Incorporating mesoscopic orientation dependent R2 from magnetic susceptibility into the Standard Model of Diffusion in White Matter

Motivation: $$$R_2$$$ in WM is orientation dependent due to microscopic magnetic anisotropy. So far, the Standard Model of diffusion (SM) has been extended to only include isotropic $$$R_2$$$ (TEdDI). Goal(s): Our goal is to characterize $$$R_2$$$ anisotropy of a multi-echo dMRI signal for long diffusion times and incorporate $$$R_2$$$ anisotropy into TEdDI (STEdDI). Approach: We simulate $$$R_2$$$ of PGSE signal in magnetized cylinders using Monte-Carlo, and fit TEdDI and STEdDI on ex vivo mouse multi-echo dMRI data acquired at 16.4T. Results: $$$R_2$$$ anisotropy outside axons are non-axially-symmetric, depends on B0 direction, gradient direction and b-value. Residuals are significantly lower with STEdDI in dMRI data. Impact: Interplay between microscopic magnetic fields and diffusion weighting affects $$$R_2$$$ in extra-axonal space. Incorporating $$$R_2$$$ anisotropy$$$\,$$$into modeling lowered the residuals and may allow rotation-free estimation of $$$R_2$$$ anisotropy, which could be useful to gain a deeper insight into brain microstructure.