What is the preferred imaging modality, MRI (Magnetic Resonance Imaging) or CT (Computed Tomography), for suspected osteomyelitis of the toe?

MRI is the Preferred Imaging Modality for Suspected Toe Osteomyelitis

MRI is highly accurate for detection of acute osteomyelitis and should be the imaging modality of choice after initial plain radiographs for suspected osteomyelitis of the toe. 1

Initial Diagnostic Algorithm

• Start with plain radiographs as the first imaging test in all suspected cases, as they are safe, inexpensive, quickly obtained, and widely available. 1, 2

• Radiographs help exclude alternative diagnoses such as fractures, degenerative changes, tumors, and can reveal radiodense foreign bodies or soft tissue gas. 1

• However, normal radiographs do not exclude osteomyelitis, especially in early presentation—bone destruction typically does not appear until 7-10 days into the disease course, and sensitivity is extremely low until >30% of osseous matrix has been destroyed. 2

• If radiographs are normal or inconclusive but clinical suspicion remains high, proceed immediately to MRI—do not delay advanced imaging waiting for X-ray changes to develop. 2, 3

Why MRI is Superior to CT

MRI is the modality of choice for suspected bone and extremity soft-tissue infections due to its inherent sensitivity for bone marrow abnormalities, superb soft-tissue contrast, and delineation of anatomic detail. 1

MRI Advantages:

• MRI has a 100% negative predictive value for excluding osteomyelitis—a normal marrow signal reliably excludes infection. 1, 4

• Noncontrast MRI has high sensitivity and specificity in the diagnosis of osteomyelitis, with confirmed high accuracy of T1-weighted signal abnormalities for assigning true positive and true negative cases. 1

• MRI is highly sensitive at depicting marrow signal changes of acute osteomyelitis (decreased T1-weighted bone marrow signal, with increased signal on fluid-sensitive sequences). 1

• MRI provides excellent evaluation of adjacent soft tissues including abscesses or fistulas, which is critical for surgical planning and determining extent of disease. 1

• IV contrast does not improve diagnosis of peripheral osteomyelitis itself, but may improve evaluation of soft tissue infections. 1

CT Limitations:

• CT is insensitive in evaluation of acute osteomyelitis. 1

• CT may be most useful in characterizing osseous changes from chronic osteomyelitis including detection of sequestrum, but this is not the primary concern in suspected acute toe osteomyelitis. 1

• While CT depicts cortical bone well and can show periosteal reaction, bone destruction, and sinus tracts, it is generally less sensitive than MRI for detecting bone infection. 5, 6

• CT is superior to MRI only for specific findings like sequestra, foreign bodies, and gas—but these are not the primary diagnostic concerns in suspected acute osteomyelitis. 1

Alternative Imaging Options (When MRI is Contraindicated)

If MRI is contraindicated (non-MRI-compatible implanted devices, severe claustrophobia), consider these alternatives:

• FDG-PET/CT has high sensitivity (81%-100%) and high specificity (87%-100%) for detection of osteomyelitis, with higher resolution allowing for more precise localization. 1

• Nuclear medicine studies (3-phase bone scan with WBC scan and sulfur colloid scan) can be used, though they have lower spatial resolution and are more complex. 1

• Three-phase bone scan alone has sensitivity of 83% but specificity of only 45%, making it inadequate as a standalone test. 1

Common Pitfalls to Avoid

• Do not rely solely on radiographs to rule out infection—early osteomyelitis may have completely normal radiographic appearance. 2, 3

• Do not choose CT over MRI for suspected acute osteomyelitis—CT is insensitive for acute disease and should be reserved for chronic osteomyelitis or when MRI is contraindicated. 1

• Do not misinterpret post-traumatic changes as infection on MRI—consider clinical context carefully. 3

• In small bones like toes, subtle changes are particularly difficult to detect on standard radiographs, making MRI even more critical. 2

Practical Implementation

The recommended imaging sequence is:

1. Plain radiographs of the affected toe (initial study)
2. MRI without contrast (if radiographs normal/inconclusive and clinical suspicion remains)
3. Consider adding IV contrast only if soft tissue involvement needs better characterization 1, 3

This approach optimizes diagnostic accuracy while avoiding unnecessary radiation exposure and ensuring timely diagnosis to prevent chronic bone destruction and resultant deformity. 1