Local Sound Speed Estimation for Pulse-Echo Ultrasound in Layered Media.

Our previous methodology in local sound speed estimation utilized time delays measured by the cross-correlation of delayed full synthetic aperture channel data to estimate the average speed of sound. However, focal distortions in this methodology lead to biased estimates of the average speed of sound, which, in turn, leads to biased estimates of the local speed of sound. Here, we demonstrate the bias in the previous methodology and introduce a coherence-based average sound speed estimator that eliminates this bias and is computationally much cheaper in practice. Because this coherence-based approach estimates the average sound speed in the medium over an equally-spaced grid in depth rather than time, we derive a refined model that relates the local and average speeds of sound as a function of depth in layered media. A fast, closed-form inversion of this model yields highly accurate local sound speed estimates. The root mean-square (RMS) error of local sound speed reconstruction in simulations of two-layer media is 4.6, and 2.5 m/s at 4 and 8 MHz, respectively. This work examines the impact of frequency, f-number, aberration, and reverberation on sound speed estimation. Phantom and in-vivo experiments in rats further validate the coherence-based sound speed estimator.