Ultrasonic fragmentation of microbubbles: a theoretical approach of the flash in flash-echo

Predicting the dynamic behavior of ultrasound insonified lipid-shelled microbubbles has been of much clinical interest. For perfusion measurements, a technique named flash-echo has been proposed. A burst of high-MI ultrasound is to destroy the contrast agent bubbles, supposedly resulting in a strong scattering signal that is visible on the B-mode image: the flash. The absence of this strong response in parts of the B-mode image indicates a (too) low perfusion. In this paper, we investigate how microbubbles collapse and fragment. An overview of fragmentation theory is given, followed by some high-speed optical observations of collapsing and fragmenting microbubbles in an ultrasonic field. Fragmentation occurs exclusively during the collapse phase. We hypothesize that fragmentation will only occur if and only if the kinetic energy of the collapsing microbubble is greater than the instantaneous bubble surface energy. In contradiction to the assumption that the Blake critical radius is a good approximation for a fragmentation threshold, our simulations show Rmax/R0<<2 for most microbubbles.