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Medical Physics 2: Radiology > X-ray Recap > Flashcards

Flashcards in X-ray Recap Deck (28)
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1

Define Rayleigh scattering

Elastic scattering off a bound electron

Scattered photons have the same energy as the incoming photon, but a different direction.

Incoming photon not ‘detected’ – no good for making an x-ray detector

2

Define Compton scattering

Inelastic scattering cross section dep on e density

Outer shell electron ejected (ionisation) and photon scattered with lower energy

3

Define the photo-electric effect

Depends heavily on Z

Inner shell electron ejected (ionisation) if energy is high enough
E = hv - o

Scatter photon in visible spectrum

4

The impact of cross-sectional area and how is it measured

Defines the probability of a interaction occurring.

Atomic cross section is measured in barns 10-28 m2

Can calculate from mass-attenuation coeff

5

Requirements for detecting X-ray photons for imaging

For efficient detection you need to find materials with a high probability of photo-electric interactions at diagnostic energies

Cross-section is boosted when the incoming photo energy matches the transition energies of the atom -> results in a k-edge

k-shell is important, needs to be in the range of 10-60keV to maximise the PE effect.

6

Two main methods of photon interaction

- Direct detectors (photons to electrons)
-Indirect detectors (radiative transitions)

7

Types of detectors

Direct:
Ionisation chambers
Semiconductors

Indirect:
Photoluminescence

8

Describe Ionisation X-ray detection

High pressure xenon used in older CT scanners

Electron-ion pairs collected from any type of ionisation events

9

What are the limitations of ionisation x-ray detectors?

-> poor resolution
-> poor response time
-> strong angular dependency
-> Low sensitivity (gas!)

10

Describe direct semiconductor detectors

High energy photo-electron generates many ionisation events: lots of free electrons to collect; measure charge generated

X-rays -> charge -> signal

charged stored per pixel -> energy integrator

a-Se is most common, requires doping to stay amorphous at room temp

high electric field to limit literal spread

11

Limits of a-Se

Low K-edge
High elec field to lim spatial spread mans inc dark current, dec SNR
Higher cost
Trapped electron issues
Temperature instability
Value dep on tot energy dep per pixel

Great for mammo! 12.7keV k-edge

12

Describe indirect detectors

Uses photoluminescence

X-rays -> light -> charge -> signal

13

Two types of photo-luminescence detector

Fluorescence: uses photon generated directly - II, FP

Phosphorescence: Electron traps, photons released later during readout - CR

14

Describe Photoluminescence

Fluorescence: excited electrons rapidly decay to the ground state

Phosphorescence : excited electrons decay to a metastable state. Transition probability to the ground state is low.

15

Ideal FP detector

Scintillator light has an output wavelength must be optimised for light detector.

CsI gives green light which match well to photodiodes

Good yield of photons for input x-ray energy range 10-100keV - P-E cross-sections

16

Types of FP scintillator

Cesium Iodide (CsI) with added Thallium (Tl) impurity (‘doping’) and Gadolinium oxysulfide (Gd2O2S) doped with Terbium (Tb) - these both emit green light at a wavelength of 545nm

17

Why is CsI preferred to GOS detectors ?

CsI has a higher efficiency:

CsI has a needle like structure which acts like a light guide.

GOS is powder based, so must be thin to minimise scattering/blur (loss of res)

18

Compare the type types of light detector

Charge couple arrays: CCD

- cheaper, are limited (4cm2)
- Req lens/opt fibre
- poorer performance

CMOS: 2D array of photodiodes
- Active panel sensors (APS)
- Lower noise performance
- Size limited -> tiled array.

19

Describe an a-Si detector

2D array of photodiodes
Readout elecs is a dead area - fill factor not 100%

Charge is accumulated -> stored -> energy integrator

20

Limitations of CsI Flat panel detector

- Spatial resolution is limited by fill factor: compromise between DQE and spatial resolution.
- Resolution degradation is limited by fill factor considerations, then light spread.
- Relatively fragile!
- Expensive

21

Describe flat panel readout

- The TFT switches are pulsed in the order A, B, C one row at a time

- Charge from each pixel element in the row passes to the pre-amplifiers (columns), the output of which are switched in turn to an ADC

- Image is built up 1 pixel at a time in a progressive scan

22

Describe Phosphorescence

- X-ray absorption results in an excited state.
-The excited electrons decay to a metastable state. Transition probability to ground state is low.
-After a delay, by natural or by forced means, the electrons drop back to the ground state and loose energy by photon emission.
- Materials which exhibit this property are called phosphors

23

Describe CR image storage and readout

-Storage phosphor: BaFBr:Eu
- Trapped electrons proportional to X-ray photons locally absorbed
- The image is stored as a spatial distribution of electrons trapped in meta-stable states

Readout:

Image can be ‘read’ out by laser, releasing the electrons to produce blue light proportional in intensity to X-ray photons absorbed

24

What are the two types of CR phosphor?

Powder phosphor:
-use a binder
-thin layer to avoid light scatter
-leads to lower eff

Needle structure
-no binder
- act like light guide, can be thicker

25

CR limitations

Reader problems
Laser spot size – limits resolution
Light spread in phosphor – limits resolution
Dirt – drop-outs, light can’t penetrate
Mechanical problems (e.g. feed rollers)

Plate / cassette problems
Cracks in plate
Fading image – read out quickly after acquisition
Storage / scatter problems
Erasure problems – ghost image

26

DR properties

Signal Transfer Properties (STP)
DR: generally linear response
CR: generally a non-linear function, e.g. log(dose)
Dynamic range
Dose range is huge: limited at the low dose end by noise and at the high end by saturation

27

Comparing receptor efficiencies

DQE = SNR out ^2 / SNR in ^2

comparing detection to noise added

note: cannot compare diff designs. dqe also varies with spatial freq.

28

Image artefacts

CR:
Plate cracks
Dirt / hair in plate
Dirt on reader
Dark line from lead coating on cassette

DR:
RF interference with reader
Ghost image due to flat fielding error