Imaging Atherosclerosis

High-resolution and accelerated multi-parametric mapping with automated characterization of vessel disease using intravascular MRI

BACKGROUND

Atherosclerosis is prevalent in cardiovascular disease, but present imaging modalities have limited capabilities for characterizing lesion stage, progression and response to intervention. This study tests whether intravascular magnetic resonance imaging (IVMRI) measures of relaxation times (T1, T2) and proton density (PD) in a clinical 3 Tesla scanner could characterize vessel disease, and evaluates a practical strategy for accelerated quantification.

METHODS

IVMRI was performed in fresh human artery segments and swine vessels in vivo, using fast multi-parametric sequences, 1-2 mm diameter loopless antennae and 200-300 μm resolution. T1, T2 and PD data were used to train a machine learning classifier (support vector machine, SVM) to automatically classify normal vessel, and early or advanced disease, using histology for validation. Disease identification using the SVM was tested with receiver operating characteristic curves. To expedite acquisition of T1, T2 and PD data for vessel characterization, the linear algebraic method ('SLAM') was modified to accommodate the antenna's highly-nonuniform sensitivity, and used to provide average T1, T2 and PD measurements from compartments of normal and pathological tissue segmented from high-resolution images at acceleration factors of R ≤ 18-fold. The results were validated using compartment-average measures derived from the high-resolution scans.

RESULTS

The SVM accurately classified ~80% of samples into the three disease classes. The 'area-under-the-curve' was 0.96 for detecting disease in 248 samples, with T1 providing the best discrimination. SLAM T1, T2 and PD measures for R ≤ 10 were indistinguishable from the true means of segmented tissue compartments.

CONCLUSION

High-resolution IVMRI measures of T1, T2 and PD with a trained SVM can automatically classify normal, early and advanced atherosclerosis with high sensitivity and specificity. Replacing relaxometric MRI with SLAM yields good estimates of T1, T2 and PD an order-of-magnitude faster to facilitate IVMRI-based characterization of vessel disease.

Synthesis of Monocyte-targeting Peptide Amphiphile Micelles for Imaging of Atherosclerosis

Atherosclerosis is a major contributor to cardiovascular disease, the leading cause of death worldwide, which claims 17.3 million lives annually. Atherosclerosis is also the leading cause of sudden death and myocardial infarction, instigated by unstable plaques that rupture and occlude the blood vessel without warning. Current imaging modalities cannot differentiate between stable and unstable plaques that rupture. Peptide amphiphiles micelles (PAMs) can overcome this drawback as they can be modified with a variety of targeting moieties that bind specifically to diseased tissue. Monocytes have been shown to be early markers of atherosclerosis, while large accumulation of monocytes is associated with plaques prone to rupture. Hence, nanoparticles that can target monocytes can be used to discriminate different stages of atherosclerosis. To that end, here, we describe a protocol for the preparation of monocyte-targeting PAMs (monocyte chemoattractant protein-1 (MCP-1) PAMs). MCP-1 PAMs are self-assembled through synthesis under mild conditions to form nanoparticles of 15 nm in diameter with near neutral surface charge. In vitro, PAMs were found to be biocompatible and had a high binding affinity for monocytes. The methods described herein show promise for a wide range of applications in atherosclerosis as well as other inflammatory diseases.

Comparison of the Diameters of the Major Epicardial Coronary Arteries by Angiogram in Asian-Indians Versus European Americans <40 Years of Age Undergoing Percutaneous Coronary Artery Intervention

Coronary artery disease (CAD) rates are higher in Asian-Indians than in Caucasians. CAD occurs at an earlier age in this group, with about a quarter of all myocardial infarctions occurring under the age of 40. Previous reports have suggested smaller coronary artery size in Asian-Indians as a major cause for increased CAD in this population. This study sought to evaluate the size of normal "atheroma-free" segments of the epicardial coronary arteries in Asian-Indians and Caucasians aged ≤40 years undergoing coronary artery intervention in other diseased segments. A total of 69 consecutive patients (41 whites, 28 Asian-Indians) aged ≤40 years were evaluated. Angiograms were analyzed using standard quality control analysis software with digital acquisition. The arteries measured were the left main, left anterior descending, left circumflex, and the right coronary artery. Conventional risk factors, including hypertension, smoking, and diabetes, that could influence coronary size were also assessed. The coronary arteries of Asian-Indian patients showed significantly smaller values in the mean diameters of the left main (2.96 mm vs 4.04 mm, p = 0.0004), left anterior descending (2.48 mm vs 3.24 mm, p = 0.0005), left circumflex (2.52 mm vs 3.06 mm, p = 0.00002), and right coronary artery (2.71 mm vs 3.65 mm, p = 0.0008) as compared with Caucasians. Even after correction for body surface area, a statistically significant difference remained in coronary artery diameters. In conclusion, statistically significant difference in the mean diameter size even after correction for body surface area in Asian-Indians has implications for predisposition to atherosclerosis, and more challenging performance of procedures such as coronary artery bypass grafting, stent implantation, or atherectomy.

