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

F-18 Fluorodeoxyglucose Uptake in Atherosclerotic Plaque in the Mediastinum Mimicking Malignancy

Atherosclerosis is an inflammatory process accumulating numerous inflammatory cells such as macrophages and lymphocytes. This cellular infiltrate has a high metabolic demand, which can be reflected on F-18 FDG positron emission tomographic (PET) imaging. The FDG uptake in atherosclerotic vessels is usually distinguishable as a result of its linear and continuous appearance and mild level of activity. However, occasionally, atherosclerotic plaques present themselves as focal "hot spots," which can be easily misinterpreted as malignancy. This report emphasizes atherosclerotic plaques as a potential pitfall while interpreting FDG scans. It also highlights the importance of radiographic correlation to avoid such an error.

Molecular imaging in nuclear cardiology

State-of-the-art techniques have been used to measure key aspects of cardiovascular pathophysiology from the birth of radionuclide cardiovascular imaging. However, during the last 30 years, there have been few innovative imaging advances to further our understanding of the complex physiologic processes. Molecular imaging now offers an array of tools to develop advanced diagnostic approaches and therapies for patients with coronary artery disease and heart failure. For example, the enhanced understanding of the pathophysiology of atheroma makes it possible to identify vulnerable plaque based on its metabolic signature or the presence of excessive apoptosis. Because the metabolic and apoptotic signals are large, it is likely that even small lesions will be visible. Of the many approaches that are being developed, 2 tracers appear most likely to be tested in the near future: (1) [18F]-fluorodeoxyglucose, to determine macrophage metabolism; and (2) radiolabeled annexin, to measure apoptosis of the inflammatory cells. Using existing techniques such as perfusion imaging, appropriate patients can be selected for treatment with novel therapies, such as stem cell transplantation or vascular gene therapy. Using positron tomography in place of single photon imaging adds the capability for the measurement of absolute perfusion and perfusion reserve to the information on regional perfusion. Flow reserve detects global decreases in perfusion and refines the determination of lesion severity available from perfusion imaging.

Fluorodeoxyglucose Uptake in the Aortic Wall at PET/CT: Possible Finding for Active Atherosclerosis

PURPOSE

To evaluate fluorine 18 fluorodeoxyglucose (FDG) uptake in the thoracic aortic wall at combined positron emission tomography (PET)/computed tomography (CT) and compare uptake with aortic wall calcification.

MATERIALS AND METHODS

Records of 85 consecutive cancer patients who underwent FDG PET/CT were evaluated retrospectively. One hour after FDG injection, CT followed by PET was performed from ear to middle of the thigh. CT, PET, and fused PET/CT images were generated. FDG uptake and calcification were evaluated visually and semiquantitatively. FDG uptake was graded according to intensity; calcification, according to thickness. Unpaired t test was used for comparison of patient age with and without FDG uptake and with and without calcification. The relationship between the score (sum of grades along all aortic segments) of positive FDG uptake and calcification and patient age was analyzed with Spearman rank correlation. Comparison of frequency of FDG uptake and calcification with age, sex, risk factors for cardiovascular disease (CVD), or history of CVD was performed with chi2 analysis.

RESULTS

Fifty patients had at least one area of FDG uptake in thoracic aortic wall, 14 of whom showed focal FDG uptake. Intermediate to intense FDG uptake tended to be observed in the descending aorta. Forty-five patients had at least one measurable aortic calcification. Thick calcification was observed most often at the aortic arch. Twelve patients had 13 uptake areas at the calcification site. Patients with positive findings were on average older (P <.05 for both increased uptake and calcification); the older patient group had higher frequency of both aortic wall uptake (P <.005) and calcification (P <.001). The calcification score correlated with age (rho = 0.60, P <.001) but the FDG uptake score did not. Women, patients with hyperlipidemia, and patients with history of CVD tended to show increased FDG uptake (P =.073,.080, and.068, respectively), whereas patients with diabetes had significantly more calcifications (P <.05).

CONCLUSION

PET/CT depicted FDG uptake commonly in the thoracic aortic wall. The FDG uptake site was mostly distinct from the calcification site and may possibly be located in areas of metabolic activity of atherosclerotic changes.

Human cellular inflammation in the pathology of acute cerebral ischaemia

Leucocytes form important effector pathways for inflammation. This article reviews the clinical evidence for the presence of a cellular inflammatory response in cerebral ischaemia, and attempts to define its temporal profile and spatial distribution. The processes involved in recruitment and activation of leucocytes in this context are addressed, and the successes and failures of interventions aimed at these processes discussed.

