Serial Cross-Sectional Mapping of Bone Marrow Microenvironment Remodelling During Fracture Repair Using Multiparametric MRI and Histopathology.

Restoring bone marrow microenvironment (BMM) homeostasis is critical for fracture healing, yet non-invasively tracking physiological events like apoptosis, angiogenesis and matrix remodelling remains challenging. This study evaluated multiparametric 7.0T MRI (T2 mapping and proton density-weighted imaging [PDWI]) for in vivo tracking of BMM spatiotemporal remodelling during natural and pharmacologically accelerated bone repair. Utilizing a rat tibial drill-hole injury model (N = 36), we compared a natural healing control with a Huoxue Qiangji Decoction (HQD) intervention group. Serial high-resolution sagittal MRIs were acquired at days 7, 14 and 21, with signal intensity ratios (SIR) normalized to ipsilateral intact marrow using ITK-SNAP. Distance correlation (dCor) analysis explored non-linear dependencies between MRI metrics and histological features (c-cas3, Masson's trichrome and day-14 CD31/Ki67 expression). Results showed distinct MRI signal trajectories, with earlier homeostatic stabilization in the accelerated healing group. Spatiotemporal mapping revealed closely mirrored non-linear dependencies between MRI and histology. Specifically, T2 mapping SIR mirrored early-stage apoptosis (dCor = 0.527) and mid-term cellular proliferation (Ki67, dCor = 0.732). Concurrently, PDWI reflected dynamic trends of mid-term angiogenesis (CD31, dCor = 0.637) and late-stage collagen deposition (dCor = 0.480). In conclusion, normalized multiparametric MRI provides complementary macroscopic insights into BMM histological progression, highlighting its potential as an objective, non-invasive translational tool for monitoring musculoskeletal healing.