Detection of coronary stenoses and quantification of the degree and spatial extent of blood flow mismatch during coronary hyperemia with myocardial contrast echocardiography

Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications

Microbubbles are increasingly being used in biomedical applications such as ultrasonic imaging and targeted drug delivery. Microbubbles typically range from 0.1 to 10 µm in size and consist of a protective shell made of lipids or proteins. The shell encapsulates a gaseous core containing gases such as oxygen, sulfur hexafluoride or perfluorocarbons. This review is a consolidated account of information available in the literature on research related to microbubbles. Efforts have been made to present an overview of microbubble synthesis techniques; microbubble stability; microbubbles as contrast agents in ultrasonic imaging and drug delivery vehicles; and side effects related to microbubble administration in humans. Developments related to the modeling of microbubble dissolution and stability are also discussed.

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.

In vivo MR angiography and velocity measurement in mice coronary arteries at 9.4 T: assessment of coronary flow velocity reserve

PURPOSE

To demonstrate the feasibility of coronary magnetic resonance (MR) angiography in living mice and to evaluate a dynamic MR angiographic method for coronary flow measurement at 9.4-T field strength.

MATERIALS AND METHODS

This study was conducted according to European law and was in full compliance with National Institutes of Health recommendations for animal care and a local institutional animal care committee. Mice were anesthetized by using isoflurane. First, time-of-flight MR angiography was performed in 10 mice to measure coronary diameters at 80-mum isotropic resolution. Second, left coronary artery (LCA) velocity measurements were performed at seven cardiac phases in nine other mice to assess the velocity curve profile. Third, coronary velocities were measured at the middiastolic phase in 13 mice at rest and during adenosine-induced hyperemia to calculate coronary flow velocity reserve (CFVR). The Pearson coefficient compared the correlation between isoflurane dose and CFVR. Paired t tests compared R-R intervals and respiratory rates between rest and hyperemia.

RESULTS

Proximal diameters were, respectively, 404 mum +/- 34 [standard deviation] and 259 mum +/- 22 for the LCAs and the right coronary arteries, which were in accordance with reported values. The velocity curve profile throughout the cardiac cycle was similar to values from the literature. Baseline and hyperemic velocities were, respectively, 19.0 cm/sec +/- 4.4 and 33.7 cm/sec +/- 4.7 (P<.001), resulting in a CFVR of 1.77 +/- 0.19. CFVR did not correlate with isoflurane dose (r = 0.05, P = .88). R-R intervals shortened by 2.5% during hyperemia (P = .04). Respiratory rates showed no difference between rest and hyperemia (P = .39).

CONCLUSION

High-spatial-resolution three-dimensional coronary MR angiography is feasible in living mice. Dynamic MR angiography depicts coronary velocity changes throughout the cardiac cycle and between rest and maximum hyperemia, providing a tool for CFVR assessment.

Prognostic value of dipyridamole stress myocardial contrast echocardiography: comparison with single photon emission computed tomography

BACKGROUND

Dipyridamole stress myocardial contrast echocardiography (MCE) can be used to detect coronary artery disease (CAD). Because it measures myocardial blood flow velocity in addition to measuring myocardial blood volume, it was hypothesized that it should have greater prognostic utility than single photon-emission computed tomography (SPECT), which measures only myocardial blood volume. Because blood flow mismatch precedes wall thickening (WT) abnormalities during demand ischemia, it was also postulated that perfusion on MCE would be superior to WT abnormalities on echocardiography for this purpose.

METHODS

The incidence of nonfatal myocardial infarction and cardiac death was determined in 261 patients with known or suspected CAD over a mean follow-up period of 14 months who underwent simultaneous dipyridamole stress MCE and 99mTc-sestamibi SPECT. Comparisons of survival curves were conducted with stratified (and unstratified) log-rank tests.

RESULTS

Abnormal results on MCE were found to be the best predictor of an adverse outcome (odds ratio, 23; 95% confidence interval, 6-201; P<.0001) and provided incremental prognostic value over clinical variables (age>60 years, the presence of >or=3 cardiac risk factors, known peripheral vascular disease, prior myocardial infarction, and left ventricular systolic function), inducible WT abnormalities, and SPECT. Prognoses were worst in patients who had both abnormal results on MCE and inducible WT abnormalities and best in those who had neither. Patients with abnormal results on MCE but no inducible WT abnormalities had intermediate outcomes.

CONCLUSION

In patients with known or suspected CAD undergoing dipyridamole stress, MCE provides powerful prognostic information that is superior to clinical variables, electrocardiography, left ventricular systolic function, WT analysis, and SPECT. MCE may therefore serve as a method of choice for myocardial perfusion assessment in patients with known or suspected CAD. Larger studies are needed to confirm these findings.

