Imaging myocardial metabolism and ischemic memory

Role of molecular imaging in prognosis, diagnosis, and treatment of gastrointestinal cancers: An update on new therapeutic methods

One of the leading causes of cancer-related death is gastrointestinal cancer, which has a significant morbidity and mortality rate. Although preoperative risk assessment is essential for directing patient care, its biological behavior cannot be accurately predicted by conventional imaging investigations. Potential pathophysiological information in anatomical imaging that cannot be visually identified can now be converted into high-dimensional quantitative image features thanks to the developing discipline of molecular imaging. In order to enable molecular tissue profile in vivo, molecular imaging has most recently been utilized to phenotype the expression of single receptors and targets of biological therapy. It is expected that molecular imaging will become increasingly important in the near future, driven by the expanding range of biological therapies for cancer. With this live molecular fingerprinting, molecular imaging can be utilized to drive expression-tailored customized therapy. The technical aspects of molecular imaging are first briefly discussed in this review, followed by an examination of the most recent research on the diagnosis, prognosis, and potential future clinical methods of molecular imaging for GI tract malignancies.

Hybrid PET/MR in Cardiac Imaging

In the last few years, technological advances in MR imaging, PET detectors, and attenuation correction algorithms have allowed the creation of truly integrated PET/MR imaging systems, for both clinical and research applications. These machines allow a comprehensive investigation of cardiovascular diseases, by offering a wide variety of detailed anatomical and functional data in combination. Despite significant pathophysiologic mechanisms being clarified by this new data, its clinical relevance and prognostic significance have not been demonstrated yet.

Strategies for Imaging Metabolic Remodeling of the Heart in Obesity and Heart Failure

PURPOSE OF REVIEW

Define early myocardial metabolic changes among patients with obesity and heart failure, and to describe noninvasive methods and their applications for imaging cardiac metabolic remodeling.

RECENT FINDINGS

Metabolic remodeling precedes, triggers, and sustains functional and structural remodeling in the stressed heart. Alterations in cardiac metabolism can be assessed by using a variety of molecular probes. The glucose tracer analog, 18F-FDG, and the labeled tracer 11C-palmitate are still the most commonly used tracers to assess glucose and fatty acid metabolism, respectively. The development of new tracer analogs and imaging agents, including those targeting the peroxisome proliferator-activated receptor (PPAR), provides new opportunities for imaging metabolic activities at a molecular level. While the use of cardiac magnetic resonance spectroscopy in the clinical setting is limited to the assessment of intramyocardial and epicardial fat, new technical improvements are likely to increase its usage in the setting of heart failure. Noninvasive imaging methods are an effective tool for the serial assessment of alterations in cardiac metabolism, either during disease progression, or in response to treatment.

Echocardiographic Ischemic Memory Molecular Imaging for Point-of-Care Detection of Myocardial Ischemia

BACKGROUND

Noninvasive molecular imaging of recent ischemia can potentially be used to diagnose acute coronary syndrome (ACS) with high accuracy.

OBJECTIVES

The authors hypothesized that bedside myocardial contrast echocardiography (MCE) ischemic memory imaging could be achieved with phosphatidylserine microbubbles (MB_PS) that are retained in the microcirculation via ischemia-associated endothelial activation.

METHODS

A dose-finding study was performed in healthy volunteers (n = 17) to establish optimal MB_PS dosing. Stable patients with ACS (n = 30) and confirmed antecedent but resolved myocardial ischemia were studied within 2 hours of coronary angiography and percutaneous coronary intervention (PCI) when indicated. MCE molecular imaging was performed 8 minutes after intravenous administration of MB_PS. MCE perfusion imaging was used to assess the status of the postischemic microcirculation.

RESULTS

Based on dose-finding studies, 0.10 or 0.15 mL of MB_PS based on body mass was selected. In patients with ACS, all but 2 underwent primary PCI. MCE molecular imaging signal intensity was greater in the postischemic risk area vs remote territory (median [95% CI]: 56 [33-66] vs 8 [2-17] IU; P < 0.001) with a receiver-operating characteristic curve C-statistic of 0.94 to differentiate post-ischemic from remote territory. Molecular imaging signal in the risk area was not related to type of ACS (unstable angina: 3; non-ST-segment elevation myocardial infarction: 14; ST-segment elevation myocardial infarction: 13), peak troponin, time to PCI, post-PCI myocardial perfusion, GRACE (Global Registry of Acute Coronary Events) score, or HEART score.

CONCLUSIONS

Molecular imaging with point-of-care echocardiography and MB_PS can detect recent but resolved myocardial ischemia. This bedside technique requires only minutes to perform and appears independent of the degree of ischemia. (Ischemic Memory Imaging With Myocardial Contrast Echocardiography; NCT03009266).

