Carotid atherosclerotic wall imaging by MRI

Approximately 750,000 strokes occur each year in the United States alone, and while one third of these patients die, half of the survivors are left with a disability. The United States estimates the direct and indirect costs of the disease to be almost $40 billion a year [1-3]. Improved methods for the diagnosis and treatment of stroke would have a tremendous impact on the personal and public health burden caused by the disease. Of the etiologies of stroke, recent attention has focused on carotid atherosclerotic plaque, because the North American Symptomatic Carotid Endarterectomy Trial (NASCET) confirmed the efficacy of carotid endarterectomy (CEA) in reducing the risk of stroke in symptomatic patients who had 70-99% ipsilateral carotid artery stenosis [4]. A recent study based on the same patient population, however, suggests that approximately 20% and 45% of strokes in the territory of symptomatic and asymptomatic carotid arteries, respectively, are unrelated to carotid stenosis [5]. Although controversial, this finding points out the importance of examining independent risk factors in addition to the presence of flow-limiting stenosis. Improved imaging methods that can characterize atherosclerotic lesions may help us understand disease progression, provide a better way to assess lesion severity than degree of stenosis, and improve our ability to select the optimal treatment for the individual patient. Because approximately 140,000 CEA are done each year in the United States, and roughly 60% of those are performed on asymptomatic patients [6], this would be a major benefit. Currently there are two predominant noninvasive imaging modalities being investigated for this purpose: ultrasonography [7] and MRI. Although carotid sonography is the most widely performed noninvasive test in the evaluation of a patient suspected of having carotid disease, the modality is highly operator dependent and presently is not capable of consistently characterizing plaque morphology [3,7,8]. In contrast, a growing number of studies are demonstrating the unique abilities of high spatial resolution MRI for evaluating atherosclerotic plaque composition both in vitro and in vivo [9-19]. The work from these groups demonstrates the potential that MR techniques have for developing into a reliable means of characterizing plaque morphology.