Effect of bubble interaction on contrast agent destruction behaviour under repeated insonation

The controllable destruction of ultrasound contrast agents plays an important role in quantitative molecular imaging, perfusion estimation, and drug delivery applications. In general, repeated insonation is required to destroy all microbubbles (MB) present. It is expected that MB destruction will depend on MB concentration due to MB interaction and varying attenuation. While attenuation is expected to reduce destruction rates for high concentrations, microbubble interaction may lead to the increased destruction of bubbles during a single pulse sequence. Therefore, we investigate the dependency of the rate of destruction events on the concentration of hard shelled MB to analyze the influence of microbubble interaction. To immobilize the MB during the repeated insonation, microbubbles were embedded in gelatine. Destruction events of single, stationary bubbles can be visualized in Doppler images. This allows the quantification of events in a sequence of frames and thus offers a possibility to analyze destruction rates. Four phantoms with concentrations ranging from 6·10⁴ to 1.5·10⁶ MB/ml were imaged with a Vevo770 system and a RMV 707B scanhead (Visualsonics) in Power Doppler mode. From the total number of colored pixels the number of destruction events was calculated. These were then normalized to the total number of events for a frame sequence to allow comparison of the different concentrations. The results show that the relative decrease of destruction events is slower for lower concentrations. Thus, the results indicate that for high concentrations the destruction threshold of MBs is lowered by neighbouring bubbles.