PET Radiotracers Explained: What Is Injected and Why It Matters
A PET scan shows where a labelled molecule went, so the tracer decides what the scan can tell you. Here is how tracers are built, made, delivered and limited.
People often arrive at a PET scan knowing only that "a radioactive injection" is involved. The injection is the most important part of the examination, because the scanner only draws a map of wherever the tracer collects, and different tracers collect in different places for different reasons. This page explains what a radiotracer is, why isotopes with half-lives from about one minute to under two hours are used, how fluorodeoxyglucose (FDG) works and where it falls short, how a few other tracers are used, and how these short-lived materials physically reach a scanner. The physics of how the labelled atom produces a signal is covered in the physics of positron emission and annihilation.
What a radiotracer is
A radiotracer, also called a radiopharmaceutical or PET drug, is a molecule that has been tagged with a positron-emitting atom. The US Food and Drug Administration (FDA) describes PET drugs as products that emit positrons through the decay of unstable nuclei and are used to generate PET images. The molecule is chosen because the body handles it in a way that tells us something: a sugar analogue shows where sugar is being consumed, a blood flow tracer shows where blood arrives, and a tracer built to stick to a particular protein shows where that protein is.
The amount of material injected is deliberately tiny. In a review of PET in clinical microdose studies, Wagner and Langer define a microdose as 100 micrograms or less and note that PET tracers are made at high specific activity (a high ratio of radioactivity to mass), so the unlabelled substance administered is usually under 10 micrograms. The idea behind this "tracer principle" is that you follow the biology without disturbing it. The word tracer therefore describes the dose as well as the molecule: enough atoms to be detected, too few to act as a medicine. The separate question of radiation exposure is covered on the radiation dose page.
Isotopes, and why half-life matters
Half-life is the time it takes for half of a radioactive sample to decay. It sets two practical limits. If it is too short, the tracer decays before it can be made, shipped and injected. If it is too long, the tracer lingers and the patient is irradiated for longer than the scan needs. The positron emitters used in PET occupy a middle range, and the values in the table below come from the International Atomic Energy Agency (IAEA) report Cyclotron Produced Radionuclides: Physical Characteristics and Production Methods.
| Isotope | Half-life | How it is made | Example use |
|---|---|---|---|
| Fluorine-18 | 109.8 minutes | Cyclotron | FDG and many other tracers |
| Carbon-11 | 20.4 minutes | Cyclotron | Choline for recurrent prostate cancer; many research tracers |
| Nitrogen-13 | 10 minutes | Cyclotron | Ammonia, a blood flow tracer for the heart |
| Oxygen-15 | 122 seconds | Cyclotron | Labelled water and gases to measure blood flow and oxygen use |
| Gallium-68 | 68 minutes | Germanium-68 generator (also cyclotron) | Somatostatin-receptor and PSMA tracers |
| Rubidium-82 | 1.25 minutes | Strontium-82 generator | Heart muscle blood flow (perfusion) |
The IAEA report gives half-lives of 270.8 days for germanium-68 and 25 days for strontium-82, which is why a generator can sit in a hospital for weeks and keep producing its much shorter-lived daughter atom on demand. It describes the germanium-68 and gallium-68 pair as ideally suited to on-demand supply and calls nitrogen-13 ammonia the most widely used nitrogen-13 compound for PET, while oxygen-15 is used to measure blood flow, blood volume and oxygen consumption. The FDA's 2023 briefing document on PET drugs lists uses by isotope, including rubidium-82 for myocardial perfusion and gallium-68 for neuroendocrine tumours and prostate cancer.
To make half-life concrete: with fluorine-18, roughly half of the radioactivity is gone after under two hours and about a tenth remains after six hours from physical decay alone. With rubidium-82, almost nothing is left within ten minutes. Short half-lives also shape what a scan can do. Oxygen-15 and rubidium-82 follow fast processes such as blood flow, while fluorine-18 allows an hour of waiting for a tracer to settle into tissue.
FDG, the running example
We use FDG as the running example across this site because it is the best known PET tracer and the one most patients meet. It is a glucose analogue: a molecule shaped enough like glucose that cells treat it as sugar, with a fluorine-18 atom added. The European Association of Nuclear Medicine (EANM) guideline for FDG tumour imaging says FDG is taken up by living cells through the membrane glucose transporters and then enters the first step of the normal glycolytic (sugar-burning) pathway.
The trick is what happens next. A review of FDG uptake in cancer by Peppicelli and colleagues explains that after FDG is chemically modified by the enzyme hexokinase, the product, FDG-6-phosphate, is not available for further metabolic activity. The authors call this a "metabolic trap" and describe it as fundamental to FDG PET. In plain terms, ordinary glucose is used up; FDG gets stuck, so the signal builds where glucose consumption is high. The EANM guideline notes that increased glucose consumption is characteristic of most cancers and is related in part to overexpressed glucose transporters and increased hexokinase activity. That is why FDG is used in cancer care, and the FDA table lists approved uses in abnormal glucose metabolism in cancer, coronary artery disease, left ventricular dysfunction and foci of epileptic seizures. Those applications are covered in PET in cancer care, PET in cardiology and PET in epilepsy surgery planning.
FDG has real limits, and the EANM guideline is frank about them. It is taken up by any tissue that burns glucose, so uptake appears normally in the brain, heart, kidneys and urinary tract about 60 minutes after injection, and also in granulation tissue (for example, wound healing), infections and other inflammation; brown fat can light up too, especially in young patients and in cold rooms. A bright spot is therefore not automatically a tumour. In the other direction, Peppicelli and colleagues note that low FDG uptake by some cancer cells may reflect a low glycolytic rate or the use of alternative fuels, so a quiet area is not automatically benign. Blood sugar matters as well. For non-diabetic adults the EANM guideline asks for at least four hours without food or drink other than plain water, and says that the sensitivity of FDG PET/CT may be reduced in diabetic patients with elevated glucose levels. Whether to proceed at a given glucose reading is a decision for the imaging team. The instructions you receive from your own centre are the ones to follow, and what to expect on scan day covers the usual sequence.
