Cardiovascular Molecular Imaging

Molecular Mechanisms of Ischemic Stroke: A Review Integrating Clinical Imaging and Therapeutic Perspectives

Ischemic stroke poses a significant global health challenge, necessitating ongoing exploration of its pathophysiology and treatment strategies. This comprehensive review integrates various aspects of ischemic stroke research, emphasizing crucial mechanisms, therapeutic approaches, and the role of clinical imaging in disease management. It discusses the multifaceted role of Netrin-1, highlighting its potential in promoting neurovascular repair and mitigating post-stroke neurological decline. It also examines the impact of blood-brain barrier permeability on stroke outcomes and explores alternative therapeutic targets such as statins and sphingosine-1-phosphate signaling. Neurocardiology investigations underscore the contribution of cardiac factors to post-stroke mortality, emphasizing the importance of understanding the brain-heart axis for targeted interventions. Additionally, the review advocates for early reperfusion and neuroprotective agents to counter-time-dependent excitotoxicity and inflammation, aiming to preserve tissue viability. Advanced imaging techniques, including DWI, PI, and MR angiography, are discussed for their role in evaluating ischemic penumbra evolution and guiding therapeutic decisions. By integrating molecular insights with imaging modalities, this interdisciplinary approach enhances our understanding of ischemic stroke and offers promising avenues for future research and clinical interventions to improve patient outcomes.

Optical detection of atherosclerosis at molecular level by optical coherence tomography: An in vitro study

There is an urgent need for contrast agents to detect the first inflammation stage of atherosclerosis by cardiovascular optical coherence tomography (CV-OCT), the imaging technique with the highest spatial resolution and sensitivity of those used during coronary interventions. Gold nanoshells (GNSs) provide the strongest signal by CV-OCT. GNSs are functionalized with the cLABL peptide that binds specifically to the ICAM-1 molecule upregulated in the first stage of atherosclerosis. Dark field microscopy and CV-OCT are used to evaluate the specific adhesion of these functionalized GNSs to activated endothelial cells. This adhesion is investigated under static and dynamic conditions, for shear stresses comparable to those of physiological conditions. An increase in the scattering signal given by the functionalized GNSs attached to activated cells is observed compared to non-activated cells. Thus, cLABL-functionalized GNSs behave as excellent contrast agents for CV-OCT and promise a novel strategy for clinical molecular imaging of atherosclerosis.

Current and novel radiopharmaceuticals for imaging cardiovascular inflammation

Cardiovascular disease (CVD) remains the leading cause of death worldwide despite advances in diagnostic technologies and treatment strategies. The underlying cause of most CVD is atherosclerosis, a chronic disease driven by inflammatory reactions. Atherosclerotic plaque rupture could cause arterial occlusion leading to ischemic tissue injuries such as myocardial infarction (MI) and stroke. Clinically, most imaging modalities are based on anatomy and provide limited information about the on-going molecular activities affecting the vulnerability of atherosclerotic lesion for risk stratification of patients. Thus, the ability to differentiate stable plaques from those that are vulnerable is an unmet clinical need. Of various imaging techniques, the radionuclide-based molecular imaging modalities including positron emission tomography and single-photon emission computerized tomography provide superior ability to noninvasively visualize molecular activities in vivo and may serve as a useful tool in tackling this challenge. Moreover, the well-established translational pathway of radiopharmaceuticals may also facilitate the translation of discoveries from benchtop to clinical investigation in contrast to other imaging modalities to fulfill the goal of precision medicine. The relationship between inflammation occurring within the plaque and its proneness to rupture has been well documented. Therefore, an active effort has been significantly devoted to develop radiopharmaceuticals specifically to measure CVD inflammatory status, and potentially elucidate those plaques which are prone to rupture. In the following review, molecular imaging of inflammatory biomarkers will be briefly discussed.

Discovery of Radioiodinated Monomeric Anthraquinones as a Novel Class of Necrosis Avid Agents for Early Imaging of Necrotic Myocardium

Assessment of myocardial viability is deemed necessary to aid in clinical decision making whether to recommend revascularization therapy for patients with myocardial infarction (MI). Dianthraquinones such as hypericin (Hyp) selectively accumulate in necrotic myocardium, but were unsuitable for early imaging after administration to assess myocardial viability. Since dianthraquinones can be composed by coupling two molecules of monomeric anthraquinone and the active center can be found by splitting chemical structure, we propose that monomeric anthraquinones may be effective functional groups for necrosis targetability. In this study, eight radioiodinated monomeric anthraquinones were evaluated as novel necrosis avid agents (NAAs) for imaging of necrotic myocardium. All (131)I-anthraquinones showed high affinity to necrotic tissues and (131)I-rhein emerged as the most promising compound. Infarcts were visualized on SPECT/CT images at 6 h after injection of (131)I-rhein, which was earlier than that with (131)I-Hyp. Moreover, (131)I-rhein showed satisfactory heart-to-blood, heart-to-liver and heart-to-lung ratios for obtaining images of good diagnostic quality. (131)I-rhein was a more promising "hot spot imaging" tracer for earlier visualization of necrotic myocardium than (131)I-Hyp, which supported further development of radiopharmaceuticals based on rhein for SPECT/CT ((123)I and (99m)Tc) or PET/CT imaging ((18)F and (124)I) of myocardial necrosis.

