X Ray Physic

8/14/2019 X Ray Physic

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X-RAYSX-RAYS

X-ray (or radiographic images) use the x-ray part of the em spectrum to

expose the subject

short wavelength (l): < 1 nm (more than x10000 shorter than visible) high energy (E=hn=hc/l - n=c/l): > 1keV

high frequency (n=c/l)

E: energy (in eV)l: wavelength

h: planks constant

c: speed of light

n: frequency

Rem: eV is the energy that an e- acquires when accelerated one meter

distance in an electric field of a potential difference of 1 V

(e is charge and V is potential difference).


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X-RAY FILMX-RAY FILM

Contains a blue-green sensitive emulsion

Large crystals

Gives optical density related to input exposure via the H&D curve

Large tonal (dynamic range) 1000:1 instead of 100:1 for norm. films

A:screen exposure type

B:direct exposure typeDT: transmission density

H: exposure

R: useful density range (2)

L: useful exposure range (


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X-RAY TUBEX-RAY TUBE

Basic x-ray tube is a vacuum tube containing a tungsten filament (anode),

bombarded by e- accelerated by kV. Photons are created from this process

The emission x-ray spectrum has a continuum and line (spike) characteristics


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Spectral properties are changed by: altering kV (electric field ie. speed of the e-)

or mA (current to filament)

Increased mA increases intensity (graph a)

Increased kV gives higher penetration - because of shorterl (graph b)


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Use of filter of various materials (Pb, Cu, Al) between source and subject

attenuates wavebands and removes low energy ls (soft x-rays)

which produce scattering. Because of shorterls the beam is more penetrating!!



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Io

x

I

xoeIIm-

=

m: linear attenuation coefficient

x

I

X-RAYSX-RAYSAttenuation by absorption


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X-RAYSX-RAYSHalf Value Thickness (HVT)

HVT is the beam power measured as thickness (x) of material

to reduce Io to Io/2 (I.e. x where the intensity is decreased by 2)

2/12/ == xorIo

put

xoo eII

m-=2/

x

e

m-=

2/1

divide by xoo eII

m-=2/

apply loge

xe m-=)2/1(log solve for x

m=

2logex


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X-RAYSX-RAYSHalf Value Thickness (HVT)

2/12/ == xforo

put

xoo eII

m-=2/

xe m-=2/1

divide by xoo eII

m-=2/

apply loge

xe m-=)2/1(log solve for x

m=

2logex

x

I

1/2

HVT is the beam power measured as thickness (x) of material

to reduce Io to Io/2 (I.e. x where the intensity is decreased by 2)


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X-RAYSX-RAYS

The attenuation mechanism is mostly related to Z (atomic number)

of the material. It includes:

Releigh scattering (proportional to Z2)

Compton scattering (no dependence on Z)

Photoelectric effect (proportional to Z3)

Pair production (proportional to Z2)

For a fixed mA the intensity I is related to the focus to film distance d by

the inverse square low: I=Io/d2


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X-RAYSX-RAYS

Z (atomic number) for:

fat ~ 5.9

muscle ~ 7.4

bone ~ 13.9

Attenuation ratio for bone to muscle is 13.9/7.4= 6.6

I.e. Bone attenuates 6.6. (~7) times more than muscleand thus gives better contrast.


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SHADOWGRAPHSSHADOWGRAPHS

Most usual mode of radiographs - skiagraphs

The image relies on different absorption of tissuethat is transparent, but there is a large change in Z

(from air to bone gives a good contrast)

Tissue would not be easily differentiated without the aid ofcompounds with different Z introduced to give shadows.

Examples: Barium Sulphate (BaSO4) to image stomach, intestines

Iodine (I2) for blood.


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SHADOWGRAPHSSHADOWGRAPHS

S

X

Film


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X-RAY TOMOGRAPHYX-RAY TOMOGRAPHY

Image of slices (tomi) of the body. Now an obsolescent

technique developed to localise an internal site (depth

localisation) in a body. Used instead of stereo radiography

that uses 2 tubes or two exposures.

Detail in plane

K is sharp


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CAT SCANNING - 1st GenerationCAT SCANNING - 1st Generation

X: X-ray sourceS: subject

D: detector

Rotation in X intervals Time ~ 4 min!!!!


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CAT SCANNING - 2nd GenerationCAT SCANNING - 2nd Generation

Single source with narrow fan of detectorswhich traversed and rotated.

Time ~ 20 sec.


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CAT SCANNING - 3rd GenerationCAT SCANNING - 3rd Generation

Moving source with more detectors.

Time ~ 4-5 sec.


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CAT SCANNING - 4th GenerationCAT SCANNING - 4th Generation

Stationary 360 degree ring of detectors

and a moving source.

Time ~ 1 sec.


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CAT SCANNING - 5th GenerationCAT SCANNING - 5th Generation

Uses no moving parts.

Tube with the patient inside 210 deg.

The detector ring is similar.

An e- beam scans around the body

in multiple adjacent tracks to generate

x-rays.

Time ~ 0.1s to a few ms or real time


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NUCLEAR MEDICINENUCLEAR MEDICINE

Uses ingested or injected radioisotopes

Measurement of the distribution and concentration shows abnormalities

Uses include Radiotherapy and Diagnosis by:

tracers (showing the functions of the organs) or

imaging(picture of an organ)

Type of emitters: a particles - not detectable outside the body

b particles - very damaging

g-rays - very penetrating, not damaging (low radiotoxicity)

Detection and imaging is with the aid of thegamma camera.


