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Background
Rheumatologic diseases are a heterogeneous group of conditions that affect joints, bones, cartilage, tendons, ligaments, and muscles. Inflammatory arthritis is a subset of rheumatologic disease that affects 2 - 3% of the population, involves joints and the tendon/ligamentous complex, and can result in significant morbidity for patients. These diseases are often imaged radiographically; however, differentiating between their overlapping presentations may be challenging, especially when imaging a single joint. In comparison, fluorine-18 fluorodeoxyglucose (FDG) PET/CT and bone scintigraphy allow evaluation of the whole-body, with PET/CT exquisitely sensitive to both articular and extraarticular manifestations of rheumatologic disease, allowing both diagnosis and monitoring of therapy.

Educational Goals / Teaching Points
The basic pathophysiology, clinical symptomatology, imaging appearance, and treatment strategies of the inflammatory arthritides will be reviewed. A series of unknown cases with associated discussion will be presented. After this presentation, the viewer will be able to describe the various imaging appearances of inflammatory arthritis on FDG PET/CT and bone scintigraphy and discuss the utility of nuclear medicine for imaging inflammatory arthropathies.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The pathophysiology and main categories of inflammatory arthritides will be presented, with attention to uptake patterns and differentiating these diseases from one another. The pathophysiology of these conditions will be used to help explain their variable distributions and imaging patterns. Each of these disease entities will be discussed individually, focusing on their appearance on both FDG PET/CT and bone scintigraphy with ancillary diagnostic imaging included when applicable. Unknown cases will include rheumatoid arthritis, seronegative spondyloarthritis, ankylosing spondylosis, psoriatic arthritis, SAPHO (with clinical photographs), and chronic recurrent multifocal osteomyelitis, along with related cases of polymyalgia rheumatica and immune checkpoint-inhibitor-related arthritis.

Conclusion
Inflammatory arthritis is an important category of rheumatologic disease that is often imaged radiographically; however, FDG PET/CT and bone scintigraphy may provide additional useful information regarding both diagnosis and treatment monitoring. Familiarity with the appearance of inflammatory arthritides on FDG PET/CT and bone scintigraphy will aid in both diagnosing these conditions and identifying incidental arthritis on oncologic examinations.