Translational applications of molecular imaging in cardiovascular disease and stem cell therapy

Cardiovascular disease (CVD) is the leading cause of mortality and morbidity worldwide. Molecular imaging techniques provide valuable information at cellular and molecular level, as opposed to anatomical and structural layers acquired from traditional imaging modalities. More specifically, molecular imaging employs imaging probes which interact with specific molecular targets and therefore makes it possible to visualize biological processes in vivo. Molecular imaging technology is now progressing towards preclinical and clinical application that gives an integral and comprehensive guidance for the investigation of cardiovascular disease. In addition, cardiac stem cell therapy holds great promise for clinical translation. Undoubtedly, combining stem cell therapy with molecular imaging technology will bring a broad prospect for the study and treatment of cardiac disease. This review will focus on the progresses of molecular imaging strategies in cardiovascular disease and cardiac stem cell therapy. Furthermore, the perspective on the future role of molecular imaging in clinical translation and potential strategies in defining safety and efficacy of cardiac stem cell therapies will be discussed.

Positron emission tomography imaging of atherosclerosis

Atherosclerosis-related cardiovascular events are the leading causes of death in the industrialized world. Atherosclerosis develops insidiously and the initial manifestation is usually sudden cardiac death, stroke, or myocardial infarction. Molecular imaging is a valuable tool to identify the disease at an early stage before fatal manifestations occur. Among the various molecular imaging techniques, this review mainly focuses on positron emission tomography (PET) imaging of atherosclerosis. The targets and pathways that have been investigated to date for PET imaging of atherosclerosis include: glycolysis, cell membrane metabolism (phosphatidylcholine synthesis), integrin α_vβ_3, low density lipoprotein (LDL) receptors (LDLr), natriuretic peptide clearance receptors (NPCRs), fatty acid synthesis, vascular cell adhesion molecule-1 (VCAM-1), macrophages, platelets, etc. Many PET tracers have been investigated clinically for imaging of atherosclerosis. Early diagnosis of atherosclerotic lesions by PET imaging can help to prevent the premature death caused by atherosclerosis, and smooth translation of promising PET tracers into the clinic is critical to the benefit of patients.

Molecular imaging in heart failure patients

This review focuses on molecular imaging using various radioligands for the tissue characterization of patients with heart failure. ¹²³I-labeled metaiodobenzylguanidine (MIBG), as a marker of adrenergic neuron function, plays an important role in risk stratification in heart failure and may be useful for predicting fatal arrhythmias that may require implantable cardioverter-defibrillator treatment. MIBG has also been used for monitoring treatment effects under various medications. Various positron emission tomography (PET) radioligands have been introduced for the quantitative assessment of presynaptic and postsynaptic neuronal function in vivo. ¹¹C-hydroxyephedrine, like MIBG, has potential for assessing the severity of heart failure. Our PET study using the β-receptor antagonist ¹¹C-CGP 12177 in patients with heart failure showed a reduction of β-receptor density, indicating downregulation, in most of the patients. More studies are needed to confirm the clinical utility of these molecular imaging modalities for the management of heart failure patients.

Multimodal iron oxide nanoparticles for hybrid biomedical imaging

Iron oxide core nanoparticles are attractive imaging agents because their material properties allow the tuning of pharmacokinetics as well as the attachment of multiple moieties to their surface. In addition to affinity ligands, these include fluorochromes and radioisotopes for detection with optical and nuclear imaging. As the iron oxide core can be detected by MRI, options for combining imaging modalities are manifold. Already, preclinical imaging strategies have combined noninvasive imaging with higher resolution techniques, such as intravital microscopy, to gain unprecedented insight into steady-state biology and disease. Going forward, hybrid iron oxide nanoparticles will help to merge modalities, creating a synergy that will enable imaging in basic research and, potentially, also in the clinic.

