Abstract
Alterations in the myelination of the cerebral cortex may underlie abnormal cortical function in a variety of brain diseases. Here, we describe a technique for investigating changes in intracortical myelin in clinical populations on the basis of cortical thickness measurements with magnetic resonance imaging (MRI) at 3 Tesla. For this, we separately compute the thickness of the shallower, lightly myelinated portion of the cortex and its deeper, heavily myelinated portion (referred to herein as unmyelinated and myelinated cortex, respectively). Our expectation is that the thickness of the myelinated cortex will be a specific biomarker for disruptions in myeloarchitecture. We show representative atlases of total cortical thickness, T, unmyelinated cortical thickness, G, and myelinated cortical thickness, M, for a healthy group of 20 female subjects. We further demonstrate myelinated cortical thickness measurements in a preliminary clinical study of 10 bipolar disorder type-I subjects and 10 healthy controls, and report significant decreases in the middle frontal gyrus in T, G, and M in the disorder, with the largest percentage change occurring in M. This study highlights the potential of myelinated cortical thickness measurements for investigating intracortical myelin involvement in brain disease at clinically relevant field strengths and resolutions.
Highlights
The cerebral cortex contains a substantial number of myelinated axons whose pattern of distribution describes its myeloarchitecture
The myelinated cortical thickness technique relies on datadriven clustering of Magnetic resonance imaging (MRI) intensity in the voxels of the cerebrum to obtain the three tissue classes: gray matter (GM), myelinated gray matter (mGM), and white matter (WM)
That this threshold underestimated the volume of the WM tissue class in the cerebrum because the additional mGM tissue class resulted in a displaced WM/mGM boundary compared to a two-tissue classification
Summary
The cerebral cortex contains a substantial number of myelinated axons whose pattern of distribution describes its myeloarchitecture. Intracortical myelin is found predominantly in the deeper layers of the cortex, and likely serves to speed the propagation of neural signals, as myelin does in the major white matter tracts. It fine-tunes the timing and synchrony of neural networks, thereby continuously optimizing cortical function (Haroutunian et al, 2014). MRI has identified deficits in cortical myelin in mental disease, with losses of intracortical myelin being observed in patients with schizophrenia (Bartzokis et al, 2009, 2012). These studies predict a future role for MRI in studies of how intracortical myelin is related to function in health and disease
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