Ultrasound contrast agents phagocytosed by neutrophils demonstrate acoustic activity

Ultrasound contrast agents are microbubbles composed of a thin lipid or albumin shell filled with air or a high molecular weight gas. These microbubbles are used for contrast-enhanced ultrasound (CEU) assessment of organ perfusion. In regions of inflammation, microbubbles are phagocytosed intact by activated neutrophils adherent to the venular wall. The authors hypothesized that microbubbles remain acoustically active following phagocytosis. Accordingly, they assessed the physical responses of both phagocytosed and free microbubbles by direct microscopic observation during delivery of repetitive single pulses of ultrasound at various acoustic pressures. Insonation results in oscillation in the bubbles volume. Microbubbles were optically recorded during insonation with a high-speed imaging system and diameter-time curves were analyzed to determine the effect of phagocytosis. Phagocytosed microbubbles retained their acoustic activity, although the intracellular environment increased viscoelastic damping experienced by microbubbles. With a pulse of high acoustic intensity (>1 MPa), phagocytosed microbubbles expanded up to 500% of their initial radii, which occasionally resulted in neutrophil rupture. Primary radiation force displaced phagocytosed microbubbles a distance of 100 microns with an acoustic pressure of -240 kPa and a pulse repetition frequency of 10 kHz, thus providing further evidence of acoustic activity. The authors conclude that phagocytosed microbubbles exhibit viscoelastic damping and yet are susceptible to acoustic destruction. They can generate non-linear echoes on the same order of magnitude as free microbubbles. These results indicate that CEU may be used to identify and assess regions of inflammation by detecting acoustic signals from microbubbles that are phagocytosed by activated neutrophils. In addition, the rapid expansion of a microbubble at high acoustic pressure may present a means to rupture a neutrophil or drug capsule at a specific site, resulting in delivery of a drug.