Investigating the resonance response of a system of two ultrasound-driven lipid encapsulated microbubbles confined within a viscoelastic vessel

Abstract

Lipid encapsulated microbubbles, strongly characterized by the initial phospholipid concentration, are clinical ultrasound imaging agents. These microbubbles are commonly used as an ultrasound contrast agent and are being developed for therapeutic applications. In this work, we investigate the vibration dynamics of two microbubbles confined within a viscoelastic vessel. We developed a finite element model to study radial microbubble dynamics in a two-bubble system, as typical clinical doses result in closely spaced bubbles. Specifically, we study the effect of the vessel wall viscosity on the resonance behaviour of microbubbles in the frequency range of 2– 8 MHz. For two identical microbubbles, we observed a decrease in 50% in resonance amplitude as we increased the vessel viscosity from 0.1 to 1 Pa s as well as an 8% shift of peak resonance activity towards higher frequencies. Furthermore, we investigated the same system consisting of microbubbles with a lower initial phospholipid concentration and showed the same decrease in amplitude; however, the direction of shift of peak resonance was towards lower frequencies by 4%. Our work suggests that microbubble resonance behaviour is greatly affected by the vessel viscosity and that the extent and direction of bubble dynamics changes are dependent on shell characteristics.