Iron oxide core oil-in-water nanoemulsion as tracer for atherosclerosis MPI and MRI imaging

PURPOSE

For early atherosclerosis imaging, magnetic oil-in-water nanoemulsion (NE) decorated with atheroma specific monoclonal antibody was designed for Magnetic Particle Imaging (MPI) and Magnetic Resonance Imaging (MRI). MPI is an emerging technique based on direct mapping of superparamagnetic nanoparticles which may advantageously complement MRI.

METHODS

NE oily droplets were loaded with superparamagnetic iron oxide nanoparticles of 7, 11 and 18nm and biofunctionalized with atheroma specific scFv-Fc TEG4-2C antibody.

RESULTS

Inclusion of nanoparticles inside NE did not change the hydrodynamic diameter of the oil droplets, close to 180nm, nor the polydispersity. The droplets were negatively charged (ζ=-30mV). In vitro MPI signal was assessed by Magnetic Particle Spectroscopy (MPS). NE displayed MRI and MPS signals confirming its potential as new contrast agent. NE MPS signal increase with NPs size close to the gold standard (Resovist). In MRI, NE displayed R₂* transversal relaxivity of 45.45, 96.04 and 218.81mM^-1s^-1 for 7, 11 and 18nm respectively. NE selectively bind atheroma plaque both in vitro and ex vivo in animal models of atherosclerosis.

CONCLUSION

Magnetic NE showed reasonable MRI/MPS signals and a significant labelling of the atheroma plaque. These preliminary results support that NE platform could selectively image atherosclerosis.

Spectral analysis assisted photoacoustic imaging for lipid composition differentiation

Recent advances in atherosclerotic plaque detection have shown that not only does lipid core size and depth play important roles in plaque rupture and thrombi formation, but lipid composition, especially cholesterol deposition, is equally important in determining lesion vulnerability. Here, we demonstrate a spectral analysis assisted photoacoustic imaging approach to differentiate and map lipid compositions within an artery wall. The approach is based on the classification of spectral curves obtained from the sliding windows along time-of-flight photoacoustic signals via a numerical k-means clustering method. The evaluation result on a vessel-mimicking phantom containing cholesterol and olive oil shows accuracy and efficiency of this method, suggesting the potential to apply this approach in assessment of atherosclerotic plaques.

The Role of Coronary Computed Tomography Angiography and Cardiac Magnetic Resonance in STEMI Patients with Normal Coronary Angiography

Usually, the diagnosis of myocardial infarction based on patient symptoms, electrocardiogram (ECG) changes, and cardiac enzymes, is not a challenge for cardiologists. The correlation between coronary anatomy and the ECG territories that present ischemic changes can help the clinician to estimate which coronary artery presents lesions upon performing a coronary angiogram. In certain situations, the diagnosis of myocardial infarction can be difficult due to the lack of correlations between the clinical and paraclinical examinations and the coronary angiogram. In some cases, patients with chest pain and ST-segment elevation on the ECG tracing present with a normal coronary angiography. In other cases, patients without important changes on the ECG can present critical lesions or even occlusions upon angiographic examination. The aim of this article is to highlight the role of noninvasive coronary magnetic resonance and multi-slice computed tomography in patients with ST-segment elevation myocardial infarction and normal coronary angiography.

In vivo label-free structural and biochemical imaging of coronary arteries using an integrated ultrasound and multispectral fluorescence lifetime catheter system

Existing clinical intravascular imaging modalities are not capable of accurate detection of critical plaque pathophysiology in the coronary arteries. This study reports the first intravascular catheter combining intravascular ultrasound (IVUS) with multispectral fluorescence lifetime imaging (FLIm) that enables label-free simultaneous assessment of morphological and biochemical features of coronary vessels in vivo. A 3.7 Fr catheter with a fiber-optic channel was constructed based on a 40 MHz clinical IVUS catheter. The ability to safely acquire co-registered FLIm-IVUS data in vivo using Dextran40 solution flushing was demonstrated in swine coronary arteries. FLIm parameters from the arterial wall were consistent with the emission of fluorophores present in healthy arterial wall (collagen, elastin). Additionally, structural and biochemical features from atherosclerotic lesions were acquired in ex vivo human coronary samples and corroborated with histological findings. Current results show that FLIm parameters linked to the amount of structural proteins (e.g. collagen, elastin) and lipids (e.g. foam cells, extracellular lipids) in the first 200 μm of the intima provide important biochemical information that can supplement IVUS data for a comprehensive assessment of plaques pathophysiology. The unique FLIm-IVUS system evaluated here has the potential to provide a comprehensive insight into atherosclerotic lesion formation, diagnostics and response to therapy.

