Thesis (Ph.D.) - Cairo University - Faculty of Science - Department of Biophysics

This work employs a point source to investigate the effects of phantom thickness and source-to-detector distance on the extrinsic sensitivity and counting efficiency of a gamma camera with a rectangular detector. Measurements are performed using 99mTc source (0.197±3.7{u00D7}10-4 GBq) and a water phantom container (as a scattering medium). The scatter fraction is determined through the analysis of the energy spectra of 99mTc for a certain energy window. Results show the direct dependence of the calculated extrinsic counting efficiency on the solid angle subtended by a point source on a rectangular detector. The increase in water thickness from 0.0 to 200 mm resulted in an increase in the scatter fraction from 0.20 to 24.12. This increase is also associated with a decrease in the extrinsic sensitivity from 121.59 to 11.15 counts/s.GBq, and a decrease in extrinsic counting efficiency from 0.039 to 0.0036%. The increase in source-to-detector distance from 0.50 to 1.10 m is accompanied by a decrease in extrinsic sensitivity from 121.59 to 115.53 counts/s.GBq and an increase in extrinsic counting efficiency from 0.039 to 0.15%. In such point source geometry, it is found that a source-to-detector distance of 0.79±0.02 m is a good compromise for an acceptable extrinsic sensitivity and a reasonable extrinsic counting efficiency. Some important differences between the present results and published measurements using a flood source are extensively discussed

Issued also as CD