Background/Purpose: The current gold standard for detection of disease progression in ankylosing spondylitis (AS) is conventional radiography. The most reliable method mSASSS includes the anterior parts of the cervical and lumbar spine. However, sensitivity to change is limited (1). MRI, that visualizes the total spine tomographically, may therefore improve detection of new bone formation. The aim of this study was to develop standardized definitions for new bone formation on MRI, to test the reliability of their detection in patients with AS, to compare this reliability with radiography, and to determine whether availability of radiographs enhances the reliability of detection on MRI.

Methods: The Canada-Denmark MRI working group developed standardized consensus based definitions for bone spurs and ankylosis observed on sagittal images of T1-weighted MRI scans. The definitions included lesions at anterior and posterior vertebral corners as well as non-corner lesions in the disc space and lesions in the lateral segments. A reference image set was generated that included examples of all these lesions as well as variations in normal anatomy. In the MRI-based score, bone spurs and ankylosis were assigned a score of 2 and 3, respectively. In exercise 1, reliability for status and change scores for lesions were assessed on baseline and 2 year scans in 55 patients with AS by 3 readers scoring in known time sequence. Discrepant scans were reviewed extensively using radiography as a reference. In exercise 2, baseline/2 year pairs of radiographs and MRIs of 25 patients with AS (numbered independently) were assessed in 3 reads. Read 1: Radiographs were assessed for syndesmophytes and ankylosis; Read 2: MRI scans were assessed for new bone; Read 3: Simultaneous assessment of radiographs and MRI scans. Reliability was assessed by intra-class correlation coefficient (ICC).

Results: ICC for 3 readers reading MRI scans in the first exercise were 0.79 and 0.23 for baseline status and 2 year change scores, respectively. In exercise 2, radiography was superior to MRI in reliably detecting new bone (Table). Simultaneous availability of radiographs enhanced the reliability of detecting new bone in the C spine by MRI but this was still inferior to radiography. ICC for detection of new bone in the thoracic spine by MRI was 0.48 and 0.36 for baseline status and 2-year change scores, respectively.

Conclusion: Standardization of MRI features, scoring methodology, and calibration of expert readers with radiography failed to show any major advantage of MRI over radiography in the reliable detection of new bone in the cervical and lumbar spine of patients with AS. Future efforts should focus on the methodology for assessment of the thoracic spine.

Reference: Wanders AJ et al. AR 2004

Table

Intra-class correlation coefficient (ICC) for detection of bone spurs and ankylosis in exercise 2