Non-invasive Imaging in Coronary Syndromes - Recommendations of the European Association of Cardiovascular Imaging and the American Society of Echocardiography, in Collaboration with the American Society of Nuclear Cardiology, Society of Cardiovascular Computed Tomography and Society for Cardiovascular Magnetic Resonance

Coronary artery disease (CAD) is one of the major causes of mortality and morbidity worldwide, with a high socioeconomic impact.(1) Non-invasive imaging modalities play a fundamental role in the evaluation and management of patients with known or suspected CAD. Imaging end-points have served as surrogate markers in many observational studies and randomized clinical trials that evaluated the benefits of specific therapies for CAD.(2) A number of guidelines and recommendations have been published about coronary syndromes by cardiology societies and associations, but have not focused on the excellent opportunities with cardiac imaging. The recent European Society of Cardiology (ESC) 2019 guideline on chronic coronary syndromes (CCS) and 2020 guideline on acute coronary syndromes in patients presenting with non-ST-segment elevation (NSTE-ACS) highlight the importance of non-invasive imaging in the diagnosis, treatment, and risk assessment of the disease.(3)(4) The purpose of the current recommendations is to present the significant role of non-invasive imaging in coronary syndromes in more detail. These recommendations have been developed by the European Association of Cardiovascular Imaging (EACVI) and the American Society of Echocardiography (ASE), in collaboration with the American Society of Nuclear Cardiology, the Society of Cardiovascular Computed Tomography, and the Society for Cardiovascular Magnetic Resonance, all of which have approved the final document.

Normal Variants and Pitfalls in Cardiac PET/CT

Positron emission tomography and/or computed tomography (PET/CT) MPI is a powerful imaging modality for the assessment of cardiovascular diseases. It offers several advantages over single-photon emission computed tomography (SPECT) MPI including robust attenuation correction and absolute quantification of radiotracer activity. PET MPI has a large evidence base and is the only clinical tool to evaluate coronary microvascular dysfunction. In addition, the clinical use and evidence base for 2-deoxy-2-[¹⁸F]fluoro-D-g1ucose (¹⁸F-FDG) cardiac PET imaging for inflammation and metabolism imaging is rising exponentially. In order to gain from the advances of this sophisticated quantitative technique, a high-quality scan is critical. It is important for readers to recognize a poor-quality scan, identify artifacts contributing to the poor image quality, and understand how to correct them prior to reporting the results. In this review, we will discuss some normal variants and pitfalls in cardiac PET/CT radionuclide MPI, myocardial viability, and inflammation imaging.

[Echocardiography in the Assessment of Postsystolic Shortening of the Left Ventricle Myocardium of the Heart]

Echocardiography allows evaluating left ventricular (LV) myocardial contractility; however, the visual assessment of contractility is subjective and requires considerable experience. Modern technologies for assessment of LV myocardial contractility, such as tissue Doppler and speckle-tracking echocardiography, provide quantitative estimation of various parameters of myocardial strain, including the LV postsystolic shortening. Several studies have demonstrated the value of postsystolic shortening for evaluation of myocardial ischemia and "ischemic memory" in patients with cardiovascular diseases. This review analyzes experimental and clinical studies that addressed LV postsystolic shortening.

Cardiac Imaging for Coronary Heart Disease Risk Stratification in Chronic Kidney Disease

Chronic kidney disease (CKD), defined as dysfunction of the glomerular filtration apparatus, is an independent risk factor for the development of coronary artery disease (CAD). Patients with CKD are at a substantially higher risk of cardiovascular mortality compared with the age- and sex-adjusted general population with normal kidney function. The risk of CAD and mortality in patients with CKD is correlated with the degree of renal dysfunction including presence of microalbuminuria. A greater cardiovascular risk, albeit lower than for patients receiving dialysis, persists even after kidney transplantation. Congestive heart failure, commonly caused by CAD, also accounts for a significant portion of the cardiovascular-related events observed in CKD. The optimal strategy for the evaluation of CAD in patients with CKD, particularly before renal transplantation, remains a topic of contention spanning over several decades. Although the evaluation of coexisting cardiac disease in patients with CKD is desirable, severe renal dysfunction limits the use of radiographic and magnetic resonance contrast agents due to concerns regarding contrast-induced nephropathy and nephrogenic systemic sclerosis, respectively. In addition, many patients with CKD have extensive and premature (often medial) calcification disproportionate to the severity of obstructive CAD, thereby limiting the diagnostic value of computed tomography angiography. As such, echocardiography, non-contrast-enhanced magnetic resonance, nuclear myocardial perfusion, and metabolic imaging offer a variety of approaches to assess obstructive CAD and cardiomyopathy of advanced CKD without the need for nephrotoxic contrast agents.

Seeing the Invisible-Ultrasound Molecular Imaging

Ultrasound molecular imaging has been developed in the past two decades with the goal of non-invasively imaging disease phenotypes on a cellular level not depicted on anatomic imaging. Such techniques already play a role in pre-clinical research for the assessment of disease mechanisms and drug effects, and are thought to in the future contribute to earlier diagnosis of disease, assessment of therapeutic effects and patient-tailored therapy in the clinical field. In this review, we first describe the chemical composition and structure as well as the in vivo behavior of the ultrasound contrast agents that have been developed for molecular imaging. We then discuss the strategies that are used for targeting of contrast agents to specific cellular targets and protocols used for imaging. Next we describe pre-clinical data on imaging of thrombosis, atherosclerosis and microvascular inflammation and in oncology, including the pathophysiological principles underlying the selection of targets in each area. Where applicable, we also discuss efforts that are currently underway for translation of this technique into the clinical arena.

Phosphatidylserine on microparticles and associated cells contributes to the hypercoagulable state in diabetic kidney disease

Background

Relatively little is known about the role of phosphatidylserine (PS) in procoagulant activity (PCA) in patients with diabetic kidney disease (DKD). This study was designed to evaluate whether exposed PS on microparticles (MPs) and MP-origin cells were involved in the hypercoagulability in DKD patients.

