  In ultrasound beamforming, focusing time delays are typically computed with a
spatially constant speed-of-sound (SoS) assumption. A mismatch between
beamforming and true medium SoS then leads to aberration artifacts. Other
imaging techniques such as spatially-resolved SoS reconstruction using
tomographic techniques also rely on a good SoS estimate for initial
beamforming. In this work, we exploit spatially-varying geometric disparities
in the transmit and receive paths of multiple sequences for estimating a mean
medium SoS. We use images from diverging waves beamformed with an assumed SoS,
and propose a model fitting method for estimating the SoS offset. We
demonstrate the effectiveness of our proposed method for tomographic SoS
reconstruction. With corrected beamforming SoS, the reconstruction accuracy on
simulated data was improved by 63% and 29%, respectively, for an initial SoS
over- and under-estimation of 1.5%. We further demonstrate our proposed method
on a breast phantom, indicating substantial improvement in contrast-to-noise
ratio for local SoS mapping.
