Directional synthetic aperture flow imaging using a dual stage beamformer approach

A new method for directional synthetic aperture flow imaging using a dual stage beamformer approach is presented. The velocity estimation is angle independent and the amount of calculations is reduced compared to full synthetic aperture, but still maintains all the advantages at the same time. The new method has been studied using the Field II simulations and experimental flow rig measurements. A linear array transducer with 7 MHz center frequency is used, and 64 elements are active to transmit and receive signals. The data is processed in two stages. The first stage has a fixed focus point. In the second stage, focal points are considered as virtual sources and data is beamformed along the flow direction. Then the velocities are estimated by finding the spatial shift between two signals. In the experimental measurements the angle between the transmit beam and flow vessel was 70° and a laminar flow with a parabolic profile was generated by a flow rig. The flow with a peak velocity of 0.1 m/s was sampled at a pulse repetition frequency of 4 kHz. The signals were transmitted and received by the experimental scanner SARUS (Synthetic Aperture Real-time Ultrasound System). A relative standard deviation of 2.3% and bias of 6.4% at 65° were achieved in the simulations, and 4.3% and 4.2% for the experimental measurements. A color flow map image was made in 48 emissions corresponding to a frame rate of 83 frames/s.