PET Radiation Dose in Context: What the Numbers Mean
A PET scan involves radiation from the tracer and, usually, from a CT. Here is how that exposure is measured, how it compares with everyday sources, and why centres limit it.
Anyone offered a PET scan is entitled to ask how much radiation is involved. The honest answer has two parts: the tracer gives a dose from inside the body, and the CT that usually comes with it gives another from outside. Neither is zero, and neither is large enough that most clinicians treat it as a reason to avoid a scan that is needed. This page explains the unit used, gives figures from published sources with their caveats, compares them with natural background radiation and other tests, and sets out what is known about pregnancy and breastfeeding. It does not replace advice from your own imaging team, who know your tracer, scanner and medical history. How the tracers work is described in PET radiotracers explained.
Effective dose in plain language
Different organs respond differently to radiation, so a single number for "how much" needs some weighting. RadiologyInfo.org, the patient information site of the Radiological Society of North America and the American College of Radiology, explains that the scientific unit for whole-body dose is the effective dose, measured in millisieverts (mSv), and that it takes into account how sensitive different tissues are to radiation. That lets doctors compare very different sources, from a scan to the radiation we receive from the ground and the sky.
The US Food and Drug Administration (FDA), in a 2023 briefing document on PET drugs, describes how this is done: organ doses are multiplied by weighting factors reflecting each tissue's share of the risk of delayed harm and added up. It also says that this exposure carries a stochastic risk, meaning a small probability of effects such as cancer later in life, as opposed to effects that appear with certainty above a threshold. That is the honest framing: a small, statistical risk, not a prediction about any one person. Effective dose is a tool for comparing and managing exposure, not a measurement of harm in an individual. RadiologyInfo adds that doses vary with body size, the reason for imaging and imaging practice, and that children's doses differ from adults'.
Typical figures
For the tracer alone, the FDA's reviewers compiled published dosimetry studies. For fluorine-18 tracers, 142 studies gave a mean effective dose of 5.4 mSv with a range of 0.5 to 18.5 mSv; for carbon-11 the mean was 2.5 mSv, for gallium-68 4.3 mSv, and for rubidium-82 1.3 mSv. These figures cover many tracers, including investigational ones, not only FDG. For FDG specifically, the European Association of Nuclear Medicine (EANM) guideline for tumour imaging cites the International Commission on Radiological Protection (ICRP) figure of 0.019 mSv per megabecquerel of tracer (a megabecquerel is a unit of radioactivity), which works out to about 3.5 mSv for an administered activity of 185 MBq.
The CT adds to the total, and how much depends on its purpose. The FDA briefing cites literature giving an average CT dose of 5.0 mSv and a total of 14 mSv for a standard FDG PET/CT that uses CT for attenuation correction only, and 15.4 mSv for the CT and 24.4 mSv in total for a diagnostic-quality CT. The EANM guideline states that the effective CT dose ranges from 1 to 20 mSv. RadiologyInfo's table lists 22.7 mSv for a whole-body PET/CT protocol. These totals differ because the tracer amounts, scanners and CT settings differ, not because the sources disagree about the physics, and they are best read as a range of about 14 to 25 mSv for whole-body FDG PET/CT, depending on the CT, rather than a single value. PET-CT and PET-MRI explains why the CT is included.
Comparisons with background and other imaging
Everyone is exposed to natural radiation. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) estimates in a report announced by the United Nations Information Service in February 2026 that the worldwide average annual effective dose from natural sources is about 3.0 mSv, the largest part from inhaled radon (1.8 mSv). The earlier global average was 2.4 mSv, and UNSCEAR says the change reflects better data rather than any real increase.
| Exposure | Approximate effective dose | Comparable to US natural background for |
|---|---|---|
| Chest X-ray | 0.1 mSv | 10 days |
| CT of the chest | 6.1 mSv | 2 years |
| PET/CT, whole-body protocol | 22.7 mSv | 7.6 years |
These "equivalent years" use RadiologyInfo's own US figure of about 3 mSv a year, so they scale with the background value you choose. They are a way of making numbers relatable, not a claim that the exposures are the same in kind. A PET/CT is at the higher end of diagnostic imaging, and a scan with a low-dose CT or a short-lived tracer such as rubidium-82 will be lower.
