Lecture 2 Flashcards
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.