title: T₂* mapping and B₀ orientation-dependence at 7 T reveal cyto- and myeloarchitecture organization of the human cortex.
abstract: 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 T₂* and orientation of white matter fibers with respect to the main magnetic field B₀. In this study we probed the potential of T₂* mapping at 7 T to provide new markers of cortical architecture. We acquired multi-echo measurements at 7 T and mapped T₂* over the entire cortex of eight healthy individuals using surface-based analysis. B₀ dependence was tested by computing the angle θ(z) between the normal of the surface and the direction of B₀, then fitting T₂*(θ(z)) using model from the literature. Average T₂* in the cortex was 32.20 +/- 1.35 ms. Patterns of lower T₂* were detected in the sensorimotor, visual and auditory cortices, likely reflecting higher myelin content. Significantly lower T₂* was detected in the left hemisphere of the auditory region (p<0.005), suggesting higher myelin content, in accordance with previous investigations. B₀ orientation dependence was detected in some areas of the cortex, the strongest being in the primary motor cortex (∆R₂*=4.10 Hz). This study demonstrates that quantitative T₂* measures at 7 T MRI can reveal patterns of cytoarchitectural organization of the human cortex in vivo and that B₀ 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.
authors.affiliation: A.A. Martinos Center for Biomedical Imaging, Dept. of Radiology, Massachusetts General Hospital, Charlestown, MA 02129, USA. jcohen@nmr.mgh.harvard.edu
authors.forename: J
authors.initials: J
authors.lastname: Cohen-Adad
authors.forename: J R
authors.initials: JR
authors.lastname: Polimeni
authors.forename: K G
authors.initials: KG
authors.lastname: Helmer
authors.forename: T
authors.initials: T
authors.lastname: Benner
authors.forename: J A
authors.initials: JA
authors.lastname: McNab
authors.forename: L L
authors.initials: LL
authors.lastname: Wald
authors.forename: B R
authors.initials: BR
authors.lastname: Rosen
authors.forename: C
authors.initials: C
authors.lastname: Mainero
mesh_terms: D000328:Adult; D001931:Brain Mapping / Q000379:methods*; D002540:Cerebral Cortex / Q000033:anatomy & histology* / Q000166:cytology; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*