Low-level internal radionuclide contamination: use of gamma camera for detection

The uncollimated gamma camera was evaluated as a screening tool for the detection of low levels of internal radionuclide contamination. The minimum detectable activity (MDA) is on the order of 1-4 nCi (37-148 Bq) for a 3-min sample and background counting time. Identification of unknown radionuclides at these low levels involves a systematic search and hence requires a more careful peaking method than organ imaging. Most gamma cameras are capable both of detecting a wide range (35 keV to 1.332 MeV) of gamma-ray energies in the presence of three to four radionuclides and also of distinguishing principal gamma-ray energies from Compton scatter. The differences in MDAs among gamma cameras are small, and vary primarily with the background activity. The actual contaminant activities lie between the calculated values based on measurements when the source is 10 cm away from the uncollimated detector head and using point sources both in air and with the tissue-equivalent scatter medium. The widely available gamma camera is capable of screening a large population, either suspected or actually contaminated, in the event of an emergency.