What is the best imaging modality to rule out osteomyelitis of the leg?

Optimal Imaging Modality for Ruling Out Osteomyelitis of the Leg

MRI is the best imaging modality for ruling out osteomyelitis of the leg, with its 100% negative predictive value and superior ability to detect early bone marrow changes. 1, 2

Diagnostic Imaging Algorithm

First-Line Imaging:

• Plain Radiographs: Always start with radiographs as they are widely available, inexpensive, and provide anatomic evaluation
  • However, radiographs have low sensitivity in early osteomyelitis and may appear normal for up to 2 weeks after infection onset
  • If radiographs show changes suggestive of osteomyelitis (cortical erosion, periosteal reaction, mixed lucency, sclerosis), treat for presumptive osteomyelitis 1

Second-Line Imaging (if radiographs are negative or equivocal):

• MRI with contrast: Highest sensitivity (82-100%) and specificity (75-96%) for osteomyelitis detection 2
  • Provides excellent evaluation of bone marrow and adjacent soft tissues
  • Can detect abscesses, fistulas, and other complications
  • Optimal sequences should include T1-weighted imaging, fluid-sensitive sequences (T2-weighted fat-saturated or STIR), and post-contrast T1-weighted fat-suppressed images 2
  • A normal marrow signal on MRI reliably excludes infection with 100% negative predictive value 1, 2

Alternative Imaging Options (if MRI is contraindicated or unavailable):

1. FDG-PET/CT:

   • High sensitivity (81-100%) and specificity (87-100%) for osteomyelitis detection 1
   • Particularly useful for multifocal infection evaluation
   • Higher resolution allows more precise localization than other nuclear medicine techniques

2. 3-Phase Bone Scan:

   • Can be used to rule out osteomyelitis with good sensitivity (83%)
   • However, low specificity (45%) limits its usefulness as a standalone test 1
   • Most accurate when bone is not affected by other conditions like osteoarthritis, recent fracture, or hardware implantation

3. Combined Nuclear Medicine Studies:

   • If 3-phase bone scan is positive, adding WBC scan and sulfur colloid scan increases specificity
   • Combined WBC scan with marrow imaging has been reported up to 90% accurate 1, 2

4. CT with IV contrast:

   • Limited sensitivity for acute osteomyelitis
   • More useful for chronic osteomyelitis to detect sequestrum, foreign bodies, and gas
   • Helpful for assessing soft tissue involvement 1

Key Diagnostic Features on MRI

• Decreased T1-weighted bone marrow signal
• Increased signal on fluid-sensitive sequences (T2-weighted fat-saturated or STIR)
• Confluent T1-weighted signal abnormality is highly specific for osteomyelitis 3
• Cortical erosion is an independent risk factor for osteomyelitis 3
• Secondary signs that increase diagnostic confidence include subperiosteal fluid collection 2

Common Pitfalls and Challenges

• Difficulty distinguishing infection from reactive inflammation
• Potential confusion between osteomyelitis and bone infarction or neoplastic processes
• Artifact from orthopedic hardware may limit evaluation 2
• Uncomplicated septic joint effusions may demonstrate abnormal marrow signal intensity that can be mistaken for osteomyelitis 4
• Discordant marrow signal intensity between individual MR sequences can occur in 8-21% of cases 5

Special Considerations

• In diabetic patients, MRI is particularly valuable but should be interpreted in conjunction with clinical findings 1
• In pediatric patients, MRI is strongly preferred over nuclear medicine studies due to radiation concerns 2
• In post-surgical settings or when hardware is present, metal artifact reduction sequences may be necessary 2

MRI's superior soft tissue contrast, multiplanar capability, and high sensitivity and specificity make it the optimal choice for ruling out osteomyelitis of the leg when available and not contraindicated.