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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.
Kauppinen et al. (Fri,) studied this question.