OBJECTIVE To compare a low field dedicated extremity magnetic resonance imaging system (E-MRI) with x ray and clinical examination, in the detection of inflammation and erosive lesions in wrist and metacarpophalangeal (MCP) joints in newly diagnosed, untreated rheumatoid arthritis (RA).

PATIENTS AND METHODS Twenty five patients (disease duration ⩽1 year) and three healthy controls entered the study. Anx ray examination and MRI (before and after intravenous injection of a contrast agent) of the 2nd–5th MCP joints and the wrist was performed. The number of erosions onx ray examination and MRI was calculated, and synovitis in the MCP joints and wrists was graded semiquantitatively.

RESULTS E-MRI detected 57 bone erosions, whereas only six erosions were disclosed byx ray examination (ratio 9.5:1). Synovial hypertrophy grades were significantly higher in RA joints with clinical signs of joint inflammation—that is, swelling and/or tenderness (median 3, 5th–95th centile 1–4) than without these clinical signs (median 2, 5th–95th centile 1–3), p<0.001. 51% of the joints without clinical signs of synovitis showed synovial hypertrophy on E-MRI. There was a positive correlation between MRI scores of synovitis and the number of erosions detected by MRI in the MCP joints (Spearmanr _s=0.31, p<0.01). No healthy controls had erosions or synovitis on MRI.

CONCLUSION Joint destruction starts very early in RA and E-MRI allows detailed evaluation of inflammatory and destructive changes in wrists and MCP joints in patients with incipient RA.