Cavitation clouds created by shock scattering from bubbles during histotripsy

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 5À20 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 negativepressure 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.

Effect of annular focusing of ultrasound on rate of stone erosion using cavitation bubbles

“…20) A HIFU waveform at the focus tends to have such a highly positive pressure owing to a nonlinear propagation followed by a focal phase shift, 18,19) as shown in Fig. 3.…”

“…18,19) Once cavitation bubbles are formed on the surface of a stone, they reflect a positive-peak-enhanced wave and convert it to a negative-peak-enhanced wave, which further generates cavitation bubbles, creating a large cloud of cavitation microbubbles upstream. 20) In sector vortex annular focusing, the positive peak pressure at the focus should be smaller because the lateral size of the focus is larger and the focal phase shift is smaller. Therefore, this focusing scheme is expected to generate large cavitation bubbles laterally and thinly.…”

Enhancement of High-Intensity Focused Ultrasound Heating by Short-Pulse Generated Cavitation

“…Although the intrinsic threshold is extremely high, an acoustic pulse with a longer duration can generate clouds of bubbles even at a lower intensity because of the mechanism called "shock scattering" [46]. The ultrasound waveform in a focal region at a high intensity has a steep and large peak-positive pressure and relatively small peak-negative pressure-for instance, +78 MPa and −15 MPa, respectively [46]-owing to the focusing of nonlinearly propagated ultrasound. Once cavitation bubbles are generated, they provide a pressure release surface converting the positive pressure to negative pressure, which exceeds the intrinsic threshold.…”

“…In histotripsy, cavitation bubbles are induced by extremely high-intensity pulses with a peak-negative pressure exceeding the threshold range and a soft tissue is eroded by the collapse of the bubbles [43][44][45]. Although the intrinsic threshold is extremely high, an acoustic pulse with a longer duration can generate clouds of bubbles even at a lower intensity because of the mechanism called "shock scattering" [46]. The ultrasound waveform in a focal region at a high intensity has a steep and large peak-positive pressure and relatively small peak-negative pressure-for instance, +78 MPa and −15 MPa, respectively [46]-owing to the focusing of nonlinearly propagated ultrasound.…”

“…Cavitation clouds are formed backward from a focal point, in other words, toward a HIFU transducer, through a shock scattering phenomena [34,46,53,54]. In the trigger HIFU heating, the mismatch between the center position of bubble clouds and focal plane of heating bursts may have caused a less predictable coagulation pattern.…”