Basis for detection of stenosis using venous administration of microbubbles during myocardial contrast echocardiography: bolus or continuous infusion?

Contrast echocardiography: a practical guideline from the British Society of Echocardiography

Ultrasound contrast agents (UCAs) have a well-established role in clinical cardiology. Contrast echocardiography has evolved into a routine technique through the establishment of contrast protocols, an excellent safety profile, and clinical guidelines which highlight the incremental prognostic utility of contrast enhanced echocardiography. This document aims to provide practical guidance on the safe and effective use of contrast; reviews the role of individual staff groups; and training requirements to facilitate its routine use in the echocardiography laboratory.

Contrast Echocardiography for Assessing Myocardial Perfusion

PURPOSE OF REVIEW

Improvements in ultrasound methods for detecting microbubble ultrasound enhancing agents have led to an increase in the use of perfusion imaging with myocardial contrast echocardiography (MCE). This technique is now beginning to play an important role in specific clinical scenarios, which is the focus of this review.

RECENT FINDINGS

MCE was originally conceived as a technique for detecting resting perfusion abnormalities related to ischemia at rest or during stress from coronary artery disease. More recently, MCE has increasingly been used in circumstances where the technique's ability to provide rapid, quantitative, or bedside assessment of perfusion is advantageous. Quantitative MCE is also increasingly being used as a research technique for evaluating pathobiology and therapy that involve changes in the myocardial microcirculation. While MCE was developed and validated decades ago, it is only now beginning to be used by an increasing number of clinicians due to improvements in imaging technology and recognition of specific situations where the technique is impactful.

Role of cardiac imaging in management of heart failure

The significant advancement in cardiac imaging in recent years led to improved diagnostic accuracy in identifying the specific causes of heart failure and also provided physicians with guidelines for appropriately managing patients with heart failure. Diseases that were once considered rare are now more easily detected with the aid of cardiac imaging. Various cardiac imaging techniques are used to evaluate patients with heart failure, and each technique plays a distinct yet complementary role. This review aimed to discuss the comprehensive role of different types of cardiac imaging in the management of heart failure.

Coronary No-Reflow after Primary Percutaneous Coronary Intervention-Current Knowledge on Pathophysiology, Diagnosis, Clinical Impact and Therapy

Coronary no-reflow (CNR) is a frequent phenomenon that develops in patients with ST-segment elevation myocardial infarction (STEMI) following reperfusion therapy. CNR is highly dynamic, develops gradually (over hours) and persists for days to weeks after reperfusion. Microvascular obstruction (MVO) developing as a consequence of myocardial ischemia, distal embolization and reperfusion-related injury is the main pathophysiological mechanism of CNR. The frequency of CNR or MVO after primary PCI differs widely depending on the sensitivity of the tools used for diagnosis and timing of examination. Coronary angiography is readily available and most convenient to diagnose CNR but it is highly conservative and underestimates the true frequency of CNR. Cardiac magnetic resonance (CMR) imaging is the most sensitive method to diagnose MVO and CNR that provides information on the presence, localization and extent of MVO. CMR imaging detects intramyocardial hemorrhage and accurately estimates the infarct size. MVO and CNR markedly negate the benefits of reperfusion therapy and contribute to poor clinical outcomes including adverse remodeling of left ventricle, worsening or new congestive heart failure and reduced survival. Despite extensive research and the use of therapies that target almost all known pathophysiological mechanisms of CNR, no therapy has been found that prevents or reverses CNR and provides consistent clinical benefit in patients with STEMI undergoing reperfusion. Currently, the prevention or alleviation of MVO and CNR remain unmet goals in the therapy of STEMI that continue to be under intense research.

Long-Term Prognostic Value of Myocardial Viability by Myocardial Contrast Echocardiography in Patients after Acute Myocardial Infarction: A Systematic Review and Meta-Analysis

Background and Objectives: According to recent guidelines, myocardial contrast echocardiography (MCE) is recommended for detecting residual myocardial viability (MV). However, the long-term prognostic value of MV as assessed by MCE in identifying major adverse cardiac events (MACE) after acute myocardial infarction (AMI) remains undefined. Materials and Methods: We searched multiple databases, including PubMed, EMBASE, and Web of Science for studies on the prognostic value of MCE for clinical outcomes in AMI patients. The primary endpoints were MACEs during follow-up. Six studies that evaluated a total of 536 patients with a mean follow-up of 36.8 months were reviewed. Results: The pooled sensitivity and specificity of MCE for predicting MACEs were 0.80 and 0.78, respectively, and the summary operating receiver characteristics achieved an area under the curve of 0.84. The pooled relative risks demonstrated that the MV evaluated by MCE after AMI was correlated with a high risk for total cardiac events (pooled relative risk: 2.07; 95% confidence interval: 1.28–3.37) and cardiac death (pooled relative risk: 2.48; 95% confidence interval: 1.03–5.96). MV evaluated by MCE was a highly independent predictor of total cardiac events (pooled hazard ratio: 2.09, 95% confidence interval: 1.14–3.81) in patients after AMI. Conclusions: Residual MV evaluated by MCE may be an effective long-term prognostic tool for predicting MACE in patients after AMI that can provide moderate predictive accuracy. The assessment of MV by MCE may become an alternative technique with the potential to rapidly provide important information for improving long-term risk stratification in patients after AMI, at the bedside in clinical practice, especially for patients who cannot tolerate prolonged examinations. The PROSPERO registration number is CRD42020167565.

High Channel Temperature Mapping Electronics in a Thin, Soft, Wireless Format for Non-Invasive Body Thermal Analysis

Hemodynamic status has been perceived as an important diagnostic value as fundamental physiological health conditions, including decisive signs of fatal diseases like arteriosclerosis, can be diagnosed by monitoring it. Currently, the conventional hemodynamic monitoring methods highly rely on imaging techniques requiring inconveniently large numbers of operation procedures and equipment for mapping and with a high risk of radiation exposure. Herein, an ultra-thin, noninvasive, and flexible electronic skin (e-skin) hemodynamic monitoring system based on the thermal properties of blood vessels underneath the epidermis that can be portably attached to the skin for operation is introduced. Through a series of thermal sensors, the temperatures of each subsection of the arrayed sensors are observed in real-time, and the measurements are transmitted and displayed on the screen of an external device wirelessly through a Bluetooth module using a graphical user interface (GUI). The degrees of the thermal property of subsections are indicated with a spectrum of colors that specify the hemodynamic status of the target vessel. In addition, as the sensors are installed on a soft substrate, they can operate under twisting and bending without any malfunction. These characteristics of e-skin sensors exhibit great potential in wearable and portable diagnostics including point-of-care (POC) devices.

