Purpose: Calculating the organs' radiation dosage in cardiac nuclear medicine procedures is essential in order to identify critical organs, radiation risk assessment and optimization dose value in the injection of radiopharmaceutical drugs. In this study, the biological distribution of 99mTc-2-Methoxyisobutylisonitrile) 99mTc-MIBI as the most common radiopharmaceutical in a cardiac study in human organs based on animal samples was investigated. Materials and Methods: After 99mTc-MIBI preparation, radiopharmaceutical was injected into 15 rats. After sacrificing rats, the uptake of radiopharmaceuticals in critical organs at 15, 30, and 45 minutes was measured using an High Purity Germanium (HPGE) detector and the percentage of injected dose per gram of organs was calculated. The cumulative activity was calculated from the radiopharmaceutical transformation diagram with time. The absorption of a radioactive complex in human organs based on animal data was calculated by applying a correction factor. The organs dose was calculated using S factor and the effective dose was calculated using tissue weighting factors. Results: The mean effective dose per unit of activity was 0.0062 mSv/MBq. The mean effective dose of 27.5 mCi radiopharmaceutical injection was 6.3 mSv. In this study, the absorbed dose in blood, heart, lung, thyroid, liver, spleen, stomach wall, muscle, and bone was calculated as 0.28, 2.92, 1.85, 24.82, 11.13, 7.03, 20.95, 1.11, 4.97, and 22.22 mGy, respectively. Conclusion: The effective dose of human organs based on the animal model in the study of cardiac nuclear medicine was evaluated by injection of 99mTc-MIBI radiopharmaceutical. The kidneys, salivary glands, thyroid, and spleen were the most critical organs that should be considered in dose optimization studies. The effective dose limit was 28% lower than the values reported in international references
