Objective-This study evaluates the ability of MRI to quantify all major carotid atherosclerotic plaque components in vivo. Methods and Results-Thirty-one subjects scheduled for carotid endarterectomy were imaged with a 1.5T scanner using time-of-flight-, T1-, proton density-, and T2-weighted images. A total of 214 MR imaging locations were matched to corresponding histology sections. For MRI and histology, area measurements of the major plaque components such as lipid-rich/necrotic core (LR/NC), calcification, loose matrix, and dense (fibrous) tissue were recorded as percentages of the total wall area. Intraclass correlation coefficients (ICCs) were computed to determine intrareader and inter-reader reproducibility. Key Words: atherosclerosis Ⅲ magnetic resonance imaging Ⅲ carotid artery Ⅲ plaque A therosclerosis and its thrombotic complications are the leading cause of morbidity and mortality in industrialized countries. Therefore, the need for new medical therapies and technology to treat and prevent cardiovascular atherosclerotic disease is enormous.Accurate information of atherosclerotic plaque morphology and plaque composition is necessary to identify the "vulnerable plaques" that are likely to cause embolic events. A noninvasive imaging modality that could provide such information would be an invaluable tool in studies of the relationship between plaque composition/morphology and plaque progression/regression. Furthermore, such imaging techniques may be used in clinical trials to monitor the effects of drugs on diseased arteries.B-Mode ultrasonography has been used widely in plaque progression/regression trials that involve either lipidlowering drugs or calcium channel blockers. 1 However, this modality is highly operator dependent, has limited soft tissue contrast, and requires a large number of subjects to detect a significant change in the intima-media thickness. 1 Intravascular ultrasound (IVUS) is used increasingly in atherosclerosis regression/progression trials that study coronary arteries. 2 Although IVUS is highly reproducible 3 and provides tomographic information about the vessel wall, 3 it is an invasive procedure and has limited capacity to discriminate between fibrous and fatty plaques. 4 Recent publications 5-11 have shown that in vivo MRI can identify the main components of the atherosclerotic plaque such as the lipid-rich/necrotic core (LR/NC), calcification, and hemorrhage. In addition, morphological information about the status of the fibrous cap 12 and the American Heart Association (AHA) lesion type 13 can be obtained noninvasively. Moreover, the tomographic orientation of MRI enables the full cross-sectional view of the vessel wall, which can be measured accurately 14 and reproducibly. 15 It has been demonstrated that ex vivo MRI of endarterectomy specimen is able to identify 16 and quantify 17,18 plaque components with high diagnostic accuracy. This study is aimed at evaluating the ability of MRI to quantify all major carotid atherosclerotic plaque components in vivo, using histolog...

Automated analysis tools are capable of providing accurate and reproducible measurements of carotid atherosclerotic burden and composition when compared with manually outlined results.

MRI is a promising noninvasive technique for characterizing atherosclerotic plaque composition in vivo, with an end-goal of assessing plaque vulnerability. Because of limitations arising from acquisition time, achievable resolution, contrast-to-noise ratio, patient motion, and the effects of blood flow, automatically identifying plaque composition remains a challenging task in vivo. In this article, a segmentation method using maximum a posteriori probability Bayesian theory is presented that divides axial, multi-contrast-weighted images into regions of necrotic core, calcification, loose matrix, and fibrous tissue. Key advantages of the method are that it utilizes morphologic information, such as local wall thickness, and coupled active contours to limit the impact from noise and artifacts associated with in vivo imaging. In experiments involving 142 sets of multi-contrast images from 26 subjects undergoing carotid endarterectomy, Numerous studies have shown that MRI exhibits high contrast for internal plaque features, but that combined information from multiple contrast weightings is critical for distinguishing all plaque components (1-6). Based on these studies, desirable combinations of contrast weightings and a set of image characteristics have emerged that can be used to segment plaque into its subcomponents. Manual segmentation using these characteristics has produced quantitative measurements of the relative volumes of necrotic cores, calcification, loose matrix, and fibrous tissue that correlate strongly with histologic assessments (5).Nevertheless, replacing subjective, manual segmentation with an automated segmentation alternative would have several benefits. Aside from saving time in image review, automated segmentation would reduce the considerable amount of training required to read these images and the corresponding inter-rater variability. Additionally, a viable, automated segmentation procedure would permit various combinations of contrast weightings and image characteristics to be objectively analyzed for accuracy in plaque characterization. Such studies have been conducted using automated segmentation of ex vivo endarterectomy specimens (6,7), but these results are difficult to translate to in vivo imaging, given the different constraints regarding acquisition time, resolution, contrast-to-noise ratio, and effects of blood flow. Recent efforts to demonstrate in vivo segmentation of some plaque components (8,9) are promising, but they have not yet been histologically validated.The goal of this work was to develop a flexible, multicontrast plaque segmentation technique that is suitable for objectively testing various approaches for measuring plaque composition in vivo and to validate the method with histology. Furthermore, the technique was developed to mimic the highly successful procedure used in manual review. Unlike other works (6,8,9) in which only the intensity values in each contrast weighted image were considered, and (7) in which spatial information was used just for minimizing pixel di...

Magnetic resonance imaging (MRI) of the arterial wall has emerged as a viable technology for characterizing atherosclerotic lesions in vivo, especially within carotid arteries and other large vessels. This capability has facilitated the use of carotid MRI in clinical trials to evaluate therapeutic effects on atherosclerotic lesions themselves. MRI is specifically able to characterize three important aspects of the lesion: size, composition and biological activity. Lesion size, expressed as a total wall volume, may be more sensitive than maximal vessel narrowing (stenosis) as a measure of therapeutic effects, as it reflects changes along the entire length of the lesion and accounts for vessel remodeling. Lesion composition (e.g. lipid, fibrous and calcified content) may reflect therapeutic effects that do not alter lesion size or stenosis, but cause a transition from a vulnerable plaque composition to a more stable one. Biological activity, most notably inflammation, is an emerging target for imaging that is thought to destabilize plaque and which may be a systemic marker of vulnerability. The ability of MRI to characterize each of these features in carotid atherosclerotic lesions gives it the potential, under certain circumstances, to replace traditional trials involving large numbers of subjects and hard end-points -heart attacks and strokes -with smaller, shorter trials involving imaging end-points. In this review, the state of the art in MRI of atherosclerosis is presented in terms of hardware, image acquisition protocols and post-processing. Also, the results of validation studies for measuring lesion size, composition and inflammation will be summarized. Finally, the status of several clinical trials involving MRI of atherosclerosis will be reviewed.