpness (geometric)–uranium decay series

unsharpness (radiographic) Ur (image) Displaypenumbra due to image detector properties andincluding collimator design for gamma camerasin nuclear medicine. The basic factors are dueto separation of the screen or collimator fromthe imaging surface, diffusion of the event inthe intensifying screen or due to collimator/gridsepta penetration. The intensifying screen alsoshows halation due to surface reflection andthe dual emulsion film shows crossover due tofilm base transparency.

unsharpness (total) Ut (image) The total unsharp-ness, incorporating geometric unsharpness, move-

ment and radiographic, is the geometrical mean ofthese values:

Ut is also called image blur.UPS (comp) Universal power supply or uninter-

ruptible power supply. An electrical power sup-ply that includes a battery to provide enoughpower to a computer during a power failure toback-up data and properly shut down.

uranium (U) (elem)

Atomic number (Z) 92Relative atomic mass (Ar) 238.03Density (ρ) kg/m3 18970Melting point (K) 1405.4K-edge (keV) 115.6Relevance to radiology: depleted uranium (235U extracted) is used as a shielding material for highactivity sources (some 99 mTc generators).

235Uranium

Half life 700 � 106 yearsDecay mode AlphaEnergy 4.7 MeVAbundance 0.72% of natural uranium

238Uranium

Half life 4.5 � 109yDecay mode mixed α, β and γDecay constant 1.54E–10 y�1

Photon Mixed

This is the start of the natural uranium decayseries, which eventually yields radon.

uranium decay series (phys, nmed) The naturaldecay series from 238U to stable lead 206Pbinvolving complex mixed α, β and γ emissions.

U U U Ut g m r� � �

U


uranium decay series–Uroselectan®

Z Element Symbol T½

92 Uranium 238U 4.5 � 109 y90 Thorium 234Th 24 d91 Protactinium 234mPa 1.2 m92 Uranium 234U 2.5 � 105 y90 Thorium 230Th 8 � 104 y88 Radium 226Ra 1602 y86 Radon 222Rn 3.8 d84 Polonium 218Po 3.0 m82 Lead 214Pb 27 m83 Bismuth 214Bi 20 m84 Polonium 214Po 160 μs82 Lead 210Pb 21 y83 Bismuth 210Bi 5.0 d84 Polonium 210Po 138 d82 Lead 206Pb Stable

(see radon).uranium (shielding) (nmed) Large activity

99Mo/99 mTc generators greater than 600 GBq,commonly carry an internal depleted uraniumshield. The requirements for packaging usingdepleted uranium as shielding should be covered in an inactive (plastic) sheath.

Density 19 050 kg m�3

K-edge 115.6 keV

urethrography (clin) Radiography of the male orfemale urethra by retrograde injection of con-trast medium or voiding of contrast mediumfrom the bladder (cystourethrography, mic-turating cystogram). Combining ascending anddescending urethrography gives information on:the anatomy of the urethra, bladder capacity,bladder anatomy, the presence of vesicouretericreflux, competence of the bladder neck and distalsphincter, presence of intraprostatic reflux andresidual urine volume. Ascending urethrogramwhere CM is instilled slowly via catheter. The CMshould pass proximal to the distal sphincter tooutline the posterior urethra; patient is supine.Descending micturating cystourethrographywhere CM is instilled via catheter and the bladderadequately filled to induce micturation. Images ofposterior urethra taken during voiding.

urine values (clin) Normal values:

Calcium �7.5 mmol/24 hCreatinine clearance 70–130 mL/mmProtein �150 mg/24 hSodium 40–210 mmol/24 hUrea 170–500 mmol/24 hUric acid 3–6 mmol/24 h

URL (comp) The unique resource locator is theaddress of a web page. The protocol for identify-ing a document on the web. For example, the URLwww.hospital.com/library/protocol.html tells thebrowser to use HITP protocol to find the servernamed www.hospital.com and ask it for the pageentitled protocol.html in the /library/ directory. AURL is unique to each user (see domain).

Urografin® (cm) Commercial preparation contain-ing varying proportions of sodium diatrizoateand meglumine diatrizoate by Schering for infu-sion urography. Ionic monomer.


Na-diatrizoate 2.2 @ 20° 146meglumine 1.4 @ 37°diatrizoate 30%Na-diatrizoate 220megluminediatrizoate 45%Na-diatrizoate 292megluminediatrizoate 60%Na-diatrizoate 1940 370megluminediatrizoate 76%

urography (clin) Radiographic examination of theurinary tract, particularly the upper parts, i.e. the kidneys and the ureters. More rarely itinvolves the lower parts, i.e. bladder and ure-thra. Contrast medium excreted close to 100%via the kidneys. Intravenous: following injec-tion of contrast medium images are taken of therenal area to demonstrate pelvi-calyceal sys-tems and renal areas, ureters and bladder. Alsocalled excretion urography. Control imagestaken prior to the injection. Drip infusion:investigation of masses within the entire renalanatomy including ureters in cases of dimin-ished renal function. Retrograde: radiographyof the urinary tract following injection of con-trast medium directly into the bladder, ureter orrenal pelvis.

Uromiro® (cm) Commercial (Merck PharmaChemicals Inc., Bracco) version iodamide.

Uroselectan® (cm) The first clinically used intra-venous contrast medium, chemically consistingof a large negatively-charged iodinated anion,and a smaller positively-charged sodium cation.When dissolved in water forming two ions foreach molecule of contrast medium. This was an

www.hospital.com/library/protocol.html

www.hospital.com


U

Uroselectan®–UUE

ionic contrast medium, having a high osmolal-ity. Similar compounds were synthesized whichhad two atoms of iodine each, thus doubling theradio-opacity for the same contrast media con-centration (see Binz, Räth).

Urovison® (cm) Generic name Na-diatrizoate meg-lumine diatrizoate. An ionic contrast mediummanufactured by Schering AG with a higher pro-portion of sodium diatrizoate than the Urografins.Iodine concentration 325 mg I/mL.

Urovist® (clin) Generic name diatrizoate. An ioniccontrast medium.

USB (comp) Universal serial bus. An industry stan-dard for connecting different compatible periph-eral devices across multiple platforms. Itsupports true plug and play and allows hot-swapping of devices (hot pluggable). It providesstandardized cabling for printers, digital cam-eras, scanners, game pads, joysticks, keyboards,mice and storage devices. USB supports simul-taneous connection of up to 127 devices byattaching peripherals through interconnectedexternal hubs. USB.1 has a limited bandwidth of12 Mbits s�1. USB 2.0 standard gives transferspeeds of up to 480 Mbits s�1 enabling fastvideoconferencing; considered a potential alter-native to IEEE 1394 (otherwise known asFireWire) for use with high-speed video. USB2.0 is backwards-compatible; older systemsthat support version 1.0 accommodated.

USB hub (comp) A multiple-socket USB connecterthat allows several USB-compatible devices tobe connected to a computer.

useful field (xray) The extent of the collimated x-ray beam. The smaller the angle τ the widerthe target track can be. In general, the smallerthe anode angle the wider the focal trackwhich increases the power rating; however,angle size also influences the field size of the x-ray beam at a given source to image distance(SID). Useful field size increases with anodeangle, however, so also does the effective focalspot size which will degrade image resolution,so a large area radiograph would be obtainedat the expense of resolution.

useful field of view (UFOV) (nmed) The collima-tor’s field diameter. These will generally beworse than the central field of view (CFOV)because of crystal edge effects. (xray) The extentof the collimated x-ray beam or extent of gammacamera field of view.

USPIO (mri) Ultra small super paramagnetic ironoxide.

