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

Advances in atheroma imaging in the carotid

Atherosclerosis affects all vascular beds, including the coronary, carotid, intracerebral, peripheral and aortic vascular beds, and is responsible for tremendous morbidity and mortality, with the most serious outcomes being myocardial infarction, stroke and death. Historically the effects of vascular narrowing and associated thrombosis have been key indicators of disease in the coronary and carotid territories, with degrees of vascular stenosis being of profound importance in carotid surgery trials. Our improving understanding of the biology of atheromatous lesions and the development of alternative therapeutic agents which can initiate actual plaque regression have created a need to attempt to image plaque itself, with the carotid artery being an achievable target. This article reviews current strategies for assessing carotid atherosclerotic disease, particularly with reference to identifying plaque components and risk of rupture, the so-called vulnerable plaque.

Childhood origins of arterial disease

PURPOSE OF REVIEW

The present article reviews the importance of classical and novel risk factors that present in childhood, track into adult life and contribute to arterial disease. The value of noninvasive techniques that can assist in characterization of preclinical atherosclerotic changes as intermediate phenotypes is also discussed.

RECENT FINDINGS

Noninvasive functional and structural techniques are now available and provide the opportunity to characterize early arterial disease long before cardiovascular complications present. By using these techniques, it has been possible to quantify the impact of conventional and novel cardiovascular risk factors seen in childhood on the development of preclinical atherosclerotic changes. Scientific interest has recently widened to include not only study of mechanisms and biomarkers of injury but also mechanisms that promote vascular repair. In this new field, characterization of endothelial progenitor cells has presented new opportunities for cardiovascular research.

SUMMARY

Atherosclerosis begins in early life. Primary prevention strategies for adult cardiovascular disease beginning in childhood have great potential as the disease process is most reversible at this stage. Several guidelines have recently been published for screening and implementation of appropriate therapeutic choices in early life.

Cardiovascular molecular imaging of apoptosis

Introduction

Molecular imaging strives to visualise processes at the molecular and cellular level in vivo. Understanding these processes supports diagnosis and evaluation of therapeutic efficacy on an individual basis and thereby makes personalised medicine possible.

Apoptosis and molecular imaging

Apoptosis is a well-organised mode of cell suicide that plays a role in cardiovascular diseases (CVD). Apoptosis is associated with loss of cardiomyocytes following myocardial infarction, atherosclerotic plaque instability, congestive heart failure and allograft rejection of the transplanted heart. Thus, apoptosis constitutes an attractive target for molecular imaging of CVD. Our current knowledge about the molecular players and mechanisms underlying apoptosis offers a rich palette of potential molecular targets for molecular imaging. However, only a few have been successfully developed so far.

Aims

This review highlights aspects of the molecular machinery and biochemistry of apoptosis relevant to the development of molecular imaging probes. It surveys the role of apoptosis in four major areas of CVD and portrays the importance and future perspectives of apoptosis imaging. The annexin A5 imaging protocol is emphasised since it is the most advanced protocol to measure apoptosis in both preclinical and clinical studies.

(18)Fluorodeoxyglucose positron emission tomography imaging of atherosclerotic plaque inflammation is highly reproducible: implications for atherosclerosis therapy trials

OBJECTIVES

This study tested the near-term reproducibility of (18)fluorodeoxyglucose positron emission tomography (FDG-PET) imaging of atherosclerosis.

BACKGROUND

It is known that FDG-PET can measure inflammation within the aorta, carotid, and vertebral arteries with histologic validation in humans and animal models of disease. By tracking changes in inflammation over time, PET could be used as a surrogate marker of antiatheroma drug efficacy. However, the short-term variability and reproducibility of the technique are unknown.

METHODS

We imaged the carotid arteries and aorta in 11 subjects with FDG-PET/computed tomography twice, 14 days apart. We assessed interobserver and intraobserver agreement and interscan variability.

RESULTS

Interscan plaque FDG variability over 2 weeks was very low; intraclass correlation coefficients (ICC) ranged between 0.79 and 0.92. Interobserver agreement was high across all territories imaged except aortic arch (ICC values from 0.90 to 0.97, arch 0.71). Intraobserver agreement was high, with ICC values between 0.93 and 0.98.

CONCLUSIONS

Spontaneous change in plaque FDG uptake is low over 2 weeks, with favorable inter- and intraobserver agreement. Power calculations suggest that drug studies using FDG-PET imaging would require few subjects compared with other imaging modalities. This study strengthens the case for FDG-PET as a noninvasive plaque imaging technique.

Cardiovascular Molecular Imaging

The goal of this review is to highlight how molecular imaging will impact the management and improved understanding of the major cardiovascular diseases that have substantial clinical impact and research interest. These topics include atherosclerosis, myocardial ischemia, myocardial viability, heart failure, gene therapy, and stem cell transplantation. Traditional methods of evaluation for these diseases will be presented first, followed by methods that incorporate conventional and molecular imaging approaches.

