Calibration of medical gamma cameras for estimation of internal contamination from137Cs.

Calibration of 22 gamma camera units was performed at 15 hospitals in southern and western Sweden to estimate¹³⁷Cs contamination in humans in a supine static geometry, with a new developed calibration protocol and phantom. The minimum detectable activities (MDAs) and the estimated committed effective doses (CEDs) were calculated for each calibration. Generic calibration factors were calculated for five predetermined groups based on the detector type and manufacturer. Group 1 and 2 included NaI-based gamma cameras from General Electrics (GEs) with a crystal thickness of 5/8'' and 3/8'' respectively. Group 3 and 4 included NaI-based gamma cameras from Siemens Healthineers with a crystal thickness of 3/8'', with a similar energy window as the GE NaI-based cameras and a dual window respectively. Group 5 included semiconductor-based gamma cameras from GE with a CdZnTe (CZT) detector. The generic calibration factors were 60.0 cps kBq^-1, 52.3 cps kBq^-1, 50.3 cps kBq^-1, 53.2 cps kBq^-1and 48.4 cps kBq^-1for group 1, 2, 3, 4, and 5 respectively. The MDAs ranged between 169 and 1130 Bq for all groups, with measurement times of 1-10 min, corresponding to a CED of 4.77-77.6μSv. A dead time analysis was performed for group 1 and suggested a dead time of 3.17μs for¹³⁷Cs measurements. The dead time analysis showed that a maximum count rate of 232 kcps could be measured in the calibration geometry, corresponding to a CED of 108-263 mSv. It has been shown that semiconductor-based gamma cameras with CZT detectors are feasible for estimating¹³⁷Cs contamination. The generic calibration factors derived in this study can be used for gamma cameras of the same models in other hospitals, for measurements in the same measurement geometry. This will increase the measurement capability for estimating internal¹³⁷Cs contamination in the recovery phase following radiological or nuclear events.