Direct video-microscopic observation of the dynamic effects of medical ultrasound on ultrasound contrast microspheres

Ultrasound Contrast: Gas Microbubbles in the Vasculature

Gas-filled microbubbles are currently in clinical use as blood pool contrast agents for ultrasound imaging. The goal of this review is to discuss the trends and issues related to these relatively unusual intravascular materials, which are not small molecules per se, not polymers, not even nanoparticles, but larger micrometer size structures, compressible, flexible, elastic, and deformable. The intent is to connect current research and initial studies from 2 to 3 decades ago, tied to gas exchange between the bubbles and surrounding biological medium, in the following areas of focus: (1) parameters of microbubble movement in relation to vasculature specifics; (2) gas uptake and loss from the bubbles in the vasculature; (3) adhesion of microbubbles to target receptors in the vasculature; and (4) microbubble interaction with the surrounding vessels and tissues during insonation.Microbubbles are generally safe and require orders of magnitude lower material doses than x-ray and magnetic resonance imaging contrast agents. Application of microbubbles will soon extend beyond blood pool contrast and tissue perfusion imaging. Microbubbles can probe molecular and cellular biomarkers of disease by targeted contrast ultrasound imaging. This approach is now in clinical trials, for example, with the aim to detect and delineate tumor nodes in prostate, breast, and ovarian cancer. Imaging of inflammation, ischemia-reperfusion injury, and ischemic memory is also feasible. More importantly, intravascular microbubbles can be used for local deposition of focused ultrasound energy to enhance drug and gene delivery to cells and tissues, across endothelial barrier, especially blood-brain barrier.Overall, microbubble behavior, stability and in vivo lifetime, bioeffects upon the action of ultrasound and resulting enhancement of drug and gene delivery, as well as targeted imaging are critically dependent on the events of gas exchange between the bubbles and surrounding media, as outlined in this review.

Quantitative ultrasound molecular imaging

Ultrasound molecular imaging using targeting microbubbles is predominantly a semi-quantitative tool, thus limiting its potential diagnostic power and clinical applications. In the work described here, we developed a novel method for acoustic quantification of molecular expression. E-Selectin expression in the mouse heart was induced by lipopolysaccharide. Real-time ultrasound imaging of E-selectin expression in the heart was performed using E-selectin-targeting microbubbles and a clinical ultrasound scanner in contrast pulse sequencing mode at 14 MHz, with a mechanical index of 0.22-0.26. The level of E-selectin expression was quantified using a novel time-signal intensity curve analytical method based on bubble elimination, which consisted of curve-fitting the bi-exponential equation [Formula: see text] to the elimination phase of the myocardial time-signal intensity curve. Ar and Af represent the maximum signal intensities of the retained and freely circulating bubbles in the myocardium, respectively; λr and λf represent the elimination rate constants of the retained and freely circulating bubbles in the myocardium, respectively. Ar correlated strongly with the level of E-selectin expression (|r|>0.8), determined using reverse transcriptase real-time quantitative polymerase chain reaction, and the duration of post-lipopolysaccharide treatment-both linearly related to cell surface E-selectin protein (actual bubble target) concentration in the expression range imaged. Compared with a conventional acoustic quantification method (which used retained bubble signal intensity at 20 min post-bubble injection), this new approach exhibited greater dynamic range and sensitivity and was able to simultaneously quantify other useful characteristics (e.g., the microbubble half-life). In conclusion, quantitative determination of the level of molecular expression is feasible acoustically using a time-signal intensity curve analytical method based on bubble elimination.

Quantitative in vivo imaging of embryonic development: opportunities and challenges

Animal models are critically important for a mechanistic understanding of embryonic morphogenesis. For decades, visualizing these rapid and complex multidimensional events has relied on projection images and thin section reconstructions. While much insight has been gained, fixed tissue specimens offer limited information on dynamic processes that are essential for tissue assembly and organ patterning. Quantitative imaging is required to unlock the important basic science and clinically relevant secrets that remain hidden. Recent advances in live imaging technology have enabled quantitative longitudinal analysis of embryonic morphogenesis at multiple length and time scales. Four different imaging modalities are currently being used to monitor embryonic morphogenesis: optical, ultrasound, magnetic resonance imaging (MRI), and micro-computed tomography (micro-CT). Each has its advantages and limitations with respect to spatial resolution, depth of field, scanning speed, and tissue contrast. In addition, new processing tools have been developed to enhance live imaging capabilities. In this review, we analyze each type of imaging source and its use in quantitative study of embryonic morphogenesis in small animal models. We describe the physics behind their function, identify some examples in which the modality has revealed new quantitative insights, and then conclude with a discussion of new research directions with live imaging.

