Predicting Mesoscopic Larmor Frequency Shifts with Diffusion MRI in Ex Vivo Pig Optic Nerve

Motivation: Larmor frequency shifts in white matter (WM) depend on orientation due to anisotropic microstructure. Accurate magnetic susceptibility estimation in brain tissue requires a model for these sub-voxel shifts. Goal(s): To validate if μQSM can predict measured WM Larmor frequency shifts by incorporating mesoscopic shifts from orientationally dispersed hollow cylinders and spherical inclusions. Approach: We acquired dMRI and MGE images of a pig optic nerve in PBS at multiple orientations using a 3T Siemens Connectom scanner, comparing sub-voxel shifts to model predictions. Results: μQSM successfully predicted tissue Larmor frequency, suggesting mesoscopic shifts from uniformly magnetized axons explain the observed changes, without invoking susceptibility anisotropy. Impact: This study elucidates the nature of Larmor frequency shifts in coherent white matter, enhancing our understanding of its microstructural origin. These insights have the potential to improve QSM techniques by achieving better estimation of tissue magnetic susceptibility.