Flow dynamics in a stenosed carotid bifurcation model--Part II: Derived indices.

Basic velocity measurements from the previous study in this issue of modelled carotid artery bifurcation disease are post-processed to derive indices potentially useful for clinical diagnosis. Selected parameters are based upon ultrasound pulse Doppler velocity measurements made at sites +/- 0.625 radius at axial distances of 0, 1, 3, 5, and 10 diameters downstream of smooth, axially symmetric constrictions of 0, 20, 40, 60, and 80% diameter reduction. Indices based on single point velocity measurements include: (a) the center-line velocity index (CVI) at the throat of the constrictor which is sensitive to all degrees of constriction (p less than 0.05), and (b) various measures of velocity disturbance (VDI1-VD14) which show greatest sensitivity when measured at one-fourth diameter from the tube wall. Cross-sectional indices include: (a) the maximum slope index (MSI) which separates constrictions of less than 60% diameter reduction from those having greater than or equal to 60% diameter reduction (p less than 0.05), and (b) the cross-sectional profile index (CPI) which separates constrictions of less than or equal to 20% diameter reduction from those having greater than 20% diameter reduction (p less than 0.05). A field profile index (FPI) utilizes data from all available sites up to 5 diameters downstream and is able to separate all constrictor groups (p less than 0.05). Presence of constrictions produces characteristic flow patterns which can be quantified using indices based on the downstream velocity spectra. Specifically, locally increased velocity is very sensitive to degree of constriction and is best detected along the center line. Disturbance effects, while less pronounced, are complementary to changes in velocity magnitude and are first seen off-axis. Multiple point profile measures (cross sectional and full field) are also sensitive to degree of constriction and are best evaluated at 1 diameter and over 5 diameters, respectively, downstream of the constriction. All of the nondimensional indices offer the advantages of reduced probe and angle dependence and particular approaches may be implemented according to the data collection capability of the instrument used.