Commercially available phospholipid encapsulated perfluoropropane microbubbles can be compressed into droplets that are submicron in size and remain in a liquid form within their shell even at body temperatures. We have demonstrated that these shelled droplets have significantly different acoustic properties than the microbubbles they are formed from, and can accumulate within a developing myocardial scar zone. Moreover, we have vaporized these droplets with diagnostic high mechanical index (MI) transthoracic ultrasound in small and large animal models of myocardial ischemia and reperfusion (I/R). We have now demonstrated droplet presence within extravascular locations including the sarcoplasm following intravenous injection after I/R. The diagnostic and therapeutic potential of selective activation/cavitation of droplets within the developing scar zone (DSZ) will be explored in this application. The central hypothesis of this project is that intravenously injected perfluoropropane droplets within the DSZ can be vaporized with high MI diagnostic ultrasound, and that subsequent background-subtracted intensities will correlate with droplet concentration. Furthermore, we project that activation and cavitation of these formed microbubbles will increase tissue nitric oxide production, resulting in a reduction in the size of the DSZ. This proposal seeks to address significant knowledge gaps that must be overcome to adequately test this hypothesis. The diagnostic ultrasound thresholds for droplet vaporization must be determined, and what specific behavior the formed microbubbles exhibit in terms of coalescence, cavitation, or re-condensation following vaporization. We will employ an ultra-high speed (>106 Megahertz frame rate) camera to detect activation (vaporization) thresholds and examine the formed microbubble behavior. We will utilize in vitro flow systems with passive cavitation detectors to determine activation and cavitation thresholds in microvascular and vascular flow conditions. We will analyze the microvascular location of droplets (vascular or extravascular) under normal conditions and following I/R in the rat cremaster muscle. We will then apply selective activation/cavitation pulses to the DSZ in a rat model of myocardial I/R. The selective activation of nitric oxide activity within the scar zone will also be verified, and how it is affected by the timing of the applied activation/cavitation impulses in relation to reperfusion. We will then assess the ability of selective activation/cavitation to quantify infarct size in a large animal model of myocardial I/R. Finally, we will assess the long-term therapeutic effect of selective activation/cavitation imaging of the DSZ following intravenous injections of perfluoropropane droplets at different time points following reperfusion in porcine models of I/R. This project will determine the potential for selective droplet activation and cavitation to detect the developing scar zone, and how this activation/cavitation process may alter left ventricular remodeling following injury.
Small nanometer sized droplets can be formulated from ultrasound contrast agents. These droplets can be re- activated by diagnostic ultrasound. We hypothesize in this project that this re-activation process can a) be utilized to detect and quantify early scar formation within the heart following a heart attack, and b) alter the natural course of scar formation.
