Duplex ultrasound may be insensitive to differentiating high-grade stenosis from complete occlusion. MR angiography (MRA), with and without contrast, is also used as a noninvasive method for evaluating arterial anatomy and has the advantage of providing images of both the cer- vical and intracranial portions of the carotid artery and its proximal intracranial branches. MRA may overestimate the degree of stenosis, and as with duplex ultrasound, there may be errors when high-grade stenosis is differentiated from com- plete occlusion. MR contrast material may cause debilitating nephrogenic systemic fibrosis in patients with renal dysfunc- tion. When concordant, the combination of duplex ultrasound and MRA is more accurate than either test alone.521 Computed tomographic angiography is another means of identifying and measuring stenosis of the extracranial carotid artery.522 Like MRA, it has the advantage of being able to evaluate the intra- cranial circulation. Disadvantages of computed tomographic angiography include radiation exposure and the need for intra- venous injection of contrast material. Atherosclerotic calcifi- cation may confound accurate measurement of stenosis with computed tomographic angiography. A variety of vascular risk factors reviewed in this guideline are associated with carotid atherosclerosis.523,524 Carotid bruit can reflect an underlying carotid stenosis. However, the sen- sitivity for detecting carotid stenosis is low. In NOMAS, aus- cultation had a sensitivity of 56% and a specificity of 98%.525 Endarterectomy for Asymptomatic Carotid Stenosis The first study with >1000 patients comparing CEA plus best medical therapy to medical therapy alone was the Asymptomatic Carotid Atherosclerosis Study (ACAS).513 The primary outcome was the composite of any stroke or death occurring in the perioperative period and ipsilateral cerebral infarction thereafter. During follow-up after 34 centers random- ized 1662 patients, the Data and Safety Monitoring Committee called a halt to the trial because of a clear benefit in favor of CEA. 