Shape mode stability of lipid-coated ultrasound contrast agent microbubbles

Abstract

Ultrasound contrast agents (UCAs) are shell encapsulated microbubbles developed originally for ultrasound imaging enhancement. More recently, UCAs are being exploited for therapeutic applications such as drug and gene delivery. Ultrasound transducer pulses can induce spherical (radial) UCA oscillations, translation, and nonspherical shape oscillations, the latter of which can lead to breakup. Breakup can facilitate drug or gene delivery, but should be minimized for imaging purposes to increase residence time and maximize diagnostic effect. Therefore, an understanding of the interplay between the acoustic driving and shape mode stability of UCAs is important for both diagnostic and therapeutic applications. The present work couples a radial model of a lipid-coated microbubble with a model for bubble translation and nonspherical shape oscillation to predict shape mode stability for ultrasound driving frequencies and pressure amplitudes of clinical interest. In addition, calculations of the stability of individual shape modes, residence time, maximum radius, and translation are provided with respect to acoustic driving parameters and compared to an unshelled bubble. The effects of shell elasticity, shell viscosity, and initial radius on stability are investigated. The results show greater stability at higher values of shell elasticity and viscosity and at smaller radius, and provide guidance for optimizing shell design and ultrasound driving parameters with respect to shape stability.