Imaging techniques for the vulnerable coronary plaque

The goal of this article is to illustrate the main invasive and noninvasive diagnostic modalities to image the vulnerable coronary plaque, which is responsible for acute coronary syndrome. The main epidemiologic and histological issues are briefly discussed in order to provide an adequate background. Comprehensive coronary atherosclerosis imaging should involve visualization of the entire coronary artery tree and plaque characterization, including three-dimensional morphology, relationship with the lumen, composition, vascular remodelling and presence of inflammation. No single technique provides such a comprehensive description, and no available modality extensively identifies the vulnerable plaque. In particular, we describe multislice computed tomography, which at present seems to be the most promising noninvasive tool for an exhaustive image-based quantification of coronary atherosclerosis.

[18F]Fluorodeoxyglucose PET in Large Vessel Vasculitis

[(18)F]fluorodeoxyglucose (FDG) PET is a noninvasive metabolic imaging modality based on the regional distribution of [18F]FDG that is highly effective in assessing the activity and extent of giant cell arteritis and Takayasu's arteritis, respectively. Metabolic imaging using [18F]FDG-PET has been shown to identify more affected vascular regions than morphologic imaging with MRI in both diseases. The visual grading of vascular [18F]FDG uptake helps to discriminate arteritis from atherosclerosis and therefore provides high specificity. High sensitivity is attained by scanning during the active inflammatory phase. Thus, [18F]FDG-PET has the potential to develop into a valuable tool in the diagnostic workup of giant cell arteritis and Takayasu's arteritis.

Molecular imaging and therapy of atherosclerosis with targeted nanoparticles

Advances in bionanotechnology are poised to impact the field of cardiovascular diagnosis and therapy for decades to come. This review seeks to illustrate selected examples of newly developed diagnostic and therapeutic nanosystems that have been evaluated in experimental atherosclerosis, thrombosis, and vascular biology. We review a variety of nanotechnologies that are capable of detecting early cardiovascular pathology, as well as associated imaging approaches and conjunctive strategies for site-targeted treatment with nanoparticle delivery systems.

Carotid plaque inflammation detected by 18F-fluorodeoxyglucose-positron emission tomography

OBJECTIVES

Inflammation is important in both the pathogenesis and outcome of atherosclerosis. Current imaging techniques provide anatomic data but no indication of plaque inflammation. We tested the hypothesis that plaque inflammation could be assessed in vivo by (18)FDG-PET and that plaque inflammation could increase the risk of recurrent vascular events and poor response to treatment in a pilot study.

PATIENTS AND METHODS

Thirteen patients (median age 66.1 years [55-82 years]) with recent carotid territory TIA or ischemic stroke and internal carotid artery (ICA) stenosis > or =50% were studied. Angiography and (18)F-fluorodeoxyglucose-positron emission tomography ((18)FDG-PET) imaging were carried out in all patients. Treatment for carotid stenosis in each patient was selected by the attending physician and consisted in medical treatment, endarterectomy or stent placement. During 6 months of follow-up, the specific end points assessed were the occurrence of any stroke, death, or re-stenosis.

RESULTS

Patients with symptomatic carotid atherosclerosis were imaged using (18)FDG-PET. Strong (18)FDG uptake (SUV> or =2.7) was seen in 11 of 13 (85%) carotid lesions. Among these patients two died during follow-up, 3 had recurrent non-fatal ipsilateral ischemic stroke and 1 patient who had undergone stenting had non-symptomatic re-stenosis in control studies. There was a significant correlation between the (18)FDG uptake and degree of ICA stenosis detected by angiography.

CONCLUSION

Carotid atherosclerotic plaques contain a variable degree of inflammation which can be assessed in vivo by means of FDG and PET. The prognostic value of this marker is, however, still unclear and needs further study.

Role of imaging end points in atherosclerosis trials: focus on intravascular ultrasound

Increasing attention has been focused on the appropriate role of surrogate markers in the development of novel anti-atherosclerotic therapies. Technological advances in imaging modalities allow for visualisation of the entire arterial wall. Intravascular ultrasound (IVUS) has been increasingly employed to precisely quantify the extent of coronary atherosclerosis. Use of IVUS has provided a number of important insights into the natural history of atherosclerosis and the remodelling changes of the arterial wall in response to plaque accumulation. More recently, clinical trials have employed serial evaluations of arterial segments by IVUS to assess the impact of medical therapies.

Imaging and molecular biomarkers of vulnerable atheromatous plaques

Rupture of a vulnerable atherosclerotic plaque is the main cause of acute coronary syndromes, myocardial infarction and death. Identification of biomarkers that accurately predict the risk of plaque rupture may be a means to establish and monitor response to therapeutic intervention. This review focuses on recent advances for the use of circulating molecular biomarkers and imaging modalities to assess atherosclerotic vulnerable plaques in both preclinical models and clinical conditions.

Inflammation and ischaemic stroke

PURPOSE OF REVIEW

Inflammation is implicated in ischaemic stroke as a general cardiovascular risk factor, a possible immediate trigger, a component (and possible exacerbating factor) of the response to tissue injury, a marker of future risk, and as a therapeutic target. Each aspect is reviewed.

