In Ultrasound-Stimulated Vibro-Acoustography (USVA) imaging method, one tries to form an image of the deformability of a tissue submitted to a low frequency (LF) f- stress field. This sound field can be locally created by mean of a focused annular array emitting two primary beams driven at the two close frequencies f1 and f2 = f$=1)+f-. The coherent acoustic emission resulting form the object vibration is detected by a sensitive hydrophone and used to form an image. In the present literature, the origin of this stress field has been essentially identified to the LF radiation pressure created by the two primary beams frequency beating. However, an other contribution of this internal stress is, according to us, the LF field distributed in the object volume and created by the nonlinear (NL) interaction of the two primary beams. The q-, q1 and q2 beams emitted by a focused four rings annular array are experimentally measured in a water tank. Amplitude and shape of the theoretical model of the NL interference beam and the experimental curves are compared. USVA images of a steel ball placed into gelatin and scans of a calcaneus bone at different depths are presented. The resolution of an elastodynamic problem in the case of a spherical element in gelatin shows that the displacement amplitude of the tissue, in force axis, is principally due to the variations of the elasticity shear modulus (mu) . USVA images represent in gray scale the displacement amplitude due to the radiation resulting from the object vibration. If we assume that the stress field is constant during the scan, we then obtain images proportional to the local elasticity shear modulus.