International Atomic Energy Agency Medical exposure in radiology: Optimization of protection Module VIII.3 - Part 1: Design considerations for the equipment

International Atomic Energy Agency

Medical exposure in radiology:Medical exposure in radiology:Optimization of protectionOptimization of protection

Module VIII.3 - Part 1: Design considerations for the equipment

!! CONTINUATION OF FILE VIII.3 Equipment !


Topic 4: Computed Topic 4: Computed TomographyTomography

Module VIII.3-Part 1. Design considerations equipment Continued 3


The CT equipmentThe CT equipment

• The CT scanner

• Hounsfield units

• Difference with projection radiology

• Latest generations of scanners



CT: “Computed” and “Tomography”CT: “Computed” and “Tomography”

CT is a tomographic imaging technique, generating cross-sectional image in the axial planeCT techniques use high kV with heavy filtrationA fan beam is passed through the patientTransmitted radiation is measured by array detectorsCT images or “sections” are maps of µThey are derived by mathematical analysis of multiple projections (use of computer): filtered back projection



Image and “Hounsfield” numberImage and “Hounsfield” number

• Pixel size depends on FOV and matrix• Matrix size 512x512 or 1024x1024

• 12 bits (4096 gray levels)

• Voxel is:• volume element=pixel area x slice thickness

• Relative attenuation coefficient µ is expressed in Hounsfield units or CT numbers

• By definition HUwater = 0, HUair = -1000

• Attention: µ will depend on kV…so will HU

HUt = 1000 x (µt - µwater) / µwater



Image display: windowingImage display: windowing

CT is a DIGITAL IMAGEWindow settings determine HOW tissue attenuation values are displayed



Window settings: optimized for mediastinumWindow settings: optimized for mediastinum



Window settings: optimized for lungWindow settings: optimized for lung



CT scannerCT scanner

• Computed Tomography (CT) was introduced into clinical practice in 1972 and revolutionized X Ray imaging by providing high quality images which reproduced transverse cross sections of the body.

• Tissues are therefore not superimposed on the image as they are in conventional projections

• The technique offered in particular improved low contrast resolution for better visualization of soft tissue, but with relatively high absorbed radiation dose

• Dose distribution different due to rotation



Dose distributions in projection RL and CTDose distributions in projection RL and CT

Conventional: dose decreases from entrance to exit, ratio

1/100 …1/1000

Head exam

Rotational scanning gives dose more concentrated at center of rotation

Also dose distribution in slices, but contribution outside imaged slice



3rd & 4th generation scanners3rd & 4th generation scanners

3rd generation: rotating fan beam and array of

detectors (rotate-rotate)

4th generation: rotating source and fixed ring of

detectors (rotate-stationary)



A look inside a rotate/rotate CTA look inside a rotate/rotate CT

X Ray Tube

Detector Arrayand Collimator



3rd & 4th generation scanners3rd & 4th generation scanners

• Slice (section) acquisition: 1-2 seconds

• Cables limit rotation to 1 revolution

• Modern scanners: slip ring technology

• Beam is highly filtered (-> 10mmAl HVL)

• Heat loading on tube very high

• Collimation • defines section thickness

• reduces scatter

• @ 1mm slice also additional collimation at detector



X Ray beam

Direction of patientmovement

Helical or spiral CTHelical or spiral CT

• Slip ring technology allows continuous rotation

• The patient can then be moved continuously through the beam, making the examination much faster

• Continuous data acquisition and table feed



New possibilities for proceduresNew possibilities for procedures

• The new helical scanning CT units allow a range of new features, such as :• CT fluoroscopy, where the patient is stationary, but

the tube continues to rotate

• multislice CT, where multiple slices can be collected simultaneously

• 3-dimensional CT and CT endoscopy

• All these new technologies and applications:• Constant increase in number of examinations

• High contribution to collective effective dose



CT contribution to number of exams and to CT contribution to number of exams and to collective effective dose collective effective dose (FRG 1990-92)(FRG 1990-92)

% freq X Ray exams FRG 1990-02 % coll effect dose X-ray FRG 1990-92

CT35%

Spine 10%

Chest5%

GI15%

Angio/DSA10%

Urography9%

Misc16%CT

4%Spine 10%

Chest18%

Dental17%

Angio/DSA1%

Extremities20%

Misc30%

FRG: Federal Republic of Germany



Factors influencing dose in CTFactors influencing dose in CT



Equipment related factorsEquipment related factors

• Wave form of generator: ~nil, low ripple

• Range of tube current settings, steps

• Beam filtration

• Beam shaper

• Focus axis distance: • shorter geometry (60 cm instead of 80) gives increase of

wCTDI, but allows less mAs

• Slice collimation• For ~1mm slices: dose increase

• Scan field and scan angle



Dose and spiral CTDose and spiral CT

• With identical protocol settings(Slice thickness, table feed, mAs product per scan or rotation, scan

length)

dose in spiral CT would be same as in sequential scanning

• For interpolation purpose: ~1 additional rotation => slight increase in dose (<10%)

