Recent advances in myocardial contrast echocardiography

Interactions of bubbles in acoustic Lichtenberg figure

The evolution of acoustic Lichtenberg figure (ALF) in ultrasound fields is studied using high-speed photography. It is observed that bubbles travel along the branch to the aggregation region of an ALF, promoting the possibility of large bubble or small cluster formation. Large bubbles move away from the aggregation region while surrounding bubbles are attracted into this structure, and a bubble transportation cycle arises in the cavitation field. A simplified model consisting of a spherical cluster and a chain of bubbles is developed to explain this phenomenon. The interaction of the two units is analyzed using a modified expression for the secondary Bjerknes force in this system. The model reveals that clusters can attract bubbles on the chain within a distance of 2 mm, leading to a bubble transportation process from the chain to the bubble cluster. Many factors can affect this process, including the acoustic pressure, frequency, bubble density, and separation distance. The larger the bubble in the cluster, the broader the attraction region. Therefore, the presence of large bubbles might enhance the process in this system. Local disturbances in bubble density could destroy the ALF structure. The predictions of the model are in good agreement with the experimental phenomena.

Diabetic Microvascular Disease: An Endocrine Society Scientific Statement

Both type 1 and type 2 diabetes adversely affect the microvasculature in multiple organs. Our understanding of the genesis of this injury and of potential interventions to prevent, limit, or reverse injury/dysfunction is continuously evolving. This statement reviews biochemical/cellular pathways involved in facilitating and abrogating microvascular injury. The statement summarizes the types of injury/dysfunction that occur in the three classical diabetes microvascular target tissues, the eye, the kidney, and the peripheral nervous system; the statement also reviews information on the effects of diabetes and insulin resistance on the microvasculature of skin, brain, adipose tissue, and cardiac and skeletal muscle. Despite extensive and intensive research, it is disappointing that microvascular complications of diabetes continue to compromise the quantity and quality of life for patients with diabetes. Hopefully, by understanding and building on current research findings, we will discover new approaches for prevention and treatment that will be effective for future generations.

Regulation of muscle microcirculation in health and diabetes

Insulin increases microvascular perfusion and substrate exchange surface area in muscle, which is pivotal for hormone action and substrate exchange, by activating insulin signaling cascade in the endothelial cells to produce nitric oxide. This action of insulin is closely coupled with its metabolic action and type 2 diabetes is associated with both metabolic and microvascular insulin resistance. Muscle microvascular perfusion/volume can be assessed by 1-methylxanthine metabolism, contrast-enhanced ultrasound and positron emission tomography. In addition to insulin, several factors have been shown to recruit muscle microvasculature, including exercise or muscle contraction, mixed meals, glucagon-like peptide 1 and angiotensin II type 1 receptor (AT(1)R) blocker. On the other hand, factors that cause metabolic insulin resistance, such as inflammatory cytokines, free fatty acids, and selective activation of the AT(1)R, are capable of causing microvascular insulin resistance. Therapies targeting microvascular insulin resistance may help prevent or control diabetes and decrease the associated cardiovascular morbidity and mortality.

Systematic method to assess microvascular recruitment using contrast-enhanced ultrasound. Application to insulin-induced capillary recruitment in subjects with T1DM

Contrast-enhanced ultrasound (CEU) is an ultrasound imaging technique used to assess tissue perfusion. Analysis of microvascular recruitment necessitates the definition of a region of interest (ROI) containing exclusively the tissues to be studied. Conventional ROI selection requires examining the images and drawing the ROI by hand, making the analysis of CEU images non-reproducible and analyst-dependent. We have designed a systematic ROI selection method that is both reproducible and analyst-independent. Microvascular blood volume (MBV) assessed in 21 sequences of images was used to correlate the systematic ROI selection method with the conventional method performed by two independent analysts (correlation of 0.88 and 0.87 respectively) and the MBV sample distribution from the systematic method was not significantly different from those obtained from the conventional one. Using the systematic method, we found no significant insulin-induced capillary recruitment in subjects with type 1 diabetes mellitus, which might be related to the observed low glucose uptake during the hyperinsulinemic euglycemic clamp compared to healthy patients.

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.

