  For beamforming ultrasound (US) signals, typically a spatially constant
speed-of-sound (SoS) is assumed to calculate delays. As SoS in tissue may vary
relatively largely, this approximation may cause wavefront aberrations, thus
degrading effective imaging resolution. In the literature, corrections have
been proposed based on unidirectional SoS estimation or
computationally-expensive a posteriori phase rectification. In this paper we
demonstrate a direct delay correction approach for US beamforming, by
leveraging 2D spatial SoS distribution estimates from plane-wave imaging. We
show both in simulations and with ex vivo measurements that resolutions close
to the wavelength limit can be achieved using our proposed local SoS-adaptive
beamforming, yielding a lateral resolution improvement of 22% to 29% on tissue
samples with up to 3% SoS-contrast (45m/s). We verify that our method
accurately images absolute positions of tissue structures down to sub-pixel
resolution of a tenth of a wavelength, whereas a global SoS assumption leads to
artifactual localizations.