RECENT FINDINGS

Evidence of epidemiological association of inflammatory markers, particularly C-reactive protein, has accrued, but the independence of inflammation from more conventional risk indicators is under question. Other inflammatory markers are associated with intermediate phenotypes such as hypertension. Tissue inflammation in atherosclerotic plaque is of probable relevance in identifying recently symptomatic carotid disease. Both humoral and cellular inflammation are evident following stroke, with evidence that these responses may exacerbate tissue injury. Blockade of interleukin-1, or of neutrophil chemotaxis, has reduced infarct volume in models of stroke but has yet to show benefit in clinical trials. Other anti-inflammatory strategies are promising.

SUMMARY

Inflammation is implicated in several aspects of acute ischaemic stroke. It remains to be established whether the inflammatory response is a truly independent risk factor in general, or whether specific anti-inflammatory interventions are beneficial either in prevention or acute treatment.

Metabolic imaging using PET

INTRODUCTION

There is growing evidence that myocardial metabolism plays a key role not only in ischaemic heart disease but also in a variety of diseases which involve myocardium globally, such as heart failure and diabetes mellitus. Understanding myocardial metabolism in such diseases helps to elucidate the pathophysiology and assists in making therapeutic decisions.

MEASUREMENT

As well as providing information on regional changes, PET can deliver quantitative information about both regional and global changes in metabolism. This capability of quantitative measurement is one of the major advantages of PET along with physiological positron tracers, especially relevant in evaluating diseases which involve the whole myocardium.

DISCUSSION

This review discusses major PET tracers for metabolic imaging and their clinical applications and contributions to research regarding ischaemic heart disease and other diseases such as heart failure and diabetic heart disease. Future applications of positron metabolic tracers for the detection of vulnerable plaque are also highlighted briefly.

Comparison of 99mTc-annexin A5 with 18F-FDG for the detection of atherosclerosis in ApoE-/- mice

PURPOSE

(99m)Tc-annexin A5, a marker of ongoing apoptosis, and (18)F-FDG, a marker of the increased metabolism of inflammatory cells, are supposed to be useful in the detection of metabolically active atheroma. This study reports a comparison of the intralesional distribution of these tracers in relation to lesion development in ApoE-/- mice.

METHODS

Male ApoE-/- mice (n = 12-14/group) were maintained on a Western-type diet after the age of 5 weeks. At 25 weeks, (99m)Tc-annexin A5 or (18)F-FDG was injected and the aortas were harvested for autoradiography (ARG) and Oil Red O staining. Regional radioactivity accumulation was compared in relation to the Oil Red O staining score (ranging from 0 to 3, a semiquantitative parameter for evaluating lesion development).

RESULTS

Both (99m)Tc-annexin A5 and (18)F-FDG showed preferential uptake into atherosclerotic lesions, with higher uptake levels for (18)F-FDG (mean, 56.07 %IDxkg/m(2)) than for (99m)Tc-annexin A5 (mean, 10.38 %IDxkg/m(2)). The regional uptake levels of each tracer correlated with the Oil Red O staining score (r = 0.65, p < 0.05 for (99m)Tc-annexin A5; r = 0.56, p < 0.05 for (18)F-FDG). The uptake ratios of advanced lesions (score >0.5) to early lesions (score <0.5) were significantly higher for (99m)Tc-annexin A5 than for (18)F-FDG (f = 4.73, p = 0.03).

CONCLUSION

Both (99m)Tc-annexin A5 and (18)F-FDG accumulate in atherosclerotic lesions and correlate with the severity of each lesion. The higher absolute uptake levels of (18)F-FDG may be advantageous for lesion detection, whereas the preferential uptake of (99m)Tc-annexin A5 in advanced lesions may be a useful indicator of late-stage lesions or vulnerable plaque transformation.

Noninvasive detection of macrophages using a nanoparticulate contrast agent for computed tomography

Sudden fibrous cap disruption of 'high-risk' atherosclerotic plaques can trigger the formation of an occlusive thrombus in coronary arteries, causing acute coronary syndromes. High-risk atherosclerotic plaques are characterized by their specific cellular and biological content (in particular, a high density of macrophages), rather than by their impact on the vessel lumen. Early identification of high-risk plaques may be useful for preventing ischemic events. One major hurdle in detecting high-risk atherosclerotic plaques in coronary arteries is the lack of an imaging modality that allows for the identification of atherosclerotic plaque composition with high spatial and temporal resolutions. Here we show that macrophages in atherosclerotic plaques of rabbits can be detected with a clinical X-ray computed tomography (CT) scanner after the intravenous injection of a contrast agent formed of iodinated nanoparticles dispersed with surfactant. This contrast agent may become an important adjunct to the clinical evaluation of coronary arteries with CT.

Vascular inflammation evaluated by [18F]-fluorodeoxyglucose positron emission tomography is associated with the metabolic syndrome

OBJECTIVES

We investigated factors for carotid artery inflammation by [(18)F]-fluorodeoxyglucose (FDG) positron emission tomography (PET).

BACKGROUND

Inflammation is present in some atherosclerotic plaques. The FDG-PET is capable of identifying and quantifying vascular inflammation within atherosclerotic plaques.