Determination of postexcitation thresholds for single ultrasound contrast agent microbubbles using double passive cavitation detection

This work presents experimental responses of single ultrasound contrast agents to short, large amplitude pulses, characterized using double passive cavitation detection. In this technique, two matched, focused receive transducers were aligned orthogonally to capture the acoustic response of a microbubble from within the overlapping confocal region. The microbubbles were categorized according to a classification scheme based on the presence or absence of postexcitation signals, which are secondary broadband spikes following the principle oscillatory response of the ultrasound contrast agent and are indicative of the transient collapse of the microbubble. Experiments were conducted varying insonifying frequencies (0.9, 2.8, 4.6, and 7.1 MHz) and peak rarefactional pressures (200 kPa to 6.2 MPa) for two types of contrast agents (Definity and Optison). Results were fit using logistic regression analysis to define pressure thresholds where at least 5% and 50% of the microbubble populations collapsed for each frequency. These thresholds were found to occur at lower pressures for Definity than for Optison over the range of frequencies studied; additionally, the thresholds occurred at lower pressures with lower frequencies for both microbubble types in most cases, though this trend did not follow a mechanical index scaling.

Controllable microfluidic synthesis of multiphase drug-carrying lipospheres for site-targeted therapy

We report the production of micrometer-sized gas-filled lipospheres using digital (droplet-based) microfluidics technology for chemotherapeutic drug delivery. Advantages of on-chip synthesis include a monodisperse size distribution (polydispersity index (sigma) values of <5%) with consistent stability and uniform drug loading. Photolithography techniques are applied to fabricate novel PDMS-based microfluidic devices that feature a combined dual hydrodynamic flow-focusing region and expanding nozzle geometry with a narrow orifice. Spherical vehicles are formed through flow-focusing by the self-assembly of phospholipids to a lipid layer around the gas core, followed by a shear-induced break off at the orifice. The encapsulation of an extra oil layer between the outer lipid shell and inner bubble gaseous core allows the transport of highly hydrophobic and toxic drugs at high concentrations. Doxorubicin (Dox) entrapment is estimated at 15 mg mL(-1) of particles packed in a single ordered layer. In addition, the attachment of targeting ligands to the lipid shell allows for direct vehicle binding to cancer cells. Preliminary acoustic studies of these monodisperse gas lipospheres reveal a highly uniform echo correlation of greater than 95%. The potential exists for localized drug concentration and release with ultrasound energy.

Insulin at physiological concentrations increases microvascular perfusion in human myocardium

Vascular endothelium regulates vascular tone and tissue perfusion in response to various physiological and pathological stimuli. Insulin and meal feeding increase microvascular perfusion and thus oxygen, nutrient, and hormone delivery to human skeletal muscle. Meal feeding also increases cardiac microvascular perfusion in healthy humans. To examine whether insulin at physiological concentrations increases microvascular perfusion in human myocardium, we studied 13 healthy, overnight-fasted, lean, young human volunteers by using myocardial contrast echocardiography (MCE) and insulin-clamp techniques. We measured cardiac microvascular blood volume (MBV), microvascular flow velocity (MFV), and microvascular blood flow (MBF) at baseline, 60 min, and 120 min after initiating insulin infusion at 1 mU.kg(-1).min(-1). MBF is the product of MBV and MFV and represents microvascular perfusion. Insulin increased myocardial MBV by 23% at 60 min (P < 0.01) and by 41% at 120 min (P = 0.001) without changing MFV. As a result, insulin-mediated myocardial MBF increased significantly at both 60 min (P < 0.01) and 120 min (P < 0.0005). Insulin also significantly increased brachial artery diameter, flow velocity, and total blood flow at 60 and 120 min (P < 0.05 for all). The changes in cardiac MBV correlated positively with quantitative insulin sensitivity check index (QUICKI) and negatively with body mass index but not with the steady-state glucose-infusion rates or the changes in brachial artery parameters. We conclude that insulin, at physiologically relevant concentrations, increases microvascular perfusion in human heart muscle by increasing cardiac MBV in healthy, insulin-sensitive adults. This insulin-mediated cardiac microvascular perfusion may play an important role in normal human myocardial oxygen and substrate physiology.

