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Difficulty: HardOrdering Imaging Studies

A 68-year-old man presents to the clinic for evaluation of a nonhealing ulcer on the plantar aspect of his right foot. He has a 15-year history of type 2 diabetes mellitus complicated by severe peripheral neuropathy. His past medical history is also significant for sick sinus syndrome, for which a legacy, non-MRI-conditional dual-chamber permanent pacemaker was placed 12 years ago. His temperature is 37.8°C (100.0°F), blood pressure is 135/85 mm Hg, and pulse is 72/min. Physical examination reveals a 2.5-cm, deep ulcer over the first metatarsal head. The ulcer base contains granulation tissue with some surrounding erythema and mild purulent exudate. A sterile metal probe is inserted into the ulcer but does not palpate bone. Laboratory studies show an erythrocyte sedimentation rate (ESR) of 88 mm/h and a leukocyte count of 11,500/mm³. Initial plain radiographs of the right foot show soft tissue swelling but no evidence of periosteal elevation, cortical disruption, or bone destruction. Given the high clinical suspicion for osteomyelitis, which of the following is the most appropriate next imaging study to evaluate this patient's condition?

  1. A
    Magnetic resonance imaging of the right foot with and without intravenous contrast
  2. Technetium-99m bone scan combined with indium-111 labeled leukocyte scintigraphyAnswer
  3. C
    Computed tomography of the right foot without contrast
  4. D
    Repeat plain radiography of the right foot in 48 hours
  5. E
    Arterial duplex ultrasonography of the right lower extremity

Answer

Technetium-99m bone scan combined with indium-111 labeled leukocyte scintigraphy
The patient presents with high clinical suspicion for diabetic foot osteomyelitis (large, deep ulcer, ESR > 70 mm/h). The standard initial imaging test, plain radiography, is negative, which is common in the first 1-2 weeks of infection. The definitive next step is advanced imaging. Magnetic resonance imaging (MRI) is the test of choice due to its high sensitivity for marrow edema. However, this patient has a legacy, non-MRI-conditional permanent pacemaker, which is an absolute contraindication to an MRI study. In patients who cannot safely undergo MRI, the most appropriate alternative imaging modality is a nuclear medicine study utilizing a technetium-99m bone scan combined with an indium-111 labeled leukocyte (WBC) scan. This combination provides high sensitivity (bone scan) and specificity (WBC scan localizing active infection).

Step-by-Step Solution

1
Synthesize the clinical presentation and initial workup.
The patient has a deep diabetic foot ulcer with an elevated ESR (88 mm/h) and a negative initial plain radiograph. Clinical suspicion for osteomyelitis is very high.
Plain radiographs are the initial test of choice but have low sensitivity (around 50-60%) for acute osteomyelitis in the first 2 weeks of infection.
2
Determine the standard next step in the diagnostic algorithm.
Magnetic resonance imaging (MRI) of the foot is typically the standard next step when initial X-rays are negative but clinical suspicion remains high.
MRI is highly sensitive for detecting early bone marrow edema characteristic of acute osteomyelitis.
3
Evaluate patient-specific contraindications.
The patient has a legacy, non-MRI-conditional pacemaker placed 12 years ago, which is an absolute contraindication to MRI.
Placing a patient with a legacy pacemaker in an MRI scanner risks fatal arrhythmias or device failure.
4
Select the best alternative imaging modality.
A combined technetium-99m bone scan and indium-111 labeled leukocyte (WBC) scintigraphy is the most appropriate alternative.
This combined nuclear medicine study offers the highest sensitivity and specificity for osteomyelitis in the absence of MRI.

Key Concept

In patients with suspected diabetic foot osteomyelitis and an absolute contraindication to MRI (such as a legacy pacemaker), a combined technetium-99m bone scan and labeled leukocyte scintigraphy is the most appropriate next diagnostic imaging study.
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Ordering Imaging Studies

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