Other tracers, at a general level
FDG measures one thing, sugar use. Many other tracers target specific molecules instead. The following examples are limited to what our sources support, and approvals differ from country to country; the FDA list reflects the position in the United States at the time of its 2023 briefing.
In the brain, fluorine-18 tracers named florbetapir, flutemetamol and florbetaben are used to assess the presence and density of amyloid plaques. The Society of Nuclear Medicine and Molecular Imaging (SNMMI) reports that in June 2025 the FDA expanded the indications of these three agents, including quantitative measurement and use in therapy monitoring, but cautions that amyloid plaques are a biomarker not established as the sole cause of Alzheimer's disease, that cost limits access, and that careful patient selection matters. A separate tracer, flortaucipir, is approved to show the density and distribution of tau tangles in people being evaluated for Alzheimer's disease, according to the FDA briefing. See PET in dementia and memory evaluation.
For movement disorders the picture is more nuanced. The FDA table lists fluorodopa for visualising dopaminergic nerve terminals in the striatum in suspected parkinsonian syndromes. Imaging of the dopamine transporter, a protein on those nerve terminals, is mostly done with single-photon (SPECT) tracers rather than PET. A 2021 review by Akdemir and colleagues reports that PET dopamine transporter tracers such as fluorine-18 FP-CIT, carbon-11 PE2I and fluorine-18 FE-PE2I exist but were not yet commercially available, with use mostly limited to research centres that have a cyclotron because of the short half-lives and higher production costs. That may have changed since; PET in movement disorders gives the wider context.
In oncology, prostate-specific membrane antigen (PSMA) tracers are approved for PSMA-positive prostate cancer lesions, and the FDA lists fluorine-18 piflufolastat, fluorine-18 flotufolastat and gallium-68 gozetotide. For neuroendocrine tumours that carry somatostatin receptors, gallium-68 DOTATATE, gallium-68 DOTATOC and copper-64 DOTATATE are listed. In the heart, rubidium-82 and nitrogen-13 ammonia assess blood flow to heart muscle. The FDA briefing also records carbon-11 choline for suspected recurrence of prostate cancer.
Where tracers come from
PET depends on a supply chain that starts with a cyclotron, a particle accelerator that fires protons or other particles at a target to create radioactive atoms. A 2022 survey of Latin America and the Caribbean, conducted by the IAEA and reported by Avila-Rodriguez and colleagues states that PET relies primarily on cyclotron-produced radionuclides and, because most positron emitters have half-lives from a few minutes to several hours, needs an on-site or nearby cyclotron within roughly one to two half-lives of travel. The same paper cites an IAEA database of approximately 1,300 cyclotron facilities for medical radionuclide production worldwide, and reports that every facility in its survey made FDG.
The chain does not end at the cyclotron. The cyclotron makes the raw radioactive atom, a radiochemistry laboratory attaches it to the right molecule, the product is checked for quality, and a dose is drawn up and delivered. In that survey, 18 of 46 institutions produced radiopharmaceuticals only for commercial distribution and 14 produced for both distribution and in-house use, so some centres buy doses from a supplier. With carbon-11, nitrogen-13 and oxygen-15, half-lives of 20 minutes or less mean production generally has to happen very close to the scanner. Generators, as the IAEA report describes, avoid this for gallium-68 and rubidium-82 by letting a long-lived parent supply the short-lived daughter right at the hospital. For the patient, one consequence is that the timing of the appointment matters.
What a tracer cannot do
A tracer shows a biological process, not a diagnosis. The same image can come from different causes, normal activity differs between people, and some tracers are validated for particular questions only. Reports are read by specialists who combine the images with scans, tests and history, which is why reading a PET report is a separate subject. And because availability and approval vary by country and by centre, a tracer named here may not be offered where you live.
Frequently asked questions
How long does the tracer stay radioactive?
Physical decay is predictable. Fluorine-18 has a half-life of 109.8 minutes, so after about six hours roughly a tenth of the activity remains from decay alone, and the body also clears some of the tracer. Shorter-lived tracers such as rubidium-82 and oxygen-15 are essentially gone within minutes.
Is the injection a drug with a pharmacological effect?
PET tracers are given in very small mass amounts; the clinical microdose literature describes usually under 10 micrograms of unlabelled substance, too little to act as a medicine. Questions about allergies, medicines you take or other health conditions belong with your imaging team before the injection.
Why do I have to wait about an hour after the injection?
The EANM guideline recommends an interval of 60 minutes between FDG injection and the start of acquisition, which gives the tracer time to be taken up by tissue. Other tracers use different waiting times, and your centre will tell you which applies.
Are all these tracers available everywhere?
No. Approval, reimbursement and local production differ between countries, and the FDA list used here describes the United States in 2023. A centre can only offer tracers it can obtain within the half-life window.
The short version
A radiotracer is a molecule tagged with a short-lived positron emitter and given in a microdose, so the scan maps a biological process such as glucose use, blood flow or receptor binding. Half-lives range from about a minute for rubidium-82 to just under two hours for fluorine-18, and they dictate whether a tracer comes from a regional cyclotron, an on-site cyclotron or a generator. FDG is trapped in cells that burn a lot of sugar, which makes it powerful and also non-specific. Which tracer you receive, and what it is used to answer, is decided by your own care team.