Image-guided therapies for myocardial repair: concepts and practical implementation

Cell- and molecule-based therapeutic strategies to support wound healing and regeneration after myocardial infarction (MI) are under development. These emerging therapies aim at sustained preservation of ventricular function by enhancing tissue repair after myocardial ischaemia and reperfusion. Such therapies will benefit from guidance with regard to timing, regional targeting, suitable candidate selection, and effectiveness monitoring. Such guidance is effectively obtained by non-invasive tomographic imaging. Infarct size, tissue characteristics, muscle mass, and chamber geometry can be determined by magnetic resonance imaging and computed tomography. Radionuclide imaging can be used for the tracking of therapeutic agents and for the interrogation of molecular mechanisms such as inflammation, angiogenesis, and extracellular matrix activation. This review article portrays the hypothesis that an integrated approach with an early implementation of structural and molecular tomographic imaging in the development of novel therapies will provide a framework for achieving the goal of improved tissue repair after MI.

Hypercortisolism is associated with increased coronary arterial atherosclerosis: analysis of noninvasive coronary angiography using multidetector computerized tomography

BACKGROUND

Observational studies show that glucocorticoid therapy and the endogenous hypercortisolism of Cushing's syndrome (CS) are associated with increased rates of cardiovascular morbidity and mortality. However, the causes of these findings remain largely unknown.

OBJECTIVE

To determine whether CS patients have increased coronary atherosclerosis.

DESIGN

A prospective case-control study was performed.

SETTING

Subjects were evaulated in a clinical research center.

SUBJECTS

Fifteen consecutive patients with ACTH-dependent CS, 14 due to an ectopic source and 1 due to pituitary Cushing's disease were recruited. Eleven patients were studied when hypercortisolemic; 4 patients were eucortisolemic due to medication (3) or cyclic hypercortisolism (1). Fifteen control subjects with at least one risk factor for cardiac disease were matched 1:1 for age, sex, and body mass index.

PRIMARY OUTCOME VARIABLES

Agatston score a measure of calcified plaque and non-calcified coronary plaque volume were quantified using a multidetector CT (MDCT) coronary angiogram scan. Additional variables included fasting lipids, blood pressure, history of hypertension or diabetes, and 24-hour urine free cortisol excretion.

RESULTS

CS patients had significantly greater noncalcified plaque volume and Agatston score (noncalcified plaque volume [mm(3)] median [interquartile ranges]: CS 49.5 [31.4, 102.5], controls 17.9 [2.6, 25.3], P < .001; Agatston score: CS 70.6 [0, 253.1], controls 0 [0, 7.6]; P < .05). CS patients had higher systolic and diastolic blood pressures than controls (systolic: CS 143 mm Hg [135, 173]; controls, 134 [123, 136], P < .02; diastolic CS: 86 [80, 99], controls, 76 [72, 84], P < .05).

CONCLUSIONS

Increased coronary calcifications and noncalcified coronary plaque volumes are present in patients with active or previous hypercortisolism. Increased atherosclerosis may contribute to the increased rates of cardiovascular morbidity and mortality in patients with glucocorticoid excess.

The Role of Atrial Structural Remodeling in Atrial Fibrillation Ablation:An Imaging Point of View for Predicting Recurrence

Atrial fibrillation (AF) is the most common arrhythmia and is associated with a significant morbidity and mortality. Invasive catheter ablation of AF has emerged as an effective therapy for patients with symptomatic AF. Atrial remodeling, particularly structural remodeling, is important not only for AF persistence but also for AF recurrence after ablation. Atrial dilation and fibrosis are two of the core processes involved in atrial structural remodeling. Increased automaticity and triggered activity occur in atrial structural remodeling, which may cause difficulty in maintaining sinus rhythm after ablation. Furthermore, an enlarged left atrium (LA) may increase the difficulty in achieving catheter stability and thereby require more energy to complete AF ablation. AF causes similar remodeling in both the left and right atria (RA), and myocardial changes in both atria influence AF recurrence. A non-invasive assessment of fibrotic structural remodeling helps predict the outcome of AF ablation. A varie ty of cardiac imaging modalities, such as two- or three-dimensional echocardiography or multi-detector row computed tomography, have been used to estimate the magnitude of atrial structural remodeling by measuring atrial volume or LA function. Furthermore, delayed enhanced cardiac magnetic resonance imaging has been used to detect not only atrial fibrosis but also the effect of the ablation point. Thus, atrial remodeling, particularly structural remodeling, plays an important role in AF recurrence. These non-invasive imaging modalities are significant tools for estimating atrial enlargement to improve patient selection for AF ablation at the point of paroxysmal AF, and for estimating atrial fibrosis to select the AF treatment including ablation strategy at the point of development to persistent or permanent AF.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

Molecular imaging: challenges of bringing imaging of intracellular targets into common clinical use

