ИСТИНА

Various clinical applications of high intensity focused ultrasound (HIFU) have different requirements to the pressure level and degree of nonlinear waveform distortion at the focus. Some applications rely on formation of high amplitude shocks in the focal waveform of the beam. On the contrary, high pressures without presence of nonlinear effects are preferable for other applications. The goal of this work was to determine parameters of the transducers that would result in either specific shock amplitude at the focus or specific peak pressures while still maintaining quasilinear waveform distortion. The hypothesis was that pressure levels required for certain degree of nonlinear effects to occur at the focus is mainly determined by a focusing angle of the transducer. Most of HIFU transducers are highly focused thus nonlinear effects accumulate mostly in the main high-amplitude diffraction lobe. This accumulation within a short focal lobe of a stronger focused transducers requires higher acoustic pressures leading to higher values of the shock amplitudes or higher pressures in quasilinear waveforms. The hypothesis was validated by simulations based on the Khokhlov-Zabolotskaya equation. The results for pressure levels achievable at the focus in a shock-formation or quasilinear regimes for different focusing angle can be used for guiding transducer design for various HIFU applications. [Work supported RSF 14-12-00974 and NIH EB007643].