Dynamics and frequency response analysis of encapsulated microbubble under nonlinear ultrasound

Abstract

The bubble dynamic behavior and frequency response of encapsulated microbubbles in nonlinear acoustic fields is significant in applications such as tumor therapy, thrombolysis, tissue destruction, ultrasonic lithotripsy, etc. The acoustic cavitation effect includes stable cavitation and transient cavitation. The transformation from stable cavitation to transient cavitation requires a certain threshold, which is also called the transient cavitation threshold. Phospholipid-coated microbubbles are commonly used to enhance acoustic cavitation. However, the acoustic effect of different coating material is not very clear. Especially when considering the nonlinear effects caused by diffraction, scattering, and reflection during ultrasonic propagation. In this paper, the bubble dynamic behavior and frequency response of microbubbles under different frequencies, acoustic pressures, and viscoelastic properties of different shell materials are analyzed by coupling the Gilmore-Akulichev-Zener model with the nonlinear model of a lipid envelope and using the KZK equation to simulate the nonlinear acoustic field. At the same time, impacts of the coated material and nonlinear acoustic effects are considered. The bubble dynamic behavior and frequency response under the actual measured sound field were compared with those simulated by the KZK equation. The results show that the nonlinearity will lead to a decrease in the velocity of the microbubble wall, and when the pressure of ultrasound increases, the main frequency component of the microbubble oscillation increases, making the radial motion of the microbubble more violent. When the frequency changes, the closer the oscillation frequency of the microbubble is to the resonant frequency, the more intense the radial motion of the microbubble is. The coating material can change the harmonic component in the oscillation frequency. When the harmonic is close to the resonance frequency, the radial motion of the microbubble is enhanced. The elasticity of the coated material has almost no effect on the microbubble's frequency response, and the encapsulated microbubble's initial viscosity and surface tension will change the encapsulated microbubble's oscillation frequency distribution. When the initial viscosity of the coated microbubble is smaller, the subharmonic component of the microbubble oscillation increases. When the frequency of the subharmonic is closer to the resonance frequency than the main frequency, the acoustic cavitation effect is significantly enhanced. On the other hand, when the initial surface tension of the encapsulated microbubble is increased, the main frequency and subharmonics component of the microbubble oscillation are enhanced, so that the acoustic cavitation effect is also enhanced. Therefore, this study could further elucidate the bubble dynamics of encapsulated microbubbles stimulated by nonlinear ultrasound and benefit the frequency response analysis of coated microbubbles under nonlinear acoustic fields.