Gamma Cameras
Sensitivity
The sensitivity is simply the count rate recorded by the camera from a given source. The sensitivity is heavily based on the design of the collimators so baseline readings taken during commissioning and manufacturers specifications are used to check against. Any decreases in the sensitivity of the camera can be an indication of changes in the crystal, drift in the PMT and associated circuitry or collimator damage.
The NEMA standard for measuring sensitivity uses a petri dish 10cm diameter. The activity is measured accurately in a dose calibrator and dispersed into the bottom to form a film approximately 3mm deep. The count rate is kept at a level where it is linear (<20,000 cps). The source is positioned 10cm away form the camera face in air.
A point source is not used as local variations in uniformity could prevent a reading that represents the true sensitivity.
Typical values:
- Low energy high resolution 70 cps / MBq
- Low energy general purpose 120 cps / MBq