Current role of contrast echocardiography in the diagnosis of cardiovascular diseases

The use of contrast echocardiography (CE) in cardiovascular medicine has grown significantly over the last 15 years. Depending on the site of injection, contrast enhancement of the right- or left-sided cardiac chambers or myocardium now can be achieved. Contrast echocardiography can improve the evaluation of patients with valvular heart disease by enhancing the Doppler signal; CE also improves detection of intracardiac or intrapulmonary shunts. In patients with coronary artery disease, enhancement of the endocardial blood-tissue boundary allows for improved visualization of endocardial wall motion, assessment of wall thickening, and calculation of ejection fraction. Contrast echocardiography promises to delineate myocardial perfusion and has the potential for quantitating coronary flow and assessing myocardial viability. These applications may add important physiologic information to the anatomic information readily available from noncontrast echocardiography. Because it can be rapidly performed at the bedside, CE may be a valuable tool for use with inpatients with acute myocardial ischemia. When CE has been used after recanalization of occluded coronary arteries, the assessment of myocardial salvage conveys information concerning reflow, stunning, and prognosis, and in the case of an angioplasty it provides immediate information regarding the success of the procedure. Contrast echocardiography can also assess myocardial areas at risk of irreversible damage and the presence or absence of collateral flow. When performed with transesophageal or epicardial echocardiography in the operating room, CE is emerging as a valuable tool in the assessment of cardioplegia distribution and graft patency as well as in the delineation of the regional supply of each graft. With the continued development of newer contrast agents and refinement of ultrasound imaging equipment, the applications of CE will continue to grow.

Evaluation of Blood Flow Reserve in Left Anterior Descending Coronary Artery Territory by Quantitative Myocardial Contrast and Doppler Echocardiography

We sought to compare the feasibility and accuracy of myocardial blood flow reserve (MBFR) measured by quantitative real-time myocardial contrast echocardiography with those of coronary flow velocity reserve (CFVR) obtained by transthoracic Doppler echocardiography for detecting left anterior descending coronary artery (LAD) stenosis. We studied 71 patients who underwent adenosine stress contrast echocardiography, transthoracic Doppler echocardiography, and quantitative coronary angiography within 1 month. An index of myocardial blood flow (A x beta) was determined by quantification of peak plateau acoustic intensity (A) and microbubble replenishment velocity (beta) by contrast echocardiography. Feasibilities of qualitative analysis of myocardial perfusion, and CFVR and MBFR measurements were 98%, 83%, and 94%, respectively. Patients with LAD stenosis had lower CFVR (1.1 +/- 0.4 vs 2.7 +/- 0.8, P < .001), MBFR (1.2 +/- 0.5 vs 2.5 +/- 0.8, P < .001), and beta reserve (1.1 +/- 0.5 vs 2.4 +/- 0.6, P < .001) than those without lesion. Sensitivities, specificities, and accuracies for detecting LAD stenosis were 64%, 93%, and 80% for qualitative analysis of myocardial perfusion; 92%, 94%, and 93% for CFVR; 84%, 87%, and 86% for MBFR; and 80%, 97%, and 89% for beta reserve. In this selected study population, CFVR was the best index for detecting LAD stenosis (odds ratio = 1.78, 95% confidence interval = 1.28-2.47).

Contrast echocardiography

Myocardial contrast echocardiography (MCE) is a noninvasive imaging technique that relies on the ultrasound detection of microbubble contrast agents. These agents are confined to the intravascular space thereby producing signal enhancement from the blood pool. This review encompasses many of the key concepts regarding the clinical application of MCE. The first section focuses on the composition, safety, and biokinetics of ultrasound contrast agents. Then we discuss new ultrasound imaging methodology that has been developed to enhance detection of contrast agent and to assess perfusion at the tissue level. Next, the clinical applications of contrast ultrasound are reviewed. These include enhancement of the cardiac chambers for better assessment of cardiac function and masses, myocardial perfusion imaging for the detection of coronary artery disease, and the assessment of myocardial viability and microvascular reflow. Finally, we discuss some of the future applications for MCE, which include molecular imaging of disease and drug/gene delivery. The overall aim of the review is to update the clinician on state-of-the-art MCE and how it can be applied in patients with cardiovascular disease.

Myocardial Contrast Echocardiography Evolving as a Clinically Feasible Technique for Accurate, Rapid, and Safe Assessment of Myocardial Perfusion

Intravenous myocardial contrast echocardiography (MCE) is a recently developed technique for assessment of myocardial perfusion. Up to now, many studies have demonstrated that the sensitivity and specificity of qualitative assessment of myocardial perfusion by MCE in patients with acute and chronic ischemic heart disease are comparable with other techniques such as cardiac scintigraphy and dobutamine stress echocardiography. Furthermore, quantitative parameters of myocardial perfusion derived from MCE correlate well with the current clinical standard for this purpose, positron emission tomography. Myocardial contrast echocardiography provides a promising and valuable tool for assessment of myocardial perfusion. Although MCE has been primarily performed for medical research, its implementation in routine clinical care is evolving. This article is intended to give an overview of the current status of MCE.

Noninvasive cardiac imaging in the evaluation of suspected acute coronary syndromes

Optimal management of patients presenting with chest pain to the emergency department is a major challenge, both in terms of a diagnostic dilemma and consumption of resources. The triage of such patients can be aided vastly by the appropriate use of noninvasive imaging. Noninvasive imaging modalities such as echocardiogram, radionuclide perfusion studies, positron emission tomography, cardiac magnetic resonance imaging and computed tomography have all been demonstrated to have favorable diagnostic and prognostic value, with an enhanced sensitivity to detect acute ischemia. A normal noninvasive evaluation in the appropriate clinical setting presents a strong argument against acute ischemia as an etiology of the chest pain. Randomized trials of both rest and stress imaging in the emergency department have confirmed a reduction in unnecessary hospitalizations and cost savings without compromising the safety of the patient. Cardiac magnetic resonance and computed tomography would provide an insight into subendocardial ischemia, the detection of which has previously been difficult, using single-photon emission tomography and echocardiography. In this review, novel hot-spot imaging modalities are discussed including infarct-avid imaging agents and ischemia-avid imaging agents, thus elucidating the pathophysiology of reperfusion-induced cell death. These agents represent work in evolution and are likely to be used routinely in the future as understanding of coronary syndromes and coronary artery disease becomes clearer.