Molecular imaging (MI) takes advantage of several new techniques to detect biomarkers or biochemical and cellular processes, with the goal of obtaining high sensitivity, specificity and signal-to-noise ratio imaging of disease. The imaging modalities bearing the most promise for MI are positron emission tomography (PET), single photon emission computer tomography (SPECT) and different optical imaging techniques with high sensitivity. Also magnetic resonance imaging (MRI) with contrast agents like ultra-small superparamagnetic iron oxide particles (USPIO), magnetic resonance spectroscopy and ultrasound imaging with contrast agents may be useful approaches. MI techniques have been used in the clinic for many years, i.e. PET imaging using (18)  F-labeled fluorodeoxyglucose. Animal studies have during the last years revealed great potential for MI also with several other agents. The focus of the present article is the challenges of clinical imaging of intracellular targets following intravenous injection of the agents. Thus, the great challenge of getting enough contrast agent into the cytosol and at the same time obtaining a low signal from tissue just outside the diseased area is discussed.

Multimodality imaging in interventional cardiology

'Multimodality' imaging--the side-by-side interpretation of data obtained from various noninvasive imaging techniques, such as echocardiography, radionuclide techniques, multidetector CT (MDCT), and MRI--allows anatomical, morphological, and functional data to be combined, increases diagnostic accuracy, and improves the efficacy of cardiovascular interventions and clinical outcomes. During the past decade, advances in software and hardware have allowed co-registration of various imaging modalities, resulting in cardiac 'hybrid' or 'fusion' imaging. In this Review, we discuss the roles of both multimodality and hybrid imaging in three broad areas of cardiology--coronary artery disease (CAD), heart failure, and valvular heart disease. In the evaluation of CAD, integration of either single-photon emission computed tomography (SPECT) or PET with CT coronary angiography provides both morphological and functional data in a single procedure. Accordingly, the functional consequences (myocardial hypoperfusion on SPECT or PET) of anatomical pathology (coronary anatomy on MDCT or MRI) can be assessed. Co-registration of PET and MRI data sets to provide cellular and molecular information on plaque composition and stability is now possible. Furthermore, novel imaging modalities have been implemented to guide electrophysiological and transcatheter-based procedures, such as cardiac resynchronization therapy (an established treatment for patients with heart failure), and transcatheter valve repair or replacement procedures.

Novel peptide targeting integrin αvβ3-rich tumor cells by magnetic resonance imaging

PURPOSE

To investigate the targeting activity of the peptide (named P1c) derived from connective tissue growth factor (CTGF) to αvβ3-rich tumor cells.

MATERIALS AND METHODS

P1c was synthesized and conjugated with ultrasuperparamagnetic iron oxide particles (USPIOs) coated with meso-2,3-dimercaptosuccinic acid (DMSA). The specific binding activity of P1c-USPIOs to αvβ3 was verified by solid phase binding assay. The combination of P1c-USPIOs with a human primary liver cancer cell (Bel 7402) with αvβ3-positive expression and uptake of P1c-USPIOs by cells was investigated by Prussian blue staining, transmission electron microscopy (TEM), and magnetic resonance imaging (MRI). The targeting activity of the probe in vivo was also evaluated using a small-animal tumor model by MRI.

RESULTS

The cell uptake of P1c-USPIOs was observed in a dose-dependent manner, whereas no significant particle uptake was found in the plain USPIOs group. The differences on T2*-weighted imaging were also found by MRI and the signal intensity (SI) was statistically reduced after coculture of Bel 7402 cells with P1c-USPIOs at a concentration of 20-80 μg/mL compared with plain USPIOs (P < 0.05). The in vivo study showed that the signal reduction was distributed mainly in the periphery and some central areas of the tumor. The tumor-to-muscle CNR (contrast-to-noise ratio) at 12 hours after the administration of the P1c-USPIOs was statistically significantly different compared to those at 0 hour, 1 hour, or the plain USPIO group (P < 0.05).

CONCLUSION

The peptide P1c might be a good candidate as a targeting carrier for drugs or tracers.

PET and PET/CT in cardiovascular disease

The present review provides an overview of the role of cardiac positron emission tomography in the diagnosis and management of cardiovascular disease. It expands on the relative advantages and disadvantages over other imaging modalities as well as the available evidence supporting its value in the diagnosis and management of patients with coronary artery disease, the assessment of myocardial viability, and evaluation of the cardiac sympathetic nervous system. Furthermore, the recent developments, such as the implementation of high-end computed tomography devices to form hybrid systems, and the advances of molecular imaging probes in experimental applications are briefly discussed.

