18F-FDG PET-CT Imaging in Sarcoidosis Molecular Mechanisms and Applications

Sarcoidosis is a complex disease with unknown etiology. Histologic hallmarks are nonnecrotizing and noncaseating granulomas, which usually affect the lungs but can grow widely. Computed tomography (CT) helps reveal typical features such as alveolar and ground-glass opacities and perivascular, subpleural nodules or active alveolitis, as well as atypical features such as bronchiectasis, pulmonary fibrosis and lung emphysema, and to differentiate with concomitant infection or neoplasms. Fluorine-18-fluorodeoxyglucose (18F-FDG) is a glucose derivative that is highly concentrated in areas with intense glucose metabolic and high inflammatory activity, since inflammatory cells such as macrophages, lymphocytes, neutrophils have an elevated metabolic activity of this compound and positron emission tomography/computed tomography (PET-CT) with FDG has proven to be a non-invasive technique which is useful to assess several infectious, inflammatory and neoplastic disorders, such as toxoplasmosis, prosthetic joint infections, spinal osteomyelitis, fever of unknown origin (FUO), vasculitis and sarcoid lesions in sarcoidosis. The molecular mechanisms of FDG PET-CT, its role in sarcoidosis and its recent advances are discussed in this review. Information about its role in assessing the disease activity, treatment response, prognosis and pitfalls are also reported.