Methotraxate-Loaded Hybrid Nanoconstructs Target Vascular Lesions and Inhibit Atherosclerosis Progression in ApoE-/- Mice

Atherosclerosis is an inflammatory disorder characterized by the progressive thickening of blood vessel walls eventually resulting in acute vascular syndromes. Here, intravenously injectable hybrid nanoconstructs are synthesized for tempering immune cell inflammation locally and systemically. Lipid and polymer chains are nanoprecipitated to form 100 nm spherical polymeric nanoconstructs (SPNs), loaded with methotrexate (MTX) and subsequently labeled with Cu⁶⁴ and fluorescent probes for combined nuclear/optical imaging. Upon engulfment into macrophages, MTX SPNs intracellularly release their anti-inflammatory cargo significantly lowering the production of proinflammatory cytokine (interleukin 6 and tumor necrosis factor α) already at 0.06 mg mL^-1 of MTX. In ApoE^-/- mice, fed with high-fat diet up to 17 weeks, nuclear and optical imaging demonstrates specific accumulation of SPNs within lipid-rich plaques along the arterial tree. Histological analyses confirm SPN uptake into macrophages residing within atherosclerotic plaques. A 4-week treatment with biweekly administration of MTX SPNs is sufficient to reduce the plaque burden in ApoE^-/- mice by 50%, kept on high-fat diet for 10 weeks. Systemic delivery of MTX to macrophages via multifunctional, hybrid nanoconstructs constitutes an effective strategy to inhibit atherosclerosis progression and induce, potentially, the resorption of vascular lesions.

Hyaluronan Nanoparticles Selectively Target Plaque-Associated Macrophages and Improve Plaque Stability in Atherosclerosis

Hyaluronan is a biologically active polymer, which can be formulated into nanoparticles. In our study, we aimed to probe atherosclerosis-associated inflammation by using hyaluronan nanoparticles and to determine whether they can ameliorate atherosclerosis. Hyaluronan nanoparticles (HA-NPs) were prepared by reacting amine-functionalized oligomeric hyaluronan (HA) with cholanic ester and labeled with a fluorescent or radioactive label. HA-NPs were characterized in vitro by several advanced microscopy methods. The targeting properties and biodistribution of HA-NPs were studied in apoe^-/- mice, which received either fluorescent or radiolabeled HA-NPs and were examined ex vivo by flow cytometry or nuclear techniques. Furthermore, three atherosclerotic rabbits received ⁸⁹Zr-HA-NPs and were imaged by PET/MRI. The therapeutic effects of HA-NPs were studied in apoe^-/- mice, which received weekly doses of 50 mg/kg HA-NPs during a 12-week high-fat diet feeding period. Hydrated HA-NPs were ca. 90 nm in diameter and displayed very stable morphology under hydrolysis conditions. Flow cytometry revealed a 6- to 40-fold higher uptake of Cy7-HA-NPs by aortic macrophages compared to normal tissue macrophages. Interestingly, both local and systemic HA-NP-immune cell interactions significantly decreased over the disease progression. ⁸⁹Zr-HA-NPs-induced radioactivity in atherosclerotic aortas was 30% higher than in wild-type controls. PET imaging of rabbits revealed 6-fold higher standardized uptake values compared to the muscle. The plaques of HA-NP-treated mice contained 30% fewer macrophages compared to control and free HA-treated group. In conclusion, we show favorable targeting properties of HA-NPs, which can be exploited for PET imaging of atherosclerosis-associated inflammation. Furthermore, we demonstrate the anti-inflammatory effects of HA-NPs in atherosclerosis.

