Topic 10: Detectors PET/SPECT Flashcards
Name two principal interaction processes and whether they are scattering or absorption processes
Photoelectric effect - an absorption process Compton Interaction - a scattering process
Compton effect
Photon collides with orbital electron. Photon is deflected - scattered photon with different direction and reduced energy. Electron knocked out of atom - compton recoil electron - the energy is used by creation of ionising trail
In the compton effect the energy of the scattered photon and KE of recoil electron varies with what?
Scattering angle.
Energy of incident photon in compton effect = what energy output?
energy of incident photon =energy of scattered photon +kinetic energy of recoil electron
The parts of a scintillation detector?
Scintillator - high energy photons enter and converts incident to light
Photodetector - When struck by light photons the photocathode ejects electrons (by compton , photoelectric effect or pair production)
Amplification electronics - amplifies, shapes and processes
Explain the process of the scintillator in the use of a scintillation detector?
Incident radiation produce electrons in scintillation crystal (by photoelec., compton)
Some electrons move to high energy state within crystal lattice
De-excitation of electrons in high energy state to ground energy state produces light
All light from one incident photon is summed.
Scintillation crystal properties?
- Density – Ability of crystal to stop radiation photon – Higher – more able to stop photon – more sensitive
- Light output – Higher light output – higher sensitivity – lower noise
- Decay constant (speed of crystal) – Time needed for crystal to reset for new photon detection
- Refractive Index – Ease at which scintillator can couple to PMT – Ideally close to that of glass (~1.5)
- Wavelength of light – Match to detector
Properties of crystal?
Density – (more able to stop photon) - sensitive
- Light output – Higher light output – higher sensitivity – lower noise
- Decay constant (speed of crystal) – Time needed for crystal to reset for new photon detection
- Refractive Index – Ease at which scintillator can couple to PMT – Ideally close to that of glass (~1.5)
- Wavelength of light – Match to detector
Disadvantageous Properties of using Sodium Iodide Crystal in Nuclear Medicine
Fragile to mechanical and thermal stress
– ΔT = 50C per hour
– Fractures -> opacifications reducing light reaching photocathode
• Hygroscopic
– Exposure to moisture causes yellowing of crystal that effects light transmission
• E > 250 keV, predominant interaction is Compton
– Larger volumes of crystal required to get good detection efficiency
Explain the photodetectors role in the photomultiplier tube.
- Typically photodetectors are incorporated into a photomultiplier tubes in the form of a photocathode.
- When struck by light photons the photocathode ejects electrons
- Electrons from the photocathode are attracted through an amplification process to the anode of the photomultiplier tube.
Explain the amplification process in the photomultiplier tube.
Amplification occurs through a series of dynodes in the photomultiplier tube
Electron (e- ) leaves photocathode – Electron focussed and accelerated towards dynode at higher potential (voltage) and gains kinetic energy – Kinetic of electron absorbed in dynode and is then released as multiple electrons – Electrons accelerated to by (10-15) successive dynodes at higher potential multiplying in number at each stage – Huge increase (x107) in the number of electrons collected at anode as a pulse of charge – Produces amplification of the starting input signal
Determination of photon energy
Scintillator: Light produced is proportional to Energy absorbed
- Photocathode: Number of Electrons Proportional to Light
- Photomultiplier Tube: Output signal Proportional to Number of Electrons
-> Output Proportional to Energy Absorbed (Requires Calibration to Measure Energy)
How do you calculate the Full Width at Half maximum?
(How well is photon energy measured? )
There is an uncertainty in the number N.
Energy signal forms a Gaussian distribution.
Mean numbers of photoelectrons = N
Standard deviation = sqrt(N)
Full-width half maximum = 2.35 X S.D. = 2.35 X sqrt(N)
How do you calculate the energy resolution ?
As the energy increases how does the FWHM and energy resolution change?
Higher energies - the FWHM increases and the Energy resolution as a percentage decreases as a proportion so it gets better.
Calculate the energy resolution.
Assume All Photon Energy Absorbed (140 keV)
Assuming a Photon yield for NaI (Tl) of 1 per 30 eV
Number of light photons = 140,000/30
The Photocathode Efficiency is 20 %
Number of Photoelectrons = (140,000 x 0.2) / 30 = 933
This number will be calibrated to energy
FWHM = 2.35 x 9331/2 = 72
Energy Resolution = (100 x 72)/933 = 7.7%
How can we tell the energy absorbed from this graph?
the pulse height is proportional to energy absorbed = which is through the photoelectric effect (complete absorption) and compton scattering
pulse height = Ein-Eout
Energy absorbed by the photoelectric effect?
- Full energy of incident photon absorbed
- Characteristic radiation may or may not be absorbed in detector
- complete absorption or characteristic could have escaped so we could get any value between full energy and characteristic energy of material
Energy absorbed due to a single compton scatter?
The energy that is absorbed on the scatter angle. The max is at 180 degrees (high energy) small angles will have low energy.
0 -> Emax
Energy absorbed due to single and multiple compton scatter?
absorbed energy due to multiple scatters summed
explain this graph
scatter component and peak and characteristic component.
The peak to scatter ratio depends on what?
1) Photon energy
2) Detector material
3) Detector size
Why do we use energy windowing?
we can use the properties of the detector to determine photon energy, and therefore remove scattered photons from the final image.
Multichannel analyser
allows the characterisation of the photon distribution for more complex corrections.
why do we use solid state semiconductors as an alternative to scintillator detectors?
scintillation detectors are bulky and relatively poor energy resolution.
So solid state semiconductor detectors have:
- better energy resolution
- Slim
- However costly
Name an example of a semiconductor?
cadmium zinc telluride
Why does the semiconductor detector have better energy resolution?
Interaction of gamma photon is measured directly as an electrical pulse.
What are the advantages of using semi-conductor detectors?
- Work essentially the same way as gas-filled detectors
- Hole electron pair collected across a potential difference
- Compared to gas (air) filled detector – One ionisation per 3-5 eV (per 34 eV in air) Þ 10 x larger signal per unit radiation energy absorbed
- More efficient absorber than gas chambers – More dense
- Size of electrical signal proportional to energy absorbed
Semi-conductor detector Disadvantages?
Some of them need cooling.
You need very pure crystals because you relying on electron holes etc. which are expensive.
Limited by the thickness, high energy photons would go straight through if its too slim, however if its too thick you’ll lose the semi-conducting properties.
You can dope them a bit but thats expensive.
What is the disadvantage of using a traditional photomultiplier tube?
When you now use PET-MR scanners you use magnetic fields, you lose positional information because the electrons in the photomultiplier tube are affected by the magnets.
So we use semi-conductor light readouts.
What are avalanche photodiodes?
Semiconductor detector sensitive to light photons.
Absorption area (low electric field) and a multiplication area (high electric field, amplification).
So its analagous to photomultiplier tubes.
Advantages of Avalanche Photo Diodes (APDs)?
Advantages
- Compact (semiconductors)
- High Quantum Efficiency (taking light and producing a signal)
- Low Bias Voltage
- Insensitivity to high magnetic fields
What are the disadvantages of avalanche photo diodes?
Disadvantages
- Lower Gain than PMTs – we need some Pre-amplification
- Therefore more Noise
- Low timing resolution (poor)
- More temperature sensitive (we have those cooling channels going through)
Silicon photomultipliers?
They are APDs but they run in Gieger mode (so you count events not doing any proportionality) which can be used to determine the energy.
Insensitive to magnetic fields
can be used for photon counting
faster than APD so suitable for TOF
Relatively low quantum efficiency
May be suitable for SPECT as well as PET.