Research methodology for in vivo measurements of resting energy expenditure, daily body temperature, metabolic heat and non-viral tissue-specific gene therapy in baboons

A large number of studies have shown that the baboon is one of the most commonly used non-human primate (NHP) research model for the study of immunometabolic complex traits such as type 2 diabetes (T2D), insulin resistance (IR), adipose tissue dysfunction (ATD), dyslipidemia, obesity (OB) and cardiovascular disease (CVD). This paper reports on innovative technologies and advanced research strategies for energetics and translational medicine with this NHP model. This includes the following: measuring resting energy expenditure (REE) with the mobile indirect calorimeter Breezing®; monitoring daily body temperature using subcutaneously implanted data loggers; quantifying metabolic heat with veterinary infrared thermography (IRT) imaging, and non-viral non-invasive, tissue-specific ultrasound-targeted microbubble destruction (UTMD) gene-based therapy. These methods are of broad utility; for example, they may facilitate the engineering of ectopic overexpression of brown adipose tissue (BAT) mUCP-1 via UTMD-gene therapy into baboon SKM to achieve weight loss, hypophagia and immunometabolic improvement. These methods will be valuable to basic and translational research, and human clinical trials, in the areas of metabolism, cardiovascular health, and immunometabolic and infectious diseases.

Spectral Imaging for Microbubble Characterization

Microbubbles stabilized by an outer lipid shell have been studied extensively for both diagnostic and therapeutic applications. The shell composition can significantly influence microbubble behavior, but performing quantitative measurements of shell properties is challenging. The aim of this study is to investigate the use of spectral imaging to characterize the surface properties of a range of microbubble formulations representing both commercial and research agents. A lipophilic dye, C-laurdan, whose fluorescence emission varies according to the properties of the local environment, was used to compare the degree and uniformity of the lipid order in the microbubble shell, and these measurements were compared with the acoustic response and stability of the different formulations. Spectral imaging was found to be suitable for performing rapid and hence relatively high throughput measurements of microbubble surface properties. Interestingly, despite significant differences in lipid molecule size and charge, all of the different formulations exhibited highly ordered lipid shells. Measurements of liposomes with the same composition and the debris generated by destroying lipid microbubbles with ultrasound (US) showed that these exhibited a lower and more varied lipid order than intact microbubbles. This suggests that the high lipid order of microbubbles is due primarily to compression of the shell as a result of surface tension and is only minimally affected by composition. This also explains the similarity in acoustic response observed between the formulations, because microbubble dynamics are determined by the diameter and shell viscoelastic properties that are themselves a function of the lipid order. Within each population, there was considerable variability in the lipid order and response between individual microbubbles, suggesting the need for improved manufacturing techniques. In addition, the difference in the lipid order between the shell and lipid debris may be important for therapeutic applications in which shedding of the shell material is exploited, for example, drug delivery.

Contrast Transthoracic Echocardiography Using 50% Glucose as a Contrast Agent for Screening of a Patent Foramen Ovale

A patent foramen ovale (PFO) is considered a risk factor for neurologic events. The goal of the study described here was to assess the feasibility, advantages, diagnostic sensitivity and accuracy of contrast transthoracic echocardiography examination (cTTE) using 50% glucose as a contrast agent in comparison with the use of agitated saline as contrast to screen for PFO. In our study, we found that the peak time, effective duration and duration of microbubbles produced by 50% glucose were all longer than those produced by the physiologic saline. The sensitivities for detection of PFO with cTTE using physiologic saline and 50% glucose as contrast were 83% (20/24) and 100% (24/24), respectively. TEE suggested a PFO in 24 patients in two groups. Use of 50% glucose as a contrast agent in cTTE examination enables ultrasound technicians to easily observe the right-to-left shunt across the PFO. However, the sensitivities for detection of PFO with cTTE using 50% glucose did not statistically significantly differ from those for physiologic saline.

Cardiovascular aging and the microcirculation of skeletal muscle: using contrast-enhanced ultrasound

Skeletal muscle is the largest and most important site of capillary-tissue exchange, especially during high-energy demand tasks such as exercise; however, information regarding the role of the microcirculation in maintaining skeletal muscle health is limited. Changes in microcirculatory function, as observed with aging, chronic and cardiovascular diseases, and exercise, likely precede any alterations that arise in larger vessels, although further investigation into these changes is required. One of the main barriers to addressing this knowledge gap is the lack of methodologies for quantifying microvascular function in vivo; the utilization of valid and noninvasive quantification methods would allow the dynamic evaluation of microvascular flow during periods of clinical relevance such as during increased demand for flow (exercise) or decreased demand for flow (disuse). Contrast-enhanced ultrasound (CEUS) is a promising noninvasive technique that has been used for diagnostic medicine and more recently as a complementary research modality to investigate the response of the microcirculation in insulin resistance, diabetes, and aging. To improve the reproducibility of these measurements, our laboratory has optimized the quantification protocol associated with a bolus injection of the contrast agent for research purposes. This brief report outlines the assessment of microvascular flow using the raw time-intensity curve incorporated into gamma variate response modeling. CEUS could be used to compliment any macrovascular assessments to capture a more complete picture of the aging vasculature, and the modified methods presented here provide a template for the general analysis of CEUS within a research setting.

Clinical practice of contrast echocardiography: recommendation by the European Association of Cardiovascular Imaging (EACVI) 2017

Contrast echocardiography is widely used in cardiology. It is applied to improve image quality, reader confidence and reproducibility both for assessing left ventricular (LV) structure and function at rest and for assessing global and regional function in stress echocardiography. The use of contrast in echocardiography has now extended beyond cardiac structure and function assessment to evaluation of perfusion both of the myocardium and of the intracardiac structures. Safety of contrast agents have now been addressed in large patient population and these studies clearly established its excellent safety profile. This document, based on clinical trials, randomized and multicentre studies and published clinical experience, has established clear recommendations for the use of contrast in various clinical conditions with evidence-based protocols.