Potential of α7 nicotinic acetylcholine receptor PET imaging in atherosclerosis

Atherosclerotic events are usually acute and often strike otherwise asymptomatic patients. Although multiple clinical risk factors have been associated with atherosclerosis, as of yet no further individual prediction can be made as to who will suffer from its consequences based on biomarker analysis or traditional imaging methods like CT, MRI or angiography. Previously, non-invasive imaging with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET was shown to potentially fill this niche as it offers high sensitive detection of metabolic processes associated with inflammatory changes in atherosclerotic plaques. However, ¹⁸F-FDG PET imaging of arterial vessels suffers from non-specificity and has still to be proven to reliably identify vulnerable plaques, carrying a high risk of rupture. Therefore, it may be regarded only as a secondary marker for monitoring treatment effects and it does not offer alternative treatment options or direct insight in treatment mechanisms. In this review, an overview is given of the current status and the potential of PET imaging of inflammation and angiogenesis in atherosclerosis in general and special emphasis is given to imaging of α7 nicotinic acetylcholine receptors (α7 nAChRs). Due to the gaps that still exist in our understanding of atherogenesis and the limitations of the available PET tracers, the search continues for a more specific radioligand, able to differentiate between stable atherosclerosis and plaques prone to rupture. The potential role of the α7 nAChR as imaging marker for plaque vulnerability is explored. Today, strong evidence exists that nAChRs are involved in the atherosclerotic disease process. They are suggested to mediate the deleterious effects of the major tobacco component, nicotine, a nAChR agonist. Mainly based on in vitro data, α7 nAChR stimulation might increase plaque burden via increased neovascularization. However, in animal studies, α7 nAChR manipulation appears to reduce plaque size due to its inhibitory effects on inflammatory cells. Thus, reliable identification of α7 nAChRs by in vivo imaging is crucial to investigate the exact role of α7 nAChR in atherosclerosis before any therapeutic approach in the human setting can be justified. In this review, we discuss the first experience with α7 nAChR PET tracers and developmental considerations regarding the "optimal" PET tracer to image vascular nAChRs.

PET/MR: Yet another Tesla?

After the successful introduction of PET/CT as a multimodality imaging technique, PET/MR has subsequently emerged as an attractive instrumentation for applications in neurology, oncology, and cardiology. Simultaneous data acquisition combining structural, functional, and molecular imaging provides a unique platform to link various aspects of cardiac performance for the non-invasive characterization of cardiovascular disease phenotypes. Specifically, tissue characterization by MR techniques with and without contrast agents allows for functional parameters such as LGE, myocardial perfusion, and T1 maps as well as an estimate of extracellular volume. PET tracers excel by their high sensitivity and specificity, thus supplementing the functional tissue characterization by MRI. Although the clinical applications are yet to be validated , the first experience with PET/MR suggests future applications in the area of vascular imaging (unstable plaque) as well as in the characterization of inflammatory processes involving the heart. Ischemic heart disease can be comprehensively assessed by integrating regional function, perfusion, and viability. Future technical improvements leading to less costly PET/MR instrumentation are necessary to support routine clinical application of this promising technique in cardiology.

Multimodal laser-based angioscopy for structural, chemical and biological imaging of atherosclerosis

The complex nature of atherosclerosis demands high-resolution approaches to identify subtle thrombogenic lesions and define the risk of plaque rupture. Here, we report the proof-of-concept use of a multimodal scanning fiber endoscope (SFE) consisting of a single optical fiber scanned by a piezoelectric drive that illuminates tissue with red, blue, and green laser beams, and digitally reconstructs images at 30 Hz with high resolution and large fields-of-view. By combining laser-induced reflectance and fluorescence emission of intrinsic fluorescent constituents in arterial tissues, the SFE allowed us to co-generate endoscopic videos with a label-free biochemical map to derive a morphological and spectral classifier capable of discriminating early, intermediate, advanced, and complicated atherosclerotic plaques. We demonstrate the capability of scanning fiber angioscopy for the molecular imaging of vulnerable atherosclerosis by targeting proteolytic activity with a fluorescent probe activated by matrix metalloproteinases. We also show that the SFE generates high-quality spectral images in vivo in an animal model with medium-sized arteries. Multimodal laser-based angioscopy could become a platform for the diagnosis, prognosis, and image-guided therapy of atherosclerosis.

Cardiac Imaging in the Diagnosis of Coronary Artery Disease

Coronary artery disease (CAD) continues to be a leading cause of morbidity and mortality worldwide. Although invasive coronary angiography has previously been the gold standard in establishing the diagnosis of CAD, there is a growing shift to more appropriately use the cardiac catheterization laboratory to perform interventional procedures once a diagnosis of CAD has been established by noninvasive imaging modalities rather than using it primarily as a diagnostic facility to confirm or refute CAD. With ongoing technological advancements, noninvasive imaging plays a pre-eminent role in not only diagnosing CAD but also informing the choice of appropriate therapies, establishing prognosis, all while containing costs and providing value-based care. Multiple imaging modalities are available to evaluate patients suspected of having coronary ischemia, such as stress electrocardiography, stress echocardiography, single-photon emission computed tomography myocardial perfusion imaging, positron emission tomography, coronary computed tomography (CT) angiography, and magnetic resonance imaging. These imaging modalities can variably provide functional and anatomical delineation of coronary stenoses and help guide appropriate therapy. This review will discuss their advantages and limitations and their usage in the diagnostic pathway for patients with CAD. We also discuss newer technologies such as CT fractional flow reserve, CT angiography with perfusion, whole-heart coronary magnetic resonance angiography with perfusion, which can provide both anatomical as well as functional information in the same test, thus obviating the need for multiple diagnostic tests to obtain a comprehensive assessment of both, plaque burden and downstream ischemia. Recognizing that clinicians have a multitude of tests to choose from, we provide an underpinning of the principles of ischemia detection by these various modalities, focusing on anatomy vs physiology, the database justifying their use, their prognostic capabilities and lastly, their appropriate and judicious use in this era of patient-centered, cost-effective imaging.