• If pitch is taken >1: less loss of information, but dose is reduced

• BUT often INCREASE:• Scanning over longer distances

• Multi-phase studies, same body scanned repeatedly



Summary on this topic CTSummary on this topic CT

• We learned about the principle of CT scanner, the different generations and the factors influencing this high dose examination


Topic 5: X Ray beam Topic 5: X Ray beam characteristicscharacteristics

Beam description, factors affecting beam quality and effect on imaging process.



Contents of this topicContents of this topic• Factors affecting X Ray beam and image

• Spectra

• Beam quality

• Tube current

• Influence of mAs

• Filtration

• Wave-form, ripple

• Geometric factors• Inverse square law

• Anode angle

• Heel effect

• Unwanted radiation• Stray radiation, scatter, leakage

• Fighting against scatter

• Enhancing contrast• Lowering kV

• Contrast agents



Factors affecting the X Ray beamFactors affecting the X Ray beam

•Tube current•kVp value•Ripple•Filtration



Photon spectra of X RaysPhoton spectra of X Rays

kVp ≤ charact. line en. kVp > charact. line en.



Beam “Quality”Beam “Quality”

• “Measures” penetrating power

• Subjectively describes the shape of continuous spectrum

• Function of:• kVp

• filtration

• high voltage supply characteristics

• Diagnostic radiology beams are POLYCHROMATIC

• ± Quantification: effective energy

• With moderate filtration: Eeff ≤ 2/3 kVp

• With heavy filtration: Eeff ≤ 1/2 kVp



Changing the tube currentChanging the tube current

Change of QUANTITYNO change of quality

Effective kV not changed



Changing current x time product (mAs)Changing current x time product (mAs)

70 kV- 50 mAs 70 kV- 80 mAs 70 kV- 25 mAs



Effect of changing kVEffect of changing kVpp

Change in QUANTITY&

Change in QUALITY - spectrum shifts to higher energy- characteristic lines appear



Importance of correct choice of kVImportance of correct choice of kV

Right image is made with correct kV; left image with too high kV, leading to loss of contrast.



Remember generator waveformsRemember generator waveforms

100%

13%

4%

Line voltage

Single phase single pulse

Single phase 2-pulse

Three phase 6-pulse

Three phase 12-pulse

0.02 s

0.01 s

kV ripple (%)



Effect of waveform or rippleEffect of waveform or ripple

• During 1/2 mains cycle kV fluctuates between min and max

• At any moment spectrum defined by kV

• The more ripple, the more low energy photons

• Less ripple, beam is harder

• We do not need these low energy photons:• Absorbed in patient or scattered

• Dose to patient increases

• Do not reach the image receptor

• HF or 12-pulse will reduce dose to patient

• And give less noisy images (less scatter)



Ripple increases image noiseRipple increases image noise

The left image is made with an old (high ripple) generator.The increased noise led to loss of detail compared to modern equipment



FiltrationFiltration

Change in QUANTITY&

Change in QUALITY spectrum shifts to higher energy

1- Spectrum out of anode2- After window tube housing

(INHERENT filtration)3- After ADDITIONAL filtration



Inherent filtrationInherent filtration

•Tube housing window 1•Inherent filtration:

–Glass envelope–Cooling oil–Beryllium window (thin)

•“Radioluscent”•You can see the filament: -->1

•Filtration expressed in equivalent Al thickness•Typically: 1mm Al•Housing: lead shield !



Additional filtrationAdditional filtration

Additional filtration: •preferably removes lower energy photons• increases effective energy Eeff

• lowers intensity• reduces patient dose !•but increases tube loading



K-edge filtrationK-edge filtration

kV

Metals with K-edges in diagnostic rangeEx: Erbium (Mo, Rh,Sn)Max absorption above K-edgeAllow modification spectrum: mammoK-edge Tin (Sn) allows lightweight shielding in protective aprons

