Destruction of contrast agent microbubbles in the ultrasound field: the fate of the microbubble shell and the importance of the bubble gas content

A Voltage-Sensitive Ultrasound Enhancing Agent for Myocardial Perfusion Imaging in a Rat Model

Echocardiographers with specialized expertise sometimes perform myocardial perfusion imaging using U.S. Food and Drug Administration-approved microbubbles in an off-label capacity, correlating microbubble replenishment in the near field with blood flow through the myocardium. This study reports the in vivo clinical feasibility of a voltage-sensitive ultrasound enhancing agent (UEA) for myocardial perfusion imaging. Four UEAs were injected into Sprague-Dawley rats while ultrasound images were collected to quantify brightness in the left ventricular (LV) cavity, septal wall, and posterior wall in systole and diastole. Formulation IV, a phase change agent nested within a negatively charged phospholipid bilayer, increased the tissue-to-cavity ratio in both systole and diastole in the septal wall, 6 dB, and in the posterior wall, 5 dB, while leaving the LV cavity at baseline. This outcome improves the signal of the myocardium relative to the LV cavity and shows promise as a myocardial perfusion UEA.

Effect of Bubble Concentration on the in Vitro and in Vivo Performance of Highly Stable Lipid Shell-Stabilized Micro- and Nanoscale Ultrasound Contrast Agents

Ultrasound (US) is a widely used diagnostic imaging tool because it is inexpensive, safe, portable, and broadly accessible. Ultrasound contrast agents (UCAs) are employed to enhance backscatter echo and improve imaging contrast. The most frequently utilized UCAs are echogenic bubbles made with a phospholipid or protein-stabilized hydrophobic gas core. While clinically utilized, applications of UCAs are often limited by rapid signal decay (<5 min) in vivo under typical ultrasound imaging protocols. Here, we report on a formulation of lipid shell-stabilized perfluoropropane (C₃F₈) microbubbles and nanobubbles with a significantly prolonged in vivo stability. Microbubbles (875 ± 280 nm) of the target size were prepared by utilizing a multiple-step centrifugation cycle, while nanobubbles (299 ± 189 nm) were isolated from the activated vial using a single centrifugation step. To provide in-depth acoustic characterization of the new construct we evaluated the effect of size and concentration on their in vitro and in vivo performance. In vitro and in vivo characterization were carried out for a range of bubble concentrations normalized by total gas volume quantified via headspace gas chromatography/mass spectrometry (GC/MS). In vitro characterization revealed that nanobubbles at different concentrations are more consistently stable over time with the highest and lowest dilutions (50-fold decrease) only differing in US signal after 8 min exposure by 10.34%, while for microbubbles the difference was 86.46%. As expected, due to the difference in hydrodynamic diameter and scattering cross section difference, nanobubbles showed lower overall initial signal intensity. In vivo experiments showed that both microbubbles and nanobubbles with similar initial peak signal intensity are comparably stable over time with 66.8% and 60.6% remaining signal after 30 min, respectively. This study demonstrates that bubble concentration has significant effects on the persistence of both microbubbles and nanobubbles in vitro and in vivo, but the effects are more pronounced in larger bubbles. These effects should be taken into account when selecting the appropriate bubble parameters for future imaging applications.

Cryo-EM Visualization of Lipid and Polymer-Stabilized Perfluorocarbon Gas Nanobubbles - A Step Towards Nanobubble Mediated Drug Delivery

Gas microbubbles stabilized with lipids, surfactants, proteins and/or polymers are widely used clinically as ultrasound contrast agents. Because of their large 1-10 µm size, applications of microbubbles are confined to the blood vessels. Accordingly, there is much interest in generating nanoscale echogenic bubbles (nanobubbles), which can enable new uses of ultrasound contrast agents in molecular imaging and drug delivery, particularly for cancer applications. While the interactions of microbubbles with ultrasound have been widely investigated, little is known about the activity of nanobubbles under ultrasound exposure. In this work, we demonstrate that cryo-electron microscopy (cryo-EM) can be used to image nanoscale lipid and polymer-stabilized perfluorocarbon gas bubbles before and after their destruction with high intensity ultrasound. In addition, cryo-EM can be used to observe electron-beam induced dissipation of nanobubble encapsulated perfluorocarbon gas.