Vascular targeting of nanoparticles for molecular imaging of diseased endothelium

This review seeks to highlight the enormous potential of targeted nanoparticles for molecular imaging applications. Being the closest point-of-contact, circulating nanoparticles can gain direct access to targetable molecular markers of disease that appear on the endothelium. Further, nanoparticles are ideally suitable to vascular targeting due to geometrically enhanced multivalent attachment on the vascular target. This natural synergy between nanoparticles, vascular targeting and molecular imaging can provide new avenues for diagnosis and prognosis of disease with quantitative precision. In addition to the obvious applications of targeting molecular signatures of vascular diseases (e.g., atherosclerosis), deep-tissue diseases often manifest themselves by continuously altering and remodeling their neighboring blood vessels (e.g., cancer). Thus, the remodeled endothelium provides a wide range of targets for nanoparticles and molecular imaging. To demonstrate the potential of molecular imaging, we present a variety of nanoparticles designed for molecular imaging of cancer or atherosclerosis using different imaging modalities.

Cysteine proteases in atherosclerosis

Atherosclerosis predisposes patients to cardiovascular diseases, such as myocardial infarction and stroke. Instigation of vascular injury is triggered by retention of lipids and inflammatory cells in the vascular endothelium. Whereas these vascular lesions develop in young adults and are mostly considered harmless, over time persistent inflammatory and remodeling processes will ultimately damage the arterial wall and cause a thrombotic event due to exposure of tissue factors into the lumen. Evidence from human tissues and preclinical animal models has clearly established the role of cathepsin cysteine proteases in the development and progression of vascular lesions. Hence, understanding the function of cathepsins in atherosclerosis is important for developing novel therapeutic strategies and advanced point of care diagnostics. In this review we will describe the roles of cysteine cathepsins in different cellular process that become dysfunctional in atherosclerosis, such as lipid metabolism, inflammation and apoptosis, and how they contribute to arterial remodeling and atherogenesis. Finally, we will explore new horizons in protease molecular imaging, which may potentially become a surrogate marker to identify future cardiovascular events.

The use of echocardiography for the non-invasive evaluation of coronary artery disease

In the Western world, there are now millions of patients who undergo clinical procedures that evaluate coronary artery status each year. Methods span from direct imaging using angiography, computerized tomography, to nuclear magnetic imaging as well as to functional studies, such as positron emission tomography. These techniques have provided significant information to physicians, but there is still need for an improved accessibility. Angiographic methods are expensive and expose the patient to significant amounts of radiation, undesirable in younger patients. Among the novel technologies for coronary diagnostics, transthoracic echocardiography (TTE) of coronary arteries has provided an important alternative, particularly in everyday practice. Diagnostic arterial TTE can allow determination of the coronary wall lumen in at least three major coronary segments (left main [LM], left arterial descending [LAD] and right coronary artery [RCA]). Coronary wall thickness using the LAD has been preliminarily shown to be related to the risk of coronary events. Since it is well ascertained that coronary lesions found in any location indicate that at least 80% of the coronary tree is affected, this is very important clinical information. Evaluation of coronary status by TTE is a novel technology providing important information in ischemic syndromes, in cases of coronary malformations and other coronary diseases. KEY MESSAGES Coronary evaluation can be carried out by a variety of both invasive and non-invasive methods, many requiring radiation exposure or patient immobility. Transthoracic echocardiography (TTE) of the coronaries can, in particular, evaluate the coronary wall thickness, and this may be directly related to the coronary disease risk. TTE is a useful method for the monitoring of coronary flow reserve and can allow the detection of coronary malformations.

Molecular Imaging of Vulnerable Atherosclerotic Plaques in Vivo with Osteopontin-Specific Upconversion Nanoprobes

Owing to the high mortality rate of cardiovascular diseases, developing novel noninvasive diagnostic methods becomes urgent and mandatory. It is well-known that the rupture of vulnerable plaques directly leads to deadly consequences. However, differentiating vulnerable plaques from stable plaques remains challenging in the clinic. In the current study, osteopontin (OPN), a secreted biomarker associated with macrophages and foamy macrophages, was selected as a target for identifying the vulnerable plaques. A dual modality imaging probe was constructed by covalently attaching an OPN antibody to NaGdF₄:Yb,Er@NaGdF₄ upconversion nanoparticles. Upon intravenous injection of the resulting probes, upconversion optical imaging was performed to visualize the plaques induced by altering the shear stress in carotid arteries of a mouse model. The imaging studies revealed that the signals of vulnerable and stable plagues induced by lowered shear stress and oscillatory shear stress, respectively, presented significantly different signal intensities, implying that the current probe and imaging strategy are potentially useful for a precise diagnosis of atherosclerosis plaques.

