PET-CT Scan: Why It's Done, How to Prepare and What Happens on the Day

A PET-CT scan combines two pictures in one visit: a PET scan, which shows how active the cells in your body are, and a CT scan, which shows the exact anatomy. You are given an injection of a radioactive sugar called FDG (fluorodeoxyglucose), rest for about an hour while it spreads, and then lie on a scanner for 20 to 40 minutes. Cancer cells and inflamed tissue use more sugar, so they appear brighter. It is mainly used to stage cancer, check whether treatment is working and look for recurrence. Preparation involves fasting for about 4 to 6 hours, avoiding strenuous exercise the day before and managing blood sugar carefully.

This guide explains what the scan shows, who needs one, how to prepare (including special advice for people with diabetes), what happens step by step, how safe it is, what to do afterwards and what the scan cannot tell you.

How a PET-CT works

Cells take up glucose for energy. Many cancers burn far more glucose than normal tissue. FDG is glucose labelled with a tiny amount of a radioactive isotope (fluorine-18). After injection, FDG collects in tissues that use a lot of glucose. The PET camera detects the signal and makes a map of activity. The CT, done in the same machine, gives a detailed anatomical picture. Fusing the two shows exactly where the active areas are.

The result is reported with a measure called SUV (standardised uptake value), which expresses how strongly an area takes up FDG. There is no universal SUV cut-off for cancer, because many benign conditions also show uptake.

Why doctors order a PET-CT

In cancer care

• Staging. To find out how far a cancer has spread, for example in lymphoma, lung cancer, oesophageal, head and neck, cervical and many other cancers.
• Finding the primary site when cancer has spread but the origin is unknown.
• Checking response to treatment, often midway through or after chemotherapy or radiotherapy.
• Detecting recurrence when tumour markers rise or symptoms return.
• Planning radiotherapy or surgery.
• Guiding a biopsy to the most active area.

Beyond cancer

• Brain: locating the source of seizures in epilepsy surgery planning, and in some cases supporting the diagnosis of dementia.
• Heart: assessing whether heart muscle that looks damaged is still alive (viability), which affects decisions about bypass or angioplasty.
• Infection and inflammation: looking for hidden infection, fever of unknown origin, vasculitis, sarcoidosis and infected implants.

Other tracers, such as PSMA for prostate cancer and DOTATATE for neuroendocrine tumours, are used for specific conditions and need separate preparation, which your centre will explain.

Who may not need one

PET-CT is not a routine health check. It is not recommended as a screening scan for healthy people, because it can pick up harmless findings that lead to unnecessary tests. It's also not usually the first test: doctors typically start with blood tests, ultrasound, X-ray, CT or MRI and use PET-CT when it will change management.

How to prepare

Follow your centre's written instructions. General principles:

Fasting

• No food for about 4 to 6 hours before the injection. Eating raises insulin and sends FDG into muscle and fat instead of the area of interest.
• Water is allowed and encouraged: drinking plenty of plain water helps the dye clear.
• No sugary drinks, juice, tea or coffee with sugar, gum, candy or glucose drips during the fasting window. (Your centre will tell you about black tea or coffee.)
• Some centres advise a low-carbohydrate meal the evening before.

Exercise

Avoid strenuous exercise and heavy lifting for 24 hours before. Muscles take up FDG and can create misleading areas of uptake.

Medicines

Continue regular medicines with water unless told otherwise. Tell the team about everything you take.

Blood sugar

Blood glucose is checked before FDG is given. Most centres want it below roughly 150 to 200 mg/dL, because high blood sugar competes with FDG and lowers image quality. If your glucose is too high, the scan may be postponed.

If you have diabetes

• Tell the centre in advance. Instructions differ for diet-controlled, oral medicines and insulin.
• Usually you take your tablets as directed, but avoid the usual breakfast. Insulin timing is adjusted so that insulin is not active at the time of injection. Do not change doses on your own; follow the centre's or your doctor's plan.
• Bring your meter and any snacks to eat after the scan.
• Book the scan for early morning if possible, so fasting is easier.

What to bring

• Referral letter, previous scans, biopsy reports, blood tests and a list of medicines
• Comfortable clothes without metal
• Someone to accompany you if you feel anxious or are elderly

Special situations

• Pregnancy: tell the staff if you may be pregnant. PET-CT is avoided in pregnancy unless essential.
• Breastfeeding: you may be asked to pause breastfeeding for some hours and express and discard milk. Follow your centre's advice.
• Claustrophobia: the scanner is open at both ends, but tell the staff if you are anxious.
• Kidney function: needed if CT contrast is planned.