Stress echocardiography in contemporary clinical cardiology: practical considerations and accreditation

Stress echocardiography is a widely utilised test in patients with known or suspected coronary artery disease (CAD), valvular heart disease and cardiomyopathies. Its advantages include the ubiquitous availability of echocardiography, lack of ionising radiation, choice of physiological or pharmacological stressors, good diagnostic accuracy and robust supporting evidence base. SE has evolved significantly as a technique over the past three decades and has benefitted considerably from improvements in overall image quality (superior resolution), machine technology (e.g. digital cine-loop acquisition and side-by-side image display) and development of second-generation ultrasound contrast agents that have improved reader confidence and diagnostic accuracy. The purpose of this article is to review the breadth of SE in contemporary clinical cardiology and discuss the recently launched British Society of Echocardiography (BSE) Stress Echocardiography accreditation scheme.

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.

Contrast Microsphere Destruction by a Continuous Flow Ventricular Assist Device: An In Vitro Evaluation Using a Mock Circulation Loop

OBJECTIVES

Transthoracic echocardiography (TTE) is fundamental in managing patients supported with ventricular assist devices (VAD). However imaging can be difficult in these patients. Contrast improves image quality but they are hydrodynamically fragile agents. The aim was to assess contrast concentration following passage through a VAD utilising a mock circulation loop (MCL).

METHODS

Heartware continuous flow (CF) VAD was incorporated into a MCL. Definity® contrast was infused into the MCL with imaging before and after CF-VAD. 5 mm2 regions of interest were used to obtain signal intensity (decibels), as a surrogate of contrast concentration.

RESULTS

Four pump speeds revealed significant reduction in contrast signal intensity after CF-VAD compared to before CF-VAD (all p < 0.0001). Combined pre- and postpump data at all speeds showed a 22.2% absolute reduction in contrast signal intensity across the CF-VAD (14.8 ± 0.8 dB prepump versus 11.6 ± 1.4 dB postpump; p < 0.0001). Mean signal intensity reduction at each speed showed an inverse relationship between speed and relative reduction in signal intensity.

CONCLUSION

Contrast microsphere transit through a CF-VAD within a MCL resulted in significant reduction in signal intensity, consistent with destruction within the pump. This was evident at all CF-VAD pump speeds but relative signal drop was inversely proportional to pump speed.

Optimally Repeatable Kinetic Model Variant for Myocardial Blood Flow Measurements with 82Rb PET

Purpose. Myocardial blood flow (MBF) quantification with ⁸²Rb positron emission tomography (PET) is gaining clinical adoption, but improvements in precision are desired. This study aims to identify analysis variants producing the most repeatable MBF measures. Methods. 12 volunteers underwent same-day test-retest rest and dipyridamole stress imaging with dynamic ⁸²Rb PET, from which MBF was quantified using 1-tissue-compartment kinetic model variants: (1) blood-pool versus uptake region sampled input function (Blood/Uptake-ROI), (2) dual spillover correction (SOC-On/Off), (3) right blood correction (RBC-On/Off), (4) arterial blood transit delay (Delay-On/Off), and (5) distribution volume (DV) constraint (Global/Regional-DV). Repeatability of MBF, stress/rest myocardial flow reserve (MFR), and stress/rest MBF difference (ΔMBF) was assessed using nonparametric reproducibility coefficients (RPC_np = 1.45 × interquartile range). Results. MBF using SOC-On, RVBC-Off, Blood-ROI, Global-DV, and Delay-Off was most repeatable for combined rest and stress: RPC_np = 0.21 mL/min/g (15.8%). Corresponding MFR and ΔMBF RPC_np were 0.42 (20.2%) and 0.24 mL/min/g (23.5%). MBF repeatability improved with SOC-On at stress (p < 0.001) and tended to improve with RBC-Off at both rest and stress (p < 0.08). DV and ROI did not significantly influence repeatability. The Delay-On model was overdetermined and did not reliably converge. Conclusion. MBF and MFR test-retest repeatability were the best with dual spillover correction, left atrium blood input function, and global DV.

A fundamental study for quantitative measurement of ultrasound contrast concentration by low mechanical index contrast ultrasonography

PURPOSE

In high mechanical index (MI) contrast ultrasonography it has been shown that the power of contrast signal intensity (CI) has a strong linear correlation with the concentration of the ultrasound contrast agent under conditions of constant applied acoustic pressure. However, it is unclear whether the linearity is preserved in low-MI contrast ultrasonography. Thus, we investigated the relationship between ultrasound contrast concentration and CI in vitro.

METHODS

Solutions of the ultrasound contrast agents Definity and Imagent were prepared at concentrations of 0.5, 2, 8, 32, and 128 μl/l. Placing a jelly block between the transducer and the solution, the solutions were transmitted using pulse subtraction imaging with an MI of 0.05, 0.1, and 0.5. CI was measured in dB in a region of interest 3 mm in height placed just below the border between the jelly and the solution. Data were plotted using double logarithm scales, where the concentration was expressed in dB as 10 × log (concentration).

RESULTS

CI in dB had a strong linear correlation with concentration in dB for both agents with any MI. Best fitted slopes were close to 1, indicating that the power of CI is proportional to the concentration.

CONCLUSIONS

In low-MI contrast ultrasonography, the power of CI is proportional to contrast concentration, and CI in dB is logarithmic to the concentration. Thus, the microbubble concentration can be quantitatively measured even in low-MI contrast ultrasonography.

Resting Myocardial Contrast Echocardiography for the Evaluation of Coronary Microcirculation Dysfunction in Patients With Early Coronary Artery Disease

BACKGROUND

Coronary microcirculation dysfunction can occur in patients with chest pain suggestive of coronary artery disease (CAD). The present study aimed to determine the diagnostic value of resting myocardial contrast echocardiography (MCE) for early CAD with myocardial microcirculation dysfunction by evaluating the continuous imaging time, peak time, and peak intensity.

HYPOTHESIS

Resting MCE is an effective and noninvasive method for evaluation of coronary microcirculation dysfunction in patients with early coronary artery disease.

