In vivo ultrasound imaging of macrophages using acoustic vaporization of internalized superheated nanodroplets

Abstract

Ultrasound (US) not only detects but also interacts with its contrast agents, potentially allowing the use of perfluorocarbon-filled nanodroplets (NDs) or microbubbles (MBs) for both the detection and treatment of deep-seated tumors. Cells labeled using MBs to allow their detection in vivo with clinical ultrasound scanner presents several advantages over PET and MRI: Ultrasound is 2–3 orders of magnitude more sensitive and can detect a single acoustically labeled cell in vivo. Furthermore, with high ultrasound intensities, drugs or genes that are loaded on NDs or MBs can be delivered with spatiotemporal control. Although the acoustic labeling of stem cells for in vivo imaging using MBs has been reported, the ability to load and image internalized low boiling point NDs has not been explored yet. We used perfluorobutane NDs to acoustically label macrophages, which were viable following post internalization and ultrasound-mediated phase change. We documented that internalized NDs are stable for at least 8 h, which is needed for macrophages to accumulate to diseased sites. Labeled macrophage accumulated in the liver of healthy rats as stable intracellular NDs were vaporized into MBs using a clinical scanner (Siemens ACUSON Sequoia, 10L4 transducer at 4 MHz, 1.4 MI) and remained visible after vaporization.