Imaging atherosclerotic plaque inflammation with [18F]-fluorodeoxyglucose positron emission tomography

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.

Evaluation of translocator protein quantification as a tool for characterising macrophage burden in human carotid atherosclerosis

Macrophage presence within atherosclerotic plaque is a feature of instability and a risk factor for plaque rupture and clinical events. Activated macrophages express high levels of the translocator protein/peripheral benzodiazepine receptor (TSPO/PBR). In this study, we investigated the potential for quantifying plaque inflammation by targeting this receptor. TSPO expression and distribution in the plaque were quantified using radioligand binding assays and autoradiography. We show that cultured human macrophages expressed 20 times more TSPO than cultured human vascular smooth muscle cells (VSMCs), the other abundant cell type in plaque. The TSPO ligands [3H](R)-1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoquinoline carboxamide ([3H](R)-PK11195) and [3H]N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide ([3H]-DAA1106) bound to the same sites in human carotid atherosclerotic plaques in vitro, and demonstrated significant correlation with macrophage-rich regions. In conclusion, our data indicate that radioisotope-labelled DAA1106 has the potential to quantify the macrophage content of atherosclerotic plaque.

Multimodality Imaging of Carotid Artery Plaques

Background and Purpose— This study’s objective was to compare 18 F -fluoro-2-deoxyglucose positron emission tomography ( 18 F-FDG PET), CT, and MRI of carotid plaque assessment.

Materials and Methods— Fifty patients with symptomatic carotid atherosclerosis underwent 18 F-FDG PET/CT and MRI. Correlations and agreement between imaging findings were assessed by Spearman and Pearson rank correlation tests, t tests, and Bland-Altman plots.

Results— Spearman ρ between plaque 18 F-FDG standard uptake values and CT/MRI findings varied from −0.088 to 0.385. Maximum standard uptake value was significantly larger in plaques with intraplaque hemorrhage (1.56 vs 1.47; P =0.032). Standard uptake values did not significantly differ between plaques with an intact and thick fibrous cap and plaques with a thin or ruptured fibrous cap on MRI. (1.21 vs 1.23; P =0.323; and 1.45 vs 1.54; P =0.727). Pearson ρ between CT and MRI measurements varied from 0.554 to 0.794 ( P <0.001). For lipid-rich necrotic core volume, the CT–MRI correlation was stronger in mildly (≤10%) than in severely (>10%) calcified plaques (Pearson ρ 0.730 vs 0.475). Mean difference in measurement ±95% limits of agreement between CT and MRI for minimum lumen area, volumes of vessel wall, lipid-rich necrotic core, calcifications, and fibrous tissue were 0.4±18.1 mm 2 ( P =0.744), −41.9 ±761.7 mm 3 ( P =0.450), 78.4±305.0 mm 3 ( P <0.001), 180.5±625.7 mm 3 ( P =0.001), and −296.0±415.8 mm 3 ( P <0.001), respectively.

Conclusions— Overall, correlations between 18 F-FDG PET and CT/MRI findings are weak. Correlations between CT and MRI measurements are moderate to strong, but there is considerable variation in absolute differences.

Multimodality cardiovascular molecular imaging, part I

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

An approach to molecular imaging of atherosclerosis, thrombosis, and vascular inflammation using microparticles of iron oxide

The rapidly evolving field of molecular imaging promises important advances in the diagnosis, characterization and pharmacological treatment of vascular disease. Magnetic resonance imaging (MRI) provides a modality that is well suited to vascular imaging as it can provide anatomical, structural and functional data on the arterial wall. Its capabilities are further enhanced by the use of a range of increasingly sophisticated contrast agents that target specific molecules, cells and biological processes. This article will discuss one such approach, using microparticles of iron oxide (MPIO).

MPIO have been shown to create highly conspicuous contrast effects on T₂^*-weighted MR images. We have developed a range of novel ligand-conjugated MPIO for molecular MRI of endothelial adhesion molecules, such as vascular cell adhesion molecule-1 (VCAM-1) and P-selectin expressed in vascular inflammation, as well as activated platelet thrombosis. This review discusses the application of ligand-targeted MPIO for in vivo molecular MRI in a diverse range of vascular disease models including acute vascular inflammation, atherosclerosis, thrombosis, ischemia-reperfusion injury and ischemic stroke. The exceptionally conspicuous contrast effects of ligand-conjugated MPIO provide a versatile and sensitive tool for quantitative vascular molecular imaging that could refine diagnosis and measure response to treatment. The potential for clinical translation of this new class of molecular contrast agent for clinical imaging of vascular syndromes is discussed.