METHODS

The present study included 20 consecutive patients without obvious clinical evidence of early CAD and 20 healthy volunteers. Resting MCE was performed to evaluate the myocardial microcirculation perfusion, and the follow-up evaluation of myocardial microcirculation perfusion was performed with technetium 99 m 2-methoxy-isobutyl-isonitrile ((99m) Tc-MIBI) single-photon emission computed tomography (SPECT).

RESULTS

Peak intensity was significantly lower in patients with high risk of CAD than in controls (P < 0.0001). The peak time and continuous imaging time were significantly higher in patients with high risk of CAD than in controls (P < 0.0001). None of the 40 subjects experienced discomfort, such as cough and chest tightness, during the resting MCE procedure, and the heart rate and blood pressure showed no abnormalities during the entire procedure. SPECT imaging showed reversible myocardial perfusion reduction in 80% (16/20) of the patients with high risk of CAD. Abnormalities of heart rate and blood pressure and adverse reactions were noted during the process of SPECT examination.

CONCLUSIONS

Resting MCE is an effective and noninvasive method for detecting abnormalities of coronary microcirculation and can help in the clinical analysis, risk assessment, and treatment of early occult CAD.

Targeted Transthoracic Acoustic Activation of Systemically Administered Nanodroplets to Detect Myocardial Perfusion Abnormalities

BACKGROUND

Liquid core nanodroplets containing condensed gaseous fluorocarbons can be vaporized at clinically relevant acoustic energies and have been hypothesized as an alternative ultrasound contrast agent instead of gas-core agents. The potential for targeted activation and imaging of these agents was tested with droplets formulated from liquid octafluoropropane (C3) and 1:1 mixtures of C3 with liquid decafluorobutane (C3C4).

METHODS AND RESULTS

In 8 pigs with recent myocardial infarction and variable degrees of reperfusion, transthoracic acoustic activation was attempted using 1.3 to 1.7 MHz low (0.2 mechanical index [MI]) or high MI (1.2 MI) imaging in real time (32-64 Hertz) or triggered 1:1 at end systole during a 20% C3 or C3C4 droplet infusion. Any perfusion defects observed were measured and correlated with delayed enhancement magnetic resonance imaging and postmortem staining. No myocardial contrast was produced with any imaging setting when using C3C4 droplets or C3 droplets during low MI real-time imaging. However, myocardial contrast was observed in all 8 pigs with C3 droplets when using triggered high MI imaging and in 5 of 6 pigs that had 1.7 MHz real time-high MI imaging. Although quantitative myocardial contrast was lower with real-time high MI imaging than 1:1 triggering, the correlation between real-time resting defect size and infarct size was good (r=0.97; P<0.001), as was the correlation with number of transmural infarcted segments by delayed enhancement imaging.

CONCLUSIONS

Targeted transthoracic acoustic activation of infused intravenous C3 nanodroplets is effective, resulting in echogenic and persistent microbubbles which provide real-time high MI visualization of perfusion defects.

Evaluation of the Accuracy of Liver Lesion DCEUS Quantification With Respiratory Gating

Confidence in the accuracy of dynamic contrast enhanced ultrasound (DCEUS) quantification parameters is imperative for the correct diagnosis of liver lesion perfusion characteristics. An important source of uncertainty in liver DCEUS acquisitions is artifacts introduced by respiratory motion. The objective of this study is to construct a respiratory motion simulation model (RMSM) of dual contrast imaging mode acquisitions of liver lesions in order to evaluate an algorithm for automatic respiratory gating (ARG). The respiratory kinetics as well as the perfusion models of the liver lesion and parenchyma used by the RMSM were solely derived from clinical data. The quality of fit (of the DCEUS data onto the bolus kinetics model) depends on the respiration amplitude. Similar trends in terms of quality of fit as a function of respiration amplitude were observed from RMSM and clinical data. The errors introduced on the DCEUS quantification under the influence of respiration were evaluated. The RMSM revealed that the error in the liver lesion DCEUS quantification parameters significantly decreased (p < 0.001) from a maximum of 32.3% to 6.2% when ARG was used. The use of RMSM clearly demonstrates the capability of the ARG algorithm in significantly reducing errors introduced from both in-plane and out-of-plane respiratory motion.

Simultaneous Assessment of Myocardial Perfusion, Wall Motion, and Deformation during Myocardial Contrast Echocardiography: A Feasibility Study

AIMS

Ultrasound contrast agents may be used for the assessment of regional wall motion and myocardial perfusion, but are generally considered not suitable for deformation analysis. The aim of our study was to assess the feasibility of deformation imaging on contrast-enhanced images using a novel methodology.

METHODS AND RESULTS

We prospectively enrolled 40 patients who underwent stress echocardiography with continuous intravenous infusion of SonoVue for the assessment of myocardial perfusion imaging with flash replenishment technique. We compared longitudinal strain (Lε) values, assessed with a vendor-independent software (2D CPA), on 68 resting contrast-enhanced and 68 resting noncontrast recordings. Strain analysis on contrast recordings was evaluated in the first cardiac cycles after the flash. Tracking of contrast images was deemed feasible in all subjects and in all views. Contrast administration improved image quality and increased the number of segments used for deformation analysis. Lε of noncontrast and contrast-enhanced images were statistically different (-18.8 ± 4.5% and -22.8 ± 5.4%, respectively; P < 0.001), but their correlation was good (ICC 0.65, 95%CI 0.42-0.78). Patients with resting wall-motion abnormalities showed lower Lε values on contrast recordings (-18.6 ± 6.0% vs. -24.2 ± 5.5%, respectively; P < 0.01). Intra-operator and inter-operator reproducibility was good for both noncontrast and contrast images with no statistical differences.

CONCLUSIONS

Our study shows that deformation analysis on postflash contrast-enhanced images is feasible and reproducible. Therefore, it would be possible to perform a simultaneous evaluation of wall-motion abnormalities, volumes, ejection fraction, perfusion defects, and cardiac deformation on the same contrast recording.