On-chip generation of microbubbles as a practical technology for manufacturing contrast agents for ultrasonic imaging

This paper presents a new manufacturing method to generate monodisperse microbubble contrast agents with polydispersity index (sigma) values of <2% through microfluidic flow-focusing. Micron-sized lipid shell-based perfluorocarbon (PFC) gas microbubbles for use as ultrasound contrast agents were produced using this method. The poly(dimethylsiloxane) (PDMS)-based devices feature expanding nozzle geometry with a 7 microm orifice width, and are robust enough for consistent production of microbubbles with runtimes lasting several hours. With high-speed imaging, we characterized relationships between channel geometry, liquid flow rate Q, and gas pressure P in controlling bubble sizes. By a simple optimization of the channel geometry and Q and P, bubbles with a mean diameter of <5 microm can be obtained, ideal for various ultrasonic imaging applications. This method demonstrates the potential of microfluidics as an efficient means for custom-designing ultrasound contrast agents with precise size distributions, different gas compositions and new shell materials for stabilization, and for future targeted imaging and therapeutic applications.

Efficacy of myocardial contrast echocardiography in the diagnosis and risk stratification of acute coronary syndrome

We examined the hypothesis that myocardial contrast echocardiography (MCE) is superior to conventional electrocardiographic, echocardiographic, and troponin I criteria for the diagnosis of acute coronary syndrome. We prospectively enrolled 114 consecutive patients (60+/-10 years of age, 73 men) who presented to the emergency room with chest pain on exertion and at rest. Exclusion criteria included an age<40 years, presence of Q wave or ST-segment elevation, and a poor echocardiographic window. Echocardiography and MCE were performed to assess regional wall motion abnormalities (RWMAs) and myocardial perfusion defects by using continuous infusion of perfluorocarbon-exposed sonicated dextrose albumin. Acute coronary syndrome was confirmed in 87 patients. There were no deaths; 46 patients had acute myocardial infarction, and 41 patients required urgent revascularization. On multiple logistic regression analysis, myocardial perfusion defect (odd ratio 87, p<0.001) was the only independent variable for diagnosing acute coronary syndrome. Myocardial perfusion defect (odd ratio 21, p=0.001) and troponin I levels (odd ratio 3, p=0.009) were independent predictors for acute myocardial infarction. The sensitivity of myocardial perfusion defect for diagnosing acute coronary syndrome was 77%, which is significantly higher than the sensitivities of ST change, troponin I increase, and RWMA (28%, 34%, and 49%, respectively), with similar specificities of 85% to 96%. In conclusion, MCE is more sensitive than the currently used electrocardiographic and troponin I criteria, and evaluation of myocardial perfusion defect by MCE complements RWMA analysis by conventional echocardiography for accurate diagnosis of acute coronary syndrome.

ACTIVE ROLE FOR THE VASCULATURE IN THE DELIVERY OF INSULIN TO SKELETAL MUSCLE

1. In the 80+ years since insulin's discovery, an enormous amount of literature has accumulated relating to its actions on body fat, glucose and protein metabolism. In particular, skeletal muscle has been extensively studied because of its major role as a site of insulin-mediated glucose disposal. Liver and adipose tissue are two other extensively studied sites of insulin action. Much less investigation has been directed towards delineating insulin's actions on cells other than myocytes, adipocytes and hepatocytes. 2. Over the past 5-10 years it has become increasingly evident that insulin exerts important actions on vascular cells. Here, we review evidence that insulin's action within muscle may be very much regulated by its ability to transit the vasculature to access the interstitial fluid (and hence the myocyte insulin receptor). Surprisingly little is known regarding the regulation of vascular events that first bring insulin to the capillary endothelium within muscle, whence presumably it transits from the vascular to the interstitial space. Recent studies suggest that insulin can increase blood flow and also influence the distribution of blood flow within skeletal muscle, potentially therefore regulating its own delivery to the capillary endothelium. Beyond insulin's ability to access the vascular lumen within skeletal muscle microvasculature lies the issue of its passing the endothelial barrier. Even less is known about the processes involved in insulin's actual transit across the endothelium. Available data do not clearly indicate whether this is a saturable, receptor-mediated process or a passive-diffusion pathway. Also, whether insulin in any manner regulates its own transit across the endothelium or its clearance via the lymphatic system is entirely unknown. 3. The aim of the present review is to identify areas where knowledge is deficient and highlight hypotheses which may lead to a better understanding of the coordinated relationship between insulin's vascular actions within muscle and its metabolic actions in that tissue. Even so, there is now sufficient evidence to indicate that insulin's vascular action within skeletal muscle is a major regulatory locus for its insulin mediated glucose disposal.

