Parallel beamforming using synthetic transmit beams

Ultrasound images generated using conventional parallel beamforming contains stationary stripes due to beam-to-beam variations across adjacent beams. Such an imaging system will thus be shift variant. This paper investigates a method of parallel beamforming which eliminates this flaw and restores the shift invariant property. The beam-to-beam variations occur because the transmit and receive beams are not aligned. The underlying idea is then to generate additional synthetic transmit beams (STB) through interpolation of the received, unfocused signal at each array element prior to beamforming. Now each of the parallel receive beams can be aligned perfectly with a transmit beam - synthetic or real - thus eliminating the distortion caused by misalignment. To investigate the performance of the method a simulation study has been conducted based on the ultrasound simulation software Field II. The results have been verified with in vitro experiments. The simulations were done with parameters similar to a standard cardiac examination with two parallel receive beams and a transmit-line density corresponding to the Rayleigh criterion, wavelength f-number. The RMS sum of the point spread function (PSF) and its variation was used as a measure of the shift variance of the imaging system. Without synthesizing additional transmit-beams, simulations showed that the peak value of the PSF varied with up to 5dB as a scatterer was translated laterally in the image. Using the proposed method the variation was reduced to 1.5dB. Without synthesized transmit beams the width of the PSF sum varied with up to 38%. With synthesized transmit beams the variation was reduced to 11%. The simulations were confirmed by results from in vitro experiments where vertical line artifacts were unnoticeable when using the proposed method.