Early Metabolic and Microstructural Responses of the Rat Hippocampus to BED-Matched Single-Fraction and Fractionated Whole-Brain Irradiation: A Multimodal MRI Study

Objectives: The biologically effective dose (BED), based on the linear–quadratic model, is widely used to equate different irradiation schedules. However, whether BED equivalence holds for early brain responses is unclear. We compared single-fraction and fractionated whole-brain irradiation (WBI) under BED-matched conditions in a rat model, using multimodal magnetic resonance imaging (MRI) and histology to characterize early hippocampal responses. Methods: We randomly assigned 27 male Wistar rats to three groups: Sham, WBI-Single (15 Gy × 1), and WBI-Fraction (6.2 Gy × 5), the latter designed to achieve a comparable BED (α/β = 2 Gy). Magnetic resonance spectroscopy (MRS), glutamate chemical exchange saturation transfer (Glu-CEST), and diffusion tensor imaging (DTI) of the hippocampus were performed pre-irradiation and at 1 and 2 wk post-irradiation on a 7-T scanner. Hematoxylin–eosin staining, Luxol Fast Blue staining, and immunohistochemistry (glial fibrillary acidic protein, ionized calcium-binding adapter molecule 1) were evaluated. Longitudinal MRI data were analyzed using linear mixed-effects models, with false discovery rate (FDR) correction across imaging endpoints. Results: Among the MRI endpoints, only myo-inositol exhibited a significant group-by-time interaction after FDR correction (p = 0.0026, q = 0.0342). At 1 wk, myo-inositol was higher in the WBI-Fraction group than in the Sham group (Tukey-adjusted p = 0.0217). At 2 weeks, glutamine was lower in the WBI-Single group than in the Sham group in an exploratory post hoc comparison (Tukey-adjusted p = 0.0382). For DTI, radial diffusivity was higher in the WBI-Single group than in the Sham and WBI-Fraction groups at 1 wk (Tukey-adjusted p = 0.0460 and 0.0110, respectively), although no significant group-by-time interactions were observed for DTI metrics. Glu-CEST showed no significant group differences or group-by-time interaction. Conclusions: Under irradiation conditions designed to achieve comparable BED, early hippocampal MRI revealed a significant group-dependent temporal pattern in myo-inositol and exploratory differences in diffusion metrics. These findings suggest that multimodal MRI may provide complementary information on early hippocampal responses to WBI, although distinct biological effects of the two irradiation schedules remain to be established.