Further literature:

Grading Carotid Stenosis Using Ultrasonic Methods
Gerhard-Michael von Reutern; Michael-Wolfgang Goertler; Natan M. Bornstein; Massimo Del Sette; David H. Evans; Andreas Hetzel; Manfred Kaps; Fabienne Perren; Alexander Razumovky; Toshiyuki Shiogai; Ekaterina Titianova; Pavel Traubner; Narayanaswamy Venketasubramanian; Lawrence K.S. Wong; Masahiro Yasaka; on behalf of the Neurosonology Research Group of the World Federation of Neurology.
Stroke. 2012;43:916-921

Abstract—The controversy as to whether Doppler ultrasonic methods should play a role in clinical decision-making in the prevention of stroke is attributable to reported disagreement between angiographic and ultrasonic results and the lack of internationally accepted ultrasound criteria for describing the degree of stenosis. Foremost among the explanations for both is the broad scatter of peak systolic velocities in the stenosis, the criterion that has so far received most attention. Grading based on a set of main and additional criteria can overcome diagnostic errors. Morphological measurements (B-mode images and color flow imaging) are the main criteria for low and moderate degrees of stenosis. Increased velocities in the stenosis indicate narrowing, but the appearance of collateral flow and decreased poststenotic flow velocity prove a high degree stenosis (±70%), additionally allowing the estimation of the hemodynamic effect in the category of high-degree stenosis. Additional criteria refer to the effect of a stenosis on prestenotic flow (common carotid artery), the extent of poststenotic flow disturbances, and derived velocity criteria (diastolic peak velocity and the carotid ratio). This multiparametric approach is intended to increase the reliability and the standard of reporting of ultrasonic results for arteriosclerotic disease of the carotid artery.