Enhanced cavitation activities from axial split foci using second/third-harmonic superimposition for focused ultrasound surgery

This study aims at enhancing cavitation heating by using dual-frequency modes of second-harmonic and third-harmonic superimposition for high-efficient focused-ultrasound surgery. By controlling the ratio of dual-frequency acoustic powers, the superimposing of two frequency pressures result in split foci along beam axial within confocal region, and the maximal peak intensity of split focus can reach about 2 times the sum of two frequency intensities, indicating strong wave interference. The 1.1 MHz/3.3 MHz and 1.1 MHz/2.2 MHz experiments implemented in BSA gel-phantom, and lesion inception time is about 0.3 s, which is 10 times shorter than that in single frequency mode. The boiling bubbles occur frequently in each lesion formation, beneficiating increase the lesion size. In 1.1 MHz/3.3 MHz modes, lesions developed in a symmetrical manner in ellipsoid shapes. The 12-s lesion size of 1.1 MHz/3.3 MHz modes is nearly two times as large as the lesion size of 1.1 MHz/2.2 MHz modes. Although in 20% DC case, the acoustic power dose is only a half that used in 80% DC case, the 12-s lesion size with 20% DC is nearly same as that with 80% DC for both 1.1 MHz/3.3 MHz and 1.1 MHz/2.2 MHz conditions. The inertial-cavitation-energy waveforms reveal that the strong inertial-cavitation activities involve in all cases.