q‐space and conventional diffusion imaging of axon and myelin damage in the rat spinal cord after axotomy

Parallel and perpendicular diffusion properties of water in the rat spinal cord were investigated 3 and 30 days after dorsal root axotomy, a specific insult resulting in early axonal degeneration followed by later myelin damage in the dorsal column white matter (WM). Results from q-space analysis (i.e. the diffusion probability density function, PDF) obtained with strong diffusion weighting were compared to conventional anisotropy and diffusivity measurements at low b-values, as well as to histology for axon and myelin damage. Q-space contrasts included the height (PZERO), full width at half maximum (FWHM), root mean square displacement (RMSD), and kurtosis excess (KE) of the PDF, which quantifies the deviation from Gaussian diffusion. Following axotomy, a significant increase in perpendicular diffusion (with decreased KE) and decrease in parallel diffusion (with increased KE) were found in lesions relative to uninjured WM. Notably, a significant change in abnormal parallel diffusion was detected from 3 to 30 days with FWHM, but not with conventional diffusivity. Also, directional FWHM and RMSD measurements exhibited different sensitivities to WM damage. When compared to histology, the increase in perpendicular diffusion was not specific to demyelination, whereas combined reduced parallel diffusion and increased perpendicular diffusion was associated with axon damage.