Production of uniformly sized serum albumin and dextrose microbubbles

Uniformly-sized preparations with average microbubble (MB) diameters from 1 to 7 μm were produced reliably by sonicating decafluorobutane-saturated solutions of serum albumin and dextrose. Detailed protocols for producing and size-separating the MBs are presented, along with the effects that changing each production parameter (serum albumin concentration, sonication power, sonication time, etc.) had on MB size distribution and acoustic stability. These protocols can be used to produce MBs for experimental applications or serve as templates for developing new protocols that yield MBs with physical and acoustic properties better suited to specific applications. Size stability and ultrasonic performance quality control tests were developed to assure that successive MB preparations perform identically and to distinguish the physical and acoustic properties of identically sized MBs produced with different serum albumin-dextrose formulations and sonication parameters. MBs can be stored at 5 °C for protracted periods (2 weeks to one year depending on formulation).

Acoustic radiation force on a spherical contrast agent shell near a vessel porous wall--theory

Contrast agent microshells (CAMSs) are under intensive investigation for their wide applications in biomedical imaging and drug delivery. In drug delivery applications, CAMSs are guided to the targeted site before fragmentation by high-intensity ultrasound waves leading to the drug release. Prediction of the acoustic radiation force used to nondestructively guide a CAMS to the suspected site is becoming increasingly important and gaining attention particularly because it increases the system efficiency. The goal of this work is to present a theoretical model for the time-averaged (static) acoustic radiation force experienced by a CAMS near a blood vessel wall. An exact solution for the scattering of normal incident plane acoustic waves on an air-filled elastic spherical shell immersed in a nonviscous fluid near a porous and nonrigid boundary is employed to evaluate the radiation force function (which is the radiation force per unit energy density per unit cross-sectional surface). A particular example is chosen to illustrate the behavior of the time-averaged (static) radiation force on an elastic polyethylene spherical shell near a porous wall, with particular emphasis on the relative thickness of the shell and the distance from its center to the wall. This proposed model allows obtaining a priori information on the static radiation force that may be used to advantage in related as drug delivery and contrast agent imaging. This study should assist in the development of improved models for the evaluation of the time-averaged acoustic radiation force on a cluster of CAMSs in viscous and heat-conducting fluids.

Influence of the bubble-bubble interaction on destruction of encapsulated microbubbles under ultrasound

Influence of the bubble-bubble interaction on the pulsation of encapsulated microbubbles has been studied by numerical simulations under the condition of the experiment reported by Chang et al. [IEEE Trans. Ultrason Ferroelectr. Freq. Control 48, 161 (2001)]. It has been shown that the natural (resonance) frequency of a microbubble decreases considerably as the microbubble concentration increases to relatively high concentrations. At some concentration, the natural frequency may coincide with the driving frequency. Microbubble pulsation becomes milder as the microbubble concentration increases except at around the resonance condition due to the stronger bubble-bubble interaction. This may be one of the reasons why the threshold of acoustic pressure for destruction of an encapsulated microbubble increases as the microbubble concentration increases. A theoretical model for destruction has been proposed.

Efficacy of perflubutane microbubble-enhanced ultrasound in the characterization and detection of focal liver lesions: phase 3 multicenter clinical trial

OBJECTIVE

The purpose of this study was to assess the efficacy and safety of contrast-enhanced ultrasound performed with perflubutane microbubbles in comparison with unenhanced ultrasound and dynamic CT in the characterization of focal liver lesions during the vascular phase of imaging and in the detection of lesions during the Kupffer phase.

SUBJECTS AND METHODS

A total of 196 patients were enrolled at 15 centers in Japan. Vascular phase images were obtained before contrast injection until 1 minute after injection. Kupffer phase images were obtained 10 minutes after injection. Dual-phase CT was performed as determined by standard clinical practice at each center. Unenhanced ultrasound, contrast-enhanced ultrasound, and CT images were read by blinded reviewers, and the results they reached regarding characterization and detection were compared with reference standard findings made by onsite investigators. The safety observation period was 72 hours after contrast administration.

