Optical and acoustical dynamics of microbubble contrast agents inside neutrophils

Acoustically active microbubbles are used for contrast-enhanced ultrasound assessment of organ perfusion. In regions of inflammation, contrast agents are captured and phagocytosed by activated neutrophils adherent to the venu lar wall. Using direct optical observation with a high-speed camera and aco ustical interrogation of individual bubbles and cells, we assessed the phys ical and acoustical responses of both phagocytosed and free microbubbles. O ptical analysis of bubble radial oscillations during insonation demonstrate d that phagocytosed microbubbles experience viscous damping within the cyto plasm and yet remain acoustically active and capable of large volumetric os cillations during an acoustic pulse. Fitting a modified Version of the Rayl eigh-Plesset equation that describes mechanical properties of thin shells t o optical radius-time data of oscillating bubbles provided estimates of the apparent viscosity of the intracellular medium. Phagocytosed microbubbles experienced a viscous damping approximately sevenfold greater than free mic robubbles. Acoustical comparison between free and phagocytosed microbubbles indicated that phagocytosed microbubbles produce an echo with a higher mea n frequency than free microbubbles in response to a rarefaction-first singl e-cycle pulse. Moreover, this frequency increase is predicted using the mod ified Rayleigh-Plesset equation. We conclude that contrast-enhanced ultraso und can detect distinct acoustic signals from microbubbles inside of neutro phils and may provide a unique tool to identify activated neutrophils at si tes of inflammation.