Why centres follow ALARA
ALARA stands for "as low as reasonably achievable". The FDA briefing applies it directly: dosing instructions for PET drugs are meant to use the lowest administered activity found adequate for diagnosis, and raising the activity above that level violates the principle. Some labels give the activity per kilogram of body weight, and centres choose the CT settings to match the question.
Patients take part in this too. The FDA notes that the prescribing information for approved PET drugs primarily instructs patients to void as often as possible during the first hour after the scan, because the bladder dose, and so the whole-body estimate, depends meaningfully on how long urine is held. Drinking fluids and emptying the bladder, if your centre asks, is a real dose-reducing step.
Pregnancy and breastfeeding: ask your centre
This is the area where sources give differing practical advice, which is itself a reason to talk to your team before the tracer is injected, and to tell them if you might be pregnant or are breastfeeding. Protocols differ by country and by centre.
On pregnancy, the EANM guideline says that for any diagnostic procedure in a woman who is known or suspected to be pregnant a clinical decision is needed, in which benefits are weighed against possible harm. The International Atomic Energy Agency (IAEA) page on PET/CT and pregnant women adds that it is not desirable for a pregnant woman to accompany someone having a scan, and that if her presence is necessary, for example to comfort a small child, she should be advised to keep her distance and contact should be as short as possible.
On breastfeeding, two sources describe different approaches for FDG. The EANM guideline says the ICRP does not recommend interrupting breastfeeding after FDG administration because little FDG reaches milk, but suggests limiting contact between mother and infant for 12 hours after injection to reduce the infant's external exposure. The IAEA page says that some FDG might be excreted in small amounts in breast milk, that normally the scan should be delayed until breastfeeding has stopped, and that if the scan is urgent, milk should be collected before the scan for later feeds and milk expressed for 2 hours after the scan should be discarded, after which normal breastfeeding can resume. Centres generally ask about both, and if yours does not, raise it yourself. Other tracers, and the CT component, can change the advice, so the plan for your scan should come from your centre.
For how a visit usually unfolds, including what staff may ask beforehand, see what to expect on scan day.
Frequently asked questions
Is a PET scan more radiation than a CT scan?
It can be, because a PET/CT combines both. By the RadiologyInfo table, a whole-body PET/CT protocol (22.7 mSv) is above a chest CT (6.1 mSv). A PET scan with only a low-dose CT for correction falls lower, in the FDA's cited example at a total of 14 mSv.
Which tracers involve the least radiation?
Per the FDA's compilation, mean effective doses were lowest for rubidium-82 (1.3 mSv) and carbon-11 (2.5 mSv) and higher for fluorine-18 (5.4 mSv). Choice of tracer is driven by the clinical question, not by dose alone.
Why does my centre ask me to empty my bladder?
Tracer that leaves through the urine collects in the bladder and irradiates it. The FDA says approved PET drug labels primarily tell patients to void often during the first hour, which lowers that dose.
Is it safe to be around others after a scan?
The IAEA recommends limiting contact with pregnant women, and the EANM suggests limiting mother and infant contact for 12 hours after FDG. How long and how strictly depends on your tracer and centre, so follow the instructions you are given.
The short version
A PET/CT delivers radiation from the tracer plus the CT, with published whole-body totals of roughly 14 to 25 mSv against natural background of about 3 mSv a year, and lower totals for many tracer-only or low-dose protocols. The exposure involves a small statistical risk, which is why centres use the lowest activity that gives a diagnostic image. Pregnancy and breastfeeding guidance varies between sources, so tell your team beforehand and follow their plan.