Experimental determination of phase velocity of perfluorocarbons: Applications to targeted contrast agents

Targeted acoustic contrast agents are designed to enhance the sensitivity a nd specificity of ultrasonic diagnoses, We have previously developed a liga nd targeted ultrasonic contrast system that is a lipid-encapsulated, liquid -perfluorocarbon emulsion. The emulsion particles are small (250 nm) and ha ve inherently low echogenicity unless bound to a surface by a pretargeted l igand through avidin-biotin interactions. We have recently proposed a simpl e acoustic transmission line model that treats the emulsion particles as a thin layer over the targeted surface, In this model, the acoustic reflectiv ity of the sample increases for perfluorocarbons with smaller velocities of longitudinal sound or lower densities, In this study, we measure and repor t the velocity of longitudinal sound for 20 perfluorocarbons using a broadb and phase spectroscopic approach for estimating phase velocities. Experimen tally determined velocities ranged from 520 +/- 2 m/sec (perfluorohexane) t o 705 +/- 5 m/s (perfluorodecalin), No measurable dispersion was observed o ver the useful bandwidth of 2 to 22 MHz. Increasing carbon backbone chain l ength and fluorine substitution with halogens of greater atomic weight incr eased the measured speed of sound. Our experimental data were consistent (R = 0.87) with a published empirical model that predicts velocity as a funct ion of molecular structure. These data provide a rational basis for optimiz ing targeted perfluorocarbon-based contrast agents and offer further insigh t into the physical mechanisms responsible for the observed enhancement of surface acoustic reflectivity.