99mTc-interleukin-2 scintigraphy for the in vivo imaging of vulnerable atherosclerotic plaques

Molecular imaging of cardiovascular disease: Current status and future perspective

Advancements in knowledge of cardiovascular disease, pharmacology, and chemistry have led to the development of newer radiopharmaceuticals and targets for new and more suitable molecules. Molecular imaging encompasses multiple imaging techniques for identifying the characteristics of key components involved in disease. Despite its limitations in spatial resolution, the affinity for key molecules compensates for disadvantages in diagnosing diseases and elucidating their pathophysiology. This review introduce established molecular tracers involved in clinical practice and emerging tracers already applied in clinical studies, classifying the key component in A: artery, specifically those vulnerable plaque (A-I) inflammatory cells [18F-FDG]; A-II) lipid/fatty acid; A-III) hypoxia; A-IV) angiogenesis; A-V) protease [18F/68Ga-FAPI]; A-VI) thrombus/hemorrhage; A-VII) apoptosis and A-VIII) microcalcification [18F-NaF]) and B: myocardium, including myocardial ischemia, infarction and myocardiopathy (B-I) myocardial ischemia; B-II) myocardial infarction (myocardial damage and fibrosis); B-III) myocarditis and endocarditis; B-IV) sarcoidosis; B-V) amyloidosis; B-VI) metabolism; B-VII) innervation imaging). In addition to cardiovascular-specific tracers tested in animal models, many radiotracers may have been developed in other areas, such as oncology imaging or neuroimaging. While this review does not cover all available tracers, some of them hold potential for future use assessing cardiovascular disease. Advances in molecular biology, pharmaceuticals, and imaging sciences will facilitate the identification of precise disease mechanisms, enabling precise diagnoses, better assessment of disease status, and enhanced therapeutic evaluation in this multi-modality era.

MR Angiography of Extracranial Carotid Disease

Magnetic resonance angiography sequences, such as time-of-flight and contrast-enhanced angiography, provide clear depiction of vessel lumen, traditionally used to evaluate carotid pathologic conditions such as stenosis, dissection, and occlusion; however, atherosclerotic plaques with a similar degree of stenosis may vary tremendously from a histopathological standpoint. MR vessel wall imaging is a promising noninvasive method to evaluate the content of the vessel wall at high spatial resolution. This is particularly interesting in the case of atherosclerosis as vessel wall imaging can identify higher risk, vulnerable plaques as well as has potential applications in the evaluation of other carotid pathologic conditions.

Vulnerable Plaque Imaging

Atherosclerotic plaques progress as a result of inflammation. Both invasive and noninvasive imaging techniques have been developed to identify and characterize plaque as vulnerable (more likely to rupture and cause a clinical event). Imaging techniques to identify vulnerable include identifying vessels with focal subendothelial collections of I) inflammatory cells; II) lipid/ fatty acid; III) local regions of hypoxia; IV) local expression of angiogenesis factors; V) local expression of protease; VI) intravascular foci of thrombus; hemorrhage (most often seen in the aftermath of a clinical event); VII) apoptosis and VIII) microcalcification. This review provides an overview of atherosclerotic plaque progression and tracers which can visualize specific molecules associated with vulnerability.

Emerging Nuclear Medicine Imaging of Atherosclerotic Plaque Formation

Atherosclerosis is a chronic widespread cardiovascular disease and a major predisposing factor for cardiovascular events, among which there are myocardial infarction and ischemic stroke. Atherosclerotic plaque formation is a process that involves different mechanisms, of which inflammation is the most common. Plenty of radiopharmaceuticals were developed to elucidate the process of plaque formation at different stages, some of which were highly specific for atherosclerotic plaque. This review summarizes the current nuclear medicine imaging landscape of preclinical and small-scale clinical studies of these specific RPs, which are not as widespread as labeled FDG, sodium fluoride, and choline. These include oxidation-specific epitope imaging, macrophage, and other cell receptors visualization, neoangiogenesis, and macrophage death imaging. It is shown that specific radiopharmaceuticals have strength in pathophysiologically sound imaging of the atherosclerotic plaques at different stages, but this also may induce problems with the signal registration for low-volume plaques in the vascular wall.

Molecular Imaging of Vulnerable Coronary Plaque with Radiolabeled Somatostatin Receptors (SSTR)

Atherosclerosis is responsible for the majority of heart attacks and is characterized by several modifications of the arterial wall including an inflammatory reaction. The silent course of atherosclerosis has made it necessary to develop predictors of disease complications before symptomatic lesions occur. Vulnerable to rupture atherosclerotic plaques are the target for molecular imaging. To this aim, different radiopharmaceuticals for PET/CT have emerged for the identification of high-risk plaques, with high specificity for the identification of the cellular components and pathophysiological status of plaques. By targeting specific receptors on activated macrophages in high-risk plaques, radiolabelled somatostatin analogues such as ⁶⁸Ga-DOTA-TOC, TATE,0 or NOC have shown high relevance to detect vulnerable, atherosclerotic plaques. This PET radiopharmaceutical has been tested in several pre-clinical and clinical studies, as reviewed here, showing an important correlation with other risk factors.