The magnitude of radiation force on ultrasound contrast agents

High-speed photography of insonified bubbles with a time resolution of 10 ns allows observations of translation due to radiation force, in addition to the visualization of radial oscillations. A modified version of the Rayleigh-Plesset equation is used to estimate the radius-time behavior of insonified microbubbles, and the accuracy of this model is verified experimentally. The translation of insonified microbubbles is calculated using a differential equation relating the acceleration of the bubble to the forces due to acoustic radiation and the drag imposed by the fluid. Simulations and experiments indicate that microbubbles translate significant distances with clinically relevant parameters. A 1.5 micron radius contrast agent can translate over 5 microns during a single 20-cycle, 2.25 MHz, 380 kPa acoustic pulse, achieving velocities over 0.5 m/s. Therefore, radiation force should be considered during an ultrasonic examination because of the possibility of influencing the position and flow velocity of the contrast agents with the interrogating acoustic beam.

Targeted imaging using ultrasound

The discipline of medical imaging is expanding to include both traditional anatomic modalities and new techniques for the functional assessment of the presence and extent of disease. Current FDA-approved ultrasound contrast agents are micron-sized bubbles with a stabilizing shell. Microbubble contrast agents can be used to estimate microvascular flow rate in a manner similar to dynamic contrast-enhanced magnetic resonance imaging (MRI). The concentration of these agents within the vasculature, reticulo-endothelial, or lymphatic systems produces an effective passive targeting of these areas. Liquid-filled nanoparticles and liposomes have also demonstrated echogenicity and are under evaluation as ultrasound contrast agents. Actively targeted ultrasound relies on specially designed contrast agents to localize the targeted molecular signature or physiologic system. These agents typically remain within the vascular space, and therefore possible targets include molecular markers on thrombus, endothelial cells, and leukocytes. The purpose of this review is to summarize the requirements, challenges, current progress, and future directions of targeted imaging with ultrasound.

Physiologic hyperinsulinemia enhances human skeletal muscle perfusion by capillary recruitment

Despite intensive study, the relation between insulin's action on blood flow and glucose metabolism remains unclear. Insulin-induced changes in microvascular perfusion, independent from effects on total blood flow, could be an important variable contributing to insulin's metabolic action. We hypothesized that modest, physiologic increments in plasma insulin concentration alter microvascular perfusion in human skeletal muscle and that these changes can be assessed using contrast-enhanced ultrasound (CEU), a validated method for quantifying flow by measurement of microvascular blood volume (MBV) and microvascular flow velocity (MFV). In the first protocol, 10 healthy, fasting adults received insulin (0.05 mU. kg(-1). min(-1)) via a brachial artery for 4 h under euglycemic conditions. At baseline and after insulin infusion, MBV and MFV were measured by CEU during continuous intravenous infusion of albumin microbubbles with intermittent harmonic ultrasound imaging of the forearm deep flexor muscles. In the second protocol, 17 healthy, fasting adults received a 4-h infusion of either insulin (0.1 mU. kg(-1). min(-1), n = 9) or saline (n = 8) via a brachial artery. Microvascular volume was assessed in these subjects by an alternate CEU technique using an intra-arterial bolus injection of albumin microbubbles at baseline and after the 4-h infusion. With both protocols, muscle glucose uptake, plasma insulin concentration, and total blood flow to the forearm were measured at each stage. In protocol 2 subjects, tissue extraction of 1-methylxanthine (1-MX) was measured as an index of perfused capillary volume. Caffeine, which produces 1-MX as a metabolite, was administered to these subjects before the study to raise plasma 1-MX levels. In protocol 1 subjects, insulin increased muscle glucose uptake (180%, P < 0.05) and MBV (54%, P < 0.01) and decreased MFV (-42%, P = 0.07) in the absence of significant changes in total forearm blood flow. In protocol 2 subjects, insulin increased glucose uptake (220%, P < 0.01) and microvascular volume (45%, P < 0.05) with an associated moderate increase in total forearm blood flow (P < 0.05). Using forearm 1-MX extraction, we observed a trend, though not significant, toward increasing capillary volume in the insulin-treated subjects. In conclusion, modest physiologic increments in plasma insulin concentration increased microvascular blood volume, indicating altered microvascular perfusion consistent with a mechanism of capillary recruitment. The increases in microvascular (capillary) volume (despite unchanged total blood flow) indicate that the relation between insulin's vascular and metabolic actions cannot be fully understood using measurements of bulk blood flow alone.