The role of hypoxia in atherosclerosis

PURPOSE OF REVIEW

It is important to address the factors involved in the progression of atherosclerosis because advanced atherosclerotic lesions are prone to rupture, leading to disability or death. Hypoxic areas are known to be present in human atherosclerotic lesions, and lesion progression is associated with the formation of lipid-loaded macrophages and increased local inflammation. Here we summarize the role of hypoxia in the development of advanced atherosclerotic lesions by promoting lipid accumulation, inflammation, ATP depletion, and angiogenesis.

RECENT FINDINGS

A recent study clearly demonstrated the presence of hypoxia in macrophage-rich regions of advanced human carotid atherosclerotic lesions. We showed that hypoxia increases the formation of lipid droplets in macrophages and promotes increased secretion of inflammatory mediators, and recent evidence indicates that lipid droplets may play a role in mediating the inflammatory response. Hypoxia also promotes lesion progression by exacerbating ATP depletion and lactate accumulation, and the presence of hypoxia in human carotid atherosclerotic lesions correlates with angiogenesis.

SUMMARY

Recent studies indicate that hypoxia may play a key role in the progression to advanced lesions by promoting lipid accumulation, increased inflammation, ATP depletion, and angiogenesis. Further understanding of the effects of hypoxia in atherosclerotic lesions could indicate potential therapeutic targets.

Hybrid in vivo FMT-CT imaging of protease activity in atherosclerosis with customized nanosensors

OBJECTIVE

Proteases are emerging biomarkers of inflammatory diseases. In atherosclerosis, these enzymes are often secreted by inflammatory macrophages, digest the extracellular matrix of the fibrous cap, and destabilize atheromata. Protease function can be monitored with protease activatable imaging probes and quantitated in vivo by fluorescence molecular tomography (FMT). To address 2 major constraints currently associated with imaging of murine atherosclerosis (lack of highly sensitive probes and absence of anatomic information), we compared protease sensors (PS) of variable size and pharmacokinetics and coregistered FMT datasets with computed tomography (FMT-CT).

METHODS AND RESULTS

Coregistration of FMT and CT was achieved with a multimodal imaging cartridge containing fiducial markers detectable by both modalities. A high-resolution CT angiography protocol accurately localized fluorescence to the aortic root of atherosclerotic apoE(-/-) mice. To identify suitable sensors, we first modeled signal kinetics in-silico and then compared 3 probes with oligo-L-lysine cleavage sequences: PS-5, 5 nm in diameter containing 2 fluorochromes, PS-25, a 25-nm version with an elongated lysine chain and PS-40, a polymeric nanoparticle. Serial FMT-CT showed fastest kinetics for PS-5 but, surprisingly, highest fluorescence in lesions of the aortic root for PS-40. PS-40 robustly reported therapeutic effects of atorvastatin, corroborated by ex vivo imaging and qPCR for the model protease cathepsin B.

CONCLUSIONS

FMT-CT is a robust and observer-independent tool for noninvasive assessment of inflammatory murine atherosclerosis. Reporter-containing nanomaterials may have unique advantages over small molecule agents for in vivo imaging.

Contrast ultrasound molecular imaging of inflammation in cardiovascular disease

The cellular immune response plays an important role in almost every major form of cardiovascular disease. The ability to image the key aspects of the immune response in the clinical setting could be used to improve diagnostic information, to provide important prognostic or risk information, and to customize therapy according to disease phenotype. Accordingly, targeted imaging probes for assessing inflammation have been developed for essentially all forms of medical imaging. Molecular imaging of inflammation with contrast ultrasound relies on the detection of targeted microbubble or other gas-filled particle contrast agents. These agents are confined to the vascular space and, hence, have been targeted to either activated leucocytes or endothelial cell adhesion molecules that are upregulated in inflammation and mediate leucocyte recruitment and adhesion. This review focuses on the inflammation-targeting strategies for ultrasound contrast agents and how they have been matched to cardiovascular disease states such as myocardial ischaemia, infarction, atherosclerosis, transplant rejection, and arteriogenesis.

