Phase‐shifted transverse relaxation orientation dependences in human brain white matter

Abstract

AbstractThis work aimed to demonstrate an essential phase shift for better quantifying and in human brain white matter (WM), and to further elucidate its origin related to the directional diffusivities from standard diffusion tensor imaging (DTI). was integrated into a proposed generalized transverse relaxation model for characterizing previously published and orientation dependence profiles in brain WM, and then comparisons were made with those without . It was theorized that anisotropic diffusivity direction was collinear with an axon fiber subject to all eigenvalues and eigenvectors from an apparent diffusion tensor. To corroborate the origin of , orientation dependences referenced by were compared with those referenced by the standard principal diffusivity direction at b‐values of 1000 and 2500 (s/mm2). These orientation dependences were obtained from ‐weighted images (b = 0) of ultrahigh‐resolution Connectome DTI datasets in the public domain. A normalized root‐mean‐square error ( ) and an ‐test were used for evaluating curve‐fittings, and statistical significance was considered to be a p of 0.05 or less. A phase‐shifted model resulted in significantly reduced compared with that without in quantifying various and profiles, both in vivo and ex vivo at multiple fields. The profiles based on manifested a right‐shifted phase ( ) at two b‐values, while those based on became free from . For all phase‐shifted and profiles, generally depended on the directional diffusivities by , as predicted. In summary, a ubiquitous phase shift has been demonstrated as a prerequisite for better quantifying transverse relaxation orientation dependences in human brain WM. Furthermore, the origin of associated with the directional diffusivities from DTI has been elucidated. These findings could have a significant impact on interpretations of prior and datasets and on future research.