UTP (comp) Unshielded twisted pair; a thin-diameter network wire popular in networkcabling installations. UTP is more pliable thanSTP and less expensive and easier to install. Itsdisadvantage is limited immunity to noise. UTPis available in voice-grade and data-grade ver-sions. Category 5 wire supports 100 Mbps datarates. Category 4 wire supports 20 Mbps andCategory 3 wire supports 16 Mbps.

UUE (comp) UUEncoded files used with Internetnews groups.


V.42bis–velocity (sound)

VV.42bis (comp) An algorithm used by modems that

can compress data by ratios of 8:1, but in thereal world this can be reduced to around 2.5 1.

V.90 (comp) Approved in February 1998, this stan-dard replaces K56FIex and X2, providing a com-mon protocol for 56K modems to access an ISPthat supports V.90. 56K modems not V.90 readymust have Flash memory and will requirefirmware upgrades provided by the manufacturer.

valvoplasty (clin) Reconstruction of deformedcardiac valve.

vapourware (comp) A name given to softwarethat is announced far in advance of any releaseand either never materializes or appears muchlater than originally advertised.

variable, continuous (math) A variable that maytake any value.

variable, dependent (math) A variable depend-ent on another. Usually placed on the y-axis.

variable, discrete (math) The opposite to continu-ous. A variable that may only take certain values.

variable focusing (us) Transmission focus withvarious focal lengths.

variable, independent (math) A variable notdependent on another, usually placed on the x-axis.

variance (stats) A measure of dispersion. Square ofstandard deviation; one of the outputs of the auto-correlation process; a measure of spectral broad-ening (i.e. spread around the mean). (us) Square ofstandard deviation; one of the outputs of theauto-correlation process; a measure of spectralbroadening (i.e. spread around the mean).

VAS (mri) Variable angle spinning.Vascoray® (clin) Meglumine iothalamate, manu-

factured by Mallinckrodt.Vasoray® (cm) Iothalamate preparation. Ionic

monomer.Vasovist® (cm) Gadolinium compound as gado-

fosvesat trisodium for MR angiography, abdom-inal or limb vessels (Schering).

vasogram (clin) Radiography of the vas deferensto determine patency using contrast medium.

vector (magnetic) (mri) When a sample is exposedto an external magnetic field a net alignment ofthe individual magnetic moments (protons) willresult in a preferred direction along the externalfield. The nuclei that make up this alignmentcontribute to a resultant magnetization vector (M).There is a longitudinal magnetization vector

(ML) and transverse magnetization vector (Mxy).A transverse component of the magnetizationvector can be produced by applying a trans-verse magnetic field (90° RF pulse or other flipangle) which will move M away from the mag-netic field; as the strength of the RF pulseincreases the longitudinal component ML of themagnetization vector decreases. M will assumea position between M zero and Mxy.

vector array (us) Linear sequenced array thatemits pulses from different starting points and(by phasing) in different directions.

vectors (math) Quantities for which both size anddirection are necessary for a full specification.Examples are velocity, acceleration and force.They are frequently represented by an arrowwhose length represents signal magnitude andangle representing the magnitude of compo-nent (e.g. sine wave angle in radians) (see scalors).

veiling glare (xray) Ratio of light intensity atcentre of image with and without lead disk.

velocity (linear) (v) (phys) The velocity of a mov-ing point, or of a body is the rate of its displace-ment or the rate at which it changes its position,in a given direction. If it moves through s m in tseconds then v � s/tm s�1. Velocity is a vectorquantity having both magnitude and direction,but when the direction is constant (a straightline) and the body covers equal distances inequal times, the velocity is uniform and ismeasured by the displacement per unit time,

V

Photon time of flightThe distance travelled by a 0.511 MeV gamma photonafter 1 nanosecond (10�9s). All electromagnetic radia-tion has a velocity of approximately 3.0 � 108m s�1.Distance travelled � (3.0 � 108) � 10�9 � 0.3 m.

Units of linear velocity

SI m s�1

CGS cm s�1 1 cm s�1 � 1 � 10�2m s�1

Other miles hr�1 1 mph � 0.44 ms�1

velocity (angular) (ω) (phys) See angular velocity.velocity (of flow) (phys) Compare to flow

Poiseuille etc.velocity (sound) (phys) Velocity of sound in

medium depends on density ρ and elasticity K:the velocity of sound v in a gas obeys:

v �Pαρ


velocity (sound)–virus

Boyle’s Law states that gas volume is inverselyproportional to pressure:

and as gas density is proportional to pressurethen sound velocity is independent of pressurechanges. From Charles’ Law at constant pressurev T, since V 1/ρ then 1/ρ T. Therefore, atconstant pressure:

Material Velocity (m s�1)

Air 330Helium 1000Water 1540Soft tissue 1540Bone 4080Aluminium 6400

VENC (mri) Velocity encoding value that is speci-fied for phase contrast MRA.

venocavography (clin) Imaging the vena cava.Inferior vena cavography : angiography of theinferior vena cava (IVC), generally performedusing a common femoral vein approach witheither an angiographic cannula or, for improvedimage quality, a 5F pigtail catheter placed inthe lower end of the vena cava. Superior venacavography : angiography (TE) of the superiorvena cava (SVC) performed either by femoralvenous approach using a pigtail catheteradvanced into the proximal SVC, or by means ofinjection of contrast medium into one, or betterboth, median cubital arm veins.

venography (clin) The major investigations whichuse iodine-based contrast material for visualizingmajor veins are: pelvic venography for pelvicveins and distal vena cava; inferior venocavogra-phy for the inferior vena cava and some hepaticand renal veins; renal venography; ascendingperipheral venography of the leg for visualizingcalf, thigh and patency of the iliac veins; periph-eral venography of the arm (see splenoportography).

ventricular angiography (clin) Radiographicexamination of the left ventricle by intra-ventricular injection of contrast medium. Theprocedure demonstrates global left ventricularfunction and regional wall motion. Right ventri-cle angiography performed percutaneously viathe femoral vein.

Verluma® (nmed) See nofetumomab.

velocity T

vP

1

vertebral angiography (clin) For the visualiza-tion of vessels supplying the spinal canal, cordand cauda equina.

vertebral arteriography (clin) Radiography ofthe vertebral and basilar artery and their majorbranches (PICA, ICA, superior cerebellar artery,posterior cerebral artery). More usually the vertebral arteries are catheterized via a transfemoral approach. Usually performed via atransfemoral approach, or counterflow (retro-grade) injection using a brachial approach (seecerebral angiography).

VGA (SVGA, XVGA) (comp) Video graphics array.The minimum standard for video displays giving640 � 480 pixel resolution with 16 colour depth.Super VGA (SVGA) extends this to 800 � 600with 256 colour depth and Extra VGA (XVGA) to1024 � 768 with a 16 or 32 million colour depth.

VHF (units) Very high frequency radio. Commonlyrefers to 30–300 MHz where:

88 to 108 MHz FM radio108 to 136 MHz Aircraft communication144 to 148 MHz 2 metre amateur radio174 to 216 MHz Television channels

These frequencies are common causes of MRIinterference for field strengths 0.3 T (12.77 MHz),1.0 T (42.582 MHz), 1.5 T (63.873 MHz) and 3 T(127.746 MHz).

vignetting (xray) Reduced energy transfer on theperiphery of the field of view.