Myocardial contrast echocardiography in the detection of coronary stenosis

The need for the capillary bed to maintain normal hydrostatic pressure results in capillary derecruitment distal to a stenosis during hyperemia. This pathophysiologic behavior is seen irrespective of the method by which coronary driving pressure is reduced. The functional consequences of the decrease in MBV are an increase in myocardial vascular resistance and a concomitant reduction in hyperemic MBF. These phenomena form the basis for the detection of CAD with MCE, which isa unique noninvasive tool that allows the separate assessment of both MBV and MBF velocity. Because of its excellent spatial and temporal resolution, portability, widespread availability,and relatively low cost, MCE should provide an attractive method for the noninvasive detection of CAD and for the quantification of stenosis severity.

Comparison of usefulness of dipyridamole stress myocardial contrast echocardiography to technetium-99m sestamibi single-photon emission computed tomography for detection of coronary artery disease (PB127 Multicenter Phase 2 Trial results)

We hypothesized that assessment of hyperemic myocardial blood flow (MBF) velocity using myocardial contrast echocardiography (MCE) can detect coronary artery disease (CAD). We also postulated that only a single MCE study during stress is required for the detection of CAD in patients with normal function at rest. Patients with known or suspected CAD referred for dipyridamole stress technetium-99m sestamibi single-photon emission computed tomographic (SPECT) studies were enrolled. MCE was performed concurrently with SPECT using continuous infusions of PB127 during intermittent harmonic power Doppler imaging at multiple pulsing intervals. MCE and SPECT were compared in 43 of 54 patients who had adequate studies using both techniques. In 15 of the 43 patients, coronary angiography was performed within 30 days of the MCE/SPECT tests. Overall concordance for classification of patients as normal versus abnormal was 84% (kappa = 0.63) between the 2 tests. When false-negative SPECT scans were corrected for results of angiography, concordance increased to 93% (kappa = 0.82). For territorial analysis, concordance between MCE and SPECT for location of perfusion defects was 65% (kappa = 0.41) and 74% (kappa = 0.61) after SPECT was corrected by angiography. In patients with normal function at rest, a single stress MCE perfusion study allowed identification of CAD with the same concordance as rest/stress perfusion studies. In conclusion, visual assessment of regional differences in MBF velocity using PB127 allows detection of CAD with good concordance compared with technetium-99m sestamibi SPECT. In patients with normal left ventricular function at rest, a single stress PB127 MCE perfusion study is adequate for the detection of CAD.

Quantification of regional myocardial perfusion using semiautomated translation-free analysis of contrast-enhanced power modulation images

Quantitative analysis of myocardial perfusion is currently based on manual tracing and frame-by-frame realignment of regions of interest. We developed a technique for semiautomated identification of myocardial regions from power modulation images as a potential tool for quantification of myocardial contrast enhancement. This approach was tested in 13 anesthetized pigs during continuous intravenous infusion of contrast at baseline, left anterior descending coronary artery occlusion, and reperfusion. Regional pixel intensity was calculated for each consecutive end-systolic frame after a high-energy ultrasound impulse, and fitted with an exponential function. Perfusion defects caused by occlusion of left anterior descending coronary artery were confirmed by a significant decrease in both postimpulse steady-state intensity and the initial rate of contrast replenishment (P <.05), which were reversed with reperfusion. Automated measurements of myocardial intensity correlated highly with conventional manual tracing (r = 0.90 to 0.97), and resulted in improved signal-to-noise ratios. This technique allows translation-free quantification of regional myocardial perfusion, without the need for manual tracing.

Approaches to the detection of coronary artery disease using myocardial contrast echocardiography

In the presence of an epicardial coronary stenosis, changes in microvascular hemodynamics after the administration of pharmacologic stress agents result in a decrease in myocardial blood volume (MBV) from capillary derecruitment. Because capillaries provide the greatest resistance to flow during maximal hyperemias, changes in capillary volume are closely coupled to changes in hyperemic myocardial blood flow (MBF). A wide array of imaging techniques are now available with myocardial contrast echocardiography to assess both the spatial and temporal aspects of myocardial perfusion, providing many options for the detection of coronary stenosis by evaluating changes in MBV, MBF, or both.

Assessment of coronary stenoses of graded severity by myocardial contrast echocardiography

BACKGROUND

Myocardial contrast echocardiography (MCE) has potential value in the assessment and quantitation of myocardial perfusion defects. However, the severity of stenosis detectable by MCE and its diagnostic accuracy remain undefined. Thus, we produced coronary stenoses of variable severity and quantified their effect on MCE.

METHODS AND RESULTS

Three grades of left anterior descending (LAD) obstructions were produced in 7 open-chest swine. The stenoses were nonflow-limiting at rest, but decreased coronary hyperemia by 31.3% +/- 4.7%, 69.9% +/- 5.3% and 98.9% +/- 1.1%, respectively. Regional myocardial blood flow (RBF) was measured with fluorescent microspheres and was expressed as the ratio of LAD and control (LCx) beds. MCE was performed with 0.3 mg/kg intravenous AF0150 during ECG-gated harmonic imaging in short-axis view. Background-subtracted peak intensity (PI) was expressed as the ratio of LAD/LCx beds. Both RBF and PI ratios progressively decreased with increasing grades of stenosis. MCE showed a significant correlation with RBF (r = 0.74; P <.0001). Ratios of both PI and RBF differed significantly from baseline when coronary hyperemia was reduced more than 50%. An LAD/LCx ratio less than 0.6 by MCE yielded 61% and 83% sensitivity and 85% and 76% specificity with stenosis that reduced coronary hyperemia more than 50% and more than 75%, respectively.

