Gamma camera count rate performance assessment without specialized equipment

Objectives Assessment of gamma camera count rate performance is ideally performed as part of acceptance testing and is also an annual requirement of certain accreditation organizations (e.g. ACR). Unfortunately the current NEMA NU 1-2007 procedure for assessing count loss is impractical as it involves a 2 day dynamic protocol that is not supported on many gamma camera systems. In this study we investigate alternative methods for measuring count loss, including a new method that requires no specialized test equipment.

Methods We propose a new variable-distance method involving two Tc-99m point sources: a low activity source (~0.6 MBq) and a high activity source (~12 MBq). The incident count rate is controlled by adjusting the distance between the source and the face of an uncollimated gamma camera. At each distance, data are separately acquired for both sources. A plot of the count rates for the high activity source against those for the low activity source indicates the count rate performance of the system, under the assumption that the low activity source causes negligible dead-time. The results of this method were compared with results obtained using the copper attenuator method (Geldenhuys 1988).

Results None of the 8 gamma cameras at our institution were capable of performing the NEMA NU 1-2007 protocol. The new variable-distance method was successfully implemented on all cameras. The slope of a linear fit to the low count rate data approximated the ratio of the activities of the two sources, confirming the principle of the technique. The maximum observed count rate and the observed count rate at 20 % count loss were 9 % and 18 % lower with the variable-distance method compared to the copper attenuator method. Count rate performance measured by both methods was not well approximated by a simple paralyzable model.

Conclusions The copper attenuator method is quick and reliable but requires specialized test equipment. When the necessary copper filters are not available, the new variable-distance method provides a viable alternative