Imaging of Peripheral Arthritis: Special Focus on Differences in Inflammatory Lesions Between Rheumatoid Arthritis and Psoriatic Arthritis.

Differentiating rheumatoid arthritis (RA) and psoriatic arthritis (PsA) remains challenging, particularly when clinical and serological markers are inconclusive. Imaging provides critical insights, with MRI and dual-energy CT iodine maps highlighting key distinctions. Both conditions share inflammatory features such as capsular synovitis, tenosynovitis, and bone marrow edema. However, periarticular inflammation is often a strong indicator of PsA. This reflects their differing inflammatory targets: RA primarily involves the synovium, whereas PsA targets the enthesis. This distinction contributes to the broader bone marrow edema seen in PsA and explains inflammatory changes at the distal interphalangeal joint and dactylitis, which are characteristic of PsA but not RA. Recognizing these inflammatory patterns and distributions is essential for accurate diagnosis and treatment guidance.