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 and signs of soft tissue infection extending to the calf, MRI is the most appropriate investigation to assess both osteomyelitis and soft tissue infection extent. 1, 2

Clinical Reasoning

This patient presents with high-risk features that demand advanced imaging:

• Ulcer size >2 cm² significantly increases the likelihood of underlying osteomyelitis 1
• First metatarsal location is one of the most common sites for pedal osteomyelitis in diabetic patients 1
• Calf warmth, swelling, and tenderness suggests soft tissue infection extending beyond the foot, requiring extent assessment 2
• Intact distal pulses indicate adequate vascular supply, making infection rather than ischemia the primary concern 2

Why MRI Over Other Options

MRI Advantages

• Superior diagnostic accuracy for osteomyelitis with sensitivity of 98% and specificity of 89% in diabetic foot infections 1
• Excellent soft tissue characterization using fluid-sensitive, fat-suppressed sequences to identify abscesses, sinus tracts, tenosynovitis, and differentiate cellulitis from edema 1
• Critical for surgical planning by defining the precise extent of bone and soft tissue involvement 1
• Can assess compartment spread - forefoot infections frequently spread to adjacent compartments, unlike hindfoot infections which remain confined 1
• Radiation-free and widely available 2

Why Not the Other Options

Duplex ultrasound (Option A): This is inappropriate as the clinical picture suggests infection, not deep vein thrombosis. While calf symptoms might raise DVT concerns, the presence of a foot ulcer with local warmth and tenderness points to ascending soft tissue infection 2

Conventional angiography (Option B): Unnecessary since distal pulses are intact, indicating adequate arterial perfusion. Angiography is reserved for neuroischemic ulcers with vascular compromise 3

CT with contrast (Option D): Inferior to MRI for soft tissue characterization and has limited sensitivity for early osteomyelitis. CT is useful for assessing bone structure and emphysema but provides poor soft tissue contrast compared to MRI 1

Clinical Algorithm

Immediate MRI of the foot should be performed: 2

• If MRI shows osteomyelitis: Treat for bone infection with prolonged antibiotic therapy (typically 6 weeks) and consider surgical debridement 1
• If MRI shows only soft tissue infection: Withhold prolonged osteomyelitis treatment; manage with appropriate antibiotics for soft tissue infection and close follow-up 2
• If MRI is equivocal or unavailable: Consider WBC SPECT/CT or [18F]FDG PET/CT as alternative advanced imaging 1, 2

Important Caveats

• Obtain deep tissue or bone cultures before starting antibiotics when possible to guide antimicrobial therapy 2
• The 2 cm ulcer size is a critical threshold - ulcers >2 cm² have significantly higher likelihood of underlying osteomyelitis 1
• MRI should be performed before nuclear medicine imaging as it provides more comprehensive anatomical information and is radiation-free 1, 2
• Forefoot osteomyelitis spreads differently than hindfoot infections - soft tissue inflammation in the forefoot can spread into adjacent compartments, making extent assessment crucial 1

Answer: C - MRI