PET/CT in inflammatory joint disorders: update and future trends

Inflammatory joint disorders are highly prevalent in most countries and represent a huge physical burden to the patients as well as major costs to health systems. Clinical evaluation is the basis for a correct diagnosis and appropriate treatment but in some indications, additional imaging may be necessary for a more precise assessment before and following therapy. Functional imaging using positron emission tomography combined with computerized tomography (PET/CT) offers high sensitivity for this purpose even in clinically silent disease. [¹⁸F]-FDG PET/CT is widely available and currently the mostly used technique, especially in rheumatoid arthritis and polymyalgia rheumatica and similar entities in elderly patients. However, [¹⁸F]-FDG lacks specificity and alternative tracers have been developed during the last years. They include macrophage targeting using the translocator protein (TSPO) or folate receptors as well as the more recently accessible ligands of the fibroblast-activation proteins (FAPIs). In some disorders, such as spondyloarthropathies and psoriatic arthritis, the bone-seeking agent ¹⁸F-sodium fluoride is more performant as these disorders involve mainly bone remodelling and formation rather than inflammation.