Experimental determination of the extrinsic sensitivity and counting efficiency of a nuclear gamma camera using a homogeneous circular planar source

Two important operational parameters of medical gamma cameras are the extrinsic counting efficiency and sensitivity. Historically, for practical reasons these two parameters have been defined using a point source of radiation at a certain distance from the detector. This definition has the disadvantage of producing measurements of limited clinical relevance since real patients are not point sources of radiation. In this work, we propose a more clinically relevant method of determining efficiency and sensitivity, using a planar, circular, homogeneous source. For this purpose, a gamma camera of 39 cm diameter, and a circular, homogeneous, 36.5 cm diameter Co-57 source with an activity of 5.5±0.7% mCi were employed. The source was placed coaxially with the detector at distances of 50, 100, and 150 cm. Data were acquired using a medium and a high-resolution collimator. The efficiency was found to depend directly on the solid angle subtended by the detector. Computed values for the sensitivity are in the range of 5–6 kcts/sec-mCi, and values for the efficiency were found to range between 0.2% and 2%. The results show an approximate 2:1 ratio in sensitivity and efficiency between the medium and high-resolution collimators regardless of source–detector distances.