Cell-Based Tracers as Trojan Horses for Image-Guided Surgery

Surgeons rely almost completely on their own vision and palpation to recognize affected tissues during surgery. Consequently, they are often unable to distinguish between different cells and tissue types. This makes accurate and complete resection cumbersome. Targeted image-guided surgery (IGS) provides a solution by enabling real-time tissue recognition. Most current targeting agents (tracers) consist of antibodies or peptides equipped with a radiolabel for Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT), magnetic resonance imaging (MRI) labels, or a near-infrared fluorescent (NIRF) dye. These tracers are preoperatively administered to patients, home in on targeted cells or tissues, and are visualized in the operating room via dedicated imaging systems. Instead of using these 'passive' tracers, there are other, more 'active' approaches of probe delivery conceivable by using living cells (macrophages/monocytes, neutrophils, T cells, mesenchymal stromal cells), cell(-derived) fragments (platelets, extracellular vesicles (exosomes)), and microorganisms (bacteria, viruses) or, alternatively, 'humanized' nanoparticles. Compared with current tracers, these active contrast agents might be more efficient for the specific targeting of tumors or other pathological tissues (e.g., atherosclerotic plaques). This review provides an overview of the arsenal of possibilities applicable for the concept of cell-based tracers for IGS.

In vivo Imaging Technologies to Monitor the Immune System

The past two decades have brought impressive advancements in immune modulation, particularly with the advent of both cancer immunotherapy and biologic therapeutics for inflammatory conditions. However, the dynamic nature of the immune response often complicates the assessment of therapeutic outcomes. Innovative imaging technologies are designed to bridge this gap and allow non-invasive visualization of immune cell presence and/or function in real time. A variety of anatomical and molecular imaging modalities have been applied for this purpose, with each option providing specific advantages and drawbacks. Anatomical methods including magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound provide sharp tissue resolution, which can be further enhanced with contrast agents, including super paramagnetic ions (for MRI) or nanobubbles (for ultrasound). Conjugation of the contrast material to an antibody allows for specific targeting of a cell population or protein of interest. Protein platforms including antibodies, cytokines, and receptor ligands are also popular choices as molecular imaging agents for positron emission tomography (PET), single-photon emission computerized tomography (SPECT), scintigraphy, and optical imaging. These tracers are tagged with either a radioisotope or fluorescent molecule for detection of the target. During the design process for immune-monitoring imaging tracers, it is important to consider any potential downstream physiologic impact. Antibodies may deplete the target cell population, trigger or inhibit receptor signaling, or neutralize the normal function(s) of soluble proteins. Alternatively, the use of cytokines or other ligands as tracers may stimulate their respective signaling pathways, even in low concentrations. As in vivo immune imaging is still in its infancy, this review aims to describe the modalities and immunologic targets that have thus far been explored, with the goal of promoting and guiding the future development and application of novel imaging technologies.

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.

Immuno-Imaging to Predict Treatment Response in Infection, Inflammation and Oncology

Background: Molecular nuclear medicine plays a pivotal role for diagnosis in a preclinical phase, in genetically susceptible patients, for radio-guided surgery, for disease relapse evaluation, and for therapy decision-making and follow-up. This is possible thanks to the development of new radiopharmaceuticals to target specific biomarkers of infection, inflammation and tumour immunology. Methods: In this review, we describe the use of specific radiopharmaceuticals for infectious and inflammatory diseases with the aim of fast and accurate diagnosis and treatment follow-up. Furthermore, we focus on specific oncological indications with an emphasis on tumour immunology and visualizing the tumour environment. Results: Molecular nuclear medicine imaging techniques get a foothold in the diagnosis of a variety of infectious and inflammatory diseases, such as bacterial and fungal infections, rheumatoid arthritis, and large vessel vasculitis, but also for treatment response in cancer immunotherapy. Conclusion: Several specific radiopharmaceuticals can be used to improve diagnosis and staging, but also for therapy decision-making and follow-up in infectious, inflammatory and oncological diseases where immune cells are involved. The identification of these cell subpopulations by nuclear medicine techniques would provide personalized medicine for these patients, avoiding side effects and improving therapeutic approaches.

Antibody-Based Tracers for PET/SPECT Imaging of Chronic Inflammatory Diseases

Chronic inflammatory diseases are often progressive, resulting not only in physical damage to patients but also social and economic burdens, making early diagnosis of them critical. Nuclear medicine techniques can enhance the detection of inflammation by providing functional as well as anatomical information when combined with other modalities such as magnetic resonance imaging, computed tomography or ultrasonography. Although small molecules and peptides were mainly used for the treatment and imaging of chronic inflammatory diseases in the past, antibodies and their fragments have also been emerging for chronic inflammatory diseases as they show high specificity to their targets and can have various biological half-lives depending on how they are engineered. In addition, imaging with antibodies or their fragments can visualize the in vivo biodistribution of the probes or help monitor therapeutic responses, thereby providing physicians with a greater understanding of drug behavior in vivo and another means of monitoring their patients. In this review, we introduce various targets and radiolabeled antibody-based probes for the molecular imaging of chronic inflammatory diseases in preclinical and clinical studies. Targets can be classified into three different categories: 1) cell-adhesion molecules, 2) surface markers on immune cells, and 3) cytokines or enzymes. The limitations and future directions of using radiolabeled antibodies for imaging inflammatory diseases are also discussed.

Molecular Imaging of Vulnerable Plaque

Molecular imaging provides multiple imaging techniques to identify characteristics of vulnerable plaque including I) Inflammatory cells (the presence and metabolic activity of macrophages), II) synthesis of lipid and fatty acid in the plaque, III) the presence of hypoxia in severely inflamed lesions, IV) expression of factors stimulating angiogenesis, V) expression of protease enzymes in the lesion, VI) development of microthrombi in late-phase lesions, VII) apoptosis, and VIII) microcalcification.

