Optimally Enhanced Heating for Focused Ultrasound Surgery with Split Foci, Dual-Frequency, or Multi Foci

' To substantially enhance heating in HIFU treatment, several methods such as split foci, multi foci, and dual-frequency modes are used. The enhanced-cavitation heating protocols are implemented experimentally in BSA gel-phantom using four-element split-focus array. Using dual frequency of 1.2 and 2.4 MHz, the superimposing of two frequency pressures at confocal region can enhance nucleation cavitation and inertial cavitation activity. When using 135 degrees phase shift combined with dual frequency of 1.2 and 2.4 MHz, the peak negative pressure reach maximum due to peak-negative pressures of two frequencies occur at same time, resulting strong cavitation activities. When using dual frequency of 1.2 and 2.4 MHz, 25-Hz pulse-repetition frequency (PRF), both 135 degrees and 180 degrees phase shift protocols, the experiment results show the largest lesion size of 10.5 x 10.5 x 11 mm(3), quickest lesion inception time of less than 0.2 s, therefore, both 135 degrees and 180 degrees phase shift protocols are most efficient in enhanced-cavitation heating. The filtered-PCD mean square waveforms reveal that the strong inertial-cavitation activities involve in those two treatments. The lesion size of four foci of 180 degrees phase shift, single frequency, 25 Hz PRF, is 2 times that of 0 degrees phase shift, single frequency even if the peak intensity of 180 degrees case is half less than that of 0 degrees phase shift case. When arrange multi foci using phased array in a style of a wavelength distance between neighbor foci in focal plane, the result is the same as that using split foci of 180 degrees phase shift and single frequency.