Nanomedicines for dysfunctional macrophage-associated diseases

Macrophages play vital functions in host inflammatory reaction, tissue repair, homeostasis and immunity. Dysfunctional macrophages have significant pathophysiological impacts on diseases such as cancer, inflammatory diseases (rheumatoid arthritis and inflammatory bowel disease), metabolic diseases (atherosclerosis, diabetes and obesity) and major infections like human immunodeficiency virus infection. In view of this common etiology in these diseases, targeting the recruitment, activation and regulation of dysfunctional macrophages represents a promising therapeutic strategy. With the advancement of nanotechnology, development of nanomedicines to efficiently target dysfunctional macrophages can strengthen the effectiveness of therapeutics and improve clinical outcomes. This review discusses the specific roles of dysfunctional macrophages in various diseases and summarizes the latest advances in nanomedicine-based therapeutics and theranostics for treating diseases associated with dysfunctional macrophages.

Multimodal laser-based angioscopy for structural, chemical and biological imaging of atherosclerosis

The complex nature of atherosclerosis demands high-resolution approaches to identify subtle thrombogenic lesions and define the risk of plaque rupture. Here, we report the proof-of-concept use of a multimodal scanning fiber endoscope (SFE) consisting of a single optical fiber scanned by a piezoelectric drive that illuminates tissue with red, blue, and green laser beams, and digitally reconstructs images at 30 Hz with high resolution and large fields-of-view. By combining laser-induced reflectance and fluorescence emission of intrinsic fluorescent constituents in arterial tissues, the SFE allowed us to co-generate endoscopic videos with a label-free biochemical map to derive a morphological and spectral classifier capable of discriminating early, intermediate, advanced, and complicated atherosclerotic plaques. We demonstrate the capability of scanning fiber angioscopy for the molecular imaging of vulnerable atherosclerosis by targeting proteolytic activity with a fluorescent probe activated by matrix metalloproteinases. We also show that the SFE generates high-quality spectral images in vivo in an animal model with medium-sized arteries. Multimodal laser-based angioscopy could become a platform for the diagnosis, prognosis, and image-guided therapy of atherosclerosis.

High wall shear stress and high-risk plaque: an emerging concept

In recent years, there has been a significant effort to identify high-risk plaques in vivo prior to acute events. While number of imaging modalities have been developed to identify morphologic characteristics of high-risk plaques, prospective natural-history observational studies suggest that vulnerability is not solely dependent on plaque morphology and likely involves additional contributing mechanisms. High wall shear stress (WSS) has recently been proposed as one possible causative factor, promoting the development of high-risk plaques. High WSS has been shown to induce specific changes in endothelial cell behavior, exacerbating inflammation and stimulating progression of the atherosclerotic lipid core. In line with experimental and autopsy studies, several human studies have shown associations between high WSS and known morphological features of high-risk plaques. However, despite increasing evidence, there is still no longitudinal data linking high WSS to clinical events. As the interplay between atherosclerotic plaque, artery, and WSS is highly dynamic, large natural history studies of atherosclerosis that include WSS measurements are now warranted. This review will summarize the available clinical evidence on high WSS as a possible etiological mechanism underlying high-risk plaque development.

Spectrum of Abdominal Aortic Disease in a Tertiary Health Care Setup: MDCT Based Observational Study

INTRODUCTION

Abdominal aortic disease is an important cause of clinical disability that requires early detection by imaging methods for prompt and effective management. Understanding regional disease pattern and prevalence has a bearing on healthcare management and resource planning. Non-invasive, conclusive imaging strategy plays an important role in the detection of disease. Multi-Detector Computed Tomography (MDCT) with its technological developments provides affordable, accurate and comprehensive imaging solution.

AIM

To evaluate regional demography of abdominal aortic disease spectrum detected using MDCT imaging data in a tertiary hospital.

MATERIALS AND METHODS

A descriptive study was conducted based on MDCT imaging data of patients who were investigated with clinical diagnosis of abdominal aortic disease, from March 2008-2010, over a period of 24 months. Patients were examined with the contrast-enhanced MDCT examination. Morphological diagnosis of the aortic disease was based on changes in relative aortic caliber, luminal irregularity, presence of wall calcification, dissection or thrombus and evidence of major branch occlusion. Patients were categorized into four groups based on imaging findings. MDCT information and associated clinical parameters were examined and correlated to management of patient. Descriptive statistical data, namely mean, standard deviation and frequency of disease were evaluated.