RESULTS

Among the 190 patients included in the characterization analysis, the accuracy of contrast-enhanced ultrasound (88.9%) was significantly greater than that of unenhanced ultrasound (68.4%) and dynamic CT (80.5%) (p < 0.001 and p = 0.008). Among the 191 patients in the detection analysis, the efficacy of contrast-enhanced ultrasound in detection of lesions was significantly higher than that of unenhanced ultrasound and dynamic CT (p < 0.001 and p = 0.008), predominantly because more metastatic lesions were detected (both p < 0.001). In particular, contrast-enhanced ultrasound was superior to dynamic CT in the detection of metastatic lesions measuring 1 cm or smaller. The incidence of adverse events was 49.2% and that of adverse drug reactions was 10.4%. All adverse drug reactions were mild.

CONCLUSION

Compared with unenhanced ultrasound and dynamic CT, contrast-enhanced ultrasound with perflubutane microbubbles improved diagnostic efficacy in both characterization and detection of focal liver lesions with no serious adverse drug reactions.

Ultrasound contrast microbubbles in imaging and therapy: physical principles and engineering

Microbubble contrast agents and the associated imaging systems have developed over the past 25 years, originating with manually-agitated fluids introduced for intra-coronary injection. Over this period, stabilizing shells and low diffusivity gas materials have been incorporated in microbubbles, extending stability in vitro and in vivo. Simultaneously, the interaction of these small gas bubbles with ultrasonic waves has been extensively studied, resulting in models for oscillation and increasingly sophisticated imaging strategies. Early studies recognized that echoes from microbubbles contained frequencies that are multiples of the microbubble resonance frequency. Although individual microbubble contrast agents cannot be resolved-given that their diameter is on the order of microns-nonlinear echoes from these agents are used to map regions of perfused tissue and to estimate the local microvascular flow rate. Such strategies overcome a fundamental limitation of previous ultrasound blood flow strategies; the previous Doppler-based strategies are insensitive to capillary flow. Further, the insonation of resonant bubbles results in interesting physical phenomena that have been widely studied for use in drug and gene delivery. Ultrasound pressure can enhance gas diffusion, rapidly fragment the agent into a set of smaller bubbles or displace the microbubble to a blood vessel wall. Insonation of a microbubble can also produce liquid jets and local shear stress that alter biological membranes and facilitate transport. In this review, we focus on the physical aspects of these agents, exploring microbubble imaging modes, models for microbubble oscillation and the interaction of the microbubble with the endothelium.

Effects of acoustic insonation parameters on ultrasound contrast agent destruction

Ultrasound contrast agents (UCAs) are used to enhance the acoustic backscattered intensity of blood and thereby assist the assessment of blood perfusion. Characterization of UCA destruction provides important information for the design of contrast-assisted perfusion imaging. High-speed optical observation of single microbubble destruction during acoustic insonation has been performed in previous studies. The results identified that pressure, center frequency and transmission phase have significant effects on the fragmentation threshold. We proposed an acoustic-based experiment method to demonstrate the relationship between different acoustic exposure conditions and the degree of UCA destruction. The method also provides a simple and convenient way to determine the microbubble destruction threshold. The experiments introduced three insonation parameters, including acoustic pressure (0 to 1 MPa), pulse frequency (1, 2.25, 5 and 7.5 MHz) and pulse length (1 to 10 cycles). The term of surviving percentage (SP) was proposed to represent the ratio of UCA backscattered power with and without acoustic insonation. The results showed that the SP decreased with decreasing pulse frequency, but with increasing transmission acoustic pressure and pulse length. In addition, there was an exponential relationship between SP and acoustic pressure, and thus the UCA destruction pressure threshold could be predicted from the fitted exponential curve. The results also show that the degree of UCA destruction was not related to mechanical index (MI). Potential applications of this method include UCA high-resolution destruction/replenishment imaging model, microbubble cavitation, sonoporation in drug delivery and gene therapy.

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.

