Acoustical response of DSPC versus DPPC lipid-coated microbubbles

Ultrasound contrast agents are widely used in clinical practice. Commercial lipid-coated microbubbles mainly consist of either DSPC or DPPC. Previous research on homemade microbubbles based on these lipids showed different shell micro-structures. In this study the acoustical behavior and shell properties are characterized by microbubble spectroscopy using the Brandaris 128 high-speed camera. DPPC microbubbles were found to be acoustically less stable. Resonance frequencies of both types were similar and both did not show compression- or expansion-only behavior. DPPC showed more nonlinear behavior, particularly more subharmonics; 10 out of 14 versus 4 out of 15 for DSPC. Moreover, the emitted acoustic pressures at the subharmonic frequency were higher for DPPC and measurable up to 2 cm from the bubble. Second harmonic behavior was similar for both types and present in ~70% of the bubbles, with pressures ~1 Pa. Shell elasticities were 0.17 (± 0.06) N/m for DSPC and 0.06 (± 0.08) N/m for DPPC. Shell viscosities increased with diameter and showed no relation with dilatation rates. Mean shell viscosities were equal at 1.3 · 10^-8 kg/s. The presence of subharmonics and the higher emitted pressures make DPPC microbubbles more useful for contrast-enhanced ultrasound imaging, e.g. carotid imaging.