What are the required pre‑scan preparations and contraindications for a patient undergoing a PET‑CT (18F‑FDG)?

PET-CT (18F-FDG) Pre-Scan Requirements

Yes, there are essential prerequisites for PET-CT scanning that must be followed to ensure diagnostic quality images and patient safety. The most critical requirements include fasting for at least 4-6 hours, blood glucose control, adequate hydration, and specific activity restrictions before the scan 1, 2.

Pre-Scan Fasting Requirements

• Patients must fast for a minimum of 4 hours before 18F-FDG injection (6 hours is preferred for optimal results) 1
• Only plain, unflavored water is permitted during the fasting period 1
• Intravenous glucose solutions and parenteral nutrition must be discontinued at least 4 hours before FDG administration 1
• Coffee and caffeinated beverages are prohibited, even if labeled "sugar-free," as they may contain traces of simple carbohydrates and cause excitant effects 1
• Prescribed medications can be taken as usual 1

Common pitfall: Patients often assume "sugar-free" beverages are acceptable—they are not, as even trace carbohydrates can affect FDG uptake 1.

Blood Glucose Requirements

Blood glucose must be measured before FDG administration and should be below 200 mg/dL (11 mmol/L) for clinical studies 1, 2:

• If glucose is ≥200 mg/dL (≥11 mmol/L), the scan should be rescheduled 1
• For research studies, stricter thresholds of 126-150 mg/dL may apply 1
• Diabetic patients may require blood glucose stabilization on the day before and day of the scan 2
• For brain imaging specifically, glucose should be <160 mg/dL (<8.9 mmol/L); acute insulin correction is not recommended as it does not improve image quality 1

Important caveat: The impact of hyperglycemia may be less significant in infection/inflammation imaging compared to oncologic indications, but glucose control remains important 3.

Hydration Requirements

• Consume 1 liter of water during the 2 hours prior to FDG injection 1
• Adequate hydration reduces FDG concentration in urine (minimizing artifacts) and decreases radiation dose to the bladder 1, 2
• If IV contrast is planned, hydration is especially critical to minimize kidney injury risk 1
• Patients should void immediately before scanning to reduce bladder activity 1

Activity and Positioning Restrictions

Patients must avoid strenuous exercise for at least 6 hours before the scan (preferably 24 hours) 1:

• After FDG injection, patients must remain seated or lying down and stay silent during the 60-minute uptake phase to minimize muscle uptake 1
• Patients should be kept warm starting 30-60 minutes before FDG injection and continuing through image acquisition to reduce brown fat activation 1
• The patient must be able to lie still for 20-45 minutes during scanning 1

Timing Considerations After Treatment

Wait at least 10-15 days after systemic therapy (chemotherapy or endocrine therapy) to avoid metabolic flare phenomenon 1:

• For radiotherapy: wait at least 3 months before scanning the treated field to avoid false positives from inflammation 1
• For brain metastases: wait minimum 1 month after radiotherapy or surgery to reduce false positive results 1
• Document any recent corticosteroid use, growth factors, biopsies, or surgical procedures as these affect FDG uptake 1

Contraindications and Special Considerations

Renal Function Assessment

• Check serum creatinine and estimated glomerular filtration rate if IV contrast is planned 1
• Volume repletion is protective against acute kidney injury, especially in patients with myeloma-related kidney disease 1

Contrast Allergy History

• Document any previous contrast reactions (idiosyncratic vs. non-idiosyncratic) 1
• Premedication protocols should be implemented for patients with prior severe reactions 1
• For severe contrast allergy history, perform unenhanced CT instead 1

Claustrophobia Screening

• Screen for claustrophobia when scheduling the study to allow premedication planning and reduce cancellations 1
• Regular sedative use in adults is not recommended, but premedication can be planned for known claustrophobic patients 1

PET/MRI-Specific Requirements

• Complete standardized MRI safety checklist for implants, pacemakers, metallic objects, and claustrophobia 1
• Remove all metallic items including dental prostheses, jewelry, and clothing with zippers 1
• Verify implant safety status: "MRI unsafe" (absolute contraindication), "MRI conditional" (relative contraindication with specific conditions), or "MRI safe" 1

Required Clinical Information

The following must be documented before scanning 1:

• Height and body weight (measured accurately for SUV calculations) 1
• Complete oncological history and known tumor sites 1
• Current medications, especially antidiabetic drugs, corticosteroids, and growth factors 1
• Recent clinical symptoms (pain, fractures, fever) 1
• Co-morbidities including chronic kidney disease, allergies, thyroid dysfunction 1
• Previous imaging results with dates and reports 1

The combination of proper fasting, glucose control, hydration, and activity restriction is essential for diagnostic quality PET-CT imaging 1.