What is the most appropriate initial 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 Initial Investigation

For this diabetic patient with a foot ulcer, calf warmth/tenderness, and swelling of the big toe, MRI of the foot should be performed immediately as the initial advanced imaging study. 1, 2

Clinical Reasoning

This presentation is highly concerning for diabetic foot osteomyelitis with soft tissue infection:

• The 2-cm ulcer over the first metatarsal is a high-risk location where pedal osteomyelitis occurs most frequently, as infection spreads contiguously from soft tissue to bone at the metatarsophalangeal joints 3
• Warmth, swelling, and tenderness extending to the calf suggests moderate-to-severe soft tissue infection that may be spreading to adjacent compartments, which forefoot infections characteristically do 3, 1
• The 4-day duration is sufficient time for infection to develop and potentially involve bone 2

Why MRI is Superior

MRI with fluid-sensitive, fat-suppressed sequences is the modality of choice for investigating diabetic foot soft-tissue complications because it provides:

• Superior soft tissue characterization compared to X-rays or CT, with the highest tissue contrast ratio 3
• High diagnostic accuracy for osteomyelitis with sensitivity of 93% and specificity of 75% 1
• Critical anatomic detail including identification of skin ulcers, sinus tracts, abscesses, and tenosynovitis 3
• Differentiation between cellulitis and edema through IV contrast enhancement patterns (cellulitis enhances, edema does not) 3, 1
• Assessment of infection extent to guide surgical planning, particularly important since forefoot infections can spread to adjacent compartments 3, 1

Why Not the Other Options

Duplex ultrasound (Option A) is not indicated here because:

• Distal pulses are intact, making significant arterial insufficiency unlikely 1
• Ultrasound does not adequately assess for osteomyelitis or deep soft tissue infection 3

Conventional angiography (Option B) is inappropriate because:

• There is no clinical evidence of critical limb ischemia requiring vascular intervention 1
• The intact distal pulses argue against significant arterial disease 1

CT with contrast (Option D) is inferior to MRI because:

• CT has lower soft tissue contrast resolution compared to MRI 3
• While CT can detect soft tissue emphysema and guide ulcer treatment planning, it is not the preferred modality for assessing osteomyelitis or soft tissue infection extent 3

Clinical Algorithm

The recommended diagnostic approach is:

1. Perform MRI immediately as the first advanced imaging study 1, 2
2. If MRI shows osteomyelitis, treat for bone infection with prolonged antibiotics 1, 2
3. If MRI shows only soft tissue infection, withhold prolonged osteomyelitis therapy but treat the soft tissue infection appropriately 1
4. If MRI is unavailable or contraindicated (e.g., pacemaker, severe claustrophobia), consider WBC SPECT/CT or [18F]FDG PET/CT as alternative advanced imaging 3, 1

Critical Pitfalls to Avoid

• Do not rely on plain X-rays alone in this acute setting, as radiographic changes of osteomyelitis require time to develop and may be normal early in the disease course 2
• Do not delay MRI while waiting for plain X-ray results if clinical suspicion for osteomyelitis is high based on ulcer size and location 2
• MRI should be performed before nuclear medicine imaging since it is widely available, radiation-free, and provides comprehensive anatomic information 3, 1
• Obtain deep tissue or bone cultures using conventional culture techniques to guide antibiotic therapy, ideally before starting antibiotics if possible 1, 2