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

MRI is the Most Appropriate Investigation

For this diabetic patient with a forefoot ulcer, swelling, warmth, and calf tenderness, MRI is the most appropriate investigation to assess for osteomyelitis and soft tissue infection extent before determining treatment strategy. 1

Clinical Reasoning

This presentation is highly concerning for diabetic foot infection (DFI) with possible osteomyelitis:

• 2 cm ulcer over first metatarsal (high-risk location for bone involvement) 1
• Swelling, warmth, and tenderness extending to calf (suggests moderate-to-severe soft tissue infection) 1
• Intact distal pulses (rules out critical ischemia as primary concern, making duplex ultrasound unnecessary) 2

Why MRI is the Correct Answer

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

MRI Advantages in This Clinical Scenario:

• Superior soft tissue characterization with higher tissue contrast than X-rays or CT 1
• Identifies critical findings: skin ulcers, sinus tracts, abscesses, tenosynovitis 1
• Differentiates cellulitis from edema (cellulitis enhances with IV contrast; edema does not) 1
• High diagnostic accuracy for osteomyelitis: sensitivity 93%, specificity 75% 1
• Assesses infection extent in forefoot: infection can spread into adjacent compartments, which MRI delineates well 1
• Radiation-free and widely available 1

Why Other Options Are Incorrect:

A. Duplex Ultrasound: While excellent for vascular assessment (sensitivity 80%, specificity 88% for soft tissue pathology) 2, this patient has intact distal pulses, making vascular insufficiency unlikely as the primary problem. The clinical picture points to infection, not ischemia.

B. Conventional Angiography: Invasive and unnecessary when pulses are intact. Reserved for cases requiring revascularization planning in critical limb ischemia 2.

D. CT with Contrast: While CT can evaluate forefoot structures and detect soft tissue gas 1, it has inferior soft tissue contrast compared to MRI and provides less information about infection extent, abscesses, and bone marrow involvement 1.

Clinical Algorithm for This Patient

1. Perform MRI of the foot immediately 1

   • If MRI shows osteomyelitis → treat for bone infection 1
   • If MRI shows only soft tissue infection → withhold prolonged antibiotic therapy for osteomyelitis 1

2. Consider advanced imaging if MRI unavailable or contraindicated: WBC SPECT/CT or [18F]FDG PET/CT 1

3. Obtain cultures from deep tissue or bone (if surgical debridement performed) using conventional culture techniques 1

Important Caveats

• The first metatarsal area is a high-risk location: osteomyelitis occurs most frequently around the first and fifth metatarsophalangeal joints 1
• Forefoot infections can spread to adjacent compartments (unlike hindfoot infections which remain confined), making extent assessment critical for surgical planning 1
• MRI should be performed before nuclear medicine imaging since it's widely available and radiation-free 1
• If MRI results are equivocal, consider WBC SPECT/CT (sensitivity 86-90%) or [18F]FDG PET/CT for clarification 1