ABSTRACT
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) T_1WI,fast spin-echo(FSE)T_2WI,gradient echo (GRE)T_2~* 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 T_1 WI and hyper- intense on T_2 WI or T_2~* WI.Twenty four hours after USPIO injection,significant T_1 enhancement(ASNR 41.91%?27.94%),negative T_2 and T_2~* 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.