Doppler mode pulse sequences mitigate glomerular capillary hemorrhage in contrast-aided diagnostic ultrasound of rat kidney

Glomerular capillary hemorrhage (GCH) induced in rat kidney by diagnostic ultrasound involving contrast agent destruction was characterized for different modes to explore possible mitigation strategies. Anesthetized hairless rats were scanned at 2.5 MHz in a water bath with contrast agent infused at 10 microl/kg/minute via tail vein. B mode flash echo imaging (FEI), color Doppler (CD) FEI and realtime Doppler imaging at 1 frame per second were tested, which had image pulse sequences of approximately 0.53 ms, 15.8 ms, and 83.5 ms duration, respectively. Bioeffects endpoints included grossly observed blood-filled tubules, histological evaluation of GCH, and detection of hematuria. B mode FEI for 1 minute induced GCH in 38.6+/-17.1% of glomeruli in histology from the scan plane for a peak rarefactional pressure amplitude (RPA) of 2.6 MPa. The threshold for GCH was approximately 1.5 MPa, confirmed by 10-minute exposure with agent infusion. Paradoxically, CD mode FEI delivered many more pulses but produced less GCH (P < 0.02), and real-time Doppler mode induced only 5.3 +/- 3.8% (P < 0.005). Hematuria results followed the GCH trends. These findings indicate a promising strategy, which is to use relatively slow ramp-up of pulse RPAs in agent-destroying image pulse sequences, for mitigating potential bioeffects in contrastaided diagnostic ultrasound.

Direct observations of ultrasound microbubble contrast agent interaction with the microvessel wall

Many thousands of contrast ultrasound studies have been conducted in clinics around the world. In addition, the microbubbles employed in these examinations are being widely investigated to deliver drugs and genes. Here, for the first time, the oscillation of these microbubbles in small vessels is directly observed and shown to be substantially different than that predicted by previous models and imaged within large fluid volumes. Using pulsed ultrasound with a center frequency of 1 MHz and peak rarefactional pressure of 0.8 or 2.0 MPa, microbubble expansion was significantly reduced when microbubbles were constrained within small vessels in the rat cecum (p<0.05). A model for microbubble oscillation within compliant vessels is presented that accurately predicts oscillation and vessel displacement within small vessels. As a result of the decreased oscillation in small vessels, a large resting microbubble diameter resulting from agent fusion or a high mechanical index was required to bring the agent shell into contact with the endothelium. Also, contact with the endothelium was observed during asymmetrical collapse, not during expansion. These results will be used to improve the design of drug delivery techniques using microbubbles.

Therapeutic effects of paclitaxel-containing ultrasound contrast agents

Drug delivery vehicles that combine ultrasonic and molecular targeting are shown to locally concentrate a drug in a region-of-interest. The drug delivery vehicles, referred to as acoustically active lipospheres (AALs), are microbubbles surrounded by a shell of oil and lipid. In a region limited to the focal area of ultrasound application, circulating AALs are deflected by radiation force to a vessel wall and can subsequently be fragmented. Ligands targeting the alphavbeta3 integrin are conjugated to the AAL shell and increase in vitro binding by 26.5-fold over nontargeted agents. Toxicity assays demonstrate that paclitaxel-containing AALs exert a greater antiproliferative effect after insonation than free paclitaxel at an equivalent concentration. Lastly, ultrasound and molecular targeting are combined to deliver a model drug to the endothelium and interstitium of chorioallantoic membrane vasculature in vivo.

Microbubble contrast agents: targeted ultrasound imaging and ultrasound-assisted drug-delivery applications

The use of microbubble contrast agents for general tissue delineation and perfusion enjoys steady interest in ultrasound imaging. Microbubbles as contrast materials require a small dosage and show excellent detection sensitivity. Targeting ligands on the surface of microbubbles permit the selective accumulation of these particles in the areas of interest, which show an up-regulated level of receptor molecules on vascular endothelium. Selective contrast imaging of inflammation, ischemia-reperfusion injury, angiogenesis, and thrombosis has been achieved in animal models. Ultrasound-assisted drug delivery and activation, performed by combining microbubble agent containing drug substances or coadministered with pharmaceutical agents (including plasmid DNA for transfection), has been achieved in multiple model systems in vitro and in vivo. Ultrasound and microbubbles-based targeted acceleration of the thrombolytic enzyme action already have reached clinical trials. Overall, microbubble targeting and ultrasound-assisted microbubble-based drug-delivery systems will offer a step toward the application of targeted personalized diagnostics and therapy.