Carotid Artery Wall Imaging: Perspective and Guidelines from the ASNR Vessel Wall Imaging Study Group and Expert Consensus Recommendations of the American Society of Neuroradiology

Identification of carotid artery atherosclerosis is conventionally based on measurements of luminal stenosis and surface irregularities using in vivo imaging techniques including sonography, CT and MR angiography, and digital subtraction angiography. However, histopathologic studies demonstrate considerable differences between plaques with identical degrees of stenosis and indicate that certain plaque features are associated with increased risk for ischemic events. The ability to look beyond the lumen using highly developed vessel wall imaging methods to identify plaque vulnerable to disruption has prompted an active debate as to whether a paradigm shift is needed to move away from relying on measurements of luminal stenosis for gauging the risk of ischemic injury. Further evaluation in randomized clinical trials will help to better define the exact role of plaque imaging in clinical decision-making. However, current carotid vessel wall imaging techniques can be informative. The goal of this article is to present the perspective of the ASNR Vessel Wall Imaging Study Group as it relates to the current status of arterial wall imaging in carotid artery disease.

Recent Advances in the Development of PET/SPECT Probes for Atherosclerosis Imaging

The rupture of vulnerable atherosclerotic plaques and subsequent thrombus formation are the major causes of myocardial and cerebral infarction. Accordingly, the detection of vulnerable plaques is important for risk stratification and to provide appropriate treatment. Inflammation imaging using 2-deoxy-2-[¹⁸F]fluoro-D-glucose (¹⁸F-FDG) has been most extensively studied for detecting vulnerable atherosclerotic plaques. It is of great importance to develop PET/SPECT probes capable of specifically visualizing the biological molecules involved in atherosclerotic plaque formation and/or progression. In this article, we review recent advances in the development of PET/SPECT probes for visualizing atherosclerotic plaques and their application to therapy monitoring, mainly focusing on experimental studies.

Nuclear Molecular Imaging for Vulnerable Atherosclerotic Plaques

Atherosclerosis is an inflammatory disease as well as a lipid disorder. Atherosclerotic plaque formed in vessel walls may cause ischemia, and the rupture of vulnerable plaque may result in fatal events, like myocardial infarction or stroke. Because morphological imaging has limitations in diagnosing vulnerable plaque, molecular imaging has been developed, in particular, the use of nuclear imaging probes. Molecular imaging targets various aspects of vulnerable plaque, such as inflammatory cell accumulation, endothelial activation, proteolysis, neoangiogenesis, hypoxia, apoptosis, and calcification. Many preclinical and clinical studies have been conducted with various imaging probes and some of them have exhibited promising results. Despite some limitations in imaging technology, molecular imaging is expected to be used both in the research and clinical fields as imaging instruments become more advanced.

Inflammation imaging of atherosclerosis in Apo-E-deficient mice using a (99m)Tc-labeled dual-domain cytokine ligand

Interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) play a critical role in initiating and accelerating atherosclerosis. This study evaluated the imaging properties of (99m)Tc-TNFR2-Fc-IL-1RA ((99m)Tc-TFI), a dual-domain cytokine radioligand that targets TNF-α and IL-1β pathways, in assessing atherosclerosis development in apolipoprotein-E-deficient (ApoE(-)(/)(-)) mice.

METHODS

The feasibility and specificity of detecting atherosclerosis with (99m)Tc-TFI SPECT imaging were investigated in ApoE(-)(/)(-) and ApoE(+)(/)(+) mice. Fifty-four ApoE(-)(/)(-) mice were fed either an atherogenic diet (AGD) or a normal diet (ND) beginning at 5 weeks of age. Eighteen Apo-E wild-type (ApoE(+)(/)(+)) mice were fed an ND. Two groups of ApoE(-)(/)(-) mice (n=12 each group) on AGD and ND were imaged three times with (99m)Tc-TFI and a high-resolution SPECT system at 20-25, 30-40, and 48-52 weeks to study the evolution of atherosclerotic plaque.

RESULTS

Focal radioactive accumulations in the aortic arch region were observed in the ApoE(-)(/)(-) mice (n=12) on AGD but not in the ApoE(+)(/)(+) mice on ND (n=10). Apo-E(-)(/)(-) mice on ND (n=11) exhibited lower radioactive uptake than ApoE(-)(/)(-) mice on AGD (P<0.05). Co-injection of an excess of cold ligand with (99m)Tc-TFI resulted in significant reduction of (99m)Tc-TFI uptake in the ApoE(-)(/)(-) mice on AGD. Longitudinal studies showed that (99m)Tc-TFI uptake in the aortas of ApoE(-)(/)(-) mice progressively increased from 20 to 48 weeks. Real-time PCR assays demonstrated that atherosclerotic aortas expressed significantly higher IL-1β and TNF-α than the aortas from wild-type controls.

CONCLUSIONS

Atherosclerotic plaques were detected by (99m)Tc-TFI imaging in ApoE(-)(/)(-) mice. (99m)Tc-TFI is promising for specific detection of inflammatory response in atherosclerotic plaques.

Molecular imaging of bacterial infections in vivo: the discrimination of infection from inflammation

Molecular imaging by definition is the visualization of molecular and cellular processes within a given system. The modalities and reagents described here represent a diverse array spanning both pre-clinical and clinical applications. Innovations in probe design and technologies would greatly benefit therapeutic outcomes by enhancing diagnostic accuracy and assessment of acute therapy. Opportunistic pathogens continue to pose a worldwide threat, despite advancements in treatment strategies, which highlights the continued need for improved diagnostics. In this review, we present a summary of the current clinical protocol for the imaging of a suspected infection, methods currently in development to optimize this imaging process, and finally, insight into endocarditis as a model of infectious disease in immediate need of improved diagnostic methods.

