Quantitative Susceptibility Mapping (QSM) and Susceptibility Weighted Imaging (SWI) are MRI techniques that measure and display differences in the magnetization that is induced in tissues, i.e. their magnetic susceptibility, when placed in the strong external magnetic field of an MRI system. SWI produces images in which the contrast is heavily weighted by the intrinsic tissue magnetic susceptibility. It has been applied in a wide range of clinical applications. QSM is a further advancement of this technique that requires sophisticated post-processing in order to provide quantitative maps of tissue susceptibility. This review explains the steps involved in both SWI and QSM as well as describing some of their uses in both clinical and research applications.

Purpose: Imaging carotid artery plaques to identify features of vulnerability typically requires a multicontrast MRI protocol. The identification of regions of inflammation with ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles requires separate pre-and postcontrast scans. We propose a method of joint waterfat separation and quantitative susceptibility mapping (QSM) to aid classification of atherosclerotic plaques and offer a positive contrast mechanism in USPIO-imaging. Methods: Ten healthy volunteers (3 women and 7 men; aged, 30.7 ± 10.7 years) were imaged at 1.5T to develop an acquisition and postprocessing protocol. Five patients (1 woman and 4 men; mean age, 71 ± 7.5 years) with moderate to severe luminal stenosis were imaged pre-and postadministration of a USPIO contrast agent.We used a multiecho gradient echo acquisition to perform water/fat separation and subsequently QSM. The results were compared with a conventional multicontrast MRI protocol, CT images, and histopathology data. Results: In the volunteer scans, a multiecho gradient echo acquisition with bipolar readout gradients demonstrated to be a reliable acquisition methodology to produce high-quality susceptibility maps in conjunction with the proposed postprocessing methodology. In the patient study, water/fat separation provided a tool to identify lipid-rich necrotic cores and QSM provided a qualitative and quantitative evaluation of plaque features and positive contrast when evaluating USPIO uptake. Plaque calcification could be identified by strong diamagnetism (−1.27 ± 0.71 ppm), while USPIO uptake demonstrated a strong paramagnetism (1.32 ± 0.61 ppm). Conclusion: QSM was able to identify multiple plaque features in a single acquisition, providing positive contrast for plaques demonstrating USPIO uptake and negative contrast for calcification. K E Y W O R D S atherosclerosis, carotid artery, magnetic resonance imaging, quantitative susceptibility mapping (QSM), USPIO, water-fat separation | 687 RUETTEN ET al. How to cite this article: Ruetten PPR, Cluroe AD, Usman A, Priest AN, Gillard JH, Graves MJ. Simultaneous MRI water-fat separation and quantitative susceptibility mapping of carotid artery plaque pre-and post-ultrasmall superparamagnetic iron oxide-uptake. Magn Reson Med. 2020;84:686-697. https ://doi.

Purpose: This study is to compare the accuracy of four different black-blood T 2 mapping sequences in carotid vessel wall. Methods: Four different black-blood T 2 mapping sequences were developed and tested through phantom experiments and 17 healthy volunteers. The four sequences were: 1) double inversion-recovery (DIR) prepared 2D multi-echo spin-echo (MESE); 2) DIR-prepared 2D multi-echo fast spin-echo (MEFSE); 3) improved motion-sensitized driven-equilibrium (iMSDE) prepared 3D FSE and 4) iMSDE prepared 3D fast spoiled gradient echo (FSPGR). The concordance correlation coefficient and Bland-Altman statistics were used to compare the sequences with a gold-standard 2D MESE, without blood suppression in phantom studies. The volunteers were scanned twice to test the repeatability. Mean and standard deviation of vessel wall T 2 , signal-to-noise (SNR), the coefficient of variance and interclass coefficient (ICC) of the two scans were compared. Results:The phantom study demonstrated that T 2 measurements had high concordance with respect to the gold-standard (all r values >0.9). In the volunteer study, the DIR 2D MEFSE had significantly higher T 2 values than the other three sequences (P < 0.01). There was no difference in T 2 measurements obtained using the other three sequences (P > 0.05). iMSDE 3D FSE had the highest SNR (P < 0.05) compared with the other three sequences. The 2D DIR MESE has the highest repeatability (ICC: 0.96, [95% CI: 0.88-0.99]). Conclusion: Although accurate T 2 measurements can be achieved in phantom by the four sequences, in vivo vessel wall T 2 quantification shows significant differences. The in vivo images can be influenced by multiple factors including black-blood preparation and acquisition method. Therefore, a careful choice of acquisition methods and analysis of the confounding factors are required for accurate in vivo carotid vessel wall T 2 measurements. From the settings in this study, the iMSDE prepared 3D FSE is preferred for the future volunteer/ patient scans.

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