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Patient Information PET/CT


Positron emission tomography (PET) scans are used to produce detailed 3-dimensional images of the inside of the body.

A PET scan helps reveal how your tissues and organs are functioning. A PET scan uses a radioactive tracer to show this activity. This scan can sometimes detect disease before it shows up on other imaging tests.

PET scans are combined with CT scans to produce even more detailed images. This is known as a PET/CT scan.

Why PET/CT scans are used

A PET/CT scan is useful in revealing or evaluating several conditions, including many cancers, heart disease and brain disorders.

Blackrock Clinic uses the referral criteria, ‘iRefer Guidelines, Making the Best Use of Clinical Radiology’.  These guidelines are considered when requesting a diagnostic radiology/nuclear medicine examination.

How PET/CT scans work

PET/CT scanners work by detecting the radiation given off by a substance injected into your arm called a radiotracer as it collects in different parts of your body.

In most PET scans a radiotracer called fluorodeoxyglucose (FDG) is used, which is similar to naturally occurring glucose (a type of sugar) so your body treats it in a similar way.

By analysing the areas in which the radiotracer may or may not be active, it is possible to work out how well certain body functions are working and identify any abnormalities.

For example, a concentration of FDG in the body’s tissues can help identify cancerous cells as cancer cells use glucose at a much faster rate than normal cells.

Preparing for a PET/CT scan

PET/CT scans are carried out on an outpatient basis. This means you won’t need to stay in hospital overnight.

It is important to arrive on time for your scan as the radiotracer used has a short shelf-life and your scan may be cancelled if you are late.

Your appointment letter will include all preparation needed before your appointment. You will be advised not to eat anything for 6 hours beforehand.

Drinking is allowed, however, you may only drink plain, unflavoured water. You should also avoid strenuous exercise for 24 hours before your appointment.

It is advised to wear loose, comfortable clothes. It may be possible to wear these during the scan, although you may be asked to change into a hospital gown.

Avoid wearing jewellery and clothes that have metal parts, such as zips, because these will need to be removed.

What happens during the scan

Radiotracer injection

Before the scan, the radiotracer is injected into a vein in your arm or hand. You will need to then rest quietly for about an hour, in an uptake room, to allow the radiotracer to be absorbed by the cells in your body.

It is important to relax, keep as still as possible, and to avoid talking or reading while you wait as these activities will affect the radiotracer uptake and the resultant scan.

You will be asked to go to the toilet before having the scan.

The scan

During the scan, you lie on a flat bed that’s moved into the centre of the large, cylindrical scanner.

You must stay still while the scan takes place.

The scan can take up to 30 minutes. Having the scan is completely painless, but you may feel uncomfortable lying still for this long.

The operating radiographer can see and hear you throughout your scan. If you feel unwell or need assistance at any point you can signal or call out to the radiographer and they will be with you immediately.

After the scan

You shouldn’t experience any side effects after having a PET scan and can go home soon afterwards.

The results of your scan will not be available on the same day, however they will be sent to your specialist within 48 hours of your scan.

Are there any risks?

The amount of radiation you receive is small and the risks are low, similar to that of other radiological tests.

The radiotracer you receive becomes less radioactive over time and will usually be passed out of your body naturally within a few hours. Drinking plenty of fluid after the scan can help flush it from your body.

As a precaution, you will be advised to avoid prolonged close contact with pregnant women, babies or young children for a few hours after having a PET scan as you will be slightly radioactive during this time.

PET images

Patient Information Download: PET/CT Scan

How to Refer for a PET/CT Scan

PET/CT (Consultant referral only).

Consultant Referral Downloads:

NB. PET/CT requires pre-approval from the health insurance companies prior to proceeding with the scan. The latest versions of the pre-approval documents are available to download here: