Evaluation of fetus absorbed dose in trimester of pregnancy in SPECT-CT imaging by considering the biological Half-life of the injected radiopharmaceutical

SPECT-CT is used to study the physiology of Cardiac in two stages, means, stress and rest. In the stress stage, a 20 mCi of 99mTc-MIBI is injected to the patient and scan is done. If the patient is a pregnant woman who is in a special condition, Imaging is performed at rest two hours after stress by re-injecting 20 mCi of 99mTc-MIBI radiopharmaceutical. Due to the fact that the developing fetus inside the uterus, it is sensitive to any radiation, so it is important to find how much radiation dose is absorbed by the fetus and it is the goal of this research. To achieve this goal, MCNPX code and ORNL phantom, were used to simulate the fetus, uterus and other tissues inside the maternal in three periods of pregnancy, means, three, six and nine months by using spherical lines. The radiopharmaceutical is distributed throughout the body via the cardiac pumping. Therefore all organs become a radioactive source. 21 organs close to the fetus were considered as volumetric sources and the absorbed dose of the fetus was calculated in three periods of pregnancies. The absorbed dose of fetus in the stress stage in three, six and nine months of pregnancy are 1.26 × 10-2, 2.9 × 10-3, 2.83 × 10-4 mGy respectively. The absorbed doses in the rest mode without urination in the mentioned periods are 2.52 × 10-2, 5.79 × 10-3, 5.68 × 10-4 mGy respectively. Furthermore the absorbed doses in the rest mode with two times urination and considering a biological half-life of 3 hours, and 25% of the injected radiopharmaceutical comes out by urinary excretion, were found 1.89 × 10-2, 4.35 × 10-3, 4.25 × 10-4 mGy in the mentioned periods, respectively. Therefore, to reduce the absorbed dose of fetal and the potential risks, it is strongly recommended to the patient to drink fluids as much as possible in the two hours interval between the scans.