Myocardial perfusion imaging using contrast echocardiography

Microbubbles are an excellent intravascular tracer, and both the rate of myocardial opacification (analogous to coronary microvascular perfusion) and contrast intensity (analogous to myocardial blood volume) provide unique insights into myocardial perfusion. A strong evidence base has been accumulated to show comparability with nuclear perfusion imaging and incremental diagnostic and prognostic value relative to wall motion analysis. This technique also provides the possibility to measure myocardial perfusion at the bedside. Despite all of these advantages, the technique is complicated, technically challenging, and has failed to scale legislative and financial hurdles. The development of targeted imaging and therapeutic interventions will hopefully rekindle interest in this interesting modality.

A primer on the methods and applications for contrast echocardiography in clinical imaging

Contrast echocardiography is broadly described as a variety of techniques whereby the blood pool on cardiac ultrasound is enhanced with encapsulated gas-filled microbubbles or other acoustically active nano- or microparticles. The development of this technology has occurred primarily in response to the need improve current diagnostic applications of echocardiography such as the need to better define left ventricular cavity volumes, regional wall motion, or the presence or absence of masses and thrombi. A secondary reason for the development of contrast echocardiography has been to expand the capabilities of echocardiography. These new applications include myocardial perfusion imaging for detection of ischemia and viability, perfusion imaging of masses/tumors, and molecular imaging. The ability to fill all of these current and future clinical roles has been predicated on the ability to produce robust contrast signal which, in turn, has relied on technical innovation with regards to the microbubble contrast agents and the ultrasound imaging paradigms. In this review, we will discuss the basics of contrast echocardiography including the composition of microbubble contrast agents, the unique imaging methods used to optimize contrast signal-to-noise ratio, and the clinical applications of contrast echocardiography that have made a clinical impact.

Ischemic memory imaging in nonhuman primates with echocardiographic molecular imaging of selectin expression

BACKGROUND

Selectins are adhesion molecules that are expressed by the vascular endothelium upon activation and may be an imaging target for detecting myocardial ischemia long after resolution. The aim of this study was to test the hypothesis that molecular imaging of selectins with myocardial contrast echocardiographic (MCE) molecular imaging could be used to detect recent brief ischemia in closed-chest nonhuman primates.

METHODS

Myocardial ischemia was produced in anesthetized adult rhesus macaques (n = 6) by percutaneous balloon catheter occlusion of the left anterior descending or circumflex coronary artery for 5 to 10 min. Three separate macaques served as nonischemic controls. MCE perfusion imaging was performed during coronary occlusion to measure risk area and at 100 to 110 min to exclude infarction. MCE molecular imaging was performed at 30 and 90 min after reperfusion using a lipid microbubble bearing dimeric recombinant human P-selectin glycoprotein ligand-1 (MB-YSPSL). Collection of blood for safety data, electrocardiography, and echocardiography were performed at baseline and before and 10 min after each MB-YSPSL injection.

RESULTS

Vital signs, oxygen saturation, electrocardiographic results, ventricular systolic function, pulmonary vascular resistance, and serum safety markers were unchanged by intravenous injection of MB-YSPSL. On echocardiography, left ventricular dysfunction in the risk area had resolved by 30 min, and there was no evidence of infarction on MCE perfusion imaging. On selectin-targeted MCE molecular imaging, signal enhancement was greater (P < .05) in the risk area than remote territory at 30 min (25 ± 11 vs 11 ± 4 IU) and 90 min (13 ± 3 vs 3 ± 2 IU) after ischemia. There was no enhancement (<1 IU) in control nonischemic subjects.

CONCLUSIONS

In primates, MCE molecular imaging of selectins using MB-YSPSL, a recombinant ligand appropriate for humans, is both safe and effective for imaging recent myocardial ischemia. This technique may be useful for detecting recent ischemia in patients with chest pain even in the absence of necrosis.

The feasibility and clinical utility of myocardial contrast echocardiography in clinical practice: results from the incorporation of myocardial perfusion assessment into clinical testing with stress echocardiography study

BACKGROUND

This prospective study investigated whether the incorporation of myocardial contrast echocardiography (MCE) into a clinical stress echocardiography service reproduces the benefits of assessing myocardial perfusion proved previously in research studies.

METHODS

MCE was performed during physiologic and pharmacologic clinical stress echocardiographic studies, and the value of myocardial perfusion to the reporting echocardiologists was categorized as of benefit (subclassified as incremental benefit over wall motion [WM] or greater confidence with WM) or of no added benefit. The presence and extent of inducible ischemia by WM and myocardial perfusion were documented and correlated with angiographic results in patients who underwent cardiac catheterization.

RESULTS

In total, 220 patients underwent simultaneous MCE during stress echocardiography by eight different operators. Overall, MCE was of benefit in 193 patients (88%), providing incremental benefit over WM in 25% and greater confidence with WM evaluation in 62%. MCE provided no added benefit in 27 patients (12%). MCE detected significantly more cases of ischemia than WM in the left anterior descending coronary artery territory (65% vs 53%, P = .02) and detected a greater ischemic burden than WM on a per patient basis (median, 5 [interquartile range, 3-8] vs 4 [interquartile range, 2-7] segments; P < .001) and across all coronary territories. MCE correctly identified a greater proportion of patients with multivessel disease than WM (76% vs 56%, P = .02) and a greater ischemic burden in patients with multivessel disease (median, 7 [interquartile range, 4-9] vs 5 [interquartile range, 1-8] segments; P < .001).

CONCLUSIONS

This prospective study is the first to demonstrate that the excellent feasibility and diagnostic utility of MCE, which have been documented in the research arena, are reproducible in the clinical arena.

Trends in myocardial contrast echocardiography: parameteric versus volumetric imaging

It is now possible to perform myocardial contrast echocardiography at the bedside with an intravenous injection of commercially available contrast media. Although myocardial contrast echocardiography is a sensitive method for the detection of coronary stenosis and myocardial viability, its diagnosis has relied largely on the subjective interpretation of regional perfusion by experienced clinicians. Thus, quantification of myocardial contrast echocardiography data and displaying comprehensive images have been necessary for its routine application. In this review, new methods for quantifying or displaying myocardial contrast echocardiography parameters will be introduced: firstly, parametric imaging that displays the parameters of myocardial blood volume, blood flow velocity and myocardial blood flow separately; and secondly, color-coded maps of myocardial blood volume established from one myocardial contrast echocardiography image. These quantitative techniques can provide comprehensive and easy-to-understand images, although the quality of the baseline image remains a critical factor.

