PURPOSE: To evaluate whether ultrasmall superparamagnetic iron particles (USPIO)-enhanced MRI is capable of assessing both synovial perfusion characteristics and the presence of synovial macrophages in a model of antigen-induced arthritis.
MATERIALS AND METHODS: Unilateral arthritis was induced in six knees of six rabbits. The contralateral knees of the rabbits served as control knees. After onset of arthritis, all 12 knees were scanned prior to and immediately following intravenous administration of USPIO using a multiphase T1-weighted (T1w) fast gradient-echo (FGRE) sequence, and T1w spin-echo (SE), T2-weighted (T2w) FSE, T2*w GRE, and short-tau inversion recovery (STIR) sequences prior to and 24 hours following USPIO administration. SI-vs.-time curves (STCs) and the early enhancement rate during the first 56 seconds (REE(56)) were calculated from SI measurements within the synovial tissue of all knees on dynamic T1w images. MR findings were correlated to histopathology.
RESULTS: REE(56) was significantly higher in the synovial tissue of arthritic knees than in the control knees (P < 0.01). Significant T1-, T2-, and T2* effects (P = 0.03-0.04) and multiple synovial vessels were visually detectable within the arthritic synovial tissue 24 hours after administration of USPIO, whereas no signal changes or synovial vessels were seen in the control knees. Histopathology revealed widened synovial blood vessels in the arthritic knees, and confirmed iron uptake by macrophages in the arthritic knees.
CONCLUSION: USPIO-enhanced MRI is capable of both assessing synovial hyperperfusion and detecting macrophages in antigen-induced arthritis in rabbits.