What is the most appropriate investigation for a diabetic patient presenting with swelling, warmth, and tenderness of the calf, a 2 cm ulcer over the first metatarsal area, and intact distal pulses?

MRI is the Most Appropriate Investigation

For this diabetic patient with a 2 cm ulcer over the first metatarsal, calf warmth/swelling, and intact pulses, MRI is the definitive investigation of choice to assess for both osteomyelitis and deep soft tissue infection extent. 1, 2

Clinical Reasoning

This presentation raises immediate concern for moderate-to-severe diabetic foot infection with possible bone involvement:

• The 2 cm ulcer over the first metatarsal is a high-risk location where >90% of pedal osteomyelitis occurs from contiguous spread 3, 1
• Warmth, swelling, and tenderness extending to the calf suggests deep soft tissue infection or ascending cellulitis requiring extent assessment 1, 2
• The 4-day duration provides sufficient time for infection to develop and potentially spread to bone 2
• Intact distal pulses effectively rule out critical limb ischemia, making vascular imaging (duplex ultrasound or angiography) unnecessary at this stage 1

Why MRI Over Other Modalities

MRI with fluid-sensitive, fat-suppressed sequences is the gold standard because it:

• Achieves 93% sensitivity and 75% specificity for osteomyelitis detection 1
• Provides superior soft tissue characterization to identify abscesses, sinus tracts, tenosynovitis, and cellulitis versus edema (cellulitis enhances with IV contrast; edema does not) 3, 1
• Determines infection extent in forefoot compartments, which is critical since forefoot infections can spread to adjacent compartments 3, 1
• Guides surgical decision-making regarding need for debridement versus conservative management 1, 2

Why Not the Other Options

• Duplex ultrasound (Option A): Only indicated if vascular insufficiency is suspected; this patient has intact pulses 1
• Conventional angiography (Option B): Reserved for revascularization planning when arterial disease is present; not a diagnostic tool for infection 1
• CT with contrast (Option D): Inferior to MRI for soft tissue detail and osteomyelitis detection, though dual-energy CT is emerging as an alternative when MRI is contraindicated 3

Diagnostic Algorithm

1. Perform MRI of the foot immediately before starting antibiotics if possible (antibiotics may reduce diagnostic yield) 2
2. If MRI shows osteomyelitis: treat for bone infection with prolonged antibiotics (typically 6 weeks) 3
3. If MRI shows only soft tissue infection: withhold prolonged osteomyelitis therapy and treat soft tissue infection appropriately 1
4. If MRI is unavailable or contraindicated (metal implants, pacemaker): consider WBC SPECT/CT or [18F]FDG PET/CT as alternatives 3, 1

Critical Caveats

• Plain X-rays should still be obtained first to identify gas in soft tissues, fractures, or obvious bone destruction, but normal X-rays do not exclude osteomyelitis (requires 40-50% bone loss to be visible, taking 2-3 weeks) 3
• Consider Charcot neuroarthropathy in the differential, as it can present with similar warmth and swelling; MRI can differentiate but may show bone marrow edema in both conditions, requiring expert radiologist interpretation 3
• Obtain deep tissue or bone cultures during any surgical debridement to guide antibiotic therapy; avoid superficial swabs which yield contaminants 3, 1
• Check inflammatory markers (ESR, CRP) and blood cultures if systemically unwell to support infection diagnosis and assess severity 3, 2

Answer: C - MRI