On the dissolution of microbubble contrast agents in an ultrasound field

Accurate understanding of the behavior of ultrasound contrast particles driven by an external sound source is crucial to enabling the development of better therapeutic and imaging techniques. Unfortunately current models are unable to fully explain, amongst other factors, changes in bubble characteristics following pulsed excitation. One aspect that has hitherto received little attention is the time dependent nature of the microbubble coating, specifically its gas permeability and variable surface tension. We show initially through nonlinear numerical simulations that acoustically driven microbubbles can dissolve more rapidly than stationary bubbles. However, the increased rate observed seems insufficient to account for the significant changes in bubble response over a few pulse cycles observed experimentally. Then, in an attempt to explain this continued discrepancy between models and experiments, we consider the impact of changes in surfactant concentration on the bubble behavior and gas diffusion. In particular, a model of surfactant mass transfer, including shedding, between the surface of the bubble and the bulk liquid is developed and subsequently coupled to a nonlinear bubble equation. Results from this model are presented that can replicate and potentially explain bubble response changes which may occur, leading to improvements in propagation modeling and quantitative imaging software.