VIGRE (mri) Gradient echo.VINNIE (mri) Velocity imaging in cine mode 2DFI

method of velocity measurement.virtual endoscopy (clin) The CT investigation of

the complete lower colon by computer identifi-cation of the lumen in axial sections, followingthis throughout the image data set and dis-playing either as a 3D ‘fly-through’ or longitu-dinal section.

virtual memory (comp) Swapping program segments between main memory and disk.When additional program pages are required inmemory it makes room by dumping them onmemory to be retrieved later.

virtual terminal (comp) A terminal emulationprogram that makes a workstation appear to bea dumb terminal connected to some remotesystem, such as a mainframe.

virus (comp) A malicious unauthorized piece ofcomputer code attached to a computer program

V


virus–visual response

or portions of a computer system that secretlycopies itself from one computer to another byshared disks and over telephone and cable lines.Information stored on the computer can bedestroyed or altered, and can also destroy systemoperability. Virus prevention software is widelyavailable and should keep new virus definitionsup to date. Most viruses are not programmed tospread themselves. They have to be sent toanother computer by e-mail, sharing or applica-tions. The worm is an exception, because it is pro-grammed to replicate itself by sending copies toother computers listed in the e-mail address bookin the computer. Common viruses are:

● Boot viruses which place some of their code inthe start-up disk sector to automatically executewhen booting. Therefore, when an infectedmachine boots, the virus loads and runs.

● File viruses which are attached to program files(files with the extension ‘exe’). When the infectedprogram runs the virus code executes.

● Macro viruses which copy their macros to tem-plates and/or other application document files.

● Trojan horse is a malicious, security-breakingprogram that is disguised as something benignsuch as a screen saver or game.

Anti-virus software (Norton, etc.) can automati-cally detect viruses at boot-up or at regular inter-vals. Virus infection can enter via the Internet oron exchangeable media.

viscosity (phys) Dynamic viscosity describes afluid’s resistance to flow. The resistance to liquid flow when subjected to shear stress. For streamline flow: F � ηA dv/dx where F isthe tangential force between two parallel layers of liquid of area. The coefficient of vis-cosity η is measured in kg m�1s�1 or N s m�2

where 10�1N sm�2 � 1 dyne-seconds cm�2 or1 poise. The SI physical unit of dynamic viscos-ity is the pascal-second (Pa�s), having no spe-cific name. It is identical to 1 kg m�1s�1. The cgsphysical unit for dynamic viscosity is the poise

(P); commonly expressed as the fraction cen-tipoise (cP).

SI unit Conversion

Pascal-second (Pa�s) 1 Pa s � 1000 mPa sMillipascal second (mPa�s)cgs unitPoise (P) 10 P � 1 Pa sCentipoises (cP) 1 cP � 1 mPa s

Viscosity of a liquid varies with temperatureand in general decreases with increasing tem-perature. The centipoise is commonly usedbecause water has a viscosity of 1.0020 cP (at20°C); very nearly unity. Comparison of waterand saline with some iodine contrast agents.

Compound Viscosity (cP)

20° 37°

Water 1.00 0.65Plasma 1.9–2.3Whole blood 3.6–5.4Ionic monomer 5 3Ionic dimer 12 6Non-ionic monomer 11 6Non-ionic dimer 25 10

viscosity (contrast medium) (cm) The clinicalimportance of viscosity-osmolality of contrastmedium addresses the haemodynamic andrheological effects of the blood plus contrastmaterial. The viscosity of non-ionic dimers hasvery little effect on blood flow or blood rheol-ogy in vivo. Non-ionic dimers seem to disruptblood rheology and the haemodynamic balanceof the body to a lesser extent than itsmonomeric non-ionic comparators. Influenceon the viscosity factor on the blood-CM mixturein vivo. It is well known that osmolality, viscos-ity, hydrophicity and solubility cannot be opti-mized simultaneously with one class ofcompounds. The optimization of the osmolalityand hydrophilicity causes an increased viscos-ity of the final solutions under the same concentrations of iodine.

visible wavelenghts (phys) 400–760 nm. Infraredis 760–1000 nm. Ultraviolet range from 100 to400 nm: UVA 315 400nm, UVB 280–315nm andUVC 100–280 nm (see visual response).

Visipaque® (cm) Commercial (GE Healthcare)preparation of iodixanol a non-ionic dimer.

Compound Viscosity Osmolality Iodine (mg I/mL)(cP) (mOsm/kg)

Visipaque 270 12.7 @ 20° 290 270(iodixanol)

6.3 @ 37°Visipaque 320 26.6 @ 20° 290 320(iodixanol)

11.8 @ 37°

visual contrast (image) See visual response.visual response (image) Low contrast detectability

and resolution capability of the human eye is


visual response–voxel

V

complex since it involves the mixed responsesfrom both cones (chromatic/photopic) and rods(achromatic/scotopic). Visual contrast has a largedynamic range by adapting to a given luminancelevel. Visual contrast sensitivity is described bythe log difference between two intensities: logIa–log Ib. An eye adapted to the average video-screen can accommodate a range of approxi-mately 1 : 30 (30 dB) or a grey scale range ofapproximately 35 shades. Integration time is thedelay required for the eye to accumulate infor-mation and varies according to viewing condi-tions and has values from 100 to 300 ms for darkadapted (scotopic) and 15–100 ms in the lightadapted eye (photopic).

VLANS (comp) Virtual LANS, a switching technol-ogy that enables logical segmentation ofswitched networks, independent of physicalgrouping or collision domains.

VME (bus) (comp) Versatile module Eurocard bus.A 32-bit bus design widely used in industry. A64-bit version VME64 is available.

volt (V) (unit) An SI unit measure of electrical poten-tial difference where 1 V � 1 J C�1 (see ampere).

Megavolts (MV:106) Static electricity (millions of volts)Kilovolts (kV:103) x-ray high voltage (20–150 kV)Volts Domestic supply and batteries

(1.5–240 V)MilliVolts (mV:10�3) Physiological signal level (ECG)

(10–100 mV)MicroVolts (μV:10�6) Radio signal strength, e.g. MRI

(1–10μV)

volume (unit) The SI unit is m3. The litre (L) isdefined as 1 kg of pure water at maximum den-sity (�4°C) and normal pressure.

● 1 L � 1 dm3 � 10�3m3;● 1 gal (UK) � 4.54 L;● 1 gal (USA) � 3.78 L.

(see litre, metre).volume coil (mri) RF coil that surrounds a portion

of the body.volume CT See helical CT.Volume contrast imaging (VCI) (us) Real time

acquisition of volumetric data. Projects 3D dataon 2D display screen as surface rendering. Usesspecific volume transducers.

volume data set (image) A 3D block of data fromCT, nuclear medicine tomography (SPECT, PET) orMRI, usually acquired as sequential axial images,that represent x,y slice and z position. The

smallest volume element is the voxel (equivalentto a pixel in display images). Data acquisitiondefine different voxel shapes. Isotropic voxelshave x, y and z axes the same. In an anisotropicdata set the voxel has one axis larger than theothers (typically the z-axis).

volume flow rate (us) Volume of fluid passing apoint per unit time (second or minute).

volume imaging (mri) Imaging techniques inwhich NMR signals are gathered from the wholeobject volume to be imaged at once, with appro-priate encoding pulse RE and gradient sequencesto encode positions of the spins. Many sequentialplane imaging techniques can be generalized tovolume imaging, at least in principle. Advantagesinclude potential improvement in signal-to-noiseratio by including signal from the whole volumeat once. Disadvantages include a bigger compu-tational task for image reconstruction and longerimage acquisition times (although the entire vol-ume can be imaged from the one set of data).Also called simultaneous volume imaging.

volume of interest (VOI) (mri) MR spectroscopy.A VOI is the volume selected for measurementor evaluation particularly in spectroscopy. ForSVS or hybrid CSI procedures, VOI is the signalgenerating the measurement volume. For SVS,VOI and voxel are identical, but for hybrid CSIthe VOI is divided into voxels.

volume of investigation (imaging volume) (ct)Entire volume of the region under investigationby scanning.

volume of investigation (imaging volume)(image) Entire volume of the region underinvestigation by scanning.

volume-selective excitation (mri) Selectiveexcitation of spins in a limited volume. Used inspectroscopy and imaging. Achieved throughspatially selective excitation. Typicallyachieved with one selective RF excitation pulse(and a magnetic field gradient along a desireddirection). A 3D volume can be selected with astimulated echo produced with three RF pulseswhose field gradients are mutually orthogonal,intersecting in the chosen region.

volume transducer (us) A 2D convex or lineartransducer that swivels in a motor driven fan-like motion acquiring image data that canbe formatted for volume contrast imaging (VCI).

voxel (ct) Volume element, representing matrixresolution and slice depth. holding the completerange of CT values from �1000 to �3000.

