Signal changes in the intervertebral discs on MRI of the thoracolumbar spine in ankylosing spondylitis.

Contrary to standard teaching in magnetic resonance imaging (MRI), recent reports have documented calcification appearing as areas of increased signal intensity (SI) on T1-weighted images. Intervertebral disc calcification is a frequent finding on radiographs in chronic ankylosing spondylitis (AS). This study was performed to investigate the appearance of variable degrees of disc calcification in MRI. Thirty-six patients with AS of variable duration underwent an MRI scan of the thoraco-lumbar spine and the MR appearances, particularly of the discs, were compared with plain radiographs. Increased SI of the discs on T1-weighted images were found in 17 of 36 patients, occurring over a range of disc levels, and correlating with disc calcification on the radiographs in 78% of cases. This group tended to be older and have a longer duration of disease than those with normal appearing discs. Four different patterns of increased signal within the discs termed Type A (marginal), Type B (annular), Type C (central) and Type D (solid) were identified. In those with less than six discs involved Type A was the most common pattern. In those with more than six discs involved Type D was the most common pattern, suggesting a progression of disc involvement with more advanced disease. Although these findings will not affect the management of the disease, they do highlight the recently described phenomenon of calcification appearing as increased SI on T1-weighted images, likely to be related to the surface area of the calcium crystal. This should lead to the consideration of calcium in the differential diagnosis of increased SI on T1-weighted images. End-plate marrow changes were a relatively frequent finding in this study but did not correlate with the signal changes seen within the discs; in a number of cases they related to variable degrees of bony bridging.