Which scan is recommended to exclude osteomyelitis in a diabetic patient with a foot ulcer and peripheral neuropathy?

Imaging for Foot Wounds to Rule Out Osteomyelitis

Start with plain radiographs of the affected foot in all patients with foot wounds concerning for osteomyelitis, then proceed to MRI if the diagnosis remains uncertain—MRI is the single most accurate imaging study for confirming or excluding osteomyelitis. 1

Initial Imaging: Plain Radiographs

• Obtain plain radiographs immediately for any patient presenting with a foot wound where osteomyelitis is suspected, looking for cortical erosion, periosteal reaction, bone destruction, mixed lucency and sclerosis, soft tissue gas, and radio-opaque foreign bodies. 1, 2

• Plain radiographs have relatively low sensitivity (28-93%) and specificity (25-92%) for osteomyelitis, and changes may not appear for 2-4 weeks after infection onset. 1

• If initial radiographs show classic changes of osteomyelitis (cortical erosion, active periosteal reaction, mixed lucency and sclerosis), treat for presumptive osteomyelitis after obtaining cultures. 1

• If initial radiographs are normal but clinical suspicion persists, treat the soft tissue infection for 1-2 weeks with appropriate wound care and off-loading, then repeat radiographs in 2-4 weeks—progressive changes on serial films have greater sensitivity and specificity. 1

Advanced Imaging: MRI as the Gold Standard

• MRI is the imaging study of choice when further evaluation is needed, particularly when the diagnosis remains uncertain after plain radiographs or when planning surgical intervention. 1, 3

• MRI demonstrates superior diagnostic accuracy with sensitivity of 77-91% and specificity of 78-100% for detecting osteomyelitis in diabetic foot wounds. 4, 5, 6

• A normal MRI reliably excludes osteomyelitis with a 100% negative predictive value—normal bone marrow signal on T1-weighted sequences effectively rules out infection. 1

• MRI provides critical additional information including soft tissue abscess detection, extent of bone and soft tissue involvement, and helps guide surgical planning. 1, 6

• Request T1-weighted and T2-weighted (or STIR) sequences as the standard protocol; T1-weighted sequences showing decreased bone marrow signal are highly specific for osteomyelitis. 1, 3

Important MRI Pitfalls to Avoid

• Do not rely solely on bone marrow edema, as reactive edema from trauma, previous surgery, or Charcot neuroarthropathy can mimic osteomyelitis and reduce specificity. 3

• MRI may have difficulty distinguishing infection from reactive inflammation, and metal hardware creates significant artifact that limits interpretation. 1

• Request interpretation by an experienced musculoskeletal radiologist when available, as accurate interpretation requires expertise. 1, 3

Alternative Imaging When MRI is Unavailable or Contraindicated

• When MRI is unavailable or contraindicated, use labeled white blood cell (WBC) scintigraphy combined with bone scan as the best alternative. 1

• FDG-PET/CT offers high specificity (92-93%) and sensitivity (81%) and should be considered when MRI cannot be performed. 1, 3, 7

• Do not use three-phase technetium bone scans alone—they have very high sensitivity (67-100%) but extremely low specificity (18-83%), resulting in unacceptably high false-positive rates. 1, 4, 5

• Indium-labeled leukocyte scintigraphy has poor accuracy (50%) in diabetic foot infections and should not be used as a standalone test. 5

Clinical Adjuncts to Imaging

• Perform a probe-to-bone (PTB) test for any foot wound—when properly conducted, it helps diagnose osteomyelitis when clinical likelihood is high or exclude it when likelihood is low. 1, 2

• Obtain inflammatory markers (ESR, CRP, or procalcitonin) to support the diagnosis, though these are nonspecific and should be used in combination with imaging and clinical findings. 2

Definitive Diagnosis: Bone Biopsy

• Bone culture and histology provide the most definitive diagnosis of osteomyelitis, particularly when the diagnosis remains uncertain despite imaging or when identifying the causative organism is critical for antibiotic selection. 1, 2

• Consider bone biopsy when there is diagnostic uncertainty after imaging, absence of culture data from soft tissue, or involvement of mid- or hind-foot (which are more difficult to treat and more likely to require high-level amputation). 1

• Bone biopsy can be performed percutaneously under fluoroscopic or CT guidance or operatively; complications are very rare. 1