V


voxel–VxD

Elementary volume element (expressed in unitsof mm3) within the scanned slice of the object,with which CT numbers are associated. The pixel

values in a CT image represent a window selec-tion of voxel values. A collection of transverseimages represents a three-dimensional volumeof voxel values. Image quality is defined by thedimensions of the sample ray used to determinethe attenuation in the voxel and spatial sampling.Uncertainty in the measured voxel attenuationappears as image noise, and the dimensionsdefine the limiting spatial resolution attainable bythe scanner.

Voxeltissue volume

Slice width

(mri) Volume element of the sample to be examined. The voxel size equals the slice

thickness � acquired dimensions x and y. The xand y dimensions are determined by the field of

view and matrix size. A 512 � 512 voxel matrixcan be interpolated as a 1024 � 1024 pixelmatrix so voxel and pixel sizes are not neces-sarily compatible (see volume of interest).

voxel bleeding (mri) Indicates cross talk of signalintensity from one voxel to an adjacent voxel.Up to 10% of a signal can appear in an adjacentvoxel. These localization artefacts primarilyappear in the image during intensity tests.Reduced by the Hanning filter.

VPS (mri) Views per segment.VQ (scan) (nmed) Ventilation/perfusion scintigra-

phy using gas or aerosol for ventilation imagingand 99 mTc-MAA for perfusion imaging. A mis-match between the two images commonlyindicates a pulmonary embolus (see PIOPED).

VRAM (comp) Video random access memory. Aspecial type of memory used on video adaptersto speed up image display rates. It can besimultaneously accessed by two devices so thedigital to analogue converter (RAMDAC) and canprovide screen updates while the video proces-sor is supplying new data.

VxD (comp) Virtual device driver. A device driverthat has access to the core of the operatingsystem for supervising hardware operationsdirectly. Mouse, serial port and parallel port use VxDs.


W

wall filter–wave (transverse)

Wwall filter (us) An electric filter that passes fre-

quencies above a set level and eliminates strong,low-frequency Doppler shifts from pulsatingheart or vessel walls. High pass filter betweendetector and FFT processor. Suppresses low fre-quency noise in Doppler spectrum eliminatingvessel and wall motion.

Walton, Ernest Thomas S. (1903–1995) Irishphysicist who with John Cockroft at theCavendish Laboratory, Cambridge produced the first artificial disintegration of a nucleus by bombarding lithium with protons using the first particle accelerator and designingnovel voltage multipliers. Awarded the 1951Nobel Prize for physics (see Cockcroft–Walton

effect).WAN (comp) Wide area network. A geographically

dispersed network that connects several LANS,.typically involves dedicated high-speed phonelines, radio-links or satellites. The Internet isprobably the largest WAN.

washout effect (mri) An effect appearing perpen-dicular to the image plane with fast bloodvelocity. Occurring during spin echo sequences.With the initial 90° pulse any blood is excitedwithin the slice volume. Blood flowing out ofthe slice before the subsequent 180° pulsereduces subsequent signal strength. This givesa low signal or no signal at all.

waste disposal (radioactive) (nmed) The follow-ing limits are accepted for the discharge ofradioactive waste from clinical centres (IAEASafety Series 70). Patient excreta are exempt fromall limitations providing exclusive toilet arrange-ments are available for patients having under-gone studies involving radioactive substances.

Discharged without control

Radio-toxicity Total Liquid Solid (L�1) (m�3)

Group 289Sr, 125I, 131I 50 kBq 5 kBq 50 kBq

Group 351Cr, 57Co, 500 kBq 50 kBq 500 kBq99Mo, 111In

Group 499 mTc 5 MBq 500 kBq 5 MBq

Controlled discharge

Radio-toxicity Total Liquid Solid (L�1) (m�3)

Group 289Sr, 125 I,131I 500 kBq 50 kBq 500 kBq

Group 351Cr, 57Co, 5 MBq 500 kBq 5 MBq99Mo, 111In

Group 499mTc 50 MBq 5 MBq 50 MBq

(see radiopharmacy).water (material) A common constituent of soft tissue

equivalent phantoms (see water phantom).

Effective atomic number (Zeff) 7.51Density (ρ) kg/m3 at 293 K 1000Ice density 920 kg m�3.Melting point (K) 273Boiling point (K) 373Specific heat capacity J kg�1 K�1 4190Thermal conductivity W m�1 K�1 0.591Relevance to radiology: Tissue equivalent material.

WATER GATE (mri) Water suppression pulsesequence.

water phantom (ct) Commonly used for qualityassurance in computed tomography, consisting ofa PMMA cylinder filled with pure water. Water-filled test objects are also used for in vitro radi-tion dose measurements

water saturation (mri) Frequency-selectivewater excitation with subsequent dephasing.This technique suppresses water signals.

water suppression (mri) In proton spectroscopythe water signal is greater than the nextstrongest signal. The water signal is sup-pressed to remove the dynamic range problem,typically with presaturation or binomial pulses.

watt W (phys) The SI unit of mechanical or electrical power where 1 W � 1 J s�1.

wave (longitudinal) (phys) Vibrations that occurin the same direction as the direction of travel.The most common example is a sound wavewhere compressions and rarefactions movealong with the speed of the waveform, eachparticle vibrating about a mean position trans-ferring energy to the next particle.

wave (transverse) (phys) Propagated by vibra-tions perpendicular to the direction of the wavetravel. Examples of these are seen on waterand electro-magnetic waves (light waves).

W


wave variables (us) Quantities that are functionsof space and time in a wave.

waveform (phys) Of a periodic quantity; the shape of a function when plotted against time.Commonly sinusoidal in shape (oscillatingwaveform) or step shaped (square waveform).The interactions of sound waves and radio-waves show the same general behaviour:

● oscillation;● signal decay;● signal resonance;● signal interference between two signals having

different frequency and phase.

The waveform characteristics of oscillating sig-nals can be analysed using a Fourier transform.

wavefront (phys) A line that joins all points on awave that have the same phase (see Huygens

principle).wavelength λ (phys) The dimension, measured

in metres, cm, mm or nm between zero-crossing points of a sinusoidal waveform.λ � c/f. Some wavelengths of the electromag-netic spectrum are:

Type Wavelength

Gamma 10�14 mx-rays 10�9mUV 400 nm

Visible

Blue 435 nmGreen 518 nmYellow 577 nmOrange 610 nmRed 760 nmIR 800 nmRadar 1 cmRadio (VHF) 3 mVLF (2–50 Hz) 1–0.06 � 108 m

(us) The ratio of the speed of sound in themedium to the centre frequency as:

The unit is millimetres per cycle, mm cycle�1.where c is metres s�1 and frequency f is Hz. So

wavelength ( )propagation speed (c)

frequency (f)λ �

wave variables–WEFT

for a soft tissue propagation velocity of1568 m s�1 and transducer frequency of 1.5 MHz(1.5 � 106Hz) the wavelength is 1.0 mm. For asimilar propagation velocity in soft tissue:

Frequency (MHz) Wavelength (mm)

2.0 0.783.5 0.445.0 0.317.5 0.21

10.0 0.1615.0 0.10

(see angstrom).wavelength matching (us) Crystal thickness

λ/2 mm. Matching layer λ/4 mm. Matching layerimpedance:

where ZT represents the transducer and ZL thematching layer.

wavelet coefficients (image) Result from filteringof the original image by spatially oriented horizontally and vertically two-dimensionalwavelet transform.

wavelet transform (image) Signal decompositioninto a set of basis functions, which are waves oflimited duration and are referred to as wavelets.

