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

TL;DR

This paper extends quantitative susceptibility mapping (QSM) to include white matter microstructure effects, significantly improving susceptibility estimation accuracy in ex-vivo mouse brains by accounting for structural anisotropy.

Contribution

The authors develop a model incorporating white matter microstructure into QSM, reducing bias and improving susceptibility estimates in brain tissue imaging.

Findings

Model improves susceptibility estimation over standard QSM

Structural anisotropy causes bias in susceptibility tensor imaging

Susceptibility values changed by up to 25% in white matter

Abstract

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) magnetic microstructure by using our previously presented model for solid cylinders with arbitrary orientations to describe axons in terms of concentric cylinders. Through computer simulations, we find that our model improves susceptibility estimation compared to QSM, and Susceptibility Tensor Imaging (STI) are substantially biased by the unaccounted-for structural anisotropy due to the mesoscopic frequency contribution, indicating the observed STI tensor might not represent susceptibility anisotropy as expected. Experiments in mouse brains acquired at ultrahigh field shows the mesoscopic contribution due to WM microstructure to be substantial. This in turn changed estimated susceptibility values up to 25% root-mean-squared-difference 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.