  Shear wave absolute vibro-elastography (S-WAVE) is an imaging technique that
generates steady-state shear waves inside the tissue using multi-frequency
excitation from an external vibration source. In this work, plane wave imaging
is introduced to reduce total acquisition time while retaining the benefit of
3D formulation. Plane wave imaging with a frame rate of 3000 frames/s is
followed by 3D absolute elasticity estimation. We design two imaging sequences
of ultrafast S-WAVE for two sets of excitation frequencies using a Verasonics
system and a motorized swept ultrasound transducer to synchronize ultrasound
acquisition with the external mechanical excitation. The overall data
collection time is improved by 83-88% compared to the original 3D S-WAVE
because of the per-channel acquisition offered by the Verasonics system. Tests
are performed on liver fibrosis tissue-mimicking phantoms and on ex vivo bovine
liver. The curl operator was previously used in magnetic resonance elastography
(MRE) to cancel out the effect of the compressional waves. In this work, we
apply the curl operator to the full 3D displacement field followed by 3D
elasticity reconstruction. The results of phantom experiment show more accurate
elasticity estimation as well as 18% less standard deviation (STD) compared to
reconstruction using the curl of a 2D displacement field and 45% less STD than
without the curl. We also compare our experimental results with a previous
method based on acoustic radiation force impulse (ARFI) and achieve closer
results to phantom manufacturer provided values with ultrafast S-WAVE.
Furthermore, the dependency of the bovine liver elasticity on the frequency of
excitation was also shown with our system.
