  3D ultrasound imaging provides more spatial information compared to
conventional 2D frames by considering the volumes of data. One of the main
bottlenecks of 3D imaging is the long data acquisition time which reduces
practicality and can introduce artifacts from unwanted patient or sonographer
motion. This paper introduces the first shear wave absolute vibro-elastography
(S-WAVE) method with real-time volumetric acquisition using a matrix array
transducer. In SWAVE, an external vibration source generates mechanical
vibrations inside the tissue. The tissue motion is then estimated and used in
solving a wave equation inverse problem to provide the tissue elasticity. A
matrix array transducer is used with a Verasonics ultrasound machine and frame
rate of 2000 volumes/s to acquire 100 radio frequency (RF) volumes in 0.05 s.
Using plane wave (PW) and compounded diverging wave (CDW) imaging methods, we
estimate axial, lateral and elevational displacements over 3D volumes. The curl
of the displacements is used with local frequency estimation to estimate
elasticity in the acquired volumes. Ultrafast acquisition extends substantially
the possible S-WAVE excitation frequency range, now up to 800 Hz, enabling new
tissue modeling and characterization. The method was validated on three
homogeneous liver fibrosis phantoms and on four different inclusions within a
heterogeneous phantom. The homogeneous phantom results show less than 8% (PW)
and 5% (CDW) difference between the manufacturer values and the corresponding
estimated values over a frequency range of 80 Hz to 800 Hz. The estimated
elasticity values for the heterogeneous phantom at 400 Hz excitation frequency
show average errors of 9% (PW) and 6% (CDW) compared to the provided average
values by MRE. Furthermore, both imaging methods were able to detect the
inclusions within the elasticity volumes.