Visualizing the atherosclerotic plaque: a chemical perspective

Atherosclerosis is the major underlying pathologic cause of coronary artery disease. An early detection of the disease can prevent clinical sequellae such as angina, myocardial infarction, and stroke. The different imaging techniques employed to visualize the atherosclerotic plaque provide information of diagnostic and prognostic value. Furthermore, the use of contrast agents helps to improve signal-to-noise ratio providing better images. For nuclear imaging techniques and optical imaging these agents are absolutely necessary. We report on the different contrast agents that have been used, are used or may be used in future in animals, humans, or excised tissues for the distinct imaging modalities for atherosclerotic plaque imaging.

In vivo and in vitro evidence that ⁹⁹mTc-HYNIC-interleukin-2 is able to detect T lymphocytes in vulnerable atherosclerotic plaques of the carotid artery

PURPOSE

Recent advances in basic science have established that inflammation plays a pivotal role in the pathogenesis of atherosclerosis. Inflammatory cells are thought to be responsible for the transformation of a stable plaque into a vulnerable one. Lymphocytes constitute at least 20 % of infiltrating cells in these vulnerable plaques. Therefore, the interleukin-2 (IL-2) receptor, being overexpressed on activated T lymphocytes, may represent an attractive biomarker for plaque vulnerability. The aim of this study was to evaluate the specificity of radiolabelled IL-2 [(99m)Tc-hydrazinonicotinamide (HYNIC)-IL-2] for imaging the lymphocytic infiltration in carotid plaques in vivo by planar and single photon emission computed tomography (SPECT)/CT imaging and ex vivo by microSPECT and autoradiography.

METHODS

For the in vivo study, ten symptomatic patients with advanced plaques at ultrasound who were scheduled for carotid endarterectomy underwent (99m)Tc-HYNIC-IL-2 scintigraphy. The images were analysed visually on planar and SPECT images and semi-quantitatively on SPECT images by calculating target to background (T/B) ratios. After endarterectomy, immunomorphological evaluation and immunophenotyping were performed on plaque slices. For the ex vivo studies, four additional patients were included and, after in vitro incubation of removed plaques with (99m)Tc-HYNIC-IL-2, autoradiography was performed and microSPECT images were acquired.

RESULTS

Visual analysis defined clear (99m)Tc-HYNIC-IL-2 uptake in seven of the ten symptomatic plaques. SPECT/CT allowed visualization in eight of ten. A significant correlation was found between the number of CD25+ lymphocytes and the total number of CD25+ cells in the plaque and the T/B ratio with adjacent carotid artery as background (Pearson's r = 0.89, p = 0.003 and r = 0.87, p = 0.005, respectively). MicroSPECT imaging showed clear (99m)Tc-HYNIC-IL-2 uptake within the plaque wall and not in the lipidic core. With autoradiography, only CD3+ lymphocytes were found to be labelled.

CONCLUSION

These in vivo and ex vivo studies confirm the specificity of (99m)Tc-HYNIC-IL-2 for imaging activated T lymphocytes in carotid plaques. (99m)Tc-HYNIC-IL-2 is a true marker for the inflamed plaque and therefore of plaque instability.

PET imaging of inflammation biomarkers

Inflammation plays a significant role in many disease processes. Development in molecular imaging in recent years provides new insight into the diagnosis and treatment evaluation of various inflammatory diseases and diseases involving inflammatory process. Positron emission tomography using (18)F-FDG has been successfully applied in clinical oncology and neurology and in the inflammation realm. In addition to glucose metabolism, a variety of targets for inflammation imaging are being discovered and utilized, some of which are considered superior to FDG for imaging inflammation. This review summarizes the potential inflammation imaging targets and corresponding PET tracers, and the applications of PET in major inflammatory diseases and tumor associated inflammation. Also, the current attempt in differentiating inflammation from tumor using PET is also discussed.

Future challenges of multimodality imaging

During the last decade, positron emission tomography/computed tomography (PET/CT) and single-photon emission computed tomography/computed tomography (SPECT/CT) have procured advances in research and clinical application of fusion imaging. The recent introduction of systems that combine PET and MRI opens new horizons for multimodality molecular imaging. These systems offer simultaneous morphologic, functional, and molecular information of a living system. Moreover, other combinations of anatomic and functional imaging modalities (for example CT and MRI or PET and optical imaging) are emerging, holding promise in basic medical research or in clinical medicine. These developments are paralleled by advances in the field of biomolecules and particles, to provide new agents useful for more than one imaging modality and to facilitate the study of the same target by different imaging devices. In the near future PET/MRI may emerge as a new powerful multimodality technique in clinical oncology, offering considerable potential for imaging applications beyond correlation of functional and anatomic images. Future developments should include the simultaneous acquisition of multifunctional data such as PET tracer uptake, MR spectroscopy, or fMRI along with high-resolution anatomic MRI.

Adverse cardiovascular effects of anabolic steroids: pathophysiology imaging

BACKGROUND

  Anabolic-androgenic steroids (AAS) are widely abused for enhancing muscle mass, strength, growth and improving athletic performance.

MATERIALS AND METHODS

  In recent years, many observational and interventional studies have shown important adverse cardiovascular effects of AAS abuse.

CONCLUSIONS

  This review discusses established and future perspectives of novel molecular imaging techniques that may serve as potential tools for early detection of AAS-associated cardiovascular disorders.

Imaging of inflamed carotid artery atherosclerotic plaques with the use of 99mTc-HYNIC-IL-2 scintigraphy in end-stage renal disease patients

PURPOSE

Identification of vulnerable plaques remains crucial for better cardiovascular risk assessment. At least 20% of inflammatory cells within unstable (vulnerable) plaques comprise T lymphocytes, which contain receptors for interleukin-2 (IL-2); those receptors can be identified by scintigraphy with radiolabelled IL-2.The aim of this study was to identify the "inflamed" (vulnerable) plaques by scintigraphy using IL-2 labelled with (99m)Tc in the selected, high cardiovascular risk group of end-stage renal disease (ESRD) patients.

METHODS

A total of 28 patients (18 men, 10 women, aged 55.2 ± 9.6 years, 17 on peritoneal dialysis, 11 on haemodialysis) underwent common carotid artery (CCA) scintigraphy with the use of (99m)Tc-hydrazinonicotinamide (HYNIC)-IL-2. In all cases, ultrasound examination of the CCA was performed and levels of selected proinflammatory factors, atherogenic markers and calcium-phosphate balance parameters were measured. Finally, the target to non-target (T/nT) ratio of IL-2 uptake in atherosclerotic plaques with intima-media thickness (IMT), classic cardiovascular risk factors and concentrations of the measured factors were compared.

RESULTS

Increased (99m)Tc-HYNIC-IL-2 uptake in atherosclerotic plaques in 38/41 (91%) cases was detected. The median T/nT ratio of focal (99m)Tc-HYNIC-IL-2 uptake in atherosclerotic plaques was 2.35 (range 1.23-3.63). The mean IMT value on the side of plaques assessed by scintigraphy was 0.79 ± 0.18 mm (median 0.8, range 0.5-1.275). Correlations between T/nT ratio and homocysteine (R = 0.22, p = 0.037), apolipoprotein B (apoB) (R = 0.31, p = 0.008), apoB to apoA-I ratio (R = 0.29, p = 0.012) and triglyceride concentration (R = 0.26, p = 0.021) were detected. A lower T/nT ratio in patients with better parameters of nutritional status (haemoglobin, albumin, adiponectin) in comparison with patients with worse nutritional parameters (3.20 ± 0.5 vs 2.16 ± 0.68, p = 0.025) was revealed as well as a difference between values of T/nT ratio in groups of patients with values of apoB, soluble CD40 ligand and asymmetric dimethylarginine above and below median (3.18 ± 0.52 vs 2.16 ± 0.68, p = 0.031). No statistically significant association was found between T/nT ratio and mean value of either IMT or classic cardiovascular risk factors.

CONCLUSION

Scintigraphy with the use of (99m)Tc-HYNIC-IL-2 can be a tool for inflamed atherosclerotic (vulnerable) plaque visualization within CCA in ESRD patients. Quantitative results of carotid artery scintigraphy with (99m)Tc-HYNIC-IL-2 correlate with serum concentration of selected cardiovascular risk markers.

Direct relationship between cell density and FDG uptake in asymptomatic aortic aneurysm close to surgical threshold: an in vivo and in vitro study

PURPOSE

Conflicting results have been reported about the clinical value of fluorodeoxyglucose (FDG) imaging in predicting the risk of rupture of abdominal aortic aneurysm (AAA). The present study tests the hypothesis that FDG uptake is low in asymptomatic noninflammatory AAA due to the low cell density in aneurysmal walls.

METHODS

Positron emission tomography (PET)/CT imaging was performed in 12 consecutive candidates for AAA surgical repair and in 12 age- and sex-matched controls. At intervention, aneurysmal walls were cut into three sequential blocks. Block A was frozen to cut three 5-μm slices for incubation with 2-3 MBq of FDG for 5 min. Block C was first incubated with the same tracer solution for the same time and subsequently frozen to cut three 5-μm slices. Autoradiographic images were coregistered with immunohistochemical pictures of cell density, type and DNA synthesis as assessed on block B.

RESULTS

No visible uptake in abdominal aorta occurred in any patient or control subject. Immunohistochemistry documented a severe loss of wall structure, with low numbers of cells. Tracer retention directly correlated with overall cell density and with prevalence of cells synthesizing DNA. The metabolic nature of FDG uptake was confirmed by the selective effect of preliminary freezing that decreased tracer content by 90% in regions with high cell density and only by 34% in cold acellular areas.

CONCLUSION

The loss of tissue structure and the marked decrease in cell density account for the low prevalence of positive findings at FDG PET imaging, at least in asymptomatic patients bearing AAAs whose diameter is close to surgical indication.

The molecular imaging approach to image infections and inflammation by nuclear medicine techniques

Inflammatory and infectious diseases are a heterogeneous class of diseases that may be divided into infections, acute inflammation and chronic inflammation. Radiological imaging techniques have, with the exception of functional MRI, high sensitivity but lack in specificity. Nuclear medicine techniques, by contrast, allow the in vivo detection in humans of different physiologic and pathologic phenomena and offer noninvasive tools to detect early pathophysiological changes before anatomical changes occur. In this review, we highlight the role of nuclear medicine in inflammation/infection with emphasis on molecular imaging for in vivo histological characterization of affected tissues for diagnostic purposes and follow-up of therapies. We also describe the clinical indications of all available radiopharmaceuticals in the light of the newly available guidelines.

Emerging applications of nanotechnology for the diagnosis and management of vulnerable atherosclerotic plaques

An estimated 16 million people in the United States have coronary artery disease (CAD), and approximately 325,000 people die annually from cardiac arrest. About two-thirds of unexpected cardiac deaths occur without prior recognition of cardiac disease. A vast majority of these deaths are attributable to the rupture of 'vulnerable atherosclerotic plaques'. Clinically, plaque vulnerability is typically assessed through imaging techniques, and ruptured plaques leading to acute myocardial infarction are treated through angioplasty or stenting. Despite significant advances, it is clear that current imaging methods are insufficiently capable for elucidating plaque composition--which is a key determinant of vulnerability. Further, the exciting improvement in the treatment of CAD afforded by stenting procedures has been buffered by significant undesirable host-implant effects, including restenosis and late thrombosis. Nanotechnology has led to some potential solutions to these problems by yielding constructs that interface with plaque cellular components at an unprecedented size scale. By leveraging the innate ability of macrophages to phagocytose nanoparticles, contrast agents can now be targeted to plaque inflammatory activity. Improvements in nano-patterning procedures have now led to increased ability to regenerate tissue isotropy directly on stents, enabling gradual regeneration of normal, physiologic vascular structures. Advancements in immunoassay technologies promise lower costs for biomarker measurements, and in the near future, may enable the addition of routine blood testing to the clinician's toolbox--decreasing the costs of atherosclerosis-related medical care. These are merely three examples among many stories of how nanotechnology continues to promise advances in the diagnosis and treatment of vulnerable atherosclerotic plaques.

Nanomedicine approaches in vascular disease: a review

Nanomedicine approaches have revolutionized the treatment of cancer and vascular diseases, where the limitations of rapid nonspecific clearance, poor biodistribution and harmful side effects associated with direct systemic drug administration can be overcome by packaging the agents within sterically stabilized, long-circulating nanovehicles that can be further surface-modified with ligands to actively target cellular/molecular components of the disease. With significant advancements in genetics, proteomics, cellular and molecular biology and biomaterials engineering, the nanomedicine strategies have become progressively refined regarding the modulation of surface and bulk chemistry of the nanovehicles, control of drug release kinetics, manipulation of nanoconstruct geometry and integration of multiple functionalities on single nanoplatforms. The current review aims to capture the various nanomedicine approaches directed specifically toward vascular diseases during the past two decades. Analysis of the promises and limitations of these approaches will help identify and optimize vascular nanomedicine systems to enhance their efficacy and clinical translation in the future.

FROM THE CLINICAL EDITOR

Nanomedicine-based approaches have had a major impact on the treatment and diagnosis of malignancies and vascular diseases. This review discusses various nanomedicine approaches directed specifically toward vascular diseases during the past two decades, highlighting their advantages, limitations and offering new perspectives on future applications.

Multiple aberrations in shared inflammatory and oxidative & nitrosative stress (IO&NS) pathways explain the co-association of depression and cardiovascular disorder (CVD), and the increased risk for CVD and due mortality in depressed patients

There is evidence that there is a bidirectional relationship between major depression and cardiovascular disorder (CVD): depressed patients are a population at risk for increased cardiac morbidity and mortality, and depression is more frequent in patients who suffer from CVD. There is also evidence that inflammatory and oxidative and nitrosative stress (IO&NS) pathways underpin the common pathophysiology of both CVD and major depression. Activation of these pathways may increase risk for both disorders and contribute to shared risk. The shared IO&NS pathways that may contribute to CVD and depression comprise the following: increased levels of pro-inflammatory cytokines, like interleukin-1β (IL-1β), IL-2, IL-6, IL-8, IL-12, tumor necrosis factor-α, and interferon-γ; T cell activation; increased acute phase proteins, like C-reactive protein, haptoglobin, fibrinogen and α1-antitrypsin; complement factors; increased LPS load through bacterial translocation and subsequent gut-derived inflammation; induction of indoleamine 2,3-dioxygenase with increased levels of tryptophan catabolites; decreased levels of antioxidants, like coenzyme Q10, zinc, vitamin E, glutathione and glutathione peroxidase; increased O&NS characterized by oxidative damage to low density lipoprotein (LDL) and phospholipid inositol, increased malondialdehyde, and damage to DNA and mitochondria; increased nitrosative stress; and decreased ω3 polyunsaturated fatty acids (PUFAs). The complex interplay between the abovementioned IO&NS pathways in depression results in pro-atherogenic effects and should be regarded as a risk factor to future clinical CVD and due mortality. We suggest that major depression should be added as a risk factor to the Charlson "comorbidity" index. It is advised that patients with (sub)chronic or recurrent major depression should routinely be assessed by serology tests to predict if they have an increased risk to cardiovascular disorders.

Molecular SPECT Imaging: An Overview

Molecular imaging has witnessed a tremendous change over the last decade. Growing interest and emphasis are placed on this specialized technology represented by developing new scanners, pharmaceutical drugs, diagnostic agents, new therapeutic regimens, and ultimately, significant improvement of patient health care. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) have their signature on paving the way to molecular diagnostics and personalized medicine. The former will be the topic of the current paper where the authors address the current position of the molecular SPECT imaging among other imaging techniques, describing strengths and weaknesses, differences between SPECT and PET, and focusing on different SPECT designs and detection systems. Radiopharmaceutical compounds of clinical as well-preclinical interest have also been reviewed. Moreover, the last section covers several application, of μSPECT imaging in many areas of disease detection and diagnosis.

Noninvasive imaging of the vulnerable atherosclerotic plaque

Atherosclerosis is an inflammatory disease, complicated by progressively increasing atherosclerotic plaques that eventually may rupture. Plaque rupture is a major cause of cardiovascular events, such as unstable angina, myocardial infarction, and stroke. A number of noninvasive imaging techniques have been developed to evaluate the vascular wall in an attempt to identify so-called vulnerable atherosclerotic plaques that are prone to rupture. The purpose of the present review is to systematically investigate the accuracy of noninvasive imaging techniques in the identification of plaque components and morphologic characteristics associated with plaque vulnerability, assessing their clinical and diagnostic value.

