  Shear-wave Elastography Imaging (SWEI) is a noninvasive imaging modality that
provides tissue elasticity information by measuring the travelling speed of an
induced shear-wave. It is commercially available on clinical ultrasound
scanners and popularly used in the diagnosis and staging of liver disease and
breast cancer. In conventional SWEI methods, a sequence of acoustic radiation
force (ARF) pushes are used for inducing a shear-wave, which is tracked using
high frame-rate multi-angle plane wave imaging (MA-PWI) to estimate the
shear-wave speed (SWS). Conventionally, these plane waves are beamformed using
a constant speed-of-sound (SoS), assuming an a-priori known and homogeneous
tissue medium. However, soft tissues are inhomogeneous, with intrinsic SoS
variations. In this work, study the SoS effects and inhomogeneities on SWS
estimation, using simulation and phantoms experiments with porcine muscle as an
abbarator, and show how these aberrations can be corrected using local
speed-of-sound adaptive beamforming. For shear-wave tracking, we compare
standard beamform with spatially constant SoS values to software beamforming
with locally varying SoS maps. We show that, given SoS aberrations, traditional
beamforming using a constant SoS, regardless of the utilized SoS value,
introduces a substantial bias in resulting SWS estimations. Average SWS
estimation disparity for the same material was observed over 4.3 times worse
when a constant SoS value is used compared to that when a known SoS map is used
for beamforming. Such biases are shown to be corrected by using a local SoS map
in beamforming, indicating the importance of and a need for local SoS
reconstruction techniques.
