Written for patients: what the scan can and cannot answer, the radiation involved, what to tell the team beforehand, and what the radiologist's report will say.

What PET-CT shows

Metabolism, laid over anatomy. The PET half shows where cells are consuming glucose unusually fast — which is what most active tumour does — and the CT half shows exactly which structure that activity sits in. What it does NOT do is tell you what the activity IS: infection, inflammation, healing tissue after surgery or radiotherapy and some benign conditions light up on the same principle, and conversely several tumour types — low-grade, mucinous and some well-differentiated cancers — take up very little tracer and can look deceptively quiet. The brain uses glucose constantly, so its normal uptake is intense and it is a poor background against which to find a lesion. A PET-CT narrows the question; a biopsy answers it, and MRI remains the better test for local soft-tissue detail.

How it works, and what it feels like

A small amount of a radioactive tracer — most often FDG, a glucose lookalike tagged with fluorine-18 — is injected into a vein. You then rest quietly for around 45 to 60 minutes while it circulates and is taken up by cells in proportion to how much glucose they are burning. Because working muscle takes up the tracer too, that resting hour matters: talking, reading, chewing, walking about or being cold all pull tracer into places that then obscure the area of interest. The scanner detects the paired photons the tracer emits and reconstructs where they came from, while a CT is acquired in the same sitting to localise the activity and correct the images. You lie still, usually with your arms above your head, for the 20 to 30 minutes of actual scanning. The injection itself is the only thing you feel.

Safety and radiation

This is the highest-dose imaging study in routine use, and it is honest to say so plainly. It carries two exposures rather than one: the injected tracer, which irradiates from inside the body until it decays and is passed out in urine, plus the CT component. Published reference figures put the combined dose of a whole-body FDG PET-CT at roughly the natural background radiation an average person absorbs over several years — appreciably more than the CT alone. That is a trade worth making when the result will change cancer treatment, and not worth making otherwise, which is why PET-CT is a specialist request and never a screening test. Fluorine-18 decays quickly — its half-life is under two hours — so most of the activity is gone within a day, and drinking plenty of water and emptying your bladder often afterwards clears it faster.

Who should not have a PET-CT scan

Tell the team before your scan if any of these apply

Pregnancy, or any chance of it: The tracer crosses the placenta and the CT adds a direct dose, so PET-CT is avoided in pregnancy other than in genuinely exceptional circumstances. Declare it when you book, not on the day — a pregnancy test is often done before the injection, and once the tracer is in there is no undoing it.
Breastfeeding: Two separate issues, and the department will give you instructions covering both. Tracer can pass into breast milk, so milk is expressed and discarded for the interval your nuclear medicine team specifies — commonly around twelve hours for FDG — with milk expressed BEFORE the scan used for those feeds. Separately, and often the bigger dose to the baby, you remain mildly radioactive yourself, so prolonged close holding is limited for the rest of the day.
Diabetes, or a high blood sugar on the day: This one materially invalidates the scan rather than merely delaying it. FDG competes with your own glucose, so a high blood sugar means less tracer reaches the tumour and a scan that has cost you a full radiation dose can come back non-diagnostic. Blood glucose is measured before the injection and the scan is postponed above the threshold your department uses. Bring your full diabetes medication list and follow their timing instructions exactly — insulin given close to the injection drives tracer into muscle and away from the target, and metformin commonly causes intense bowel uptake that obscures the abdomen, so it is sometimes held for a day or two beforehand. Never change your own diabetes treatment without being told to.
Reduced kidney function, if diagnostic CT contrast is used: Many PET-CTs use a low-dose CT for localisation alone and need no contrast at all. Where a full contrast-enhanced CT is combined with it, every check that applies to an ordinary CT applies here too: a recent creatinine or eGFR, any previous reaction to iodinated contrast, and your medication list.
Close contact with pregnant women and young children afterwards: You remain a source of radiation for the rest of the day after the injection. Avoid prolonged close contact with anyone who is pregnant and with infants and small children for the period the department specifies, sit apart from others on the journey home, and do not bring a small child along as your companion to the appointment.
Claustrophobia, or an inability to lie still for 20–30 minutes: The PET-CT bore is longer than a CT gantry and the scan runs far longer, usually with the arms raised. Movement blurs the images and misregisters the PET against the CT, which can make the study uninterpretable. Say so in advance — positioning aids, arm supports and, where necessary, medication arranged with your doctor all work better than discovering the problem after the tracer has been given.

When a PET-CT is ordered

Staging a newly diagnosed cancer — how far it has spread
Restaging after treatment, and assessing response to chemotherapy
Distinguishing residual active disease from post-treatment scarring
Finding the primary site when a secondary deposit is found first
Characterising a solitary lung nodule found on CT
Lymphoma staging and interim response assessment
Planning the target volume for radiotherapy
Selected cases of fever or inflammation of unknown origin

Contrast and preparation

Two different injections can be involved and they do quite different jobs. The radiotracer is what the PET half detects and it is always given. Iodinated CT contrast is given only when a diagnostic-quality CT is wanted at the same sitting. The six hours of fasting is for the tracer, not the contrast — food raises both blood sugar and insulin, and each of those pulls FDG away from what the scan is looking for. Plain water is encouraged throughout, and you will be asked to keep drinking and to empty your bladder regularly afterwards.

What your report will cover

The report describes the pattern of tracer uptake through the whole body read against the CT anatomy: where uptake is abnormal, in which structure, how big that structure is on CT, and how intense the uptake is — usually reported as a maximum standardised uptake value, or SUVmax. Nodal stations, liver, lungs, adrenals and skeleton are commented on specifically because those are the common sites of spread. Normal physiological uptake in the brain, heart, kidneys, bladder, bowel and working muscle is described as such so it is not mistaken for disease. Where a previous PET-CT exists, the direction of change between the two is often the single most useful line in the report.

How it works

The path from booking to results

Book in the app

Choose your tests, pick a slot that works, and pay when it suits you.

We come to you

A phlebotomist visits home at your chosen time — or drop by a partner lab instead.

Processed at an NABL lab

Every sample is handled at one of our accredited labs, tracked end to end.

Report + doctor review

Your digital report lands with a complimentary read-through from a doctor.

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All assessments

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