What to expect on the day of a PET-CT scan

The first PET appointment often feels strange because so much of it is waiting. You are given a tracer, asked to rest quietly for about an hour, and only then scanned. Knowing the order of events takes some of the mystery out. This page walks through the usual sequence for an FDG PET-CT, the tracer used in most cancer scans, and notes where centres differ. Other tracers have their own rules.

One caveat before anything else. Everything here is general. Your appointment letter is the instruction that applies to you, and the imaging team has the final word. If the letter says something different from this page, follow the letter. For what the scan is for, see PET in cancer care, and for the machine itself, PET-CT and PET-MRI.

Before you arrive: preparation

FDG behaves like sugar, so the aim of preparation is to keep your blood sugar and insulin low and your muscles quiet. European guidelines say adults without diabetes should have nothing to eat, no simple carbohydrates and no drinks other than plain water for at least 4 hours before the FDG injection (EANM procedure guidelines, 2014). In practice, centres set their own rules. Cancer Research UK mentions about 4 to 6 hours, while Memorial Sloan Kettering asks for 6 hours with water only (Cancer Research UK; Memorial Sloan Kettering). Both permit water.

A typical sequence of preparation looks like this. The exact numbers are the part that varies.

1. Read the appointment letter and note the fasting start time.
2. Stop eating at the stated time, and drink only plain water if the letter allows it.
3. Avoid strenuous exercise for the period the centre specifies. Guidelines say at least 6 hours and preferably 24, and both Cancer Research UK and Memorial Sloan Kettering mention 24 hours.
4. Take regular medicines as the centre advises, and tell the team about any other health conditions, pregnancy or breastfeeding.

Exercise and movement matter because muscles that are working can take up FDG. That is why you are asked to stay seated or lying down and silent during the uptake period, which the guidelines say minimises FDG uptake in muscles.

Water, and diabetes

Drinking water is usually encouraged. The EANM guidelines explain that good hydration lowers the concentration of FDG in the urine, which means fewer image artefacts, and they give 1 litre of water over the 2 hours before injection as one example (EANM, 2014). Whether your centre uses that amount is its decision.

Diabetes needs particular care because both fasting and insulin affect how FDG is distributed. RadiologyInfo.org says people with diabetes receive special instructions, and Cancer Research UK advises phoning the number on the appointment letter, as diet may need adjusting and the appointment time can change (RadiologyInfo.org). Medication timing, particularly insulin, differs by person and centre. Do not change your diabetes medicines on your own. Contact the department and your diabetes team beforehand.

Arrival, the blood sugar check and the injection

On arrival you confirm your identity, answer a short questionnaire, and have your blood glucose measured with a finger prick or blood sample. The EANM guideline gives a clinical threshold: if plasma glucose is below 11 mmol/L (about 200 mg/dL) the scan can go ahead, and if it is at or above that, the study should be rescheduled. Memorial Sloan Kettering states its own acceptable range as 70 to 199 mg/dL (Memorial Sloan Kettering). A rescheduled appointment is a quality decision, not a failure on your part.

A small cannula is then placed in a vein and the tracer is injected. RadiologyInfo.org notes you may feel a cold sensation moving up the arm.

The uptake wait

This is the longest stretch. The tracer needs time to travel through the body and gather in tissue. The EANM guideline recommends about 60 minutes between injection and the start of scanning, with an accepted range of 55 to 75 minutes, and says you should stay seated or lying down and silent (EANM, 2014). Cancer Research UK likewise describes resting about an hour in a quiet room and moving as little as possible, and Memorial Sloan Kettering says to expect about 60 minutes.

Memorial Sloan Kettering also asks patients to stay warm on the day, and to try to relax and limit movement while waiting. Follow whatever your own centre says about reading, phones and conversation.

Just before the scan you will be asked to empty your bladder.

Lying still for the scan

You lie on a table that slides slowly through a doughnut-shaped ring. A CT image is acquired first, which RadiologyInfo.org says takes under two minutes, followed by the PET portion, which that page puts at 20 to 30 minutes. Guidelines say you should be able to lie still for the whole examination, which they give as 20 to 45 minutes (EANM, 2014). Memorial Sloan Kettering cites 15 to 45 minutes depending on the scan. Cancer Research UK says the scan is painless, not especially noisy, and that you can ask to move if you become stiff.

Overall time in the department varies. Cancer Research UK suggests around 2 hours in total, while RadiologyInfo.org says up to three hours if additional tracers or drugs are used. A technologist may ask for a few extra pictures, which RadiologyInfo.org says does not necessarily mean a problem was found.

If you dislike enclosed spaces

Some people feel anxious inside a scanner. RadiologyInfo.org recognises that those with a fear of closed spaces may feel anxious during the exam. Cancer Research UK advises telling the department in advance, and notes your doctor can arrange medicine to help you relax. If sedation is used, plan for someone to take you home.

Children, and people who come with you

Children have their own protocols. The joint SNMMI and EANM paediatric guideline describes fasting for 4 to 6 hours with plain water encouraged, a glucose check before injection, and an uptake time of about 60 minutes. It generally avoids sedation or anaesthesia and asks that the study be explained to the child and the parents or caregivers, both verbally and in writing. For small children, it suggests helping them pass urine before scanning, for example by holding a baby upright, and changing a diaper just before the images are taken (SNMMI/EANM paediatric FDG PET/CT guideline, 2021).

Caregivers should ask the centre who may be present in the scanning room. Centres differ on contact rules afterwards, as the next section shows.

After the scan

Most people go home and carry on with normal activities, unless told otherwise. RadiologyInfo.org says the tracer loses radioactivity through decay and may leave the body in urine or stool during the first few hours or days, and it suggests drinking plenty of water. Memorial Sloan Kettering gives the same advice to stay hydrated for the rest of the day.

Guidance about close contact varies. Cancer Research UK advises avoiding close contact with pregnant women, babies and young children for the rest of that day. Memorial Sloan Kettering specifies 1 hour after the scan for skin-to-skin contact or sharing a bed with a pregnant person or anyone under 18. Your own letter decides. The doses involved are discussed in radiation dose in context.

If you breastfeed, speak to the department before the day, because some centres suggest a pause and expressed milk. Results are usually sent to the referring doctor rather than given on the day. Cancer Research UK says results usually arrive within 1 to 2 weeks. When you do see a written report, reading a PET report explains the common terms.

The short version

A PET-CT visit follows a predictable order: fasting, a blood sugar check, a tracer injection, roughly an hour of quiet rest, then a scan you lie still for. Timings and contact advice vary between centres and tracers, so the letter you were sent matters more than any general guide. If something in it worries you, a call to the department beforehand is the simplest fix.