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

Purpose: Recent studies indicate that T-1 in white matter (WM) is influenced by fiber orientation in B-0. The purpose of the study was to investigate the interrelationships between axon fiber orientation in corpus callosum (CC) and T-1 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-1 plots from WM were computed using fractional anisotropy and fiber-to-field-angle maps. T-1 and fiber-to-field angle were measured in five sections of CC to estimate the effects of inherently varying fiber orientations on T-1 within the same tracts in vivo. Ex vivo rat-brain preparation encompassing posterior CC was rotated in B-0 and T-1, and diffusion MRI images acquired at 9.4 T. T-1 angular plots were determined at several rotation angles in B-0.Results: Angular T-1 plots from global WM provided reference for estimated fiber orientation-linked T-1 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-1, matching that estimated from WM T-1 data. In CC, where large and giant axons are numerous, the measured T-1 change is about 2-fold greater than the estimated one. Ex vivo rotation of the same midsagittal CC region of interest produced angular T-1 plots at 9.4 T, matching those observed at 7 T in vivo.Conclusion: These data causally link axon fiber orientation in B-0 to the T-1 relaxation anisotropy in WM.

Automated summary

Recent studies have shown that the T-1 relaxation time in white matter is affected by fiber orientation. This study investigated the relationship between axon fiber orientation in the corpus callosum and T-1 relaxation time in vivo and ex vivo. The results showed that changes in axon orientation in the corpus callosum were associated with corresponding changes in T-1 relaxation time. The study also demonstrated that the same results could be obtained from ex vivo experiments as in vivo experiments.