Quantitative fat and R2* mapping in vivo to measure lipid‐rich necrotic core and intraplaque hemorrhage in carotid atherosclerosis

Abstract

Purpose The aim of this work was to quantify the extent of lipid‐rich necrotic core (LRNC) and intraplaque hemorrhage (IPH) in atherosclerotic plaques. Methods Patients scheduled for carotid endarterectomy underwent four‐point Dixon and T1‐weighted magnetic resonance imaging (MRI) at 3 Tesla. Fat and R2* maps were generated from the Dixon sequence at the acquired spatial resolution of 0.60 × 0.60 × 0.70 mm voxel size. MRI and three‐dimensional (3D) histology volumes of plaques were registered. The registration matrix was applied to segmentations denoting LRNC and IPH in 3D histology to split plaque volumes in regions with and without LRNC and IPH. Results Five patients were included. Regarding volumes of LRNC identified by 3D histology, the average fat fraction by MRI was significantly higher inside LRNC than outside: 12.64 ± 0.2737% versus 9.294 ± 0.1762% (mean ± standard error of the mean [SEM]; P < 0.001). The same was true for IPH identified by 3D histology, R2* inside versus outside IPH was: 71.81 ± 1.276 s−1 versus 56.94 ± 0.9095 s−1 (mean ± SEM; P < 0.001). There was a strong correlation between the cumulative fat and the volume of LRNC from 3D histology (R2 = 0.92) as well as between cumulative R2* and IPH (R2 = 0.94). Conclusion Quantitative mapping of fat and R2* from Dixon MRI reliably quantifies the extent of LRNC and IPH. Magn Reson Med 78:285–296, 2017. © 2016 International Society for Magnetic Resonance in Medicine

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

“…Within a single sequence QSM allows for identification of plaque features such as calcification, USPIO uptake, and potentially paramagnetic intraplaque hemorrhage, which was not present in our patient cohort. Furthermore, a previous study has shown the feasibility of using Dixon‐based water‐fat separation to identify the LRNC of the plaque . In our study, 1 of the plaques, where histology was performed contained a large LRNC, whose presence was confirmed using IDEAL water‐fat separation in addition to QSM analysis.…”

“…In addition, the IDEAL water‐fat separation was able to detect a large LRNC in one of the patients, the presence of which was confirmed on the histological analysis of the carotid endarterectomy specimen with the Elastic Van Gieson histochemical stain, where it could be identified by its yellow/pink coloring and the presence of cholesterol crystals. For MRI, we used a similar analysis to Koppal et al to identify the LRNC as the LRNC was identified by a heightened fat fraction when compared with “normal” plaque regions and the sternocleidomastoid muscle (Figure ). In the same plaque, QSM was able to identify both calcification and USPIO uptake, as described previously (Figure ).…”

“…Furthermore, previous studies have used Dixon‐based water‐fat separation techniques to identify LRNC, another feature of atherosclerotic plaques . Therefore, the addition of IDEAL to estimate Δ B in our proposed methodology not only serves to remove artifacts from QSM but allows for the identification of an additional feature of advanced plaques using a single sequence.…”

Improving Assessments of Hemodynamics and Vascular Disease

“…For example, in the study by Koppal et al [63], a 4-point Dixon sequence is used with an OP, IP, OP, IP scheme at echo times of n ⇤ 3.6 [ms]. The use of an additional echo enables the estimation of T ⇤ 2 relaxation (a measure of phasedispersion) times for each voxel.…”

“…Paper IV built upon the work of Koppal et al [63], where they used a fourpoint Dixon sequence to quantify FF and R ⇤ 2 signal in atherosclerotic plaques of the carotid artery and validated these signals against histological findings. This work was limited in scope by the need for manual segmentation of the plaque boundaries.…”