Slow-Flow Ultrasound Localization Microscopy Using Recondensation of Perfluoropentane Nanodroplets.

Ultrasound localization microscopy (ULM) is an emerging, super-resolution imaging technique for detailed mapping of the microvascular structure and flow velocity via subwavelength localization and tracking of microbubbles. Because microbubbles rely on blood flow for movement throughout the vascular space, acquisition times can be long in the smallest, low-flow microvessels. In addition, detection of microbubbles in low-flow regions can be difficult because of minimal separation of microbubble signal from tissue. Nanoscale, phase-change contrast agents (PCCAs) have emerged as a switchable, intermittent or persisting contrast agent for ULM via acoustic droplet vaporization (ADV). Here, the focus is on characterizing the spatiotemporal contrast properties of less volatile perfluoropentane (PFP) PCCAs. The results indicate that at physiological temperature, nanoscale PFP PCCAs with diameters less than 100 nm disappear within microseconds after ADV with high-frequency ultrasound (16 MHz, 5- to 6-MPa peak negative pressure) and that nanoscale PFP PCCAs have an inherent deactivation mechanism via immediate recondensation after ADV. This "blinking" on-and-off contrast signal allowed separation of flow in an in vitro flow phantom, regardless of flow conditions, although with a need for some replenishment at very low flow conditions to maintain count rate. This blinking behavior allows for rapid spatial mapping in areas of low or no flow with ULM, but limits velocity tracking because there is no stable bubble formation with nanoscale PFP PCCAs.