RESULTS

A total of 90 out of 210 patients (43%) were detected with the abdominal aortic abnormality defined by imaging criteria. Group I, comprising of patients with atherosclerosis -including those with complications, constituted 65.5% of the patients. Group II represented patients with aneurysms (45.5%). Group III, consisting of 32.2% of the patients, contained those with dissections. The rest of the patients, including patients with aorto-arteritis, were classified as group IV. Eight patients with aneurysm and one patient with aorto-arteritis were considered for surgical treatment. Ten patients with dissection underwent endovascular procedure. Rest of the patients was managed conservatively.

CONCLUSION

Aortic disease was observed in 43% of investigated patients. Atherosclerosis with and without aortic aneurysm constituted the largest group. MDCT provided comprehensive information about the lesion and associated complications. In view of the wider availability and desired imaging qualities, MDCT provided optimal information for diagnosis and management of aortic pathology. Majority of our patients (90%) were treated conservatively.

[18F]FDG Uptake in the Aortic Wall Smooth Muscle of Atherosclerotic Plaques in the Simian Atherosclerosis Model

Atherosclerosis is a self-sustaining inflammatory fibroproliferative disease that progresses in discrete stages and involves a number of cell types and effector molecules. Recently, [¹⁸F]fluoro-2-deoxy-D-glucose- ([¹⁸F]FDG-) positron emission tomography (PET) has been suggested as a tool to evaluate atherosclerotic plaques by detecting accumulated macrophages associated with inflammation progress. However, at the cellular level, it remains unknown whether only macrophages exhibit high uptake of [¹⁸F]FDG. To identify the cellular origin of [¹⁸F]FDG uptake in atherosclerotic plaques, we developed a simian atherosclerosis model and performed PET and ex vivo macro- and micro-autoradiography (ARG). Increased [¹⁸F]FDG uptake in the aortic wall was observed in high-cholesterol diet-treated monkeys and WHHL rabbits. Macro-ARG of [¹⁸F]FDG in aortic sections showed that [¹⁸F]FDG was accumulated in the media and intima in the simian model as similar to that in WHHL rabbits. Combined analysis of micro-ARG with immunohistochemistry in the simian atherosclerosis model revealed that most cellular [¹⁸F]FDG uptake observed in the media was derived not only from the infiltrated macrophages in atherosclerotic plaques but also from the smooth muscle cells (SMCs) of the aortic wall in atherosclerotic lesions.

Cap inflammation leads to higher plaque cap strain and lower cap stress: An MRI-PET/CT-based FSI modeling approach

Plaque rupture may be triggered by extreme stress/strain conditions. Inflammation is also implicated and can be imaged using novel imaging techniques. The impact of cap inflammation on plaque stress/strain and flow shear stress were investigated. A patient-specific MRI-PET/CT-based modeling approach was used to develop 3D fluid-structure interaction models and investigate the impact of inflammation on plaque stress/strain conditions for better plaque assessment. 18FDG-PET/CT and MRI data were acquired from 4 male patients (average age: 66) to assess plaque characteristics and inflammation. Material stiffness for the fibrous cap was adjusted lower to reflect cap weakening causing by inflammation. Setting stiffness ratio (SR) to be 1.0 (fibrous tissue) for baseline, results for SR=0.5, 0.25, and 0.1 were obtained. Thin cap and hypertension were also considered. Combining results from the 4 patients, mean cap stress from 729 cap nodes was lowered by 25.2% as SR went from 1.0 to 0.1. Mean cap strain value for SR=0.1 was 0.313, 114% higher than that from SR=1.0 model. The thin cap SR=0.1 model had 40% mean cap stress decrease and 81% cap strain increase compared with SR=1.0 model. The hypertension SR=0.1 model had 19.5% cap stress decrease and 98.6% cap strain increase compared with SR=1.0 model. Differences of flow shear stress with 4 different SR values were limited (<10%). Cap inflammation may lead to large cap strain conditions when combined with thin cap and hypertension. Inflammation also led to lower cap stress. This shows the influence of inflammation on stress/strain calculations which are closely related to plaque assessment.

