Development of high intensity focused ultrasound transducers to deliver specified shock wave amplitudes at the focus

Abstract

High intensity focused ultrasound (HIFU) is currently emerging into many clinical applications. Certain HIFU modalities, histotripsy, for example, rely on the formation of high amplitude shocks in the focal waveform of the beam. These shocks develop at different focal pressure levels depending on the geometry of the HIFU transducer. The goal here was to determine optimal transducer parameters that would result in specified shock amplitudes and corresponding peak negative focal pressures. The hypothesis was that pressure level for shocks to form is mainly determined by the F-number of the transducer. As nonlinear effects accumulate almost entirely in the focal lobe of HIFU beams, shocks will form at the higher pressures for lower F-number transducers with shorter focal lobe and thus will have higher amplitudes. Simulations based on the Khokhlov-Zabolotskaya-Kuznetsov equation have shown that for typical HIFU transducers with 1–3MHz frequencies, geometries with F-number close to 1 are optimal for generating waveforms with about 70 MPa shocks and 12 MPa peak negative pressures. For lower F-number transducers, higher amplitude shocks and peak negative pressures will be formed unless cavitation occurs proximal to the focus to attenuate the focal beam. [Work supported by the grants MK-5895.2013.2, RFBR-13-02-0018, NIH-EB007643, and T32-DK007779.]