METHODS

The FDG-PET imaging was performed in 216 consecutive patients (63 +/- 9 years, men:women 147:69) for cancer screening. Vascular inflammation in carotid atherosclerosis was quantified by measuring the standardized uptake value (SUV) of FDG into the artery.

RESULTS

Multiple stepwise regression analysis revealed significant relationships between SUV and waist circumference (p < 0.001), hypertensive medication (p < 0.001), carotid intima-media thickness (p < 0.001), high-density lipoprotein cholesterol (p < 0.01, inversely), homeostasis model assessment of insulin resistance (p < 0.05), or high sensitivity C-reactive protein (p < 0.05). Age- and gender-adjusted SUV of FDG was significantly higher (p < 0.0001) in proportion to the accumulation of the number of the components of the metabolic syndrome. Thus, the metabolic syndrome was associated with increased FDG uptake in carotid atherosclerosis.

CONCLUSIONS

Our present study may suggest that the metabolic syndrome is associated with inflammation in carotid atherosclerosis. (Detection of Plaque Inflammation by Positron Emission Tomography (PET); http://www.clinicaltrials.gov/ct/show/NCT00114504; NCT00114504).

Dual cardiac-respiratory gated PET: implementation and results from a feasibility study

PURPOSE

Spatial resolution in myocardial imaging is impaired by both cardiac and respiratory motion owing to motional blurring. We investigated the feasibility of a dual cardiac-respiratory gated positron emission tomography (PET) acquisition using a clinical PET/computer tomography (CT) scanner. We describe its implementation and present results on the respiratory motion observed.

METHODS

The correlation between diaphragmatic excursion measured by real-time magnetic resonance imaging (MRI) and the expansion of the chest measured with an elastic belt was studied in six subjects. PET list mode acquisitions were then performed in 12 patients, six of them injected with 13N-ammonia and six with 18F-FDG. In parallel, the ECG and respiratory signals of the patients were recorded and the list mode file correspondingly sorted using a dual gated approach. Respiratory motion of the heart was quantified by measuring the displacement between the inspiratory and expiratory images in the diastolic phase by means of intensity-based non-rigid image registration.

RESULTS

The correlation between diaphragmatic excursion and expansion of the chest was excellent (R2=0.91), validating the ability of the elastic belt to provide an adequate respiratory trigger. Respiratory signals corresponding to the chest expansion showed a large inter-patient variability, requiring adapted algorithms in order to define suitable respiratory gates. Dual gated PET series were successfully acquired for both groups of patients, showing better resolved myocardial walls. The average respiratory motion of the heart measured by PET was 4.8 mm, with its largest component in the craniocaudal direction. Moreover, a deformation of the heart with respiration was observed, with the inferior wall moving significantly more than the anterior.

CONCLUSION

Dual gated cardiac PET studies were performed successfully and showed better resolved myocardial walls as compared with ungated acquisitions. The respiratory motion of the heart presented a significant elastic component and was of the same magnitude as the spatial resolution of current PET cameras.

The aging of the heart and blood vessels: a consideration of anatomy and physiology in the era of computed tomography, magnetic resonance imaging, and positron emission tomographic imaging methods with special consideration of atherogenesis

Physicians have long told their patients that the doctor's job is to help patients "get as old as they can." As physicians, we have been aided in this objective by many other scientists in other disciplines. The entity of aging and its related changes blends imperceptibly with a variety of age-related diseases. However, these entities do appear to be separate though interrelated. Curing disease is important and a goal that we all work toward to add years to life expectancy. Here, we consider aging as it affects the heart and great vessels and as it serves to influence and support, if not cause, age-related cardiac diseases. This relationship is drawn as cardiac mechanics, hemodynamics, perfusion, metabolism and innervation, anatomy, and pathophysiology are each considered. The effects of aging are presented in 2 sections related to the early and recent "spikes" in aging related information. The latter is largely based in recent developments in chemistry, genetic engineering, molecular biology and the new imaging methods. The purpose of this manuscript is to present these new imaging methods, especially PET, and their impact on the second "spike." This is emphasized particularly in the second half of this review. As a method of demonstrating these imaging tools and their finest potential application, we decided to "showcase" atherosclerosis as the age-related disease for which these methods have made their greatest impact, for which yet more is promised, and for which the influence on longevity is most obvious. The application of positron emission tomography and other imaging methods to the characterization and image identification of atherosclerotic plaques and particularly the "vulnerable" plaque is emphasized. Yet, even with the eradication of coronary disease, the potential for very long life would not be likely. Only with the identification and eradication of the causative factors of aging can this possibility have a chance of becoming reality.

