Table 4

Mean percentage of false positive controls and of principal MRI lesions responsible for misclassification, and mean number (range) of single MRI lesions on sacroiliac joint MRI and spine MRI

CohortA (n=62)B (n=88)
Groupnr-axSpAASNSBPHCnr-axSpAASNSBP
Number of subjects1991420312433
SIJ MRI +: n (%)14.5 (76.3)9.0 (100.0)1.0 (7.1)1.5 (7.5)14.3 (46.0)24.0 (100.0)5.3 (15.9)
Most important SIJ lesion: %
 BMO69.033.350.0100.035.113.552.4
 FI00005.36.314.3
 ER31.055.650.0056.159.428.6
 NBF011.1003.520.84.8
Mean number of SIJ quadrants (range)
 BMO21.1 (0–52)21.5 (0–52)1.3 (0–3)*4.7 (3–7)*8.0 (0–39)11.4 (0–42)5.8 (0–21)
 FI6.5 (0–30)22.6 (0–51)1.0 (0–4)1.5 (0–9)16.0 (0–49)23.6 (0–57)7.5 (0–29)*
 ER5.6 (0–26)16.1 (0–38)2.3 (0–5)0 (0)5.7 (0–27)12.2 (0–38)2.4 (0–8)*
 NBF0 (0)1.9 (0–20)0 (0)0 (0)0.2 (0–8)3.3 (0–31)0.1 (0–2)
Spine MRI +: n (%)9.8 (51.3)6.5 (72.2)4.8 (33.9)5.3 (26.3)15.0 (48.4)17.5 (72.9)9.3 (28.0)
Most important spine lesion: %
 BMO79.576.952.652.423.347.116.2
 FI17.923.126.323.866.745.762.2
 ER2.6004.85.002.7
 NBF0021.119.05.07.118.9
Mean number of spine lesions (range)
 BMO9.2 (0–30)15.5 (0–51)2.8 (1–6)*4.8 (0–29)*5.9 (0–52)9.4 (0–36)2.4 (0–12)*
 FI8.2 (0–37)12.2 (0–43)4.7 (0–14)4.0 (0–12)13.3 (0–61)25.7 (0–124)8.6 (0–27)
 CIL6.3 (0–18)8.8 (0–22)2.2 (0–5)*3.9 (0–18)*3.8 (0–26)5.4 (0–22)1.8 (0–12)
 CFL5.9 (0–22)8.8 (0–31)3.9 (0–14)3.7 (0–12)10.8 (0–34)18.9 (0–55)7.1 (0–18)
Combined MRI +: n (%)17.0 (89.5)9.0 (100.0)4.5 (32.1)5.0 (25.0)21.0 (67.7)23.8 (99.0)8.0 (24.2)
Decision based on SIJ MRI only: %42.633.35.645.028.627.434.4
Decision based on spine MRI only: %13.2038.940.026.2037.5
Decision based on SIJ and spine MRI: %44.166.755.615.045.272.628.1
Most important SIJ lesion: %
 BMO70.652.861.165.033.314.737.5
 FI02.8006.03.23.1
 ER13.233.30032.163.221.9
 NBF011.1003.618.90
 No relevant SIJ lesion16.2038.935.025.0037.5
Most important spine lesion: %
 BMO32.422.261.150.021.424.221.9
 FI11.85.633.3032.121.137.5
 ER000001.10
 NBF1.50015.01.24.20
 No relevant spine lesion54.472.25.635.045.249.540.6
  • MRI data are the mean numbers (percentage or range) over four readers.

  • *p Value ≤0.05 (Wilcoxon test) for NSBP versus nr-axSpA and HC versus nr-axSpA for cohort A and cohort B separately.

  • AS, ankylosing spondylitis; BMO, Bone marrow oedema; CIL, corner inflammatory lesion (in central spinal MRI slices); CFL, corner fatty lesion (in central spinal MRI slices); ER, erosion; FI, fat infiltration; HC, healthy control; NBF, new bone formation; nr-axSpA, non-radiographic axial spondyloarthritis; NSBP, non-specific back pain; SIJ, sacroiliac joint.