CT Scan for Osteomyelitis of the Foot
CT is generally not the preferred imaging modality for diagnosing osteomyelitis of the foot and has limited sensitivity for acute bone infection, though it can detect chronic changes such as sequestra (dead bone fragments) and cortical destruction. 1
Why CT Has Limited Utility for Foot Osteomyelitis
The American College of Radiology explicitly states that CT is generally insensitive in the evaluation of acute osteomyelitis and should not be the primary imaging modality when osteomyelitis is suspected. 1
CT has lower sensitivity compared to MRI for detecting early bone marrow changes that characterize acute osteomyelitis. 1
While CT provides excellent visualization of cortical bone detail, it cannot reliably detect the marrow edema and inflammatory changes that appear early in bone infection. 2, 1
What CT Can Show in Osteomyelitis
CT does have specific capabilities that may be useful in certain clinical scenarios:
Chronic osteomyelitis findings: CT excels at detecting sequestra (necrotic bone fragments), periosteal reaction, cortical destruction, and sinus tracts—all features of chronic rather than acute infection. 2, 1, 3
Soft tissue gas and foreign bodies: CT is superior to MRI for identifying soft-tissue gas (which can signal necrotizing infection) and radiodense foreign bodies. 2
Cortical bone changes: CT shows bone erosions and cortical thickening more clearly than plain radiographs. 3
When to Use CT with Contrast
If CT must be used (when MRI is contraindicated), intravenous contrast is recommended:
The American College of Radiology recommends CT with IV contrast over non-contrast CT for suspected osteomyelitis, as contrast helps assess soft tissue involvement and identify abscess formation. 1
CT with IV contrast receives a rating of 7 ("usually appropriate") for suspected osteomyelitis with cellulitis or wounds. 1
However, contrast does not improve diagnostic sensitivity for acute osteomyelitis itself—it only enhances evaluation of associated soft tissue infections. 1
The Superior Alternative: MRI
For diabetic foot osteomyelitis specifically, the International Working Group on the Diabetic Foot recommends MRI when advanced imaging is needed. 2
MRI has 95.6% sensitivity and 80.7% specificity for osteomyelitis in adults, with a 100% negative predictive value for excluding infection. 4, 1
MRI detects decreased T1-weighted bone marrow signal with increased signal on fluid-sensitive sequences—the hallmark of osteomyelitis—which CT cannot visualize. 2, 3
A systematic review confirmed MRI, PET, and SPECT all have superior diagnostic accuracy compared to CT for osteomyelitis. 4
Practical Algorithm for Foot Osteomyelitis Imaging
Start with plain radiographs to exclude alternative diagnoses (fracture, Charcot arthropathy) and assess for chronic changes like periosteal reaction or bone destruction. 2, 1
Proceed to MRI (with or without contrast) as the next imaging study if osteomyelitis is still suspected—this is the gold standard. 2, 1, 3
Consider CT with IV contrast only if:
Alternative nuclear medicine studies (FDG-PET/CT or labeled white blood cell scans) should be considered if both MRI and CT are inadequate, particularly with extensive hardware. 2, 1
Common Pitfalls to Avoid
Do not rely on CT alone to rule out osteomyelitis—a negative CT does not exclude acute bone infection, as early marrow changes are invisible on CT. 1
Avoid dual-phase CT (without and with contrast)—the American College of Radiology rates this as "usually not appropriate" due to unnecessary radiation exposure. 1
In the diabetic foot specifically, perform a probe-to-bone test clinically; in high-risk patients, a positive test is largely diagnostic and may obviate the need for advanced imaging. 2
Remember that in pediatric patients, radiation exposure from CT is a significant concern, making MRI even more strongly preferred. 2, 1