What happens on the day

1. Check-in and history. You are weighed, your height is recorded and your blood sugar is checked.
2. Injection. FDG is injected into a vein. It doesn't hurt more than a normal injection.
3. Uptake period (about 45 to 60 minutes). You rest quietly in a dimly lit room, with no talking on the phone, reading or walking around. Movement and chatting can make muscle uptake. You may be asked to drink water and empty your bladder before the scan.
4. Scan (20 to 40 minutes). You lie on a narrow table and pass slowly through the scanner. You must stay still. The CT portion is quick; the PET portion takes longer. Sometimes intravenous or oral contrast is given for the CT.
5. After the scan. You may be asked to wait for the technologist to check images. Most people go home within 2 to 3 hours of arrival in total.

The whole visit usually takes 2 to 3 hours.

Is it safe? Radiation and precautions

A PET-CT delivers a radiation dose that is higher than a single X-ray and comparable to or somewhat higher than a CT of the chest and abdomen, in total about several years of natural background radiation, depending on the protocol. The radioactivity from FDG decays quickly (half-life about 110 minutes) and leaves the body through urine. The doctor orders a PET-CT only when the information is expected to benefit your care.

After the scan:

• Drink plenty of water and pass urine frequently for the next several hours to flush out the tracer.
• Limit close contact (prolonged, within about one metre) with pregnant women, infants and young children for about 4 to 6 hours, or as advised by your centre.
• Wash your hands after using the toilet and flush twice.
• You can usually return to normal activities, and driving is fine unless you were sedated.
• Breastfeeding guidance as per the centre.

Allergic reactions to FDG are extremely rare. Reactions to CT contrast are possible and the team is prepared for them.

Reading the report

The radiologist or nuclear medicine physician describes areas of increased uptake, their size, location and SUV, and compares them with earlier scans if available. The report should answer your doctor's question, for example: Where is the disease? Has it spread? Has it responded? Is there evidence of recurrence?

What a "hot spot" does and does not mean

Increased uptake is not the same as cancer. Benign causes include:

• Infection and inflammation, including tuberculosis, pneumonia and abscesses
• Recent surgery, biopsy or radiotherapy
• Sarcoidosis and other inflammatory diseases
• Brown fat activated by cold
• Muscle activity, especially in the neck from tension
• Healing fractures
• Benign tumours

And absence of uptake does not always mean absence of cancer

Some tumours take up little FDG, such as certain low-grade, slow-growing cancers (some neuroendocrine tumours, mucinous tumours, early prostate cancer and some lung adenocarcinomas). Very small lesions (under about 5 to 8 mm) may be missed. The brain normally takes up a lot of sugar, which makes brain tumours harder to assess with FDG.

That's why doctors read PET-CT alongside other tests, and why a biopsy may still be needed to confirm a diagnosis.

Common questions about the experience

• Will I feel anything after the injection? Most people feel nothing. A metallic taste or warmth is occasionally reported.
• Will I be radioactive? Briefly, at a low level. The precautions above cover it.
• Can I eat afterwards? Yes, as soon as the scan is finished. Take your diabetes medicines as advised.
• How soon will the report be ready? Usually the same day or the next, depending on the centre.
• Can I do it with a pacemaker or implants? Yes in most cases; tell the staff.
• Can children have a PET-CT? Yes, when needed, with special low-dose protocols.

Frequently asked questions

Why is a PET-CT scan done?

Mainly to detect, stage and monitor cancer, and also to evaluate some brain, heart and inflammatory conditions.

How should I prepare for a PET-CT?

Fast 4 to 6 hours, drink water, avoid strenuous exercise for 24 hours, and manage diabetes as instructed by the centre.

How long does a PET-CT take?

About 2 to 3 hours in total, with 20 to 40 minutes on the scanner.

Is a PET-CT painful?

No, apart from the injection. You must lie still.

Is PET-CT better than CT or MRI?

It shows function, which CT and MRI don't. It is used when metabolic information changes the plan. It complements, rather than replaces, other scans.

Can PET-CT detect all cancers?

No. Some cancers take up little FDG and small lesions may be missed.

Is radiation from PET-CT harmful?

The dose is higher than a plain X-ray but is justified when needed. Drinking water and following precautions limit exposure to others.

When to see a doctor

A PET-CT is ordered by a doctor, usually an oncologist or specialist. See a doctor first if you have unexplained weight loss, a lump, persistent fever, night sweats, persistent cough, blood in stool or urine, or a rise in tumour markers. Do not book a PET-CT as a health check on your own.

Care at Prakash Hospital Noida

Whether you live in Sector 18, Sector 62, Greater Noida West, or anywhere nearby, Prakash Hospital Noida is a trusted name for oncology and advanced diagnostics in Noida. Our specialists decide when a PET-CT will help, guide your preparation and explain the findings in the context of your treatment.

To book a consultation, call the number.

Sources: Society of Nuclear Medicine and Molecular Imaging procedure guidelines for FDG PET/CT; EANM guideline for FDG PET/CT tumour imaging. Preparation details vary by centre; follow your centre's instructions.

Tags: #PETCTScan #CancerImaging #PrakashHospitalNoida