Methods

DKD patients (n = 90) were divided into three groups based on urinary albumin excretion rate, defined as normoalbuminuria (No-A) (<30 mg/24 h), microalbuminuria (Mi-A) (30-299 mg/24 h) or macroalbuminuria (Ma-A) (>300 mg/24 h), and compared with healthy controls (n = 30). Lactadherin was used to quantify PS exposure on MPs and their original cells. Healthy blood cells (BCs) and human umbilical vein endothelial cells (HUVECs) were treated with 25, 5 or 2.5 mmol/L glucose as well as 3-12 mg/dL uric acid and cells were evaluated by clotting time and purified coagulation complex assays. Fibrin production was determined by turbidity. PS exposure and fibrin strands were observed using confocal microscopy.

Results

Using flow cytometry, we found that PS+ MPs (derived from platelets, erythrocytes, HUVECs, neutrophils, monocytes and lymphocytes) and BCs were significantly higher in patients than in controls. Furthermore, the number of PS+ MPs and BCs in patients with Ma-A was significantly higher than in patients with No-A. Similarly, we observed markedly elevated PS exposure on HUVECs cultured with serum from patients with Ma-A versus serum from patients with Mi-A or normoalbuminuria. In addition, circulating PS+ MPs cooperated with PS+ cells, contributing to markedly shortened coagulation time and dramatically increased FXa/thrombin generation and fibrin formation in each DKD group. Confocal microscopy images demonstrated colocalization of fibrin with PS on HUVECs. Moreover, blockade of exposed PS on MPs and cells with lactadherin inhibited PCA by ∼80%. In vitro, BCs and endothelial cells exposed more PS in hypoglycemia or hyperglycemia. Interestingly, reconstitution experiments showed that hypoglycemia-treated cells could be further activated or injured when recovery is obtained reaching hyperglycemia. Moreover, uric acid induced PS exposure on cells (excluding platelets) at concentrations >6 mg/dL. Linear regression analysis showed that levels of PS+ BCs and microparticles were positively correlated with uric acid and proteinuria, but negatively correlated with glomerular filtration rate.

Conclusions

Our results suggest that PS+ MPs and MP-origin cells play procoagulant roles in patients with DKD. Blockade of PS could become a novel therapeutic modality for the prevention of thrombosis in these patients.

The Cardiorenal Axis: Myocardial Perfusion, Metabolism, and Innervation

PURPOSE OF THE REVIEW

Cardiorenal syndrome (CRS), defined as concomitant heart and kidney disease, has been a focus of attention for nearly a decade. As more patients survive severe acute and chronic heart and kidney diseases, CRS has emerged as an "epidemic" of modern medicine. Significant advances have been made in unraveling the complex mechanisms that underlie CRS based on classification of the condition into five pathophysiologic subtypes. In types 1 and 2, acute or chronic heart disease results in renal dysfunction, while in types 3 and 4, acute or chronic kidney diseases are the inciting factors for heart disease. Type 5 CRS is defined as concomitant heart and kidney dysfunction as part of a systemic condition such as sepsis or autoimmune disease.

RECENT FINDINGS

There are ongoing efforts to better define subtypes of CRS based on historical information, clinical manifestations, laboratory data (including biomarkers), and imaging characteristics. Systematic evaluation of CRS by advanced cardiac imaging, however, has been limited in scope and mostly focused on type 4 CRS. This is in part related to lack of clinical trials applying advanced cardiac imaging in the acute setting and exclusion of patients with significant renal disease from studies of such techniques in chronic HF. Advanced cardiac nuclear imaging is well poised for assessment of the pathophysiology of CRS by offering a myriad of molecular probes without the need for nephrotoxic contrast agents. In this review, we examine the current or potential future application of advanced cardiac imaging to evaluation of myocardial perfusion, metabolism, and innervation in patients with CRS.

Molecular Imaging of the Heart

Multimodality cardiovascular imaging is routinely used to assess cardiac function, structure, and physiological parameters to facilitate the diagnosis, characterization, and phenotyping of numerous cardiovascular diseases (CVD), as well as allows for risk stratification and guidance in medical therapy decision-making. Although useful, these imaging strategies are unable to assess the underlying cellular and molecular processes that modulate pathophysiological changes. Over the last decade, there have been great advancements in imaging instrumentation and technology that have been paralleled by breakthroughs in probe development and image analysis. These advancements have been merged with discoveries in cellular/molecular cardiovascular biology to burgeon the field of cardiovascular molecular imaging. Cardiovascular molecular imaging aims to noninvasively detect and characterize underlying disease processes to facilitate early diagnosis, improve prognostication, and guide targeted therapy across the continuum of CVD. The most-widely used approaches for preclinical and clinical molecular imaging include radiotracers that allow for high-sensitivity in vivo detection and quantification of molecular processes with single photon emission computed tomography and positron emission tomography. This review will describe multimodality molecular imaging instrumentation along with established and novel molecular imaging targets and probes. We will highlight how molecular imaging has provided valuable insights in determining the underlying fundamental biology of a wide variety of CVDs, including: myocardial infarction, cardiac arrhythmias, and nonischemic and ischemic heart failure with reduced and preserved ejection fraction. In addition, the potential of molecular imaging to assist in the characterization and risk stratification of systemic diseases, such as amyloidosis and sarcoidosis will be discussed. © 2019 American Physiological Society. Compr Physiol 9:477-533, 2019.

