Vaporization dynamics of superheated phase-change contrast agents—Simulations versus experiments

Superheated perfluorocarbon droplets and other similar phase change contrast agents offer exciting new opportunities for ultrasound imaging and therapy. Their small size allows them to more easily perfuse the microvasculature of tumors than commonly used microbubble contrast agents; their typically longer circulation half-lives and greater surface-to-volume ratios are also highly advantageous for clinical use. These droplets can potentially be “activated” (vaporized) by an externally applied ultrasound pulse. Controlling the timing and location of droplet activation, while minimizing the risk of spontaneous vaporization, is vitally important to avoid potential safety concerns in vivo, such as vascular occlusion. The activating pulse, however, must fall within a range of pressures that can be safely used on patients. The aim of this study is to simulate the radial dynamics and acoustic signature of an activated nanodroplet and compare these to recent experimental temporal measurements of individual vaporizing nanodroplets. The accuracy of the ideal gas approximation for these droplets will be discussed, as well as the impact of shell properties on the radial dynamics and what insight can be gained from the simulations in terms of control of the activation process.