Cardiac PET-CT

Integrated positron emission tomography computed tomography (PET/CT) scanners allow a true integration of the structure and function of the heart. Myocardial perfusion PET provides a high sensitivity (91%) and specificity (89%) for the diagnosis of obstructive coronary artery disease (CAD). But, as with single photon emission CT, relative perfusion PET often uncovers only the territory subtended by the most severe coronary stenosis, leading to underestimation of the extent of CAD. In contrast, quantitative PET provides a noninvasive assessment of myocardial blood flow and coronary flow reserve and improves detection of preclinical and multivessel coronary atherosclerosis. Similarly, CT coronary angiography is an accurate means to image the entire continuum of anatomic coronary atherosclerosis from nonobstructive to obstructive CAD. However, not all coronary stenoses are hemodynamically significant and <50% of the patients with obstructive CAD on CT angiography demonstrate stress induced perfusion defects. Stress PET data complement the anatomic information on the CT angiogram by providing instant readings about the ischemic burden of coronary stenoses. Thus, combined PET/CT may be potentially superior to CT angiography alone for the guiding revascularization decisions. Further, fusion of the PET and CT angiogram images allows identification of the culprit stenosis in patients presenting with chest pain. Finally, the advances in molecular imaging and image fusion may soon make noninvasive detection of vulnerable coronary plaques a clinical reality. In summary, integrated PET/CT is a powerful new noninvasive modality that offers the potential for refined diagnosis and management of the entire spectrum of coronary atherosclerosis.

Emerging approaches for imaging vulnerable plaques in patients

The diagnosis of vulnerable plaques, which have the propensity to develop atherothrombosis, remains an elusive goal in clinical medicine. The most accepted features of vulnerable plaques, such as a large lipid core, increased inflammatory milieu and thin fibrous caps, have been well characterized through pathological studies. The ability to image a vulnerable plaque in susceptible patients would theoretically result in useful prognostic information that can be used to either monitor or treat patients at risk more aggressively. Several invasive techniques, such as integrated backscatter, virtual histology, palpography, optical coherence tomography and thermal heterogeneity, have been validated ex vivo and are now being evaluated in clinical studies. Non-invasive techniques, such as nuclear imaging, show promise in identifying increased metabolic activity and characteristic features of vulnerable plaques in patients. Natural history and intervention studies will need to be performed to determine whether identifying and treating vulnerable plaques will lead to improved clinical outcomes.

99mTc-Annexin A5 for noninvasive characterization of atherosclerotic lesions: imaging and histological studies in myocardial infarction-prone Watanabe heritable hyperlipidemic rabbits

PURPOSE

Apoptosis is commonly observed in advanced atherosclerotic lesions. 99mTc-annexin A5 (99mTc-annexin V) has been proposed as a potential tracer for imaging apoptosis in atherosclerotic plaques. Accordingly, we determined the usefulness of 99mTc-annexin A5 as an atherosclerosis imaging tracer in a rabbit model (myocardial infarction-prone Watanabe heritable hyperlipidemic rabbits; WHHLMI rabbits) of spontaneous atherosclerosis.

METHODS

The WHHLMI and control rabbits were injected intravenously with 99mTc-annexin A5. After in vivo planar imaging, the radioactivity in the aorta was measured. Autoradiography, TUNEL staining, Azan-Mallory staining and immunohistological studies were performed serially throughout the aorta.

RESULTS

99mTc-Annexin A5 accumulation in the aorta of the WHHLMI rabbits was 5.6-fold higher than in that of control rabbits. Autoradiography showed heterogeneous multifocal accumulation of 99mTc-annexin A5 in WHHLMI rabbits. 99mTc-Annexin A5 accumulation was highest in the atheromatous lesions (6.2+/-2.5, %IDxBW/mm2x10(3)), followed in decreasing order by neointimal (4.9+/-1.3), fibroatheromatous (4.5+/-1.9), and collagen-rich lesions (3.3+/-1.4). The regional 99mTc-annexin A5 accumulation was significantly correlated with the TUNEL-positive cell density, macrophage density and "vulnerability index," an index of the morphological destabilized characteristics. The in vivo imaging clearly visualized the atherosclerotic lesions in WHHLMI rabbits.

CONCLUSION

The present study in WHHLMI rabbits showed higher 99mTc-annexin A5 accumulation in grade IV atheroma than in other more stable lesions. 99mTc-Annexin A5 may be useful in identifying atheroma that is at higher risk for rupture and possibly in assessing the response to anti-atherosclerotic therapy.

Molecular cardiovascular imaging using scintigraphic methods

Molecular cardiovascular imaging plays an increasingly important role both in basic research and in clinical diagnosis. Scintigraphic methods have long been used to study pathophysiological changes on a cellular and molecular level, and they are likely to remain important molecular imaging modalities in the foreseeable future. This article provides an overview over current developments in cardiovascular molecular imaging using scintigraphic methods. The focus lies on imaging of cardiac innervation, plaque instability, hypoxia and angiogenesis, gene expression and stem and progenitor cell migration and proliferation.

Diagnostic and clinical perspectives of fusion imaging in cardiology: is the total greater than the sum of its parts?

Positron emission tomography, cardiovascular magnetic resonance and multislice computed tomography have contributed to changing our pathophysiological understanding of many conditions. Clinically, they have provided new tools for the identification of preclinical disease and a better understanding of how disease progresses. The application of these imaging modalities to preclinical disease and the use of these techniques in patients with overt cardiovascular disease are reviewed.

