Investigation on the sensitivity and the counting efficiency of a gamma camera in cylindrical source geometry

Purpose: The counting efficiency and the sensitivity are two important characteristic properties of a gamma camera used in nuclear medicine imaging. Because of practical reasons, counting efficiency and the sensitivity have been characterized with a point source of radiation at a certain distance from the detector. However this way has the disadvantage of producing measurements of limited clinical relevance since real patients are not point sources of radiation. In this work, it was aimed to determine the counting efficiency and sensitivity of a gamma camera with a cylindrical source and to investigate the variation of scatter contribution with increasing source thickness. Methods: The counting efficiency and sensitivity of a dual-head clinical gamma camera were determined using homogeneous 99mTc source thickness of 2–18 cm and an activity of 4 mCi at small source–detector distances (5–25 cm) for both LEGP and LEHR collimators. The energy spectra were acquired. The asymmetric window (133–153 keV) was considered and the counts in this energy window were integrated. Scattered radiation effect was evaluated by both analyzing the energy spectrum of 99mTc for the scatter fraction and plotting the counting efficiency and sensitivity. Results: The scattered fraction values increased with increasing source thickness from 0,19 to 4,52 and from 0,27 to 4,62 for LEHR and LEGP collimator. Calculated sensitivity decreased with increasing source thickness, from 55 counts/sMBq to 24 counts/sMBq and from 98 counts/sMBq to 41 counts/sMBq for LEHR and LEGP collimator. The efficiency decreases with increasing source thickness, from 0,19 % to 0,08 % and from 0,35 % to 0,14 % for LEHR and LEGP collimator. Calculated efficiency increased with increasing source-to-detector distance, from 0,19 % to 0,54 % and from 0,35 % to 0,97 % for LEHR and LEGP collimator. Conclusions: The scattered fraction values increased with increasing source thickness. Calculated sensitivity shows a decreasing trend with increasing source thickness and increasing source-to-detector distance. The efficiency decreases with increasing source thickness and efficiency increases with increasing source to detector distance. Increasing source to detector distances results with increasing counting efficiency but decreasing sensitivity.