Molecular imaging in atherosclerosis

Atherosclerosis is the major cause of cardiovascular disease, which still has the leading position in morbidity and mortality in the Western world. Many risk factors and pathobiological processes are acting together in the development of atherosclerosis. This leads to different remodelling stages (positive and negative) which are both associated with plaque physiology and clinical presentation. The different remodelling stages of atherosclerosis are explained with their clinical relevance. Recent advances in basic science have established that atherosclerosis is not only a lipid storage disease, but that also inflammation has a fundamental role in all stages of the disease. The molecular events leading to atherosclerosis will be extensively reviewed and described. Further on in this review different modalities and their role in the different stages of atherosclerosis will be discussed. Non-nuclear invasive imaging techniques (intravascular ultrasound, intravascular MRI, intracoronary angioscopy and intravascular optical coherence tomography) and non-nuclear non-invasive imaging techniques (ultrasound with Doppler flow, electron-bean computed tomography, coronary computed tomography angiography, MRI and coronary artery MR angiography) will be reviewed. After that we focus on nuclear imaging techniques for detecting atherosclerotic plaques, divided into three groups: atherosclerotic lesion components, inflammation and thrombosis. This emerging area of nuclear imaging techniques can provide measures of biological activity of atherosclerotic plaques, thereby improving the prediction of clinical events. As we will see in the future perspectives, at present, there is no special tracer that can be called the diagnostic tool to diagnose prospective stroke or infarction in patients. Nevertheless, we expect such a tracer to be developed in the next few years and maybe, theoretically, it could even be used for targeted therapy (in the form of a beta-emitter) to combat cardiovascular disease.

Small-animal SPECT and SPECT/CT: application in cardiovascular research

Preclinical cardiovascular research using noninvasive radionuclide and hybrid imaging systems has been extensively developed in recent years. Single photon emission computed tomography (SPECT) is based on the molecular tracer principle and is an established tool in noninvasive imaging. SPECT uses gamma cameras and collimators to form projection data that are used to estimate (dynamic) 3-D tracer distributions in vivo. Recent developments in multipinhole collimation and advanced image reconstruction have led to sub-millimetre and sub-half-millimetre resolution SPECT in rats and mice, respectively. In this article we review applications of microSPECT in cardiovascular research in which information about the function and pathology of the myocardium, vessels and neurons is obtained. We give examples on how diagnostic tracers, new therapeutic interventions, pre- and postcardiovascular event prognosis, and functional and pathophysiological heart conditions can be explored by microSPECT, using small-animal models of cardiovascular disease.

Advanced carotid plaque imaging

Treatment of carotid artery stenosis by endarterectomy or stenting can significantly reduce stroke risk. In clinical practice, indication for surgery or stenting is primarily based on the degree of stenosis, but there is growing awareness that pathophysiological features within a vulnerable plaque play a key role in predicting stroke risk. Important molecular processes associated with plaque vulnerability are inflammation, lipid accumulation, proteolysis, apoptosis, angiogenesis and thrombosis. The rapidly emerging field of molecular and functional imaging strategies allows identification of pathophysiological processes in carotid artery stenosis. We aimed to review the literature regarding the current most promising advanced imaging techniques in carotid artery disease. Various advanced imaging methods are available, such as high-resolution magnetic resonance imaging (HR-MRI), single photon emission computed tomography (SPECT), positron emission tomography (PET) and near-infrared fluorescence (NIRF). Radionuclide and fluorescent tracers that identify inflammation, apoptosis and proteolysis, such as FDG, MMP probes and Annexin A5, are promising. A combination of activity of molecular processes and detailed anatomic information can be obtained, providing a powerful tool in the identification of the vulnerable plaque. With these developments, we are entering a new era of imaging techniques in the selection of patients for carotid surgery.

Identification of Inflamed Atherosclerotic Plaque using 123I-Labeled Interleukin-2 Scintigraphy in High-Risk Peritoneal Dialysis Patients: A Pilot Study

Background

Patients with end-stage renal disease (ESRD) suffer from markedly increased cardiovascular morbidity and mortality. Common carotid artery (CCA) intima-media thickness (IMT) assessment and CCA plaque identification using ultrasound are well-recognized tools for identification and monitoring of atherosclerosis. A new method for monitoring the inflammatory status of plaque, namely radiolabeled interleukin-2 (IL-2) scintigraphy, was proposed recently. The aim of this pilot study was to perform 123I-labeled-IL-2 carotid plaque scintigraphy in ESRD patients treated with peritoneal dialysis and to correlate obtained results with ultrasound assessment of CCA and selected inflammatory markers.

Methods

CCA-IMT was measured and CCA plaques were identified by ultrasound in 10 patients (5 women, 5 men; mean age 62.4 ± 10.4 years; median peritoneal dialysis duration 32.5 months, range 12 – 55 months) with advanced cardiovascular comorbidity. Following CCA ultrasound, 123I-labeled IL-2 carotid plaque scintigraphy was performed. Several biomarkers of inflammation and atherosclerosis were also measured in all patients.

Results

Mean target/non-target ratio for focal 123I-IL-2 uptake within the plaque was 3.15 ± 0.54, and mean IMT from the site of the scintigraphy analysis was 0.975 ± 0.337 mm. Highly significant correlation was found between CCA-IMT and a target/non-target ratio for focal 123I-IL-2 uptake in a corresponding artery ( R = 0.92, p = 0.01). However, no significant correlations were found between target/non-target ratio for focal 123I-IL-2 uptake and levels of measured biomarkers.

Conclusions

Our preliminary results suggest potential for identification of an inflamed (vulnerable) plaque using IL-2 scintigraphy in ESRD patients with cardiovascular comorbidities.

