The effect of cavitation-induced pressure on bubble cloud density in histotripsy

Tissue ablation in histotripsy is achieved by generating clouds of cavitation or boiling bubbles in the tissue. The rate of tissue ablation increases for clouds with a greater number density of nucleated bubbles. Therefore, controlling density may offer a mechanism to improve treatment efficacy. Experiments demonstrate several trends in density with ultrasound frequency, transducer f-number, and other variables. This presentation will describe one potential mechanism governing the density of cavitation bubbles nucleated during a focused ultrasound pulse. In particular, the rapid expansion of a cavitation bubble generates a partial cancellation of the incident pressure in the vicinity of the bubble, mitigating potential nucleation of other bubbles. We demonstrate this effect through a single-bubble numerical model, and further evaluate dependences of the pressure field with nuclei size, pressure amplitude, pulse frequency, and medium properties. The single-bubble model is further extended to consider the evaluate the effect of transducer focusing, and the combined pressure fields of multiple bubbles during cavitation nucleation. The predictions from simulation show good agreement with experimentally reported trends with frequency and transducer f-number, supporting the role of the mechanism in limiting the nucleation density in histotripsy.