What is the role of X-ray in diagnosing sacral iliitis due to spondyloarthritis?

Role of X-ray in Diagnosing Sacroiliac Iliitis Due to Spondyloarthritis

Radiographs of the pelvis are the recommended first-line imaging modality for evaluation of suspected sacroiliitis in spondyloarthritis, despite their limited sensitivity for early disease detection. 1

Initial Imaging Approach

• X-rays of the sacroiliac joints demonstrate:

  • Chronic erosions
  • Sclerotic changes
  • Ankylosis
  • Structural sequelae of inflammatory sacroiliitis 1

• Technical considerations:

  • Anteroposterior view of the whole pelvis is recommended to evaluate both sacroiliac joints and hips 1
  • No significant benefit from adding oblique views 1
  • No superiority between anteroposterior and Ferguson views 1

Limitations of X-ray in Sacroiliitis

• Low sensitivity for early disease detection:

  • Inflammatory changes precede radiographic findings by 3-7 years 1
  • Variable reported sensitivity (19%-72%) and specificity (47%-84.5%) 1
  • Misses more than half of patients with structural changes when compared to low-dose CT 1
  • Fair to moderate interobserver agreement for radiographic findings 1

• Unable to demonstrate active inflammation:

  • Only shows structural consequences of inflammation 1
  • Cannot detect bone marrow edema (active inflammation) 1

When X-rays Are Negative or Equivocal

When radiographs are negative or equivocal but clinical suspicion remains high:

1. MRI of sacroiliac joints is the next appropriate imaging method 1, 2

   • Detects inflammatory changes 3-7 years before radiographic evidence 1
   • Should include:
     • T1-weighted sequences
     • Fat-suppressed fluid-sensitive sequences (T2-weighted fat-suppressed or STIR) 1
     • Coronal oblique and axial oblique planes 2

2. CT of sacroiliac joints (without contrast) may be helpful when:

   • Patient is unable to undergo MRI 1
   • Need to identify subtle erosions and soft-tissue ossification 1
   • Better assessment of complex SI joint anatomy 1
   • Low-dose CT is a reasonable alternative to minimize radiation exposure 2

Additional Imaging Considerations

• For comprehensive evaluation, radiographs of the spine (cervical and lumbar) should be performed alongside sacroiliac joint imaging 1
• Thoracic spine radiographs are less useful due to overlying structures but may be obtained for specific symptoms 1

Pitfalls and Caveats

• Relying solely on radiographs may lead to missed diagnoses in early disease
• 41.3% of radiography reports gave incorrect diagnoses compared to CT as reference standard 1
• In pediatric patients, normal developmental features (flaring, blurring, irregular articular surface) can mimic disease on imaging 2
• Ultrasound, bone scintigraphy, and PET/CT are not routinely recommended for initial evaluation of suspected sacroiliitis 1

X-ray remains the cornerstone initial imaging modality for suspected sacroiliitis despite its limitations, with MRI being the preferred next step when radiographs are negative or equivocal but clinical suspicion remains high.