Background:The differentiation between rheumatoid arthritis (RA) and psoriatic arthritis (PsA) is sometimes a challenge for rheumatologists in daily clinical practice. Imaging techniques such as MRI could be a helpful tool for this purpose.Objectives:To examine the value of 3 Tesla (T) magnetic resonance imaging (MRI) with a high-resolution 16-channel hand coil for the differentiation between RA and PsA.Methods:A total of 17 patients with active PsA and 27 patients with active RA were evaluated by 3T MRI. Images were analyzed by three readers according to the outcome measures for RA clinical trials (OMERACT) and RA and PsA MRI scores for the presence and intensity of the following MRI features: synovitis, flexor tenosynovitis, bone edema, bone erosion, periarticular inflammation, bone proliferation, and joint space narrowing. A receiver operating characteristics (ROC) curve was established for a calculated prediction model comprising age, gender, and the imaging features ‘periarticular inflammation’ and ‘erosion’ of the metacarpophalangeal (MCP) joint of the 5th finger.Results:PsA could be differentiated from RA by extracapsular inflammatory changes (PsAMRIS sub-score ‘periarticular inflammation’), with a minimal odds ratio (OR) for the outcome ‘not RA’ of 0.06 (p< 0.01) at all MCP joints. The calculated ROC curve had an area under the curve (AUC) of 98.1%.Conclusion:3T MRI showed a strong association of extracapsular inflammatory changes with PsA at the MCP joint level, and consequently allowed differentiation between PsA and RA.Figure 1.Receiver operating characteristics (ROC) curve with different thresholds for the calculated prediction model for the outcome RA. Area under the curve (AUC) = 98.1%.Figure 2.51-year-old female patient with PsA. MR images show flexor tenosynovitis (FS), synovitis (Syn), and periarticular inflammation (PI). A. Sagittal PD fat-saturation of D5. PI at the volar and dorsal aspects at the MCP, PIP, and DIP levels. FS at the PIP and DIP joint levels. Black asterisks indicate PI. Black arrow points to FS. B. Coronal STIR with bone edema (BE) at the proximal portion of PIP3 and 5 accompanied by PI at PIP3 and MCP, PIP and DIP5. Asterisks indicate BE. Arrowheads point to PI. C. Transversal T2 fat-saturation with FS and PI at MCP5. Arrowhead indicates FS, arrow points to volar PI. D. Transversal T1 fat-saturation following iv contrast, with FS and PI at MCP5. Arrowhead indicates FS, arrows points to volar PI.Disclosure of Interests:Philipp Sewerin Grant/research support from: AbbVie Deutschland GmbH & Co. KGBristol-Myers Squibb Celgene GmbHLilly Deutschland GmbHNovartis Pharma GmbH Pfizer Deutschland GmbHRheumazentrum Rhein-Ruhr, Consultant of: AMGEN GmbH AbbVie Deutschland GmbH & Co. KG Biogen GmbHBristol-Myers Squibb Celgene GmbH Chugai Pharma arketing Ltd. / Chugai Europe GmbHHexal Pharma Janssen-CilagGmbH Johnson & Johnson Deutschland GmbHLilly Deutschland GmbH / Lilly Europe / Lilly Global Novartis Pharma GmbH Pfizer Deutschland GmbH Roche Pharma Rheumazentrum Rhein-Ruhr Sanofi-Genzyme Deutschland GmbH Swedish Orphan Biovitrum GmbH UCB Pharma GmbH, Speakers bureau: AMGEN GmbH AbbVie Deutschland GmbH & Co. KG Biogen GmbHBristol-Myers Squibb Celgene GmbH Chugai Pharma arketing Ltd. / Chugai Europe GmbHHexal Pharma Janssen-CilagGmbH Johnson & Johnson Deutschland GmbHLilly Deutschland GmbH / Lilly Europe / Lilly Global Novartis Pharma GmbH Pfizer Deutschland GmbH Roche Pharma Rheumazentrum Rhein-Ruhr Sanofi-Genzyme Deutschland GmbH Swedish Orphan Biovitrum GmbH UCB Pharma GmbH, Daniel Abrar: None declared, Alexander Lautwein: None declared, Stefan Vordenbäumen: None declared, Ralph Brinks: None declared, Christine Goertz: None declared, Miriam Frenken: None declared, Matthias Schneider Grant/research support from: GSK, UCB, Abbvie, Consultant of: Abbvie, Alexion, Astra Zeneca, BMS, Boehringer Ingelheim, Gilead, Lilly, Sanofi, UCB, Speakers bureau: Abbvie, Astra Zeneca, BMS, Chugai, GSK, Lilly, Pfizer, Sanofi, Benedikt Ostendorf: None declared, Christoph Schleich: None declared