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

This 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.