T2* mapping and Bo orientation-dependence at 7 T reveal cyto- and myeloarchitecture organization of the human cortex

Ultra-high field MRI (>= 7 T) has recently shown great sensitivity to depict patterns of tissue microarchitecture. Moreover, recent studies have demonstrated a dependency between T-2* and orientation of white matter fibers with respect to the main magnetic field B-o. In this study we probed the potential of T-2* mapping at 7 T to provide new markers of cortical architecture. We acquired multi-echo measurements at 7 T and mapped T-2* over the entire cortex of eight healthy individuals using surface-based analysis. B-o dependence was tested by computing the angle theta(z) between the normal of the surface and the direction of B-o, then fitting T-2* (theta(z)) using model from the literature. Average T-2* in the cortex was 32.20 +/- 1.35 ms. Patterns of lower T-2* were detected in the sensorimotor, visual and auditory cortices, likely reflecting higher myelin content. Significantly lower T-2* was detected in the left hemisphere of the auditory region (p<0.005), suggesting higher myelin content, in accordance with previous investigations. B-o orientation dependence was detected in some areas of the cortex, the strongest being in the primary motor cortex (Delta R-2* = 4.10 Hz). This study demonstrates that quantitative T-2* measures at 7 T MRI can reveal patterns of cytoarchitectural organization of the human cortex in vivo and that B-o orientation dependence can probe the coherency and orientation of gray matter fibers in the cortex, shedding light into the potential use of this type of contrast to characterize cyto-/myeloarchitecture and to understand the pathophysiology of diseases associated with changes in iron and/or myelin concentration. (c) 2012 Elsevier Inc. All rights reserved.