Imaging the Cytokine Receptor CXCR4 in Atherosclerotic Plaques with the Radiotracer 68Ga-Pentixafor for PET

⁶⁸Ga-pentixafor is a radiotracer for PET that binds with nanomolar affinity to CXCR4. The CXCR4 receptor is expressed at the surface of inflammatory cells. The objective of the study was to analyze the ability of radiolabeled pentixafor to detect CXCR4 expression on inflammatory cells present in atherosclerotic plaques of an experimental rabbit model. Methods: Atherosclerotic plaques were induced by endothelial abrasion of the right carotid artery and abdominal aorta of 7 rabbits fed an atherogenic diet. Five noninjured rabbits fed a chow diet were used as controls. Rabbits were imaged on a PET/MR system after injection of ⁶⁸Ga-pentixafor (15 MBq/kg). Vascular signal was quantified as tissue-to-background ratio (TBR). Biodistribution and autoradiographic studies were performed 1 h after injection of ¹²⁵I-pentixafor (7.5 MBq/kg). In addition, blocking studies were performed in 2 atherosclerotic rabbits with preinjection of the CXCR4 inhibitor AMD3100. Tracer uptake was quantified on arterial cryosections using autoradiography and compared with CXCR4 and RAM-11 (macrophage) expression on adjacent histologic sections. Results: One hour after injection of ⁶⁸Ga-pentixafor, strong signals were detected in vivo with PET/MR imaging in atherosclerotic plaques of the abdominal aorta and right carotid artery as compared with normal control arteries (mean TBR = 1.95 ± 0.51 vs. 1.22 ± 0.25 and mean TBR = 1.24 ± 0.38 vs. 0.96 ± 0.37, respectively; P < 0.05 for both). Blocking studies with preinjection of a CXCR4 inhibitor reduced ¹²⁵I-pentixafor uptake in atherosclerotic plaques by approximately 40%. ¹²⁵I-pentixafor uptake in the vessel wall on autoradiographies was located in macrophage-rich regions of atherosclerotic plaques and correlated with the intensity of CXCR4 expression on corresponding cryosections (r² = 0.61; P < 0.05). Conclusion:⁶⁸Ga-pentixafor allows for the noninvasive detection of CXCR4 expression in the vessel wall with PET and emerges as a potential alternative to ¹⁸F-FDG for the assessment of macrophage infiltration in atherosclerotic plaques.

Plaque angiogenesis and its relation to inflammation and atherosclerotic plaque destabilization

PURPOSE OF REVIEW

The review discusses the recent literature on plaque angiogenesis and its relation to inflammation and plaque destabilization. Furthermore, it discusses how plaque angiogenesis can be used to monitor atherosclerosis and serve as a therapeutic target.

RECENT FINDINGS

Histopathologic studies have shown a clear relationship between plaque angiogenesis, intraplaque hemorrhage (IPH), plaque vulnerability, and cardiovascular events. Hypoxia is a main driver of plaque angiogenesis and the mechanism behind angiogenesis is only partly known. IPH, as the result of immature neovessels, is associated with increased influx of inflammatory cells in the plaques. Experimental models displaying certain features of human atherosclerosis such as plaque angiogenesis or IPH are developed and can contribute to unraveling the mechanism behind plaque vulnerability. New imaging techniques are established, with which plaque angiogenesis and vulnerability can be detected. Furthermore, antiangiogenic therapies in atherosclerosis gain much attention.

SUMMARY

Plaque angiogenesis, IPH, and inflammation contribute to plaque vulnerability. Histopathologic and imaging studies together with specific experimental studies have provided insights in plaque angiogenesis and plaque vulnerability. However, more extensive knowledge on the underlying mechanism is required for establishing new therapies for patients at risk.

Coronary computed tomography angiography (CCTA) and cardiac magnetic resonance (CMR) imaging in the assessment of patients presenting with chest pain suspected for acute coronary syndrome

Acute chest pain is an important clinical challenge and a major reason for presentation to the emergency department. Although multiple imaging techniques are available to assess patients with suspected acute coronary syndrome (ACS), considerable interest has been focused on the use of non-invasive imaging options as coronary computed tomography angiography (CCTA) and cardiac magnetic resonance (CMR). According to several recent evidences, CCTA has been shown to represent a useful tool to rapidly and accurately diagnose coronary artery disease (CAD) in patients with low to intermediate cardiovascular risk. CCTA examination has the unique ability to non-invasively depict the coronary anatomy, not only allowing visualization of the lumen of the arteries in order to detect severe stenosis or occlusion responsible of myocardial ischemia, but also allows the assessment of coronary artery wall by demonstrating the presence or absence of CAD. However, routine CCTA is not able to differentiate ischemic from non-ischemic chest pain in patients with known CAD and it does not provide any functional assessment of the heart. Conversely, CMR is considered the gold standard in the evaluation of morphology, function, viability and tissue characterization of the heart. CMR offers a wide range of tools for diagnosing myocardial infarction (MI) at least at the same time of the elevation of cardiac troponin values, differentiating infarct tissue and ischemic myocardium from normal myocardium or mimicking conditions, and distinguishing between new and old ischemic events. In high-risk patients, with acute and chronic manifestations of CAD, CMR may be preferable to CCTA, since it would allow detection, differential diagnosis, prognostic evaluation and management of MI.