Simulation, Experiments, and Modeling of Cloud Cavitation with Application to Burst Wave Lithotripsy

Abstract

Modeling, numerical simulations, and experiments are used to investigate the dynamics of cavitation bubble clouds induced by strong ultrasound waves. A major application of this work is burst wave lithotripsy (BWL), recently proposed method of lithotripsy that uses pulses (typically 10 wavelengths each) of highintensity, focused ultrasound at a frequency of O(100) kHz and an amplitude of O(1) MPa to break kidney stones. BWL is an alternative to standard shockwave lithotripsy (SWL), which uses much higher amplitude shock waves delivered at a typically much lower rate. In both SWL and BWL, the tensile component of the pressure can nucleate cavitation bubbles in the human body. For SWL, cavitation is a significant mechanism in stone communition, but also causes tissue injury. By contrast, little is yet known about cavitation in BWL. To investigate cloud cavitation in BWL, two numerical tools are developed: a model of ultrasound generation from a medical transducer, and a method of simulating clouds of cavitation bubbles in the focal region of the ultrasound. The numerical tools enable simulation of the cavitation growth and collapse of individual bubbles, their mutual interactions, and the resulting bubble-scattered acoustics. The numerics are implemented in a massively parallel framework to enable large-scale, three-dimensional simulations. Next, the numerical tools are applied to bubble clouds associated with BWL. Additionally, laboratory experiments are conducted in vitro in order to calibrate and validate the simulations. A major feature of the resulting bubble clouds is that the cloud size is similar to the ultrasound wavelength. This results in an anisotropic structure where the bubbles closest to the wave source grow to larger size and oscillate more rapidly. A new scaling parameter is introduced to characterize the nonlinear bubble cloud dynamics that generalizes the cloud interaction parameter of d'Agostino and Brennen (1989) defined for weak (linearized), bubble cloud dynamics excited uniformly by long-wavelength pressure waves. The mechanisms leading to the observed bubble dynamics are identified. The results further show that bubble clouds can scatter a large portion of incident ultrasound and consequently shield distal regions, including kidney stones, from irradiation. This energy shielding is quantified, and the simulations show that even a thin layer of bubbles can scatter up to 90% of the incident wave energy. A strong correlation is identified between the magnitude of energy shielding and the amplitude of the bubble-scattered acoustics. The correlation may be of use to control cavitation in the human body in real time by ultrasound monitoring for better outcomes of BWL.