Magnetic Resonance Imaging of $$T_2$$- and Diffusion Anisotropy Using a Tiltable Receive Coil

Abstract

The anisotropic microstructure of white matter is reflected in various MRI contrasts. Transverse relaxation rates can be probed as a function of fibre-orientation with respect to the main magnetic field, while diffusion properties are probed as a function of fibre-orientation with respect to an encoding gradient. While the latter is easy to obtain by varying the orientation of the gradient, as the magnetic field is fixed, obtaining the former requires re-orienting the head. In this work we deployed a tiltable RF-coil to study $$T_2$$ T 2 - and diffusional anisotropy of the brain white matter simultaneously in diffusion-$$T_2$$ T 2 correlation experiments.

Measuring compartmental T2-orientational dependence in human brain white matter using a tiltable RF coil and diffusion-T2 correlation MRI

“…were scanned on a T 300 mT/m Connectom scanner equipped with a modified 20-channel head/neck tiltable coil (Siemens Healthineers, Erlangen, Germany). Qualitative preliminary observations of apparent orientation-dependence using this setup have previously been reported in Tax et al. (2020b) .…”

Orientation dependent proton transverse relaxation in human brain white matter: The magic angle effect on a cylindrical helix

“…Here, ߠ is the angle between an axon fiber and ‫ܤ‬ , ‫ܣ‬ and ‫ܤ‬ (݅ ൌ 1,2,3) the model parameters (i.e., trigonometric function coefficients). As revealed recently, 7,18 both functions are mathematically equivalent albeit with different coefficients. More importantly, these coefficients are not mutually independent as cos4ߠ (or sin ସ ߠ) can be expressed by cos2ߠ (or sin ଶ ߠ) and vice versa; thus, any proposed biophysical interpretations of fitted model parameters will become ambiguous.…”

“…For instance, myelinated axons in the human brain white matter (WM) are anisotropic and inhomogeneous in nature and water proton longitudinal (i.e., ܴ ଵ =1 ܶ ଵ ⁄ ) and transverse (i.e., ܴ ଶ =1 ܶ ଶ ⁄ ) relaxation rates have been revealed depending on orientations of axon fibers although the reported ܴ ଵ relaxation anisotropy at 3T is much smaller than that of ܴ ଶ . [4][5][6][7] Based on water proton transverse magnetization dephasing induced by applied directional diffusion gradients, diffusion tensor imaging (DTI) can provide axon orientation information in WM at an image voxel size level. 8,9 Generally, three orthogonal translational diffusivities (i.e., eigenvalues) and the corresponding directions (i.e., eigenvectors) relative to an external static magnetic field ‫ܤ‬ can be determined based on the standard DTI model.…”

“…23 A general form of MAE function has long been available 24 and lately reformatted to gain further insight into proton transverse relaxation orientation dependences. 25 As both stemmed from thermally driven Brownian motions, molecular translational diffusion and rotational diffusion (or reorientation) should be intimately linked, 7,26,27 with the former probed by DTI and the latter by MR relaxation measurements. To our knowledge, no direct connection exists between the two different measurements for studying the same anisotropic microstructures in WM.…”

Orientation dependent proton transverse relaxation in the human brain white matter: The magic angle effect on a cylindrical helix

“…In the past, an ultrashort transverse relaxation time (ܶ ଶ ~10-15 ‫)ݏߤ‬ of semisolid lipid CH2 protons was observed by quantitative magnetization transfer (qMT) imaging, 31 revealing the comparable orientation dependence with respect to that of surface water based on multiple orientation-dependent transverse relaxation studies in literature. 4,7,17,18,32 The aim of this work was thus to introduce an angle offset ߝ , determined by DTI diffusivities, into a cylindrical helix model based on the generalized MAE function for characterizing anisotropic transverse relaxation of ordered water and semisolid CH2 protons in WM. The proposed theoretical framework was validated by a high-resolution Connectome DTI dataset and then applied to previously published anisotropic ܴ ଶ and ܴ ଶ ‫כ‬ profiles at 3T in vivo from the human brain WM of neonates, healthy and diseased adults.…”