High-speed photography during ultrasound illustrates potential therapeutic applications of microbubbles

Ultrasound contrast agents consist of microscopically small encapsulated bubbles that oscillate upon insonification. To enhance diagnostic ultrasound imaging techniques and to explore therapeutic applications, these medical microbubbles have been studied with the aid of high-speed photography. We filmed medical microbubbles at higher frame rates than the ultrasonic frequency transmitted. Microbubbles with thin lipid shells have been observed to act as microsyringes during one single ultrasonic cycle. This jetting phenomenon presumably causes sonoporation. Furthermore, we observed that the gas content can be forced out of albumin-encapsulated microbubbles. These free bubbles have been observed to jet, too. It is concluded that microbubbles might act as a vehicle to carry a drug in gas phase to a region of interest, where it has to be released by diagnostic ultrasound. This opens up a whole new area of potential applications of diagnostic ultrasound related to targeted imaging and therapeutic delivery of drugs such as nitric oxide.

Ultrasound-microbubble-induced neovascularization in mouse skeletal muscle

Ultrasound-microbubble (US-MB) interactions stimulate neovascularization in rat gracilis muscle (GM). We examined microvascular remodeling (MVR) in GMs of C57BL/6 and balb/C mice following ultrasonic MB destruction. A range of MB dosages were administered IV, and exposed GMs received US. Muscles harvested 3, 7 and 14 d posttreatment were stained for vascular markers and assessed for changes in microvessel number, diameter and length. Muscles receiving a low MB dose (LMBD) and US showed significant increases in microvascular density after 3 d, returning to sham levels after one week. A MB dose producing maximum capillary disruptions was then established. This high MB dose (HMBD) facilitated significant MVR in C57BL/6 mice after one week. Balb/C GMs exhibited neovascularization 3 d, but not 7 or 14 d, following US-HMBD treatment. We conclude that HMBD in C57BL/6 mice induces a more sustained neovascularization response compared to balb/C or LMBD-treated C57BL/6 muscles; however, this response is still impermanent.

A method for radiation-force localized drug delivery using gas-filled lipospheres

We have developed a method using ultrasound and acoustically active lipospheres (AALs) that might be used to deliver bioactive substances to the vascular endothelium. The AALs consist of a small gas bubble surrounded by a thick oil shell and enclosed by an outermost lipid layer. The AALs are similar to ultrasound contrast agents: they can be nondestructively deflected using ultrasound radiation force, and fragmented with high-intensity ultrasound pulses. The lipid-oil complex might be used to carry bioactive substances at high concentrations. An optimized sequence of ultrasound pulses can deflect the AALs toward a vessel wall then disrupt them, painting their contents across the vascular endothelium. This paper presents results from a series of in vitro and ex vivo experiments demonstrating localization of a fluorescent model drug. In experiments using a human melanoma cell (A2085) monolayer, a specific radiation force-fragmentation ultrasound pulse sequence increased cell fluorescence more than 10-fold over no ultrasound or fragmentation pulses alone, and by 50% over radiation force pulses alone. We observe that dye transfer is limited to cells that are in the region of ultrasonic focus, indicating that the application of radiation force pulses to bring the delivery vehicle into proximity with the cell is required for successful adhesion of the vehicle fragments to the cell membrane. We also demonstrate dye transfer from flowing AALs, both in a mimetic vessel and in excised rat cecum. We believe that this method could be successfully used for drug delivery in vivo.

[Ultrasound contrast agents]

Ultrasound contrast agents consist of tiny gas bubbles encapsulated by a stabilising membrane or shell. When combined with recent contrast-specific ultrasound techniques, they provide substantial enhancement of vessels and solid organs. The clinical use and the diagnostic value of ultrasound contrast agents are in principle comparable to those of contrast agents for CT and MRI. They add an additional dimension of information to sonography, which results in considerable improvement of diagnostic accuracy in many cases. This paper reviews the physicochemical properties of various microbubble contrast agents, discusses non-linear bubble behaviour and contrast-specific imaging techniques. An overview of the most important radiological clinical applications in the liver, kidney and spleen is given.

