Comparison between maximum radial expansion of ultrasound contrast agents and experimental postexcitation signal results.

Experimental postexcitation signal data of collapsing Definity microbubbles are compared with the Marmottant theoretical model for large amplitude oscillations of ultrasound contrast agents (UCAs). After taking into account the insonifying pulse characteristics and size distribution of the population of UCAs, a good comparison between simulated results and previously measured experimental data is obtained by determining a threshold maximum radial expansion (Rmax) to indicate the onset of postexcitation. This threshold Rmax is found to range from 3.4 to 8.0 times the initial bubble radius, R0, depending on insonification frequency. These values are well above the typical free bubble inertial cavitation threshold commonly chosen at 2R0. The close agreement between the experiment and models suggests that lipid-shelled UCAs behave as unshelled bubbles during most of a large amplitude cavitation cycle, as proposed in the Marmottant equation.