FDG-PET can distinguish inflamed from non-inflamed plaque in an animal model of atherosclerosis

The presence of activated macrophages is an important predictor of atherosclerotic plaque rupture. In this study, our aim was to determine the accuracy of (18)F- fluorodeoxyglucose (FDG) microPET imaging for quantifying aortic wall macrophage content in a rabbit model of atherosclerosis. Rabbits were divided into a control group and two groups post aortic balloon injury: 6 months high-cholesterol diet (HC); and 3 months HC followed by 3 months low-cholesterol diet plus statin (LCS). In vivo and ex vivo microPET, ex vivo well counting and histological quantification of the atherosclerotic aortas were performed for all groups. Macrophage density was greater in the HC group than the LCS group (5.1 +/- 1.4% vs. 0.6 +/- 0.7%, P < 0.001) with a trend towards greater macrophage density in LCS compared to controls (P = 0.08). There was a strong correlation across all groups between macrophage density and standardized uptake value (SUV) derived from ex vivo microPET (r = 0.95, P < 0.001) and well counting (r = 0.96, P < 0.001). Ex vivo FDG SUV was significantly different between the three groups (P < 0.001). However, the correlation between in vivo microPET FDG SUV and macrophage density was insignificant (r = 0.16, P = 0.57) with no statistical differences in FDG SUV seen between the three groups. This study confirms that in an animal model of inflamed and non-inflamed atherosclerosis, significant differences in FDG SUV allow differentiation of highly inflamed atherosclerotic aortas from those stabilized by statin therapy and low cholesterol diet and controls.

Glucose metabolism is required for oxidized LDL-induced macrophage survival: role of PI3K and Bcl-2 family proteins

OBJECTIVE

Oxidized low-density lipoprotein (oxLDL) induces survival of colony stimulating factor-1 (CSF-1)-dependent macrophages in vitro. Because atherosclerotic lesion-associated macrophages take up large amounts of glucose, we investigated whether, and how, oxLDL promotes glucose uptake and how glucose metabolism regulates oxLDL-induced macrophage survival.

METHODS AND RESULTS

OxLDL-induced macrophage survival required glucose metabolism. OxLDL stimulated 2 phases of glucose uptake, namely acute and chronic, which required PI3K but not MEK1/2 activity. PI3K appeared to regulate glucose transport via glucose transporter affinity and/or mobilization. OxLDL also maintained levels of the prosurvival proteins, Bcl-2 and Bcl-x(L), after CSF-1 had been removed through a combination of mechanisms including transcription, translation, and protein stabilization. Significantly, inhibition of glucose metabolism reduced Bcl-2 and Bcl-x(L) protein levels. MEK1/2 and PI3K activities were also required for oxLDL-induced Bcl-2 and Bcl-x(L) mRNA upregulation.

CONCLUSIONS

These results suggest that oxLDL enhances macrophage survival in the absence of CSF-1 by inducing PI3K-dependent glucose uptake, which is metabolized to maintain Bcl-2 and Bcl-x(L) protein levels.

Cardiac positron emission tomography

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

Molecular imaging in atherosclerosis, thrombosis, and vascular inflammation

Appreciation of the molecular and cellular processes of atherosclerosis, thrombosis, and vascular inflammation has identified new targets for imaging. The common goals of molecular imaging approaches are to accelerate and refine diagnosis, provide insights that reveal disease diversity, guide specific therapies, and monitor the effects of those therapies. Here we undertake a comparative analysis of imaging modalities that have been used in this disease area. We consider the elements of contrast agents, emphasizing how an understanding of the biology of atherosclerosis and its complications can inform optimal design. We address the potential and limitations of current contrast approaches in respect of translation to clinically usable agents and speculate on future applications.

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.

Pre-clinical and clinical evaluation of nuclear tracers for the molecular imaging of vulnerable atherosclerosis: an overview

Cardiovascular diseases (CVD) are the leading cause of mortality worldwide. Despite major advances in the treatment of CVD, a high proportion of CVD victims die suddenly while being apparently healthy, the great majority of these accidents being due to the rupture or erosion of a vulnerable coronary atherosclerotic plaque. A non-invasive imaging methodology allowing the early detection of vulnerable atherosclerotic plaques in selected individuals prior to the occurrence of any symptom would therefore be of great public health benefit. Nuclear imaging could allow the identification of vulnerable patients by non-invasive in vivo scintigraphic imaging following administration of a radiolabeled tracer. The purpose of this review is to provide an overview of radiotracers that have been recently evaluated for the detection of vulnerable plaques together with the biological rationale that initiated their development. Radiotracers targeted at the inflammatory process seem particularly relevant and promising. Recently, macrophage targeting allowed the experimental in vivo detection of atherosclerosis using either SPECT or PET. A few tracers have also been evaluated clinically. Targeting of apoptosis and macrophage metabolism both allowed the imaging of vulnerable plaques in carotid vessels of patients. However, nuclear imaging of vulnerable plaques at the level of coronary arteries remains challenging, mostly because of their small size and their vicinity with unbound circulating tracer. The experimental and pilot clinical studies reviewed in the present paper represent a fundamental step prior to the evaluation of the efficacy of any selected tracer for the early, non-invasive detection of vulnerable patients.

