Recondensation of nano-sized perfluorocarbon droplets after acoustic droplet vaporization for blinking ultrasound localization microscopy

Ultrasound localization microscopy (ULM) is a promising super-resolutionimaging technique based on the build-up of subwavelength localizations of gas-filled ultrasound contrast agents (UCAs) within the microvascular space. Slow flow regions can reduce localization rates and diminish the isolation of microbubble signals from tissue. This work compares the blinking contrast dynamics of nano-sized perfluorocarbon droplets to the slowly changing contrast dynamics of traditional gas-filled UCAs. A 16-MHz linear array connected to a Verasonics research ultrasound system was used to capture high-frame rate acquisitions of contrast agents flowing through an in vitro- flow phantom. Nano-sized droplets produced an on and off contrast signal due to vaporization/recondensation events which enhanced microbubble separation from tissue. This allows for higher localization rates for droplets at slow flow speeds, since a new population of bubbles was localized with each ultrasound pulse and was independent of flow speed. This could improve ULM at slow flow speeds, where UCAs require long acquisition times for adequate microbubble movement via flow to produce sufficient localization rates. Preliminary in-vivo experiments comparing UCAs to nano-sized droplets for contrast-enhanced imaging of a mouse kidney will also be discussed. [This work was supported by NIH grant R21 HD097485.]