Elsevier

NeuroImage

Volume 185, 15 January 2019, Pages 27-34
NeuroImage

Depth-dependent intracortical myelin organization in the living human brain determined by in vivo ultra-high field magnetic resonance imaging

https://doi.org/10.1016/j.neuroimage.2018.10.023Get rights and content
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Highlights

  • Intracortical myelin is a key determinant of optimal brain function.

  • We used a new approach to study intracortical myelin using ultra-high field MRI.

  • A new non-linearity index was used as a measure of intracortical organization.

  • Intracortical myelin concentration correlated with non-linearity indices.

  • Intracortical myelin concentration correlated with subcortical fractional anisotropy.

Abstract

Background

Intracortical myelin is a key determinant of neuronal synchrony and plasticity that underpin optimal brain function. Magnetic resonance imaging (MRI) facilitates the examination of intracortical myelin but presents with methodological challenges. Here we describe a whole-brain approach for the in vivo investigation of intracortical myelin in the human brain using ultra-high field MRI.

Methods

Twenty-five healthy adults were imaged in a 7 Tesla MRI scanner using diffusion-weighted imaging and a T1-weighted sequence optimized for intracortical myelin contrast. Using an automated pipeline, T1 values were extracted at 20 depth-levels from each of 148 cortical regions. In each cortical region, T1 values were used to infer myelin concentration and to construct a non-linearity index as a measure the spatial distribution of myelin across the cortical ribbon. The relationship of myelin concentration and the non-linearity index with other neuroanatomical properties were investigated. Five patients with multiple sclerosis were also assessed using the same protocol as positive controls.

Results

Intracortical T1 values decreased between the outer brain surface and the gray-white matter boundary following a slope that showed a slight leveling between 50% and 75% of cortical depth. Higher-order regions in the prefrontal, cingulate and insular cortices, displayed higher non-linearity indices than sensorimotor regions. Across all regions, there was a positive association between T1 values and non-linearity indices (P < 10−5). Both T1 values (P < 10−5) and non-linearity indices (P < 10−15) were associated with cortical thickness. Higher myelin concentration but only in the deepest cortical levels was associated with increased subcortical fractional anisotropy (P = 0.05).

Conclusions

We demonstrate the usefulness of an automatic, whole-brain method to perform depth-dependent examination of intracortical myelin organization. The extracted metrics, T1 values and the non-linearity index, have characteristic patterns across cortical regions, and are associated with thickness and underlying white matter microstructure.

Keywords

Ultra-high field
Neuroimaging
Cortical depth-levels
Myeloarchitecture

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