The Potential of Clinical Phenotyping of Heart Failure With Imaging Biomarkers for Guiding Therapies: A Focused Update

The need for noninvasive assessment of cardiac volumes and ejection fraction (EF) ushered in the use of cardiac imaging techniques in heart failure (HF) trials that investigated the roles of pharmacological and device-based therapies. However, in contrast to HF with reduced EF (HFrEF), modern HF pharmacotherapy has not improved outcomes in HF with preserved EF (HFpEF), largely attributed to patient heterogeneity and incomplete understanding of pathophysiological insights underlying the clinical presentations of HFpEF. Modern cardiac imaging methods offer insights into many sets of changes in cardiac tissue structure and function that can precisely link cause with cardiac remodeling at organ and tissue levels to clinical presentations in HF. This has inspired investigators to seek a more comprehensive understanding of HF presentations using imaging techniques. This article summarizes the available evidence regarding the role of cardiac imaging in HF. Furthermore, we discuss the value of cardiac imaging techniques in identifying HF patient subtypes who share similar causes and mechanistic pathways that can be targeted using specific HF therapies.

Myocardial Viability: Survival Mechanisms and Molecular Imaging Targets in Acute and Chronic Ischemia

Myocardial responses to acute ischemia/reperfusion and to chronic ischemic conditions have been studied extensively at all levels of organization. These include subcellular (eg, mitochondria in vitro); intact, large animal models (eg, swine with chronic coronary stenosis); as well as human subjects. Investigations in humans have used positron emission tomographic metabolic and myocardial blood flow measurements, assessment of gene expression and anatomic description of myocardium obtained at the time of coronary artery revascularization, ventricular assist device placement, or heart transplantation. A multitude of genetic, molecular, and metabolic pathways have been identified, which may promote either myocyte survival or death or, most interestingly, both. Many of these potential mediators in both acute ischemia/reperfusion and adaptations to chronic ischemic conditions involve the mitochondria, which play a central role in cellular energy production and homeostasis. The present review is focused on operative survival mechanisms and potential myocardial viability molecular imaging targets in acute and chronic ischemia, especially those which impact mitochondrial function.

Lipid Use and Misuse by the Heart

The heart utilizes large amounts of fatty acids as energy providing substrates. The physiological balance of lipid uptake and oxidation prevents accumulation of excess lipids. Several processes that affect cardiac function, including ischemia, obesity, diabetes mellitus, sepsis, and most forms of heart failure lead to altered fatty acid oxidation and often also to the accumulation of lipids. There is now mounting evidence associating certain species of these lipids with cardiac lipotoxicity and subsequent myocardial dysfunction. Experimental and clinical data are discussed and paths to reduction of toxic lipids as a means to improve cardiac function are suggested.

Cardiac molecular imaging to track left ventricular remodeling in heart failure

Cardiac left ventricular (LV) remodeling is the final common pathway of most primary cardiovascular diseases that manifest clinically as heart failure (HF). The more advanced the systolic HF and LV dysfunction, the worse the prognosis. The knowledge of the molecular, cellular, and neurohormonal mechanisms that lead to myocardial dysfunction and symptomatic HF has expanded rapidly and has allowed sophisticated approaches to understanding and management of the disease. New therapeutic targets for pharmacologic intervention in HF have also been identified through discovery of novel cellular and molecular components of membrane-bound receptor-mediated intracellular signal transduction cascades. Despite all advances, however, the prognosis of systolic HF has remained poor in general. This is, at least in part, related to the (1) relatively late institution of treatment due to reliance on gross functional and structural abnormalities that define the "heart failure phenotype" clinically; (2) remarkable genetic-based interindividual variations in the contribution of each of the many molecular components of cardiac remodeling; and (3) inability to monitor the activity of individual pathways to cardiac remodeling in order to estimate the potential benefits of pharmacologic agents, monitor the need for dose titration, and minimize side effects. Imaging of the recognized ultrastructural components of cardiac remodeling can allow redefinition of heart failure based on its "molecular phenotype," and provide a guide to implementation of "personalized" and "evidence-based" evaluation, treatment, and longitudinal monitoring of the disease beyond what is currently available through randomized controlled clinical trials.

Cardiac Metabolism in Perspective

The heart is a biological pump that converts chemical to mechanical energy. This process of energy conversion is highly regulated to the extent that energy substrate metabolism matches energy use for contraction on a beat-to-beat basis. The biochemistry of cardiac metabolism includes the biochemistry of energy transfer, metabolic regulation, and transcriptional, translational as well as posttranslational control of enzymatic activities. Pathways of energy substrate metabolism in the heart are complex and dynamic, but all of them conform to the First Law of Thermodynamics. The perspectives expand on the overall idea that cardiac metabolism is inextricably linked to both physiology and molecular biology of the heart. The article ends with an outlook on emerging concepts of cardiac metabolism based on new molecular models and new analytical tools. © 2016 American Physiological Society. Compr Physiol 6:1675-1699, 2016.

Prospective Evaluation of 18F-Fluorodeoxyglucose Uptake in Postischemic Myocardium by Simultaneous Positron Emission Tomography/Magnetic Resonance Imaging as a Prognostic Marker of Functional Outcome

BACKGROUND

The immune system orchestrates the repair of infarcted myocardium. Imaging of the cellular inflammatory response by (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography/magnetic resonance imaging in the heart has been demonstrated in preclinical and clinical studies. However, the clinical relevance of post-MI (18)F-FDG uptake in the heart has not been elucidated. The objective of this study was to explore the value of (18)F-FDG positron emission tomography/magnetic resonance imaging in patients after acute myocardial infarction as a biosignal for left ventricular functional outcome.

