Functional imaging in extrapulmonary sarcoidosis: FDG-PET/CT and MR features.

Sarcoidosis is a multiorgan granulomatous disease of unknown etiology that primarily involves the lungs and the lymphatic system. Extrapulmonary sarcoidosis is common, occurring in 30 to 50% of patients. In this review, we describe and illustrate the role of F-FDG PET/CT and MR imaging in patients with extrapulmonary sarcoidosis. FDG-PET/CT and MR can improve the accuracy of the diagnosis of extrapulmonary involvement, specify the respective contributions of active and fibrotic components of lesions, guide the selection of the biopsy site, provide prognostic information, and guide therapeutic management. We focus on suggestive patterns that help to improve lesion characterization, especially when these lesions are clinically occult. In cardiac sarcoidosis, the combined use of FDG-PET/CT and cardiac MR may provide optimal detection of the disease by enabling the differentiation between patients with active granulomatous inflammation and those with fibrous lesions. In cases with central nervous system involvement, the T2 hypointensity of the dural and parenchymal lesions is helpful for identifying sarcoidosis. Granulomatous bone marrow infiltration in the axial skeleton can be sensitively detected by both FDG-PET/CT and MR. Muscular sarcoidosis can have a characteristic appearance with the "dark star" sign on MR and a thick linear FDG uptake that predominantly involves the lower legs, designated as the "tiger man" sign. Extrathoracic lymphadenopathy is commonly observed on FDG-PET/CT imaging; however, its features are not specific, and the differentiation of extrathoracic lymphadenopathy from metastatic disease, tuberculosis, or lymphoma may be difficult. Familiarity with the functional imaging features in extrapulmonary sarcoidosis in various anatomical locations plays a crucial role in the diagnosis and management of patients.