Automatic respiratory gating for contrast ultrasound evaluation of liver lesions

Dynamic contrast-enhanced ultrasound (DCEUS) has been used in radiology for many years for lesion detection and characterization. In recent years, more emphasis has been placed on tumor perfusion quantification with DCEUS. To ensure accuracy in both quantitative and qualitative evaluation of liver tumors with DCEUS, sources of noise in clinical data must be identified and, if possible, removed. One of the major sources of such noise is respiratory motion. A new automatic respiratory gating (ARG) algorithm is presented and evaluated with clinical data. The results of the evaluation demonstrate the potential of the ARG algorithm for clinical use as a fast and easy-to-implement method for removing respiratory motion from DCEUS loops.

Echocardiographic evaluation of the effects of stem cell therapy on perfusion and function in ischemic cardiomyopathy

BACKGROUND

Small animal models of ischemic left ventricular (LV) dysfunction are important for the preclinical optimization of stem cell therapy. The aim of this study was to test the hypothesis that temporal changes in LV function and regional perfusion after cell therapy can be assessed in mice using echocardiographic imaging.

METHODS

Wild-type mice (n = 25) were studied 7 and 28 days after permanent ligation of the left anterior descending coronary artery. Animals were randomized to receive closed-chest ultrasound-guided intramyocardial delivery of saline (n = 13) or 5 × 10(5) multipotential adult progenitor cells (MAPCs; n = 12) on day 7. LV end-diastolic and end-systolic volumes, LV ejection fraction, and stroke volume were measured using high-frequency echocardiography. Multiplanar assessments of perfusion and defect area size were made using myocardial contrast echocardiography.

RESULTS

Between days 7 and 28, MAPC-treated animals had 40% to 50% reductions in defect size (P < .001) and 20% to 30% increases in total perfusion (P < .01). Perfusion did not change in nontreated controls. Both LV end-diastolic and end-systolic volumes increased between days 7 and 28 in both groups, but LV end-systolic volume increased to a lesser degree in MAPC-treated compared with control mice (+4.2 ± 7.9 vs +19.2 ± 22.0 μL, P < .05). LV ejection fraction increased in the MAPC-treated mice and decreased in control mice (+3.0 ± 4.3% vs -5.6 ± 5.9%, P < .01). There was a significant linear relation between the change in LV ejection fraction and the change in either defect area size or total perfusion.

CONCLUSIONS

High-frequency echocardiography and myocardial contrast echocardiography in murine models of ischemic LV dysfunction can be used to assess the response to stem cell therapy and to characterize the relationship among spatial flow, ventricular function, and ventricular remodeling.

Qualitative and quantitative contrast enhanced ultrasonography of the pancreas using bolus injection and continuous infusion methods in normal dogs

Quantitative contrast enhanced ultrasound is a major breakthrough for ultrasound imaging in recent years. However, contrast enhancement of the pancreas is brief with bolus injection. To assess if continuous infusion of Sonazoid^® can prolong the duration of pancreatic enhancement over bolus injections, eight adult dogs received bolus injection and continuous infusion of Sonazoid^® on separate days. Contrast enhanced ultrasound of the pancreatic parenchyma and proximal descending duodenum was performed, and time intensity curves reflecting tissue perfusions were generated. Perfusion parameters- time to initial upslope, peak time, time to wash-out and peak intensity were calculated and evaluated. Fast wash-in to intense peak, followed by rapid wash-out was observed for time intensity curves of bolus injection. With continuous infusion, contrast wash-in to peak intensity was gradual, followed by long plateau and slow wash-out. Median contrast enhancement durations of the pancreas and duodenum were significantly prolonged by continuous infusion from 11 sec (range, 10 to 23 sec) and 16 sec (range, 3 to 43 sec) at bolus injection to 205 sec (range, 170 to 264 sec, P<0.01) and 193 sec (range, 169 to 216 sec, P<0.05), respectively. Median peak intensity of the pancreas was 100.9 MPV (range, 80.2 to 124.3 MPV) at bolus injection and 77.6 MPV (range, 58.2 to 99.5 MPV, P<0.05) at continuous infusion. Prolonged continuous imaging is afforded by continuous infusion of contrast agent. Peak intensity of the pancreas was slightly diminished in continuous infusion, but offered adequate imaging subjectively.

Echocardiography in the era of multimodality cardiovascular imaging

Echocardiography remains the most frequently performed cardiac imaging investigation and is an invaluable tool for detailed and accurate evaluation of cardiac structure and function. Echocardiography, nuclear cardiology, cardiac magnetic resonance imaging, and cardiovascular-computed tomography comprise the subspeciality of cardiovascular imaging, and these techniques are often used together for a multimodality, comprehensive assessment of a number of cardiac diseases. This paper provides the general cardiologist and physician with an overview of state-of-the-art modern echocardiography, summarising established indications as well as highlighting advances in stress echocardiography, three-dimensional echocardiography, deformation imaging, and contrast echocardiography. Strengths and limitations of echocardiography are discussed as well as the growing role of real-time three-dimensional echocardiography in the guidance of structural heart interventions in the cardiac catheter laboratory.

Comparison of sulfur hexafluoride microbubble (SonoVue)-enhanced myocardial contrast echocardiography with gated single-photon emission computed tomography for detection of significant coronary artery disease: a large European multicenter study

OBJECTIVES

The purpose of this study was to compare sulfur hexafluoride microbubble (SonoVue)-enhanced myocardial contrast echocardiography (MCE) with single-photon emission computed tomography (SPECT) relative to coronary angiography (CA) for assessment of coronary artery disease (CAD).

BACKGROUND

Small-scale studies have shown that myocardial perfusion assessed by SonoVue-enhanced MCE is a viable alternative to SPECT for CAD assessment. However, large multicenter studies are lacking.

METHODS

Patients referred for myocardial ischemia testing at 34 centers underwent rest/vasodilator SonoVue-enhanced flash-replenishment MCE, standard (99m)Tc-labeled electrocardiography-gated SPECT, and quantitative CA within 1 month. Myocardial ischemia assessments by 3 independent, blinded readers for MCE and 3 readers for SPECT were collapsed into 1 diagnosis per patient per technique and were compared to CA (reference standard) read by 1 independent blinded reader.

