Estimation of damping coefficient based on the impulse response of echogenic liposomes

Abstract

Echogenic liposomes (ELIP) are under development as theragnostic ultrasound contrast agents for the diagnosis and treatment of cardiovascular disease. ELIP formulations have a phospholipid bilayer shell and are echogenic due to the presence of air; however, the exact location of the entrapped air has not been fully ascertained. Air pockets could either be stabilized by lipid monolayers within the liposome, or by the lipid bilayer shell. Our goal is to develop a more complete understanding of the encapsulation and shell properties of ELIP. This study demonstrates a method to estimate the damping coefficient using experimentally measured radius-time curves of the impulse response of individual ELIP using optical methods. The non-dimensional damping coefficient and the natural frequency of oscillation were estimated based on 140 individual impulse responses as measured with the Brandaris 128 fast-framing camera (15 Mfps) at 37°C. The damping coefficient was in agreement with the damping coefficient as measured previously using a broadband acoustic technique [Raymond et al., Ultrasound Med Biol. 40(2), 410–421 (2014)]. However, the natural frequency of oscillation was lower than previously reported.