MI: Image Acquisition Flashcards
Name the main components of a gamma camera detector.
Collimator Scintillator Crystal (Light Guide) Photomultiplier Tubes Computer/Electronics
What is the function of the collimators in a gamma camera detector?
Allow points from source to correspond to points on the image (gamma emission is isotropic).
How does the geometry of the collimator effect its sensitivity, resolution and energy of the source detected?
Thickness increases with energy (higher energy scans require thicker septa)
Increased length increase resolution, decreases sensitivity.
Increased width decreases resolution, increases sensitivity.
What is the purpose of the scintillator crystal?
Converts gamma photon into light photons (NaI(Tl) converts about 1 gamma to about 38 light photons per keV so signal proportional to the energy deposited in crystal).
What is the lifetime of the excited state of a NaI(Tl) crystal?
230ns
What are the properties of a NaI(Tl) crystal?
Transparent - allows light transmission
High atomic no. & Density - efficient absorber (9mm thick crystal will absorb 84% of 14keV)
Quick relaxation time - crystal can only process one photon at a time, quick relaxation time allows for high count rate detection.
Hygroscopic - reacts with water and yellows. Must be sealed in Al.
Fragile - mechanical and thermal stress can break crystal - room must be air conditioned.
What is the trade off with crystal thickness?
If thicker
- increased stopping efficiency.
- decreased spatial resolution as increased chance of multiple scattering events so photon energy reduced and not recorded, and fewer photons at the tube nearest events.
What is the purpose of a photomultiplier tube in a gamma camera detector?
Turns light photons into electrons (used for signal processing).
Increases signal - contains ~10 dynodes each creating 5-10x as many electrons - rough gain of ~10^7.
Allows location of event based on signal intensity at surrounding PMTs.
How is the quantum efficiency of a PMT calculated?
QE = (No. Photoelectrons/No. Incident photons) x 100%
What produces the shape of the photopeak on a gamma camera spectrum?
Statistical variations in no. of light photons and photoelectrons produced.
How is the energy resolution of a gamma camera calculated?
Energy Res. = (FWHM/Energy at photopeak) x 100%
What is the function of the energy window in a gamma camera?
Records only isotope of interest, and removes scatter. Typical windows are set at 15-20%.
How does the camera process the x-y position of the incident gamma ray?
Amount of light received by PMT is related to proximity to incident photon.
Spatial resolution is improved by factoring in the signal from adjacent PMTs.
Signal thresholding increases localisation accuracy and decreases dead time.
Correction circuits used to correct for spatial distortion, and for the drift in energy calibration.
