Cavitation histotripsy at the surface of soft material using the dual-frequency excitation

Cavitation histotripsy has been applied for the disintegration at the interface of soft tissue and fluid in a well-controlled manner. The dual-frequency excitation was proposed and evaluated in this study. The acoustic field generated by the dual-frequency (1.09+1.11 MHZ) excitation and associated bubble dynamics at the focus was simulated using Khokhlov-Zabolotskaya-Kuznetsov equation and Gilmore model, respectively, and compared with those by the single-frequency (1.1 MHz) excitation at the same power. Their beam sizes and distribution of pulse-average acoustic intensity are similar. It shows that the dual-frequency excitation at the duty cycle of 2% and pulse repetition frequency of 1 Hz after 20 s exposure can increase the erosion area and volume on the surface of Alginate gel phantom by ∼2 fold (p < 0.05). The effects of operating parameters of the cavitation histotripsy with the dual-frequency excitation on the erosion were investigated. Small frequency difference (< 0.1 MHz), high modulation depth (100%), moderate pulse repetition frequency (100 Hz), large duty cycle, and long interval time between pulses are preferred. Meanwhile, the acoustic emission signals during the ultrasound exposure were collected using passive cavitation detection to calculate their spectra using the short-time Fourier transform. It is shown that higher scattering and inertial cavitation levels, especially at small frequency differences and high modulation depth can be produced by the dual-frequency excitation. Erosion capabilities of both excitations were also evaluated using the ex vivo porcine kidney.