Fig. 5.

Fifty-six-year-old patient with osteoarthritis. Radiography can easily be falsely judged as positive because osteophytes can mimic erosion and sclerosis (arrows) and even partial ankylosis (arrowheads) when bone proliferation is superimposed over the joint but is in fact external to the articular surface and is around the joint. On MRI, it may be harder to see the bridging osteophytes (arrow), especially compared to CT. Also, osteoarthritis can show small bone marrow lesions with fat deposition (arrowhead in T1) or edema (arrowhead in STIR), especially near osteophytes. In this case, the radiography is ‘false-positive’ but the MRI is ‘true-negative’ for sacroiliitis