CONCLUSION

MCE with intravenous AF0150 during vasodilation correctly depicted the progressive reduction of flow ratios produced by graded coronary stenoses. A significant reduction of PI ratio was observed with stenosis causing more than 50% reduction of coronary hyperemia. An MCE ratio in stenosed/control beds could be selected, which exhibited good sensitivity and specificity in the identification of coronary stenosis.

Real-time perfusion imaging with low mechanical index pulse inversion Doppler imaging

OBJECTIVES

We sought to determine how successful pulse inversion Doppler (PID) imaging would be in detecting myocardial perfusion defects during dobutamine stress echocardiography.

BACKGROUND

By transmitting multiple pulses of alternating polarity (PID) at a low mechanical index, myocardial contrast enhancement from intravenously injected microbubbles can be detected using real-time frame rates. Pulse inversion Doppler imaging was performed in 117 patients during dobutamine stress echocardiography by using an intravenous bolus of a perfluorocarbon-filled, albumin-(Optison: n = 98) or liposome- (Definity: n = 19) encapsulated microbubble and a mechanical index of <0.3. The visual identification of myocardial contrast defects and wall motion abnormalities was determined by blinded review. Forty of the patients had quantitative angiography (QA) performed to correlate territorial contrast defects with stenosis diameter >50%.

RESULTS

There was a virtual absence of signal from the myocardium before contrast injections in all patients. Bright myocardial opacification at peak stress was observed in at least one coronary artery territory at frame rates up to 25 Hz in 114 of the 117 patients during dobutamine stress echocardiography. Regional myocardial contrast defects at peak stress were observed in all 30 patients with >50% stenosis in at least one vessel (13 with single-vessel and 17 with multivessel disease). Contrast defects were observed in 17 territories subtended by >50% diameter stenosis that had normal wall motion at peak stress. Overall agreement between QA and myocardial contrast enhancement on a territorial basis was 83%, as compared with 72% for wall motion.

CONCLUSIONS

Pulse inversion Doppler imaging allows the detection of myocardial perfusion abnormalities in real-time during stress echocardiography and will further add to the quality and sensitivity of this test.

Quantitative assessment of myocardial perfusion during graded coronary artery stenoses by intravenous myocardial contrast echocardiography

OBJECTIVES

The purpose of this study was to examine whether coronary stenoses of variable severity could be quantitatively assessed by analysis of myocardial perfusion as determined by intravenous (IV) myocardial contrast echocardiography.

BACKGROUND

Recently, new contrast agents and imaging technology have been developed that may enable improved assessment of myocardial perfusion by IV contrast injection.

METHODS

Variable obstruction of the left anterior descending (LAD) coronary artery in dogs was produced by a screw occluder. Coronary artery flow was measured with a transit time flowmeter during baseline, pharmacological vasodilation, a non-flow-limiting stenosis at rest in conjunction with vasodilation, a flow-limiting stenosis, and total occlusion. Myocardial contrast echocardiography was performed after IV injection of the contrast agent NC 100100. Time-intensity curves were obtained off-line for the LAD risk area and the adjacent left circumflex (LCx) territory, and peak background-subtracted video intensity was determined. Fluorescent microspheres were injected at each intervention for determination of regional myocardial blood flow.

RESULTS

During non-flow-limiting stenosis, flow limiting stenosis and total occlusion, LAD/LCx ratios of peak myocardial videointensity and blood flow decreased proportionately. Both LAD/LCx ratios of video intensity and blood flow identified the non-flow-limiting and the flow-limiting stenoses as well as total occlusion of the LAD artery. A significant correlation between LAD/LCx video intensity and blood flow ratios was observed (r = 0.83, p < 0.0001).

CONCLUSIONS

The degree of blood flow mismatch between ischemic and normal myocardial regions during graded coronary stenoses can be estimated in the dog by quantitative assessment of myocardial perfusion produced by IV myocardial contrast echocardiography.

Assessment of myocardial perfusion with contrast echocardiography at rest and with stress: an emerging technology

Over the past 20 years, there has been considerable progress in the field of myocardial contrast echocardiography (MCE). What began as a modality limited to selected cardiac catherization laboratories may soon become a rapid and accurate bedside tool for assessing myocardial perfusion. Because MCE via intravenous contrast injection can be performed at the bedside and avoids the use of radiation exposure, it offers multiple potential clinical applications, including assessment of reperfusion after fibrinolytic therapy, postinfarction risk area, and myocardial viability. The addition of perfusion data to wall motion may augment the results of stress echocardiography. This report describes the technologic advances in contrast agents and related imaging technologies that enable myocardial perfusion to be assessed by echocardiography. In addition, the latest clinical studies of myocardial perfusion by MCE are presented.

Quantification of mitral regurgitation in the cardiac catheterization laboratory with contrast echocardiography

BACKGROUND

There is no method of quantifying the severity of mitral regurgitation (MR) from injection of tracer directly into the left ventricular (LV) cavity, a method commonly used in the cardiac catheterization laboratory.

