Quantifying Activation of Perfluorocarbon-Based Phase-Change Contrast Agents Using Simultaneous Acoustic and Optical Observation

Phase-change contrast agents in the form of nanoscale droplets can be activated into microbubbles by ultrasound, extending the contrast beyond the vasculature. This article describes simultaneous optical and acoustical measurements for quantifying the ultrasound activation of phase-change contrast agents over a range of concentrations. In experiments, decafluorobutane-based nanodroplets of different dilutions were sonicated with a high-pressure activation pulse and two low-pressure interrogation pulses immediately before and after the activation pulse. The differences between the pre- and post-interrogation signals were calculated to quantify the acoustic power scattered by the microbubbles activated over a range of droplet concentrations. Optical observation occurred simultaneously with the acoustic measurement, and the pre- and post-microscopy images were processed to generate an independent quantitative indicator of the activated microbubble concentration. Both optical and acoustic measurements revealed linear relationships to the droplet concentration at a low concentration range <10⁸/mL when measured at body temperature. Further increases in droplet concentration resulted in saturation of the acoustic interrogation signal. Compared with body temperature, room temperature was found to produce much fewer andlarger bubbles after ultrasound droplet activation.