Estimation and display for Vector Doppler Imaging using planewave transmissions

Vector Doppler Imaging (VDI) reveals the blood flow velocity vector at each pixel. We present a multiple-angle planewave transmission (PWT) scheme that enables vector velocity imaging at a high frame-rate. The method employs a fast, aliasing-resistant velocity vector estimator and captures transitory flow dynamics, as demonstrated on a carotid artery with a 5 MHz linear array using a novel synthetic particle flow visualization method. Conventional color Doppler processing applied to several PWT angles produces a set of Doppler images. These are combined into a small weighted least squares (WLS) problem for each pixel, conditioned for bias due to aliasing. Weights account for signal quality at each PWT angle. The WLS covariance provides metrics to qualify pixels. To visualize the resulting velocity vector image, we use a novel technique which synthesizes a moving field of points representing particles entrained in the fluid. Each particle is generated by Bernoulli trial at detected flow pixels, and given motion by the velocity vector estimate. The motion is scaled down so the viewer may easily perceive flow in “real-time slow-motion”. Particles migrate across the image from frame to frame, under conservation rules mimicking fluid flow. The user controls displayed density of particles that overlay the detected flow regions, which are color-coded for velocity magnitude. Using a Verasonics VDAS acquisition system and a Philips L7-4 transducer, we demonstrate in vivo VDI on neck vasculature. PWT ensembles collected at seven angles are processed with the VDI algorithm giving an imaging rate of 30 fps. Video display reveals dynamics of the flow field and shows good detection of flow during diastole.