Mapping of ATP, glucose, glycogen, and lactate concentrations within the arterial wall

OBJECTIVE

In large- and medium-sized arteries, the diffusion distances for oxygen and nutrients are long. This has been suggested to make these vessels prone to develop local energy metabolic deficiencies that could contribute to atherogenesis. To validate this hypothesis, we introduced a new method to measure energy metabolites within the arterial wall at high spatial resolution.

METHODS AND RESULTS

Bioluminescence imaging was used to quantify local metabolite concentrations in cryosections of snap frozen (in vivo) and incubated pig carotid artery rings. Incubation at hypoxia resulted in increased lactate concentrations, whereas ATP, glucose, and glycogen concentrations were decreased, especially in the mid media, where concentrations of these metabolites were close to zero. In snap frozen arteries, glycogen concentrations were markedly higher in deep layers of the media than toward the lumen. ATP, glucose, and lactate were more homogenously distributed.

CONCLUSIONS

Bioluminescence imaging is a new and powerful tool to assess arterial wall energy metabolism at high spatial resolution. Our experiments demonstrate heterogeneous distributions of energy metabolites under hypoxic in vitro conditions. Furthermore, we show that glycogen concentrations are higher in deep medial layers in vivo. This might represent a local adaptation to a low-oxygen microenvironment.

Imaging of carotid artery disease: from luminology to function?

There have been tremendous advances in our ability to image atheromatous disease, particularly in the carotid artery, which is accessible and large enough to image. The repertoire of methodology available is growing, giving anatomical information on luminal narrowing which is approaching the level at which conventional carotid angiography will become very uncommon as CT and contrast-enhanced MR angiographic techniques become the norm. More exciting is the tentative ability to perform functional plaque imaging addressing enhancement patterns and macrophage activity using MR or positron-emission tomography techniques. These techniques, once rigorously evaluated, may, in addition to complex mathematical modelling of plaque, eventually allow us to assess true plaque risk. Time will best judge whether we will be able to move from the use of simple luminology to assessment of plaque function.

Towards understanding acute destabilization of vulnerable atherosclerotic plaques

BACKGROUND

The current wisdom is that destabilization of human atheromatous fibroinflammatory plaques may result in thrombosis and is responsible for most acute ischemic syndromes. This paradigm has led to vigorous research to understand the pathogenesis of plaque vulnerability and subsequent rupture, to find reliable systemic serological markers and to identify imaging techniques in order to determine vulnerability of individual plaques.

METHODS

Research examining the pathobiology of the vulnerable plaque and its subsequent destabilization is described. Investigations are based on the current understanding of vascular cell and molecular biology and clinical paradigms of acute coronary syndromes.

RESULTS

It is apparent that there are three steps that need to be considered. These are transformation of a stable plaque into a vulnerable plaque, destabilization of a vulnerable plaque and regulation of the complications following destabilization, the most serious being acute occlusive thrombosis. In vitro cell and molecular vascular biology studies, and animal model studies that alter specific gene(s) expression, have provided new knowledge on putative mechanisms leading to plaque vulnerability and on subsequent destabilization of the plaque. These studies show that several local and systemic factors, including inflammation, matrix disruption, lipid deposition, cell necrosis and apoptosis are likely to play a role in vulnerability, destabilization and clinical syndromes.

CONCLUSION

Plaque vulnerability and destabilization is of multifactoral etiology with inflammation, cap matrix and necrotic lipid core remodeling being important pathobiological processes associated with vulnerability and destabilization. Identifying gene-environment interactions, improving imaging techniques and improving our understanding of the mechanisms underlining plaque pathogenesis via animal models are essential elements for understanding human plaque vulnerability and destabilization.

Clinical biomarkers in drug discovery and development

Biomarkers enable the characterization of patient populations and quantitation of the extent to which new drugs reach intended targets, alter proposed pathophysiological mechanisms and achieve clinical outcomes. In genomics, the biomarker challenge is to identify unique molecular signatures in complex biological mixtures that can be unambiguously correlated to biological events in order to validate novel drug targets and predict drug response. Biomarkers can stratify patient populations or quantify drug benefit in primary prevention or disease-modification studies in poorly served areas such as neurodegeneration and cancer. Clinically useful biomarkers are required to inform regulatory and therapeutic decision making regarding candidate drugs and their indications in order to help bring new medicines to the right patients faster than they are today.