What is the FDG (Fluorodeoxyglucose) PET (Positron Emission Tomography) imaging protocol for Pyrexia of Unknown Origin (PUO)?

FDG-PET Imaging Protocol for Pyrexia of Unknown Origin (PUO)

FDG-PET/CT is recommended as a valuable diagnostic tool for patients with pyrexia of unknown origin, with sensitivity of 80-100% and specificity of 66.7-79.2%, capable of identifying the source of fever in approximately 48% of cases and leading to treatment modifications in up to 53% of patients. 1

Patient Selection and Timing

• FDG-PET/CT should be considered after standard initial workup fails to identify the cause of fever persisting for at least 3 weeks with temperature higher than 38.3°C (100.9°F) 2
• The examination should ideally be performed within 3 days of initiation of oral glucocorticoid therapy to prevent suppression of inflammatory activity 2
• FDG-PET/CT has higher diagnostic yield in patients with elevated inflammatory markers (CRP, ESR) and is not recommended in patients with normal inflammatory markers 2, 3
• Early implementation in the diagnostic workup may improve outcomes by allowing for timelier diagnosis 1

Patient Preparation

• Patients should fast for at least 4-6 hours prior to FDG injection to minimize physiologic glucose uptake 2
• Consider myocardial suppression preparation (high-fat, low-carbohydrate diet for 24 hours before the scan, plus prolonged fasting) when there is potential cardiac etiology to reduce physiological myocardial uptake 2
• Adequate hydration should be maintained before and after FDG injection 2
• Blood glucose levels should be checked before FDG administration, with optimal levels below 150-180 mg/dL 2

Acquisition Protocol

• Whole-body imaging from skull base to mid-thigh is standard, with extension to include the brain and lower extremities when clinically indicated 2, 1
• Image acquisition typically begins 60 minutes after FDG injection 2
• For suspected renal involvement, consider diuretic administration and delayed imaging (at least 1 hour post-diuretic) to improve detection of renal inflammatory foci 4
• Low-dose CT should be performed for attenuation correction and anatomical correlation 2

Interpretation Considerations

• Physiologic FDG uptake in brain, heart, urinary tract, and bowel must be distinguished from pathologic uptake 2, 4
• Common causes of PUO identified by PET/CT include endocarditis (11%), systemic juvenile idiopathic arthritis (5%), and inflammatory bowel disorder (5%) 1
• False positives can occur due to recent valve implantation, inadequate suppression of myocardial FDG uptake, or prior use of certain surgical adhesives 1
• Extended periods of antibiotic therapy prior to PET/CT may reduce inflammatory activity and lead to false negative results 1

Clinical Impact and Performance

• FDG-PET/CT has demonstrated high diagnostic performance with sensitivity ranging from 80% to 100% and specificity from 66.7% to 79.2% in pediatric patients 1
• In adult patients with bacteremia of unknown origin, FDG-PET/CT had a high clinical impact in 47.3% of cases 1
• A negative FDG-PET/CT can predict favorable prognosis through spontaneous remission of fever and potentially allows a watchful waiting approach 2
• Cost-effectiveness of FDG-PET/CT, particularly when performed early in the diagnostic work-up, has been demonstrated in both FUO and inflammation of unknown origin 2

Limitations and Pitfalls

• Small or mobile lesions (such as small vegetations in endocarditis) may be missed by PET/CT 1
• False positives can occur in areas of physiologic uptake, particularly in the urinary tract 4
• Most studies on FDG-PET/CT in PUO are retrospective with relatively small patient numbers, which may introduce selection bias 1
• FDG-PET/CT should be considered as a second-line investigation for diagnostic evaluation of PUO, especially in suspected non-infectious inflammatory disorders 5