Phase-change contrast agents are rapidly developing as an alternative to microbubbles for ultrasound imaging and therapy. These agents are synthesized and delivered as liquid droplets and vaporized locally to produce image contrast. They can be used like conventional microbubbles but with the added benefit of reduced size and improved stability. Droplet-based agents can be synthesized with diameters on the order of 100 nm, making them an ideal candidate for extravascular imaging or therapy. However, their synthesis requires low boiling point perfluorocarbons (PFCs) to achieve activation (i.e., vaporization) thresholds within FDA approved limits. Minimizing spontaneous vaporization while producing liquid droplets using conventional methods with low boiling point PFCs can be challenging. In this study, a new method to produce PFC nanodroplets using spontaneous nucleation is demonstrated using PFCs with boiling points ranging from -37 to 56 °C. Sometimes referred to as the ouzo method, the process relies on saturating a cosolvent with the PFC before adding a poor solvent to reduce solvent quality, forcing droplets to spontaneously nucleate. This approach can produce droplets ranging from under 100 nm to over 1 μm in diameter. Ternary plots showing solvent and PFC concentrations leading to droplet nucleation are presented. Additionally, acoustic activation thresholds and size distributions with varying PFC and solvent conditions are measured and discussed. Finally, ultrasound contrast imaging is demonstrated using ouzo droplets in an animal model.
Keywords: Ultrasound; and phase-change; contrast agents; nanodroplets.