Novel concepts in atherogenesis: angiogenesis and hypoxia in atherosclerosis

The clinical complications of atherosclerosis are caused by thrombus formation, which in turn results from rupture of an unstable atherosclerotic plaque. The formation of microvessels (angiogenesis) in an atherosclerotic plaque contributes to the development of plaques, increasing the risk of rupture. Microvessel content increases with human plaque progression and is likely stimulated by plaque hypoxia, reactive oxygen species and hypoxia-inducible factor (HIF) signalling. The presence of plaque hypoxia is primarily determined by plaque inflammation (increasing oxygen demand), while the contribution of plaque thickness (reducing oxygen supply) seems to be minor. Inflammation and hypoxia are almost interchangeable and both stimuli may initiate HIF-driven angiogenesis in atherosclerosis. Despite the scarcity of microvessels in animal models, atherogenesis is not limited in these models. This suggests that abundant plaque angiogenesis is not a requirement for atherogenesis and may be a physiological response to the pathophysiological state of the arterial wall. However, the destruction of the integrity of microvessel endothelium likely leads to intraplaque haemorrhage and plaques at increased risk for rupture. Although a causal relation between the compromised microvessel structure and atherogenesis or between angiogenic stimuli and plaque angiogenesis remains tentative, both plaque angiogenesis and plaque hypoxia represent novel targets for non-invasive imaging of plaques at risk for rupture, potentially permitting early diagnosis and/or risk prediction of patients with atherosclerosis in the near future.

PET and MRI in cardiac imaging: from validation studies to integrated applications

INTRODUCTION

Positron emission tomography (PET) is the gold standard for non-invasive assessment of myocardial viability and allows accurate detection of coronary artery disease by assessment of myocardial perfusion. Magnetic resonance imaging (MRI) provides high resolution anatomical images that allow accurate evaluation of ventricular structure and function together with detection of myocardial infarction.

OBJECTIVE

Potential hybrid PET/MR tomography may potentially facilitate the combination of information from these imaging modalities in cardiology. Furthermore, the combination of anatomical MRI images with the high sensitivity of PET for detecting molecular targets may extent the application of these modalities to the characterization of atherosclerotic plaques and to the evaluation of angiogenetic or stem cell therapies, for example.

DISCUSSION

This article reviews studies using MRI and PET in parallel to compare their performance in cardiac applications together with the potential benefits and applications provided by hybrid PET/MRI systems.

Comparison of synthetic high density lipoprotein (HDL) contrast agents for MR imaging of atherosclerosis

Determining arterial macrophage expression is an important goal in the molecular imaging of atherosclerosis. Here, we compare the efficacy of two synthetic, high density lipoprotein (HDL) based contrast agents for magnetic resonance imaging (MRI) of macrophage burden. Each form of HDL was labeled with gadolinium and rhodamine to allow MRI and fluorescence microscopy. Either the 37 or 18 amino acid peptide replaced the apolipoprotein A-I in these agents, which were termed 37pA-Gd or 18A-Gd. The diameters of 37pA-Gd and 18A-Gd are 7.6 and 8.0 nm, respectively, while the longitudinal relaxivities are 9.8 and 10.0 (mM s)(-1). 37pA has better lipid binding properties. In vitro tests with J774A.1 macrophages proved the particles possessed the functionality of HDL by eliciting cholesterol efflux and were taken up in a receptor-like fashion by the cells. Both agents produced enhancements in atherosclerotic plaques of apolipoprotein E knockout mice of approximately 90% (n = 7 per agent) and are macrophage specific as evidenced by confocal microscopy on aortic sections. The half-lives of 37pA-Gd and 18A-Gd are 2.6 and 2.1 h, respectively. Despite the more favorable lipid interactions of 37pA, both agents gave similar, excellent contrast for the detection of atherosclerotic macrophages using MRI.

Quantification of inflammation within rabbit atherosclerotic plaques using the macrophage-specific CT contrast agent N1177: a comparison with 18F-FDG PET/CT and histology

Macrophages play a key role in atherosclerotic plaque rupture. The iodine-based contrast agent N1177 accumulates in macrophages, allowing for their detection with CT. In this study, we tested whether the intensity of enhancement detected with CT in the aortic wall of rabbits injected with N1177 correlated with inflammatory activity evaluated with (18)F-FDG PET/CT and macrophage density on histology.

