Passive acoustic mapping of magnetic microbubbles in an in vitro flow model

Abstract

Magnetic microbubbles can be successfully retained near a vascular target and simultaneously imaged using conventional B-mode ultrasound. When further modified to carry a drug, they could enable significant enhancements in targeted drug delivery for applications such as sonothrombolysis, where stable cavitation has been shown to play a key role. However, the effect of the increased proximity of the microbubbles under the effect of the magnetic field on their acoustic response remains unknown. Passive acoustic mapping is a method that enables real-time spatiotemporal monitoring of cavitation dynamics in an arbitrary plane or volume within the field of view of the ultrasound probe, and classification of the type of cavitation activity on the basis of the spatial distribution of frequency-domain emissions. In the present work, PAM is used to investigate the effect of bubble proximity and flow rate on the type, sustainability, intensity, and spatial distribution of cavitation activity observed for both magnetic and non-magnetic microbubbles excited by 0.5 MHz therapeutic ultrasound in an in vitro flow model. It is hoped that this study will not only yield a new method for real-time monitoring of drug delivery using magnetically trapped microbubbles, but will also help elucidate complex bubble–bubble interactions in therapeutic ultrasound fields.