Abstract
PurposeTo investigate the effects on quantitative susceptibility mapping (QSM) and susceptibility tensor imaging (STI) of the frequency variation produced by the microstructure of white matter (WM).MethodsThe frequency offsets in a WM tissue sample that are not explained by the effect of bulk isotropic or anisotropic magnetic susceptibility, but rather result from the local microstructure, were characterized for the first time. QSM and STI were then applied to simulated frequency maps that were calculated using a digitized whole-brain, WM model formed from anatomical and diffusion tensor imaging data acquired from a volunteer. In this model, the magnitudes of the frequency contributions due to anisotropy and microstructure were derived from the results of the tissue experiments.ResultsThe simulations suggest that the frequency contribution of microstructure is much larger than that due to bulk effects of anisotropic magnetic susceptibility. In QSM, the microstructure contribution introduced artificial WM heterogeneity. For the STI processing, the microstructure contribution caused the susceptibility anisotropy to be significantly overestimated.ConclusionMicrostructure-related phase offsets in WM yield artifacts in the calculated susceptibility maps. If susceptibility mapping is to become a robust MRI technique, further research should be carried out to reduce the confounding effects of microstructure-related frequency contributions. Magn Reson Med 73:1258–1269, 2015. © 2014 Wiley Periodicals, Inc.
Highlights
Phase images of the human brain acquired at high-field strengths using gradient echo (GE) MRI show exquisite tissue contrast (1–4)
This involved characterizing for the first time the frequency offsets in white matter (WM) that are not explained by the effect of bulk isotropic or anisotropic magnetic susceptibility, but rather result from the local microstructure
The relative isotropic magnetic susceptibility and absolute anisotropic magnetic susceptibility of a fresh sample of optic nerve were quantified by imaging the sample at multiple orientations to the B0-field and analyzing the field perturbation produced outside the sample
Summary
Phase images of the human brain acquired at high-field strengths using gradient echo (GE) MRI show exquisite tissue contrast (1–4). In most studies involving GE phase imaging, it is assumed that the dominant source of phase contrast is the variation in isotropic magnetic susceptibility across different tissues (5). Development of a plethora of sophisticated techniques for inverting phase measurements to yield three-dimensional (3D) maps of the isotropic magnetic susceptibility (6–12). These “quantitative susceptibility mapping” (QSM) methods take advantage of the simple, Fourier relationship connecting the underlying distribution of isotropic magnetic susceptibility to the induced dipolar magnetic fields whose effect can be measured in phase images (13,14). By understanding the impact of the additional phase contrast mechanisms on susceptibility mapping can the true value of QSM and other related methods be fully appreciated
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