METHODS

Atherosclerotic plaques were induced in the aorta of New Zealand White rabbits (n = 7) by a repeated balloon injury (4 wk apart) and 4 mo of hyperlipemic diet. Noninjured rabbits, fed a chow diet, were used as controls (n = 3). A CT scan of the aorta (n = 10) was acquired in each rabbit before, during, and at 2 h after intravenous injection of N1177 (250 mg of iodine/kg). One week later, the same rabbits underwent PET/CT 3 h after injection of (18)F-FDG (37 MBq/kg [1 mCi/kg]). CT enhancement was calculated as the difference in aortic wall densities between images obtained before and images obtained at 2 h after injection of N1177. Mean standardized uptake values were measured on PET axial slices of the aorta in regions of interest encompassing the vessel wall. Macrophage density was measured by immunohistology (anti-RAM-11 antibody) on corresponding aortic cross-sections.

RESULTS

N1177-enhanced CT measured stronger enhancement in the aortic wall of atherosclerotic rabbits than in control rabbits (10.0 +/- 5.2 vs. 2.0 +/- 2.1 Hounsfield units, respectively; P < 0.05). After the injection of (18)F-FDG, PET detected higher standardized uptake values in the aortic wall of atherosclerotic rabbits than in control rabbits (0.61 +/- 0.12 vs. 0.21 +/- 0.02; P < 0.05). The intensity of enhancement in the aortic wall measured with CT after injection of N1177 correlated with (18)F-FDG uptake on PET/CT (r = 0.61, P < 0.001) and macrophage density on immunohistology (r = 0.63, P < 0.001).

CONCLUSION

The intensity of enhancement detected with CT in the aortic wall of rabbits injected with N1177 correlates with intense uptake of (18)F-FDG measured with PET and with macrophage density on histology, suggesting a role for N1177 in noninvasive identification of high-risk atherosclerotic plaques with CT.

Evaluation of alphavbeta3 integrin-targeted positron emission tomography tracer 18F-galacto-RGD for imaging of vascular inflammation in atherosclerotic mice

BACKGROUND

(18)F-Galacto-RGD is a positron emission tomography (PET) tracer binding to alpha(v)beta(3) integrin that is expressed by macrophages and endothelial cells in atherosclerotic lesions. Therefore, we evaluated (18)F-galacto-RGD for imaging vascular inflammation by studying its uptake into atherosclerotic lesions of hypercholesterolemic mice in comparison to deoxyglucose.

METHODS AND RESULTS

Hypercholesterolemic LDLR(-/-)ApoB(100/100) mice on a Western diet and normally fed adult C57BL/6 control mice were injected with (18)F-galacto-RGD and (3)H-deoxyglucose followed by imaging with a small animal PET/CT scanner. The aorta was dissected 2 hours after tracer injection for biodistribution studies, autoradiography, and histology. Biodistribution of (18)F-galacto-RGD was higher in the atherosclerotic than in the normal aorta. Autoradiography demonstrated focal (18)F-galacto-RGD uptake in the atherosclerotic plaques when compared with the adjacent normal vessel wall or adventitia. Plaque-to-normal vessel wall ratios were comparable to those of deoxyglucose. Although angiogenesis was not detected, (18)F-galacto-RGD uptake was associated with macrophage density and deoxyglucose accumulation in the plaques. Binding to atherosclerotic lesions was efficiently blocked in competition experiments. In vivo imaging visualized (18)F-galacto-RGD uptake colocalizing with calcified lesions of the aortic arch as seen in CT angiography.

CONCLUSIONS

(18)F-Galacto-RGD demonstrates specific uptake in atherosclerotic lesions of mouse aorta. In this model, its uptake was associated with macrophage density. (18)F-Galacto-RGD is a potential tracer for noninvasive imaging of inflammation in atherosclerotic lesions.

Retrospective study of coronary uptake of 18F-fluorodeoxyglucose in association with calcification and coronary artery disease: a preliminary study

OBJECTIVE

To determine whether focal 18F-fluorodeoxyglucose (FDG) uptake could be detected along the course of coronary arteries in patients with known coronary artery disease (CAD) and/or coronary artery calcification (CAC) by PET/computed tomography (CT) using a new patient preparation protocol that reduces background myocardial FDG uptake.

MATERIALS AND METHODS

In this retrospective, Health Insurance Portability and Accountability Act-compliant study approved by our institutional internal review board, 60 FDG-PET/CT studies performed for noncardiac indications were reviewed and CAC and focal FDG uptake were determined. Cardiac histories were obtained. Age range was 21-88 years (64+/-16 years); 35 women/25 men; six had CAD documented by myocardial perfusion imaging, cardiac catheterization, or history of percutaneous coronary intervention or coronary artery bypass grafting. Chi-square probabilities were calculated.

