White Matter Tract Specific R2 Reflects Fiber Size and Microstructure

Motivation: T2 (or R2=1/T2) contrast is widely used for the study of neurological disease. Understanding how the relaxation relates to tissue microstructure may improve its clinical utility. Goal(s): To explore white matter microstructural correlates to R2. Approach: R2 mapping in healthy volunteers using Gradient Echo Sampling of Spin Echo at 7T. Atlas based analysis of fiber tracts, and analysis of corpus callosum subsections. Results: R2 variation was about 25% across the white matter and correlated inversely with the diffusion MRI metric of axonal diameter. Impact: T2 contrast is complex and influenced by factors including tissue iron and lipid content. Tissue microstructure, in particular white matter fiber thickness, may also play a role. Studying their relative contribution will help understand the nature of pathological tissue changes.