RESULTS

Of 628 enrolled patients who received SonoVue (71% males; mean age: 64 years; >1 cardiovascular [CV] risk factor in 99% of patients) 516 patients underwent all 3 examinations, of whom 161 (31.2%) had ≥70% stenosis (131 had single-vessel disease [SVD]; 30 had multivessel disease), and 310 (60.1%) had ≥50% stenosis. Higher sensitivity was obtained with MCE than with SPECT (75.2% vs. 49.1%, respectively; p < 0.0001), although specificity was lower (52.4% vs. 80.6%, respectively; p < 0.0001) for ≥70% stenosis. Similar findings were obtained for patients with ≥50% stenosis. Sensitivity levels for detection of SVD and proximal disease for ≥70% stenosis were higher for MCE (72.5% vs. 42.7%, respectively; p < 0.0001; 80% vs. 58%, respectively; p = 0.005, respectively).

CONCLUSIONS

SonoVue-enhanced MCE demonstrated superior sensitivity but lower specificity for detection of CAD compared to SPECT in a population with a high incidence of CV risk factors and intermediate-high prevalence of CAD. (A phase III study to compare SonoVue® enhanced myocardial echocardiography [MCE] to single photon emission computerized tomography [ECG-GATED SPECT], at rest and at peak of low-dose Dipyridamole stress test, in the assessment of significant coronary artery disease [CAD] in patients with suspect or known CAD using Coronary Angiography as Gold Standard-SonoVue MCE vs SPECT; EUCTR2007-003492-39-GR).

The value of quantitative real-time myocardial contrast echocardiography for detection of angiographically significant coronary artery disease

BACKGROUND

Real-time (RT) myocardial contrast echocardiography (MCE) is a novel method for the assessment of regional myocardial perfusion. We sought to evaluate the feasibility and diagnostic accuracy of quantitative RT-MCE in predicting significant coronary stenosis, with reference to quantitative coronary angiography.

HYPOTHESIS

RT-MCE can identify anatomically significant coronary artery stenosis in selected patients. RT-MCE is probably an effective method for detection of angiographically significant coronary artery stenosis.

METHODS

Thirty-five patients (mean age, 59.94 ± 10.63 years; 25 males) scheduled for coronary angiography underwent RT-MCE at rest, and shortly afterward underwent gated single-photon emission computed tomography (gated-SPECT). Coronary angiography was performed within 1 week after RT-MCE in all patients. The observing indexes included the images of RT-MCE that were analyzed quantitatively from microbubble replenishment curves for myocardial perfusion by using the Q-Lab software. The sensitivity and specificity of RT-MCE for quantitative detection of coronary artery disease (CAD) were obtained. The receiver operator characteristic (ROC) curves were used to assess the differences of accuracy in ischemic segments with A, β and A × β respectively. The sensitivity and specificity of gated-SPECT and RT-MCE for assessment of CAD were calculated using a 4-score method.

RESULTS

A total of 513 segments among 595 segments in 35 patients were obtained. The cutoffs for A, β and A × β were 4.58, 0.64, and 2.73, and the sensitivity and specificity of quantitative RT-MCE for detection of CAD were 86.0%, 80.2%, 88.9%, and 84.1%, 64.6%, 79.9%, respectively. Meanwhile, the sensitivity and specificity of semiquantitative analysis for assessment of CAD were 66.7% and 61.8%. The ROC curve area of A and A × β was 0.91 and 0.90 in the middle segments. The ROC area of A was 0.52 in the base segments. The sensitivity and specificity of gated-SPECT for assessment of CAD were 84.8% and 82.7%, respectively. The sensitivity of multi-indexes RT-MCE increased. The sensitivity was 89.1%, 90.4%, and 96.3% by A + β, A + A × β, and β + A × β.

CONCLUSIONS

Quantitative RT-MCE is an effective method for the detection of coronary artery stenosis. Quantitative RT-MCE is segmented for assessment to ischemic myocardium. RT-MCE with multi-indexes has a valuable application for assessment of CAD surpassing SPECT.

Coronary autoregulation is abnormal in syndrome X: insights using myocardial contrast echocardiography

BACKGROUND

Syndrome X in women is thought to be caused by coronary microvascular dysfunction, the exact site of which is unknown. The aim of this study was to characterize the microvascular site of dysfunction in these patients using myocardial contrast echocardiography.

METHODS

Women with exertional angina, positive test results on stress imaging, but no coronary artery disease (the study group, n = 18) and age-matched control women also with no coronary artery disease (n = 17) were enrolled. Myocardial contrast echocardiography was performed at rest and during dipyridamole-induced hyperemia. Mean microbubble velocity (β) and myocardial blood volume (A) were measured, and myocardial blood flow (A · β) was computed. In addition, plasma concentrations of eicosanoids, female sex hormones, and C-reactive protein were measured.

RESULTS

Rest β and myocardial blood flow (A · β) were higher in the study compared with the control women (1.61 ± 0.68 vs. 0.74 ± 0.44, P = .0001, and 157 ± 121 vs. 54 ± 54, P = 0.0001, respectively) despite similar heart rates and systolic blood pressures. After the administration of dipyridamole, whereas the changes in A and A · β were not significantly different between the two groups, β reserve (the ratio of stress β to rest β) was markedly lower in the study group (1.48 ± 0.62 vs. 2.78 ± 0.94, P = .0001). Blood hematocrit, eicosanoids, female sex hormones, glucose, and C-reactive protein were not different between the two groups.

CONCLUSIONS

Coronary autoregulation is abnormal in patients with syndrome X (higher resting β and myocardial blood flow and lower β reserve), which suggests that the coronary resistance vessels are the site of microvascular abnormality.

What is coronary blood flow reserve? Insights using myocardial contrast echocardiography

This review will briefly describe the principles of myocardial contrast echocardiography, and then discuss the clinical and experimental observations that led to the use of this approach to investigate the pathophysiological basis of coronary blood flow reserve. The insights offered by myocardial contrast echocardiography are unique and novel, and highlight the importance of the myocardial capillaries in determining coronary blood flow reserve in health and disease.

