Bubble mediated focused ultrasound: Nucleation, cavitation dynamics and lesion prediction

Cavitation is often the result of rapid and intense hyperthermia using high-intensity focused ultrasound (HIFU) to treat malignant tumors deep in tissue. With proper parameters, cavitation bubbles are very efficient in transforming acoustic energy into heat. In vivo and in vitro studies have shown that bubble enhanced heating can potentially increase HIFU efficacy. However, sporadic nucleation of bubbles and subsequent nonlinear oscillations allow uncontrolled cavitation to induce irregular lesions and other unpredictable and thus undesirable effects. This work takes a modeling approach to investigate the physical mechanisms necessary for developing bubble-mediated HIFU therapy with a satisfactory degree of predictability and controllability.; Controlled nucleation is possible with a transient vapor cavity generated from a laser heated gold nano-particle in combination with proper acoustic conditions. Preliminary experimental studies in optically transparent gel phantoms have been conducted and results show that the combined use of laser, HIFU and nano-particles reduces the cavitation threshold significantly, and the temporal and spatial characteristics of cavitation events can be controlled by the sound-light interaction mediated with the particles. A mathematical model for particle heating, vapor bubble nucleation and the subsequent bubble dynamics is constructed to predict cavitation thresholds of laser and HIFU energy, and results are comparable with experiments. A parametric study is presented to investigate the effects of particle size and laser pulse length for the optimization of this technique to generate cavitation on demand.; In another effort, the interactions of acoustic propagation and the thermal effects in the presence of cavitation bubbles (mostly gas) and boiling bubbles (mostly vapor) are modeled. By introducing temperature dependent viscosity, surface tension coefficient and vapor pressure into the nonlinear gas bubble dynamics, the variation of the bubble radial oscillations, size distributions and subsidiary heating due to the temperature elevation are studied. A continuum bubbly medium is constructed with effective sound speed and attenuation derived from the temperature dependent nonlinear gas bubble When boiling occurs, the vapor bubbles are simply modeled as pressure release cavities. An iterative simulation for acoustic, cavitation(boiling) and thermal fields is then constructed to model the lesion developments, with updating time scales estimated from intrinsic space-time scales related to the evolution of bubble clusters observed from experiments. Results indicate that cavitation enhances ultrasonic absorption and creates larger and wider lesions in a shorter time. The scattering due to the impedance mismatch in the cavitation zone is responsible for the irregular lesions. The heating from cavitation bubbles also accelerates the emerging of vapor bubbles and boiling. The current scheme has exhibited its effectiveness in predicting the position, shape and size of an evolving lesion during bubble-mediated HIFU therapy, given the thermal and acoustical properties of the target tissue and the ultrasonic insonation parameters such as focal intensity and exposure duration.