High Framerate Imaging of Ultrasound Contrast Agents

Phospholipid-coated ultrasound contrast agents may deflate or even collapse because of stress resulting from ultrasound-induced oscillations. In this work we investigate the behavior of isolated contrast agent microbubbles during prolonged ultrasound excitation. Isolated microbubbles placed in a thin capillary tube were excited with hundreds of ultrasound pulses at a low mechanical index, and their oscillations were recorded using the Brandaris-128 ultra high-speed camera. Results show that microbubbles undergo an irreversible, non-destructive deflation process. Such deflation seems to occur in discrete steps rather than as a continuous process; furthermore, the dynamics of the bubble changes during deflation: radial oscillations, both symmetric and asymmetric around the resting radius of the bubble, occur at various stages of the deflation process. Strongly asymmetric oscillations, such as compression-only and expansion-only behavior, were also observed: notably, expansion-only behavior is associated with a rapid size reduction, whereas compression-only behavior mostly occurs without a noticeable change of the bubble radius. We hypothesize that bubble deflation results from at least two distinct phenomena, namely diffusive gas loss and lipid material shedding from the encapsulating shell. Appeared as a correspondence paper on IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 59, no. 12, 2012, by J. Viti, R. Mori, F. Guidi, M. Versluis, N. de Jong, and P. Tortoli. © IEEE 2012 Nonlinear osci llations of deflating bubbles 43 43 3.