Exploiting gradient‐echo frequency evolution: Probing white matter microstructure and extracting bulk susceptibility‐induced frequency for quantitative susceptibility mapping

Abstract

AbstractPurposeThis work is to investigate the microstructure‐induced frequency shift in white matter (WM) with crossing fibers and to separate the microstructure‐related frequency shift from the bulk susceptibility‐induced frequency shift by model fitting the gradient‐echo (GRE) frequency evolution for potentially more accurate quantitative susceptibility mapping (QSM).MethodsA hollow‐cylinder fiber model (HCFM) with two fiber populations was developed to investigate GRE frequency evolutions in WM voxels with microstructural orientation dispersion. The simulated and experimentally measured TE‐dependent local frequency shift was then fitted to a simplified frequency evolution model to obtain a microstructure‐related frequency difference parameter () and a TE‐independent bulk susceptibility‐induced frequency shift (). The obtained was then used for QSM reconstruction. Reconstruction performances were evaluated using a numerical head phantom and in vivo data and then compared to other multi‐echo combination methods.ResultsGRE frequency evolutions and ‐based tissue parameters in both parallel and crossing fibers determined from our simulations were comparable to those observed in vivo. The TE‐dependent frequency fitting method outperformed other multi‐echo combination methods in estimating in simulations. The fitted , , and QSM could be improved further by navigator‐based B0 fluctuation correction.ConclusionA HCFM with two fiber populations can be used to characterize microstructure‐induced frequency shifts in WM regions with crossing fibers. HCFM‐based TE‐dependent frequency fitting provides tissue contrast related to microstructure () and in addition may help improve the quantification accuracy of and the corresponding QSM.