Doppler ultrasound spectral shape in the poststenotic velocity field

The shape of averaged and smoothed pulsed Doppler spectra from the shear layer between jet and recirculation zones, one diameter downstream of three acrylic stenosis (27, 47 and 65% diameter reduction) in a steady flow dialysis tubing model, were found to be complex. Their shape contrasted with the shape of spectra from regions of jet breakup. The latter had a shape suggesting a Gaussian distribution of apparent velocities consistent with varying magnitude and direction in turbulent flow. In contrast, the least complex spectra from the shear layer had a double-peaked shape suggestive of two distinct major velocity components or streamline groups. A subtraction method was used to resolve the least complex spectra into two relatively narrow Gaussian components. More complex spectra could be resolved into multiple components by serially subtracting individual Gaussian components. Comparison with the pattern of particles revealed by flow visualization studies suggests that most of the components are due to axially directed streamline groups of different velocities but that some may be due to radially directed streamline groups. Broadened spectral components, which are apparent at higher flow rates, are consistent with development of vortices in the shear layer.