Incorporating white matter microstructure in the estimation of magnetic susceptibility in ex-vivo mouse brain

Accurate estimation of microscopic magnetic field variations induced in biological tissue can be valuable for mapping tissue composition in health and disease. Here, we present an extension to Quantitative susceptibility mapping (QSM) to account for local white matter (WM) microstructure by using our previously presented model for solid cylinders with arbitrary orientations to describe axons in terms of concentric cylinders. We show how multi-gradient echo (MGE) and diffusion MRI (dMRI) images can be combined to estimate an apparent scalar susceptibility. Experiments in mouse brains acquired at ultrahigh field shows the mesoscopic contribution due to WM microstructure to be substantial, with a magnitude up to 70% of the total frequency shift in highly anisotropic WM. This in turn changed estimated susceptibility values up to 56% in WM compared to standard QSM. Our work underscores how microstructural field effects impact susceptibility estimates, and should not be neglected when imaging anisotropic tissue such as brain WM.Comment: 33 pages, 7 figure