PET-CT Scan: How It Works, How to Prepare and What to Expect

A PET-CT scan shows not just what organs look like, but how active the cells inside them are. It is mainly used in cancer care, and careful preparation helps make the results accurate.

Imagine being told you need a PET-CT scan to check whether a lymphoma has spread. You are told not to eat, to avoid exercise, and that you will be radioactive for a few hours. It sounds alarming, but the process is straightforward once explained.

What a PET-CT scan is

Positron emission tomography (PET) uses a small amount of a radioactive tracer, most commonly a form of glucose called FDG. Cells that use a lot of glucose, such as many cancer cells and inflamed tissue, take up more tracer and show as bright spots on the images.,The PET scan is combined with a CT scan in the same machine, so the activity can be matched precisely to the body’s structures.

Why it is used

• Finding out whether cancer has spread (staging)
• Checking how well cancer treatment is working
• Looking for cancer recurrence
• Investigating some causes of fever or inflammation
• Some heart and brain conditions, using different tracers
• Some prostate and neuroendocrine cancers use specialised tracers

How to prepare

• Fast for about four to six hours before the scan, drinking only water
• Avoid strenuous exercise for 24 hours, as active muscles take up tracer
• Tell the department if you have diabetes, as high blood sugar affects results; you will be given specific instructions about food and medicines
• Tell them if you are or might be pregnant or are breastfeeding
• Wear warm, comfortable clothes without metal, as cold can activate a type of fat that takes up tracer

What happens on the day

Your blood sugar will usually be checked. A small cannula is placed in a vein, and the tracer is injected. You then rest quietly in a warm room for about an hour while the tracer spreads through your body. You will be asked not to talk much, read or use your phone, as muscle activity can affect where the tracer goes.,You then lie on a couch that moves slowly through the scanner, which is shaped like a ring rather than a tunnel. The scan itself usually takes 20 to 40 minutes. You need to lie still and breathe normally. The whole appointment typically lasts two to three hours.

Radiation safety

A PET-CT involves a radiation dose from both the tracer and the CT scan. The dose is higher than a standard X-ray but is considered justified when the scan helps guide important decisions about your care.,The tracer loses its radioactivity quickly. For the rest of the day, you may be advised to avoid close, prolonged contact with pregnant women and young children, and to drink plenty of fluids to help flush the tracer out. Follow the specific advice given by your department.

Understanding results

A radiologist or nuclear medicine specialist reviews the scan and writes a report, usually within a few days. Bright areas do not always mean cancer; infection, inflammation, healing tissue and normal organs such as the brain, heart, kidneys and bladder also take up tracer. Your specialist team will explain what the findings mean for you, often at a planned clinic appointment.

Coping with scan anxiety

• Ask the team to explain each step
• Bring music or a calming playlist for the waiting time if allowed
• Tell staff if you feel claustrophobic; the scanner is open-ended and they can help you relax
• Bring someone to keep you company before and after
• Ask when and how you will get results, so you are not left waiting without a plan

PET-CT vs other scans

• CT: detailed pictures of structure, quick and widely available
• MRI: detailed soft-tissue images without radiation
• PET-CT: shows activity as well as structure, useful for staging many cancers
• Your team chooses the scan that best answers their question

Frequently asked questions

Can I drive afterwards?

Usually yes, unless you were given sedation.

Can I breastfeed after the scan?

You may be advised to express and discard milk for a short time. Ask your department.

Will I feel the tracer?

No. It does not cause any sensation or side effects in almost all people.

But the word “radioactive” scares me

The dose is carefully controlled and short-lived. Your team will only recommend a PET-CT if the information it gives is important for your care.

Related: Having a biopsy

Sources: Royal College of Radiologists, British Nuclear Medicine Society, Radiological Society of North America (RadiologyInfo), UK Health Security Agency.