Atherosclerotic plaque imaging by PET/CT; can inactive, active and mixed plaques be discerned?

Abstract

Atherosclerosis is one of the leading causes ofmorbidity and mortality in the world. Rupture ofatherosclerotic plaques and formation of thrombi arethe primary mechanisms of myocardial infarction orcerebrovascularaccidents.Atherosclerosisisthereforea dynamic inflammatory disorder and the biologicalcomposition and inflammatory state of an atheroscle-rotic plaque, rather than the degree of stenosis or itssize, are the major determinants of acute clinicalevents [1, 2]. Noninvasive techniques to detectvulnerable atherosclerotic plaque are criticallyneeded. In recent years, radionuclide imaging [3–6],magnetic resonance imaging (MRI) [7–13], and com-puted tomography (CT) have been put forward as themost relevant imaging modalities to identify athero-sclerotic vascular disease and the vulnerable plaque[14–30]. Positron emission tomography with fluoro-deoxyglucose (FDG-PET) has become one of theadvanced imaging approaches for the detection ofvulnerable plaques [31]. Ogawa et al. [32] showed thatmacrophages are responsible for the accumulation ofFDG in atherosclerotic lesions whereby FDG uptake iscorrelated with macrophage density. Because vulner-able plaques are rich in macrophages, FDG imagingshould be useful for selective detection of suchplaques. In recent years, multislice CT has been shownto provide excellent anatomical and morphologicalimages both of the large vessels and the coronaryarteries [18]. In addition, differences in plaque com-position and distribution—both in acute coronarysyndromes and stable coronary artery disease—canadequately be visualized with multislice-CT [29, 30].Hybrid imaging with both PET and CT, being acombined functional and structural whole-body imagingmodality, could therefore offer the optimal potentialto characterize plaque composition [33, 34]. Tatsumiet al. [35] showed in 85 patients who underwent FDGPET/CT imaging that FDG uptake was commonlydepicted in the thoracic aortic wall. The FDG uptakesite was mostly distinct from the calcification site andwas located in areas of metabolic activity (i.e.,macrophages) of atherosclerotic changes. Ben-Haimet al. [36] evaluated FDG uptake in atheroscleroticplaques in 50 asymptomatic patients. Repeat PET/CTstudies were performed to assess changes in patternsof FDG uptake and CT calcifications. FDG-avid focirepresent a dynamic process of transient inflamma-tion, whereas CT calcifications indicate stableatherosclerosis. Hybrid imaging with PET/CT createstherefore incremental value over both imagingmodalities alone. Rudd et al. [37] showed thatatherosclerosis imaging with FDG-PET/CT wasuseful for tracking inflammation within plaques.