METHODS AND RESULTS

We prospectively enrolled 49 patients with ST-segment-elevation myocardial infarction and performed (18)F-FDG positron emission tomography/magnetic resonance imaging 5 days after percutaneous coronary intervention and follow-up cardiac magnetic resonance imaging after 6 to 9 months. In a subset of patients, (99m)Tc-sestamibi single-photon emission computed tomography was performed with tracer injection before revascularization. Cellular innate immune response was analyzed at multiple time points. Segmental comparison of (18)F-FDG-uptake and late gadolinium enhancement showed substantial overlap (κ=0.66), whereas quantitative analysis demonstrated that (18)F-FDG extent exceeded late gadolinium enhancement extent (33.2±16.2% left ventricular myocardium versus 20.4±10.6% left ventricular myocardium, P<0.0001) and corresponded to the area at risk (r=0.87, P<0.0001). The peripheral blood count of CD14(high)/CD16(+) monocytes correlated with the infarction size and (18)F-FDG signal extent (r=0.53, P<0.002 and r=0.42, P<0.02, respectively). (18)F-FDG uptake in the infarcted myocardium was highest in areas with transmural scar, and the standardized uptake valuemean was associated with left ventricular functional outcome independent of infarct size (Δ ejection fraction: P<0.04, Δ end-diastolic volume: P<0.02, Δ end-systolic volume: P<0.005).

CONCLUSIONS

In this study, the intensity of (18)F-FDG uptake in the myocardium after acute myocardial infarction correlated inversely with functional outcome at 6 months. Thus, (18)F-FDG uptake in infarcted myocardium may represent a novel biosignal of myocardial injury.

Simplified protocol of cardiac 18F-fluorodeoxyglucose positron emission tomography viability study in normoglycemic patients with known coronary artery disease

PURPOSE

The purpose was to evaluate quality of 18F-fluorodeoxyglucose positron emission tomography (18F-FDG-PET) myocardial scans and its correlation with background glucose (BG) after simplified 5% intravenous glucose load protocol.

METHODS

An intravenous glucose load protocol was applied in 69 normoglycemic patients with confirmed coronary artery disease. The blood glucose level was measured every 15 min.

RESULTS

Eighty-four percent of images were optimal, 8.7% suboptimal, and 7.3% uninterpretable. The quality of 18F-FDG-PET was BG independent and body mass index dependent (P=.0007).

CONCLUSIONS

Simplified glucose load protocol is a safe and efficient method of preparation for FDG cardiac viability study in patients with normoglycemia.

The feasibility of imaging myocardial ischemic/reperfusion injury using (99m)Tc-labeled duramycin in a porcine model

When pathologically externalized, phosphatidylethanolamine (PE) is a potential surrogate marker for detecting tissue injuries. (99m)Tc-labeled duramycin is a peptide-based imaging agent that binds PE with high affinity and specificity. The goal of the current study was to investigate the clearance kinetics of (99m)Tc-labeled duramycin in a large animal model (normal pigs) and to assess its uptake in the heart using a pig model of myocardial ischemia-reperfusion injury.

METHODS

The clearance and distribution of intravenously injected (99m)Tc-duramycin were characterized in sham-operated animals (n=5). In a closed chest model of myocardial ischemia, coronary occlusion was induced by balloon angioplasty (n=9). (99m)Tc-duramycin (10-15mCi) was injected intravenously at 1hour after reperfusion. SPECT/CT was acquired at 1 and 3hours after injection. Cardiac tissues were analyzed for changes associated with acute cellular injuries. Autoradiography and gamma counting were used to determine radioactivity uptake. For the remaining animals, (99m)Tc-tetrafosamin scan was performed on the second day to identify the infarct site.

RESULTS

Intravenously injected (99m)Tc-duramycin cleared from circulation predominantly via the renal/urinary tract with an α-phase half-life of 3.6±0.3minutes and β-phase half-life of 179.9±64.7minutes. In control animals, the ratios between normal heart and lung were 1.76±0.21, 1.66±0.22, 1.50±0.20 and 1.75±0.31 at 0.5, 1, 2 and 3hours post-injection, respectively. The ratios between normal heart and liver were 0.88±0.13, 0.80±0.13, 0.82±0.19 and 0.88±0.14. In vivo visualization of focal radioactivity uptake in the ischemic heart was attainable as early as 30min post-injection. The in vivo ischemic-to-normal uptake ratios were 3.57±0.74 and 3.69±0.91 at 1 and 3hours post-injection, respectively. Ischemic-to-lung ratios were 4.89±0.85 and 4.93±0.57; and ischemic-to-liver ratios were 2.05±0.30 to 3.23±0.78. The size of (99m)Tc-duramycin positive myocardium was qualitatively larger than the infarct size delineated by the perfusion defect in (99m)Tc-tetrafosmin uptake. This was consistent with findings from tissue analysis and autoradiography.

CONCLUSION

(99m)Tc-duramycin was demonstrated, in a large animal model, to have suitable clearance and biodistribution profiles for imaging. The agent has an avid target uptake and a fast background clearance. It is appropriate for imaging myocardial injury induced by ischemia/reperfusion.

