X-ray Recap Flashcards
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
Define Compton scattering
Inelastic scattering cross section dep on e density
Outer shell electron ejected (ionisation) and photon scattered with lower energy
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
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
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.
Two main methods of photon interaction
- Direct detectors (photons to electrons)
- Indirect detectors (radiative transitions)
Types of detectors
Direct:
Ionisation chambers
Semiconductors
Indirect:
Photoluminescence
Describe Ionisation X-ray detection
High pressure xenon used in older CT scanners
Electron-ion pairs collected from any type of ionisation events
What are the limitations of ionisation x-ray detectors?
- > poor resolution
- > poor response time
- > strong angular dependency
- > Low sensitivity (gas!)
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
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
Describe indirect detectors
Uses photoluminescence
X-rays -> light -> charge -> signal
Two types of photo-luminescence detector
Fluorescence: uses photon generated directly - II, FP
Phosphorescence: Electron traps, photons released later during readout - CR
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.
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
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
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)
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.
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
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
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
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
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
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
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
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
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.
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
