What are the recommended investigations for diagnosing sacroilitis?

Recommended Investigations for Diagnosing Sacroiliitis

Conventional radiography of the sacroiliac joints should be the first imaging modality for the evaluation of suspected sacroiliitis, followed by MRI if radiographs are negative or equivocal but clinical suspicion remains high. 1

Initial Evaluation

First-Line Imaging

• Radiography of the sacroiliac joints: Standard anteroposterior view of the pelvis is recommended as the initial imaging modality 1

  • Demonstrates chronic erosions, sclerotic changes, and ankylosis as sequelae of inflammatory sacroiliitis 1
  • Limitations include low sensitivity (19%-72%) for early disease detection and fair to moderate interobserver agreement 1

• Complementary radiography of the spine: Should be performed alongside sacroiliac joint radiographs 1

  • At minimum, cervical and lumbar spine radiographs should be obtained 1
  • Thoracic spine radiographs are less useful due to overlying structures but may be obtained for specific symptoms 1

Special Considerations

• In young patients and those with short symptom duration (< 3 years), MRI of the sacroiliac joints may be considered as an alternative first imaging method since inflammatory changes can precede radiographic findings by 3-7 years 1

Second-Line Imaging (When Radiographs Are Negative/Equivocal)

MRI of Sacroiliac Joints

• Highest rated second-line investigation (rated 8/9 in appropriateness) 1

• Protocol should include:

  • T1-weighted sequences 1
  • Fat-suppressed fluid-sensitive sequences (T2-weighted fat-saturated or STIR images) 1
  • Both active inflammatory lesions (bone marrow edema, capsulitis, synovitis, enthesitis) and structural lesions (erosions, sclerosis, fat deposition, ankylosis) should be evaluated 1

• MRI without contrast is generally sufficient 1

  • Sensitivity of 79% and specificity of 89% for diagnosis of axial spondyloarthritis 1

• MRI with contrast may be considered in select cases 1

  • May improve detection of subtle inflammatory lesions and differentiation of synovitis from joint fluid 1
  • However, contrast has not been shown to significantly increase diagnostic accuracy 1

CT of Sacroiliac Joints

• Appropriate when MRI cannot be performed (rated 7/9 in appropriateness) 1
• Non-contrast CT is recommended 1
• Helpful for identifying subtle erosions and structural changes 1, 2
• Provides better assessment of complex anatomy of the SI joints 1
• Superior to conventional radiography in detecting sacroiliitis 2

Not Routinely Recommended Investigations

• MRI of the spine: Not generally recommended for initial diagnosis of axial spondyloarthritis 1

  • May be considered if sacroiliac joints are normal but clinical suspicion remains high 1

• Bone scintigraphy/SPECT: Not routinely recommended (rated 4/9 in appropriateness) 1

  • Low to moderate sensitivity and variable specificity 1
  • May help localize pain source but not helpful for establishing inflammatory diagnosis 1

• Ultrasound: Not recommended for diagnosis of sacroiliitis 1

• PET/CT: Not routinely obtained as initial imaging modality 1

  • Utility in diagnosis of axial spondyloarthritis remains uncertain 1

Common Pitfalls and Caveats

• Radiographs may miss more than half of patients with structural changes of axial spondyloarthritis when compared to CT 1
• Bone marrow edema on MRI can be seen in non-inflammatory conditions (degenerative changes, postpartum patients, chronic back pain, athletes) and in up to 30% of healthy controls 1
• Deep bone marrow edema lesions extending at least 1 cm deep to the articular surface are more specific for axial spondyloarthritis 1
• Interobserver agreement for radiographic findings of sacroiliitis is only fair to moderate 1
• Early sacroiliitis may be radiographically occult for 3-7 years after symptom onset 1