The effect of lipid monolayer in-plane rigidity on in vivo microbubble circulation persistence

The goal of this study was to increase in vivo microbubble circulation persistence for applications in medical imaging and targeted drug delivery. Our approach was to investigate the effect of lipid monolayer in-plane rigidity to reduce the rate of microbubble dissolution, while holding constant the microbubble size, concentration and surface architecture. We first estimated the impact of acyl chain length of the main diacyl phosphatidylcholine (PC) lipid and inter-lipid distance on the cohesive surface energy and, based on these results, we hypothesized that microbubble stability and in vivo ultrasound contrast persistence would increase monotonically with increasing acyl chain length. We therefore measured microbubble in vitro stability to dilution with and without ultrasound exposure, as well as in vivo ultrasound contrast persistence. All measurements showed a sharp rise in stability between DPPC (C16:0) and DSPC (C18:0), which correlates to the wrinkling transition, signaling the onset of significant surface shear and gas permeation resistance, observed in prior single-bubble dissolution studies. Further evidence for the effect of the wrinkling transition came from an in vitro and in vivo stability comparison of microbubbles coated with pure DPPC with those of lung surfactant extract. Microbubble stability against dilution without ultrasound and in vivo ultrasound contrast persistence showed a monotonic increase with acyl chain length from DSPC to DBPC (C22:0). However, we also observed that stability dropped precipitously for all measurements on further increasing lipid acyl chain length from DBPC to DLiPC (C24:0). This result suggests that hydrophobic mismatch between the main PC lipid and the lipopolymer emulsifier, DSPE-PEG5000, may drive a less stable surface microstructure. Overall, these results support our general hypothesis of the role of in-plane rigidity for increasing the lifetime of microbubble circulation.

Myocardial Contrast Echocardiography in the Evaluation of Hypertensive Heart Disease

Myocardial contrast echocardiography (MCE) has an established role in left ventricular assessment by improving the ventricular opacification and endocardial border definition especially in patients with sub-optimal echocardiographic images. With advances in cardiac ultrasound imaging technology and the development of new contrast agents, the clinical utility of this technique has greatly expanded to include assessment of coronary reperfusion in the setting of acute myocardial infarction, determination of myocardial viability within infarct zones as well as assessment of coronary microcirculation and flow reserve in patients with microvascular coronary disease. Improvements in image quality with intravenous contrast agents can facilitate image acquisition and enhance delineation of regional wall motion abnormalities at peak levels of exercise. Numerous studies have confirmed the clinical utility of contrast enhancement during echocardiographic studies, particularly in patients undergoing stress testing. In this paper, we explore the evidence in support of MCE and its potential clinical applications. Our review aims to summarize (1) the basic principles of myocardial contrast echocardiography including recent advances in the ultrasound technology and contrast agents (2) its clinical applications in the diagnosis of cardiovascular diseases and finally, (3) its potential role in risk stratification and assessment of microvascular perfusion in patients with hypertensive heart disease.

Preparation of suspensions of phospholipid-coated microbubbles by coaxial electrohydrodynamic atomization

The use of phospholipid-coated microbubbles for medical applications is gaining considerable attention. However, the preparation of lipid-coated microbubble suspensions containing the ideal size and size distribution of bubbles still represents a considerable challenge. The most commonly used preparation methods of sonication and mechanical agitation result in the generation of polydisperse microbubbles with diameters ranging from less than 1 microm to greater than 50 microm. Efforts have been made via distinctly different techniques such as microfluidic and electrohydrodynamic bubbling to prepare lipid-coated microbubbles with diameters less than 10 microm and with a narrow size distribution, and recent results have been highly promising. In this paper, we describe a detailed investigation of the latter method that essentially combines liquid and air flow, and an applied electric field to generate microbubbles. A parametric plot was constructed between the air flow rate (Qg) and the lipid suspension flow rate (Ql) to identify suitable flow rate regimes for the preparation of phospholipid-coated microbubbles with a mean diameter of 6.6 microm and a standard deviation of 2.5 microm. The parametric plot has also helped in developing a scaling equation between the bubble diameter and the ratio Qg/Ql. At ambient temperature (22 degrees C), these bubbles were very stable with their size remaining almost unchanged for 160 min. The influence of higher temperatures such as the human body temperature (37 degrees C) on the size and stability of the microbubbles was also explored. It was found that the mean bubble diameter fell rapidly to begin with but then stabilized at 1-2 microm after 20 min.

The application of microbubbles for targeted drug delivery

Interest in microbubbles as vehicles for drug delivery has grown in recent years, due in part to characteristics that make them well suited for this role and in part to the need the for localized delivery of drugs in a number of applications. Microbubbles are inherently small, allowing transvascular passage, they can be functionalized for targeted adhesion, and can be acoustically driven, which facilitates ultrasound detection, production of bioeffects and controlled release of the cargo. This article provides an overview of related microbubble biofluid mechanics and reviews recent developments in the application of microbubbles for targeted drug delivery. Additionally, related advances in non-bubble microparticles for drug delivery are briefly described in the context of targeted adhesion.

