From tracer to image in six steps
A PET scan does not photograph the body. It finds where a tiny amount of radioactive tracer has gone and draws that as a map. Here is the whole chain, in order, with plain words for each link.
PET here means positron emission tomography, the medical imaging method. This guide is for patients, families, students and teachers, and it names its public sources.
Start at step 1.0 ↓The six stages
Every PET scan follows the same chain, whether the target is a tumour, a heart muscle or a region of the brain.
- 1.0
The tracer
It starts with a molecule the body already knows how to handle, tagged with a radioactive atom. In FDG, the most widely used tracer, the molecule is a glucose look-alike and the tag is fluorine-18. The amount given is a trace.
How tracers work → - 2.0
Uptake
After the injection the tracer travels in the blood and collects wherever the body is busy using what it imitates. Active tissue takes up more glucose, so it takes up more FDG. People usually rest for a while so the tracer can spread and settle before scanning begins.
What scan day looks like → - 3.0
Decay
A fluorine-18 atom is unstable. When it decays it gives off a positron, the antimatter twin of the electron. The positron does not get far: in tissue it travels a few millimetres at most before it meets an electron.
The physics of decay → - 4.0
Annihilation
When a positron meets an electron, both disappear and their mass is released as energy in the form of two gamma photons. Each carries 511 keV, and they leave in almost exactly opposite directions. This pair is the signal a PET scanner is built to catch.
Radiation, in context → - 5.0
Detection
A ring of detectors surrounds the patient. When two of them register photons within a few billionths of a second of each other, the scanner counts one coincidence. The decay must have happened somewhere along the straight line between those two detectors.
Why scanners are combined → - 6.0
The image
Millions of those lines are collected. Software works out where along them the activity most likely was, corrects for photons absorbed or scattered on the way, and builds a three-dimensional map. The slices you see are that map, drawn in colour.
How reports are read →
Having a scan, or supporting someone who is
Start with the practical side: what the day involves, what the radiation means, and what the words in a report are trying to say.
Studying or teaching it
A classroom explainer with a glossary, discussion questions and a lesson outline, plus the physics and the history behind it.
All guides
The technology
- How Positron Emission and Annihilation Let PET Locate a Tracer
- How a positron becomes two photons, and why that lets a scanner find where it happened.
- PET Radiotracers Explained: What Is Injected and Why It Matters
- What a radiotracer is, how FDG and other tracers differ, and where they come from.
- PET-CT and PET-MRI: Why Scanners Are Combined, and What Changes
- Why PET is paired with CT or MRI, and what each combination adds.
- PET Radiation Dose in Context: What the Numbers Mean
- What the radiation from a scan means, in plain units and fair comparisons.
- A Short History of Positron Imaging, From a Cosmic-Ray Track to PET-CT
- A short, sourced timeline from the first positron work to modern hybrid scanners.
Clinical uses
- PET in cancer care: what the scan can and cannot answer
- How PET helps with staging, treatment response and follow-up, and where it falls short.
- PET in Dementia and Memory Evaluation: What Scans Can and Cannot Say
- What FDG, amyloid and tau PET can show in memory problems, and what they cannot.
- PET in Epilepsy Surgery Planning: Finding Where Seizures Begin
- How PET helps teams locate a seizure focus before surgery.
- PET in Movement Disorders: Parkinson's and Atypical Parkinsonism
- What PET adds when doctors assess Parkinson's disease and related conditions.
- PET in Cardiology: Heart Blood Flow, Viability and Inflammation
- Heart PET for blood flow and viable muscle, and how it compares with other tests.
- PET in Psychiatry and Brain Research: A Research Tool
- PET as a research window into brain chemistry, and why it is not a routine psychiatric test.
Your scan
- What to expect on the day of a PET-CT scan
- The usual order of a scan day, from preparation to going home.
- Reading a PET report: what the common terms mean
- Common report words, including SUV, explained without the jargon.
For learners
- PET for students and teachers: physics, biology and a glossary
- A classroom explainer with a glossary, discussion questions and a lesson outline.
- Further reading on PET imaging
- Public sources worth reading next, from imaging societies and health agencies.
Begin where the signal begins. Read about the tracer →