Infection Imaging: MRI & Nuclear Medicine

I would request MRI of the foot as the next investigation. On MRI, I would look for features of osteomyelitis: T1 hypointensity in the metatarsal head (marrow fat replacement), T2/STIR hyperintensity in the same location, soft tissue enhancement, and critically, a sinus tract connecting the ulcer to the bone. The presence of a sinus tract is highly specific for osteomyelitis. I would also assess for extent of soft tissue infection and abscess formation. Clinically, a positive probe-to-bone test through the ulcer has approximately 90% PPV for osteomyelitis.

The progressive lucency with elevated inflammatory markers raises concern for PJI. My investigation would include: (1) Joint aspiration - withhold antibiotics for 2 weeks, obtain under fluoroscopy or US guidance, send for cell count (WCC greater than 4200/microL suspicious for hip PJI), differential (greater than 80% PMN), and culture including prolonged culture. (2) Imaging - combined labelled WBC scan with sulphur colloid marrow scan is the gold standard with greater than 90% accuracy. Mismatched uptake (WBC without matching marrow) indicates infection. Plain bone scan has low specificity as it remains positive for years post-surgery.

On MRI, acute osteomyelitis would show: T1 hypointensity in the metaphyseal marrow (replacing normal fat signal), T2/STIR hyperintensity in the same region, periosteal reaction and soft tissue oedema on fat-saturated sequences, and post-contrast enhancement. I might see a Brodie's abscess with the pathognomonic penumbra sign (T1 hyperintense rim). Differentiating from Ewing sarcoma: Both can cause aggressive periosteal reaction (onion-skin), but infection typically has more soft tissue oedema/cellulitis pattern rather than a discrete mass, responds to antibiotics clinically, has elevated inflammatory markers, and lacks the Lodwick III permeative bone destruction pattern. Biopsy may be needed if uncertainty persists.