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NUCLEAR MEDICINE - MEASURESNUCLEAR MEDICINE - MEASURES

Half-life (t 1/2 ) of the radiochemicals (ie. measure of radiochemicals) iscalculated, in disintegrations s-1 (1Bq=1 s-1 ), by:

t 1/2 = loge2/l

Biological effects are measured by the absorbed dose D,in J kg-1 or gray Gy, by:

D=energy/mass

Damage effectis measured as a quality factor Q, eg:for x-ray, g-ray and b

particles Q=1, for slow neutrons Q=3, fora particles Q=10.

where l is the decay constant


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NUCLEAR MEDICINE - MEASURESNUCLEAR MEDICINE - MEASURES

Dose equivalent, in Sv (sievert), is calculated by:

Dose equivalent= D*Q

Annual significant natural dose ~ 1-3 msV

Additional artificial dose ~ 0.25 mSV


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RADIOACTIVE TRACERSRADIOACTIVE TRACERS

A variety of radioactive tracers (isotopes) provide diagnostic informationfor specific purposes (e.g. blood, urine, organs, liver functions, tumours, etc.).

The effective half-life of a g-emitter (Te) is related by its half-life (Tr)and its biological half-life (Tb) by the relationship:

1/ Te = 1/Tb + 1/Tr


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GAMMA CAMERASGAMMA CAMERAS

They use a scintillator as the detector (with excellent quantum

efficiency) for counting the incident g radiation.

The scintillator emits light

The light emitted is amplified by a photomultiplier tube


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SIMPLE COLLIMATORSIMPLE COLLIMATOR

Designed to measure overall activity

(eg. From the thyroid)

K: shielding

V: output signalB: background

J: subject

g: g ray emission

D: detector (usually a PMT or SPD)


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RECTILINEAR SCANNER TYPERECTILINEAR SCANNER TYPE

Includes a lead septa that gives a

focal spot for scanning action by singleprobe method to synthesize a picture

at low resolution.

Q: septa

F: focal spot

L: lightguide

x,y: scanning directions


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THE USUAL GAMMA CAMERATHE USUAL GAMMA CAMERA

Is a fixed array of multiple detectors

(PMTs* or SPDs*), with a pineholeaperture. The computed output is

viewed on CRTs real time.

P: Pinhole aperture

M: multiple detectors

*Photomultipliers,Silicon Photodiodes


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ROTATING TYPE FOR EMISSIONROTATING TYPE FOR EMISSION

TOMOGRAPHYTOMOGRAPHY

A scanning system records sectorscans in a variety of planes which are

then combined by S/W to give a

composite view. It uses a rotating

gamma camera or a multi-crystalscanner that has better resolution

A: multiple position scans

B: array of multiple detectors


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ULTRASOUND IMAGINGULTRASOUND IMAGING

Uses frequencies greater than 20kHz (ie. the audible limit) such

as 1 MHz for biomedical diagnostic use.

The method depends on the detection ofreflections of about 1%

in magnitude at body tissue.

Applications include: brain scan, foetal size and development,

cardiography, tissue abnormalities, stones and others.

Advantages are that: it can differentiate different type of tissue,

no tissue damage, no side-effects.


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ULTRASOUND - PROPERTIESULTRASOUND - PROPERTIES

Its velocity c, depends on the medium density, measured in

m s-1 (see table on the back of the handout).

Sound image resolution depends on l. Increased resolutionis achieved by reducing l, but penetration is decreased.

Reflection strength depends on density mismatch in the body,

e.g. between bone and muscle. The presence of air blocks

aids the transmission of the ultrasound. Use of a jelly between the transducer and the skin to assist the

transmission of ultrasound.

The absorption of ultrasound in tissue depends on frequency, temperature,

density etc and is given by:

xoeII

m-=

k

Where k is the attenuation coefficient and x the thickness.


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ULTRASOUND GENERATIONULTRASOUND GENERATION

Sonar Contains a transmitter

and a receiver (T).

Voltage is applied to

emit ultrasounds.

Received energy is

converted back to voltage.

Emits beams of variable

widths (pulses).

Note that otherwise we can

generate continuous ultrasound.

D=ctD: range, c:velocity, t:time of travel


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ULTRASOUNDULTRASOUND

SCANNING TECHNIQUESSCANNING TECHNIQUES

A type scan Measures with a static transducer.

The echo time is measured by

synchronizing the CRT display to

the transceiver.

Applications: brain scan.

S: subject

C: transducerJ: jelly

T:transmitter, R:receiver

P: pulse rate generator

Q: time base generatorK: CRT display


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B type scan (moving) Uses a rocked transducer and

pulsed emissions to increase theprobability of obtaining normal

reflections.

Applications: tumours and stones.

Positions 1-3 to give composite

image.


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Time Position (TP) type scan Doppler type scan

It is a modified B scan type suitably

pulsed with low speed time.

Applications: determination of foetal

heart rates.

Uses continuous waves.

Frequency changes due to the velocity

(V) of the subject or the source.

Movement towards the receiver

gives a higher frequency.

Frequency change Dn=2nV/c

n: frequency

V: velocity of subject or source

c: velocity of ultrasound

Applications: heart functions, blood flow

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