FDG is the tracer used in most PET CT scans. Learn what it is, how it works, how long it stays in the body and whether it is safe.
FDG (fluorodeoxyglucose, with fluorine-18) is a sugar-like molecule with a tiny radioactive tag. It is injected before a PET CT scan and gathers in cells that use a lot of glucose, such as many cancer cells, the brain and active inflammation. It is safe, short-lived and leaves the body mainly through urine.
How Does FDG Work?
Cells take up glucose for energy. FDG enters cells in the same way but cannot be fully used, so it gets trapped inside. A PET scanner detects the tiny radioactivity of fluorine-18 and shows where FDG has gathered. Cancer cells, which often use more glucose, appear bright. See what a PET CT scan is.
FeatureFDGFull nameFluorine-18 fluorodeoxyglucoseBehaves likeGlucose (sugar)Half-lifeAbout 110 minutesMain route out of bodyUrineTypical injected amountVery small, measured in microgramsWhich Tissues Take Up FDG?
- Many tumours and active lymph nodes
- The brain, always, since it uses lots of glucose
- The heart muscle, depending on fasting
- Infection and inflammation
- The bladder and kidneys, as FDG passes out in urine
- Muscles, if you move or talk after injection
Why Fasting and Blood Sugar Matter
If your blood sugar or insulin is high, normal tissues may take up FDG, and the tumour signal can fall. That is why you fast for 4 to 6 hours and have your blood sugar checked. See PET CT preparation and PET CT for diabetic patients.
Is FDG Safe?
FDG is used in millions of scans worldwide. Allergic reactions are extremely rare. The radiation is low and short-lived, because fluorine-18 loses half its activity in under two hours. After 10 half-lives (about a day), almost none is left. Drinking water and passing urine help clearance. Read PET CT radiation and safety and PET CT side effects.
Other PET Tracers
Not every PET CT uses FDG. PSMA tracers are used for prostate cancer, and other tracers are used for neuroendocrine tumours, amyloid in the brain, and heart perfusion. Your doctor selects the tracer for your question. See PSMA PET CT.
A Practical Example
Lata, 55, has a PET CT with FDG for lymphoma. She fasts for 6 hours, her blood sugar is 98 mg/dL and she rests quietly after the injection. The scan shows clear uptake only in the diseased nodes and normal organs. Had she eaten sweets earlier, the image could have been poorer. A well-prepared patient gets a clear and reliable scan.
Key Facts at a Glance
- FDG is a sugar-like tracer used in most PET CT scans
- It is safe and short-lived
- Drink water after the scan
- Prepare as instructed for the best images
Questions to Ask Your Doctor
- Does my condition need FDG or a different tracer?
- How should I manage my diabetes on the scan day?
- Should I stop any medicine?
- How will the result guide treatment?
Common Mistakes to Avoid
- Eating sweets or taking glucose before the scan.
- Talking or walking during the uptake period.
- Skipping the blood sugar check.
Booking, Timing and Report
Ask your doctor for the best timing of the scan. Book a morning slot so that fasting is easier, arrange someone to accompany you, and allow 2 to 3 hours at the centre. Carry biopsy, scan and blood reports and a medicine list. Reports are usually ready within 24 to 48 hours, and images can be given on a CD or digital link for your doctor.
After the Scan: What to Do
Drink plenty of water and empty your bladder often for the first few hours. You can eat normally. If your centre advises it, keep a small distance from pregnant women and infants for a few hours. Take your report to the doctor who advised the scan.
Frequently Asked Questions
What is FDG?
A radioactive sugar-like tracer used in PET CT scans.
Is FDG safe?
Yes. It has a long safety record and a low dose.
How long does FDG stay in the body?
Most radioactivity fades within a day; it leaves via urine.
Does FDG cause allergy?
Very rarely.
Can diabetic patients have FDG?
Yes, with controlled blood sugar.
Why does the brain always light up?
The brain uses a lot of glucose.
Is FDG used for all cancers?
No. Some cancers need different tracers.
Can I stay near children after FDG?
Keep some distance for a few hours if your centre advises.
Does FDG affect pregnancy?
Tell the staff if you are pregnant; the scan is usually avoided.
Book Your Test at Ganesh Diagnostic
Ganesh Diagnostic & Imaging Centre in Rohini, Delhi offers this test with trained radiology staff and clear reports. You can book an appointment online, check our branches across Delhi NCR, contact us for current rates, or explore all our services. Reports can be downloaded online through our patient portal, so you do not need to make a second trip just to collect them.
This article is for general information only. It does not replace advice from your doctor. Always follow your doctor's instructions for your test.
