Abstract
BackgroundIntracortical 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. MethodsTwenty-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. ResultsIntracortical 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). ConclusionsWe 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.
Highlights
Half the human brain comprises myelinated axons that connect brain regions with each other
To aid interpretation of this new metric, we provide information on how this nonlinearity index relates to standard Magnetic resonance imanging (MRI) measures of cortical thickness and sub-cortical white matter fractional anisotropy (FA) measured using diffusion weighted imaging
In addition to estimating myelin concentration, we computed the non-linearity index, a novel metric that assesses the spatial distribution of myelin along the cortical ribbon
Summary
Half the human brain comprises myelinated axons that connect brain regions with each other. Myelin is formed by oligodendrocytes that sheath and insulate neuronal axons (Nave & Werner, 2014). Intracortical myelin is sensitive to experience-dependent neuronal activity throughout the lifespan actively contributing to brain plasticity and remodeling (Fields, 2015; Purger et al, 2016). These features suggest that myeloarchitecture is relevant to the fine-tuning of cortical circuits, a notion that is supported by the association between intracortical myelin and individual variability in cognitive function (Grydeland et al, 2013)
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