Method for microbubble characterization using primary radiation force

Medical ultrasound contrast agents (UCAs) have evolved from straight image enhancers to pathophysiological markers and drug delivery vehicles. However, the exact dynamic behavior of the encapsulated bubbles composing UCAs is still not entirely known. In this article, we propose to characterize full populations of UCAs, by looking at the translational effects of ultrasound radiation force on each bubble in a diluted population. The setup involves a sensitive, fully programmable transmitter/receiver and two unconventional, real-time display modes. Such display modes are used to measure the displacements produced by irradiation at frequencies in the range 2-8 MHz and pressures between 150 kPa and 1.5 MPa. The behavior of individual bubbles freely moving in a water tank is clearly observed, and it is shown that it depends on the bubble physical dimensions as well as on the viscoelastic properties of the encapsulation. A new method also is distilled that estimates the viscoelastic properties of bubble encapsulation by fitting the experimental bubble velocities to values simulated by a numerical model based on the modified Herring equation and the Bjerknes force. The fit results are a shear modulus of 18 MPa and a viscosity of 0.23 Pas for a thermoplastic PVC-AN shell. Phospholipid shell elasticity and friction parameter of the experimental contrast agent are estimated as 0.8 N/m and 1 10(-7) kg/s, respectively (shear modulus of 32 MPa and viscosity of 0.19 Pas, assuming 4-nm shell thickness).

Experimental investigations of nonlinearities and destruction mechanisms of an experimental phospholipid-based ultrasound contrast agent

OBJECTIVES

We sought to characterize the acoustical behavior of the experimental ultrasound contrast agent BR14 by determining the acoustic pressure threshold above which nonlinear oscillation becomes significant and investigating microbubble destruction mechanisms.

MATERIALS AND METHODS

We used a custom-designed in vitro setup to conduct broadband attenuation measurements at 3.5 MHz varying acoustic pressure (range, 50-190 kPa). We also performed granulometric analyses on contrast agent solutions to accurately measure microbubble size distribution and to evaluate insonification effects.

RESULTS

Attenuation did not depend on acoustic pressure less than 100 kPa, indicating this pressure as the threshold for the appearance of microbubble nonlinear behavior. At the lowest excitation amplitude, attenuation increased during insonification, while, at higher excitation levels, the attenuation decreased over time, indicating microbubble destruction. The destruction rate changed with pressure amplitude suggesting different destruction mechanisms, as it was confirmed by granulometric analysis.

CONCLUSIONS

Microbubbles showed a linear behavior until 100 kPa, whereas beyond this value significant nonlinearities occurred. Observed destruction phenomena seem to be mainly due to gas diffusion and bubble fragmentation mechanisms.

Acoustic activation of targeted liquid perfluorocarbon nanoparticles does not compromise endothelial integrity

Perfluorocarbon nanoparticles consisting essentially of liquid perfluoro-octyl bromide (PFOB) core surrounded by a lipid monolayer can serve as highly specific site-targeted contrast and therapeutic agents after binding to cellular biomarkers. Based on previous findings that ultrasound applied at 2 MHz and 1.9 mechanical index (MI) for a 5-min duration dramatically enhances the cellular interaction of targeted PFOB nanoparticles with melanoma cells in vitro without inducing apoptosis or other harmful effects to cells that are targeted, we sought to define mechanisms of interaction and the safety profile of ultrasound used in conjunction with liquid perfluorocarbon nanoparticles for targeted drug delivery, as compared with conventional microbubble ultrasound contrast agents under identical insonification conditions. Cell-culture inserts were used to grow a confluent monolayer of human umbilical vein endothelial cells. Definity in conjunction with continuous wave ultrasound (2.25 MHz for 1 and 5 min) increased the permeability of monolayer by four to six times above the normal, decreased transendothelial electrical resistance (a sign of reduced membrane integrity), and decreased cell viability by approximately 50%. Histological evaluation demonstrated extensive disruptions of cell monolayers. Nanoparticles (both nontargeted and targeted) elicited no changes in these different measures under similar insonification conditions and did not disrupt cell monolayers. We hypothesize that ultrasound facilitates drug transport from the perfluorocarbon nanoparticles not by cavitation-induced effects on cell membrane but rather by direct interaction with the nanoparticles that stimulate lipid exchange and drug delivery.

