Abstract
Shear wave elasticity imaging (SWEI) is a non-invasive 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, we 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 the 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 the need for local SoS reconstruction techniques.
Highlights
Shear wave elasticity imaging (SWEI) is a non-invasive imaging technique that maps shear-wave speed (SWS) in tissues
SWEI is performed in two steps: First in the vicinity of soft tissue to be imaged, a remote “push” is generated using acoustic radiation force (ARF) to induce shear-waves
These profiles are shown as a function of time across the phantom width, and they were computed from the estimated displacements, for the beamforming of which we first used different constant SoS values (Figures 3(I,a–f))
Summary
Shear wave elasticity imaging (SWEI) is a non-invasive imaging technique that maps shear-wave speed (SWS) in tissues. SWEI is performed in two steps: First in the vicinity of soft tissue to be imaged, a remote “push” is generated using acoustic radiation force (ARF) to induce shear-waves. These shear-waves are observed using ultrasound imaging to capture lateral shear-wave travel speed [1], to relate this to the underlying tissue shear modulus. Several data acquisition sequences have been proposed in the literature to mitigate phase aberration effects in shear-wave tracking [9]; [10]; [11] These methods target mediums with slight variations in SoS, whereas in a clinical setting, several layers with largely varying thicknesses and SoS may exist between the ultrasound transducer and the location of measurement. To alleviate large phase aberration effects, knowing the local SoS distribution would be essential
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