Relative intensity



Geometrical propertiesGeometrical properties

Inverse square lawAnode angleHeel effect



Inverse square law: dose reductionInverse square law: dose reduction

Dose = 1

Dose = 1/9

Dose = 1/4

Fluency of beam constant

Dose (or doserate) per cm2

proportional to 1/x2



Reduces geometric unsharpnessReduces patient dose

Inverse square lawInverse square law

• Same dose D at image receptor

• Increase at entrance point: • Left: (40/20)2 => 4 D

• Right (120/100)2 => 1.44 D

Front

Image Receptor

Patient

Front

Image Receptor

Patient

Focus20cm

20cm

20cm

100cm



Anode angle and resolutionAnode angle and resolution

Geometrically: size of the effective focal spot directly related to the sine of the angle of the anode. A the angle of the anode is decreased, the effective focal spot is also decreased.Penumbra and geometrical unsharpness decreases



Heel effectHeel effectPlot of intensity shows variation along anode to cathode axis

Heel effect lower by large FID: not the full cone of radiation (C to

B) is used



Unwanted radiationUnwanted radiation



Leakage radiationLeakage radiation

•Radiation transmitted thtrough tube housing• Image taken on film• Cathode side• Need for Radiation protection

•At installation•In use•After maintainance

Leakage



Stray radiationStray radiation

• Is the sum of the leakage and scattered radiation

• Scattered radiation has been deviated after leaving the tube

• Scatter occurs from all material exposed to the radiation: patient, table, filters,room walls



Scattered RadiationScattered Radiation

• Effect on image quality • increasing of noise

• loss of contrast

• Effect on patient dose • increasing of superficial and depth dose

• Possible reduction through :• use of grid

• limitation of the field to the useful portion

• limitation of the irradiated volume (e.g.:breast compression in mammography)

• use of air gap



Anti-scatter gridAnti-scatter grid

Source of X Rays

LeadScattered X Rays

Useful X RaysFilm and cassette

Patient




• Radiation emerging from the patient• primary beam : contributes to the image

• scattered radiation does reach the detector and contributes to background without information, and contrast is lowered

• scattered radiation also contributes to unnecessary dose

• the grid (between patient and film) eliminates most of scattered radiation on image receptor




BUT:to keep same doseon image receptor

increase dose

Grid NOT recommended for

extremities orpediatric radiology



Scatter reduction by reducing field sizeScatter reduction by reducing field size



Scatter removal by air gapScatter removal by air gap



Scatter removal by narrow beam geometryScatter removal by narrow beam geometry



Contrast agentContrast agent

• Tissue characteristics and photon energy determine photoelectric absorption

• Basic idea to enhance contrast: lower kV• cf. mammography

• But lower kV, lower penetration

• How to improve contrast?

• Introducing contrast agents: Ba, Iodine• High atomic number : photoelectric ~ Z3

• K-edge falls within diagnostic imaging kV



Summary on Summary on what we learned about the X Ray beam what we learned about the X Ray beam (1)(1)

• Which factors influence the X Ray spectrum?• tube potential

• kVp value

• wave shape of tube potential

• anode track material• W, Mo, Rh etc.

• X Ray beam filtration• inherent + additional

• How to enhance contrast• Lower kV

• Use of contrast agents: Barium, Iodine



Summary on Summary on what we learned about the X Ray beam what we learned about the X Ray beam (2)(2)

• We became familiar with geometrical properties• Inverse square law• Anode angle• Heel effect

• We learned about some unwanted effects• Leakage radiation• Stray radiation• Scattered radiation• How to “remove” unwanted scatter radiation

• Reduce field size• Grid• Air gap• Narrow beam geometry


Topic 6: Special applicationsTopic 6: Special applications

Dental equipment, Pediatric Equipment



Summary of special applications Summary of special applications

• Pediatric radiology• Specificity

• Requirements on equipment, rooms and accessories

• Positioning and immobilization

• Considerations for the use of the equipment

• Protective shielding

• Dental radiology• Low dose high frequency

• The equipment

• Radioprotection and patient dose



Equipment for pediatric roomEquipment for pediatric room



Specificity of pediatric radiologySpecificity of pediatric radiology

• Longer life expectance• Risk of late detrimental radiation effect greater

• Estimated 2-3 x greater than @ 30-40y or 5-7 x greater than after 50y

• Justification and optimization even more important

• Smaller size require adapted radiographic techniques and exposure factors

• Positioning and lack of co-operation requires short exposures

• Functional differences (e.g. higher heart rate, faster respiration, inability to stop breathing on command, increased intestinal gas etc.)



General, equipment and installation General, equipment and installation considerations considerations (1)(1)

• Short exposure times can improve image quality. So the generator should have enough power to allow short exposure times (3 milliseconds) and the timer should allow short exposure times.