Optical and acoustical dynamics of microbubble contrast agents inside neutrophils

Acoustically active microbubbles are used for contrast-enhanced ultrasound assessment of organ perfusion. In regions of inflammation, contrast agents are captured and phagocytosed by activated neutrophils adherent to the venular wall. Using direct optical observation with a high-speed camera and acoustical interrogation of individual bubbles and cells, we assessed the physical and acoustical responses of both phagocytosed and free microbubbles. Optical analysis of bubble radial oscillations during insonation demonstrated that phagocytosed microbubbles experience viscous damping within the cytoplasm and yet remain acoustically active and capable of large volumetric oscillations during an acoustic pulse. Fitting a modified version of the Rayleigh-Plesset equation that describes mechanical properties of thin shells to optical radius-time data of oscillating bubbles provided estimates of the apparent viscosity of the intracellular medium. Phagocytosed microbubbles experienced a viscous damping approximately sevenfold greater than free microbubbles. Acoustical comparison between free and phagocytosed microbubbles indicated that phagocytosed microbubbles produce an echo with a higher mean frequency than free microbubbles in response to a rarefaction-first single-cycle pulse. Moreover, this frequency increase is predicted using the modified Rayleigh-Plesset equation. We conclude that contrast-enhanced ultrasound can detect distinct acoustic signals from microbubbles inside of neutrophils and may provide a unique tool to identify activated neutrophils at sites of inflammation.

Predictors of systolic augmentation from left ventricular preexcitation in patients with dilated cardiomyopathy and intraventricular conduction delay

BACKGROUND

VDD pacing can enhance systolic function in patients with dilated cardiomyopathy and discoordinate contraction; however, identification of patients likely to benefit is unclear. We tested predictors of systolic responsiveness on the basis of global parameters as well as directly assessed mechanical dyssynchrony.

METHODS AND RESULTS

Twenty-two DCM patients with conduction delay were studied by cardiac catheterization with a dual-sensor micromanometer to measure LV and aortic pressures during sinus rhythm and LV free-wall pacing. Pacing enhanced isovolumetric (dP/dt(max)) and ejection-phase (pulse pressure, PP) systolic function by 35+/-21% and 16.4+/-11%, respectively, and these changes correlated directly (r=0.7, P=0.001). %DeltadP/dt(max) was weakly predicted by baseline QRS (r=0.6, P<0.02), more strongly by baseline dP/dt(max) (r=0.7, P=0.001), and best by bidiscriminate analysis combining baseline dP/dt(max) < or =700 mm Hg/s and QRS > or =155 ms to predict %DeltadP/dt(max) > or =25% and %DeltaPP > or =10% (P<0.0005, chi(2)), with no false-positives. Benefit could not be predicted by %DeltaQRS. To test whether basal mechanical dyssynchrony predicted responsiveness to LV pacing, circumferential strains were determined at approximately 80 sites throughout the LV by tagged MRI in 8 DCM patients and 7 additional control subjects. Strain variance at time of maximal shortening indexed dyssynchrony, averaging 28.0+/-7.1% in normal subjects versus 201.4+/-84.3% in DCM patients (P=0.001). Mechanical dyssynchrony also correlated directly with %DeltadP/dt(max) (r=0.85, P=0.008). Conclusions-These results show that although mechanical dyssynchrony is a key predictor for pacing efficacy in DCM patients with conduction delay, combining information about QRS and basal dP/dt(max) provides an excellent tool to identify maximal responders.

Contrast Echocardiography in Clinical Practice

After FDA approval of the new-generation contrast agent Optison (Mallinckrodt Medical, St. Louis, MO) January 1998, the use of contrast in echocardiograhy has become an invaluable tool. A review of 100 patients revealed contrast to be useful for endocardial border definition and wall segment analysis, enhancement of pulsed Doppler, and chamber opacification for the detection of thrombi. Evaluation of wall segments by two observers before and after injection of the contrast agent revealed an increase in the number of wall segments visualized by 4.8. Postinjection readings were consistent between the two observers. Routine contrast echocardiography may provide a more diagnostic study.

