What type of MRI (Magnetic Resonance Imaging) should I order to rule out osteomyelitis?

MRI Protocol for Ruling Out Osteomyelitis

Order a standard MRI of the area of interest with T1-weighted and fluid-sensitive sequences (T2-weighted or STIR); contrast is optional for acute osteomyelitis but helpful for detecting abscesses. 1, 2

Essential MRI Sequences

The minimum required sequences are:

• T1-weighted images to detect decreased bone marrow signal (dark marrow indicates infection) 1, 2, 3
• Fluid-sensitive sequences (either T2-weighted or STIR) to detect increased signal in infected bone marrow 1, 2, 4

These two sequences alone provide 82-100% sensitivity and 75-96% specificity for diagnosing osteomyelitis. 1

Contrast Administration Decision

You do not need IV gadolinium contrast for diagnosing acute osteomyelitis itself, as noncontrast MRI has high sensitivity and specificity. 1, 5 However, add contrast if you suspect:

• Soft tissue abscess formation (contrast significantly improves abscess detection) 1
• Need for surgical planning requiring detailed soft tissue evaluation 2, 5
• Septic arthritis (decreased femoral head enhancement helps distinguish from transient synovitis) 1

In pediatric patients, some studies show 100% identification of osteomyelitis using fluid-sensitive sequences alone, making contrast optional unless evaluating for complications. 1

Field of View Considerations

Image the entire extremity rather than just the suspected area when:

• The patient has bacteremia or fever of unknown origin (11-20% have contralateral or additional sites of infection) 1
• The patient is a young child, particularly under 6 years old (multifocal osteomyelitis occurs in 19% of cases) 1
• Clinical localization is uncertain 1

For localized suspected infection in older children and adults, imaging the area of interest is sufficient. 1, 2

Critical Diagnostic Pitfalls to Avoid

Do not diagnose osteomyelitis based solely on bone marrow edema, as this finding has only 49-75% specificity. 6, 4 The following conditions mimic osteomyelitis on MRI:

• Recent fractures or trauma (reactive marrow edema) 5, 6, 7
• Charcot neuroarthropathy in diabetic patients 5
• Bone infarction (especially in sickle cell disease patients) 1
• Ewing sarcoma in children 1

Improve specificity by requiring all three findings:

1. Decreased T1-weighted signal (dark marrow) 2, 3, 7
2. Increased T2/STIR signal (bright marrow) 2, 4, 3
3. Secondary signs: cortical erosion, ulcer tract to bone, or subperiosteal fluid 1, 3

This combination increases specificity from 49% to 83% without losing sensitivity. 4, 3

When MRI Cannot Be Performed

If MRI is contraindicated (pacemaker, severe claustrophobia, metallic implants causing artifact), order FDG-PET/CT, which has 81-100% sensitivity and 87-100% specificity with superior localization compared to bone scan. 1, 2, 5

Alternatively, use a 3-phase bone scan combined with WBC scan and sulfur colloid scan, though this requires multiple studies and has lower spatial resolution. 1

Special Population Considerations

In children under 5 years old, MRI detects concurrent infections beyond the primary site in over 50% of cases (septic arthritis with osteomyelitis or vice versa), making it particularly valuable. 1

In diabetic foot infections, request expert musculoskeletal radiologist interpretation when available, as second reads by specialists significantly improve diagnostic accuracy in this challenging population. 5