Histological Validation of Diffusion MRI Axon Biomarkers In Human Post-Mortem Traumatic Spinal Cord Injury
Abstract
Purpose
To validate axon-specific diffusion MRI biomarkers in human traumatic spinal cord injury (SCI) by correlating diffusion tensor imaging (DTI) and diffusion basis spectrum imaging (DBSI) metrics with histological axon density.
Methods
Post-mortem spinal cord tissue from a donor with a traumatic SCI was imaged ex-vivo at 7T using high-resolution multi-shell diffusion-weighted MRI. Cord segments were slice-matched to histology using a custom cutting mold. Tissue sections were stained for phosphorylated neurofilament (SMI-31) and axons were segmented using a custom automated image analysis pipeline to generate axon density maps. Regions of interest (ROIs) were manually defined in lesion tissue and major white matter tracts, yielding 180 matched MRI–histology region pairs. Diffusion data was denoised, distortion-corrected, and processed to create DTI and DBSI metric maps. After slice-wise MRI–histology registration, diffusion metrics were extracted per ROI. Spearman correlations were calculated between diffusion metrics and histological axon density.
Results
SCI lesion ROIs showed reduced axon density with decreased DTI fractional anisotropy and axial diffusivity compared to normal-appearing white matter. Across all ROIs, axon density correlated strongly with DTI fractional anisotropy (ρ=0.71) and DTI axial diffusivity (ρ=0.57). DBSI metrics demonstrated stronger associations with axonal pathology, with axon density showing strong positive correlations with DBSI fiber fraction (ρ=0.76) and DBSI axial diffusivity (ρ=0.64). DBSI restricted fraction (cellularity) was increased in lesion ROIs and was strongly negatively correlated with axon density (ρ=−0.56). Diffusivity metrics associated with non-axonal water compartments showed weaker or negligible correlations.
Conclusion
This study provides the first histological validation of DBSI metrics in human traumatic SCI. DBSI fiber fraction and axial diffusivity, along with DTI fractional anisotropy and axial diffusivity, demonstrate strong associations with axon density, supporting their use as axon-informative diffusion biomarkers. Our findings illustrate metric specificity and support translation to in-vivo MRI for monitoring axonal damage in SCI.