Positron Emission Tomography/Computed Tomography (PET/CT) is a diagnostic tool used mainly in oncology, neurology, cardiology, infection and inflammation, and in planning surgery and treatment.

PET/CT combines two different techniques in one scanner, giving clinicians two sets of related information about the body – functional and structural – from one examination.

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If you’ve never had a PET/CT scan you might not know what to expect and this brief guide is designed to answer the questions that might be in your mind.

  • It’s important that you tell us before the scan if you:
  • are diabetic.
  • are (or might be) pregnant, breast feeding or in contact with young children.
  • weigh more than 100kg.
  • are booked for other appointments on the same day.
  • suffer from allergies or asthma.
  • have ever received chemotherapy or radiotherapy.
  • have a follow up appointment with your doctor.
  • Unless we tell you otherwise, please don't have anything to eat or drink, except water, for six hours before your appointment. However, it’s quite helpful if you drink 4 or 5 glasses of plain water.
  • Please let us know if you have any disabilities so that we can ensure we are able to offer you the highest quality service.
  • Please confirm your appointment by phone 24 hours before your scan and arrive in plenty of time.
  • You’re welcome to bring a friend or a relative with you but for safety reasons we won’t normally allow them into the examination room. Female companions must not be pregnant.
  • It’s really important that you arrive by the time we tell you. We will need to give you an injection (see preparation for the scan section below) which has an extremely short shelf life and if you’re late we might not be able to use it and your appointment will have to be cancelled.
  • We may also give you a medicine to enhance the scan results. If we think this is necessary we’ll discuss it with you before your scan, however you will not be able to drive afterwards so you will need to make arrangements for someone to collect you.
  • Don’t forget to bring your appointment letter with you
  • Once you’ve checked in at reception, a member of the radiology will meet you, explain the procedure, go through your safety questionnaire with you and ask you to sign a consent form. You’ll have the opportunity to ask any questions about the scanning process.
  • We might ask you to change into a hospital gown. We’ll provide somewhere to store your personal possessions.
  • Throughout the procedure, you will be looked after by the radiology team. They will explain what’s happening and will be there if you experience any discomfort.
  • We'll need to inject you with a radioactive tracer. It’s a minor injection (no worse than having a blood test).
  • After the injection, you’ll need to lie still without talking for around an hour to allow the tracer to be absorbed into your body.
  • Then we’ll ask you to empty your bladder, following which you’ll be ready for the scan.
  • The technologist operating the scanner will be able to see and hear you throughout the procedure.
  • We’ll ask you to lie down on the scanner bed and we'll make sure you're comfortable so you can stay as still as possible. You won’t feel anything.
  • Most scans take between 30 minutes and one hour.
  • You’re welcome to bring a friend or a relative with you, but for safety reasons, we won’t normally allow them into the examination room.
  • Do not drive if you have been instructed not to.
  • Drink plenty of fluids to help flush the tracer from your body.
  • We strongly recommend that you don’t have close contact with any pregnant women or young children for eight hours following the scan.
  • A radiologist will examine the images shortly after your visit and send a report to your doctor or consultant, normally within a few days.
  • For ethical and professional reasons, we cannot discuss results with you. Only your doctor or consultant can do this.


PET/CT combines two scanning techniques: positron emission tomography (PET) and computerised tomography (CT).

CT images provide anatomical information while the PET study gives an indication of activity and function. Overlaying the PET image onto the CT image produces a fusion of anatomy with activity.

Technically speaking, this term is applied when an unstable nuclide, in this case fluorine 18 attached to a glucose-like compound (FDG), emits a positively charged particle called a positron. The positron travels only 1 mm in tissue before reacting with an electron, a negatively charged particle, to produce two 511keV gamma rays (photons) at 180° to each other. These two photons are emitted simultaneously from within the patient. Using a sophisticated PET/CT "camera", we're able to detect these events.

The scanner surrounds the area of the patient being imaged. As the photons are emitted from the patient, they interact with a sensitive camera. The computer in the camera calculates where the positron originated.

By collecting millions of events, the PET scanner is able to create an image of the body that demonstrates where the greatest accumulation of labelled glucose is. This image is then superimposed onto the CT image.

The PET/CT scan requires an injection of a radioactive form of sugar. This injection is given via a small needle, typically into an arm vein.

The pharmaceutical is called FDG, an abbreviation for fluorodeoxyglucose which is taken up by cells like normal glucose. More active cells, such as some inflammatory and cancer cells will take up more FDG than the surrounding tissue.

These areas will produce greater numbers of photons, acting like a “beacon”, to show up abnormal sites. The injection is radioactive, but only for a short time.

The radioactive component of the PET/CT scan is called Fluorine–18. Radioactivity is often described in terms of its half-life which is defined as the time taken for radioactivity to fall to half its initial level. Fluorine has a half-life of 110 minutes. This means that the level of radioactivity remaining in your system halves every 110 minutes. Therefore it's safe to say that 8 hours after a scan there would be an insignificant amount of radioactivity left in the patient.