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Difficulty: MediumOrdering Imaging Studies

A 65-year-old man presents to the outpatient clinic for evaluation of a non-healing foot ulcer that he first noticed 4 weeks ago. His medical history is significant for poorly controlled type 2 diabetes mellitus, hypertension, and stage 4 chronic kidney disease. His current medications include insulin glargine, amlodipine, and sodium bicarbonate. He reports no fevers, chills, or systemic symptoms. Vital signs are within normal limits. Physical examination reveals a 2.5-cm, painless ulceration on the plantar aspect of the right first metatarsal head. The ulcer has a hyperkeratotic border and mild surrounding erythema. The wound base contains granulation tissue, but a sterile metal probe gently inserted into the ulcer easily contacts hard, gritty bone. Pedal pulses are palpable bilaterally. Laboratory studies show a serum creatinine of 2.9 mg/dL and an estimated glomerular filtration rate (eGFR) of 21 mL/min/1.73 m². Which of the following is the most appropriate initial diagnostic imaging study for this patient?

  1. Plain radiography of the right footAnswer
  2. B
    Magnetic resonance imaging of the right foot without intravenous contrast
  3. C
    Magnetic resonance imaging of the right foot without and with intravenous gadolinium contrast
  4. D
    Computed tomography of the right foot without intravenous contrast
  5. E
    Three-phase technetium-99m bone scintigraphy

Answer

Plain radiography of the right foot
The correct answer is plain radiography of the foot. In any patient with a suspected diabetic foot infection and possible osteomyelitis (strongly suggested in this scenario by the positive probe-to-bone test), plain radiography is the most appropriate initial imaging study. Even though plain radiographs have low sensitivity for detecting early acute osteomyelitis—as cortical bone destruction may not become radiographically apparent for 2 to 3 weeks—they are crucial as a first-line test. Radiographs can rapidly rule out the presence of radiopaque foreign bodies, detect soft-tissue gas indicating a potentially limb-threatening necrotizing infection, identify underlying structural deformities such as Charcot neuroarthropathy, and establish a baseline for monitoring disease progression.

Step-by-Step Solution

1
Analyze the clinical presentation and physical examination findings.
The patient is a diabetic male with a chronic foot ulcer and a positive probe-to-bone test, which is highly predictive of underlying osteomyelitis.
Establishing the primary clinical suspicion dictates the diagnostic pathway.
2
Evaluate the patient's laboratory values for potential contraindications.
The patient has stage 4 chronic kidney disease with an eGFR of 21 mL/min/1.73 m², indicating severe renal impairment.
Renal function directly impacts the safety profile of certain imaging contrast agents, particularly gadolinium.
3
Determine the most appropriate initial imaging study according to clinical guidelines.
Evidence-based guidelines mandate that plain radiography (X-ray) should be the first imaging modality performed for suspected osteomyelitis.
Plain radiographs are rapid, cost-effective, and essential for identifying soft-tissue gas, foreign bodies, or baseline bony destruction before more expensive tests are considered.

Key Concept

Plain radiography is the most appropriate initial diagnostic imaging study for all patients with suspected diabetic foot osteomyelitis. Advanced imaging, such as MRI, is reserved for cases where radiographs are negative or equivocal.
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Ordering Imaging Studies

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