Scattered radiation effects on the extrinsic sensitivity and counting efficiency of a gamma camera

An evaluation of the effect of scattered radiation on the performance of a gamma camera is carried out using a specially designed home-made homogeneous circular planar flood source filled with 0.2±0.01 GBq of 99mTc solution as a scattering medium. The scatter effects are assessed by analyzing the energy spectrum of 99mTc for the scatter fraction and calculating the camera's extrinsic counting efficiency and sensitivity, for five flood source thicknesses (12, 50, 100, 150 and 200 mm ) and three source-to-detector distances (0.7, 0.9 and 1.1 m). Results showed an increase in the scatter fraction from 0.29 to 22.96 as the source thickness increased. This increase was associated with a decrease in the extrinsic sensitivity from 121.36 to 49.58 counts/s GBq, and a decrease in counting efficiency as from 3.78 to 1.55%. With the increase in source-to-detector distance, the extrinsic sensitivity decreases from 121.36 to 118.77 counts/s GBq, while the counting efficiency increases from 3.78 to 11.66%. It was found that a source-to-detector distance of 0.96±9×10−3 m is a good compromise for an acceptable extrinsic sensitivity and a reasonable counting efficiency.

► Measurement and calculation of gamma camera extrinsic sensitivity and counting efficiency.
► Measurements are in the presence of a thick phantom, to study scattering effects.
► Calculations are for a circular source and a rectangular detector.
► A thicker source is suggested for use in gamma camera quality assurance.
► A compromise source-to-detector distance for attaining both high sensitivity and efficiency is suggested.