Mapping Human Cortical AreasIn VivoBased on Myelin Content as Revealed by T1- and T2-Weighted MRI

Abstract

Non-invasively mapping the layout of cortical areas in humans is a continuing challenge for neuroscience. We present a new method of mapping cortical areas based on myelin content as revealed by T1-weighted (T1w) and T2-weighted (T2w) MRI. The method is generalizable across different 3T scanners and pulse sequences. We use the ratio of T1w/T2w image intensities to eliminate the MR-related image intensity bias and enhance the contrast to noise ratio for myelin. Data from each subject was mapped to the cortical surface and aligned across individuals using surface-based registration. The spatial gradient of the group average myelin map provides an observer-independent measure of sharp transitions in myelin content across the surface—i.e. putative cortical areal borders. We found excellent agreement between the gradients of the myelin maps and the gradients of published probabilistic cytoarchitectonically defined cortical areas that were registered to the same surface-based atlas. For other cortical regions, we used published anatomical and functional information to make putative identifications of dozens of cortical areas or candidate areas. In general, primary and early unimodal association cortices are heavily myelinated and higher, multi-modal, association cortices are more lightly myelinated, but there are notable exceptions in the literature that are confirmed by our results. The overall pattern in the myelin maps also has important correlations with the developmental onset of subcortical white matter myelination, evolutionary cortical areal expansion in humans compared to macaques, postnatal cortical expansion in humans, and maps of neuronal density in non-human primates.

The impact of correction on MP2RAGE cortical T1 and apparent cortical thickness at 7T

“…For a comparison of the intersubject and scan–rescan COV with that of other (quantitative) contrasts (e.g., T 2 *), we refer the reader to Haast et al (2016). The reduction of the superior‐to‐inferior gradient of low‐to‐high T 1 —that led to the initially high COV across regions—resulted in a cortical T 1 distribution that is more comparable with previous studies at lower field strengths and are less affected by ${\mathrm{normalB}}_1^{+}$ effects, revealing the typical myelin‐related cortical pattern (Glasser & Van Essen, 2011; Lutti, Dick, Sereno, & Weiskopf, 2014). However, even after taking the ${\mathrm{normalB}}_1^{+}$ spatial profile into account, residual T 1 errors persisted across the whole cortex.…”

Cortical pathology in multiple sclerosis detected by the T1/T2‐weighted ratio from routine magnetic resonance imaging

“…The main idea of mapping the cortical ratio of the T1w and T2w signal was 2‐fold12: (1) by dividing the T1w by the T2w signal value, quantification of the signal intensity becomes possible to some degree, as this procedure levels off the bias field and scales the image intensity; as a result, the T1w/T2w ratio constitutes a potential marker of cortical tissue integrity independent of atrophy; and (2) although indirect, there is evidence that the T1w signal of both cerebral WM and cortex mainly results from myelin 12, 24, 25…”

“…This method is based on the ratio of T1‐ and T2‐weighted (w) image signal intensities 12. Heretofore, the histological substrate has been unclear, although myelin content has been suggested based on indirect evidence 12, 13, 14.…”

“…This method is based on the ratio of T1‐ and T2‐weighted (w) image signal intensities 12. Heretofore, the histological substrate has been unclear, although myelin content has been suggested based on indirect evidence 12, 13, 14. Yet 3 features of this method are particularly attractive for MS research: (1) with regard to precision, it seems to enable mapping of human cortical areas across the whole brain; (2) with regard to feasibility, it can be applied to large cohorts of patients and it is based on the ratio of intensity values from 2 conventional sequences, namely the T1w and T2w sequences, which are part of many clinical MRI protocols; and (3) with regard to functional relevance, its measures have been described to be related with performance variability in normal subjects 15…”

Tonotopic maps in human auditory cortex using arterial spin labeling

“…Recently, “myelin” imaging using MRI has been proposed for that purpose (De Martino et al, 2015). Primary sensory areas, such as early visual, somatosensory and auditory cortex, have been hypothesized to have higher myelin content than the surrounding brain areas, and they are detectable with T 1 and T 2 * MRI contrasts (Bock et al, 2009; Cohen‐Adad et al, 2012; De Martino et al, 2015; Dick et al, 2012; Geyer et al, 2011; Glasser and Van Essen, 2011; Sereno et al, 2013; Sigalovsky et al, 2006). Alternatively, susceptibility weighted imaging (SWI) can be used to probe iron and myelin differences in the cortex and, additionally, to locate veins.…”