Phase-Conversion Nanoparticle Contrast Agents

Abstract

Ultrasound contrast agents composed of encapsulated microbubbles are now widely used in many parts of the world for an array of clinical applications. During diagnostic echocardiography, contrast agents are used to better delineate endocardial borders and intracavitary masses. Echocardiographic laboratories that have specialized skill and knowledge can also apply commercially produced microbubble contrast agents in an off-label fashion for myocardial perfusion imaging to detect rest or stress-induced ischemia in coronary artery disease, microvascular reflow, tissue viability, and tumor perfusion. See Article by Porter et al The ultrasound contrast agents that have been used in humans vary in terms of their composition, including the content of their gas core (perfluorocarbons, SF6) although all are high-molecular weight gases with relatively low solubility and diffusivity to improve stability in vivo.1 There are also differences in encapsulation strategy (albumin, lipid), although a common thread has been that the viscoelastic properties must be appropriate to allow nonlinear oscillation even during low mechanical index (MI) imaging.1 Commercially produced agents also vary in terms of size and size dispersion. It is important that the vast majority of microbubbles be smaller than the dimensions of the microvessels through which they travel, and yet, very small (sub-micron) bubbles are less stable because of surface tension and contribute relatively little to signal enhancement. In this issue of Circulation: Cardiovascular Imaging , Porter et al2 report results from a collaborative study where signal enhancement during myocardial contrast echocardiography (MCE) was examined with a novel phase-shifting nanodroplet. This class of agents differ from microbubbles in that they are almost an order of magnitude smaller in diameter, have a predominately liquid-phase core, and require activation whereby diameter increases upon conversion of the core to gas phase (vaporization) during the pressure oscillations of the acoustic field.3 By virtue of …