METHODS AND RESULTS

We used a previously validated mathematical model that derives regurgitant fraction (RF) from the relative tracer washout from the left atrial (LA) and LV cavities. Thirty-nine patients referred for diagnostic cardiac catheterization with clinical evidence of possible MR were included in the study. Five milliliters of a microbubble mixture was power-injected into the LV during simultaneously performed contrast echocardiography. Relative changes in background-subtracted video intensity were measured from the LV and LA, and the resultant model-derived RF was correlated with the severity of MR on cineangiography. The severity of MR ranged from 0 to 4+ on cineangiography with corresponding model-derived RF of 0 to 0.69 on contrast echocardiography. A close linear relation was noted between angiographic severity of MR and model-derived RF on contrast echocardiography (y = 0.1x + 0.03, r = 0.89, P <.001). Contrast echocardiography was more sensitive than cineangiography for detecting mild MR.

CONCLUSIONS

We describe a new method of measuring the severity of MR in the cardiac catheterization laboratory. Apart from being quantitative, this method can be safely used during cardiac catheterization in patients in whom iodinated contrast agents may be potentially harmful.

Quantification of the physiological relevance of a coronary stenosis using myocardial contrast echocardiography

MCE can be used in the catheterization laboratory or in the operating room to provide rapid assessments of the functional significance of a coronary stenosis from direct arterial injections of microbubbles. In the past few years, the development of more stable microbubble contrast agents, and a better understanding of the interactions between ultrasound and microbubbles have led to the development of a truly non-invasive approach to quantify MBF using venous infusions. Furthermore, additional insights into the physiology of coronary stenosis, particularly as it affects MBV, have been obtained using MCE.

Role of intravenous ultrasound contrast in stress echocardiography

Intravenous newer generation perfluorocarbon containing microbubbles have been shown to enhance endocardial borders, especially during harmonic imaging. Although this significantly improves the detection of wall-motion abnormalities during stress echocardiography, intermittent imaging consistently results in myocardial contrast following intravenous infusions or injections of perfluorocarbon microbubbles. Detection of myocardial perfusion abnormalities during both exercise and pharmacologic stress echocardiography appears to be feasible clinically with either intravenous injections or continuous infusions of microbubbles using intermittent harmonic imaging. Accelerated intermittent harmonic imaging allows one to rapidly acquire both myocardial perfusion and wall motion during exercise and dobutamine stress echocardiography.

Echocardiographic determination of risk area size in a murine model of myocardial ischemia

Genetically altered mice are useful to understand cardiac physiology. Myocardial contrast echocardiography (MCE) assesses myocardial perfusion in humans. We hypothesized it could evaluate murine myocardial perfusion before and after acute coronary ligation. MCE was performed before and after this experimental myocardial infarction (MI) in anesthetized mice by intravenous injection of contrast microbubbles and transthoracic echo imaging. Time-video intensity curves were obtained for the anterior, lateral, and septal myocardial walls. After MI, MCE defects were compared with the area of no perfusion measured by Evans blue staining. In healthy animals, intramyocardial contrast was visualized in all the cardiac walls. The anterior wall had a higher baseline video intensity (53 +/- 17 arbitrary units) than the lateral (34 +/- 13) and septal (27 +/- 13) walls (P < 0.001) and a lower increase in video intensity after contrast injection [50 +/- 17 vs. 60 +/- 24 (lateral) and 65 +/- 29 (septum), P < 0.01]. After MI, left ventricular (LV) dimensions were enlarged, and the shortening fraction was decreased. A perfusion defect was imaged with MCE in every mouse, with a correlation between MCE perfusion defect size (35 +/- 13%) and the nonperfused area by Evans blue (37 +/- 16%, y = 0.77x + 6.1, r = 0.93, P < 0. 001). Transthoracic MCE is feasible in the mouse and can accurately detect coronary occlusions and quantitate nonperfused myocardium.

Assessment of flow mismatch with pharmacologic stress test on myocardial contrast echocardiography in a model of critical stenosis: adenosine triphosphate and dipyridamole

Although adenosine triphosphate (ATP) is a favorable vasodilator because of its short-acting duration, the agent's effectiveness in facilitating the diagnosis of myocardial ischemia with myocardial contrast echocardiography (MCE) is not fully understood. The goal of this study was to examine the efficacy of intravenous ATP administration (0.15 to 0.30 mg/kg/min for 5 minutes) in diagnosing the flow mismatch with MCE. To achieve this, a critical stenosis was produced in the left circumflex artery in 10 anesthetized dogs. The peak intensity ratio of risk area to control area was reduced by ATP from 0.51 +/- 0.19 to 0.31 +/- 0.12 (P <.05). Systolic wall thickening of the risk area did not change significantly (32.8% +/- 9.8% to 27.5% +/- 12.8%). These changes did not differ from those obtained after dipyridamole. We conclude that MCE with intravenous ATP administration is as useful as the dipyridamole method for diagnosing critical coronary stenosis.

Prediction of short- and intermediate-term prognoses of patients with acute myocardial infarction using myocardial contrast echocardiography one day after recanalization

OBJECTIVES

This study sought to determine whether microvascular integrity in the risk area (RA) for myocardial infarction (MI) one day after recanalization predicts the outcome in patients with first acute MI.

BACKGROUND

Immediately after recanalization, microcirculation in the RA is modified by both hyperemic response and microvascular impairment.

