[en] Objective: To investigate the feasibility of ultrasmall superparamagnetic iron oxide- enhanced (USPIO)-enhanced MR imaging for monitoring synovitis of antigen-induced arthritis in rabbit model and explore the optimal MR imaging sequences. Methods: Nine female white rabbits with antigen (0.5 ml mBSA, 2 mg/ml) induced arthritis of the right knees were used in the study. The left knees of these rabbits and both knees of another 3 rabbits served as the control. Nine to 28 days (mean 21.3 d) after successful model induction, all knees were imaged before and 24 h after intravenously injection of USPIO (0.3 ml/kg), among which 2 rabbits were also imaged at 48 and 72 h after administration of USPIO respectively. The MR protocol included spin-echo (SE) T₁WI, fast spin-echo (FSE) T₂WI, gradient echo (GRE) T₂^*WI and short tau inversion recovery (STIR). Images were analyzed quantitatively and qualitatively based on signal characteristics and patterns of the synovium. Paired t-test was used for the analysis of the signal intensity of inflammatory synovial membrane before and 24 h after injection of USPIO. MR findings were correlated with histopathology. Results: Arthritis was successfully induced in all 9 right knees with intraarticular injection of mBSA. Pathological examination revealed hyperplasia of synovium with infiltration of USPIO-loaded-macrophages. MR depicted synovial thickening (thickness 2.07±0.97 mm) and joint effusion. Synovium and joint fluid appeared as slightly hypo- or iso-intense on T₁WI and hyper- intense on T₂WI or T₂^*WI. Twenty four hours after USPIO injection, significant T₁ enhancement (ΔSNR 41.91%±27.94%), negative T₂ and T₂^* enhancement (SNR-34.92%±11.77% and -57.24%±16.05%) were demonstrated in the region of synovial inflammation respectively. The signal at 48 h and 72 h changed less than that at hour 24. No signs of arthritis occurred in all left knees and in all knees of the artificial model group. Conclusion: Iron oxide phagocytized into macrophages can be a root cause resulted in signal change on USPIO-enhanced MR images. The gradient echo sequence should be the optimal sequence to be used in USPIO-enhanced MR imaging in antigen-induced arthritis. (authors)