Influence of lipid shell physicochemical properties on ultrasound-induced microbubble destruction

We present the first study of the effects of monolayer shell physicochemical properties on the destruction of lipid-coated microbubbles during insonification with single, one-cycle pulses at 2.25 MHz and low-duty cycles. Shell cohesiveness was changed by varying phospholipid and emulsifier composition, and shell microstructure was controlled by postproduction processing. Individual microbubbles with initial resting diameters between 1 and 10 microm were isolated and recorded during pulsing with bright-field and fluorescence video microscopy. Microbubble destruction occurred through two modes: acoustic dissolution at 400 and 600 kPa and fragmentation at 800 kPa peak negative pressure. Lipid composition significantly impacted the acoustic dissolution rate, fragmentation propensity, and mechanism of excess lipid shedding. Less cohesive shells resulted in micron-scale or smaller particles of excess lipid material that shed either spontaneously or on the next pulse. Conversely, more cohesive shells resulted in the buildup of shell-associated lipid strands and globular aggregates of several microns in size; the latter showed a significant increase in total shell surface area and lability. Lipid-coated microbubbles were observed to reach a stable size over many pulses at intermediate acoustic pressures. Observations of shell microstructure between pulses allowed interpretation of the state of the shell during oscillation. We briefly discuss the implications of these results for therapeutic and diagnostic applications involving lipid-coated microbubbles as ultrasound contrast agents and drug/gene delivery vehicles.

Porous PLGA microparticles: AI-700, an intravenously administered ultrasound contrast agent for use in echocardiography

The production and characterization of AI-700, an intravenously administered ultrasound contrast agent under investigation for myocardial perfusion echocardiography, are described. The product consists of small, porous microparticles filled with decafluorobutane gas, and formulated as a dry powder. Small scale spray drying studies demonstrated that porous PLGA microparticles could be produced with varying porosity using ammonium bicarbonate as a volatile pore-forming agent. The porous microparticles of AI-700 were created aseptically by spray drying a water-in-oil emulsion containing poly-d,l-lactide-co-glycolide, 1,2-diarachidoyl-sn-glycero-3-phosphocholine, and ammonium bicarbonate using a two-chamber spray dryer. The porous microparticles were further formulated into a dry powder drug product (AI-700) containing decafluorobutane gas and excipients. The dry powder was reconstituted with sterile water prior to evaluation. Microscopy demonstrated that the microparticles were sphere-shaped and internally porous. The microparticles were appropriately sized for intravenous administration, having an average diameter of 2.3 mum. Zeta-potential analysis demonstrated that the microparticles would be expected to be stable post-reconstitution. The microparticles retained encapsulated gas post-reconstitution, had high acoustic potency that was stable over time and were physically stable upon exposure to high-power ultrasound, as used clinically. AI-700 has the characteristics desirable for an intravenously administered ultrasound contrast agent for myocardial perfusion echocardiography.

Different effects of microbubble destruction and translation in Doppler measurements

In flow measurements in which microbubbles are involved, the amplitude and phase of the received echo signal are noticeably influenced by the transmitted ultrasound intensity. Previous studies have shown that, when such intensity is progressively increased, the Doppler spectrum is accordingly distorted, i.e., it is asymmetrically broadened toward the negative frequency side. Such deformation has been attributed to radiation force, which pushes the microbubbles into the sound propagation direction, thus yielding additional phase delays in the received echoes. However, the possible contribution of microbubble destruction to this spectral deformation has not been considered yet. In this paper, this issue is investigated by analyzing the experimental spectra produced by two different types of microbubbles suspended in a moving fluid and insonified in pulsed wave (PW) mode at programmable pulse repetition frequency (PRF) and pressure. Conditions are created in which either the radiation force or the destruction mechanism is expected to be dominant. Effects produced by the two phenomena on the Doppler spectrum are shown to be different. When the PRF is low (2 kHz), so that, according to theoretical simulations, the radiation force effect is negligible, a 26 dB noise floor increase is observed for a 13 dB pressure increment. For a higher PRF (16 kHz), the same pressure increase not only affects the noise floor, but also causes the bubbles to deviate from their original streamlines, yielding a Doppler bandwidth increase by a factor of 5. It is concluded that asymmetrical spectral broadening is mainly due to radiation force, and microbubble destruction mainly results in an increased noise floor without affecting the spectral shape.

Blunt Abdominal Trauma:Does the Use of a Second-Generation Sonographic Contrast Agent Help to Detect Solid Organ Injuries?

