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

Purpose: Recent studies indicate that T in white matter (WM) is influenced by fiber orientation in B . The purpose of the study was to investigate the interrelationships between axon fiber orientation in corpus callosum (CC) and T relaxation time in humans in vivo as well as in rat brain ex vivo.

Methods: Volunteers were scanned for relaxometric and diffusion MRI at 3 T and 7 T. Angular T plots from WM were computed using fractional anisotropy and fiber-to-field-angle maps. T and fiber-to-field angle were measured in five sections of CC to estimate the effects of inherently varying fiber orientations on T within the same tracts in vivo. Ex vivo rat-brain preparation encompassing posterior CC was rotated in B and T , and diffusion MRI images acquired at 9.4 T. T angular plots were determined at several rotation angles in B .

Results: Angular T plots from global WM provided reference for estimated fiber orientation-linked T 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 T , matching that estimated from WM T data. In CC, where large and giant axons are numerous, the measured T change is about 2-fold greater than the estimated one. Ex vivo rotation of the same midsagittal CC region of interest produced angular T plots at 9.4 T, matching those observed at 7 T in vivo.

Conclusion: These data causally link axon fiber orientation in B to the T relaxation anisotropy in WM.