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

Abstract

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.

Data-driven characterization and correction of the orientation dependence of magnetization transfer measures using diffusion MRI

“…For instance, the corticospinal tract (CST), which axons are larger than in other tracts 35 , could be highly myelinated and thus increase the measures mean at low θ a angles, as CST is roughly aligned with B 0 . With that in mind and taking inspiration from Kauppinen et al 36 , we also try a 1), the single-fiber selection mask is created as the intersection of a tissue mask, a FA threshold mask and a NuFO equals 1 mask. In the second step (2), the angle θa between B0 and the principal eigenvector e1 of the diffusion tensor is calculated for each selected voxel, and put in angle bins of width ∆ • .…”

Anisotropic longitudinal water proton relaxation in white matter investigated ex vivo in porcine spinal cord with sample rotation

“…Recent work has further drawn attention to anisotropic longitudinal relaxation characteristics in human WM in vivo 15 – 18 as well as in ex-vivo tissue samples with a high degree of structural order 17 , 19 , 20 . This is remarkable because no anisotropy of longitudinal relaxation or MT was found in early investigations 21 .…”

“…However, inconsistencies also exist between studies reporting orientation dependence. Kauppinen et al 15 – 17 obtained anisotropy at 3 T for human WM fibers, with a maximum at a fiber-to-field angle ≈ 50° and an orientation-dependent variation, of about 7%. Efforts to reproduce and elucidate this trend ex vivo, in rat brain at 9.4 T, with other acquisition techniques were only partially successful and showed a different orientation dependence 17 .…”

“…Kauppinen et al 15 – 17 obtained anisotropy at 3 T for human WM fibers, with a maximum at a fiber-to-field angle ≈ 50° and an orientation-dependent variation, of about 7%. Efforts to reproduce and elucidate this trend ex vivo, in rat brain at 9.4 T, with other acquisition techniques were only partially successful and showed a different orientation dependence 17 . Other ex vivo measurements at the same field strength showed no observable anisotropy with inversion recovery, but a high anisotropy of the transverse relaxation time 19 , 20 .…”

Radiation damping at clinical field strength: Characterization and compensation in quantitative measurements

“…37 This underscores the degree of precision required to identify the gross signature of cortical layers using relaxation measurements. Small effect sizes are also characteristic of other MRI experiments, for example, arterial spin labeling 16 or MT 22,37 and relaxation anisotropy measurements in white matter, 38,39 indicating that their performance may be impacted by RD. Damping effects were also observed in reference measurements of T 1 of blood with a standard transmit-receive (Tx/Rx) head coil.…”