OBJECTIVE

The objective of our study was to prospectively evaluate whether a second-generation sonography contrast agent (SonoVue) can improve the conspicuity of solid organ injuries (liver; spleen; or kidney, including adrenal glands) in patients with blunt abdominal trauma.

SUBJECTS AND METHODS

Two hundred ten consecutive hemodynamically stable trauma patients underwent both abdominal sonography and CT at admission. The presence of solid organ injuries and the quality of sonography examinations were recorded. Patients with false-negative sonography findings for solid organ injuries in comparison with CT results underwent control sonography. If a solid organ injury was still undetectable, contrast-enhanced sonography was performed. Findings of admission, control, and contrast-enhanced sonograms were compared with CT results for their ability to depict solid organ injuries. Contrast-enhanced sonography was also performed in patients in whom a vascular injury (pseudoaneurysm) was shown on admission or control CT.

RESULTS

CT findings were positive for 88 solid organ injuries in 71 (34%) of the 210 patients. Admission, control, and contrast-enhanced sonograms had a detection rate for solid organ injury of 40% (35/88), 57% (50/88), and 80% (70/88), respectively. The improvement in the detection rate between control and contrast-enhanced sonography was statistically significant (p = 0.001). After exclusion of low-quality examinations, contrast-enhanced sonography still missed 18% of solid organ injuries. Five vascular liver (n = 1) and spleen (n = 4) injuries (pseudoaneurysms) were detected on CT; all were visible on contrast-enhanced sonography.

CONCLUSION

Contrast-enhanced sonography misses a large percentage of solid organ injuries and cannot be recommended to replace CT in the triage of hemodynamically stable trauma patients. However, contrast-enhanced sonography could play a role in the detection of pseudoaneurysms.

Extra-low acoustic power harmonic images of the liver with perflutren: novel imaging for real-time observation of liver perfusion

OBJECTIVE

The features of images below the extra-low mechanical index level were studied to elucidate a suitable mechanical index level for observing real-time and continuous harmonic images of rabbit livers with VX-2 tumors with the use of perflutren.

METHODS

Eight New Zealand White rabbits, 2 with healthy livers and 6 with VX-2 tumors, were examined by harmonic imaging (1.85 and 3.7 MHz) at a frame rate of 17 Hz under various mechanical index levels.

RESULTS

Real-time enhanced images of the liver were observed continuously in all rabbits. Vascular images were more clearly visualized at the low mechanical index level (mechanical index, 0.18) than at any other level. However, predominant enhanced images of the whole liver were observed only at the extra-low mechanical index level (mechanical index, 0.06). In VX-2 tumors, tumor vessels were shown more clearly at a low acoustic power level than at an extra-low level. The histologically proved area of viable tumor was enhanced as a stain in the tumor nodule at an extra-low mechanical index level.

CONCLUSIONS

Harmonic imaging under extra-low mechanical index levels with perflutren could provide real-time and continuous enhanced images of the liver, which would contribute to improvement of the diagnostic ability of contrast-enhanced sonography in liver diseases.

Microbubble-augmented ultrasound declotting of thrombosed arteriovenous dialysis grafts in dogs

PURPOSE

Transcutaneous low-frequency ultrasound (LFUS) can effectively lyse clots in the presence of microbubbles. This study was designed to test the commercially available human albumin microspheres injectable suspension octafluoropropane formulation, Optison, to establish efficacy and assess US parameters of intensity and wave modes in a canine model of a thrombosed arteriovenous (dialysis) graft.

MATERIALS AND METHODS

Arteriovenous grafts in five dogs were cannulated, temporarily ligated, and thrombosed. Different declotting techniques were randomized to treat nine groups. Control groups involved direct saline (4.5 mL) clot injection in 0.5-1.0-mL increments. One group underwent peripheral intravenous microbubble injection (13.5 mL). Six groups underwent direct incremental clot injection of 4.5 mL of microspheres with LFUS for 30 minutes in 3-5-minute increments with use of various intensity settings in continuous-wave and pulsed-wave (PW) modes. At each increment, angiography was used to grade flow, declotting, and overall success.

RESULTS

One hundred four procedures showed success in all 24 high-intensity PW modes (1.2-2.0 W/cm(2)); only one of 20 control experiments was successful (P <.0001). Medium-intensity modes yielded intermediate success rates. Lowest-intensity direct-injection groups and intravenous and control groups ranked lower. Results at 30 minutes were better than at 15 minutes (P <.0001).

CONCLUSIONS

LFUS with direct injection of microbubbles is effective in lysing moderate-sized clots and recanalizing thrombosed arteriovenous grafts. It best succeeds at the higher range of intensity settings tested in PW mode. Further development is justified.