METHODS

Fifty consecutive patients who underwent serial myocardial contrast echocardiography before and one day after recanalization (day 2) were studied. All patients had a completely occluded lesion in the left anterior descending coronary artery alone, and underwent successful reperfusion therapy. The relative size of the initial RA (RA ratio) and peak gray scale ratio (PGSR) within the RA on day 2 were determined. Patients were followed for a median of 22 months to evaluate clinical outcome.

RESULTS

On day 2, PGSR was a median of 0.46. Study patients were subdivided into two groups, group A of 24 patients with acceptable opacification (PGSR > 0.46 on day 2) and group B of 26 patients without it. Major cardiac events (cardiac death, nonfatal MI and repeat admission for congestive heart failure) were more frequently observed in group B (28% vs. 4%, Cox hazard ratio=8.5, p=0.05, 95% confidence interval [CI] 1.03 to 69.9). The median value of the RA ratio was 0.45. Patients (n=15) with RA ratio > 0.45 on day 1 and PGSR on day 2 < or = 0.46 exhibited a 10.7-fold relative risk for major cardiac events (p=0.005, 95% CI 2.06 to 55.8) and a 3.69-fold relative risk for composite cardiac events (major cardiac events and target lesion revascularizations) after the initial intervention (p=0.004, 95% CI 1.51 to 9.04).

CONCLUSIONS

The assessment of both the size of the initial RA and microvascular integrity on day 2 enables precise determination of the efficacy of reperfusion therapy and prediction of the short- and intermediate-term prognoses of patients with recanalized MI.

Integrated MRI assessment of regional function and perfusion in canine myocardial infarction

A single integrated examination using regional measurements of perfusion from contrast-enhanced MRI and three-dimensional (3D) strain from tissue-tagged MRI was developed to differentiate infarcted myocardium from adjacent tissue with functional abnormalities. Ten dogs were studied at baseline and 10 days after a 2-hour occlusion of the left anterior descending coronary artery (LAD). Strain was determined using a 3D finite element model. Two-dimensional measurements of hypoenhancing regions were highly correlated with myocardial viability (r = 0.96). Signal intensity versus time curves obtained from contrast-enhanced MRI were used for quantitative perfusion analysis. The remote and adjacent noninfarcted tissue of the dogs with LAD occlusion, as well as the infarcted tissue, exhibited abnormal deformation patterns as compared to normal dogs (positive predictive value (PPV) of strain determination of infarction = 66%). Integration of contrast-enhanced MRI results with 3D strain analysis enabled the delineation of the myocardial infarction (PPV = 100%) from functionally compromised myocardium. This integrated cardiac examination shows promise for noninvasive serial assessment of potentially jeopardized noninfarcted myocardium to study the process of infarct remodeling and expansion.

Correlation between quantitative angiographic lesion severity and myocardial contrast intensity during a continuous infusion of perfluorocarbon-containing microbubbles

The purpose of this study was to determine whether quantitative measurements of myocardial videointensity (MVI) during continuous intravenous infusions of microbubbles could detect differences in coronary artery stenosis severity during dobutamine stress echocardiography. Coronary artery stenoses were created in seven dogs by progressively tightening a snare around the coronary artery. Intravenous infusions of perfluorocarbon microbubbles were given during dobutamine stress. The initial rate of myocardial contrast enhancement (slope), peak myocardial contrast (peak MVI) at the longest pulsing interval, and the product (slope * peak MVI) were compared as ratios in the stenosed versus adjacent normal perfusion beds. Twenty-two coronary stenoses were compared (range 16% to 80% in diameter). There was a strong correlation between both slope ratios and slope * peak MVI ratios and percent stenosis (r = -0.89 for both, p<0.001). The rate of contrast replenishment during a continuous infusion of microbubbles can be used to determine both the presence and severity of coronary stenoses during stress echocardiography.

Detection of myocardial perfusion defects by contrast echocardiography in the setting of acute myocardial ischemia with residual antegrade flow

Although myocardial contrast echocardiography accurately demarcates area at risk during total coronary occlusion, the ability of MCE to delineate area at risk in the presence of residual antegrade flow is unknown. We hypothesized that perfusion defects in myocardial segments supplied by severe coronary stenoses with residual antegrade flow could be detected by MCE using intravenous FS069. We studied 13 open-chest dogs using an intravenous injection of FS069 during intermittent harmonic imaging. Images were collected at baseline, during acute ischemia with residual antegrade flow, physiologic hyperemia (release of stenosis), and total coronary occlusion. Regional myocardial blood flow was assessed using colored microspheres. MCE risk area during acute ischemia with residual antegrade flow and total occlusion was planimetered and compared with pathologic risk area (area unstained by monastral blue). Background-subtracted peak videointensity in the risk area was assessed for all flow states. Regional myocardial blood flow confirmed expected flow states, being significantly greater during physiologic hyperemia (4.16 +/- 1.22 ml/min/g) than at baseline (0.71 +/- 0.19 ml/min/g) and significantly diminished during coronary stenosis with residual antegrade flow (0.20 +/- 0.16 ml/min/g) and total occlusion (0.09 +/- 0.06 ml/min/g; p < 0.0001). Myocardial risk area by MCE during coronary stenosis with residual antegrade flow correlated well with pathologic risk area determined by monastral blue staining (r = 0.86). Peak videointensity during coronary stenosis (111 +/- 27) was significantly less than at baseline (157 +/- 50) but greater than during total occlusion (81 +/- 34; p < 0.0001). In conclusion, intravenous FS069 in conjunction with intermittent harmonic imaging delineates area at risk in ischemic myocardium supplied by a coronary stenoses with residual antegrade flow. The presence of a perfusion defect on MCE does not necessarily imply that the coronary artery is totally occluded.