• The use of mobile X Ray units in pediatrics could raise special problems (low power:motion blurring).

• The generator should be of high frequency to improve the accuracy and reproducibility of exposures.




• Automatic exposure control (AEC) devices should be used with caution in pediatrics (they should be adapted specifically to pediatrics)..

• Careful manual selection of exposure factors usually results in lower doses

• X Ray rooms for pediatrics should be designed for improving the child’s cooperation (control panel with easy patient visibility and audio communication, etc.).

• Fast film-screen combinations have advantages (reduction of dose)




• Low-absorbing materials in cassettes, tabletops, etc. specially important in pediatrics radiology.

• The antiscatter grid in pediatrics gives limited improvement in image quality and increases patient dose given the smaller irradiated volume (and mass) the scattered radiation is less

• Antiscatter grid should be removable in pediatric equipment, particularly fluoroscopic systems




• Image intensifiers should have high conversion factors for reducing patient dose in fluoroscopic systems.

• Additional tube filtration may allows dose reductions.

• Pulsed fluoroscopy is recommended since it allows patient dose reduction as in adults

• For CT examinations, the use of specific technical radiographic parameters for CT examinations should be promoted (lower mAs than for adults, and lower kV in some cases)



Dental radiologyDental radiology



Dental radiologyDental radiology

• High frequency (~1/4 of all examinations) but low dose technique

• Wide range of entrance doses:0.5 - 150mGy

• Dental radiology contributed for 1% of collective dose from medical diagnostics

• Image quality often very low

• Many of them in private practice with no medical physicist or RP officer and have no medical physics support available

• Organs at risk: parathyroid, thyroid, larynx, parotid glands



Intra-oral X-ray dental equipmentIntra-oral X-ray dental equipment

Some technical parameters:

-Tube 65kV 7.5 mA-Filtration: 2mm Al

-Focus skin distance: 20cm-Field size: 6cm diameter



Panoramic dental X-ray equipmentPanoramic dental X-ray equipment

Some technical parameters:

-Tube 60-80kVkV -Current 4-10mA



Image receptors in dental radiologyImage receptors in dental radiology

• Small films (2 x 3 or 3 x 4 cm) in light-tight envelopes (no screen)

• Digital intraoral sensors. • Compared with category E film, the radiation dose

is reduced by 60%.

Intraoral Radiology

Panoramic Radiology

• Film-screen combination• Digital sensors. • Compared with film-screen sensitivity class 200,

the radiation dose is reduced by 50-70%.



What we learned about X Ray equipmentWhat we learned about X Ray equipment

• We studied X Ray production (the tube, the generator,…) and the variety of radiological equipment (for plain radiography, fluoroscopy, CT, …)

• We learned about the basic interactions leading to the image formation

• Image intensifiers may drastically improve image quality and reduce staff dose

• and the beam characteristics, including the « undesirable » apects (scatter, leakage)



What we learned about X Ray equipment What we learned about X Ray equipment (ctnd)(ctnd)

• Specific examinations need dedicated equipment (dental, pediatric, mammographic)

• Digital images can enhance contrast and extract regions of interest

• Equipment used by non-radiologists (surgeons, cardiologists,…) require special technical requirements



Where to get more information Where to get more information (1)(1)

• The Physics of Diagnostic Imaging, David J. Dowsett, Patrick A. Kenny and R. Eugene Johnston, Chapman & Hall Medical, ISBN 0-412-40170-1

• Equipment for diagnostic radiology, E. Forster, MTP Press, 1993

• The Essential Physics of Medical Imaging, Williams and Wilkins. Baltimore:1994

• Imaging systems in medical diagnostics, Krestel ed., Siemens, 1990

• The AAPM/RSNA Physics Tutorial for Residents General Overview of Fluoroscopic Imaging, B. A. Schueler, Radiographics Vol 20, 1115-1126,(2000)



Where to get more information Where to get more information (2)(2)

• European Guidelines on quality criteria for CT. Report EUR 167262, Office for Official Publications of the European Communities, 1999, Luxemburg

• Radiation exposure in Computed Tomography; 4th revised Edition, December 2002, H.D.Nagel, CTB Publications, D-21073 Hamburg

• Rational use of diagnostic imaging in pediatrics. WHO, 1987

• European guidelines on quality criteria for diagnostic radiographic images. Report EUR16261 (1996)

• Radiation protection and quality assurance in dental radiology. Radiation protection 81. European Commission.(1995) CG-89-95-971-EN-C

Documents

International Atomic Energy Agency Medical exposure in radiology: Optimization of protection Module VIII.3 - Part 1: Design considerations for the equipment