Layer-by-layer capsules for magnetic resonance imaging and drug delivery

Layer-by-layer (LbL) self-assembled polyelectrolyte capsules have demonstrated their unique advantages and capability in drug delivery applications. These ordered micro/nano-structures are also promising candidates as imaging contrast agents for diagnostic and theranostic applications. Magnetic resonance imaging (MRI), one of the most powerful clinical imaging modalities, is moving forward to the molecular imaging field and requires the availability of advanced imaging probes. In this review, we are focusing on the design of MRI visible LbL capsules, which incorporate either paramagnetic metal-ligand complexes or superparamagnetic iron oxide (SPIO) nanoparticles. The design criteria cover the topics of probe sensitivity, biosafety, long-circulation property, targeting ligand decoration, and drug loading strategies. Examples of MRI visible LbL capsules with paramagnetic or superparamagnetic moieties were given and discussed. This carrier platform can also be chosen for other imaging modalities.

Imaging of apoptosis in the heart with nanoparticle technology

Apoptosis plays an important role in the loss of cardiomyocytes in both ischemic injury and heart failure. Pioneering work with single photon emission computed tomography imaging of (99)Tc-annexin showed that cell death in the heart could be imaged in vivo. Over the last 5 years a significant amount of experience with annexin-labeled magnetic nanoparticles, principally AnxCLIO-Cy5.5, has also been gained. Here, we review the experience with AnxCLIO-Cy5.5 in the heart and compare this experience to that of earlier studies with (99)Tc-annexin. The imaging of apoptosis with AnxCLIO-Cy5.5 provides valuable insights not only into molecular imaging in the heart but, more broadly, into the use of nanoparticle technology for molecular imaging in general.

Efficacy of contrast-enhanced US and magnetic microbubbles targeted to vascular cell adhesion molecule-1 for molecular imaging of atherosclerosis

PURPOSE

To evaluate whether microbubbles targeted to vascular cell adhesion molecule-1 (VCAM-1) (CD106) coupled with a magnetic guidance system could improve the efficacy of contrast-enhanced molecular ultrasonography (US) of atherosclerosis in the aorta.

MATERIALS AND METHODS

The animal research committee at Southern Medical University approved all experiments. Adherence of magnetic VCAM-1-targeted microbubbles, control inactive magnetic microbubbles, and nonmagnetic VCAM-1-targeted microbubbles to VCAM-1-Fc was determined in vitro by using a flow chamber at variable shear stress (1-24 dyne/cm(2)) under magnetic field guidance. Attachment of microbubbles under magnetic field guidance was determined in vivo with fluorescent microscopy and contrast-enhanced US of the abdominal aorta in wild-type (C57BL/6) or apolipoprotein E (APOE)-deficient mice on a regular or hypercholesterolemic diet. General factorial analysis of variance was used to compare the targeted effect of the microbubbles among different animal groups to identify significant differences.

RESULTS

Attachment was noted for magnetic and nonmagnetic microbubbles but not for inactive magnetic microbubbles; firm attachment at high shear stress (16-20 dyne/cm(2)) was achieved only with magnetic microbubbles. Fluorescence intensity and video intensity were significantly higher in magnetic microbubbles with magnetic field guidance than in inactive magnetic microbubbles and nonmagnetic microbubbles (P < .05). Video intensity from retained magnetic microbubbles in APOE-deficient mice was significantly greater than that in wild-type mice (mean video intensity for APOE-deficient mice: 28.25 [interquartile range, or IQR, 26.55-29.20] with a hypercholesterolemic diet and 16.10 [IQR, 14.15-18.75] with a regular diet; mean video intensity for wild-type mice: 9.55 [IQR, 8.85-10.5] with a hypercholesterolemic diet and 2.90 [IQR, 1.25-3.85] with a regular diet; P < .001).

CONCLUSION

Use of a magnetic targeted microbubble system results in greater attachment to endothelial VCAM-1 in atherosclerotic aortas in conditions of high shear stress and improved detection of early inflammatory changes of atherosclerosis.

Novel iodinated tracers, MIBG and BMIPP, for nuclear cardiology

With the rapid growth of molecular biology, in vivo imaging of such molecular process (i.e., molecular imaging) has been well developed. The molecular imaging has been focused on justifying advanced treatments and for assessing the treatment effects. Most of molecular imaging has been developed using PET camera and suitable PET radiopharmaceuticals. However, this technique cannot be widely available and we need alternative approach. ¹²³I-labeled compounds have been also suitable for molecular imaging using single-photon computed tomography (SPECT) ¹²³I-labeled meta-iodobenzylguanidine (MIBG) has been used for assessing severity of heart failure and prognosis. In addition, it has a potential role to predict fatal arrhythmia, particularly for those who had and are planned to receive implantable cardioverter-defibrillator treatment. ¹²³I-beta-methyl-iodophenylpentadecanoic acid (BMIPP) plays an important role for identifying ischemia at rest, based on the unique capability to represent persistent metabolic alteration after recovery of ischemia, so called ischemic memory. Since BMIPP abnormalities may represent severe ischemia or jeopardized myocardium, it may permit risk analysis in CAD patients, particularly for those with chronic kidney disease and/or hemodialysis patients. This review will discuss about recent development of these important iodinated compounds.

