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

Purpose: To overcome limitations of prior orientation-dependent R2 and R2* formalisms in white matter (WM) with a novel framework based on magic angle effect. Methods: A cylindrical helix model was developed embracing both anisotropic rotational and translational diffusions of ordered water in WM, with the former characterized by an axially symmetric system. Both R2 and R2* were divided into isotropic (R2i) and anisotropic parts, R2a*f(ε-ε0,α), with α denoting a funnel opening angle and ε0 an orientation (ε) offset relative to DTI-derived primary diffusivity direction. The proposed framework (Fit A) was compared with prior model without ε0 (Fit B) and applied to published R2 and R2* in WM of underdeveloped, healthy, and diseased conditions. Goodness of fit was characterized by root-mean-square error (RMSE). F-test and Pearson correlation coefficient (PCC) were used with statistical significance set to P ≤ .05. Results: Fit A significantly outperformed Fit B as demonstrated by reduced RMSEs in myelin water (i.e., 0.349 vs. 0.724). The fitted ε0 was in good agreement with the calculated ε0 from DTI directional diffusivities. Significant positive (R2i) and negative (α and R2a) correlations were found with aging (demyelination) in adults while ε0 showed a weak positive correction (PCC=0.11, P= .28). Compared to those from healthy adult WM, the fits of R2i, R2a, and α from neonates were considerably reduced but ε0 increased, consistent with limited myelination. Conclusion: The developed framework can better characterize anisotropic transverse relaxation in WM, shedding new light on myelin microstructural alterations at the molecular level.

Magic-angle semimetals

Breakthroughs in two-dimensional van der Waals heterostructures have revealed that twisting creates a moiré pattern that quenches the kinetic energy of electrons, allowing for exotic many-body states. We show that cold atomic, trapped ion, and metamaterial systems can emulate the effects of a twist in many models from one to three dimensions. Further, we demonstrate at larger angles (and argue at smaller angles) that by considering incommensurate effects, the magic-angle effect becomes a single-particle quantum phase transition (including in a model for twisted bilayer graphene in the chiral limit). We call these models “magic-angle semimetals”. Each contains nodes in the band structure and an incommensurate modulation. At magic-angle criticality, we report a nonanalytic density of states, flat bands, multifractal wave functions that Anderson delocalize in momentum space, and an essentially divergent effective interaction scale. As a particular example, we discuss how to observe this effect in an ultracold Fermi gas.

Use of Fat-Suppressed T2-Weighted MRI Images to Reduce the Magic Angle Effect in Peroneal Tendons

Introduction. Peroneal tendon disorders pose a diagnostic conundrum to the clinician. Magnetic resonance imaging (MRI) is widely used to assess tendon pathology. A recognized artifact of MRI, the magic angle effect (MAE), can lead to spurious results and inappropriate management. The aim of this study is to assess whether T2 fat-suppressed sequences (T2FSs) reduce the frequency of MAE compared with proton density fat-suppressed (PDFS) images. Methods. MRI scans of 18 patients were prospectively assessed for MAE. The peroneal tendons were assessed at 5 defined levels on PDFS and T2FS images. The frequency of MAE in the peroneal tendons were compared between the 2 scan sequences. Results. In the peroneus brevis tendon, 17/72 levels, on PDFS scans, showed MAE compared with 2/72 levels on the T2FS scans, demonstrating a reduction in the MAE by 85% (P = .0003). In the peroneus longus tendon 14/72 levels, on PDFS scans, demonstrated MAE compared with 4/72 on T2FSs, demonstrating a reduction of 71% (P = .02). Conclusion. The inclusion of T2-weighted sequences is useful in MRI scanning for peroneal tendons to mitigate the MAE artifact, avoid potential misdiagnosis, and guide subsequent management of peroneal tendon disorders. Levels of Evidence: Level IV: Case series