Estimating cortical myelin content based on ratio between T1-weighted and T2-weighted MRIs as done in this study could be influenced by iron, which often co-localized with myelin in the cerebral cortex (Fukunaga et al. 2010)). Iron can thus be another contributing factor to MRI contrast showing especially high contrast portions in T2* maps (Stuber et al. 2014). In our analysis, we expect the major fraction of MRI contrast was due to the variation in the myelin content because both the NCANDA and the HCP studies acquired T2-weighted sequences with high sensitivity to lipids. The T2-weighted MRIs also enabled reliable extraction of the brain pial surfaces, and the T1-weighted MRIs were enabled accurate identification of gray/white matter boundaries; together, this image information enhanced accuracy in estimating cortical myelin content on the brain surface. However, not all structural MRI studies share these MRI characteristics and thus must be carefully considered before applying the proposed approach.
