[Volumes of synovial membrane and joint effusion determined by MR imaging. Markers of severity and/or activity in rheumatoid arthritis?].

Volumes of synovial membrane and joint effusion were determined by MRI in 36 knees with gonarthritis and five healthy knees. In 18 knees MRI was performed before and immediately after arthrocentesis. The difference between MRI-determined and syringe-determined volumes of aspirated joint fluid was 0-7 ml, median 2 ml, corresponding to 0-18%, median 7%, of the pre-aspiration effusion volume. Synovial membrane volumes, determined before and after arthrocentesis varied 0-10 ml, median 3 ml (0-17%, median 7%). No significant systematic misinterpretation of the borderline between joint fluid and synovium was found. The interobserver variation was < 15% in all measurements of synovial volumes > 10 ml and effusion volumes > 5 ml. Patient repositioning resulted in variations < 10%. The synovial volumes in clinically active, clinically inactive and healthy knees were statistically significantly different (median 79 ml, 21 ml and 3 ml, respectively). We conclude that effusion volumes, and in all probability also synovial membrane volumes, can be determined by MRI with a maximal error of approximately 20%. The synovial volume is related to the inflammatory activity of the joint. The results encourage further studies of the value of effusion and synovial membrane volumes as markers of the activity and/or severity of joint inflammation.