Résumé:
Supersonic imaging is a new ultrasound based and non invasive technique for the investigation of soft tissues visco-elastic properties. Using the acoustic radiation force, elastic shear waves can be created in soft tissues by focusing an ultrasound beam at a given location during a long application time (about 100 µs). These waves are sensitive to the visco-elastic properties of tissues. Under FDA acoustic output limitations, these shear wave sources create weak displacements in the medium preventing robust elasticity reconstruction. It is proposed here to create shear sources at different locations highly faster than the shear wave propagation speed. A supersonic regime is then established giving the ability to remotely control the shape and the amplitude of the waves induced: Due to constructive interference, this supersonic mode creates shear waves of much higher amplitude (up to 20 times higher) than in the single push mode. Moreover, it enables versatile shaping of the shear wave (as for example plane, divergent or focused shear waves) and acts as a shear beamformer inside the medium. These shear waves are generated and then imaged with a single ultrasound transducer controlled by an ultrafast imaging system. This system is able to reach frame rates up to 5000 images/s and to compute a complete movie of the shear wave propagation in less than a tenth of a second. This movie is used, by applying an inverse problem algorithm, to recover the visco-elastic map of the medium. Experiments were conducted in soft tissues mimicking phantoms. Shear waves of different amplitude and shape has been generated and imaged with the new supersonic mode. First in vivo results are given to illustrate the advantage of the supersonic imaging technique.

Mots-clés: Supersonic shear imaging, Elastography, Viscoelasticity