T₂ contrast variation in human brain at 7 T and its potential contributors

Magnetic susceptibility-weighted MRI (or T₂ ^*-weighted MRI) at 7 T and higher field strengths has shown superb sensitivity to study normal and pathological levels of non-heme (tissue) iron and myelin in the brain. However, macroscopic field perturbations originating from venous vasculature and tissue-air interfaces lead to image artifacts, posing strong confounds to the interpretation of T₂ ^* contrast. Use of T₂-based rather than the more common T₂ ^*-based contrast to study susceptibility perturbations may alleviate these adverse effects, but it is technically challenging at high fields. The latter relates to the difficulty in performing accurate RF refocusing in the presence of increased B₀- and B₁-non-uniformity, and limits on RF power deposition. To overcome this, we employed the Gradient Echo Sampling of Spin Echo (GESSE) method to study R₂ (=1/T₂) variations at 7 T in healthy human brain. Our results indicate that sensitivity of R₂ to tissue iron, and associated tissue contrast, is largely preserved across subcortical structures, cortical functional areas, and between the cortex and superficial white matter, with substantially reduced sensitivity to macroscopic susceptibility effects. Therefore, R₂ as measured by GESSE may complement current R₂ ^*- and χ-based approaches for quantification of brain tissue iron and myelin. In deep white matter, R₂ was found to exhibit fiber bundle specificity, and showed significant correlations with documented fiber diameter and inferred orientation dependence with respect to the B₀. These results comprehensively chart multiple main contributors to R₂ contrast at 7 T across the whole brain, extending previous studies that have done so in specific brain areas or at lower field. Quantitative interpretation of R₂ contrast in terms of tissue iron and myelin content needs to take all these contributors into account.