Acoustic characterization of echogenic liposomes: Frequency-dependent attenuation and backscatter

Abstract

Ultrasound contrast agents (UCAs) are used clinically to aid detection and diagnosis of abnormal blood flow or perfusion. Characterization of UCAs can aid in the optimization of ultrasound parameters for enhanced image contrast. In this study echogenic liposomes (ELIPs) were characterized acoustically by measuring the frequency-dependent attenuation and backscatter coefficients at frequencies between 3 and 30 MHz using a broadband pulse-echo technique. The experimental methods were initially validated by comparing the attenuation and backscatter coefficients measured from 50-μm and 100-μm polystyrene microspheres with theoretical values. The size distribution of the ELIPs was measured and found to be polydisperse, ranging in size from 40 nm to 6 μm in diameter, with the highest number observed at 65 nm. The ELIP attenuation coefficients ranged from 3.7 ± 1.0 to 8.0 ± 3.3 dB/cm between 3 and 25 MHz. The backscatter coefficients were 0.011 ± 0.006 (cm str)(-1) between 6 and 9 MHz and 0.023 ± 0.006 (cm str)(-1) between 13 and 30 MHz. The measured scattering-to-attenuation ratio ranged from 8% to 22% between 6 and 25 MHz. Thus ELIPs can provide enhanced contrast over a broad range of frequencies and the scattering properties are suitable for various ultrasound imaging applications including diagnostic and intravascular ultrasound.

Impulse response method for characterization of echogenic liposomes

“…21 Previously published transmission electron microscopy images have shown liposomes on the order of 2 lm diameter 24 as well as vesicles in the nanometer size range that could not be resolved using the optical methods described here. 25,26 Atomic force microscopy 20 and fluorescence microscopy studies 26 also indicate a broad size distribution, with particle sizes ranging over three orders of magnitude from tens of nanometers to microns.…”

“…Multilamellar vesicles, in which the lipid monolayer immediately adjacent to the gas bubble is surrounded by one or more bilayers, have been observed in transmission electron microscopy images of ELIP. 25 The spread of the shell viscosity estimates for ELIP of similar size may indicate the presence of multilamellar vesicles, where the apparent viscosity depends on the number of lamallae. Another factor affecting the interpretation of the variation in the shell viscosity estimates is the uncertainty, which can be approximated as the random error associated with separate measurements on the same ELIP.…”

“…21,25 For low dilatation rates (< 0.5 Â 10 -6 s -1 ), the average value of the shell viscosity in this study [j s ¼ (2.1 6 1.0) Â 10 -8 kg/s, n ¼ 23] is in agreement with our previously reported bulk acoustic measurements in the linear regime [j s ¼ (2.0 6 0.14) Â 10 -8 kg/s]. 21 Moreover the shell viscosity estimates derived from damping measurements on 106 individual ELIP in this study indicate an apparent increase of the shell viscosity with increasing radius and a decrease with increasing dilatation rate, indicative of shearthinning behavior.…”

“…21,24,25 Differences in the characteristics of nanoscale versus microscale ELIP vesicles may play a role in the scattering properties of individual ELIP. This study found that smaller ELIP are characterized by a substantially lower shell viscosity, which may contribute to increased nonlinear scattering, especially at higher frequencies.…”

Monodisperse bubbles and droplets for medical applications

“…The ideal contrast agent scatters ultrasound efficiently while it absorbs little energy. Thus, the efficiency is characterized by its scattering to attenuation ratio (STAR) 89,132,[138][139][140][141] .…”