Rotation-Free Estimation of Anisotropic Transverse Relaxation & Larmor Frequency with Diffusion MRI: A Monte Carlo Study in Axonal Phantoms

Motivation: Estimating susceptibility-related tissue parameters with MRI in white matter requires imaging at multiple orientations which makes it practically infeasible. Goal(s): This study aims to estimate such anisotropic susceptibility-related parameters without rotating the sample. Approach: We propose a novel model extending The Standard Model of Diffusion to a multi-spin-echo diffusion MRI signal with multiple delayed read-outs per echo. We validate our model (SMPR) using Monte Carlo simulations in realistic white matter axonal microstructures segmented from electron microscopy. Results: The SMPR model effectively estimated all parameters in the intra-axonal space from a single sample orientation. Impact: Modeling a multi-spin-echo diffusion MRI signal with delayed read-outs per echo enables estimation of orientation-dependent susceptibility parameters without imaging multiple sample orientations. This approach could improve sensitivity to tissue characteristics, aiding studies of myelination and other white matter features.