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FDG PET/CT — principles, preparation and pitfalls
FDG is a glucose analogue that enters the cell through GLUT transporters, is phosphorylated by hexokinase and is trapped because it does not continue down glycolysis — so any tissue that consumes a lot of glucose, malignant or inflammatory, "lights up".
Why it matters in the exam
Many PET questions in the exam are not about a specific tumour but about physiology and pitfalls: a patient who ate before the study, a diabetic with high glucose, uptake in brown fat, in muscle after exercise, in bowel after metformin, in marrow after G-CSF, or in the thymus of a young adult after chemotherapy. Preparation: fasting of at least 4–6 hours, blood glucose before injection preferably below 200 mg/dL (per EANM; above 200 mg/dL postponement is considered), rest in a warm quiet room, and an interval of about 60 minutes between injection and imaging.
Normal physiological uptake is seen in the brain (high glucose use, so FDG is poorer for brain metastases), the heart (highly variable), the kidneys and bladder (FDG is excreted in urine, unlike glucose), Waldeyer ring and bowel. Insulin before the study diverts FDG to muscle and fat. The SUV normalises uptake to activity and body weight; it is useful for comparing studies of the same patient but is affected by glucose, imaging time, weight and lesion size (a lesion smaller than about 8 mm looks fainter than it is — the partial volume effect).
False positives: infection, granulomas (sarcoidosis, tuberculosis), changes after surgery or radiation (wait about 6–8 weeks after surgery and about 3 months after radiation), a vaccine reaction in axillary nodes, and a fresh fracture. False negatives: tumours with low glycolytic activity — lung adenocarcinoma in situ, well-differentiated neuroendocrine tumour, renal cell carcinoma, prostate cancer, mucinous carcinoma, lobular breast carcinoma, well-differentiated HCC — small lesions, or high glucose.
Key points
- FDG is trapped in the cell after phosphorylation by hexokinase.
- Fast 4–6 hours and keep glucose below 200 mg/dL.
- Physiological uptake: brain, heart, kidneys, bladder, bowel, brown fat.
- False positives: infection, granulomas, post-surgical or post-radiation change.
- False negatives: low-glycolysis tumours and small lesions.
Summary table
| Situation | Effect on FDG | What to do |
|---|---|---|
| High glucose or recent insulin | Lower tumour uptake, muscle uptake | Postpone if above 200 mg/dL |
| Brown fat | Symmetric uptake in the neck and between the ribs | Warm room, sometimes a beta-blocker |
| Metformin | Diffuse uptake in the colon | Interpret with care or stop for two days |
| G-CSF or chemotherapy | Uptake in marrow, spleen and thymus | Do not read as metastases |
| Tumour with low glycolysis | False negative | Another agent: PSMA, DOTATATE |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Nuclear medicine, Oncology, Diagnostic radiology.
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Sources
- EANM Procedure Guidelines for Tumour Imaging: FDG PET/CT Version 2.0, 2015 — Patient preparation, acquisition and SUV standardisation
- SNMMI Procedure Standard for Tumour Imaging with F-18 FDG PET/CT, 2023 — Interpretation, physiological uptake and pitfalls
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.