Quantitative Measurements in Vibro-acoustography

. Vibro-acoustography (VA) is an acoustic image modality based on the response of tissue under excitation of two ultrasound beams with different-frequencies. The aim of this paper is to provide a quantitative analysis with experimental measurements of the difference-frequency (DF) pressure generated in VA. This analysis considered the radiation force , the induced acoustic emission, and nonlinear wave mixing (parametric array and interaction of sound-with-sound). An experimental setup was devised employing a confocal ultrasound transducer driven by two sinusoidal signals at frequencies 3.2 ± f_ / 2 MHz, with f_ = 50, 60, 70, and 100 kHz, generating a pressure field over a metallic sphere placed at the transducer focus into a water tank. The DF pressure was measured as function of the distance r from the spherical target to an acoustic hydrophone placed along the transducer axis. The results showed the main contribution to the VA signal is due to the parametric array and the nonlinear scattering of sound-by-sound where the last one is responsible for the VA image contrast. The estimated acoustic emission and the linear scattering components had amplitudes less than -80dB of the detected signal. The statistical analyzes showed that the nonlinear decay model, which predicts a spatial variation ) ln 1 ( 1 r r   , is more efficient to explain the experimental data (R_square>0.86) when compared with the acoustic emission theory model ( 1  r ), that presented poor correlation coefficients.