Influence of microbubble shell properties on ultrasound signal: Implications for low-power perfusion imaging.

Low mechanical index perfusion imaging relies on the detection of signals produced by microbubble oscillation at low acoustic powers that results in minimal microbubble destruction. We hypothesized that the optimal acoustic power for real-time imaging would differ for microbubbles with different shell characteristics. Three microbubble agents with varying shell elastic properties according to their polymer composition were studied. Differences in the elastic properties of these microbubbles was demonstrated by: (1) measurement of their bulk modulus and (2) evaluation of their acoustic lability by microscopic visualization of microbubble destruction during insonification at incremental acoustic powers. The ultrasound signal generated by these microbubbles at various mechanical indexes and the degree of microbubble destruction during continuous imaging was determined both in an in vitro flow system and during in vivo imaging in an open-chest canine model. Both studies indicated that optimal power for achieving maximal signal intensity with minimal microbubble destruction was influenced by the shell elastic properties. We conclude that the acoustic power for maximizing acoustic signal without destroying microbubbles during low mechanical index imaging varies according to shell characteristics.