Plaque stabilization: Can we turn theory into evidence?

Coronary events often result from thrombi that form because of physical disruption of the atherosclerotic plaque. The dynamic nature of the plaque offers the opportunity to intervene to modify plaque biology with lifestyle changes and, if needed, pharmacologic measures. Inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase (statins) reduce levels of serum cholesterol and decrease the incidence of coronary events, but some of the benefits of statins may not depend on their effects on circulating lipids. Indeed, increasing evidence suggests that statins may also enhance plaque stability. Such evidence includes results of preclinical studies with experimental atherosclerosis as well as imaging data and analyses of proinflammatory and prothrombotic mediators in clinical trials. Currently, however, no studies have demonstrated conclusively the mechanisms underlying the unexpected magnitude and rapidity of statin benefits. This article reviews the evolution of the concept of plaque stabilization and reexamines the evidence for the role of statins in that process.

La tomographie d'émission de positons en médecine interne : applications actuelles et perspectives d'avenir

PURPOSE

Fluorodeoxyglucose positron emission tomography (FDG-PET) is a promising imaging technique that has already proven effective in modifying patient care in oncology. Fluorodeoxyglucose still remains the main radiopharmaceutical agent routinely used for PET imaging. A growing interest has recently lead to broaden PET research on benign disorders. The field of inflammatory or immune diseases and globally the field of internal medicine could also be impacted by FDG-PET.

MAIN POINTS

Great vessels vasculitides and fever of unknown origin have both been studied by several teams and could become indications for PET. In addition, current indications now extend to paraneoplastic syndromes. It is thus possible to foresee that the clinical applications for PET will continue to expand in these patients.

PERSPECTIVES AND PROJECTS

In the future, inflammatory arthritis, chronic inflammatory bowel diseases, systemic erythematous lupus, histiocytosis, or pulmonary and retroperitoneal fibrosis might benefit from PET even if, available data remains scarce to this day. Although PET will probably alter the landscape of patient management in internal medicine in the near future, additional clinical research is still needed to ascertain the exact role of PET.

Quantitative assessment of the atherosclerotic burden of the aorta by combined FDG-PET and CT image analysis: a new concept

INTRODUCTION

Our objective was to develop a technique for quantifying the extent of atherosclerosis in the aorta by combining standard uptake values (SUVs) in the aortic wall with volumetric data provided by computed tomography (CT).

METHODS

Eighteen patients who had both 18-fluorodeoxyglucose (FDG)-positron emission tomography (PET) and contrast-enhanced CT of the chest and abdomen were selected. All had homogeneous diffuse FDG wall uptake in four segments of the aorta. We divided the patients into three groups according to their age and measured FDG uptake in all four segments of the aorta by calculating the mean SUV for each segment. On each axial CT image, region-of-interest tracings along the inner and outer wall contours of the aorta were generated. The inner surface area was subtracted from the outer surface area. The net area values for each segment were subsequently multiplied by slice thickness to calculate arterial wall volume. By multiplying SUV with wall volume, we were able to calculate the atherosclerotic burden (AB) for each segment of the aorta. We compared the aortic wall volumes, SUVs and AB values in each arterial segment for each age group.

RESULTS

In each aortic wall segment, AB values, SUVs and wall volumes increased with age (P<.05).

CONCLUSION

AB can be used as an indicator of the extent of the atherosclerotic process in the aorta through the use of both metabolic and morphologic data provided by FDG-PET and CT, respectively. This may allow for the optimal screening, diagnosis and management of patients with this common and potentially lethal disorder.

In vivo 18F-fluorodeoxyglucose positron emission tomography imaging provides a noninvasive measure of carotid plaque inflammation in patients

OBJECTIVES

Given the importance of inflammation in atherosclerosis, we sought to determine if atherosclerotic plaque inflammation could be measured noninvasively in humans using positron emission tomography (PET).

BACKGROUND

Earlier PET studies using fluorodeoxyglucose (FDG) demonstrated increased FDG uptake in atherosclerotic plaques. Here we tested the ability of FDG-PET to measure carotid plaque inflammation in patients who subsequently underwent carotid endarterectomy (CEA).

METHODS

Seventeen patients with severe carotid stenoses underwent FDG-PET imaging 3 h after FDG administration (13 to 25 mCi), after which carotid plaque FDG uptake was determined as the ratio of plaque to blood activity (target to background ratio, TBR). Less than 1 month after imaging, subjects underwent CEA, after which carotid specimens were processed to identify macrophages (staining with anti-CD68 antibodies).

RESULTS

There was a significant correlation between the PET signal from the carotid plaques and the macrophage staining from the corresponding histologic sections (r = 0.70; p < 0.0001). When mean FDG uptake (mean TBR) was compared with mean inflammation (mean percentage CD68 staining) for each of the 17 patients, the correlation was even stronger (r = 0.85; p < 0.0001). Fluorodeoxyglucose uptake did not correlate with plaque area, plaque thickness, or area of smooth muscle cell staining.