Microbubble ultrasound contrast agents: a review

The superior scattering properties of gas bubbles compared with blood cells have made microbubble ultrasound contrast agents important tools in ultrasound diagnosis. Over the past 2 years they have become the focus of a wide and rapidly expanding field of research, with their benefits being repeatedly demonstrated, both in ultrasound image enhancement, and more recently in drug and gene delivery applications. However, despite considerable investigation, their behaviour is by no means fully understood and, while no definite evidence of harmful effects has been obtained, there remain some concerns as to their safety. In this review the existing theoretical and experimental evidence is examined in order to clarify the extent to which contrast agents are currently understood and to identify areas for future research. In particular the disparity between the conditions considered in theoretical models and those encountered both in vitro, and more importantly in vivo is discussed, together with the controversy regarding the risk of harmful bio-effects.

New method of sentinel node identification with ultrasonography using albumin as contrast agent: a study in pigs

The purpose of our study was to verify in animals the possibility of using albumin-enhanced ultrasonography as a modality for sentinel node detection. The nine pigs were injected subcutaneously in the neck with albumin, five with 5% solution and four with 25% solution, and then the regional lymph nodes were observed over time. It was found that, where the 5% solution had been injected, the lymph nodes showed no change, but where the 25% solution had been used, a high echo 1 to 5 mm in size was seen at the hilus of the nearest lymph node. Examination of the excised pathologic specimens of lymph nodes demonstrated that this echo was due to albumin accumulated in the efferent lymphatics. This finding suggested that this technique of ultrasonography using albumin as a contrast agent was an effective new method of identifying sentinel nodes.

In vitro acoustic characterisation of four intravenous ultrasonic contrast agents at 30 MHz

The acoustic properties of four ultrasonic contrast agents (Optison, Definity, SonoVue and Sonazoid) were studied at 30 MHz using a Boston Scientific ClearView Ultra intravascular ultrasound (US) scanner modified to allow access to the unprocessed US data. A range of contrast agent concentrations were studied using either saline or glucose as the diluent of choice. Mean backscatter power was measured over regions-of-interest (ROI) at distances of 1, 1.5, 2, 3, 4 and 5 mm from the centre of the intravascular probe and normalised to the US data collected from a standard glass reflector. For all of the agents, the mean backscatter power at 30 MHz varied in a linear manner with concentration between 0.01 million microbubbles/mL and 1 million microbubbles/mL. Furthermore, for two of the agents, mean backscatter enhancement was detectable at concentrations as low as 2 microbubbles/sample volume.

Thresholds for inertial cavitation in albunex suspensions under pulsed ultrasound conditions

Stabilized microbubbles used as echo-contrast agents can be destroyed by ultrasonic irradiation. We have identified two pressure thresholds at which these microbubbles undergo inertial cavitation (here, defined as the collapse of gas bubbles followed by emission of an acoustic broadband noise). The first threshold (P1) corresponds to the pressure at which all the microbubbles in a cavitation field lose their property as an effective scatterer because of fragmentation or deflation. The second threshold (P2) is associated with the acoustic reactivation of the remnants of the contrast agents and is related to the onset of more violent inertial cavitation. P1 and P2 were measured as a function of the concentration of Albunex (Molecular Biosystems Inc., San Diego, CA) contrast agent, the number of transmitting acoustic cycles, and the pulse repetition frequency (PRF). The ultrasound frequency used was 1.1 MHz, and the peak negative acoustic pressures ranged from 0 to 8 MPa. Our results, measured in Isoton II (Coulter Diagnostics, Miami, FL) and whole blood solutions, showed that P1 increased with increasing Albunex concentration and decreased with increasing PRF, whereas P2 decreased with increasing Albunex concentration and was independent of the PRF. Both P1 and P2 decreased with increasing number of acoustic cycles N for N < 10 and were independent of the number of cycles for N > 10. Ultrasound images of Albunex acquired by a commercial scanner showed echo enhancement not only at pressure levels below P1 but also at levels above P2. The threshold P2 was achieved at ultrasound energies above the diagnostic level. Inertial cavitation produced at P2 was associated with a higher level of hemolysis compared with P1. The results of this investigation have potential significance for both diagnostic and therapeutic ultrasound applications.