Recent advances in metabolic imaging

Abnormalities in myocardial substrate metabolism play a central role in the manifestations of most forms of cardiac disease such as ischemic heart disease, heart failure, hypertensive heart disease, and the cardiomyopathy due to either obesity or diabetes mellitus. Their importance is exemplified by both the development of numerous imaging tools designed to detect the specific metabolic perturbations or signatures related to these different diseases, and the vigorous efforts in drug discovery/development targeting various aspects of myocardial metabolism. Since the prior review in 2005, we have gained new insights into how perturbations in myocardial metabolism contribute to various forms of cardiac disease. For example, the application of advanced molecular biologic techniques and the development of elegant genetic models have highlighted the pleiotropic actions of cellular metabolism on energy transfer, signal transduction, cardiac growth, gene expression, and viability. In parallel, there have been significant advances in instrumentation, radiopharmaceutical design, and small animal imaging, which now permit a near completion of the translational pathway linking in-vitro measurements of metabolism with the human condition. In this review, most of the key advances in metabolic imaging will be described, their contribution to cardiovascular research highlighted, and potential new clinical applications proposed.

Ischaemic memory imaging using metabolic radiopharmaceuticals: overview of clinical settings and ongoing investigations

"Ischaemic memory" is defined as a prolonged functional and/or biochemical alteration remaining after a particular episode of severe myocardial ischaemia. The biochemical alteration has been reported as metabolic stunning. Metabolic imaging has been used to detect the footprint left by previous ischaemic episodes evident due to delayed recovery of myocardial metabolism (persistent dominant glucose utilization with suppression of fatty acid oxidation). β-Methyl-p-[(123)I]iodophenylpentadecanoic acid (BMIPP) is a single-photon emission computed tomography (SPECT) radiotracer widely used for metabolic imaging in clinical settings in Japan. In patients with suspected coronary artery disease but no previous myocardial infarction, BMIPP has shown acceptable diagnostic accuracy. In particular, BMIPP plays an important role in the identification of prior ischaemic insult in patients arriving at emergency departments with acute chest pain syndrome. Recent data also show the usefulness of (123)I-BMIPP SPECT for predicting cardiovascular events in patients undergoing haemodialysis. Similarly, SPECT or PET imaging with (18)F-FDG injected during peak exercise or after exercise under fasting conditions shows an increase in FDG uptake in postischaemic areas. This article will overview the roles of ischaemic memory imaging both under established indications and in ongoing investigations.

Vitamin C further improves the protective effect of GLP-1 on the ischemia-reperfusion-like effect induced by hyperglycemia post-hypoglycemia in type 1 diabetes

Background

It has been reported that hyperglycemia following hypoglycemia produces an ischemia-reperfusion-like effect in type 1 diabetes. In this study the possibility that GLP-1 has a protective effect on this phenomenon has been tested.

Methods

15 type 1 diabetic patients underwent to five experiments: a period of two hours of hypoglycemia followed by two hours of normo-glycemia or hyperglycemia with the concomitant infusion of GLP-1 or vitamin C or both. At baseline, after 2 and 4 hours, glycemia, plasma nitrotyrosine, plasma 8-iso prostaglandin F2alpha, sCAM-1a, IL-6 and flow mediated vasodilation were measured.

Results

After 2 h of hypoglycemia, flow mediated vasodilation significantly decreased, while sICAM-1, 8-iso-PGF2a, nitrotyrosine and IL-6 significantly increased. While recovering with normoglycemia was accompanied by a significant improvement of endothelial dysfunction, oxidative stress and inflammation, a period of hyperglycemia after hypoglycemia worsens all these parameters. These effects were counterbalanced by GLP-1 and better by vitamin C, while the simultaneous infusion of both almost completely abolished the effect of hyperglycemia post hypoglycemia.

Conclusions

This study shows that GLP-1 infusion, during induced hyperglycemia post hypoglycemia, reduces the generation of oxidative stress and inflammation, improving the endothelial dysfunction, in type 1 diabetes. Furthermore, the data support that vitamin C and GLP-1 may have an additive protective effect in such condition.

Quantitative PET imaging detects early metabolic remodeling in a mouse model of pressure-overload left ventricular hypertrophy in vivo

We proposed that metabolic remodeling in the form of increased uptake of the myocardial glucose analog (18)F-FDG precedes and triggers the onset of severe contractile dysfunction in pressure-overload left ventricular hypertrophy in vivo. To test this hypothesis, we used a mouse model of transverse aortic constriction (TAC) together with PET and assessed serial changes in cardiac metabolism and function over 7 d.

METHODS

Scans of 16 C57BL/6 male mice were obtained using a small-animal PET device under sevoflurane anesthesia. A 10-min transmission scan was followed by a 60-min dynamic (18)F-FDG PET scan with cardiac and respiratory gating. Blood glucose levels were measured before and after the emission scan. TAC and sham surgeries were performed after baseline imaging. Osmotic mini pumps containing either propranolol (5 mg/kg/d) or vehicle alone were implanted subcutaneously at the end of surgery. Subsequent scans were taken at days 1 and 7 after surgery. A compartment model, in which the blood input function with spillover and partial-volume corrections and the metabolic rate constants in a 3-compartment model are simultaneously estimated, was used to determine the net myocardial (18)F-FDG influx constant, Ki. The rate of myocardial glucose utilization, rMGU, was also computed. Estimations of the ejection fractions were based on the high-resolution gated PET images.

