Detection of region-specific fiber damage within the injured spinal cord using the spherical mean technique

This study evaluated whether diffusion MRI measures from diffusion tensor imaging (DTI) and the spherical mean technique (SMT) can detect region-specific, fine-grained tissue damage and white matter (WM) tract disruptions after spinal cord injury. Diffusion MRI data were acquired from the cervical spinal cord of monkeys before and after a controlled unilateral dorsal column lesion at C5 using a 9.4 T scanner and assessed ~16 weeks post-injury. Post-mortem silver staining served as the reference standard for regional fiber damage. Both DTI- and SMT-derived parametric maps identified the location and severity of WM damage and showed good agreement with histological results. The MRI measures captured consistent severity patterns at the lesioned segment: damage was greatest in the dorsal WM, followed by ventral and then lateral regions. Along the dorsal column tract, damage was most severe at the lesion epicenter and decreased rostrally and caudally. Among diffusion metrics, fractional anisotropy (FA) demonstrated the most significant changes across regions at and around the lesion site, including changes in some regions of caudal segments where neither SMT-derived axonal volume fraction (V_ax) nor the histology-based fiber-density indices showed significant differences. In contrast, V_ax showed the strongest regional agreement with histology; its reduction reflected histology-detected regional fiber damage more closely than FA, yielding an increase in regional correlation by more than 24%. Overall, SMT provides V_ax as a promising biomarker that is more strongly associated with fiber density and may offer a more robust assessment of regional axonal loss across different injury severities.