Influence of bubble size distribution on the echogenicity of ultrasound contrast agents: a study of SonoVue

RATIONALE AND OBJECTIVES

To study the relative contributions of different bubble size classes to SonoVue's echogenicity in fundamental acoustic imaging modes. SonoVue is a contrast agent, previously known as BR1, with a bubble size distribution extending from approximately 0.7 to 10 microm.

METHODS

A model for the acoustic response of SonoVue was determined and validated for a set of experimental data. This model was used to simulate the acoustic response of a standard batch of SonoVue as the sum of responses of non-overlapping bubble size classes.

RESULTS

The simulation was first validated for a standard SonoVue bubble size distribution. When this distribution was considered as five size classes with equal numbers of bubbles, it was found that bubbles smaller than 2 microm accounted for 60% of the total number but contained only 5% of the total gas volume. The simulation results indicated marked differences in the acoustic contributions from these classes, with 80% of the acoustic efficacy provided by bubbles 3 to 9 microm in diameter. The study also compared bubble distributions in number, surface, and volume, with the distribution computed in terms of acoustic efficacy.

CONCLUSIONS

This study shows why bubble volume is a much better indicator of SonoVue's efficacy than is bubble count. A low threshold in diameter was found for SonoVue microbubbles at approximately 2 microm, under which size bubbles do not contribute appreciably to the echogenicity at medical ultrasound frequencies.

B-mode enhancement at phase-inversion US with air-based microbubble contrast agent: initial experience in humans

Pulse- or phase-inversion ultrasonography (US) sums the signals returned from two 180 degrees ultrasound pulses. Linear scattering from tissue results in a signal void while nonlinear signals from microbubbles stand out. The technique was applied with a US contrast agent in 39 human subjects. B-mode enhancement of vessels and organ parenchyma was seen in all cases. Enhancement occurred from flowing and stationary microbubbles. The flow-independent enhancement of normal and abnormal tissue represents a major advance in contrast material-enhanced US with many potential applications especially in tumor imaging.

Quantification of microbubble destruction of three fluorocarbon-filled ultrasonic contrast agents

The assessment of myocardial blood velocity using ultrasonic contrast agents is based on the premise that the vast majority of contrast microbubbles within a myocardial region can be destroyed by an acoustic pulse of sufficient magnitude. Determination of the period of time after destruction that a region of myocardium needs to reperfuse may be used to assess myocardial blood velocity. In this study, we investigated the acoustic pressure sensitivity of three solutions of intravenous fluorocarbon-filled contrast agents and the magnitude of acoustic pulse required to destroy the contrast agent microbubbles. A novel tissue-mimicking phantom was designed and manufactured to investigate the relationships between mean integrated backscatter, incident acoustic pressure and number of frames of insonation for three fluorocarbon-filled contrast agents (Definity(R), Optison(R), and Sonazoid(R), formerly NC100100). Using a routine clinical ultrasound (US) scanner (Acuson XP-10), modified to allow access to the unprocessed US data, the contrast agents were scanned at the four acoustic output powers. All three agents initially demonstrated a linear relationship between mean integrated backscatter and number of frames of insonation. For all three agents, mean integrated backscatter decreased more rapidly at higher acoustic pressures, suggesting a more rapid destruction of the microbubbles. In spite of the fact that there was no movement of microbubbles into or out of the beam, only the results from Definity(R) suggested that a complete destruction of the contrast agent microbubbles had occurred within the total duration of insonation in this study.

Contrast media research

This selective review highlights research in contrast media development and application in the field of diagnostic radiology in 1998 and 1999. The focus is on research published in Investigative Radiology, supplemented with work from other publications in the few areas not extensively covered by the journal. Studies continue to be performed, although at a low level, examining safety issues. Most preclinical investigations have focused on MR and ultrasound agents. In MR, the research effort is concentrated on the development of targeted agents; in ultrasound, work is focused on the characterization of basic contrast mechanisms. The demonstration of clinical applications is still dominated by work with MR, both in disease models and human investigations. The use of extracellular gadolinium chelates to enhance visualization of blood vessels (the field of contrast-enhanced MR angiography) is the largest single new clinical application of contrast media to emerge in several years. New clinical applications continue to be pursued with contrast media in CT, ultrasound, and x-ray angiography. As intravenously injected ultrasound contrast agents come to market, trials demonstrating clinical applications and subsequent scientific publications will increase in number.