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

Best Imaging for Suspected Osteomyelitis

MRI with contrast enhancement is the preferred imaging modality for suspected osteomyelitis after initial radiographs due to its superior sensitivity (82-100%) and specificity (75-96%) for detecting bone marrow changes. 1

Imaging Algorithm for Osteomyelitis

First-Line Imaging

• Plain radiographs should always be the initial imaging test performed
  • Provides anatomic evaluation of the affected site
  • Can reveal changes of chronic osteomyelitis, gas, or foreign bodies
  • Note: Early acute osteomyelitis (<14 days) may show normal radiographs or only mild soft tissue swelling 2, 1

Second-Line Imaging

• MRI with contrast is the modality of choice after radiographs
  • Highest sensitivity and specificity for osteomyelitis detection
  • 100% negative predictive value - a normal marrow signal reliably excludes infection 2
  • Optimal sequences should include:
    • T1-weighted imaging
    • Fluid-sensitive sequences (T2-weighted fat-saturated or STIR)
    • Post-contrast T1-weighted fat-suppressed images 1
  • MRI findings suggestive of osteomyelitis:
    • Decreased T1-weighted bone marrow signal
    • Increased signal on fluid-sensitive sequences
    • Cortical erosion (strongest independent risk factor, RR = 1.7) 3
    • Confluent signal on T1-weighted images 3

Special Scenarios

When MRI is Contraindicated

1. CT with IV contrast (rated 7/9 by ACR) 1

   • Useful for detecting:
     • Periosteal reaction
     • Bone destruction
     • Necrotic bone (sequestra)
     • Sinus tracts
   • Superior to MRI for diagnosis of sequestra, foreign bodies, and gas 2
   • Limitation: May underestimate lesion size by up to 37% compared to actual extent of disease 1

2. Nuclear medicine studies:

   • FDG-PET/CT: High sensitivity (81-100%) and specificity (87-100%) 2
     • Higher resolution allows for more precise localization
     • Advantages in evaluation of multifocal infection
   • Combined WBC scan with sulfur colloid marrow imaging: Up to 90% accurate 2
     • Particularly useful in cases with altered bone marrow distribution
   • 3-phase bone scan: High sensitivity (83%) but low specificity (<50%) 2
     • Even lower specificity in setting of prior surgery or trauma

With Orthopedic Hardware

• MRI with metal artifact reduction sequences is recommended 1
• If MRI not feasible, consider:
  • FDG-PET/CT
  • Combined WBC scan with sulfur colloid marrow imaging 2

Clinical Pitfalls and Considerations

• Diabetes is an independent risk factor for osteomyelitis (RR = 2.4) and should lower the threshold for advanced imaging 3
• MRI may have difficulty distinguishing:
  • Infection from reactive inflammation
  • Osteomyelitis from bone infarction in sickle cell disease
  • Osteomyelitis from Ewing sarcoma 1
• Consider whole-body imaging for suspected multifocal osteomyelitis, especially in children under 6 years 1
• Secondary signs that increase diagnostic confidence include subperiosteal fluid collection 1
• In pediatric patients, MRI is strongly preferred over nuclear medicine studies due to radiation concerns 1

By following this evidence-based approach to imaging selection, clinicians can optimize the diagnosis of osteomyelitis, leading to earlier treatment and improved patient outcomes.