24.3 PET Scanning

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Understand what a tracer is and how it works inside the body
  2. Recall that PET scanning uses a tracer that decays by β⁺ (beta-plus) decay
  3. Understand what annihilation is and that mass–energy and momentum are conserved during it
  4. Explain how positrons from the tracer annihilate with electrons in the tissue, producing two gamma-ray photons travelling in opposite directions
  5. Calculate the energy of the gamma-ray photons produced during annihilation
  6. Understand how the gamma-ray photons are detected outside the body and used to create an image of tracer concentration

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1. What Is a Tracer?

A tracer is a special substance that is introduced into the human body — either by injection or by being swallowed. It contains radioactive nuclei, which means its atoms are unstable and will break down over time, releasing radiation.

Once inside the body, the tracer travels through the bloodstream and gets absorbed by the specific tissue or organ being studied. Because the tracer is radioactive, doctors can detect exactly where it has gone inside the body. This allows them to study the structure and functioning of organs without needing surgery.

Think of it like a glowing dye — except instead of visible light, the tracer gives off radiation that special detectors can pick up.

Key point: A tracer must be absorbed by the tissue being studied. It acts as a "signal" that tells doctors where that tissue is and how active it is.

2. Tracers Used in PET Scanning — β⁺ Decay

Positron Emission Tomography, or PET scanning, is a medical imaging technique that uses a specific type of tracer — one that undergoes β⁺ decay (also called beta-plus decay).

What is β⁺ decay?

In β⁺ decay, an unstable nucleus breaks down and emits a positron. A positron is the antiparticle of an electron — it has the same mass as an electron but carries a positive charge instead of a negative one.

A common tracer used in PET scanning is fluorodeoxyglucose — this is essentially a glucose (sugar) molecule with a radioactive fluorine atom (fluorine-18) attached to it.

  • Glucose is naturally used by active cells in the body (especially cancer cells and brain cells), so the tracer gets absorbed in the areas of high activity.
  • The fluorine-18 atom undergoes β⁺ decay, emitting a positron.

The nuclear equation for this decay is:

918F→818O++10β+νe{}^{18}_{9}\text{F} \rightarrow {}^{18}_{8}\text{O} + {}^{0}_{+1}\beta + \nu_e

This means fluorine-18 decays into oxygen-18, releasing a positron (β⁺) and a neutrino (νₑ — a tiny, near-massless particle).

Why is a short half-life important? The tracer must decay quickly so that the patient is not exposed to radiation for a long time. Fluorine-18 has a half-life of about 110 minutes, which is short enough to be safe but long enough for the scan to be completed.

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