RESULTS

Fifteen of the 60 patients studied showed focal FDG uptake along the course of coronary arteries; fourteen of these showed significant CAC scores and four had documented CAD. The concordance of focal FDG coronary artery uptake was 58% with CAC, 77% with cardiac history, and 90% with extracardiac vascular focal FDG uptake. There were 20 focal FDG and coronary artery lesions in 15 patients and 11 of the 20 colocalized with calcification. No preferred anatomic location was found (junction, end of calcification or along coronary artery).

CONCLUSION

This limited retrospective study shows that with a new patient preparation to reduce high FDG uptake by the myocardium on PET/CT scans, foci of increased FDG cardiac uptake can be seen in association with CAC and in patients with a history of CAD. These results provide initial evidence that it may be possible to use FDG-PET/CT to monitor sites of coronary plaque formation. Larger and invasive prospective studies will be necessary to fully determine the suitability of this technique for that purpose.

Translation of myocardial metabolic imaging concepts into the clinics

Flexibility in myocardial substrate metabolism for energy production is fundamental to cardiac health. This loss in plasticity or flexibility leads to overdependence on the metabolism of an individual category of substrates, with the predominance in fatty acid metabolism characteristic of diabetic heart disease and the accelerated glucose use associated with pressure-overload left ventricular hypertrophy being prime examples. There is a strong demand for accurate noninvasive imaging approaches of myocardial substrate metabolism that can facilitate the crosstalk between the bench and the bedside, leading to improved patient management paradigms. In this article potential future applications of metabolic imaging, particularly radionuclide approaches, for assessment of cardiovascular disease are discussed.

Coronary CT angiography (CCTA) and advances in CT plaque imaging

The goal of this review is to highlight current advances in the non-invasive detection of clinically significant atherosclerotic disease including the so-called vulnerable plaque with computed tomography. Atherosclerotic disease encompasses stages of plaque progression, stabilization, and even regression. Traditionally, the focus of diagnostic imaging has been the detection of lumen-occluding atheroma. However, advances in our understanding of the pathophysiology of atherosclerotic plaque have shown that, in certain stages of plaque progression, plaque is "vulnerable" and able to cause acute coronary syndromes despite "non-significant" vascular occlusion at baseline. This provides a rationale to improve our non-invasive imaging technology. Presented here are improvements in soft-tissue resolution with technical advancements as well as contrast-enhancement and lately even nanotechnology-based technology which are geared to detect the clinically elusive vulnerable plaque and provide an opportunity for preventative therapy.

Multimodality imaging of atherosclerotic plaque activity and composition using FDG-PET/CT and MRI in carotid and femoral arteries

PURPOSE

To evaluate the relationship between atherosclerotic plaque inflammation, as assessed by FDG-positron emission tomography/computed tomography (FDG-PET/CT), and plaque morphology and composition, as assessed by magnetic resonance imaging (MRI), in the carotid and femoral arteries.

MATERIALS AND METHODS

Sixteen patients underwent FDG-PET/CT and MRI (T2-weighted (T2W) and proton density weighted (PDW)) of the carotid and femoral arteries. For every image slice, two observers determined the corresponding regions of the FDG-PET/CT and MRI image sets by matching CT and T2W axial images. Each plaque was then classified into one of three groups according to the CT appearance and T2W/PDW signal: (1) collagen, (2) lipid-necrotic core and (3) calcium. Arterial FDG uptake was measured for each plaque and normalized to vein FDG activity to produce a blood-normalized artery activity called the target to background ratio (TBR). The vessel wall thickness (VWT), the vessel wall area and the total vessel wall area were measured from the T2W MR images.

RESULTS

The TBR value was higher in the lipid-necrotic core group compared to the collagen and calcium groups, (p<0.001). The lipid-necrotic core group demonstrated a significant TBR variation according to the median of the VWT (TBR=1.26+/-0.25 vs. 1.50+/-0.12). There was no correlation with other morphological MR parameters.

CONCLUSIONS

This study demonstrates the complementary value of non-invasive FDG-PET/CT and MR imaging for the evaluation of atherosclerotic plaque composition and activity. Lipid-rich plaques are more inflamed than either calcified or collagen-rich plaques.