An ultrasound contrast agent targeted to P-selectin detects activated platelets at supra-arterial shear flow conditions

OBJECTIVES

To evaluate targeting of a microbubble contrast agent to platelets under high shear flow using the natural selectin ligand sialyl Lewis.

MATERIALS AND METHODS

Biotinylated polyacrylamide Sialyl Lewis or biotinylated carbohydrate-free polymer (used as a control) were attached to biotinylated microbubbles via a streptavidin linker. Activated human platelets were isolated and attached to fibrinogen-coated culture dishes. Fibrinogen-coated dishes without platelets or platelet dishes blocked by an anti-P-selectin antibody served as negative control substrates. Dishes coated by recombinant P-selectin served as a positive control substrate. Microbubble adhesion was assessed by microscopy in an inverted parallel plate flow chamber, with wall shear stress values of 40, 30, 20, 10, and 5 dynes/cm2. The ratio of binding and passing microbubbles was defined as capture efficiency.

RESULTS

There was no significant difference between the groups regarding the number of microbubbles in the fluid flow at each shear rate. Sialyl Lewis-targeted microbubbles were binding and slowly rolling on the surface of activated platelets and P-selectin-coated dishes at all the flow conditions including 40 dynes/cm2. Capture efficiency of targeted microbubbles to activated platelets and recombinant P-selectin decreased with increasing shear flow: at 5 dynes/cm2, capture efficiency was 16.11% on activated platelets versus 21.83% on P-selectin, and, at 40 dynes/cm2, adhesion efficiency was still 3.4% in both groups. There was neither significant adhesion of Sialyl Lewis-targeted microbubbles to control substrates, nor adhesion of control microbubbles to activated platelets or to recombinant P-selectin.

CONCLUSIONS

Microbubble targeting using sialyl Lewis, a fast-binding ligand to P-selectin, is a promising strategy for the design of ultrasound contrast binding to activated platelets under high shear stress conditions.

Multimodality cardiovascular molecular imaging, part I

In Part I of this consensus article, the imaging methodology, evolving imaging technology, and development of novel targeted molecular probes relevant to the developing field of cardiovascular molecular imaging were reviewed. Novel reporter gene and reporter probe imaging approaches for tracking of cardiac transgene expression were also discussed and have important future implications for evaluation of gene- and cell-based therapies for the failing heart. The current role of metabolic and receptor imaging was also briefly reviewed, as these represent the beginning of our clinical application of molecular imaging within the cardiovascular system. Part II will summarize the available targeted imaging probes as well as specific future applications of molecular imaging for identification and evaluation of critical pathophysiological processes of the cardiovascular system.

Cardiac positron emission tomography

Positron emission tomography (PET) is a powerful, quantitative imaging modality that has been used for decades to noninvasively investigate cardiovascular biology and physiology. Due to limited availability, methodologic complexity, and high costs, it has long been seen as a research tool and as a reference method for validation of other diagnostic approaches. This perception, fortunately, has changed significantly within recent years. Increasing diversity of therapeutic options for coronary artery disease, and increasing specificity of novel therapies for certain biologic pathways, has resulted in a clinical need for more accurate and specific diagnostic techniques. At the same time, the number of PET centers continues to grow, stimulated by PET's success in oncology. Methodologic advances as well as improved radiotracer availability have further contributed to more widespread use. Evidence for diagnostic and prognostic usefulness of myocardial perfusion and viability assessment by PET is increasing. Some studies suggest overall cost-effectiveness of the technique despite higher costs of a single study, because unnecessary follow-up procedures can be avoided. The advent of hybrid PET-computed tomography (CT), which enables integration of PET-derived biologic information with multislice CT-derived morphologic information, and the key role of PET in the development and translation of novel molecular-targeted imaging compounds, have further contributed to more widespread acceptance. Today, PET promises to play a leading diagnostic role on the pathway toward a future of high-powered, comprehensive, personalized, cardiovascular medicine. This review summarizes the state-of-the-art in current imaging methodology and clinical application, and outlines novel developments and future directions.

Clinical cardiovascular molecular imaging

Molecular imaging holds the promise of becoming a key diagnostic modality in cardiovascular medicine by allowing visualization of specific targets and pathways that precede or underlie changes in morphology, physiology, and function. As such, molecular imaging aims at detecting precursors or early stages of cardiovascular disease and at monitoring and guiding novel, increasingly specific and versatile cardiovascular therapies. Imaging of myocardial metabolism and autonomic innervation are already used in current practice, and a wide variety of other targets and probes is on the horizon. This focused review provides an overview of the opportunities and challenges that molecular imaging faces to fulfill its promises in clinical cardiovascular medicine.

