In this study, we monitored biological response in neutronexposed normal mouse brains using DW-MRI and 18F-FDG-PET. DW-MRI and 18F-FDG-PET were sensitive enough to detect physiological changes that were not identified in the H&E results. These results suggest that DW-MRI and 18F-FDG-PET can be used as imaging biomarkers to provide a quantitative indicator of complex in vivo pathological changes. Assessment of radiation exposure using imaging biomarkers is non-invasive and can be used repeatedly in clinical practice. In addition, it should be applied not only for evaluating the therapeutic effects of tumors, but also for evaluating the degree of tissue damage by radiation exposure.