Quantum dot mediated imaging of atherosclerosis

The progression of atherosclerosis is associated with leukocyte infiltration within lesions. We describe a technique for the ex vivo imaging of cellular recruitment in atherogenesis which utilizes quantum dots (QD) to color-code different cell types within lesion areas. Spectrally distinct QD were coated with the cell-penetrating peptide maurocalcine to fluorescently-label immunomagnetically isolated monocyte/macrophages and T lymphocytes. QD-maurocalcine bioconjugates labeled both cell types with a high efficiency, preserved cell viability, and did not perturb native leukocyte function in cytokine release and endothelial adhesion assays. QD-labeled monocyte/macrophages and T lymphocytes were reinfused in an ApoE(-/-) mouse model of atherosclerosis and age-matched controls and tracked for up to four weeks to investigate the incorporation of cells within aortic lesion areas, as determined by oil red O (ORO) and immunofluorescence ex vivo staining. QD-labeled cells were visible in atherosclerotic plaques within two days of injection, and the two cell types colocalized within areas of subsequent ORO staining. Our method for tracking leukocytes in lesions enables high signal-to-noise ratio imaging of multiple cell types and biomarkers simultaneously within the same specimen. It also has great utility in studies aimed at investigating the role of distinct circulating leukocyte subsets in plaque development and progression.

Emergence of targeted molecular imaging in atherosclerotic cardiovascular disease

Atherosclerosis, a systemic disease, remains one of the leading causes of morbidity and mortality in the world. Our improved understanding of the molecular mechanisms underlying atherosclerotic lesion progression and sudden transformation into unstable plaques, indicate complex interactions of lipid metabolism, inflammatory processes and genetic predisposition. Currently, novel imaging approaches to visualize the process of atherosclerosis, particularly at the molecular level, are actively being developed. Important targets include inflammatory and endothelial cells, as well as apoptosis and angiogenesis. The next decade should solidify the role of targeted molecular imaging in all aspects of cardiovascular medicine.

Phosphatidylinostitol-3 kinase and phospholipase C enhance CSF-1-dependent macrophage survival by controlling glucose uptake

Colony stimulating factor-1 (CSF-1)-dependent macrophages play crucial roles in the development and progression of several pathological conditions including atherosclerosis and breast cancer metastasis. Macrophages in both of these pathologies take up increased amounts of glucose. Since we had previously shown that CSF-1 stimulates glucose uptake by macrophages, we have now investigated whether glucose metabolism is required for the survival of CSF-1-dependent macrophages as well as examined the mechanism by which CSF-1 stimulates glucose uptake. Importantly, we found that CSF-1-induced macrophage survival required metabolism of the glucose taken up in response to CSF-1 stimulation. Kinetic studies showed that CSF-1 stimulated an increase in the number of glucose transporters at the plasma membrane, including Glut1. The uptake of glucose induced by CSF-1 required intact PI3K and PLC signalling pathways, as well as the downstream effectors Akt and PKC, together with a dynamic actin cytoskeleton. Expression of constitutively active Akt partially restored glucose uptake and macrophage survival in the absence of CSF-1, suggesting that Akt is necessary but not sufficient for optimal glucose uptake and macrophage survival. Taken together, these results suggest that CSF-1 regulates macrophage survival, in part, by stimulating glucose uptake via Glut1, and PI3K and PLC signalling pathways.

Imaging of inflamed and vulnerable plaque in coronary arteries with 18F-FDG PET/CT in patients with suppression of myocardial uptake using a low-carbohydrate, high-fat preparation

PET/CT imaging with (18)F-FDG has been used to detect inflammation in carotid and aortic plaque; its use in detecting coronary plaque has been limited by avid (18)F-FDG uptake by the myocardium. We investigated whether (18)F-FDG PET/CT could be used to image inflammation in coronary arteries as a potential noninvasive method to detect vulnerable plaque.

METHODS

We retrospectively studied 32 patients treated for malignancy who underwent (18)F-FDG PET/CT and concomitant cardiac catheterization. As part of the recently described protocol, all patients were instructed to eat a low-carbohydrate, high-fat meal the night before and drink a vegetable oil drink the morning of the study. We reviewed the patients' baseline characteristics and their (18)F-FDG PET/CT scans for adequacy of myocardial uptake suppression and correlated the presence of angiographically apparent plaque with (18)F-FDG uptake in the major coronary arteries. Two independent observers assessed the angiographic images and (18)F-FDG PET scans.

