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 microstructures. 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 similar to 70% of the bubbles, with pressures similar to 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.