Interaction between therapeutic ultrasound propagation and cavitation bubbles

Abstract

In medical applications of high intense focused ultrasound using pressure pulses or continuous wave signals, cavitation is considered to play a significant role for physical and biological effects. To further develop therapeutic applications it is essential to improve the understanding of these cavitation related effects. In this paper a numerical model is presented to simulate the interactions between ultrasonic waves and cavitation bubbles. The FDTD model is based on a two-phase continuum approach for bubbly liquids and combines nonlinear ultrasound propagation with cavitation bubble activity. Experimental and numerical investigations are presented demonstrating the influence of cavitation bubbles on ultrasound propagation. Measurements with a fiber optic hydrofone for pulsed piezoelectric transducers show significant variations in focal pressure waveforms after the first tensile phase of the wave for different gas content. It is supposed that these changes are caused by cavitation effects. Calculations with different bubble densities confirm these experimental results and demonstrate that the first positive pressure part of the wave is not affected by bubble activity. Increasing the gas content leads to a truncated tensile part followed by augmented pressure oscillations. Further on, simulation results for the evolution and impact of cavitation bubble clouds in CW applications are presented.