The influence of boiling bubble on the acoustic field and bubble dynamics in histotripsy: A numerical investigation

Boiling histotripsy (BH) is a non-invasive treatment technique that leverages cavitation effects to disintegrate soft tissue. However, the influence of boiling bubble on the acoustic field and bubble dynamics, which are key mechanisms underlying tissue destruction in BH, is not yet fully understood. This study aims to elucidate the acoustic field distribution and dynamics of a single bubble, demonstrate the generation of intrinsic cavitation bubbles, and predict the associated cavitation damage using a numerical model. The Westervelt equation was employed to simulate the nonlinear propagation of ultrasound pulses in biological tissues, considering varying boiling bubble sizes at the focus and initial acoustic pressures on the transducer surface. Moreover, the Keller-Miksis equation model, combined with the Voigt model, was used to simulate the nonlinear oscillation of bubbles. The presence of a boiling bubble results in significant acoustic reflection at the bubble interface, particularly a notably strong negative pressure. Simulation results show that intrinsic cavitation occurs in a bubble radius of 0.4 mm at an initial acoustic pressure of 2 MPa. Increasing the boiling bubble radius to 1.1 mm leads to the extension of cavitation bubbles toward the second layer of the standing wave field. Additionally, the initial acoustic pressure seems to influence intrinsic cavitation more significantly than the boiling bubble radius. Bubble dynamics, including its maximum bubble radii and expansion duration, is significantly affected by the rarefactional wave. This effect is attributed not only to the peak negative pressure but also to the waveform. In conclusion, the presence of a boiling bubble at the focus substantially modifies the acoustic field distribution, the formation of intrinsic cavitation clouds, and the characteristics of bubble dynamics. The boiling bubble radius and the initial acoustic pressure on the transducer surface are critical factors in modifying cavitation-associated tissue fractionation. Appropriate parameter settings for ultrasonic pulses may enhance the efficacy and reduce the potential complications in BH treatment.