Influence of binding on the vibrational responses of targeted lipid-coated microbubbles

Microbubbles for molecular ultrasound imaging are targeted to disease-specific biomarkers. One of the main challenges is to acoustically distinguish free microbubbles from those that are bound to their molecular target. In this study we used the Brandaris 128 high-speed camera to compare the responses to ultrasound of two types of lipid-coated microbubbles (DSPC and DPPC) in their free and bound state aiming to acoustically discriminate them. We found larger shrinkage for DPPC than for DSPC microbubbles in both their free and bound condition, indicating higher acoustical stability for DSPC-based microbubbles. Upon binding the stability of both types did not change significantly. Frequencies of maximum response were similar for DSPC and DPPC microbubbles, whether they were free or bound. In their free state, more DPPC than DSPC microbubbles responded at the subharmonic and the second harmonic frequency, which did not change upon binding. Interestingly, the maximum relative radial excursions at the second harmonic frequency were higher for DSPC microbubbles when they had bound. In conclusion, bound DSPC and DPPC microbubbles had lower maximum relative radial excursions at the fundamental frequency, and had higher maximum relative radial excursions at the second harmonic frequency than the free microbubbles. We found no differences between bound DSPC and bound DPPC microbubbles. The higher maximum relative radial excursions of particularly bound DSPC microbubbles at the second harmonic frequency may provide opportunities to acoustically discriminate them from free microbubbles.