Myocardial ischaemia and viability: the pivotal role of echocardiography

Echocardiography has a central role for the diagnosis and management of patients with known or suspected coronary artery disease. Besides the fact that it provides an essential role in the differential diagnosis of patients presenting with chest pain in the emergency department, echocardiography provides a comprehensive non-invasive haemodynamic and functional assessment of those patients. Stress echocardiography in many institutions is now the preferred stress modality associated with imaging as it is cost-effective and does not use ionizing radiation. It is used for assessing patients with known or suspected coronary artery disease, risk stratification and for assessing myocardial viability. The recent introductions of ultrasound contrast agents as well as deformation imaging techniques have eliminated the last limitations of stress echocardiography such as image quality and quantification, respectively.

Free fatty acids induce insulin resistance in both cardiac and skeletal muscle microvasculature in humans

CONTEXT

Insulin recruits microvasculature in both cardiac and skeletal muscle, which increases the endothelial exchange surface area. Plasma concentrations of free fatty acids (FFAs) are elevated in patients with diabetes, which impairs insulin-mediated skeletal muscle microvascular recruitment.

OBJECTIVE

The objective of the study was to examine whether elevated FFAs likewise cause insulin resistance in cardiac muscle microvasculature.

SETTING

The study was conducted at the General Clinical Research Center at the University of Virginia.

METHODS

Twenty-two healthy, young adults were studied twice in random order after an overnight fast. Each subject received a 5-h systemic infusion of either saline or Intralipid/heparin with a 1 mU/min · kg euglycemic insulin clamp superimposed for the last 2 h. Cardiac and forearm skeletal muscle microvascular blood volume (MBV) and flow velocity were measured and microvascular blood flow (MBF) calculated before and at the end of the insulin infusion.

RESULTS

Insulin significantly increased MBV and MBF in both cardiac (P < 0.0001 for both) and skeletal (P = 0.008 and < 0.03, respectively) muscle. Microvascular flow velocity increased slightly but significantly in the skeletal (P = 0.04) but not in cardiac muscle. Lipid infusion lowered insulin-stimulated whole-body glucose disposal and abolished insulin-mediated increases in MBV and MBF in both cardiac and skeletal muscle. Whole-body insulin sensitivity predicted skeletal but not cardiac muscle microvascular responses to insulin. Insulin even decreased skeletal muscle MBV during lipid infusion in subjects who were moderately sensitive to insulin metabolically.

CONCLUSIONS

In conclusion, high plasma concentrations of FFAs cause insulin resistance in cardiac as well as skeletal muscle microvasculature in healthy humans. This may contribute to the association of cardiac complications with metabolic insulin resistance in diabetes.

Assessment of tissue perfusion by contrast-enhanced ultrasound

Contrast-enhanced ultrasound (CEUS) with microbubble contrast agents is a new imaging technique for quantifying tissue perfusion. CEUS presents several advantages over other imaging techniques in assessing tissue perfusion, including the use of microbubbles as blood-pool agents, portability, availability and absence of exposure to radiation or nuclear tracers. Dedicated software packages are necessary to quantify the echo-signal intensity and allow the calculation of the degree of tissue contrast enhancement based on the accurate distinction between microbubble backscatter signals and native tissue background. The measurement of organ transit time after microbubble injection and the analysis of tissue reperfusion kinetics represent the two fundamental methods for the assessment of tissue perfusion by CEUS. Transit time measurement has been shown to be feasible and has started to become accepted as a clinical tool, especially in the liver. The loudness of audio signals from spectral Doppler analysis is used to generate time-intensity curves to follow the wash-in and wash-out of the microbubble bolus. Tissue perfusion may be quantified also by analysing the replenishment kinetics of the volume of microbubbles after their destruction in the imaged slice. This allows to obtain semiquantitative parameters related to local tissue perfusion, especially in the heart, brain, and kidneys.

Roles of myocardial blood volume and flow in coronary artery disease: an experimental MRI study at rest and during hyperemia

OBJECTIVE

To validate fast perfusion mapping techniques in a setting of coronary artery stenosis, and to further assess the relationship of absolute myocardial blood volume (MBV) and blood flow (MBF) to global myocardial oxygen demand.

METHODS

A group of 27 mongrel dogs were divided into 10 controls and 17 with acute coronary stenosis. On 1.5-T MRI, first-pass perfusion imaging with a bolus injection of a blood-pool contrast agent was performed to determine myocardial perfusion both at rest and during either dipyridamole-induced vasodilation or dobutamine-induced stress. Regional values of MBF and MBV were quantified by using a fast mapping technique. Color microspheres and (99m)Tc-labeled red blood cells were injected to obtain respective gold standards.

RESULTS

Microsphere-measured MBF and (99m)Tc-measured MBV reference values correlated well with the MR results. Given the same changes in MBF, changes in MBV are twofold greater with dobutamine than with dipyridamole. Under dobutamine stress, MBV shows better association with total myocardial oxygen demand than MBF. Coronary stenosis progressively reduced this association in the presence of increased stenosis severity.

CONCLUSIONS

MR first-pass perfusion can rapidly estimate regional MBF and MBV. Absolute quantification of MBV may add additional information on stenosis severity and myocardial viability compared with standard qualitative clinical evaluations of myocardial perfusion.

Investigation of the relationship of nonlinear backscattered ultrasound intensity with microbubble concentration at low MI

The aim of this study was to measure the relationship of image intensity with contrast agent concentration. In vitro experiments were performed with a flow phantom and a sulphur hexafluoride filled microbubble contrast agent (SonoVue) at different concentrations (0.004 per thousand to 4 per thousand) covering the range commonly encountered in clinical practice. The concentration of microbubbles in the contrast agent solutions was confirmed optically. Images were collected with a diagnostic ultrasound system (iU22, Phillips Medical Systems, Bothell, WA, USA) and with a nonlinear imaging technique (power modulation) at low mechanical index (MI=0.05) to avoid bubble destruction. The mean intensity within a region of interest was measured to produce time-intensity curves from linearized (absolute scale) data. The relationship of linearized image intensity to contrast agent concentration was found to be linear up to 1 per thousand and reached a plateau at approximately 2 per thousand. To operate in the linear range of the intensity-concentration relationship the contrast agent dose should be adjusted to avoid those high values in vivo and the highest dynamic range of the ultrasound system should be used to avoid unnecessary signal saturation.