What is the best initial imaging test for osteomyelitis (infection of the bone)?

Best Imaging for Osteomyelitis

MRI is the best initial advanced imaging test for suspected osteomyelitis, with the highest sensitivity (95.6%) and specificity (80.7%), and a 100% negative predictive value that reliably excludes infection when marrow signal is normal. 1, 2

Initial Imaging Algorithm

Start with plain radiographs in all cases as the first imaging test, despite their limitations, because they are widely available, low cost, and help exclude alternative diagnoses such as fractures or tumors. 1, 3, 4

• Radiographs show cortical erosions, periosteal reaction, bone sclerosis, mixed lucency, and bone destruction in established osteomyelitis. 3, 4
• Critical pitfall: Early acute osteomyelitis (<14 days) typically shows normal radiographs or only mild soft tissue swelling, as bone destruction requires 7-10 days to appear and >30% osseous matrix loss to be visible. 3, 4
• Radiographic sensitivity is extremely low until disease is advanced, so normal radiographs do NOT exclude osteomyelitis. 4

When Radiographs Are Normal or Equivocal

Proceed directly to MRI without contrast as the next imaging study when clinical suspicion remains high after normal or equivocal radiographs. 1, 3

• MRI demonstrates bone marrow edema with decreased T1-weighted signal and increased T2-weighted/STIR signal intensity. 3, 5
• MRI provides superior evaluation of soft tissue involvement, abscesses, fistulas, and extent of bone involvement compared to all other modalities. 1, 6
• Contrast administration does NOT improve diagnosis of peripheral osteomyelitis itself, but may enhance evaluation of soft tissue infections and abscesses. 1
• A normal marrow signal on MRI has 100% negative predictive value and definitively excludes osteomyelitis. 7, 3

Alternative Advanced Imaging Options

Consider nuclear medicine studies (PET, SPECT, or WBC scan) when MRI is contraindicated, infection is suspected to be multifocal, or hardware/chronic bone alterations are present. 1, 7

• FDG-PET demonstrates high sensitivity (85.1%) and specificity (92.8%), with diagnostic accuracy similar to MRI. 1, 2
• SPECT has 95.1% sensitivity and 82.0% specificity, comparable to MRI performance. 1, 2
• Technetium-99m HMPAO WBC scintigraphy shows 87.3% sensitivity and 94.7% specificity, approaching MRI accuracy. 2
• Three-phase bone scan alone has poor specificity (<50%) and should NOT be used as a standalone test, especially after trauma or surgery where osseous remodeling causes false positives. 1
• Combined WBC scan with sulfur colloid marrow imaging improves accuracy to 90% by distinguishing infection from bone turnover. 1

CT Scanning Role

CT is NOT the preferred test for diagnosing osteomyelitis but has specific utility for detecting sequestra, cortical erosions, and guiding percutaneous biopsy. 1, 6

• CT shows cortical bone detail, periosteal reaction, bone destruction, and soft tissue fluid collections better than radiographs. 1, 6
• CT is useful for identifying necrotic bone fragments (sequestra) that serve as a nidus for chronic infection. 1
• CT findings are often nonspecific, particularly after recent trauma or surgery. 1
• CT has inferior sensitivity (69.7%) compared to MRI for detecting bone infection. 2

Ultrasound Limitations

Ultrasound has limited benefit for diagnosing osteomyelitis and should NOT be relied upon for this purpose. 1

• US may detect juxtacortical fluid collections and fistulous tracts, but these findings are nonspecific for osteomyelitis. 1
• US is useful for guiding joint aspiration in suspected septic arthritis but cannot adequately assess bone marrow. 1

Special Situations

Post-Surgical or Hardware-Related Infection

MRI remains challenging in chronic post-traumatic osteomyelitis due to marrow signal heterogeneity from prior surgery or trauma. 1

• FDG-PET shows 94% sensitivity and 87% specificity in trauma patients with metallic hardware (when imaged ≥6 months post-surgery). 1
• Avoid FDG-PET in the acute post-operative period (<3-4 months) as tracer accumulation from normal healing cannot be distinguished from infection. 1
• WBC scans remain useful after trauma because leukocytes are not incorporated into areas of bone turnover. 1

Diabetic Foot Osteomyelitis

The diagnostic algorithm is identical, with MRI remaining the preferred advanced imaging modality showing similar accuracy in diabetic patients. 3, 2

• Probe-to-bone test should be performed first: positive test in high-risk patients is largely diagnostic. 3
• If initial radiographs are negative, treat soft tissue infection for 2 weeks, then repeat radiographs 2-4 weeks later if suspicion persists. 3

Key Clinical Pitfalls to Avoid

• Never rely solely on radiographs to exclude osteomyelitis in early disease. 7, 4
• Do not delay MRI waiting for radiographic changes to develop when clinical suspicion is high. 4
• Avoid three-phase bone scan as a standalone test due to poor specificity. 1
• Do not misinterpret post-traumatic or post-surgical bone changes as infection on any imaging modality. 1, 7
• Remember that normal WBC count does NOT influence likelihood of osteomyelitis and should not guide imaging decisions. 3