Rapid shrinkage of lipid-coated bubbles in pulsed ultrasound

Many researchers have observed rapid shrinkage of lipid-coated microbubbles subjected to brief, MHz ultrasound pulses. The shrinkage is sometimes, but not always, accompanied by the shedding of visible fragments of the coat. It has been suggested that the shedding of the lipid coat alone is sufficient to explain the rapid shrinkage, as that loss increases bubble surface tension and, thus, internal pressure, increasing gas loss even between pulses. We have determined, however, that the shedding of the coat lipid must also entrain some of the gas content of the bubble, to account for the observed shrinkage rates. The evidence for this is that insonated bubbles typically shrink much faster than the Epstein-Plesset (diffusion) limit for gas dissolution and diffusion, whereas uncoated quiescent bubbles shrink more slowly. We have also modeled the diffusion of gas in the moving liquid surrounding the bubble and find no advective enhancement of diffusive loss of gas from the bubble. Thus, bubble gas loss through diffusion alone is insufficient to account for rapid shrinkage.