WBC (clin) White blood cell, a mixed leukocytesample used for labelling with radionuclide(99 mTc or 111In).

webcam (comp) A video camera/computer combin-ation that takes live images and sends them toa web browser.

weber (Wb) (unit) A measure of magnetic flux.One tesla (1 T) at 90° to an area of 1 m2 pro-duces a flux of 1 Wb.

web server (comp) The computer that holds andserves the web pages. When a request is madefor www.anywebpage.com, the server that holdsthis site will pick up the request and deliver thecorrect page to your browser.

Weber Fechner law (image) This involves the con-stancy under moderate luminance of the smallestdetectable (visible) difference. Visual sensation(brightness) is assumed to be proportional to thelogarithm of the stimulus (luminance). In practiceit is found to be only approximately true over alimited range of luminances

WEFT (mri) Water eliminated Fourier transform.

Z Z ZM T L� �( )

www.anywebpage.com


W

weight (units) The ambiguity in the meaning of theword ‘weight’ needs to differentiate betweenwhether mass or force is meant by using kilogram

when mass is intended and Newton where forceis intended. A gravitational force can also bedefined as an agency which tends to change themomentum of a body, and can be measured asthe rate of change of momentum it produces;being proportional to the rate of increase ofmomentum mv. Force is therefore given as:

F � d(mv)/dt � m dv/dt � ma

The weight of an object will differ according tothe value of ‘g’ (the gravitational constant). Awayfrom gravity (in space or an accelerating forceequal to g) g is then zero, so the body has massbut is weightless.

weighted CTDI (CTDIw) See CTDIw.weighting factor (radiation: wR) (dose) (ICRP60)

A dimensionless value used for weightingabsorbed dose according to the radiation’s bio-logical effect; the absorbed dose is multiplied toreflect the higher biological effectiveness ofhigh LET radiations compared with low LETradiations. Represents the relative biologicaleffectiveness (RBE) of that radiation for indu-cing stochastic effects at low doses. Previouslycalled the quality factor. The radiation weight-ing factor (wR) is independent of the tissueweighting factor (wT). Current values are:

Photons of all energies 1.0Electrons of all 1.0 (may be higher for DNA energies irradiated by Auger electrons)Neutrons 5–20 (depends on energy)Alpha-particles 20

weighting factor (tissue wT) (dose) (ICRP60) A fac-tor that indicates the ratio of the risk of stochas-tic effects attributable to irradiation of a givenorgan or tissue to the total risk when the wholebody is uniformly irradiated. WT is independentof the radiation type or energy. Represents therelative contribution of the organ or tissue to thetotal detriment resulting from uniform irradiationof the whole body. A weighting factor represent-ing radiosensitivity. A selection of the values:

ICRP60 (1990) ICRP (proposed)

Tissue or wT Tissue or wT

organ organ

Gonads 0.20 Bone marrow 0.12Bone marrow (red) 0.12 Breast 0.12

weight–Wiener spectrum

weighting factor (tissue wT) (Contd.)

ICRP60 (1990) ICRP (proposed)

Tissue or wT Tissue or wT

organ organ

Colon 0.12 Colon 0.12Lung 0.12 Lung 0.12Stomach 0.12 Stomach 0.12Bladder 0.05 Bladder 0.05Breast 0.05 Esophagus 0.05Liver 0.05 Gonads 0.05Esophagus 0.05 Liver 0.05Thyroid 0.05 Thyroid 0.05Skin 0.01 Bone surface 0.01Bone surface 0.01 Brain 0.01Remainder 0.05 Kidneys 0.01

Salivary glands 0.01Skin 0.01Remainder 0.10

(see tissue weighting factor).well-counter (nmed) A laboratory instrument

used for counting small sample activities (e.g.kidney function from a blood sample). Typicallya scintillation detector (NaI :Tl) with a centralhole, in which the enclosed sample is placedfor counting. The efficiency approaches 4π.

whisper sequences (mri) Sequences with lownoise gradient pulses.

white balance (comp) An electronic process usedin camcorders and video cameras to calibrate thepicture for accurate colour display in differentlighting conditions. For example, sunlight versusindoor incandescent. White balancing should beperformed prior to any recording by pointing thecamera at a white object for reference.

WHO category (nmed) A segregation of labelledradiopharmaceuticals according to patient dose.There are three groups denoting high, mediumand low radiation dose.

wide area network (WAN) (comp) See WAN.Wiener spectrum (di) Image noise measured quan-

titatively by using the Wiener spectrum whichplots the noise of a system as a function of its fre-quency content. The Wiener spectrum is theFourier transform of the noise autocorrelationfunction. In general, the frequency range of 0.2 to1 Lp mm�1 is relevant to radiology since thesefrequencies are easily visible and their effectnoticed. The modulation transfer function (MTF)and the Wiener spectrum (WS) is related as:

WSNA

G� α � � �(MTF)2 2

W


where α is the absorption coefficient of the detec-tor material, N is the photons absorbed and G isthe system gain. The Wiener spectrum shown isfrom a film/screen combination. The backgroundnoise is predominantly low frequency, identifiedas film grain and phosphor structure. High fre-quency noise is increased when detail (thin)intensifying screens are used and is related topoor photon absorption (quantum efficiency).

WI-FI (comp) Abbreviation for wireless fidelity,relating to the current wireless networkingtechnology standard 802.11. Originally, WI-FIcertification was applicable only to productsusing the 802.11b standard. Currently WI-FIcan apply to products that use any 802.11standard which is part of an evolving set ofwireless network standards known as the802.11 family. An alternative low-poweredalternative wireless technology is Bluetooth®.Many public facilities offer access to WI-FI net-works, known as wireless hot spots. A WI-FInetwork can be susceptible to unauthorizedaccess. An interconnected area of hot spots andnetwork access points is a hot zone.

Williams, Francis Henry (1852–1936)Considered America’s first radiologist. Graduatedfrom MIT, then studied medicine and obtaineda post at Boston City Hospital. Designed fastexposure x-ray machines and published hisfirst medical radiograph in February 1896. Hespecialized in chest radiology and for thisdeveloped high kV imaging techniques withbetter beam geometry using Rollins recognizedsafety precautions.

window (xray) The tube exit for the x-ray beam inthe housing, which holds the fixed filtration. (di)

Wiener spectrum–word

Truncating an infinite length impulse responseis equivalent to multiplying it by a finite lengthwindow function which determines how muchof the response can be seen. (ct) See display win-

dow. (us) An anechoic region appearing beneathecho frequencies presented on a Doppler spectral display.

window centre (ct) Centre of the selected displaywindow given in Hounsfield units (HU).

window level (ct) The central value of the window(in HU) used for the display of the reconstructedimage on the image monitor of the CT scanner.

window setting (ct) The setting of the % windowlevel and the window width, selected for opti-mization of the grey scale levels in the dis-played CT image.

window width (ct) The range of CT numberswithin which the entire grey scale is displayedon the image monitor of the CT scanner; widthof the selected display window given inHounsfield units (HU).

windowing (ct) Displaying only part (usually 256levels) of the complete voxel depth (usually4096 levels).

