Medical gamma cameras in radiological emergency preparedness: Determination of calibration factors and MDA for the general electric discovery 670 NM/CT pro.

A survey was performed of the available gamma camera models and whole-body counters in Sweden showing that there are about 75 gamma cameras and 15 whole-body counters currently in operation in Sweden. One of the most common gamma camera models (GE Discovery 670 NM/CT Pro), with the collimators removed, was calibrated for 152 Eu, 137 Cs, 60 Co and 40 K in three different measurement geometries (supine, close-up sitting and distant sitting) for six different phantom sizes (12-110 kg). Minimum detectable activities (MDAs) were calculated for the gamma camera and a typical whole-body counter, both at the Sahlgrenska University Hospital in Gothenburg, Sweden. An energy window of 30 to 510 keV was used for calibration of the gamma camera. The calibration factors for this gamma camera for supine and close-up sitting geometry, including all phantom sizes, were 138-208 cps/kBq for 152 Eu, 63-83 cps/kBq for 137 Cs and 99-126 cps/kBq for 60 Co; the MDAs were 50-73 Bq for 152 Eu, 125-198 Bq for 137 Cs and 83-105 Bq for 60 Co. ICRP dose coefficients for members of the public were used to calculate the committed effective doses (CEDs) corresponding to the MDAs, showing that CEDs down to a few μSv can be estimated with this gamma camera for inhalation of aerosols of absorption type M. The distant sitting geometry was used to enable the estimation of higher contamination levels, and a hypothetical maximum CED was calculated. This was shown to be 256-2000 mSv, depending on the radionuclide and phantom size. However, further investigations are needed on the dead time losses for higher activity levels for the radionuclides studied. The results show that the use of gamma cameras for radiological triage and, in some cases, to estimate the internal activity of relevant radionuclides in radiological and nuclear (RN) events is feasible.