Axon fiber orientation as the source of <scp>T<sub>1</sub></scp> relaxation anisotropy in white matter: A study on corpus callosum in vivo and ex vivo
Risto A. Kauppinen, Jeromy Thothard, Henri Leskinen, Pramod Kumar Pisharady, Eppu Manninen, Mikko I. Kettunen, Christophe Lenglet, Olli Gröhn, Michael Garwood, Mikko J. Nissi
Abstract
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.