FAPI PET in the Diagnosis and Management of Inflammatory Diseases

Fibroblast activation protein (FAP) is a serine protease overexpressed in activated fibroblasts during tissue remodeling, chronic inflammation, and fibrosis. While initially developed as a target for cancer imaging, FAP inhibitor (FAPI)-based PET imaging has shown increasing promise in a broad range of non-neoplastic inflammatory diseases. This review provides a comprehensive summary of recent advances in the application of FAPI PET/CT and PET/MR in inflammatory conditions, including immune-mediated inflammatory arthritis, thyroid-associated disorders, cardiovascular inflammatory disorders, immunoglobulin G4-related disease (IgG4-RD), infectious diseases, and others. Across these disease categories, FAPI PET demonstrates high target-to-background contrast and enables sensitive detection of fibroblast activation, which correlates with inflammatory and fibrotic burden. In conditions such as rheumatoid arthritis, Takayasu arteritis, IgG4-RD, and Crohn disease, FAPI PET has shown added diagnostic value over 18F-FDG PET, particularly in evaluating disease activity, subclinical involvement, and fibrotic changes. Furthermore, its whole-body capability facilitates accurate lesion localization, biopsy site selection, and dynamic treatment monitoring. Despite encouraging results, further research is warranted to address tracer heterogeneity, refine disease-specific imaging protocols, and validate its utility through large-scale prospective trials. Overall, FAPI PET is an emerging molecular imaging modality with significant potential for personalized diagnosis, disease stratification, and clinical decision-making in inflammation-driven conditions.