Acoustic radiation force in vivo: a mechanism to assist targeting of microbubbles

The goal of targeted imaging is to produce an enhanced view of physiological processes or pathological tissue components. Contrast agents may improve the specificity of imaging modalities through selective targeting, and this may be particularly significant when using ultrasound (US) to image inflammatory processes or thrombi. One means of selective targeting involves the attachment of contrast agents to the desired site with the use of a specific binding mechanism. Because molecular binding mechanisms are effective over distances on the order of nanometers, targeting effectiveness would be greatly increased if the agent is initially concentrated in a particular region, and if the velocity of the agent is decreased as it passes the potential binding site. Ultrasonic transmission produces a primary radiation force that can manipulate microbubbles with each acoustic pulse. Observations demonstrate that primary radiation force can displace US contrast agents from the center of the streamline to the wall of a 200-microm cellulose vessel in vitro. Here, the effects of radiation force on contrast agents in vivo are presented for the first time. Experimental results demonstrate that radiation force can displace a contrast agent to the wall of a 50-microm blood vessel in the mouse cremaster muscle, can significantly reduce the velocity of flowing contrast agents, and can produce a reversible aggregation. Acoustic radiation force presents a means to localize and concentrate contrast agents near a vessel wall, which may assist the delivery of targeted agents.

Optical and acoustical observations of the effects of ultrasound on contrast agents

Optimal use of encapsulated microbubbles for ultrasound contrast agents and drug delivery requires an understanding of the complex set of phenomena that affect the contrast agent echo and persistence. With the use of a video microscopy system coupled to either an ultrasound flow phantom or a chamber for insonifying stationary bubbles, we show that ultrasound has significant effects on encapsulated microbubbles. In vitro studies show that a train of ultrasound pulses can alter the structure of an albumin-shelled bubble, initiate various mechanisms of bubble destruction or produce aggregation that changes the echo spectrum. In this analysis, changes observed optically are compared with those observed acoustically for both albumin and lipid-shelled agents. We show that, when insonified with a narrowband pulse at an acoustic pressure of several hundred kPa, a phospholipid-shelled bubble can undergo net radius fluctuations of at least 15%; and an albumin-shelled bubble initially demonstrates constrained expansion and contraction. If the albumin shell contains air, the shell may not initially experience surface tension; therefore, the echo changes more significantly with repeated pulsing. A set of observations of contrast agent destruction is presented, which includes the slow diffusion of gas through the shell and formation of a shell defect followed by rapid diffusion of gas into the surrounding liquid. These observations demonstrate that the low-solubility gas used in these agents can persist for several hundred milliseconds in solution. With the transmission of a high-pulse repetition rate and a low pressure, the echoes from, contrast agents can be affected by secondary radiation force. Secondary radiation force is an attractive force for these experimental conditions, creating aggregates with distinct echo characteristics and extended persistence. The scattered echo from an aggregate is several times stronger and more narrowband than echoes from individual bubbles.

New ultrasound contrast agents for left ventricular and myocardial opacification

Until recently, the use of contrast agents with 2-dimensional echocardiography has been limited to the detection of intracardiac shunts or abnormal venous connections. The advent of commercially available transpulmonary contrast agents and progress in imaging technology changed this situation. New indications for contrast echocardiography include improved assessment of ventricular function by endocardial border enhancement and the assessment of myocardial perfusion. The major advantage of novel contrast agents is their persistence in circulation, due to the content of a gas that is poorly soluble in plasma or a specific microcapsule wall composition. These features, in conjunction with advanced imaging techniques (intermittent harmonic imaging, harmonic power Doppler, pulse inversion Doppler) allow the detection of minute amounts of the agents in myocardium. There are more than 10 echocardiographic contrast agents undergoing clinical or late preclinical tests. Apart from commercially available Albunex, Levovist, Optison, such agents as EchoGen, Quantison, NC100100 and PESDA have been successfully used in humans. Initial clinical data demonstrating the feasibility of myocardial perfusion studies in patients have been presented for PESDA, Optison, Quantison and NC100100. Early attempts are being made for therapeutic applications of microbubbles, including ultrasound-intensified thrombolysis, tissue targeting and drug delivery. Rapid progress in microbubble technology and imaging techniques has raised a wide interest of the clinicians for contrast echocardiography, which may soon become an established technique for the evaluation of myocardial perfusion, competitive for radionuclide imaging.