FDG Fundamentals
Fluorine-18 Fluorodeoxyglucose (F-18 FDG) is the primary radiotracer used in PET imaging.
Learning Objectives
- Explain the mechanism of FDG uptake and metabolic trapping
- Describe normal physiologic biodistribution
- Identify common variants and pitfalls
- Understand patient preparation requirements
Mechanism of Action
FDG is a Glucose Analog.
- Transport: Enters cell via glucose transporters (GLUT-1, GLUT-3).
- Phosphorylation: Phosphorylated by Hexokinase to FDG-6-Phosphate.
- Trapping: Unlike glucose, FDG-6-P cannot be metabolized further (isomerase step is blocked). It also cannot exit the cell (trapped).
Result: Accumulates in cells with high glycolytic rates (malignant tumors, brain, activated inflammatory cells).
Biodistribution
Normal/Physiologic Uptake
| Organ | Intensity | Reason |
|---|---|---|
| Brain | Very High | Obligate glucose user (gray matter) |
| Bladder/Kidneys | High | Renal excretion path |
| Liver | Moderate | Metabolic activity (Reference organ) |
| Heart | Variable | Prefers fatty acids fasting, but takes glucose fed |
| Bowel | Variable | Peristalsis, lymphoid tissue |
| Muscles | Low | Unless exercised/insulin driven |
Normal Variants
- Brown Fat: Neck/supraclavicular uptake in cold/anxious patients.
- Thymus: In younger patients (<40y) or thymic rebound post-chemo.
- Salivary Glands: Mild to moderate uptake.
- Larynx: Uptake from talking.
Patient Preparation
Goal: Minimize competitive inhibition by glucose and minimize insulin levels (which drive FDG into muscle).
- Fasting: NPO for 4-6 hours.
- Hydration: Drink water (helps clear background).
- Glucose Level: Ideally < 150-200 mg/dL.
- Avoid Strenuous Activity: 24h prior (reduces muscle uptake).
- Warmth: Keep patient warm to prevent brown fat activation.
SUV (Standardized Uptake Value)
A semi-quantitative measure of activity.
- SUV > 2.5: Often used as a Cutoff for malignancy (but non-specific).
- Background: Liver mean SUV is typically 2.0 - 2.5.
- Factors affecting SUV: Scan time, extravasation, glucose level, body composition.
Pitfall: Infection and Inflammation (e.g., Pneumonia, Sarcoidosis, Post-surgical change) show high FDG uptake due to activated leukocytes, leading to False Positives for cancer.