What type of MRI (Magnetic Resonance Imaging) do you order for concerns of osteomyelitis?

MRI for Suspected Osteomyelitis

For suspected osteomyelitis, MRI of the specific area of interest with contrast enhancement is the preferred imaging modality after initial radiographs due to its superior sensitivity (82-100%) and specificity (75-96%) for detecting bone marrow changes and soft tissue involvement. 1, 2

Initial Imaging Approach

1. Plain radiographs should be the first imaging test performed

   • Radiographs provide anatomic evaluation of the affected site
   • Can reveal changes of chronic osteomyelitis, gas, or foreign bodies
   • May suggest alternative diagnoses (fracture, tumor, etc.)
   • Note: Early acute osteomyelitis (<14 days) may show normal radiographs or only mild soft tissue swelling 1

2. MRI with contrast should follow if osteomyelitis is still suspected after radiographs

   • Contrast-enhanced MRI is particularly valuable for:
     • Detecting abscesses (critical for surgical planning)
     • Evaluating soft tissue involvement
     • Identifying fistulas and sinus tracts
     • Differentiating septic arthritis from other conditions 2

MRI Protocol Considerations

• Sequences to include:

  • T1-weighted imaging (shows decreased bone marrow signal in infection)
  • Fluid-sensitive sequences (T2-weighted fat-saturated or STIR)
  • Post-contrast T1-weighted fat-suppressed images 1

• Field of view considerations:

  • For suspected isolated osteomyelitis: Focus on the area of interest
  • For suspected multifocal infection: Consider larger field of view
  • In pediatric patients: Consider imaging both extremities, as studies show 11% of patients with osteomyelitis had contralateral extremity findings 1

Special Clinical Scenarios

• Post-surgical settings or hardware present:

  • Contrast-enhanced MRI is strongly recommended (rated 9/9 by ACR) 2
  • Metal artifact reduction sequences may be necessary

• Pediatric patients:

  • MRI is preferred over nuclear medicine studies due to radiation concerns 1
  • In children <6 years, multifocal osteomyelitis is more common 1

• When MRI is contraindicated:

  • Consider CT with IV contrast
  • Nuclear medicine studies (3-phase bone scan with or without labeled leukocyte scan) can be used 2

Diagnostic Features on MRI

• Primary signs of osteomyelitis:

  • Decreased T1-weighted bone marrow signal
  • Increased signal on fluid-sensitive sequences
  • Confluent signal on T1-weighted images 3
  • Cortical erosion 3

• Secondary signs that increase diagnostic confidence:

  • Presence of cutaneous ulcer
  • Sinus tract
  • Cortical interruption (highest positive predictive value) 4
  • Subperiosteal fluid collection 1

Common Pitfalls

• Difficulty distinguishing infection from reactive inflammation
• Challenges in differentiating osteomyelitis from bone infarction in sickle cell disease 1
• Potential confusion between osteomyelitis and Ewing sarcoma (look for sharply demarcated margins on T1, presence of soft-tissue mass, and cortical breach) 1
• Artifact from orthopedic hardware may limit evaluation 1

MRI's superior ability to separate soft tissue disease from underlying bone marrow abnormalities makes it significantly more specific (96% vs 65%) and accurate (94% vs 71%) than bone scintigraphy for diagnosing osteomyelitis 5.