Role of lipid coating properties on nonlinear microbubble dynamics

Abstract

Although nonlinear microbubble dynamics are exploited in many contrast-enhanced ultrasound imaging modalities, the role of lipid coating properties on nonlinear dynamics is not fully understood. Our studies show that microbubble vibrations differ between bubbles with homogeneous lipid distribution (main component DPPC, C16) and bubbles with heterogeneous lipid distribution (main component DSPC, C18). Relative to DPPC microbubbles, DSPC microbubbles were shown to elicit more second harmonic emissions, but exhibit a lower propensity to initiate subharmonic emissions. This suggests that homogeneous microstructures favor subharmonic emissions, whereas heterogeneous microstructures produce higher second harmonic emissions. Additionally, the nonlinear “compression-only” oscillation behavior is hypothesized and theoretically shown to relate to buckling of the lipid coating. High-speed fluorescence recordings revealed for the first time formation of hot spots (i.e., high local concentrations of lipids) in the coating of oscillating microbubbles during the compression phase, suggesting evidence of buckling/folding of the coating. However, hot spots were not correlated to compression-only behavior, but were related to the level of compression of the microbubbles during insonification, with a relative compression threshold of 15–20%. This suggests that compression-only behavior is not directly related to buckling of the lipid coating. This work sheds insight into the role of lipid coating properties on nonlinear microbubble behavior.