What type of scan is used for diagnosing infections?

Diagnostic Imaging for Infection Detection

MRI with contrast is the gold standard for diagnosing infections, with 96% sensitivity and 94% specificity, and should be the first-line imaging modality when infection is suspected. 1

Optimal Imaging Selection Based on Anatomical Location

Spine Infections

• First-line: MRI without and with IV contrast

  • Highest sensitivity (96%) and specificity (94%) for spine infections 1
  • Excellent for detecting epidural abscesses, discitis, and osteomyelitis
  • Provides optimal visualization of intraspinal contents including epidural space and spinal cord 1
  • IV contrast increases lesion conspicuity and helps define the extent of infection

• Second-line options (when MRI is contraindicated):

  • CT scan: 79% sensitivity, 100% specificity, but only 6% sensitivity for epidural abscess 1
  • FDG-PET/CT: Increasingly used for spine infection assessment with sensitivity of 94.8% and specificity of 91.4% 1

Joint Prosthesis Infections

• First-line: MRI with contrast (when possible)
• Alternative approaches:
  • Sequential combination of bone and labeled leukocyte scans: 88% sensitivity, 78% specificity 1
  • Labeled leukocyte/marrow imaging: Can achieve up to 100% sensitivity and specificity in some studies 1

Musculoskeletal Infections

• Superficial infections: Ultrasound (readily available, cost-effective) 2
• Diffuse or deep infections: CT scan (quick, widely available) 2
• Suspected bone involvement: MRI (gold standard for acute osteomyelitis) 2

Nuclear Medicine Options for Specific Scenarios

When MRI is Contraindicated or Inconclusive

1. FDG-PET/CT:

   • Increasingly replacing conventional nuclear medicine studies 3
   • Higher resolution, better sensitivity for low-grade infections 3
   • Sensitivity of 98% for detecting infectious foci 4

2. Three-phase bone scan:

   • Moderate-to-high sensitivity (81.4%) but low specificity (40.7%) for spine infection 1
   • Most useful in unviolated bone without underlying osseous abnormalities 5

3. WBC scans:

   • Labeled leukocyte imaging: Best for "complicating osteomyelitis" (infected prostheses, diabetic foot) 5
   • Often requires complementary bone marrow imaging for accuracy 5
   • Limited utility in spine infections (often presents as nonspecific photopenic defect) 5

4. Gallium scan:

   • Less sensitive (73%) but more specific (61%) than bone scan 1
   • Dual gallium/bone scan increases specificity to 81% 1
   • Preferred nuclear medicine study for spinal osteomyelitis 5

Common Pitfalls and Limitations

• Post-surgical evaluation: Distinguishing normal post-operative changes from infection is challenging, especially within 6 weeks of surgery 1
• WBC scans: Decreased sensitivity in low-grade infections and those with limited neutrophilic component 1
• Bone scans: Specificity decreases significantly in the presence of underlying osseous abnormalities 5
• Antimicrobial therapy: Recent treatment can decrease sensitivity of WBC scans 3
• Anatomic localization: Nuclear medicine studies provide limited anatomic detail compared to CT/MRI 3

Special Considerations for Urinary Tract Infections

• CT scan is the method of choice for diagnosis and follow-up of emphysematous urinary tract infections, which are rare but potentially fatal 6
• These infections should be suspected in diabetic patients with UTIs and worsening renal function 6

By selecting the appropriate imaging modality based on the suspected site of infection, clinical context, and availability, clinicians can optimize diagnostic accuracy while minimizing unnecessary testing.