Windows® (comp) Windows 3.1x, Windows NT,Windows 95, Windows 98. The most recentbeing Windows XP and Windows Vista. Anoperating system for small computers producedby Microsoft Inc. It can also form the basis of aserver and networking.

winsock (comp) A program that providesWindows with a standard protocol for commu-nication with the Internet.

wipe test (nmed) These should be performed on aregular basis in general radiation laboratories.Any room authorized to work with radioactivematerials should document monthly wipes.Typically wipe tests of bench and sink areas tobe over an area of 100 cm2 are performed usinga polypropylene alcohol swab. Wiping benchand sink areas is done as a series of evenlyspaced wipes. The swab is then transferred tothe appropriate container for counting. If betaemitting radioisotopes are suspected, then a liquid scintillation counter is necessary. Witha combination of beta and gamma emittingcontamination, then gamma followed by betacounting will be required.

WLM (dose) See working level month (WLM).word (comp) A collection of bytes that form the

basis of the computer memory. Word sizes canoccupy 2, 4 or 8 bytes deep (16, 32, 64 bits).

0.1

Screen/Film

Detail

Conventional

1

10

Noi

se in

tens

ity

100

1 10 Hz


W

WORD®–World Wide Web (‘WWW’ or ‘the web’)

WORD® (comp) A word processing package pro-duced by Microsoft.

work (W ) (phys) When a force F moves a body acertain distance d. The work done is F � d(measured in joules (J)). Work W is the force Fmultiplied by the distance d moved by the force:W � F � d joules. The capacity to do workinvolves either kinetic or potential energy. Workdone by constant pressure, as seen in pumpssuch as the heart, is W � � � P � V where P isforce per unit area and V the volume.

Units of work and energy

SI joule (J) 1 J � 1 W s � 1 N m � 1 m2kg s�2

eVkeVMeV1.60218 � 10�19J1.60218 � 10�16J1.60218 � 10�13JCGS erg 1 erg � 1 cm2g s�2 � 10�7 J

The WLM equals 170 working level hours. Thisterm is used for epidemiological and dosimetricestimates of risk associated with alpha levelexposure. A value of 1 WLM will deliver an annualdose of 10 mSv of alpha radiation to localizedregions in the bronchial epithelium. Some ura-nium miners in the 1900s received �300 WLMgiving a 50% incidence of lung cancer. (see radon).

workload (dose) Safety thresholds for fluo-roscopy systems are measured as mA.minutesper week (mA min w�1). It can be estimatedfrom the number of patients examined and thetime spent for each examination; this is theworkload, divided into low, medium and high.

● Low workload is �30 mA min w�1 which istypical for mobile C-arm studies. Theatres andintensive care units need no special shielding.

● Medium workload is 30–300 mA min w�1 andapplies to more intensive C-arm use (orthopaedicapplications). A certain amount of shielding isrequired and the room has restricted access.

● High workload �300 mA min w�1 applies tofixed fluoroscopy installations. The room shouldbe effectively shielded and warning lights fitted.

Fluoroscopy workloads should be reviewedfrom time to time as part of a quality assurance programme.

workstation (comp) For the purposes of thisguide, a personal computer in a network; alsocalled client.

World Federation of Nuclear Medicine andBiology (WFNMB) Formed to develop coopera-tion between groups, societies and associationsformed on a national level and active in the roleof nuclear medicine and biology and to promotethe development of nuclear medicine and biol-ogy. It also prepares and recommends theorganization of a unified programme of teach-ing and training in the field of nuclear medicineand biology (www.wfnmb.org).

World Federation for Ultrasound in Medicineand Biology (WFUMB) A group of affiliatedorganizations for the promotion of the applica-tion of ultrasound (www.wfumb.org).

World Wide Web (‘WWW’ or ‘the web’) (comp)A network of servers on the Internet usinghypertext-linked databases and files.Developed in 1989 by Tim Berners-Lee (Britishcomputer scientist), and is now the primaryplatform of the Internet.

Cardiac stress testUsing the formula W � P � V. Cardiac output, atrest, is approximately 5 litres of blood per minute.Since 1 L � 10�3m3 blood pumped per second is8.3 � 10�5m3. The average left ventricular systolic/diastolic pressure is 100 mmHg or 1.33 � 104 pascals(where 1 mmHg � 1.33 � 102Pa). The average rightventricular pressure is 20 mmHg or 2.66 � 103Pa,then:

Work done by left ventricle is 1.1 J and work doneby right ventricle is 0.22 J giving a total of 1.32 J percontraction or a power rating of 1.32 W.

Under exercise conditions cardiac output can rise to30 L with average pressures at 120 and 25 mmHg.This would increase the work done to approximately10 J or a power rating of 20 W (120 beats per min).

workgroup (comp) A grouping (or segment) ofworkstations, server(s) and any networkdevices dedicated to similar functions, usingsimilar applications, and/or sharing commonresources, and serving as a subnetwork entity.Members may have a common geography orfunction (engineering, marketing, manufactur-ing, administration).

working level month (WLM) (dose) The workinglevel month was introduced so that both durationand level of exposure can be taken into account.

www.wfnmb.org

www.wfumb.org

W


worm–WYSIWYG

worm (comp) Launches an application that destroysinformation on the hard drive. It also sends acopy of the virus to everyone in the computer’s e-mail address book (see virus).

WORM (write once, read many times) (comp)A compact disk (CD-R) having a format thatcannot be erased. A worm is also a term formalicious software.

wrap-around (mri) The high frequency signal isunder-sampled below the Nyquist frequency somismapping high frequencies into lower fre-quency spectrum causing frequency wrap-around (left/right or top/bottom swapping) oraliasing. A common artefact in the MRI imageoccurring in phase and frequency encodedirections. Prevented by over-sampling and

filtering. (us) The shift of Doppler information ona spectral display to the wrong side of the baseline (caused by aliasing).

Wright, Arthur W First American physicist toproduce a radiograph of inanimate objects onJanuary 27th 1896 (see Frost).

wR (dose) See weighting factor (radiation).wT (dose) See weighting factor (tissue).www (comp) See World Wide Web.WYSIWYG (comp) An acronym for ‘what you see

is what you get’, WYSIWYG refers to an application that shows on the screen exactlywhat will appear on the document when it isprinted. This includes colours, fonts and graph-ics, as well as text.


X

Xx (mri) Dimension in the stationary (laboratory)

frame of reference in the plane orthogonal (atright angles) to the direction of the static mag-netic field (Bo or Ho) z and orthogonal to y, theother dimension in this plane.

x (mri) Dimension in the rotating frame of refer-ence in the plane at right angles to the direc-tion of the static magnetic field (Bo or Ho) z,commonly defined to be in the direction of the magnetic vector of the exciting RF field (B1).

X2 (comp) The competition to K56Flex developedby 3Com with the same download speeds.Requires the host or ISP to be digitally termi-nated. Not interoperable with K56Flex sorequires support from ISPs.

X-chromosome (dose) The larger of the two sexchromosomes. The X-and Y-chromosomedetermine the individual’s sex. Genes carriedon the X-chromosome produce sex-linked phe-nomena.

Xenetix® (cm) Iodine x-ray contrast material con-taining iobitridol (Guerbet), introduced in 1994.


Xenetix-250 4@ 37° 585 250Xenetix-300 6@ 37° 695 300Xenetix-350 10@ 37° 915 350

xenon (Xe) (elem)

Atomic number (Z) 54Relative atomic mass (Ar) 131.30Density (ρ) kg/m3 5.5Melting point (K) 161.2K-edge (keV) 34.5Relevance to radiology: as a dense inert gas for ionchambers.