CONCLUSIONS

We established that FDG-PET imaging can be used to assess the severity of inflammation in carotid plaques in patients. If subsequent natural history studies link increased FDG-PET activity in carotid arteries with clinical events, this noninvasive measure could be used to identify a subset of patients with carotid atherosclerosis in need of intensified medical therapy or carotid artery intervention to prevent stroke.

Simvastatin attenuates plaque inflammation: evaluation by fluorodeoxyglucose positron emission tomography

OBJECTIVES

We investigated whether simvastatin attenuates plaque inflammation by using 18F-fluorodeoxyglucose positron emission tomography (18FDG-PET) co-registered with computerized tomography.

BACKGROUND

Inflammation plays a key role in progression and destabilization of atherosclerotic plaque. 18F-fluorodeoxyglucose PET is a promising tool for visualizing inflammation of atherosclerotic plaque. Antiinflammatory action is one of the pleiotropic effects of statins.

METHODS

Forty-three consecutive subjects, who underwent 18FDG-PET for cancer screening and had 18FDG uptakes in the thoracic aorta and/or the carotid arteries, were randomized to either statin group receiving simvastatin (n = 21) or diet group receiving dietary management only (n = 22). The maximum standardized uptake values (SUVs) were measured in individual plaques, and were averaged for analysis of the subjectwise results. The responses were assessed after 3-month treatments.

RESULTS

Positron emission tomography revealed 117 and 123 18FDG-positive plaques in the statin and diet groups, respectively. Simvastatin, but not diet alone, attenuated plaque (18)FDG uptakes and decreased the SUVs (p < 0.01). Simvastatin reduced low-density lipoprotein cholesterol (LDL-C) by 30% (p < 0.01) and increased high-density lipoprotein cholesterol (HDL-C) by 15% (p < 0.01), whereas LDL-C and HDL-C levels were not changed in the diet group. In the statin group, the decrease in the SUV was well correlated with the HDL-C elevation (p < 0.01) but not with the LDL-C reduction.

CONCLUSIONS

18F-fluorodeoxyglucose PET visualized plaque inflammation and simvastatin attenuated it. The LDL-C-independent effects of simvastatin may participate in the beneficial effect. 18F-fluorodeoxyglucose PET has a potential for visually monitoring plaque inflammation and the therapeutic effectiveness of statins.

Detection of inflamed atherosclerotic lesions with diadenosine-5',5'''-P1,P4-tetraphosphate (Ap4A) and positron-emission tomography

Diadenosine-5',5'''-P(1),P(4)-tetraphosphate (Ap(4)A) and its analog P(2),P(3)-monochloromethylene diadenosine-5',5'''-P(1),P(4)-tetraphosphate (AppCHClppA) are competitive inhibitors of adenosine diphosphate-induced platelet aggregation, which plays a central role in arterial thrombosis and plaque formation. In this study, we evaluate the imaging capabilities of positron-emission tomography (PET) with P(2),P(3)-[(18)F]monofluoromethylene diadenosine-5',5'''-P(1),P(4)-tetraphosphate ([(18)F]AppCHFppA) to detect atherosclerotic lesions in male New Zealand White rabbits. Three to six months after balloon injury to the aorta, the rabbits were injected with [(18)F]AppCHFppA, and microPET imaging showed rapid accumulation of this radiopharmaceutical in the atherosclerotic abdominal aorta, with lesions clearly visible 30 min after injection. Computed tomographic images were coregistered with PET images to improve delineation of aortoiliac tracer activity. Plaque macrophage density, quantified by immunostaining with RAM11 against rabbit macrophages, correlated with PET measurements of [(18)F]AppCHFppA uptake (r = 0.87, P < 0.0001), whereas smooth-muscle cell density, quantified by immunostaining with 1A4 against smooth muscle actin, did not. Biodistribution studies of [(18)F]AppCHFppA in normal rats indicated typical adenosine dinucleotide behavior with insignificant myocardial uptake and fast kidney clearance. The accumulation of [(18)F]AppCHFppA in macrophage-rich atherosclerotic plaques can be quantified noninvasively with PET. Hence, [(18)F]AppCHFppA holds promise for the noninvasive characterization of vascular inflammation.

Functional Imaging of Carotid Atheromatous Plaques

Atherosclerotic plaque rupture within the internal carotid artery is an important cause of transient ischemic attack (TIA) and stroke. Conventional imaging techniques such as ultrasound and angiography provide information about the structural consequences of such plaques in terms of luminal stenosis. Most clinical trials of carotid surgery and stenting and based on these imaging methods. Techniques aimed at imaging the biological 'functional' status of the plaque are now emerging. Most of these are based on the premise that inflammatory activity is an index of plaque stability. In this article we review potential imaging targets from the known molecular biological pathways of atherosclerosis. Both conventional imaging techniques and the newer methods are discussed. Recent data from position emission tomography (PET) imaging and from the use of superparamagnetic iron oxide particles with magnetic resonance (MR) imaging are shown.

