High speed optical observations and simulation results of lipid based microbubbles at low insonation pressures

In this investigation, a high speed camera was used to measure the radial oscillations of the commercial contrast agent SonoVue™ and in-house microbubble preparations of differing lipid compositions at low insonation pressures. It was found that a significant proportion (~75%) of the microbubbles shrank during insonation, possibly due to gas diffusion and/or excess lipid shedding. Microbubble shrinkage was found to be dependant on excitation pressure and was seen to increase for bubble sizes close to their resonance frequency. The elasticity shell parameter (Gs) in a modified nonlinear Rayleigh-Plesset (RP) model was tuned in order to fit its response to the measured oscillations (before shrinkage). It was found that, for SonoVue™, the shell elasticity varies proportionally to the initial microbubble diameter, suggesting that it is not a bulk property of the material as previously assumed. It was also determined that for cases where the bubble shrinks, the shell elasticity parameter decreases. As a result of this, the RP model was adapted to take into account the changing elasticity of the bubbles and give a good fit to the data. © 2006 Crown.