Effects of the nonlinearity of medium on the directivity of low-frequency parametric ultrasound scattering

Low-frequency parametric ultrasound, generated during the nonlinear propagation of modulated ultrasound, exhibits sharper directivity and propagates further compared to ultrasound at the same frequency directly radiated from a source with identical aperture. It also offers improved penetration depth compared to high-frequency ultrasound. Low-frequency ultrasound imaging utilizing this parametric ultrasound has been proposed [Nomura et al., Ultrasonics, 89, 64–73 (2018)], with ongoing research into imaging deeper regions and closely spaced objects. Since echoes and scattered waves of parametric ultrasound in this method consist of components generated before incidence on targets and those generated from the scattered primary waves at objects, the advantageous directivity of parametric ultrasound may not be fully utilized. In this study, we numerically compare the generation amounts of these two components. Our simulation evaluated the influence of nonlinearity on echo signals by artificially removing the medium's nonlinearity after the primary wave strikes the object. The results indicate that the generation of parametric ultrasound is limited after the scattering of primary and secondary parametric ultrasound waves by the object. This suggests that the advantageous characteristics of low frequency with improved directivity are not fully exploited.