The adapted mSASSS results for the other reader are similar (see online supplementary figure S1). We next examined the association of changes in computed syndesmophyte measurements with the main readers ratings of changes on CT scan (physique 4). CT volume measure (1.84) and the CT height measure (1.22) than either the MRI measure (0.50) or radiography (0.29). Conclusions CT-based syndesmophytes measurements experienced very good longitudinal validity and better sensitivity to change than radiography or MRI. This method shows promise for longitudinal clinical studies of syndesmophyte development and growth. Ankylosing spondylitis (AS) is an inflammatory arthritis affecting primarily the sacroiliac joints and spine.1 Growth of syndesmophytes at the intervertebral disk space (IDS) is a characteristic feature of AS. Because syndesmophytes represent progressive irreversible structural damage and are more easily detected than changes in the facet or sacroiliac joints, monitoring of their development has been a central focus of many studies. Studies of the pathogenesis of AS have tested associations of biomarkers and FSCN1 genetic polymorphisms with the extent and size of syndesmophytes.2C8 Similarly, vertebral inflammation as seen on MRI has been examined for associations with the development of new syndesmophytes.9C12 The impact of tumour necrosis factor- inhibitors around the progression of syndesmophytes has been investigated, with implications for understanding the role of cytokines in the pathogenesis of AS as well as for clinical care.13C15 These studies used plain RS-1 radiographs and semi-quantitative ratings as the method to detect and score syndesmophytes. The main limitations of this methodology are a result of the use of a two-dimensional (2D) technique to assess a 3D structure, with problems of projection, penetration and overlying shadows, resulting in poor visualisation of syndesmophytes. Semiquantitative rating methods also have limited sensitivity to change.16,17 These problems are accentuated when the goal is to detect syndesmophyte growth, because growth is typically slow. Possibly as a result of these issues, much research has been inconclusive. Whether tumour necrosis factor- antagonists influence spinal fusion remains unresolved.13C15,18 Despite several studies, the relationship between inflammation and syndesmophyte development was recently characterised as enigmatic.19 Similarly, the search for biomarkers has produced few strong predictors of syndesmophyte growth. With the aim of improving the assessment of syndesmophyte growth, we developed a computer algorithm measuring syndesmophytes on lumbar spine CT scans.20,21 The algorithm exploits the complete 3D information of CT scans and assesses syndesmophytes along the entire vertebral rim in a fully quantitative way. The method has very good reliability and cross-sectional validity.22 In this study, we assessed the longitudinal validity of the algorithm over 2 years, and compared its sensitivity to change to that of the modified Stoke AS Spine Score (mSASSS) and an MRI-based measure of chronic spine damage. METHODS Patients We enrolled patients at the National Institutes of Health and Johns Hopkins Medical Institutions in this prospective longitudinal study. Inclusion criteria were age 18 years or older, diagnosis of AS by the modified New York criteria,23 and a Bath AS Radiology Index (BASRI) Lumbar Spine Score of 0, 1, 2, or 3 (ie, excluding patients with completely fused lumbar spines).24 We ensured representation of patients with different degrees of structural damage by enrolling at least five patients in each BASRI category. We excluded patients who were pregnant or experienced contraindications to MRI. The RS-1 study protocol was approved by the institutional review boards of both centres, and all patients provided written knowledgeable consent. CT scanning Patients were scanned at baseline, 12 months 1 and 12 months 2. They were scanned on either a Philips Brilliance 64 (slice thickness 1.5 mm) or a GE Lightspeed Ultra scanner (slice thickness 1.25 mm). For both scanners, voltage and current parameters were 120 kVp and 300 mAs respectively. Patients were scanned from T10 to L4, providing 4 IDSs for processing: T11CT12, T12CL1, L1CL2, L2CL3. Radiography and MRI scanning Radiographs of the lumbar spine were taken at RS-1 baseline, 12 months 1 and 12 months 2. Patients underwent lumbar spine MRI scans at baseline and 12 months 1 on either a 1.5 T Signa Excite (GE) or.