Ultrasound elastography unveils the shear wave field: Prospects for medical and geophysical imaging

Transient elastography is an ultrasonic imaging method primarily applied in medical imaging. Through correlation of echography images it reconstructs the particle velocities and displacements of the transversal wave-field inside a medium of interest. Elastography measurements are therefore not limited to an acquisition surface. We present developments in processing and experimental design that broaden the application horizon for elastography imaging. First, we propose a multiple source-array for shear wave beamforming that adaptively recovers the impulse response of the medium through time-reversal. This allows for targeted shear wave focusing and thus significantly enhances the SNR compared to single source elastography. Due to the time-reversal robustness the focusing holds true for transmission through a rigid barrier as well, which we show by applying the method to a human skull model. Second, we present a geophysical laboratory experiment. Combining a friction test-bed with ultrasound elastography enables us to recover the effect of rupture dynamics on shear wave propagation. Albeit the use of hydro-gels our densely sampled wave-field perfectly illuminates typical rupture phenomena. These include super-shear fronts, sub-Rayleigh ruptures, and stick-slip behavior. In the future, passive elastography, an equivalent to seismic coda correlation, could be used for monitoring local elasticity during the rupture.