Acoustic investigation of pressure-dependent resonance and shell elasticity of lipid-coated monodisperse microbubbles

Abstract

In this study, frequency-dependent attenuation was measured acoustically for monodisperse lipidcoated microbubble suspensions as a function of excitation pressure and radius. The resonance frequency was identified from the attenuation spectra and had an inverse relationship with mean microbubble diameter and excitation pressure. A reduction in the estimated shell elasticity constant from 0.50 N/m to 0.29 N/m was observed as the excitation pressure was increased from 25 kPa to 100 kPa, respectively, which suggests a nonlinear relationship exists between lipid shell stiffness and applied strain. These findings support the viewpoint that lipid shells coating microbubbles exist as heterogeneous mixtures that undergo dynamic and rapid variations in mechanical properties under applied strains. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865805] Lipid-coated microbubbles (LCMB) have proven to be very effective contrast agents for diagnostic ultrasound imaging. 1-5 The scattering cross-section of LCMB is several orders of magnitude greater than tissue, resulting in an exceptional contrast-to-tissue ratio in diagnostic ultrasound images. The scattering cross-section of LCMB can be maximized when driven at resonance; thus, determining the resonance frequency of LCMB has been the subject of numerous studies, both theoretical and experimental. 6-10 Theoretical models have shown that the resonance frequency depends upon both the microbubble radius and the viscoelastic properties of the outer shell. Early models of encapsulated microbubbles defined the outer shell as a viscoelastic solid 11-14 or a viscous Newtonian fluid, 15,16 with material properties that were uniform throughout the shell. However, several studies have reported measurements of pressure-dependent attenuation in suspensions of lipid-coated microbubbles, 17-20 suggesting that the shell viscoelastic properties are in fact non-Newtonian. We hypothesized that the pressure-dependent behavior may be due to non-uniform shell viscoelastic properties, which may vary when the microbubbles are driven acoustically. Fluorescent images of lipid-coated microbubbles have shown that the phase of the lipid shell can be spatially homogeneous or heterogeneous, depending upon the length of the lipid hydrocarbon chains. 21,22 Furthermore, documented studies of lipid-coated microbubbles during gradual dissolution (over seconds) have reported that the lipid shell can fold, buckle, and shed as the lipid shell is compressed. 23,24 While informative, it remains unclear if these changes in the shell microstructure will occur during microbubble oscillation at megahertz (MHz) frequencies. Provided these structural changes do occur, they may impact the shell viscoelastic properties, which may be reflected by changes in the frequency response of microbubbles to acoustic excitation.Historically, researchers have combined theoretical predictions and experimental measurements of the frequencydependent attenuation coefficient to estimate the shell viscoelastic properties. 15,25-2...

Focal areas of increased lipid concentration on the coating of microbubbles during short tone-burst ultrasound insonification

“…For another type of DSPC-containing microbubble, 10 μm diameter BR14 microbubbles were resonant at 1 MHz (< 40 kPa insonification) [37]. Although it is known that several factors have an influence on the resonance frequency, such as the applied P_ [15, 52] and composition of the lipid coating [19], our finding that smaller microbubbles were at resonance at 1 MHz could also be explained by the difference in diameter as measured with fluorescence and bright field. Microbubbles appear larger in bright field than in fluorescence, because complex images are obtained in bright field due to refraction, i .…”

Influence of the pressure-dependent resonance frequency on the bifurcation structure and backscattered pressure of ultrasound contrast agents: a numerical investigation

“…The other method could be the use of centrifugation techniques to stack the micobubbles in narrow size ranges [51], then the driving frequency can be chosen so that it drives the majority of each stack into pressure-dependent resonance regime. Through using monodisperse microbubbles [23,52,53], one may fully take advantage of exciting all the microbubbles in the pressuredependent resonance regime.…”

Pressure dependence of the attenuation and sound speed in a bubbly medium: Theory and experiment

“…[28]) where P is pressure and V is the MB volume. Experimental investigation of the pressure and frequency dependence of the attenuation of bubbly media has been limited to few studies of coated MBs suspensions [25,29]. Importantly, the pressure dependence of the sound speed in bubbly media has not been experimentally investigated.…”

“…To experimentally explore the pressure-dependent changes of the sound speed and attenuation for coated MBs, monodisperse lipid shell MBs were produced using flow-focusing in a microfluidic device as previously described [29,37]. Figure 2 shows the size distribution of the MBs in our experiments.…”