127Xenon

Production (cyclotron) 127 I(d,2n)127 XeDecay scheme 127Xe (γ 172, 203, 375 keV) →127Xe 127I stableHalf life 36.4 daysDecay constant 0.0190 d�1

Photons (abundance) 172 (0.23)203 (0.68)

Gamma ray constant 5.8 � 10�2 mSv hr�1GBq�1@ 1 mUses Lung ventilation imaging as part

of routine V/Q studies

x–x-ray (loading)

133Xenon

Production (fission)Decay scheme (β�) 133Xe 133Xe (γ 81 keV) → 133Cs

stableHalf life 5.3 daysGamma ray constant 1.2 � 10�2 mSv hr�1GBq�1

@ 1 mHalf value layer 0.035 mm PbDecay constant 0.132 d�1

Photons (abundance) 30–36 keV(0.46)81 keV (0.366)

Uses Lung ventilation imaging as part of routine V/Q studies.

xenon gas detector (ct) Gas ionization detectorfilled with the noble gas xenon at high pres-sure in order to achieve high x-ray absorption.

X-linked (dose) X-chromosome linked deleteriousor lethal mutations are produced in males whereonly one X-chromosome exists (male XY; femaleXX). The Y-chromosome frequently has no allelefor genes on the X-chromosome and geneticdamage on the X are not always dominated byan allele on the Y so are left exposed. X-linkedrecessive mutations are mostly lethal. They haveyet to be seen in Japanese A-bomb survivors.

x-ray (xray) The name given by Wilhelm Röntgenin 1895 to penetrating radiation produced whenhigh energy electrons strike matter. Unlikegamma radiation (produced by nuclear reac-tions), x-rays have a broad spectrum of photonenergies. X-ray and gamma rays cannot be dis-tinguished by energy since there is an overlap.X-rays can be produced as an indirect result ofnuclear decay [see electron capture, electron cas-

cade, conversion electron, Auger electron).x-ray attenuation (ct) The attenuation of the inci-

dent x-ray intensity by Compton scatter and pho-

toelectric effect.; influenced by beam homogeneity.Lower photon energies are preferentiallyabsorbed changing beam quality. and causing abeam hardening artifact in computed tomography.(see linear absorption coefficient, linear attenuation

coefficient).x-ray (loading) (xray) The thermal loadability of an

x-ray tube identifies the limitations for shortmedium and long exposure times determined bythe anode heating and cooling characteristics.Short-term loadability is determined by thedimensions of the focal track, its length, width andspeed of rotation. It determines loading for thevery short exposure times of 0.1 s or less. In gen-eral, the smaller the anode angle the wider the

92235

92133

38101U(n, ) Xe ( Sr)f �

X


track and the higher the short-term loadability.Short-term loadability is critical for chest radio-graphy employing exposure times of 1 to 2 ms.Long-term loadability is determined by the cool-ing curve of the anode and rapidly restoring heatcapacity. Fluoroscopy and CT push long-termloadability to its limits. Series loadability for anexposure run considers a series of short expo-sures over a certain time period and the intervalbetween each series. Only the elements close tothe target area play a significant role in theimmediate heat storage capacity, Series loadabil-ity is determined by the diameter of the focaltrack and focus length.

x-ray production (xray) The efficiency E variesaccording to: x-ray tube design, generator per-formance, the anode material atomic number Zspecific heat c and heat conductivity λ for theworking temperature tmax. For a fixed anode:

E � Z � tmax � λ

For a rotating anode where x is the targetthickness:

Examples for the two anode materials tungstenand molybdenum:

Anode Z tmax λ Fixed Rotating

Mo 42 2167 1.38 1.26 1.7W 74 2757 1.30 2.65 3.7

Bremsstrahlung production efficiency Eb isrelated to the anode atomic number andapplied kilovoltage: Eb � K (kV � Z). K is a con-stant, typically 1.1 � 10�9. For tungstenZ � 74, the efficiencies at selected kVs are:

20 kV 0.16%60 kV 0.48%100 kV 0.82%140 kV 1.14%

X-ray production is a very inefficient process,most of the electrical energy is lost as heat. Theapproximate balance is:

Electronic beam intensity 100%Heat and light production 99%X-radiation from anode surface 1%Remaining after inherent filtration 0.5%Remaining after added filtration 0.1%(available x-rays for imaging)

E Z t Lxc� � �max

x-ray (loading)–x-ray tube (current)

x-ray quantum (ct) Smallest amount of energycarried by radiation and interacting with mat-ter; the energy of the x-ray quantum is deter-mined by the frequency or the wave length.

x-ray (spectrum) (xray) The overall x-ray inten-sity plotted against the photon energy. Theimportant features are: the absorption of lowerenergies by the skin of anode material and x-ray tube exit window, the line spectra (charac-

teristic radiation) of the anode material (i.e.molybdenum, tungsten). The spectrum peaks atthe effective energy.

x-ray tube (xray) The component parts of a singlebearing x-ray tube: anode, stem, filament/cath-ode assembly, electron beam (tube current) andreal/effective focal spots are identified in thediagram. The high voltage supply (typically150 kV maximum) is divided as �75 kV and�75 kV to the cathode and anode respectively.The entire x-ray assembly is encased in anevacuated glass or metal/ceramic envelope.

x-ray tube (current) (xray) This varies between50 and 400 mA for conventional radiographyand up to 1000 mA for fluorography, DSA andCT. Emitted electron density (tube current) canbe increased by increasing the temperature orsurface area of the filament. Mammography x-ray tubes operate at lower voltages (from 25to 30 kVp) placing their operating region belowthe saturation region so increasing filament cur-rent, at fixed voltage, will not influence tubecurrent under these conditions. Filament currentis limited to prevent tube damage. Increasingtube current mA does not influence beam qual-ity, its penetration is unaltered since only theintensity or quantity of x-ray photons increases

0 20

Eeff

40 60 80 100


Pho

ton

abun

danc

e


X

as Q mA. The overall quantity of x-ray pho-tons produced by an x-ray tube depends on:

● anode material atomic number (Z);● applied kilovoltage (kV);● tube current (mA).

The beam intensity is the product of:Z � kV2 � mA. This intensity value is influencedby the degree of beam filtration.

x-ray tube (kilovotage) (xray) Operating tubevoltage (peak kilovoltage kVp) is determined byuse. Mammographic tubes are designed for lowvoltage work (20–30 kVp), modern CT up to140 kVp and some high voltage chest x-raytubes can approach 180 kVp.

x-ray tube (lifetime) (xray) Principally deter-mined by the filament current. For short expo-sures the filament temperature approaches2500° but would be lower for continuous use.

x-ray tube loadability (xray) The amount of heatenergy deposited in the anode during an x-rayexposure. The thermal loadability for short,medium and long exposure times depends onpeak kV, waveform, tube current, exposure timeand rate of exposures per unit time. The x-raytube loadability is considered when designingthe x-ray tube for a specific task, i.e. chest radi-ography, fluoroscopy (screening), volume CT. Thethermal loadability of an x-ray tube for short,medium and long exposures is determined bythe anode’s heat capacity and heat loss (coolingrate). A high loadability/high-output tube hastwo distinguishing characteristics:

1 an anode disc with a large diameter, providinggreater heat radiation and greater heat storage;

0 20 40 60 80 100 120


Pho

ton

abun

danc

e

Eeff

x-ray tube (current)–x-ray tube rating (thermal)

2 a high conduction through larger surface areasleeve bearings.

