Quantitative <scp>T1</scp> is sensitive to cortical remyelination in multiple sclerosis: A postmortem <scp>MRI</scp> study

Remyelination of cortical lesions in people with multiple sclerosis (pwMS) has been shown to be extensive. In this work, we aimed to assess whether postmortem quantitative MRI (qMRI) can help detect those areas. We imaged six fixed whole brains of deceased pwMS by 3T-MRI using magnetization transfer ratio (MTR, 570 μm isotropic), myelin water fraction (MWF, 1000 μm isotropic), quantitative T1 (qT1, 670 μm isotropic), quantitative susceptibility mapping (QSM, 330 μm isotropic) and radial diffusivity (RD, 1300 or 1400 μm isotropic) maps. Immunohistochemistry for myelin proteins was performed in 129 tissue blocks including the cortex and enabled the detection of cortical demyelination (DM), cortical remyelination (RM), and normal-appearing cortex (NAC). We identified 25 DM, 25 RM, and for each of these areas, a corresponding NAC near the lesion. Wilcoxon paired tests showed that: (a) qT1 and RD were higher and QSM lower in DM versus NAC (all p < 0.001), whereas RD was higher and QSM lower in RM versus NAC (p = 0.048 and p < 0.01 respectively); (b) mean qT1 in RM did not differ from mean qT1 in NAC (p = 0.074); (c) MWF and MTR were not different between DM and RM. We compared the delta between DM versus NAC (∆DM) and the delta between RM versus NAC (∆RM) using a Mann-Whitney test, in which RM showed a partial recovery of qT1 only (∆qT1 DM > ∆qT1 RM, p = 0.045). Mixed-effect models confirmed the findings obtained using univariate analyses. qT1 and QSM, but not RD, correlated with MBP intensity (r = -0.28, p < 0.01 and r = 0.29, p < 0.01 respectively). A Bonferroni correction was performed for multiple testing. Our data show that qT1 is altered in demyelinated but not in remyelinated cortical areas, while QSM and RD are affected by any cortical abnormalities. Accordingly, qT1 might be considered a potential imaging biomarker of cortical RM.