Advances in molecular imaging of atherosclerotic vascular disease

PURPOSE OF REVIEW

Molecular imaging aims to illuminate vital molecular and cellular aspects of disease in vivo, and is rapidly translating into the clinical arena. Advantages of this field include enabling serial biological studies in living subjects, assessment of pharmaceutical efficacy, and in-vivo characterization of clinical diseases. Here we present recent exciting advances in molecular imaging of atherosclerotic vascular disease.

RECENT FINDINGS

Atherosclerosis molecular imaging approaches are now available for magnetic resonance, nuclear, computed tomography, ultrasound, and near-infrared fluorescence imaging. Advances in agent synthesis and detection technology are now enabling in-vivo imaging of endothelial cell activation, macrophages, cellular metabolism, protease activity, apoptosis, and osteogenic activity. Several agents show clinical utility for the detection of high-risk plaques.

SUMMARY

Molecular imaging is actively unraveling the biological basis of atherosclerosis in living subjects. In the near-term, molecular imaging will play an important role in assessing novel atherosclerosis pharmacotherapies in clinical trials. Longer term, molecular imaging should enable accurate identification of high-risk plaques responsible for myocardial infarction, stroke, and ischemic limbs.

Molecular Imaging of Myocardial Injury: A Magnetofluorescent Approach

The role of molecular imaging in enhancing the understanding of myocardial injury and repair is rapidly expanding. Moreover, in recent years magnetic resonance and fluorescence-based approaches have been added to the molecular imaging armamentarium and have been used to image selected molecular and cellular targets in the myocardium. Apoptosis, necrosis, macrophage infiltration, myeloperoxidase activity, cathepsin activity, and type 1 collagen have all been imaged in vivo with a magnetofluorescent (MRI and/or fluorescence) approach. This review highlights the potential of these and other magnetofluorescent agents, with particular focus on their role in ischemic heart disease.

Chronic left ventricular failure: the role of imaging in diagnosis and planning of conventional and novel therapies

Heart failure is the leading cause of hospitalisation in the UK, and its prevalence is expected to increase further in the future due partly to an aging population. Although pharmacological agents remain the mainstay of therapy, an increasing number of surgical and novel minimally invasive interventions have been developed for the treatment of both acute and chronic heart failure. Imaging is essential for diagnosis, guiding therapeutic options, and monitoring therapy and its complications. As a result, radiologists should be familiar with the pathogenesis, treatment options, and imaging-related issues pertaining to the management of these patients.

Incorporation of an apoE-derived lipopeptide in high-density lipoprotein MRI contrast agents for enhanced imaging of macrophages in atherosclerosis

Magnetic resonance (MR) imaging is becoming a pivotal diagnostic method to identify and characterize vulnerable atherosclerotic plaques. We previously reported a reconstituted high-density lipoprotein (rHDL) nanoparticle platform enriched with Gd-based amphiphiles as a plaque-specific MR imaging contrast agent. Further modification can be accomplished by inserting targeting moieties into this platform to potentially allow for improved intraplaque macrophage uptake. Since studies have indicated that intraplaque macrophage density is directly correlated to plaque vulnerability, modification of the rHDL platform may allow for better detection of vulnerable plaques. In the current study we incorporated a carboxyfluoresceine-labeled apolipoprotein E-derived lipopeptide, P2fA2, into rHDL. The in vitro macrophage uptake and in vivo MR efficacy were demonstrated using murine J774A.1 macrophages and the apolipoprotein E knock-out (apoE(-/-)) mouse model of atherosclerosis. The in vitro studies indicated enhanced association of murine macrophages to P2fA2 enriched rHDL (rHDL-P2A2) nanoparticles, relative to rHDL, using optical techniques and MR imaging. The in vivo studies showed a more pronounced and significantly higher signal enhancement of the atherosclerotic wall 24 h after the 50 micromol Gd/kg injection of rHDL-P2A2 relative to administration of rHDL. The normalized enhancement ratio for atherosclerotic wall of rHDL-P2A2 contrast agent injection was 90%, while that of rHDL was 53% 24 h post-injection. Confocal laser scanning microscopy revealed that rHDL-P2A2 nanoparticles co-localized primarily with intraplaque macrophages. The results of the current study confirm the hypothesis that intraplaque macrophage uptake of rHDL may be enhanced by the incorporation of the P2fA2 peptide into the modified HDL particle.