Methods for quantifying ultrasound backscatter and two-dimensional video intensity: implications for contrast-enhanced sonography

Quantification of acoustic backscatter energy is believed to be useful for assessing "tissue character" and for quantifying the regional concentration of echo contrast. Measurement of ultrasonic video intensity has been the traditional means of quantifying backscatter energy, with "integrated backscatter" considered the gold standard. The purpose of this work is to review the commonly used methods for quantifying ultrasonic backscatter and to describe the difference between detected backscatter energy and the intrinsic tissue backscatter coefficient. Many of the quantification pitfalls that can lead to erroneous conclusions will also be discussed. A set of eight rubber phantoms with backscatter coefficient from -6 dB to +15 dB relative to liver were imaged at 2.5, 3.5, and 5.0 MHz. Methods for calculating the acoustic backscatter intensity from calibrated video intensity measurements and for calculating the tissue backscatter coefficient are described and tested using equipment from two different manufacturers. A commercially available automatic "acoustic densitometry" system with on-board quantitative integrated backscatter is also evaluated. Ultrasound attenuation and ultrasound system factors were found to strongly influence the detected backscatter intensity using either calibrated video intensity or on-board integrated backscatter. Special system transfer functions and attenuation correction were found to be useful in converting video intensity and integrated backscatter to a measure of the intrinsic tissue backscatter coefficient. With these correction factors, the correlation between the measured tissue backscatter coefficient and the phantom backscatter coefficient was excellent (r = 0.99, intercept 0.0, regression slope essentially 1.0) at all three imaging frequencies with traditional video intensity or on-board integrated backscatter. Uncalibrated video intensity and on-board integrated backscatter have limitations when used in isolation for tissue characterization. Rigorous attention to the imaging parameters and the use of calibration functions are necessary before video intensity measurement or integrated backscatter can be used reliably to measure the tissue backscatter coefficient.

Doppler energy: a new acquisition technique for the transthoracic detection of myocardial perfusion defects with the use of a venous contrast agent

AIMS

This animal experiment was designed to study whether the new technique of Doppler energy imaging could display myocardial perfusion abnormalities with the use of a combination of transthoracic imaging and right atrial injection of a myocardial contrast agent.

METHODS AND RESULTS

A series of 11 pigs were studied during (1) normal perfusion, (2) dipyridamole-induced coronary dilatation, and (3) during and after temporary occlusion of the left anterior descending or circumflex artery after a right atrial injection of 8 ml Levovist, 400 mg/ml. Short-axis views were obtained with the four following imaging modes: gray scale imaging (two-dimensional and M-mode), Doppler energy imaging (two-dimensional and M-mode). Visual inspection and off-line video densitometry (results expressed in arbitrary videointensity units 0 to 255) with digital background subtraction were performed. Doppler energy was significantly more sensitive in detecting the presence of contrast than gray scale imaging (background subtracted peak videointensity 32 +/- 17 versus 17 +/- 12, p < 0.001). Mean background-subtracted videointensity increased during dipyridamole-induced coronary hyperemia (40 +/- 14 versus 31 +/- 9, p < 0.003) using Doppler energy technique. Doppler energy imaging consistently detected absent perfusion (background subtracted videointensity -6 +/- 6) and immediate reperfusion (background subtracted peak videointensity 29 +/- 15, p < 0.001).

CONCLUSIONS

With the use of a galactose-based contrast agent, Doppler energy data acquisition was superior to standard gray scale imaging in transthoracic evaluation of regional myocardial perfusion, absence of perfusion, and reperfusion.

Usefulness of myocardial contrast echocardiography for the assessment of serial changes in risk area in patients with acute myocardial infarction

Serial myocardial contrast echocardiograms were recorded to investigate the time course of microvascular integrity in the risk area for first acute myocardial infarction. Serial changes in the risk area were categorized into 4 main types according to the potential for recovery from microvascular impairment.

Hemodynamic characteristics, myocardial kinetics and microvascular rheology of FS-069, a second-generation echocardiographic contrast agent capable of producing myocardial opacification from a venous injection

OBJECTIVES

We sought to 1) study the effects of FS-069 on cardiac and systemic hemodynamic function, myocardial blood flow, left ventricular wall thickening and pulmonary gas exchange when injected intravenously; and 2) compare the myocardial kinetics and microvascular rheology of FS-069 and Albunex when injected directly into a coronary artery.

BACKGROUND

FS-069 is a second-generation echocardiographic contrast agent composed of perfluoropropane-filled albumin microspheres; it is capable of consistent and reproducible myocardial opacification from a venous injection.

METHODS

Nine dogs were used to study the effects of FS-069 on hemodynamic function, pulmonary gas exchange, left ventricular wall thickening and myocardial blood flow and to characterize its myocardial kinetics when injected intravenously. These dogs were also used to compare the myocardial kinetics of FS-069 with those of Albunex during intracoronary injections. Nine Sprague-Dawley rats were used to compare the microvascular rheology of these two contrast agents, and in vitro modeling was performed to assess whether the microvascular findings of FS-069 can explain its echocardiographic behavior during direct coronary injections.

