Non-linearities under highly focused high-intensity ultrasound fields

High-intensity focused ultrasound (HIFU) has seen widespread clinical adoption as a therapeutic tool, as it may be targeted noninvasively and without ionizing radiation. The converging sound waves induce thermal (e.g., ablation) and mechanical (e.g., radiation force) effects that may be localized to manipulate or destroy tissue. In addition to these primary effects, finite-amplitude acoustic influences (e.g., second harmonic generation) become relevant due to the high-pressure levels near the focal region. One of the ways these nonlinear effects occur is when a pressure wave develops at the difference frequency due to the nonlinearity of the medium when an acoustic beam is driven by a signal that contains two high but slightly different frequencies. This is termed as “scattering of sound by sound,” an implication of finite-amplitude propagation is the existence of sum and difference frequencies. Using highly focused ultrasound beams as carrier waves at high amplitudes, we have observed that these nonlinear effects can be localized at scales below its wavelength with sufficient amplitudes. These effects were observed for a water reference medium, and thus are likely to be even more prominent in tissue. Exploring these higher-order acoustic effects for the diagnosis and treatment of human diseases is warranted.