In vivo magnetic resonance imaging of injected mesenchymal stem cells in rat myocardial infarction; simultaneous cell tracking and left ventricular function measurement

To determine whether magnetic resonance imaging (MRI) can enable magnetically labeled mesenchymal stem cell (MSC) tracking and simultaneous in vivo functional data acquisition in rat models of myocardial infarction. Superparamagnetic iron oxide-laden human MSCs were injected into rat myocardium infarcted by cryoinjury 3 weeks after myocardial infarction. The control group received cell-free media injection. Before injection and for 3 months after, in vivo serial MRI was performed. Electrocardiography-gated gradient echo sequence MRI and cine MRI were performed for in vivo cell tracking and assessing cardiac function using left ventricular ejection fraction (LVEF), respectively. MRI revealed a persistent signal-void representing iron-laden MSCs until ten post-injection weeks. Serial follow-up MRI revealed that LVEF was significantly higher in the MSC injection group than in the control group. We conclude that MRI enables in vivo tracking of injected cells and evaluation of the long-term therapeutic potential of MSCs for myocardial infarction.

Targeted imaging of myocardial damage

Molecular imaging agents can be targeted to a specific receptor or protein on the cardiomyocyte surface, or to enzymes released into the interstitial space, such as cathepsins, matrix metalloproteinases and myeloperoxidase. Molecular imaging of the myocardium, however, requires the imaging agent to be small, sensitive (nanomolar levels or better), and able to gain access to the interstitial space. Several novel agents that fulfill these criteria have been used for targeted molecular imaging applications in the myocardium. Magnetic resonance, fluorescence, and single-photon emission CT have been used to image the molecular signals generated by these agents. The use of targeted imaging agents in the myocardium has the potential to provide valuable insights into the pathophysiology of myocardial injury and to facilitate the development of novel therapeutic strategies.

Imaging atherosclerotic plaque inflammation

Inflammation within atherosclerotic plaques is one of the main drivers of atherosclerotic plaque rupture, which frequently leads to clinical events such as myocardial infarction and stroke. Current gold standard techniques such as X-ray angiography and ultrasound can rapidly report on luminal encroachment but give no readout on inflammatory state of the plaque. We summarize several alternative imaging techniques--CT, MRI, and nuclear imaging--that are close to the clinical arena, and we provide the relative advantages of each.

Reduced nicotinamide adenine dinucleotide fluorescence lifetime separates human mesenchymal stem cells from differentiated progenies

The metabolic changes of human mesenchymal stem cells (hMSCs) during osteogenic differentiation were accessed by reduced nicotinamide adenine dinucleotide (NADH) fluorescence lifetime. An increase in mean fluorescence lifetime and decrease in the ratio between free NADH and protein-bound NADH correlated with our previously reported increase in the adenosine triphosphate (ATP) level of hMSCs during differentiation. These findings suggest that NADH fluorescence lifetime may serve as a new optical biomarker for noninvasive selection of stem cells from differentiated progenies.

Fundamentals of molecular imaging: rationale and applications with relevance for radiation oncology

Molecular imaging allows for the visualization and quantification biologic processes at cellular levels. This article focuses on positron emission tomography as one readily available tool for clinical molecular imaging. To prove its clinical utility in oncology, molecular imaging will ultimately have to provide valuable information in the following 4 pertinent areas: staging; assessment of extent of disease; target delineation for radiation therapy planning; response prediction and assessment and differentiation between treatment sequelae and recurrent disease. These issues are addressed in other contributions in this issue of Seminars in Nuclear Medicine. In contrast, this article will focus on the biochemical principles of cancer metabolism that provide the rationale for positron emission tomography imaging in radiation oncology.

Imaging of atherosclerotic cardiovascular disease

Atherosclerosis is characterized by thickening of the walls of the arteries, a process that occurs slowly and 'silently' over decades. This prolonged course of disease provides a window of opportunity for diagnosis before symptoms occur. But, until recently, only advanced atherosclerotic disease could be observed. Now, developments in imaging technology offer many enticing prospects, including detecting atherosclerosis early, grouping individuals by the probability that they will develop symptoms of atherosclerosis, assessing the results of treatment and improving the current understanding of the biology of atherosclerosis.

Molecular imaging in cardiovascular magnetic resonance imaging: current perspective and future potential

The development of novel imaging agents and techniques is allowing some biological events to be imaged in vivo with magnetic resonance imaging (MRI) at the cellular and subcellular level. In this paper, the use of novel gadolinium chelates and superparamagnetic iron oxide nanoparticles for molecular MRI of the cardiovascular system is extensively reviewed. The physical properties of these imaging agents and the pulse sequences best suited to their visualization are extensively discussed. The application of molecular MRI in diseases of the vasculature and myocardium is then reviewed. The clinical experience to date, as well as the promise and potential impact of molecular MRI, is extensively discussed.