RESULTS

A total of 95% of patients had 2 or more coronary disease risk factors, and 25% had unstable symptoms; 30% of index catheterizations resulted in intervention. In 20 of 32 patients (63%), myocardial suppression was good (12) or adequate (8). Inadequate suppression was due to self-reported dietary nonadherence. Patients with good, adequate, and poor suppression had maximal myocardial standardized uptake values of 2.8 +/- 0.7, 5.0 +/- 1.3, and 17.0 +/- 9.7, respectively. We identified (18)F-FDG uptake in 15 patients in 1 or more coronary segments. A trend to significance in correlation between presence of angiographic disease and signal in the vessel was observed (P = 0.07; 80 vessels examined). A total of 7 patients with significant coronary artery disease had aortic (18)F-FDG uptake.

CONCLUSION

In this retrospective study, we demonstrated the potential use of (18)F-FDG PET in imaging of inflammation in coronary arteries. The potential of (18)F-FDG PET is also being investigated in a prospective study.

Advances in molecular imaging of atherosclerotic vascular disease

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Noninvasive imaging of atheromatous carotid plaques

Atherothrombosis is a systemic disease of the arterial wall that affects the carotid, coronary, and peripheral vascular beds, and the aorta. This condition is associated with complications such as stroke, myocardial infarction, and peripheral vascular disease, which usually result from unstable atheromatous plaques. The study of atheromatous plaques can provide useful information about the natural history and progression of the disease, and aid in the selection of appropriate treatment. Plaque imaging can be crucial in achieving this goal. In this Review, we focus on the various noninvasive imaging techniques that are being used for morphological and functional assessment of carotid atheromatous plaques in the clinical setting.

Clinical feasibility of molecular imaging of plaque inflammation in atherosclerosis

Despite substantial advances in the diagnosis and management of coronary artery disease, acute coronary events continue to occur in many patients. It has been increasingly realized that the lesions responsible for acute events may not necessarily be critically obstructive and hence not be associated with inducible ischemia. Various morphologic features of plaque vulnerability have been described by CT angiography, intravascular ultrasound, and optical coherence tomography. The culprit plaques often demonstrate large plaque and necrotic core volumes, positive vascular remodeling, and attenuation of fibrous plaque caps. The remaining obligatory component of plaque vulnerability is fibrous cap inflammation; molecular imaging is best suited for identification of monocyte-macrophage infiltration. Whereas multiple candidate targets have been evaluated in preclinical molecular imaging studies, only (18)F-FDG and (99m)Tc-annexin-A5 have been recently used in the settings of acute vascular events. These 2 imaging strategies have demonstrated the clinical feasibility of imaging for detection of inflammation.

Increased metabolic activity in abdominal aortic aneurysm detected by 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT)

OBJECTIVES

Abdominal aortic aneurysms (AAAs) are associated with an inflammatory cell infiltrate and enzymatic degradation of the vessel wall. The aim of this study was to detect increased metabolic activity in the wall of the AAA with 18F-fluorodeoxyglucose ((18)F-FDG), mediated by glucose transporter protein (GLUTs), using a dedicated hybrid PET/64-detector CT. DESIGN, METHOD AND MATERIALS: 14 patients (All male, mean age 73.6 years, range 61-82) with AAA under surveillance underwent PET/CT scanning with 175 MBq of intravenous (18)F-FDG. The maximum aneurysm diameter and calcification score were determined on the attenuation correction CT. A volume of interest was placed on the aneurysm sac and the maximum Standardised Uptake Value (SUV(max)) measured.

RESULTS

The mean aneurysm diameter was 5.4 cm (SD+/-0.8). Two aneurysms had the CT characteristics of inflammatory aneurysms. Twelve aneurysms showed increased FDG uptake (SUV(max)>2.5). There was no significant difference in FDG uptake between heavily calcified aneurysms and non-heavily calcified aneurysms (t-test). There was a significant increase in the FDG uptake in the two inflammatory aneurysms compared to the other twelve aneurysms (t-test; P=0.04).

CONCLUSION

The findings in this study offer in vivo evidence that the AAA wall shows increased glucose metabolism, mediated by the GLUTs: this increased metabolic activity as detected by PET/CT may be present in most AAAs.

Systemic infection, inflammation and acute ischemic stroke

Extensive evidence implicates inflammation in multiple phases of stroke etiology and pathology. In particular, there is growing awareness that inflammatory events outside the brain have an important impact on stroke susceptibility and outcome. Numerous conditions, including infection and chronic non-infectious diseases, that are established risk factors for stroke are associated with an elevated systemic inflammatory profile. Recent clinical and pre-clinical studies support the concept that the systemic inflammatory status prior to and at the time of stroke is a key determinant of acute outcome and long-term prognosis. Here, we provide an overview of the impact of systemic inflammation on stroke susceptibility and outcome. We discuss potential mechanisms underlying the impact on ischemic brain injury and highlight the implications for stroke prevention, therapy and modeling.