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Duplex ultrasound in carotid artery disease: some diagnostic considerations.

Loss of pressure across a stenosis depends on the geometry of that obstruction, but flow only depends on geometry when both perfusion pressure and peripheral resistance are constant. As cerebrovascular resistance is generally low then flow over a stenosis will be dominated by stenotic resistance. A relatively modest reduction in systolic perfusion pressure will produce a large increase in stenotic resistance. Evaluation of a 'haemodynamically significant' or 'critical' or 'dynamic' stenosis should include consideration of variations in cardiac function and blood pressure as well as local vascular dynamics. Examination of the ultrasound image characteristics of the arterial lesion gives additional structural information of the contents and surface of the lesion and any movement relative to the vessel wall. This may give guidance in separating clinically stable from unstable lesions, and also in following progression or regression of disease. The physical forces which are applied to some lesions may cause the release of material from the substance of the lesion into the arterial lumen or cause damage resulting in progression of the lesion itself.