Impulse response method for characterization of echogenic liposomes
Journal of the Acoustical Society of America , Volume 137 - Issue 4 p. 1693- 1703
An optical characterization method is presented based on the use of the impulse response to characterize the damping imparted by the shell of an air-filled ultrasound contrast agent (UCA). The interfacial shell viscosity was estimated based on the unforced decaying response of individual echogenic liposomes (ELIP) exposed to a broadband acoustic impulse excitation. Radius versus time response was measured optically based on recordings acquired using an ultra-high-speed camera. The method provided an efficient approach that enabled statistical measurements on 106 individual ELIP. A decrease in shell viscosity, from 2.1 × 10<sup>-8</sup> to 2.5 × 10<sup>-9</sup> kg/s, was observed with increasing dilatation rate, from 0.5 × 10<sup>6</sup> to 1 × 10<sup>7</sup> s<sup>-1</sup>. This nonlinear behavior has been reported in other studies of lipid-shelled UCAs and is consistent with rheological shear-thinning. The measured shell viscosity for the ELIP formulation used in this study [κ<inf>s</inf> = (2.1 ± 1.0) × 10<sup>-8</sup> kg/s] was in quantitative agreement with previously reported values on a population of ELIP and is consistent with other lipid-shelled UCAs. The acoustic response of ELIP therefore is similar to other lipid-shelled UCAs despite loading with air instead of perfluorocarbon gas. The methods described here can provide an accurate estimate of the shell viscosity and damping for individual UCA microbubbles.