Inflammation imaging in atherosclerosis

Inflammation is important at many stages of atherosclerotic plaque development. We highlight several imaging modalities that can quantify the degree of plaque inflammation noninvasively. Imaging of this type might allow testing of novel antiatherosclerosis drugs, identification of patients at risk of plaque rupture, and deeper insight into the biology of the disease. The imaging modalities are discussed in relation to their potential use in these areas.

PET characteristics of a dedicated breast PET/CT scanner prototype

A dedicated breast PET/CT system has been constructed at our institution, with the goal of having increased spatial resolution and sensitivity compared to whole-body systems. The purpose of this work is to describe the design and the performance characteristics of the PET component of this device. Average spatial resolution of a line source in warm background using maximum a posteriori (MAP) reconstruction was 2.5 mm, while the average spatial resolution of a phantom containing point sources using filtered back projection (FBP) was 3.27 mm. A sensitivity profile was computed with a point source translated across the axial field of view (FOV) and a peak sensitivity of 1.64% was measured at the center of the FOV. The average energy resolution determined on a per-crystal basis was 25%. The characteristic dead time for the front-end electronics and data acquisition (DAQ) was determined to be 145 ns and 3.6 micros, respectively. With no activity outside the FOV, a peak noise-equivalent count rate of 18.6 kcps was achieved at 318 microCi (11.766 MBq) in a cylindrical phantom of diameter 75 mm. After the effects of exposing PET detectors to x-ray flux were evaluated and ameliorated, a combined PET/CT scan was performed. The percentage standard deviations of uniformity along axial and transaxial directions were 3.7% and 2.8%, respectively. The impact of the increased reconstructed spatial resolution compared to typical whole-body PET scanners is currently being assessed in a clinical trial.

Phage display selection of peptides that home to atherosclerotic plaques: IL-4 receptor as a candidate target in atherosclerosis

Imaging or drug delivery tools for atherosclerosis based on the plaque biology are still insufficient. Here, we attempted to identify peptides that selectively home to atherosclerotic plaques using phage display. A phage library containing random peptides was ex viv screened for binding to human atheroma tissues. After three to four rounds of selection, the DNA inserts of phage clones wer sequenced. A peptide sequence, CRKRLDRNC, was the most frequently occurring one. Intravenously injected phage displaying the CRKRLDRNC peptide was observed to home to atherosclerotic aortic tissues of low-density lipoprotein receptor-deficient (Ldlr^−/–) mice at higher levels than to normal aortic tissues of wild-type mice. Moreover, a fluorescein- or radioisotope-conjugated synthetic CRKRLDRNC peptide, but not a control peptide, homed in vivo to atherosclerotic plaques in Ldlr^−/– mice, while homing of the peptide to other organs such as brain was minimal. The homing peptide co-localized with endothelial cells, macrophages and smooth muscle cells a mouse and human atherosclerotic plaques. Homology search revealed that the CRKRLDRNC peptide shares a motif of interleukin-receptor (IL-4) that is critical for binding to its receptor. The peptide indeed co-localized with IL-4 receptor (IL-4R) at atherosclerotic plaques. Moreover, the peptide bound to cultured cells expressing IL-4R on the cell surface and the binding was inhibited by the knock-down of IL-4R. These results show that the CRKRLDRNC peptide homes to atherosclerotic plaques through binding to IL-4R as its target and may be a useful tool for selective drug delivery and molecular imaging of atherosclerosis.

Application of PET/SPECT imaging in vascular disease

BACKGROUND

Nuclear medicine imaging differs from other imaging modalities by showing physiological processes instead of anatomical details.

OBJECTIVE

To describe the current applications of positron emission tomography (PET) and single photon emission computed tomography (SPECT) as a diagnostic tool for vascular disease as relevant to vascular surgeons.

METHODS

A literature search identified articles focussing on vascular disease and PET or SPECT using the Pubmed database. Manual cross referencing was also performed.

RESULTS

PET and SPECT may be used to assess plaque vulnerability, biology of aneurysm progression, prosthetic graft infection, and vasculitis. The ability to combine computerized tomography scanning or magnetic resonance imaging with PET or SPECT adds detailed anatomical information and enhances the potential of nuclear medicine imaging in the investigation of vascular disease.

DISCUSSION

Considerable further information will be needed to define whether and where PET or SPECT will fit in a clinical strategy. The necessary validation studies represent an exciting challenge for the future but also may require the development of interdisciplinary imaging groups to integrate expertise and optimize nuclear diagnostic potential.

Atherosclerosis imaging on the molecular level

On the basis of clinical observations that acute coronary events often result from rupture of atherosclerotic plaques at sites with no or minor luminal narrowing, the search for techniques by which to identify vulnerable, rupture-prone lesions has developed into a quest for the holy grail of cardiovascular medicine. Vulnerable plaques may show characteristic morphologic features, but they may still differ in their biology and their activity, which ultimately leads to rupture. As a consequence, considerable efforts have been undertaken to identify biologic mechanisms of atherosclerotic lesions by use of molecular-targeted radiolabeled probes. A variety of approaches aiming at plaque inflammation, apoptosis, smooth muscle cell proliferation, extracellular matrix activation, or platelet binding have been introduced. Nevertheless, molecular imaging of atherosclerosis is still a work in progress. Challenges related to the best targeting approach, to translation of animal model results to the clinical setting, to adequate imaging methodology for visualization of coronary artery biology, and to a suitable target patient population will need to be overcome. But the field is steadily moving ahead and getting closer to the ultimate goal of an improved clinical risk assessment through in vivo assessment of vascular biology.