Coherence-Based Optimization Using Cumulative Spatial Lags to Estimate Sound Speed in Plane Wave Images of Coherent and Incoherent Targets

A globally constant sound speed of 1540 m/s is often assumed when beamforming ultrasound images, despite variations in tissue properties that lead to different sound speeds. As a result, a globally constant sound speed that is not representative of the majority of local sound speeds can degrade image quality, resulting in inaccurate target depths, sizes, and shapes, thus reducing the overall effectiveness of diagnostic and interventional ultrasound imaging. In this paper, we developed a more general coherence-based optimization method to estimate sound speed that can be applied to both coherent and incoherent targets in ultrasound images with plane wave transmissions. Sound speeds that produced minimum values of the lateral full-width at half maximum (FWHM) of point targets and the contrast of anechoic targets were considered optimal. Our method estimated sound speeds that deviated from optimal values by 5-10 m/s. Our approach was benchmarked against a speckle brightness maximization approach, which achieved deviations of 57-160 m/s from the optimal sound speed values. The lower deviations with our approach resulted in a 46%-63% improvement in lateral FWHM and 1.09 dB contrast enhancement relative to the corresponding results of the speckle brightness maximization method. The proposed approach has the potential to improve accuracy in diagnostic and interventional ultrasound imaging.