Radial diffusivity predicts demyelination in ex vivo multiple sclerosis spinal cords

Abstract

Objective Correlation of diffusion tensor imaging (DTI) with histochemical staining for demyelination and axonal damage in multiple sclerosis (MS) ex vivo human cervical spinal cords. Background In MS, demyelination, axonal degeneration, and inflammation contribute to disease pathogenesis to variable degrees. Based upon in vivo animal studies with acute injury and histopathologic correlation, we hypothesized that DTI can differentiate between axonal and myelin pathologies within humans. Methods DTI was performed at 4.7 Tesla on 9 MS and 5 normal control fixed cervical spinal cord blocks following autopsy. Sections were then stained for Luxol fast blue (LFB), Bielschowsky silver, and hematoxylin and eosin (H&E). Regions of interest (ROIs) were graded semi-quantitatively as normal myelination, mild (<50%) demyelination, or moderate-severe (>50%) demyelination. Corresponding axonal counts were manually determined on Bielschowsky silver. ROIs were mapped to co-registered DTI parameter slices. DTI parameters evaluated included standard quantitative assessments of apparent diffusion coefficient (ADC), relative anisotropy (RA), axial diffusivity and radial diffusivity. Statistical correlations were made between histochemical gradings and DTI parameters using linear mixed models. Results: Within ROIs in MS subjects, increased radial diffusivity distinguished worsening severities of demyelination. Relative anisotropy was decreased in the setting of moderate-severe demyelination compared to normal areas and areas of mild demyelination. Radial diffusivity, ADC, and RA became increasingly altered within quartiles of worsening axonal counts. Axial diffusivity did not correlate with axonal density (p=0.091). Conclusions Increased radial diffusivity can serve as a surrogate for demyelination. However, radial diffusivity was also altered with axon injury, suggesting that this measure is not pathologically specific within chronic human MS tissue. We propose that radial diffusivity can serve as a marker of overall tissue integrity within chronic MS lesions. This study provides pathologic foundation for on-going in vivo DTI studies in MS.

Neurite dispersion: a new marker of multiple sclerosis spinal cord pathology?

“…In contrast, lesional matter shows hypointense NDI and ODI maps and hyperintense IVF relative to nonlesional areas, in line with recent findings in vivo 39. DTI metrics highlight focal pathology as well, with increased diffusivity and changes in FA, similarly to what has been previously reported 36. Our results suggest that quantitative diffusion MRI metrics are well positioned to detect areas of focal pathology in the spinal cord.…”

“…Both correlation and linear regression analyses confirm once more that DTI is very sensitive to multiple sclerosis pathology, as shown by strong changes of FA, AD, RD, and MD noticeable on simple visual inspection 36. However, DTI metrics appear as surrogate biomarkers of multiple sclerosis pathology with relatively poor specificity, as they are jointly influenced by demyelination, neuroaxonal loss and even changes in the glial component.…”

“…CV is higher in gray matter compared to white matter. DTI metrics show known trends,36 such as higher diffusion anisotropy in white as compared with gray matter.…”

“…Neurofilament immunostaining demonstrates neuronal cell bodies, potentially reducing the gray/white matter contrast of NSF, while differences in myelination between dendrites and axons may increase the gray/white matter contrast of NDI. Regarding DTI metrics, known contrasts are observed between controls’ gray and white matter 27, 36…”

“…Histological images and patch‐wise metrics were co‐registered in Matlab to the corresponding MRI slices with a landmark‐guided nonlinear approach,36 which maximized the Dice coefficient37 between the outlines of the specimens in MRI and histology 38. Landmark points were drawn on the mean b = 0 image and on the downsampled histological images.…”