MRI Protocol for Foot Osteomyelitis
Standard MRI with T1-weighted and T2/STIR sequences is the recommended imaging protocol for diagnosing foot osteomyelitis when clinical assessment and plain radiographs leave diagnostic uncertainty. 1
When to Order MRI
- MRI should be performed when the diagnosis of foot osteomyelitis remains in doubt despite clinical assessment, plain X-rays, and laboratory findings (ESR, CRP, or PCT). 1
- MRI is not needed in all patients—reserve it for cases where the diagnosis is uncertain after initial workup. 1
- MRI provides high sensitivity (90-97%) and specificity (85-93%) for detecting osteomyelitis, while also delineating soft tissue involvement and extent of bone infection. 2, 3
Essential MRI Sequences
The core protocol should include:
- T1-weighted sequences to assess bone marrow signal intensity—confluent decreased T1 signal in a medullary distribution has 95% sensitivity and 91% specificity for osteomyelitis. 4
- T2-weighted or STIR (Short Tau Inversion Recovery) sequences to detect bone marrow edema and assess the marrow ROI/joint fluid ROI ratio, which augments diagnostic sensitivity. 1
An abbreviated protocol using only coronal T1-weighted and sagittal T2-weighted FMPIR sequences is non-inferior to full protocols and reduces scan time from 22 minutes to 8 minutes without compromising diagnostic accuracy (AUC 0.873 vs 0.843). 5
Advanced Sequences for Specific Scenarios
- Dynamic contrast-enhanced MRI (DCE-MRI) should be considered when distinguishing Charcot neuroarthropathy from osteomyelitis, as it improves specificity—wash-in rate >1.280 has 100% specificity and 76% sensitivity. 1, 6
- Diffusion-weighted imaging (DWI) can be added when diagnostic uncertainty persists, with ADC values <1.57 × 10⁻³ showing 88.2% sensitivity and 80% specificity. 6
- MR angiography or neurography may be useful in selected patients with possible neuro-osteoarthropathy to better differentiate from infection. 1
- Gadolinium contrast is helpful for detecting areas of bone devitalization before surgery and identifying abscesses. 7
Key Diagnostic MRI Findings
Look for these features on MRI:
- Decreased T1 marrow signal intensity in a confluent pattern with medullary (not just subcortical) distribution—this combination is highly predictive of osteomyelitis. 4
- Bone marrow edema on fluid-sensitive sequences (T2/STIR) combined with low T1 signal. 7
- Secondary findings that increase diagnostic confidence: ulcer, sinus tract, cellulitis, or abscess adjacent to abnormal bone. 7
- Increased marrow ROI/joint fluid ROI ratio on T2/STIR sequences. 1
Critical Pitfalls to Avoid
- Do not rely solely on bone marrow edema—reactive edema from trauma, previous surgery, or Charcot neuroarthropathy can mimic osteomyelitis and lower MRI specificity. 1
- Hazy reticulated patterns or purely subcortical signal changes on T1-weighted images are not specific for osteomyelitis—none of these patterns correlated with confirmed infection in one study. 4
- Request expert musculoskeletal radiologist interpretation when available, as second reads by specialists improve diagnostic accuracy. 1
- In patients with Charcot joints, standard MRI may be less specific than combined bone/WBC scintigraphy—consider nuclear imaging alternatives if MRI is equivocal. 8
Alternatives When MRI is Contraindicated
- FDG-PET/CT or ⁹⁹ᵐTc-exametazime HMPAO-labeled WBC scintigraphy offer the highest specificity (0.92 for both) and should be considered when MRI cannot be performed. 1
- Three-phase bone scintigraphy combined with In-111 labeled WBC scintigraphy is more specific than MRI in the setting of Charcot joints. 8