Short-term loadability is a measure for veryshort exposure time of 0.1 s or less (typicalrequirement for chest radiography) is deter-mined by the track area which is directly bom-barded by the electrons; this represents theregion with the highest thermal load. Factorswhich influence this are anode rotation speedand focal track size. Long-term loadability isdetermined by the anode cooling rate, achievedby rapidly restoring the heat capacity of theanode by providing suitable cooling (heat loss)for the anode. This is a typical requirement forfluoroscopy and volume CT.

Conventional High rating

Anode diameter 133 mm 200 mmFocal track diameter 113 mm 180 mmShort term loadability 30 W 85 WLong term loadability 1.3 kW 3.2 kW

■ Reference: Schreiber, 1990. (Philips Medical)

(see heat storage, x-ray tube (rating)).

x-ray tube power (ct) The product of the x-raytube voltage in kV and the tube current in mA.

x-ray tube rating (electrical) (xray) Electricalrating concerns:

● maximum voltage;● maximum current;● maximum power.

An exposure of 100 kVp at 300 mA can exposea 0.8 � 2 mm target area to 30 kW. If the x-ray tube rating is exceeded, the anode will bepermanently damaged. Likewise, if filamentcurrents are exceeded, tube life will be curtailed. The power rating is typically speci-fied at 100 kVp for an exposure time of 0.1 sec-ond: Power (kW) � 100 kV Imax (A) at 0.1 sec(see rating).

x-ray tube rating (thermal) (xray) The thermalrating for x-ray anodes range from 250 kJ to3.5 MJ and above for some recent CT x-raytubes. Thermal rating concerns heat loss during:

● very short exposures;● longer exposures or serial exposures.

The table below shows some heat ratings fortypical x-ray tubes, anode heat capacity and

X


anode heat dissipation are shown in heat units, joules per minute and watts.

Use Anode Anode Anode heat heat diameter capacity dissipation (mm)

Conventional 300 kHU 60 kHU/min 80210 kJ 44.4 kJ/min

740 WCT 6.3 MHU 840 kHU/min 120

4.7 MJ 621.6 kJ/min10.3 kW

Fluoroscopy 300 kHU 908 kHU/min 200210 kJ 672 kJ/min

11.2 kW

(see x-ray tube loadability).

x-ray tube rating (thermal)


Y

Yalow, Rosalyn S–90Yttrium

YYalow, Rosalyn S American chemist who in

1951–1955 introduced the concept of radioim-munoassay using 131I labelled insulin. She wonthe 1977 Nobel Prize for this work.

ytterbium (Yb) (elem) Rare earth element.

Atomic number (Z) 70Relative atomic mass (Ar) 173.04Density (ρ) kg/m3 6970Melting point (K) 1097K-edge (keV) 61.3Relevance to radiology: 169Yb has been used in portablex-ray units where electrical supplies are not available.

yttrium (Y) (elem)

Atomic number (Z) 39Relative atomic mass (Ar) 88.91Density (ρ) kg/m3 4600Melting point (K) 1768K-edge (keV) 17.0Relevance to radiology: in the form of complexcompounds as intensifying screens.

90Yttrium (nmed) As a therapy agent.

Decay scheme (β �) 90Y 90Y (β � mean energy750–935 keV) : 90Zr stable

Half life 2.7 daysDecay constant 0.0108 h�1Photons (abundance) pure β�2.27 MeV

ZRadiological Sciences Dictionary 355

z-axis–Zip® drive

Zz-axis (ct) Axis of the coordinate system coincid-

ing with the axis of rotation. The z-axisincreases with helical acquisition on multisliceCT machines.

z-axis efficiency (ct) The x-ray beam z-axis effi-ciency is the fraction of the dose profile seen bythe detector element in the z-axis as:

Typical z-axis dose efficiencies for three modelscanners are shown in the table below. In theseexamples the highest z-axis efficiency is givenby 16-slice for thin slices of 1.5 mm and less:

Scanner Slice Geometric z-axis Overall number efficiency efficiency dose

(%) (%) efficiency (%)

Dual 2 � 1 mm 80 63 50Quad 4 � 1 mm 78 72 54Sixteen 16 � 1 mm 75 93 65

(see geometrical efficiency).z-direction (ct) Direction of the z-axis defined by

couch/table direction.z-interpolation (ct) A procedure for data prepro-

cessing necessary before image reconstructionfrom data obtained from spiral CT. Z-interpolationyields a data set representing a single planarslice by applying an interpolation method (usuallylinear) (see 180° interpolation, 360° interpolation).

180°Interpolation

360°Interpolation Helical pitch �1.0

FWHMFWHM

SSP

SDP

Z score (mri) Statistical evaluation to t-test. The Zscore is used to calculate a differential imagefrom the activated and non-activated images.Significance weighting is used on the difference.

Z-transform (di) A valuable set of techniques for frequency analysis of digital signals. Closelyrelated to the Fourier transform. A Z-transformis specifically concerned with sampled digitalsignals whereas Fourier techniques originatedwith analogue signals. The Z-transform allowsa simple method for achieving deconvolutionanalysis (see Fourier transform, Laplace transform,convolution).

zero-crossing detector (us) An analogue detec-tor that yields mean Doppler shift as a functionof time.

zero filling (image) Data interpolation, expandinga raw data matrix with zeroes.

zero shift (us) See baseline shift.Zevalin (nmed) Dual kits for the preparation of

111Indium (scintigraphy) and 90Yttrium (therapy)labelled ibitumomab tiuxetan; an immunocon-jugate between thiourea and the IgG1 mono-clonal antibody ibitumomab and the chelatingagent tiuxetan. Directed against CD20 antigenon the surface of normal and malignant B lym-phocytes. Indicated for non-Hodgkins lymphoma.

zinc (Zn) (elem)

Atomic number (Z) 30Relative atomic mass (Ar) 65.37Density (ρ) kg/m3 7140Melting point (K) 692.6Specific heat capacity J kg�1 K�1 388Thermal conductivity W m�1K�1 116K-edge (keV) 9.6Relevance to radiology: Important constituent of alloys(brass and bronze). 65Zn has been used for followingzinc metabolism.

ZIP (comp) The common standard for compressingfiles so they take up less space on disk. Zippedfiles have the extension .zip and are compressedand decompressed using Winzip or PKZip. (mri)Zero filling interpolation in the slice-selectiondirection for 3D measurements. Enables imagereconstruction of intermediate 3D partitionsbetween those normally reconstructed.

Zip® drive (comp) Small size removable disk storage medium, replacing 3½� floppy disks.Storage capacity up to 100 M-bytes per disk;transfer rates up to 1.4 M-bytes s�1 and aver-age seek times of 29 ms.


zirconium (Zr)–zooming

Z

zirconium (Zr) (elem)

Atomic number (Z) 40Relative atomic mass (Ar) 91.22Density (ρ) kg/m3 6500Melting point (K) 2125Specific heat capacity J kg�1 K�1 278Thermal conductivity W m�1K�1 22.7K-edge (keV) 17.9Relevance to radiology: as a component alloy for anodeconstruction and as a complex compound withpiezoelectric properties used for ultrasound transducers.

zonography (xray) Narrow angle linear tomogra-phy (�10° swings) which is used for thick section high contrast images.

zoom factor (ct) Ratio of the diameter of the fieldof measurement and the diameter of the vol-ume displayed; together with the image centrethe zoom factor determines which region of theobject is reconstructed.

zoom reconstruction (ct) Image reconstructionwith a large zoom factor; provides the magnifi-cation of regions within the image. The zoomreconstruction provides higher geometrical res-olution since it decreases the pixel size fromthe voxel data set.

zooming (di) Image enlargement with recon-struction giving improved resolution. For animage intensifier, it is changing the input fieldsize to improve resolution.


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