Comparing the diagnostic utility of sacroiliac spectral CT and MRI in axial spondyloarthritis

Objective: To compare the clinical value of sacroiliac spectral CT and MRI in diagnosing axial spondyloarthritis (SpA). Methods: 137 patients with low back pain and suspected axial SpA were recruited. 76 patients were diagnosed with axial SpA, and 49 patients were diagnosed with nonspecific low back pain (nLBP). Each patient underwent spectral CT and MRI examinations of the sacroiliac joints. Water-and calcium-based material decomposition images were reconstructed for quantitative analysis. The marrow-to-muscle ratios of water and calcium concentrations and short tau inversion recovery (STIR) signal intensity, as well as Hounsfield units in the ilium and sacrum were compared between nLBP and patients with axial SpA. Results: Spectral CT is comparable with MRI for the detection of bone marrow oedema, and it is superior to MRI for detection of osseous sclerosis and erosions. MRI is superior to spectral CT in detecting enthesitis and synovitis. There were statistically significant differences in STIR signal intensity, water concentration and calcium concentration ratios as well as CT values between nLBP and patients with axial SpA (p<0.05) in the ilium. There was a statistically significant but weak correlation between ratios of water concentration and STIR signal intensity in both the ilium and sacrum (p<0.05). Overall, the iliac water concentration was most sensitive for detection of patients with SpA. The positive likelihood ratio of the STIR signal intensity ratio was the highest. The diagnostic odds ratio of the calcium concentration ratio was the highest, and its negative likelihood ratio was the lowest. Conclusion: Spectral CT not only depicts findings of chronic sacroiliitis (i.e. bone erosion and sclerosis), but also can detect and quantify the extent of marrow oedema in patients with SpA with activity sacroiliitis. The sensitivity of MRI for diagnosis of early synovitis and enthesitis remains superior. The combination of spectral CT and MRI may thus improve diagnostic accuracy in the diagnosis of axial SpA. Advances in knowledge: Spectral CT can measure both calcium and water concentration of the sacroiliac joints. The combination of spectral CT and MRI may thus improve diagnostic accuracy in the diagnosis of axial SpA.