The MRI View of Synovitis and Tenosynovitis in Inflammatory Arthritis

MRI scanning is the current gold standard modality for imaging synovitis and tenosynovitis in patients with inflammatory arthritis. Inflamed synovial membrane within the joints and investing tendon sheaths appears thickened on T1-weighted sequences and enhances postcontrast. On T2-weighted sequences, synovitis and synovial effusions typically show a high signal. Studies have shown correlations between the degree of inflammation and vascularity of synovium obtained at biopsy and postcontrast enhancement on matching dynamic MRI scans. Scoring systems have been devised that are based on quantifying synovial membrane thickening and signal intensity on static postcontrast scans and have been validated in multireader settings. Moderate to high reliability has been demonstrated with trained readers and quantification of synovitis in this way is being used increasingly as an outcome measure in clinical trials to assess response to therapy. MRI-observed synovitis is almost invariable in those with active rheumatoid arthritis, but recent studies have also demonstrated its presence in patients in clinical remission, emphasizing the sensitivity of this technique and the importance of subclinical joint inflammation. MRI-observed synovitis has been validated against other imaging modalities, including power Doppler ultrasound, and has also been investigated in normal subjects (where mild enhancement can rarely occur). Studies over 1-2 years have suggested that MRI synovial membrane volume and postcontrast enhancement on dynamic imaging can predict the development of erosions. In the long term, an overall score of inflammation incorporating synovitis, tenosynovitis, and bone edema may be a more useful MRI predictor of aggressive erosive disease.