What is the most appropriate way to assess a patient with diabetes presenting with toe swelling, discharge, and a 2 cm ulcer on the first metatarsal?

Assessment of Diabetic Foot Ulcer with Suspected Osteomyelitis

MRI is the most appropriate imaging modality to assess this patient with a diabetic foot ulcer, swelling, and discharge over the first metatarsal. 1, 2, 3

Initial Clinical Assessment

Begin with plain radiographs in three views with a radio-opaque marker at the ulcer site as the mandatory first imaging study for all non-superficial diabetic foot infections. 3 However, recognize that plain films have poor sensitivity (61.9%) and may not show osteomyelitis changes for 2-3 weeks, so normal early radiographs do not exclude bone infection. 3

• Perform a probe-to-bone test during wound examination, as a positive result is largely diagnostic of osteomyelitis. 3
• Check inflammatory markers (ESR and CRP), as markedly elevated values suggest osteomyelitis. 3
• The combination of positive probe-to-bone test, elevated ESR, and positive X-rays makes osteomyelitis highly probable, but if probe-to-bone is the only positive test, advanced imaging with MRI should be performed, especially in high-risk patients. 1

Why MRI is the Optimal Choice

MRI with fluid-sensitive, fat-suppressed sequences is the modality of choice for investigating soft-tissue complications in diabetic foot infections. 1, 2 The evidence supporting MRI is compelling:

• MRI has the highest diagnostic accuracy for osteomyelitis with 93-96.4% sensitivity and 75-83.8% specificity, significantly outperforming plain radiographs (60% sensitivity, 81% specificity), technetium bone scans (100% sensitivity, 25% specificity), and indium-labeled leukocyte scintigraphy (80% sensitivity, 29% specificity). 3, 4, 5
• MRI provides superior soft tissue contrast compared to X-rays or CT, allowing identification of skin ulcers, sinus tracts, abscesses, tenosynovitis, and differentiation between cellulitis (which enhances with IV contrast) and simple edema (no enhancement). 1, 2
• The first metatarsal area is a high-risk location for osteomyelitis, as pedal osteomyelitis occurs most frequently around the fifth and first metatarsophalangeal joints from contiguous spread of soft tissue infection. 1
• Forefoot infections can spread into adjacent compartments, making extent assessment critical for surgical planning, which MRI accomplishes better than other modalities. 1, 2

Why the Other Options Are Inappropriate

Duplex ultrasound (Option A), conventional angiography (Option B), and MRA (Option C) are for evaluating arterial perfusion, not infection. 3 These vascular imaging studies would only be indicated if:

• Distal pulses were absent
• Ankle-brachial index <0.9
• Toe-brachial index <0.75

Since this patient has intact distal pulses, vascular imaging is not indicated. 3

CT with contrast (Option D) is not recommended as a primary imaging modality for diabetic foot osteomyelitis. 3 While CT may guide interventions for ulcer prevention and can detect soft tissue emphysema, it has lower tissue contrast ratio than MRI for soft tissue complications. 1

Clinical Algorithm

Follow this stepwise approach:

1. Obtain plain radiographs immediately as the initial screening study. 3
2. Perform MRI of the foot to definitively assess for osteomyelitis and soft tissue infection extent. 2, 3
3. If MRI shows osteomyelitis, treat for bone infection with prolonged antibiotics (typically 6 weeks). 1, 2
4. If MRI shows only soft tissue infection, withhold prolonged antibiotic therapy for osteomyelitis but treat the soft tissue infection appropriately. 2
5. If MRI is unavailable, contraindicated, or shows equivocal results, consider WBC SPECT/CT or [18F]FDG PET/CT as alternative advanced imaging. 1, 2, 3

Critical Pitfalls to Avoid

• Do not delay antibiotic therapy to obtain imaging - start empirical IV antibiotics immediately after obtaining wound cultures in moderate diabetic foot infections. 3
• Do not obtain superficial wound swabs - deep tissue specimens or bone cultures obtained aseptically provide reliable microbiological data. 3
• Do not order vascular imaging based solely on diabetes - clinical assessment of pulses and perfusion guides the need for vascular studies. 3
• MRI should be performed before nuclear medicine imaging since it is widely available, radiation-free, and provides the most comprehensive assessment. 2