Simultaneous MRI water-fat separation and quantitative susceptibility mapping of carotid artery plaque pre- and post-ultrasmall superparamagnetic iron oxide-uptake

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 water-fat 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.