In-vivo magnetic resonance imaging (MRI) of laminae in the human cortex

Abstract

The human neocortex is organized radially into six layers which differ in their myelination and the density and arrangement of neuronal cells. This cortical cyto-and myeloarchitecture plays a central role in the anatomical and functional neuroanatomy but is primarily accessible through invasive histology only. To overcome this limitation, several non-invasive MRI approaches have been, and are being, developed to resolve the anatomical cortical layers. As a result, recent studies on large populations and structure-function relationships at the laminar level became possible. Early proof-of-concept studies targeted conspicuous laminar structures such as the stria of Gennari in the primary visual cortex. Recent work characterized the laminar structure outside the visual cortex, investigated the relationship between laminar structure and function, and demonstrated layer-specific maturation effects. This paper reviews the methods and in-vivo MRI studies on the anatomical layers in the human cortex based on conventional and quantitative MRI (excluding diffusion imaging). A focus is on the related challenges, promises and potential future developments. The rapid development of MRI scanners, motion correction techniques, analysis methods and biophysical modeling promise to overcome the challenges of spatial resolution, precision and specificity of systematic imaging of cortical laminae. Need for non-invasive mapping of cortical laminationThe neocortex of the human brain consists of six layers which differ in their pattern of myelination (Nieuwenhuys, 2013;Vogt and Vogt, 1919) and the density and arrangement of neuronal cells (Brodmann, 1909;Zilles and Amunts, 2010). Those differences gave rise to the broad research field of myelo-and cyto-architecture, which is based on invasive histology. To facilitate studies of the human brain in large populations and longitudinally, non-invasive methods are much needed for visualizing intracortical anatomy in-vivo, since histological methods can only be applied ex-vivo. A promising approach is magnetic resonance imaging (MRI) as it is non-invasive and highly sensitive to the presence of myelin (and iron) in the cortex (Stüber et al., 2014). It is expected to reflect the myeloarchitecture rather than the cyto-architecture, although there is a strong correspondence between the two . Scope of the reviewThis paper reviews the recent in-vivo MRI studies on the anatomical layers in the human neocortex. An additional focus is on the related challenges, promises and potential future developments. Studies using diffusion weighted MRI are excluded from this review, since they are covered by Assaf et al. (2017) in the same special issue on "Prospects for cortical laminar MRI: functional and anatomical imaging of cerebral cortical layers". MRI studies of cortical layers Studies of the stria of GennariAs the cortex is only 2-4 mm thick and convoluted (Fischl and Dale, 2000), mapping cortical layers requires sub-millimeter isotropic resolution. Another challenge is that the differences in myelo-ar...

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