What is the preferred imaging modality for diagnosing sacroiliitis?

Preferred Imaging Modality for Diagnosing Sacroiliitis

For diagnosing sacroiliitis, conventional radiography of the sacroiliac joints is recommended as the first-line imaging modality, with MRI being the preferred next step if radiographs are negative but clinical suspicion remains high. 1

Initial Imaging Approach

Conventional Radiography

• Radiographs of the pelvis are the established first-line imaging modality for evaluating suspected sacroiliitis 1
• Anteroposterior view of the whole pelvis is recommended to evaluate both sacroiliac joints and hips 1
• Radiographs demonstrate chronic structural changes:
  • Erosions
  • Sclerotic changes
  • Ankylosis

Limitations of Radiography

• Low sensitivity for early disease (19-72%) 1
• Limited specificity (47-84.5%) 1
• Fair to moderate interobserver agreement 1
• Unable to demonstrate active inflammation 1
• May miss more than half of patients with structural changes compared to CT 1

Second-Line Imaging

MRI of Sacroiliac Joints

• Recommended when radiographs are negative but clinical suspicion of axial spondyloarthritis remains high 1
• Can detect inflammatory changes 3-7 years before radiographic findings appear 1
• Should be considered as initial imaging in:
  • Young patients
  • Patients with short symptom duration 1
• MRI evaluation should include:
  • Active inflammatory lesions (primarily bone marrow edema)
  • Structural lesions (erosions, new bone formation, sclerosis, fat infiltration) 1, 2
• Recommended sequences include:
  • Coronal oblique T1-weighted and fluid-sensitive sequences
  • Perpendicular axial oblique sequence
  • Sequence for optimal evaluation of bone-cartilage interface 2

Other Imaging Modalities

CT Scan

• Not routinely recommended as initial imaging for suspected sacroiliitis 1
• May provide additional information on structural damage when:
  • Radiography is negative
  • MRI cannot be performed 1
• Excels at evaluating structural lesions 2
• Low-dose CT is a reasonable alternative when MRI is unavailable 2

Bone Scintigraphy

• Not recommended for diagnosis of sacroiliitis 1
• Low to moderate sensitivity and variable specificity 1
• Limited diagnostic value for early diagnosis 1

Ultrasound

• Not routinely recommended for diagnosis of sacroiliitis 1
• Limited utility in evaluating sacroiliac joints 1

Special Considerations

Pediatric Patients

• Knowledge of normal imaging features in children is essential
• Normal findings like flaring, blurring, or irregular appearance of the articular surface can simulate disease 2

Post-Procedural Evaluation

• For evaluating SI joint fusion devices, CT is the gold standard 3
• Provides detailed assessment of implant position relative to neural structures 3

Clinical Algorithm for Imaging Selection

1. Start with conventional radiography of sacroiliac joints for initial evaluation
2. If radiographs are negative but clinical suspicion remains high:
   • Proceed to MRI of sacroiliac joints
3. If MRI is contraindicated or unavailable:
   • Consider low-dose CT as an alternative
4. For young patients or those with short symptom duration:
   • Consider MRI as the initial imaging modality

This approach optimizes early detection of sacroiliitis while minimizing unnecessary radiation exposure and healthcare costs.