Table 4.

Diagnostic utility of fat infiltration of the SIJ in AS versus NSBP patients: mean sensitivity and specificity over 4 readers for cohort A/B (in parentheses for cohort A: AS vs NSBP and HC combined).

VariableSensitivitySpecificityPositive LRNegative LR
FI per se
  FI in ≥ 1 SIJ quadrant0.70 (0.70)/0.800.72 (0.74)/0.772.48 (2.71)/3.510.42 (0.40)/0.26
  FI in ≥ 2 SIJ quadrants10.70 (0.70)/0.800.73 (0.77)/0.782.60 (3.06)/3.630.41 (0.39)/0.26
FI with additional MRI lesions
  FI w/o BME or erosion0.03 (0.03)/0.070.82 (0.84)/0.920.14 (0.16)/0.881.19 (1.16)/1.01
  FI w/BME0.58 (0.58)/0.610.91 (0.91)/0.866.41 (6.46)/4.220.47 (0.47)/0.46
  FI w/erosion0.68 (0.68)/0.670.98 (0.99)/0.8935.10 (53.10)/6.310.33 (0.33)/0.37
Morphological features of FI
  FI w/distinct border0.53 (0.53)/0.490.97 (0.98)/0.9020.47 (30.98)/5.020.49 (0.48)/0.56
  FI w/distinct border + BME0.40 (0.40)/0.360.99 (0.99)/0.9131.20 (47.20)/3.920.61 (0.61)/0.71
  FI w/distinct border + erosion0.50 (0.50)/0.401.00 (1.00)/0.95NC (NC)/7.610.50 (0.50)/0.63
  Homogeneous FI0.58 (0.58)/0.580.97 (0.97)/0.9317.94 (22.62)/8.480.44 (0.44)/0.45
  Homogeneous FI + BME0.45 (0.45)/0.420.99 (0.99)/0.9335.10 (35.40)/6.140.56 (0.56)/0.62
  Homogeneous FI + erosion0.55 (0.55)/0.461.00 (1.00)/0.95NC (NC)/8.750.45 (0.45)/0.57
  Subchondral FI0.70 (0.70)/0.750.85 (0.86)/0.834.55 (5.01)/4.330.35 (0.35)/0.30
  Subchondral FI + BME0.58 (0.58)/0.580.94 (0.94)/0.868.97 (9.69)/4.220.45 (0.45)/0.49
  Subchondral FI + erosion0.68 (0.68)/0.640.99 (0.99)/0.8952.65 (79.65)/6.010.33 (0.33)/0.41
  FI with ≥ 1 feature20.73 (0.73)/0.750.83 (0.84)/0.814.19 (4.62)/3.990.33 (0.33)/0.30
  FI with ≥ 1 feature + BME0.60 (0.60)/0.580.93 (0.93)/0.868.51 (8.85)/4.000.43 (0.43)/0.50
  FI with ≥ 1 feature + erosion0.70 (0.70)/0.640.99 (0.99)/0.8954.60 (82.60)/6.010.30 (0.30)/0.41
  FI with ≥ 2 features20.58 (0.58)/0.590.97 (0.98)/0.9222.42 (33.93)/7.060.44 (0.43)/0.45
  FI with ≥ 2 features + BME0.45 (0.45)/0.430.99 (0.99)/0.9235.10 (53.10)/5.150.56 (0.55)/0.62
  FI with ≥ 2 features + erosion0.55 (0.55)/0.471.00 (1.00)/0.95NC (NC)/8.940.45 (0.45)/0.56
Anatomical distribution of FI
  ≥ 1 FI in all 4 sacral quadrants0.45 (0.45)/0.580.90 (0.91)/0.924.39 (4.83)/6.910.61 (0.61)/0.46
  ≥ 1 FI in all 4 iliac quadrants0.35 (0.35)/0.420.95 (0.96)/0.956.82 (9.18)/7.890.69 (0.68)/0.61
  ≥ 1 FI in all 4 upper SIJ quadrants0.43 (0.43)/0.420.94 (0.95)/0.967.37 (9.12)/11.040.61 (0.60)/0.60
  ≥ 1 FI in all 4 lower SIJ quadrants0.43 (0.43)/0.570.93 (0.95)/0.946.03 (8.36)/9.330.62 (0.61)/0.46
  • Differences in positive or negative LR despite identical mean sensitivity and specificity data result from rounding differences.

  • 1 FI in ≥ 2 SIJ quadrants on the same slice or in ≥ 1 SIJ quadrant on ≥ 2 adjacent slices.

  • 2 Morphological features of FI: FI with distinct border, homogeneous FI, subchondral FI. AS: ankylosing spondylitis; BME: bone marrow edema; FI: fat infiltration; HC: healthy control; LR: likelihood ratio; MRI: magnetic resonance imaging; NC: not calculable; NSBP: nonspecific back pain; SIJ: sacroiliac joint; SpA: spondyloarthritis.