PET Scan: What to Expect and How to Prepare

PET stands for positron emission tomography. A small amount of a radioactive substance, called a tracer, is injected into a vein. It travels in the bloodstream and collects in the body according to how active the tissue is. The scanner detects the energy the tracer gives off and turns it into images showing where that activity is concentrated.

The tracer used most often is a simple sugar labelled so it can be detected — cells that are using more energy take up more of it. This is why a PET scan can show how something is behaving, which other scans generally cannot.

Nearly all PET scanners in use are combined PET-CT scanners. A low-dose CT is taken in the same session, so that anything the PET highlights can be located precisely within the body. The machine is a single ring you pass through, open at both ends and shorter than an MRI tunnel.

PET is used to help work out whether and where disease is active, to help plan treatment, and to see how something has responded to treatment. It is also used in the assessment of some heart and brain conditions.

The range of tracers and of reasons for scanning keeps widening, so two people having a PET scan on the same day may be having quite different examinations. This page describes what the appointment involves. What the scan is looking for, and what the result might mean, are conversations to have with the doctor who referred you — not questions a web page should attempt.

Preparation for a PET scan is stricter than for most other imaging, because the tracer follows your body’s own activity. Eating changes that activity, and so does exercise. The facility will give you instructions specific to the tracer being used and the question being asked; those instructions are the ones to follow.

• Expect to fast for several hours beforehand. Plain water is usually not only allowed but encouraged, so that you arrive well hydrated.
• Ask whether to avoid strenuous exercise beforehand — muscles that have been working hard take up the tracer and can make images harder to read.
• Ask what to do about your usual medicines, and confirm rather than assume.
• Wear comfortable, loose clothing without metal — zips, studs and underwiring all affect the CT images, and you will usually change into a gown anyway.
• Allow far more time than you think. Plan for most of a morning or afternoon, and arrange transport if you would rather not drive afterwards.

The last three affect how the images are interpreted or how the appointment is run. The first three affect what you do after the scan: a small amount of radioactivity remains in your body for a few hours, so the facility may ask you to limit close contact with young children or to pause breastfeeding for a period. They will tell you exactly what applies — the arrangements depend on the tracer, so this page does not state a time.

• your referral, if it has not already been sent to the facility
• your Medicare card, and any concession or DVA card
• any previous imaging and reports, especially from another practice — comparison with earlier scans is genuinely useful here
• a list of your current medicines
• warm clothing you can layer, as waiting rooms and scan rooms are often cool
• a support person for the trip home, particularly if sedation has been arranged

You will usually be asked to put your phone and belongings aside during the quiet rest period, so bring less rather than more, and check with the facility if you were hoping to read or listen to something.

1. You check in and hand over your referral, Medicare card and any concession card.
2. A technologist or nurse goes through your history, confirms your fasting, and explains the appointment.
3. You change into a gown and remove metal items.
4. A small cannula is placed in a vein, usually in your arm or the back of your hand, and the tracer is injected through it. Some scans also involve a drink of contrast material for the CT part.
5. You rest quietly in a chair or bed while the tracer distributes — usually around 90 minutes.
6. You empty your bladder, and are then taken through to the scanner.

You will be asked not to move around, not to read aloud or talk much, and in some cases not to use your phone. This sounds strict, but it is the reason the images work: active muscles, including the ones used for talking, take up the tracer and can obscure what the scan is meant to show.

You are usually alone in a quiet room during this period, with staff nearby and a call button within reach. Many people find it the hardest part of the appointment, precisely because it is empty time — bring that expectation with you rather than being surprised by it. If you would find being alone distressing, say so when you book.

You lie on your back, usually with your arms positioned above your head or by your sides as the technologist directs. The bed moves through the ring in stages. The CT images are taken first and are quick; the PET images take longer, because the scanner is collecting a faint signal.

Staying still is what keeps the images sharp, and it is the main thing asked of you. The scanner is quiet compared with an MRI and the ring is short, so most people find it manageable, but say something if you are uncomfortable once positioned — it is far easier to adjust before the scan starts than to repeat it.

Afterwards you wait a few minutes while the images are checked. Occasionally more images are needed, which is a quality check rather than a finding.

You will feel the needle much as you would for a blood test, and nothing from the injection after that. The tracer is given in a very small amount and is not associated with the kind of reactions sometimes seen with CT contrast.

The parts people find hard are the fasting, the long quiet wait and lying still, rather than pain. If you find enclosed spaces difficult, tell the facility when you book — a mild sedative can sometimes be arranged in advance, and that affects whether you can drive home.

This is worth stating plainly rather than glossing over. The tracer is radioactive, which is how the scanner sees it, and the CT part uses X-rays. Everyone is also exposed to ionising radiation continuously from natural sources — the ground, building materials, food and cosmic rays — and the dose from a PET-CT scan is commonly described as comparable to a few years of that ordinary background exposure.

At the doses used in medical imaging, exposure to ionising radiation is associated with a small increase in the lifetime risk of cancer. That is why a PET scan is requested by a specialist for a specific question, rather than done routinely: the scan is meant to be justified by what it will show.

The tracer loses its activity quickly and passes out of your body, mostly in urine. You will be asked to drink plenty of water afterwards to help that along, and to empty your bladder regularly for the rest of the day.

For a few hours you will give off a small amount of radiation, which is why the facility may ask you to keep some distance from young children or pregnant people for a period. Follow the specific advice they give you — it depends on the tracer used, and they know which one you had.

PET images take careful reading, and the report is often written alongside your previous imaging and your treatment history. Ask the facility when the report is expected to be sent, and to whom.

Make an appointment with the doctor who referred you to go through it. If you are having the scan as part of ongoing treatment, that appointment may already be booked — check before you leave, so you are not waiting without a date.

The cannula is removed before you leave. You can eat and drink as soon as the scan is over, and unless you have been told otherwise you can return to your usual activities.

Drink water steadily for the rest of the day. If you were given instructions about breastfeeding, or about time spent close to young children or a pregnant person, follow those exactly — they are short-lived and specific to the tracer you were given.

If sedation was arranged, you will need someone to take you home and you should not drive. Contact your doctor if you feel unwell afterwards. If you become seriously unwell, seek urgent medical care — in an emergency, call 000.

PET scans are arranged by referral, usually from a specialist, and are performed at hospitals and at radiology practices with nuclear medicine facilities. There are fewer of them than there are CT or ultrasound providers, so where you can be scanned may be limited.

Whether a Medicare benefit applies to a particular PET scan depends on the item, the condition being investigated and your circumstances — PET has eligibility rules of its own, set out in the Medicare Benefits Schedule. Because those rules are specific and change from time to time, this page does not restate them.

• Ask the specialist who referred you whether your scan attracts a Medicare benefit.
• Ask the facility what you will pay, and whether any part is billed separately, before the appointment.
• Services Australia publishes current information on Medicare and bulk billing, and MBS Online publishes the Medicare Benefits Schedule itself. Both are linked in the sources below.