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.
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.
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.
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.
