Clinical study has been showing that Single photon emission computed tomography SPECT/CT has several advantages for diagnostic purposes compared to SPECT alone. the main reason is that this hybrid scanner provides us the anatomical and functional imaging for us, so it substantially increases radiation exposure to patients. Consequently, it is fundamental to evaluate and balance the radiation protection and diagnostic needs. Estimating radiation dose, lifetime attributable, and cancer risk is a crucial role in considering new recommendations of the International Commission on Radiological Protection (ICRP). Based on the examination of the SPECT/ CT in this study the lifetime attributable risk is less than 0.27%/0.37% for men/women older than 16 years. We want to justify the question that whether it is essential to acquire additional CT for attenuation correction for diagnostic purposes.in this case, the exposure lave should not the more than the national diagnostic reference level.
