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Abstract:
At 7T, MRI can now provide human brain images of structure, function and connectivity with isotropic voxels smaller than one millimeter, and thus much smaller than the cortical thickness. This resolution, achievable in scan times of less than one hour, allows visualization of myeloarchitectural layer structure, intracortical variations in functional activity – recorded in changes in BOLD signal or cerebral blood volume CBV – and intracortical axonal orientational structure via dMRI.
While recent improvements in radiofrequency receiver coils now enable excellent image data to be obtained at 3T, scanning at the ultra-high field of 7T offers further gains in signal-to-noise and speed of image acquisition, with structural image resolution up to about 300 micrometers. These improvements throw into sharp question the strategies that have become conventional for the analysis of functional imaging data, especially the practice of spatial smoothing of raw functional data prior to further analysis. Creation of a native cortical map for each human subject that provides a reliable individual parcellation into cortical areas related to Brodmann Areas enables a strikingly different approach to functional image analysis.
Experiments using proton beam microscopy have enabled a quantitative relationship to be established between measured brain tissue iron and myelin content on the one hand, and quantitative values of T1, T2* and quantitative magnetic susceptibility, on the other.
The proposed parcellation approach involves surface registration of the cortices of groups of subjects using maps of the longitudinal relaxation time T1 as an index of myelination, and methods for inferring statistical significance that do not entail spatial smoothing. The outcome is a far more precise comparison of like-with-like cortical areas across subjects, which can greatly increase experimental power, discriminate activity in neighboring cortical areas, and allow correlation of functional activity and connectivity with specific cytoarchitecture.
Furthermore, the high spatial resolution at 7T of fMRI using BOLD contrast and CBV measurement using VASO now allows cortical layer-dependent analyses to be performed, which may offer far more powerful methods of assessing neuronal causality, and more convincing modelling of brain mechanisms than current graph-based methods that require gross over-simplification of brain activity patterns in order to be computationally tractable.
