Assessment of Carotid Stenosis A hemodynamically significant carotid stenosis produces a pressure drop across the lesion, a flow reduction distal to the lesion, or both. This generally corresponds to a 60% diameter- reducing stenosis as reflected by catheter angiography as mea- sured with the North American method. This method was first described in publications from the Joint Study of Extracranial Arterial Occlusive Disease of the 1960s516 and has been used in multiple trials carried out in North America. This method measures the minimal residual lumen at the level of the stenotic lesion compared with the diameter of the more distal internal carotid artery where the walls of the artery first become paral- lel. It uses the following formula: stenosis=(1 N/D) 100%, where N is the diameter at point of maximum stenosis and D is the diameter of the arterial segment distal to the stenosis where the arterial walls first become parallel. This method is in contrast to the European method, which estimates stenosis of the internal carotid bulb. Because the randomized trials of CEA for symptomatic and asymptomatic disease in North America used catheter angiog- raphy, this has become the gold standard against which other imaging technologies are compared. Historically, catheter angiography carried an 1% risk of causing a stroke in patients with atherosclerotic disease.513,517 519 The complication rate has been dropping over the past several years, and the perma- nent stroke complication rate is <0.2%.519 Duplex ultrasound is the noninvasive method of screening the extracranial carotid artery for an atherosclerotic stenosis with the lowest cost and risk. Although there can be considerable variation in the accu- racy of duplex scanning among laboratories,520 certification programs are available that set standards for levels of perfor- mance and accuracy. Duplex ultrasound may be insensitive to differentiating high-grade stenosis from complete occlusion. 