Axon fiber orientation as the source of T1 relaxation anisotropy in white matter: A study on corpus callosum in vivo and ex vivo

Abstract

PurposeRecent studies indicate that T1 in white matter (WM) is influenced by fiber orientation in B0. The purpose of the study was to investigate the interrelationships between axon fiber orientation in corpus callosum (CC) and T1 relaxation time in humans in vivo as well as in rat brain ex vivo.MethodsVolunteers were scanned for relaxometric and diffusion MRI at 3 T and 7 T. Angular T1 plots from WM were computed using fractional anisotropy and fiber‐to‐field‐angle maps. T1 and fiber‐to‐field angle were measured in five sections of CC to estimate the effects of inherently varying fiber orientations on T1 within the same tracts in vivo. Ex vivo rat‐brain preparation encompassing posterior CC was rotated in B0 and T1, and diffusion MRI images acquired at 9.4 T. T1 angular plots were determined at several rotation angles in B0.ResultsAngular T1 plots from global WM provided reference for estimated fiber orientation–linked T1 changes within CC. In anterior midbody of CC in vivo, where small axons are dominantly present, a shift in axon orientation is accompanied by a change in T1, matching that estimated from WM T1 data. In CC, where large and giant axons are numerous, the measured T1 change is about 2‐fold greater than the estimated one. Ex vivo rotation of the same midsagittal CC region of interest produced angular T1 plots at 9.4 T, matching those observed at 7 T in vivo.ConclusionThese data causally link axon fiber orientation in B0 to the T1 relaxation anisotropy in WM.