Nuclear Cardiology: Present and Future

Nuclear cardiology has made significant advances since the first reports of planar scintigraphy for the evaluation of left ventricular perfusion and function. While the current "state of the art" of gated myocardial perfusion single-photon emission computed tomographic (SPECT) imaging offers invaluable diagnostic and prognostic information for the evaluation of patients with suspected or known coronary artery disease (CAD), advances in the cellular and molecular biology of the cardiovascular system have helped to usher in a new modality in nuclear cardiology, namely, molecular imaging. In this review, we will discuss the current state of the art in nuclear cardiology, which includes SPECT and positron emission tomographic evaluation of myocardial perfusion, evaluation of left ventricular function by gated myocardial perfusion SPECT and gated blood pool SPECT, and the evaluation of myocardial viability with PET and SPECT methods. In addition, we will discuss the future of nuclear cardiology and the role that molecular imaging will play in the early detection of CAD at the level of the vulnerable plaque, the evaluation of cardiac remodeling, and monitoring of important new therapies including gene therapy and stem cell therapy.

Non-specific binding of [18F]FDG to calcifications in atherosclerotic plaques: experimental study of mouse and human arteries

PURPOSE

[(18)F]FDG has been used as an inflammation marker and shown to accumulate in inflammatory atherosclerotic plaques. The aim of this study was to investigate the uptake and location of [(18)F]FDG in atherosclerotic plaque compartments.

METHODS

The biodistribution of intravenously administered [(18)F]FDG was analysed in atherosclerotic LDLR/ApoB48 mice (n=11) and control mice (n=9). Digital autoradiography was used to detect the ex vivo distribution in frozen aortic sections. In vitro binding of [(18)F]FDG in human atherosclerotic arteries was also examined.

RESULTS

The uptake of [(18)F]FDG was significantly higher in the aorta of atherosclerotic mice as compared with the control mice. Autoradiography of excised arteries showed higher [(18)F]FDG uptake in the plaques than in the healthy vessel wall (mean ratio +/-SD 2.7+/-1.1). The uptake of [(18)F]FDG in the necrotic, calcified sites of the advanced atherosclerotic lesions was 6.2+/-3.2 times higher than that in the healthy vessel wall. The in vitro studies of human arterial sections showed marked binding of [(18)F]FDG to the calcifications but not to other structures of the artery wall.

CONCLUSION

In agreement with previous studies, we observed [(18)F]FDG uptake in atherosclerotic plaques. However, prominent non-specific binding to calcified structures was found. This finding warrants further studies to clarify the significance of this non-specific binding in human plaques in vivo.

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.

Non-invasive imaging of atherosclerotic plaque macrophage in a rabbit model with F-18 FDG PET: a histopathological correlation

Background

Coronary atherosclerosis and its thrombotic complications are the major cause of mortality and morbidity throughout the industrialized world. Thrombosis on disrupted atherosclerotic plaques plays a key role in the onset of acute coronary syndromes. Macrophages density is one of the most critical compositions of plaque in both plaque vulnerability and thrombogenicity upon rupture. It has been shown that macrophages have a high uptake of ¹⁸F-FDG (FDG). We studied the correlation of FDG uptake with histopathological macrophage accumulation in atherosclerotic plaques in a rabbit model.

Methods

Atherosclerosis was induced in rabbits (n = 6) by a combination of atherogenic diet and balloon denudation of the aorta. PET imaging was performed at baseline and 2 months after atherogenic diet and coregistered with magnetic resonance (MR) imaging. Normal (n = 3) rabbits served as controls. FDG uptake by the thoracic aorta was expressed as concentration (μCi/ml) and the ratio of aortic uptake-to-blood radioactivity. FDG uptake and RAM-11 antibody positive areas were analyzed in descending aorta.

Results

Atherosclerotic aortas showed significantly higher uptake of FDG than normal aortas. The correlation of aortic FDG uptake with macrophage areas assessed by histopathology was statistically significant although it was not high (r = 0.48, p < 0.0001). When uptake was expressed as the ratio of aortic uptake-to-blood activity, it correlated better (r = 0.80, p < 0.0001) with the macrophage areas, due to the correction for residual blood FDG activity.

Conclusion

PET FDG activity correlated with macrophage content within aortic atherosclerosis. This imaging approach might serve as a useful non-invasive imaging technique and potentially permit monitoring of relative changes in inflammation within the atherosclerotic lesion.

Radionuclide imaging for the detection of inflammation in vulnerable plaques

Imaging of atheromatous plaques has traditionally centered on assessing the degree of luminal stenosis. More recently it has become clear that the vulnerable atherosclerotic plaques responsible for the majority of life-threatening syndromes are characterized by high numbers of inflammatory cells and proteins. This has highlighted the urgent need for suitable imaging techniques that can identify and quantify levels of inflammation within atheromatous lesions. Positron emission tomography and single-photon emission computed tomography imaging hold promise in this regard. Tracer compounds capable of assessing macrophage recruitment, foam cell generation, matrix metalloproteinase production, macrophage apoptosis, and macrophage metabolism have been developed and tested in the carotid and peripheral circulation. The identification of inflamed lesions within the coronary circulation, however, remains elusive owing to small plaque size, cardiac and respiratory motion, and lack of a suitable specific nuclear tracer.