RESULTS

Mice undergoing TAC surgery exhibited an increase in the Ki (580%) and glucose utilization the day after surgery, indicating early adaptive response. On day 7, the ejection fraction had decreased by 24%, indicating a maladaptive response. Average Ki increases were not linearly associated with increases in rMGU. Ki exceeded rMGU by 29% in the TAC mice. TAC mice treated with propranolol attenuated the rate of (18)F-FDG uptake, diminished mismatch between Ki and rMGU (9%), and rescued cardiac function.

CONCLUSION

Metabolic maladaptation precedes the onset of severe contractile dysfunction. Both are prevented by treatment with propranolol. The early detection of metabolic remodeling may offer a metabolic target for modulation of hypertrophy.

Evidence that hyperglycemia after recovery from hypoglycemia worsens endothelial function and increases oxidative stress and inflammation in healthy control subjects and subjects with type 1 diabetes

Currently there is debate on whether hypoglycemia is an independent risk factor for atherosclerosis, but little attention has been paid to the effects of recovery from hypoglycemia. In normal control individuals and in people with type 1 diabetes, recovery from a 2-h induced hypoglycemia was obtained by reaching normoglycemia or hyperglycemia for another 2 h and then maintaining normal glycemia for the following 6 h. Hyperglycemia after hypoglycemia was also repeated with the concomitant infusion of vitamin C. Recovery with normoglycemia is accompanied by a significant improvement in endothelial dysfunction, oxidative stress, and inflammation, which are affected by hypoglycemia; however, a period of hyperglycemia after hypoglycemia worsens all of these parameters, an effect that persists even after the additional 6 h of normoglycemia. This effect is partially counterbalanced when hyperglycemia after hypoglycemia is accompanied by the simultaneous infusion of vitamin C, suggesting that when hyperglycemia follows hypoglycemia, an ischemia-reperfusion-like effect is produced. This study shows that the way in which recovery from hypoglycemia takes place in people with type 1 diabetes could play an important role in favoring the appearance of endothelial dysfunction, oxidative stress, and inflammation, widely recognized cardiovascular risk factors.

Targeted metabolic imaging to improve the management of heart disease

Tracer techniques are powerful methods for assessing rates of biological processes in vivo. A case in point is intermediary metabolism of energy providing substrates, a central feature of every living cell. In the heart, the tight coupling between metabolism and contractile function offers an opportunity for the simultaneous assessment of cardiac performance at different levels in vivo: coronary flow, myocardial perfusion, oxygen delivery, metabolism, and contraction. Noninvasive imaging techniques used to identify the metabolic footprints of either normal or perturbed cardiac function are discussed.

Unfavourable consequences of chronic cardiac HIF-1α stabilization

AIMS

The hypoxia-inducible factor-1 (HIF-1) is the master modulator of hypoxic gene expression. The effects of chronically stabilized cardiac HIF-1α and its role in the diseased heart are not precisely known. The aims of this study were as follows: (i) to elucidate consequences of HIF-1α stabilization in the heart; (ii) to analyse long-term effects of HIF-1α stabilization with ageing and the ability of the HIF-1α overexpressing hearts to respond to increased mechanical load; and (iii) to analyse HIF-1α protein levels in failing heart samples.

METHODS AND RESULTS

In a cardiac-specific HIF-1α transgenic mouse model, constitutive expression of HIF-1α leads to changes in capillary area and shifts the cardiac metabolism towards glycolysis with a net increase in glucose uptake. Furthermore, Ca(2+) handling is altered, with increased Ca(2)(+) transients and faster intracellular [Ca(2+)] decline. These changes are associated with decreased expression of sarcoplasmic/endoplasmic reticulum calcium ATPase 2a but elevated phosphorylation of phospholamban. HIF-1α transgenic mice subjected to transverse aortic constriction exhibited profound cardiac decompensation. Moreover, cardiomyopathy was also seen in ageing transgenic mice. In parallel, we found an increased stabilization of HIF-1α in heart samples of patients with end-stage heart failure.

CONCLUSION

Changes induced with transgenic cardiac HIF-1α possibly mediate beneficial effects in the short term; however, with increased mechanical load and ageing they become detrimental for cardiac function. Together with the finding of increased HIF-1α protein levels in samples from human patients with cardiomyopathy, these data indicate that chronic HIF-1α stabilization drives autonomous pathways that add to disease progression.

Microvascular angina: assessment of coronary blood flow, flow reserve, and metabolism

Microvascular angina (MVA) is an often overlooked cause of significant chest pain. Decreased myocardial perfusion secondary to dysregulated blood flow in the microvasculature can occur in the presence or absence of obstructive epicardial coronary artery disease. The corresponding myocardial ischemia and angina is now a well-established diagnosis, made by detection of decreased coronary flow reserve (CFR). Although low CFR and MVA are associated with poor prognosis, there is initial evidence for reversibility of this abnormal vascular regulation with aggressive medical therapy and control of associated risk factors. Current assessment of MVA is carried out predominantly during cardiac catheterization; however, noninvasive techniques to assess CFR are being developed, including PET, MRI, and CT modalities. Quantitative tracer techniques or imaging of metabolic disturbances reflecting ischemia will likely enhance diagnostic approaches for such patients as well as allow more frequent monitoring of response to therapy.

