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The vibro-acoustography imaging method consists of forming an image of the deformability of a tissue submitted to a low frequency fLF stress field. This sound field can be created locally by means of a focused annular array emitting two primary beams driven at two close frequencies fa and fb=fa+fLF. In the existing literature, the origin of this stress field has been identified as the low frequency radiation pressure of the two primary beams. However, this work intends to show that another contribution to this internal stress is the low frequency field distributed in the object volume and created by the nonlinear interferences of the two primary beams. This nonlinear field was calculated in the case of multiple ring annular arrays and compared with the qLF beam experimentally measured in a water tank. The agreement between the theoretical and experimental curves provides information on the possibility that this nonlinear effect takes place in vibro-acoustography. [Copyright &y& Elsevier]

Abstract: It is now accepted that an effective way to investigate the elastic properties of soft tissues is to generate a localized transient acoustic radiation force and to follow the associated displacements in the time/space domain. Shear waves induced by this stress field are particularly interesting in this kind of medium because they are governed by the shear elastic modulus μ, which is directly linked to the Young modulus, and spatial distribution and temporal evolution of the transient motion induced must therefore be obtained in detail. We report here a model based on the elastodynamic Green’s function formalism to describe these displacements. 3D simulation of radiation force in homogenous elastic media was performed and the displacement curves computed at different radial distances for different temporal force profiles. Amplitude and duration of displacement were found to be reliable parameters to characterize the elastic properties of the medium. Experimental measurements were performed in a homogeneous agar-gelatin tissue-mimicking phantom, and two transducers were used to generate the radiation force and follow the induced displacements. Displacements obtained from different lateral locations around the applied force axis were then used to reconstruct the shear-wave propagation in a scan plane as a function of time. The experimental displacements/curves agreed with the theoretical profiles obtained by the elastodynamic Green’s function formalism. [Copyright &y& Elsevier]