What is the best imaging diagnostic approach for ankylosing spondylitis?

Best Imaging Diagnostic Approach for Ankylosing Spondylitis

Conventional radiography of the sacroiliac joints should be used as the first imaging method for diagnosing sacroiliitis in suspected ankylosing spondylitis, followed by MRI of the sacroiliac joints if radiographs are negative or equivocal but clinical suspicion remains high. 1

Diagnostic Algorithm for Ankylosing Spondylitis

Step 1: Initial Imaging

• Conventional radiography of the sacroiliac (SI) joints
  • First-line imaging modality for suspected ankylosing spondylitis 1
  • Advantages: Widely available, relatively inexpensive
  • Limitations: Low sensitivity for early disease (may lag behind clinical symptoms by 7+ years) 1

Step 2: If Radiographs are Negative/Equivocal but Clinical Suspicion Remains

• MRI of the sacroiliac joints
  • Best examination for assessment of acute inflammatory changes 1
  • Recommended protocol:
    • T1-weighted sequences
    • Fluid-sensitive sequences (T2-weighted fat-suppressed or STIR) 1
    • Both active inflammatory lesions (primarily bone marrow edema) and structural lesions (erosions, new bone formation, sclerosis, fat infiltration) should be evaluated 1
  • Consider as first imaging method in:
    • Young patients
    • Patients with short symptom duration 1

Step 3: Alternative When MRI Cannot Be Performed

• CT of the sacroiliac joints without contrast
  • Helpful when radiographic findings are equivocal 1
  • Better than radiography for detecting subtle erosions and reparative changes 1
  • Limitations: Lacks sensitivity for direct inflammatory changes and exposes patients to radiation 1

Imaging Features to Evaluate

On Radiography

• Erosions
• Joint space narrowing or widening
• Subchondral sclerosis
• Ankylosis

On MRI

• Active inflammatory lesions:

  • Bone marrow edema (appears as high signal on STIR sequences)
  • Enhancement after gadolinium administration (if used)
  • Capsulitis, synovitis, enthesitis

• Structural lesions:

  • Erosions
  • Fat infiltration
  • Sclerosis
  • New bone formation

Clinical Correlation with Imaging

• Subchondral edema and enhancement on MRI correlate with elevated C-reactive protein (CRP) levels, indicating active disease 2
• Subchondral fatty bone marrow changes may be seen in both active and chronic sacroiliitis and correlate with higher radiographic scores 2
• MRI inflammatory activity (particularly extensive bone marrow edema in the spine) might predict good clinical response to anti-TNF-alpha treatment 1

Monitoring Disease Progression

• For monitoring disease activity: MRI of the SI joints and/or spine may be used 1
• For long-term monitoring of structural changes: Conventional radiography of the SI joints and/or spine, not more frequently than every second year 1
• For predicting disease severity: Initial conventional radiography of the lumbar and cervical spine to detect syndesmophytes, which predict development of new syndesmophytes 1

Important Considerations and Pitfalls

• Contrast enhancement in MRI has not been shown to significantly increase diagnostic accuracy for sacroiliitis and is generally not necessary 1
• STIR sequences are usually sufficient to detect inflammation 1
• MRI of the spine is not generally recommended for initial diagnosis of axial spondyloarthritis but may provide additional information 1
• In clinically probable AS without MRI evidence of sacroiliitis, MRI of the thoracic spine can increase diagnostic sensitivity 3
• Scintigraphy and ultrasound are not recommended for diagnosis of sacroiliitis 1

By following this diagnostic algorithm, clinicians can optimize the detection of ankylosing spondylitis at both early and established stages of disease, allowing for timely intervention to prevent disease progression and improve patient outcomes.