Background Focal fat deposition is frequently seen on magnetic resonance imaging (MRI) of the spine in patients with ankylosing spondylitis (AS)1. Fat metaplasia in bone marrow is known as one of disease progression in AS2.
Objectives To evaluate clinical characteristics and the correlation between bone marrow fat-signal fraction (FF), Bath AnkylosingSpondylitis Disease Activity Index (BASDAI), Bath AnkylosingSpondylitis Functional Index (BASFI) and modified Stoke Ankylosing Spondylitis Spine Score (mSASSS).
Methods A total of 55 patients with AS who underwent a spine MRI at our institution from March 2015 to December 2015 were retrospectively evaluated. Quantification of vertebral bone marrow was assessed on the FF maps using the three-point Dixon technique3. FF was quantified after measuring the signal intensity within the region of interest (ROI) over anterosuperior and anteroinferior corner of bone marrow where there was no contamination from the bony cortex or syndesmophyte. The ROIs were drawn twice in sagittal FF map and average was used. mSASSSs were independently recorded by two musculoskeletal radiologists. Correlation, univariate and multivariate regression analyses were done for identifying significant predictors of FF.
Results Patients had a mean age of 35.6±10.5 years, and 43 were male (78.2%). A significant correlation was found between FF, mSASSS, BASDAI, and BASFI (p=0.005, p=0.021, and p=0.050)in Pearson correlation analysis. Multivariate regression analysis showed that decrease total BASDAI associated with to increase FF (p=0.005). Measurements of mSASSS had excellent intraobserver (mean intraclass correlation coefficient [ICC] = 0.997 [95% confidence interval (CI): 0.994–0.999] and interobserver agreement (mean ICC =0.941–0.995).However, erythrocyte sedimentation rate (ESR) and C reactive protein (CRP) did not correlated with FF.
Conclusions The quantitative assessment of FF using MRI might be another useful novel imaging technique to correlateactivity index of AS.
Chiowchanwisawakit, P., Lambert, R. G., Conner-Spady, B. & Maksymowych, W. P. Focal fat lesions at vertebral corners on magnetic resonance imaging predict the development of new syndesmophytes in ankylosing spondylitis. Arthritis and rheumatism, 2215–2225, doi:10.1002/art.30393 (2011).
Maksymowych, W. P., Wichuk, S., Chiowchanwisawakit, P., Lambert, R. G. & Pedersen, S. J. Fat metaplasia and backfill are key intermediaries in the development of sacroiliac joint ankylosis in patients with ankylosing spondylitis. Arthritis & rheumatology, 2958–2967, doi:10.1002/art.38792 (2014).
Eggers, H., Brendel, B., Duijndam, A. & Herigault, G. Dual-echo Dixon imaging with flexible choice of echo times. Magnetic resonance in medicine, 96–107, doi:10.1002/mrm.22578 (2011).
Disclosure of Interest None declared