The physical principle of the supersonic shear imaging (SSI) technique was described for the first time in Bercoff et al. . It can be decomposed in three main stages: excitation by acoustic radiation force, acquisition by ultrafast imaging and calculation of speed maps. It is the combination of an excitation by acoustic radiation force and an ultrafast ultrasound imaging of the resulting shear waves which characterize the specificity of this elastography mode.
23.2 Radiation Force Excitation
The basis of the SSI technique is to use radiation force to excite the medium. It is this slight displacement of the medium which will then propagate in the form of a mechanical wave if the medium is solid, or create a flux if the medium is liquid. It is therefore the essential component of the SSI that allows the generation of the shear wave and replaces the techniques based on external vibrators.
23.2.1 Radiation Force
The radiation pressure is a volumetric or surface force which results from a transfer of momentum between a wave and its propagation medium. Originally discovered in acoustics by Rayleigh, it could only be reliably observed with piezoelectric transducers ...