Background: Finding a suitable method for rapid, accurate and reliable estimation of absorbed dose has high priority in management of the radiation exposed persons. Shortly after radiation exposure, apoptosis is a major detriment in proliferative tissues such as the hematopoietic system. Therefore, quantification of apoptosis in these tissues could be useful for rapid estimation of radiation exposure. Annexin V (ANX) is considered as a biological probe for detection of apoptotic cells. The aim of the present study is to investigate the potential suitability of apoptosis quantification for estimation of radiation exposure. Materials and Methods: In order to determine the biological distribution of ANX within the mice body after radiation exposure, mice whole body irradiated with 2, 4, 6 and 8 Gy (60Co gamma rays). Ten hours later, ANX conjugated with super paramagnetic iron oxide nanoparticles (SPIO-ANX) was administered intravenously and magnetic resonance imaging was conducted 3 hours later. Results: Average signal intensities in the regions of interest (ROIs) of the femur bone marrow, liver and testis were calculated and normalized to parafemoral muscle signals. SPIO-ANX accumulated in bone marrow of irradiated groups and significantly decreased the normalized mean of signal intensity for bone marrow in comparison with control group (p< 0.01). Conclusion: Tracing and quantification of SPIO-ANX in bone marrow can be used as an indicator for radiation exposure. However, development and optimization of the assay are necessary for discrimination between different radiation doses.