A phantom for determination of calibration coefficients and minimum detectable activities using a dual-head gammacamera for internal contamination monitoring following radiation emergency situations

The purpose of this study was to derive calibration coefficients (in terms of cps kBq^-1) and minimum detectable activities, MDA, (in terms of kBq and corresponding dose rate) for the dual head gamma camera part of an SPECT/CT-instrument when used for in vivo internal contamination measurements in radiation emergency situations. A cylindrical-conical PMMA phantom with diameters in the range of 7-30 cm was developed in order to simulate different body parts and individuals of different sizes. A series of planar gamma camera investigations were conducted using an SPECT/CT modality with the collimators removed for ¹³¹I and ¹³⁷Cs, radionuclides potentially associated with radiation emergencies. Energy windows of 337-391 and 490-690 keV were selected for ¹³¹I and ¹³⁷Cs, respectively. The measurements show that the calibration coefficients for ¹³⁷Cs range from 10 to 19 cps kBq^-1 with MDAvalues in the range of 0.29-0.55 kBq for phantom diameters of 10-30 cm. The corresponding values for ¹³¹I are 12-37 cps kBq^-1 with MDAvalues of 0.08-0.26 kBq. An internal dosimetry computer program was used for the estimation of minimum detectable dose rates. A thyroid uptake of 0.1 kBq ¹³¹I (representing MDA) corresponds to an effective dose rate of 0.6 μSv d^-1. A ¹³⁷Cs source position representing the colon with an MDA of 0.55 kBq corresponds to an effective dose rate was 1 μSv y^-1. This method using a simple phantom for the determination of calibration coefficients, and MDA levels can be implemented within the emergency preparedness plans in hospitals with nuclear medicine departments. The derived data will help to quickly estimate the internal contamination of humans following radiation emergencies.