Purpose: This 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).
Methods: A 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.
Results: GRE 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.
Conclusion: A 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.
Keywords: QSM; TE-dependent frequency; bulk susceptibility-induced frequency; gradient-echo frequency evolution; microstructure-induced frequency.
© 2023 International Society for Magnetic Resonance in Medicine.