Cavitation clouds created by shock scattering from bubbles during histotripsy

TLDR

Histotripsy uses short, high‑amplitude ultrasound pulses to generate a localized cavitation cloud that mechanically disrupts tissue. The study investigates how cavitation clouds form during histotripsy, hypothesizing that backscattering of shockwaves from a single bubble produces large negative pressure that initiates the cloud. Using 1‑MHz focused pulses on a transparent phantom, the authors observed that the positive‑pressure phase inverts upon scattering and superimposes on the incident negative phase, creating sustained negative pressure that repeatedly elongates the bubble cloud toward the transducer, a process amplified by finite‑amplitude distortion; they tested modified waves that reduced positive pressure while preserving negative pressure, which suppressed cloud formation. Clouds began from single bubbles formed in the initial pulse cycles and grew along the acoustic axis opposite the propagation direction, and the modified waves that maintained negative pressure but lowered positive pressure suppressed cloud formation, confirming the hypothesis.

Abstract

Histotripsy is a therapy that focuses short-duration, high-amplitude pulses of ultrasound to incite a localized cavitation cloud that mechanically breaks down tissue. To investigate the mechanism of cloud formation, high-speed photography was used to observe clouds generated during single histotripsy pulses. Pulses of 5−20 cycles duration were applied to a transparent tissue phantom by a 1-MHz spherically focused transducer. Clouds initiated from single cavitation bubbles that formed during the initial cycles of the pulse, and grew along the acoustic axis opposite the propagation direction. Based on these observations, we hypothesized that clouds form as a result of large negative pressure generated by the backscattering of shockwaves from a single bubble. The positive-pressure phase of the wave inverts upon scattering and superimposes on the incident negative-pressure phase to create this negative pressure and cavitation. The process repeats with each cycle of the incident wave, and the bubble cloud elongates toward the transducer. Finite-amplitude propagation distorts the incident wave such that the peak-positive pressure is much greater than the peak-negative pressure, which exaggerates the effect. The hypothesis was tested with two modified incident waves that maintained negative pressure but reduced the positive pressure amplitude. These waves suppressed cloud formation which supported the hypothesis.