Ultrafast imaging of beamformed shear waves induced by the acoustic radiation force. Application to transient elastography

Several tissue characterization techniques based on acoustic radiation force have been proposed to remotely excite soft tissues with ultrasound. Recently, in transient elastography, an ultrafast echographic device was developed to follow the propagation inside soft tissues of very low frequency shear waves (/spl sim/100 Hz). These waves were generated using a vibrating system located at the body surface. In this paper, it is proposed to couple transient elastography and remote excitation techniques by replacing the usual external vibrating system by the use of the ultrasonic radiation force. In our experiments, the same probe allows both to generate and detect shear waves propagation by using an unusual emission-reception sequence. Our fully programmable electronics allow to induce and image, as desired, several shear sources at different times and locations in order to compute a real "shear wave beamforming" and optimize the shear source shape to the studied medium. An elasticity image of the whole medium can be recovered using only a few pushing beams (1 to 5). Measurements have been performed in several soft tissue mimicking phantom. Green's function based simulations have been used to support our experimental results.