The peculiar interactions of microbubbles and microvessels

The application of microbubbles for both diagnostic macro- and molecular imaging and therapeudic ultrasound requires an understanding of the coupled interactions involving microbubble dynamics with the surrounding compliant microvessel. In this study, ultra-high speed microphotography was used to directly observe transient behaviors of microbubbles in microvessels of ex vivo rat mesenteries. Definity® microbubbles were perfused through the vasculature, and then excited by a 2-μs long ultrasound pulse with a center frequency of 1 MHz with peak negative pressures between 0.8-7.2 MPa. These amplitudes span the diagnostic to therapeutic pressure levels. During insonation, ultrahigh speed images were captured with 50-ns exposure time and 150-ns or 300-ns interframe time. The recorded images show a wonderful assortment of microbubble dynamics, including oscillation, translation, jetting, coalescence and fragmentation. These microbubble behaviors were coupled with the dynamic responses of the vessel wall, which showed distention, invagination, and even rupture.