For each follow-up visit, noise removal (Coupe et al. 2008) was applied to the corresponding MRIs, which were then non-rigidly registered to their baseline modality using ANTs (Version: 2.1.0) (Avants et al. 2008). The midthickness surface of the baseline was then used to record the initial MRI intensity and low frequency values at each vertex of that follow-up scan using again the HCP mapping procedure. To improve longitudinal consistency of our analysis, the multiplicative inhomogeneity (Sled et al. 1998) of the MRI intensity values on the surface were then matched to that of the baseline by computing at each vertex the ratios between the low-frequency values at baseline and follow-up, and applying that ratio to the MRI intensity values of the follow-up scan. The reliability of myelin content from this approach (called the longitudinal approach) was compared with that of the myelin content estimated independently for each visit (called the cross-sectional approach).
