Sound Speed and Virtual Source Correction in Synthetic Transmit Focusing

In beamforming, retrospective change in sound speed and recalculation of focusing delays is attractive both for improving image quality and for using it in an iterative image quality optimization process. Modifying the speed of sound retrospectively for focused transmits is challenging because the transmit focus position is a function of sound speed error. The virtual source model is a common way to calculate the transmit focusing delays where using the correct transmit focus position is imperative. In this paper, we provide the methods necessary to perform a retrospective sound-speed correction by compensating the receive grid and by calculating the effective transmit focus needed to perform proper synthetic transmit focusing. To evaluate the efficacy of our method, we simulate wave propagation and measure the resolution of in vitro images using both phased and curvilinear arrays. The results of the suggested virtual source estimation method match the simulated wave propagation for multiple F-numbers and both positive and negative sound speed errors. We compare beamformed images using correct/incorrect sound speeds and correct/incorrect virtual source positions. The results demonstrate that the Corrected Virtual Source (CVS) method generates artifact-free images with superior quality compared to images with incorrect sound speed. Furthermore, the image beamformed with the correct sound speed, but incorrect virtual source position, exhibits image artifacts and inferior focusing quality compared to the CVS image.