RESULTS

There were no effects of 30 rapid venous injections of FS-069 (every 20 s) on cardiac output; mean aortic, pulmonary or left atrial pressures; and peak positive and negative first derivative of left ventricular pressure (dP/dt). Similarly, there were no effects of this agent on radiolabeled microsphere-measured regional myocardial blood flow, left ventricular wall thickening or pulmonary gas exchange. When injected intravenously, the myocardial transit of this agent resembled a gamma-variate form. When diluted FS-069 was injected directly into the coronary artery; however, its transit resembled the integral of gamma-variate function, with persistent myocardial opacification lasting several minutes, which was different from that of Albunex. Intravital microscopy revealed that, unlike Albunex, when no bubbles are entrapped within the microcirculation after an arterial injection, a very small fraction of the diluted, larger FS-069 microbubbles are entrapped. In vitro modeling confirmed that this small fraction of microbubbles can result in persistent myocardial opacification.

CONCLUSIONS

FS-069 produces no changes in hemodynamic function, myocardial blood flow, left ventricular wall thickening or pulmonary gas exchange when injected intravenously in large amounts. When diluted FS-069 is injected into the coronary artery, a very small fraction of the larger bubbles are entrapped within the microcirculation, resulting in a persistent contrast effect. Thus, although FS-069 is a safe intravenous echocardiographic contrast agent, it cannot provide information on myocardial blood flow when injected directly into a coronary artery.

Transesophageal echocardiography

This article presents an overview of the benefits and efficacy of transesophageal echocardiography (TEE) in the critically ill patient. The echocardiographic evaluation of ventricular function both regional and global, is discussed with special emphasis on ischemic heart disease; assessment of preload, interrogation of valvular heart disease (prosthetic and native) and its complications; endocarditis and its complications; intracardiac and extracardiac masses, including pulmonary embolism; aortic diseases (e.g., aneurysan, dissection, and traumatic tears); evaluation of patent foramen ovale and its association with central and peripheral embolic events; advancements in computer technology; and finally, the effect of TEE on critical care.

Myocardial perfusion and function in dogs with moderate coronary stenosis

MRI studies of first-pass contrast enhancement with polylysine-Gd-DTPA and myocardial tagging using spatial modulation of magnetization (SPAMM) were performed to assess the feasibility of a combined regional myocardial blood flow and 2D deformation exam. Instrumented closed-chest dogs were imaged at a baseline control state (Cntl) followed by two interventions: moderate coronary stenosis (St) achieved by partial occlusion of the left anterior descending (LAD) and moderate coronary stenosis with dobutamine loading (StD). Hypoperfusion of the anterior region (ANT) of the myocardium (LAD distribution) relative to the posterior wall (POS) based on the upslope of the signal intensity time curve from the contrast-enhanced MR images was demonstrated only with dobutamine loading (ANT:POS Cntl = 1.077 +/- 0.15 versus ANT:POS StD = 0.477 +/- 0.11, P < 0.03) and was confirmed with radiolabeled microspheres measurements (ANT:POS Cntl = 1.18 +/- 0.2 ml/min/g versus ANT:POS StD = 0.44 +/- 0.1 ml/min/g; P < 0.002). Significant changes in regional myocardial shortening were only seen in the StD state (P < 0.02); the anterior region showed impaired myocardial shortening with dobutamine loading (P = NS), whereas the nonaffected POS region showed a marked increase in shortening when compared with Cntl (Cntl = 0.964 +/- 0.02 versus StD = 0.884 +/- 0.03; P < 0.001). These results demonstrate that an integrated quantitative assessment of regional myocardial function and semiquantitative assessment of myocardial blood flow can be performed noninvasively with ultrafast MRI.

Stress echocardiography, contrast echocardiography, and tissue characterization: applications for the future

During the last three decades the application of ultrasonography has expanded rapidly. The information available to the clinician from ultrasound imaging today is vastly more significant than it was in the early years of the development of this technology. In addition to automatic information, there is an increasing potential to provide functional, dynamic perfusion and even cellular information about the heart. This article attempts to summarize briefly the advances in these areas.

Insights into the assessment of myocardial perfusion offered by different cardiac imaging modalities

Myocardial perfusion may be very broadly defined as the tightly regulated nutrient delivery to cardiac tissue. The different components of perfusion are myocardial blood flow, oxygen delivery, myocardial oxygen consumption, and myocardial blood volume. Historically, focus has been placed mostly on the assessment of blood flow. In many instances, knowledge of flow without information about these other aspects is inadequate. This review discusses the various cardiac imaging techniques used for the assessment of myocardial perfusion that represent diverse physiologic measures of "perfusion." Their strengths and limitations are discussed as is their relevance to specific clinicopathologic conditions. Significant work still needs to be performed before all the aspects of myocardial perfusion can be precisely measured in human beings.

Assessment of coronary microcirculation with myocardial contrast echocardiography: current and future clinical applications

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Myocardial contrast echocardiography in coronary artery disease: potential applications using venous injections of contrast

The feasibility of studying myocardial perfusion with myocardial contrast echocardiography using intra-aortic or intracoronary injections of contrast medium has been established both in animal models and humans. However, the assessment of myocardial perfusion using venous injection is dependent on the availability of contrast agents that can opacify the left ventricular myocardium following a venous injection. Such agents are currently being evaluated in animal models. Using data from left atrial injections of contrast, this review briefly highlights the principles that govern the study of myocardial perfusion from venous injections of contrast. The value of such an approach in the setting of chronic coronary artery disease and acute myocardial infarction is discussed.