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 MDA values 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 MDA values 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.