Lecture 2 Flashcards
(35 cards)
What is the statistical process in the scintillation detector?
- Gamma ray photon
- Photoelectron in the crystal
- Light photons
- Photoelectrons in the cathode
- Electrons multiplied in the PMT
- Signal proportional to the original photon energy but with statistical variation
What is the acceptance window usually set to?
10% below to 10% above the photopeak
Why are acceptance windows used?
Photons scattered in the patient have a lower energy. Energy acceptance windows reduce the contribution of scattered radiation to the image and hence reduces blurring.
What does the pulse train from the PMT tell us?
What do the size of the pulses depend on?
It tells us the energy spectrum.
Size of pulses depends on:
- number of photoelectrons released from the photocathode
- high voltage
- tube gain ( set by adjusting the voltage on the final few dynodes)
What is the purpose of tuning the PMT’s?
Each PMT should produce the same energy spectrum ( photopeak position) it irradiated by the same source.
How do you set the gain of each PMT?
Set high voltage to set the gain approximately
Adjust the gain of each PMT individually
How is automatic PMT tuning carried out?
- The counts are compared above and below the peak in each tube position
- The gain of each tube is modified to try to centre the peak at the appropriate energy- this will affect neighbouring tubes to some extent
- The process iterates until some termination criteria is met.
Give 4 reasons for the image not being uniform
- Spatial variations in PMT response
- Varying depth of scintillation event within crystal
- Variations in internal reflections
- variations in crystal
What is a correction map?
What are the three primary corrections?
Correction maps are applied to reduce the various detrimental effects. They are applied on the fly during acquisition as mappings stored in e.g. 128 x 128 matrices. The three primary corrections are energy, X direction and Y direction spatial corrections. For a given X, Y position the event is shifted in X, Y and E to the true corrected event.
How are energy correction maps created?
- Irradiation the uncollimated detector with a uniform field of photons
- Iterative process similar to PMT tuning but modifies the correction on a pixel by pixel basis
- Often narrow the energy window as correction improves.
How are spatial correction maps created?
- Place phantom over crystal face - comprises of a series of accurately spaces lead strips. Individual phantoms are used for x and y direction.
- Irradiate the detector with a uniform field of photons
- acquire an image- may use a rectangular matrix to increase the number of samples perpendicular to the lines.
- Measure the shift in X or Y for each position and store as a correction map.
In solid state detectors, what are the 4 steps in which there is direct conversion of gamma ray photons to an electrical signal?
- Gamma ray photon
- Photoelectron
- Electron hole pairs in the semi conductor
- Signal proportional to the original photon energy but with statistical variation
Describe planar/ static acquisition
Bog standard. The bigger the matrix size, the better the image quality. If you’re after counts rather than spatial resolution then use lower matrix size.
Describe whole body scanning
A static scan where you move the camera or patient
What are the two ways in which you can do dynamic ‘cine’ acquisition?
- single phase- all the frames are the same length
2. Three phase- 3 dynamic phases with three different frame times in each phase.
Describe gated acquisition
A cine acquisition where a short burst of frames is taken each time something happens e.g. triggered by R-wave. Normally acquire 16,28 or 32 frames. Used to look at respiratory motion in PET
What is bad bead rejection?
If a bad beat comes in at the wrong time it violates the red frames so we reject it and only add up the good beats.
How are projections taken in SPECT imaging?
The camera is rotates around the patient which creates a series of images around the patient.
Name 3 features of PET radionuclide
- have an excess of protons
- decay by positron decay
- require an accelerator for production.
Describe beta decay and the equation
A neutron transforms into a proton, electron and anti neutrino. This can occur with a free neutron or in a nucleus.
Describe positron decay
a proton effectively transforms into a neutron, positron, and neutrino. This does not happen with free protons and can only occur in an atom where there is enough energy to enable the process.
e.g. p + energy –> n + e+ + neutrino
Describe positron annihilation
A positron is emitted from the nucleus
The positron interacts through Coulomb interactions and slows down
Annihilation with an electron to produce 2 gamma photons
Rest mass of an electron is 511keV- if the electron and positron are both at rest then the available energy is 1022keV.
Conservation of momentum results in the 2 photons being at 180 degrees to result in zero net momentum.
Describe coincidence detection
If we can detect the two events in coincidence then we can say that something happened on the line between them. If we know that those were the two photons that can from annihilation we can draw a line between then and say that annihilation came from the midpoint. This is the fundamental idea that gives us PET images.
How are the lines of response created in SPECT?
What is a problem with this?
Camera rotated around the patient and lines of response created using a collimator. The collimator tells us that something happened in a particular position and the path it took.
Degradation with distance.
