In vitro characterization of echogenic liposomes by acoustic scattering at 3.5–15.0 MHz

Abstract

Echogenic liposomes (ELIP) are phospholipid vesicles that are being developed as ultrasound contrast agents and as vehicles for drug delivery. These particles, when conjugated with antibodies or peptides, can be used for targeted diagnostic imaging and therapy. Evaluating the acoustic properties of ELIP will allow for optimization of its utility as an ultrasound contrast agent. The diameter of the liposomes ranges from 0.25–10.0 μm. In order to measure the backscattering coefficient in vitro, the backscattered power was compared to that reflected from a perfect reflector, a planar air–water interface. The backscatter coefficient and attenuation of ELIP were evaluated as a function of concentration (0.8–9.9×108/ml) and frequency (3.5–15.0 MHz). By comparing the values of measured backscatter coefficient to a theoretical model that treats the gas within the liposomes as a free air bubble, a size estimate of the encapsulated gas is provided. Finally, calculation of the scattering to attenuation ratio (STAR) gives a value with which to evaluate the efficacy of ELIP as a contrast agent in cardiovascular ultrasound imaging. The backscatter coefficient shows promise as a sensitive method for determining whether the liposomes are left intact or if they are destroyed during imaging.