What is the best imaging modality for suspected coccyx (tailbone) osteomyelitis (bone infection)?

Imaging for Suspected Coccyx Osteomyelitis

MRI is the most appropriate imaging modality for suspected coccyx osteomyelitis due to its superior sensitivity and specificity for detecting early bone infection and evaluating surrounding soft tissue involvement. 1, 2

Diagnostic Approach Algorithm

First-Line Imaging

• Plain radiographs should be obtained initially to evaluate for anatomic abnormalities, bone destruction, and to exclude alternative diagnoses such as fractures or tumors, though they have limited sensitivity in early osteomyelitis 1, 2
• MRI without and with IV contrast should follow as the primary diagnostic modality when clinical suspicion for coccyx osteomyelitis exists, as it provides:
  • Highest sensitivity (92-100%) and specificity (96%) for detecting early osteomyelitis 1, 3
  • Superior evaluation of bone marrow changes (decreased T1 signal, increased T2/STIR signal) 1, 3
  • Excellent assessment of surrounding soft tissue involvement including abscess or fistulas 1, 2

Alternative Imaging Options (When MRI is Contraindicated)

• Nuclear medicine studies:

  • 3-phase bone scan has high sensitivity (83%) but poor specificity (45%) for osteomyelitis 1
  • Combined approach with WBC scan and sulfur colloid scan increases specificity for infection 1
  • FDG-PET/CT offers high sensitivity (81-100%) and specificity (87-100%) with better resolution than other nuclear medicine techniques 1, 2

• CT scan:

  • Less sensitive than MRI for early osteomyelitis but useful for detecting:
    • Cortical bone destruction
    • Sequestra (necrotic bone fragments)
    • Sinus tracts
    • Soft tissue gas or abscesses 1, 4

Strengths and Limitations of Each Modality

MRI

• Strengths:
  • Highest sensitivity and specificity for early osteomyelitis 1, 3
  • Excellent soft tissue contrast and anatomical detail 4, 5
  • No ionizing radiation 1
• Limitations:
  • May be difficult to distinguish infection from reactive inflammation 1
  • Contraindicated in patients with certain implanted devices 1
  • Potential artifacts from orthopedic hardware 1

Nuclear Medicine Studies

• Strengths:
  • Bone scan can detect changes within 1-2 days of symptom onset 1
  • Useful for detecting multifocal infection 4, 6
  • Alternative when MRI is contraindicated 1
• Limitations:
  • Lower specificity, especially with recent surgery or trauma 1
  • Limited spatial resolution 1
  • Involves radiation exposure 1

CT Scan

• Strengths:
  • Better than MRI for detecting sequestra, foreign bodies, and gas 1
  • Useful when MRI is contraindicated 2
• Limitations:
  • Insensitive for early osteomyelitis 1
  • Radiation exposure 1

Ultrasound

• Strengths:
  • Useful for detecting associated soft tissue collections 1
  • Can guide aspiration for diagnostic sampling 1, 2
• Limitations:
  • Limited benefit in direct diagnosis of osteomyelitis 1
  • Limited visualization of deeper structures 1

Common Pitfalls to Avoid

• Relying solely on plain radiographs to exclude infection, as they may appear normal in early osteomyelitis 2, 4
• Failing to obtain contrast-enhanced MRI when evaluating for associated soft tissue infection 1, 2
• Misinterpreting post-surgical or post-traumatic changes as infection on imaging studies 2
• Using CT as primary diagnostic tool for early osteomyelitis due to its lower sensitivity compared to MRI 1, 4