Although MRI is widely used to diagnose stenotic carotid arteries, it also detects characteristics of the atherosclerotic plaque itself, including its size, composition, and activity. These features are emerging as additional risk factors for stroke that can be feasibly acquired clinically. This paper summarizes the state of evidence for a clinical role for MRI of carotid atherosclerosis. 1. Introduction Atherosclerotic disease is the leading cause of death and disability in the United States and worldwide [1, 2]. Outside of the coronary circulation, the carotid arteries are likely the most clinically significant site of atherosclerosis. Estimates place carotid atherosclerosis as the cause of as many as 20% of all ischemic strokes [3]. This association led to 1.35 million carotid endarterectomy (CEA) procedures between 1998 and 2008 in the United States in patients deemed at high risk for stroke, in addition to 90,000 carotid stenting (CAS) procedures [4]. At present, the risk of stroke, on which clinical indications for CEA or CAS are based, is determined primarily from the percentage of stenosis of the vessel due to blockage of the lumen by the atherosclerotic plaque. Large trials of CEA including the North American Symptomatic Carotid Endarterectomy Trial have shown a 17% reduction in absolute risk of stroke over two years in patients with recent cerebrovascular symptoms and high-grade carotid stenosis [5]. In asymptomatic patients, studies such as the Asymptomatic Carotid Atherosclerosis Study have shown more modest benefits of CEA in patients with high-grade stenosis [6]. Advances in treatment since completion of these studies, including widespread use of statins, have further eroded the perceived benefit of intervention in asymptomatic patients leading many to advocate only medical therapy in the absence of cerebrovascular symptoms [7, 8]. On the other hand, studies have shown that in high-risk patients without any measurable stenosis, advanced plaques are present in a high proportion [9]. Thus, basing decisions on stenosis alone leads to overtreatment of select populations, whereas other populations with lower degrees of stenosis may be undertreated. These controversies and the potential to better manage both symptomatic and asymptomatic patients have fueled the efforts of numerous researchers to identify additional markers of stroke risk that better identify patients who will benefit from intervention. Magnetic resonance imaging (MRI) is emerging as the best candidate for augmenting stenosis with additional diagnostic features pertinent to
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