Studying cyto and myeloarchitecture of the human cortex at ultra-high field with quantitative imaging: R1, R2* and magnetic susceptibility

Abstract

A B S T R A C TIn this manuscript, the use of quantitative imaging at ultra-high field is evaluated as a mean to study cyto and myelo-architecture of the cortex. The quantitative contrasts used are the longitudinal relaxation rate (R 1 ), apparent transverse relaxation rate (R 2 *) and quantitative susceptibility mapping (QSM).The quantitative contrasts were computed using high resolution in-vivo (0.65mm isotropic) brain data acquired at 7 T.The performance of the different quantitative approaches was evaluated by visualizing the contrast between known highly myelinated primary sensory cortex regions and the neighbouring cortex. The transition from the inner layers to the outer layers (from white matter to the pial surface) of the human cortex, which is known to have varying cyto-and myelo architecture, was evaluated.The across cortex and through depth behaviour observed for the different quantitative maps was in good agreement between the different subjects, clearly allowing the differentiation between different Brodmann regions, suggesting these features could be used for individual cortical brain parcellation. While both R 1 and R 2 * maps decrease monotonically from the white matter to the pial surface due to the decrease of myelin and iron between these regions, magnetic susceptibility maps have a more complex behaviour reflecting its opposing sensitivity to myelin and iron concentration.

Large-Scale Gradients in Human Cortical Organization

“…However, recent advances in high-resolution and quantitative MRI [21,22] together with improved processing tools (e.g., [23,24]) facilitate noninvasive assessment of microstructural features in the human brain. MRI only provides indirect measures of microstructural components, and its spatial resolution is still low compared with histological methods.…”

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

“…Being able to detect the myelo-architecture and cortical profiles invivo throughout the entire brain offers new opportunities for parcellating the human cortex (Dick et al, 2012;Geyer et al, 2011;Marques et al, 2017) -dubbed "in-vivo Brodmann mapping" ), a particular aspect of "in-vivo histology" using MRI (hMRI, Clare, 2006, Deistung et al, 2013;Weiskopf et al, 2015). Cortical profiles of quantitative values, such as T1, T2* or susceptibility, may thereby help to detect variations of myelin and iron on an intracortical level.…”

“…Quantitative anatomical MRI yields parameter maps of relaxation times, susceptibility or (MT) saturation effects in standardized physical units that are comparable across time points and sites (Deistung et al, 2013;Marques et al, 2017;Weiskopf et al, 2013Weiskopf et al, , 2015. This can significantly reduce the measurement variability in multi-center studies .…”

“…Various contrasts were explored and also calibrated for measuring the myelin volume fraction (e.g., T1, T2*, MT and T2 based metrics; Stüber et al, 2014) or the related macromolecular tissue volume (Mezer et al, 2013). A recent study on myeloarchitecture of the human cortex at different cortical depths showed, however, that a extrapolation from ex-vivo results obtained for a single specimen (Stüber et al, 2014) to in-vivo whole brain data cannot easily be performed (Marques et al, 2017). Whereas a linear relationship between R2*/susceptibility and myelin volume could be confirmed throughout the cortex the results were less clear for R1 which is thought to be modulated by the existing myelin surface fraction rather than its volume (Marques et al, 2017).…”

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

“…However, the effect of this correction scheme has not yet been extensively characterized across cortical regions. Based on the initial results by Marques et al (2017) and the ${\mathrm{normalB}}_1^{+}$ correction methodology, the difference in the degree of T 1 (w) signal changes depends on the apparent T 1 value and, therefore, vary between WM and GM, even if the voxels are very close to each other. As such, the correction also affect the CNR between cortical GM and WM, and we expect this to propagate toward differences in the cortical surface reconstructions and, therefore, apparent cortical thickness.…”