What is the imaging pattern on PET (Positron Emission Tomography) CT in patients with Pyrexia of Unknown Origin (PUO)?

PET/CT Imaging Patterns in Pyrexia of Unknown Origin (PUO)

[18F]FDG PET/CT demonstrates high diagnostic yield in patients with PUO, with a sensitivity of 84% and specificity of 62%, capable of identifying the underlying cause in approximately 56% of cases. 1, 2

Key Imaging Patterns in PUO

Common Causes and Their Imaging Patterns

• Infections (most common cause):

  • Focal increased [18F]FDG uptake in abscesses (including splenic)
  • Linear or focal uptake in endocarditis (11% of PUO cases) 2
  • Increased uptake in osteomyelitis showing bone involvement
  • Diffuse or focal uptake in infected prosthetic devices

• Non-infectious inflammatory diseases:

  • Linear uptake along large vessel walls in vasculitis
  • Segmental bowel wall uptake in inflammatory bowel disease (5% of PUO cases) 2
  • Symmetrical joint uptake in systemic juvenile idiopathic arthritis (5% of PUO cases) 2
  • Multifocal lymph node and organ uptake in sarcoidosis

• Malignancies:

  • Focal or multifocal intense uptake in lymphoma (especially Hodgkin's and aggressive non-Hodgkin's)
  • Focal uptake in solid tumors (colorectal cancer, sarcomas)
  • Diffuse bone marrow uptake in hematologic malignancies

Diagnostic Performance

PET/CT demonstrates excellent performance in PUO evaluation:

• Sensitivity: 84% (79-89%)
• Specificity: 62% (49-75%)
• Diagnostic yield: 56% (50-61%)
• Diagnostic accuracy: 76%
• Helpfulness in clinical management: 61% 1

The high negative predictive value (80.9-100%) is particularly valuable, as a negative scan essentially rules out focal pathology as the cause of fever 2, 3.

Technical Considerations

Optimal Timing and Protocol

• Perform [18F]FDG PET/CT within 3 days of initiating glucocorticoid therapy to avoid false negatives 1
• Consider myocardial suppression preparation when cardiac etiology is suspected 1
• Early implementation in the diagnostic workup improves outcomes by allowing timelier diagnosis 2

Interpretation Pitfalls

False positives can occur due to:

• Recent surgical procedures
• Foreign body reactions
• Inadequate suppression of physiological myocardial FDG uptake
• Normal physiological uptake in certain organs (brain, heart, urinary tract) 2

False negatives may occur with:

• Prior extended antibiotic therapy (suppresses inflammatory activity)
• Small or mobile lesions
• Systemic diseases without focal manifestations 2

Clinical Applications

PET/CT is particularly valuable when:

• Conventional imaging is negative or inconclusive
• Initial basic investigations have failed to identify the cause
• Inflammatory markers (CRP, ESR) are elevated 1, 2

Important caveat: PET/CT has insufficient evidence for evaluation of PUO with normal inflammatory markers 1.

Impact on Patient Management

The results of [18F]FDG PET/CT can:

• Guide further investigations and biopsy
• Direct specific treatment when the cause is established
• Lead to treatment modifications in approximately 53% of cases 2
• Predict favorable prognosis through spontaneous resolution when negative 1

Emerging Technologies

PET/MRI shows potential but currently has insufficient data to support routine use in PUO evaluation 1, 2.