Apparent Diffusion Coefficient as an Imaging Biomarker for Spinal Disease Activity in Axial Spondyloarthritis

Background A quantifiable imaging measure to gauge the intensity of individual inflammatory lesions in axial spondyloarthritis (SpA) has not been well established. Previous studies have shown that diffusion-weighted (DW) MRI reflects disease activity in axial SpA. Purpose To determine the association between apparent diffusion coefficient (ADC) at MRI of discovertebral lesions and disease activity in individuals with axial SpA. Materials and Methods In this prospective study, 243 study participants (mean age ± standard deviation, 43.2 years ± 13.5) with back pain who fulfilled the Assessment of SpondyloArthritis International Society criteria for SpA were recruited from four rheumatology centers between April 2014 and March 2018. There were 132 men (mean age, 41.4 years ± 13.3) and 111 women (mean age, 45.3 years ± 13.4). Clinical, biochemical, and radiologic parameters were collected. All participants underwent whole-spine MRI by using a short inversion time inversion-recovery sequence and DW imaging. Two independent readers identified the presence of discovertebral lesions. ADCs were measured and normalized with normal bone marrow. Regression analysis was performed to determine association between the mean, maximum, and normalized mean and maximum ADCs of the discovertebral lesions and disease activity and functional parameters (Bath Ankylosing Spondylitis Disease Activity Index [BASDAI], Bath Ankylosing Spondylitis Functional Index [BASFI], and Bath Ankylosing Spondylitis Global Index [BASGI]). Results Ninety-one discovertebral lesions (five cervical, 61 thoracic, 25 lumbar) were present in 55 of the 243 study participants (22.6%). After adjusting for confounding factors, increased maximum ADC was independently associated with increased BASFI (regression coefficient [β] = 1.94 [×10^-3 mm²/sec], P = .04). Increased normalized maximum ADC was independently associated with BASDAI question 2 (ie, back pain score) (β = 0.45, P = .01), mean stiffness score (β = 0.41, P = .04), and BASGI (β = 0.43, P = .04). Increased normalized mean ADC was independently associated with BASDAI question 2 (β = 0.61, P = .04). Conclusion Apparent diffusion coefficients at MRI of discovertebral lesions were associated with disease activity, functional impairment, and patient global assessment in axial spondyloarthritis. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Guermazi and Roemer in this issue.