Optical and acoustic characterization of vapourized perfluorocarbon droplets as ultrasound contrast agents

Submicron perfluorocarbon droplets offer a new potential ultrasound contrast agent for extravascular diagnosis. Acoustic detection is performed by vapourizing the droplet with ultrasound, converting it into a micron-sized bubble. In this study, we use optical and acoustic methods to investigate the size, stability and echogenicity of these newly created microbubbles in the first 1 s after conversion. Suspensions of droplets with mean diameter of 415 ± 20 nm were vapourized with a single 10-cycle pulse from a focused 7.5 MHz transducer. Post-conversion droplets were examined optically, with a microscope, and acoustically, with low intensity ultrasound pulses at 1.75 MHz. Optically, micron-sized bubbles with mean diameter of 1.79 ± 0.67 ¿m were observed as soon as 1 ms after excitation. The bubbles grew in size to a mean diameter of 6.43 ± 1.80 ¿m in the first 100 ms, and then stabilized in size for at least 900 ms. Acoustically, post-conversion droplets were shown to scatter ultrasound efficiently at both the fundamental and second harmonic bands and were successfully detected with pulse-inversion imaging. In the first 100 ms after conversion the scattered fundamental signal was shown to further increase, while the harmonic signal decreased before stabilizing for a time period of at least 900 ms. This characteristic acoustic behavior of post-conversion droplets may serve as a basis for the development of an ultrasound imaging technique utilizing both regular B-mode and non-linear modes for converted-droplet specific imaging.