Purpose: Radiation therapy is widely used for the treatment of brain tumors, but itmay lead to severe cognitive impairments. Previous studies have shown that ionizingirradiation induces demyelination, blood-brain barrier alterations, and impaired neuro-genesis in animal models. Hence, noninvasive and sensitive biomarkers of irradiationinjury are needed to investigate these effects in patients and improve radiation therapyprotocols. Methods and Materials: The heads of 3-month-old male C57BL/6RJ mice (15 controlmice and 15 irradiated mice) were exposed to radiation doses of 3 fractions of 5 Gyfrom a60Co source with a medical irradiator. A longitudinal study was performedto investigate cranial irradiation-induced (3 fractions of 5 Gy) microstructural tissuealterations using water diffusion magnetic resonance imaging and magnetic resonancespectroscopy in different areas of the mouse brain (cortex, thalamus, striatum, olfac-tory bulbs [OBs], hippocampus, and subventricular zone [SVZ]). In addition to thequantification of standard non-Gaussian diffusion parameters, apparent diffusion coefficient (ADC0) and kurtosis (K), we evaluated a new composite diffusion metric,designated the S-index (ie, “signature index”). Results: We observed a significant decrease in the S-index in the SVZ from 1 month to8 months after brain irradiation (P<.05). An interesting finding was that, along with adecrease in taurine levels (up toe15% at 2 months,P<.01), a delayed S-index dropwas observed in the OBs from 4 months after irradiation and maintained until the end ofour experiment (P<.0001). These observations suggest that S-index variations revealedthe irradiation-induced decline of neurogenesis that was further confirmed by a decreasein neural stem cells in the SVZ and in newborn neurons in the OBs of irradiated animals. Conclusions: This study demonstrates that diffusion magnetic resonance imaging,especially through the S-index approach, is a relevant imaging modality to monitorbrain irradiation injury and probe microstructural changes underlying irradiation-induced cognitive deficits.