Molecular imaging agents: impact on diagnosis and therapeutics in oncology

Imaging has become a crucial tool in oncology throughout the course of disease detection and management, and is an integral part of clinical trials. Anatomical and functional imaging led the way, providing valuable information used in the diagnosis of disease, including data regarding the size and location of the tumour and on physiological processes such as blood flow and perfusion. As understanding of cancer pathogenesis has advanced through the identification of genetic, biochemical and cellular alterations in evolving tumours, emphasis has been put on developing methods to detect and serially monitor such alterations. This class of approaches is referred to as molecular imaging. Molecular imaging offers the potential for increasingly sensitive and specific visualisation and quantification of biological processes at the cellular and molecular level. These approaches have become established as essential tools for cancer research, early cancer detection and staging, and monitoring and predicting response to targeted therapies. Here, we discuss recent advances in the development of molecular imaging agents and their implementation in basic cancer research as well as in more rationalised approaches to cancer care.

Imaging myocardial metabolic remodeling

Myocardial metabolic remodeling is the process in which the heart loses its ability to utilize different substrates, becoming dependent primarily on the metabolism of a single substrate such as glucose or fatty acids for energy production. Myocardial metabolic remodeling is central to the pathogenesis of a variety of cardiac disease processes such as left ventricular hypertrophy, myocardial ischemia, and diabetic cardiomyopathy. As a consequence, there is a growing demand for accurate noninvasive imaging approaches of various aspects of myocardial substrate metabolism that can be performed in both humans and small-animal models of disease, facilitating the crosstalk between the bedside and the bench and leading to improved patient management paradigms. SPECT, PET, and MR spectroscopy are the most commonly used imaging techniques. Discussed in this review are the strengths and weaknesses of these various imaging methods and how they are furthering our understanding of the role of myocardial remodeling in cardiovascular disease. In addition, the role of ultrasound to detect the inflammatory response to myocardial ischemia will be discussed.

Tracing cardiac metabolism in vivo: one substrate at a time

In the myocardial cell, a series of enzyme-catalyzed reactions results in the efficient transfer of chemical energy into mechanical energy. The goals of this article are to emphasize the ability of noninvasive imaging techniques using isotopic tracers to detect the metabolic footprints of heart disease and to propose that cardiac metabolic imaging is more than a useful adjunct to current myocardial perfusion imaging studies. A strength of metabolic imaging is in the assessment of regional myocardial differences in metabolic activity, probing for 1 substrate at a time. We hope that new and developing methods of cardiac imaging will lead to the earlier detection of heart disease and improve the management and quality of life for patients afflicted with ischemic and nonischemic heart muscle disorders.

Effects of mitral valve surgery on myocardial energetics in patients with severe mitral regurgitation

BACKGROUND

Hemodynamically significant mitral regurgitation (MR) may alter left ventricular (LV) myocardial energy requirements. The effects of MR and subsequent corrective mitral valve (MV) surgery on myocardial energetics are not well understood. A better understanding of myocardial energetics and the LV responses to changes in preload and afterload may assist with the understanding of mitral regurgitation and its effect on the LV. We sought to determine the effects of MV surgery on forward stroke work, myocardial oxidative metabolism, and myocardial efficiency.

METHODS AND RESULTS

Prospectively enrolled patients with chronic, severe, nonischemic mitral regurgitation underwent echocardiography, radionuclide angiography, and C-11 acetate positron emission tomography to measure LV volumes, ejection fraction, and oxidative metabolism before and 1 year after MV surgery. Forward and total stroke work corrected for oxidative metabolism was used to estimate efficiency using the work metabolic index. Fourteen patients (age, 59+/- 8 years) with myxomatous MV were enrolled. One year after MV surgery, there was a reduction in LV end-diastolic and end-systolic volumes (231+/-86 to 131+/-21 mL; P<0.01 and 98+/-53 to 55+/-17 mL; P<0.01). Forward stroke volume increased (58.1+/-15.0 to 75.5+/-23 mL; P<0.01), LV ejection fraction was preserved without a significant change in oxidative metabolism. Forward work metabolic index improved (4.99+/-1.32 x 10(6) to 6.59+/-2.45 x 10(6) mm Hg x mL/m(2); P=0.02). This was not at the expense of total work metabolic index, which was preserved.

CONCLUSIONS

MV surgery has a beneficial effect on forward stroke volume and forward work metabolic index without adverse effects on oxidative metabolism or total work metabolic index.

Molecular imaging targets of cardiac remodeling

Left ventricular (LV) remodeling is a major determinant of the clinical course and outcome of systolic heart failure (HF). Activation of neurohormonal and inflammatory cytokine pathways and their effects on intracellular signal transduction cascades through stimulation of membrane-bound receptors mediate LV remodeling. Although major advances have been made in clinical management of HF through large randomized trials, its prognosis remains poor. Interindividual differences, often genetically based, are increasingly recognized as important determinants of LV remodeling. Identification of the influence of these individual factors on the clinical course of HF has stimulated a search for specific pathophysiologic mechanisms that operate at the individual level and can be targeted directly. This article summarizes the current application of molecular imaging techniques to the understanding of the cellular and molecular mechanisms involved in LV remodeling in an attempt to provide the tools necessary for personalized, truly "evidence-based" assessment, serial evaluation, and monitoring of HF.