Abstract
Phase-change contrast agents (PCCAs) provide a dynamic platform to approach problems in medical ultrasound (US). Upon US-mediated activation, the liquid core vaporizes and expands to produce a gas bubble ideal for US imaging and therapy. In this study, we demonstrate through underlying theory, high-speed microscopy, and US interrogation that PCCAs composed of highly volatile perfluorocarbons (PFCs) exhibit unique acoustic behavior that can be differentiated from tissue and standard microbubble contrast agents. Experimental results show that when activated with short pulses PCCAs will over-expand (6.3× to 7.7× the droplet diameter, PFC dependent) due to momentum of expansion, and undergo unforced, under-damped radial oscillation while settling to a final bubble diameter (5.1× to 5.5× the droplet diameter). Oscillation frequency is inversely related to droplet size—near 100 kHz for droplets ≥ 4 μm in diameter, and 2.5 MHz for droplets near 500 nm. Results from in vitro vessel phantoms using confocal piston transducers with an “activate high” (8 MHz, 2 cycles), “listen low” (1 MHz) scheme show that droplet-specific signals can be detected in both time and frequency domain, and that the magnitude of the acoustic “signature” increases with PFC volatility. These signatures may aid in development of droplet-specific detection techniques.
Published Version
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