PET Scanning

Objectives

Lesson outcomes

Describe how radioactive tracers are used to study tissue activity.
Explain why beta-plus emitting tracers are used in PET scanning.
Explain electron-positron annihilation and the production of two opposite gamma-ray photons.
Calculate the energy of annihilation photons and describe how arrival times are used to form an image.

Syllabus

CIE 9702 syllabus points

6 linked

24.3.1 understand that a tracer is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied
24.3.2 recall that a tracer that decays by β+ decay is used in positron emission tomography (PET scanning)
24.3.3 understand that annihilation occurs when a particle interacts with its antiparticle and that mass–energy and momentum are conserved in the process
24.3.4 explain that, in PET scanning, positrons emitted by the decay of the tracer annihilate when they interact with electrons in the tissue, producing a pair of gamma-ray photons travelling in opposite directions
24.3.5 calculate the energy of the gamma-ray photons emitted during the annihilation of an electron-positron pair
24.3.6 understand that the gamma-ray photons from an annihilation event travel outside the body and can be detected, and an image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photons

Lesson Notes

Student guidance and lesson notes

Overview

This lesson applies nuclear physics to positron emission tomography. You will connect radioactive tracers, beta-plus decay, electron-positron annihilation, gamma-ray detection, and arrival-time processing to the image produced by a PET scanner.

What You Need to Know

A tracer is a substance containing radioactive nuclei that can be introduced into the body and absorbed by the tissue being studied.
PET uses tracers that decay by beta-plus decay, producing positrons.
A positron annihilates when it meets an electron. Mass-energy and momentum are conserved.
In PET, each annihilation event produces two gamma-ray photons travelling in opposite directions.
The energy of the photons can be found from the mass-energy released by an electron-positron pair.
Detectors outside the body record the gamma-ray photons, and processing their arrival times helps locate the tracer concentration.

How to Work Through It

Start by recalling radioactive decay, half-life, gamma radiation, and mass-energy equivalence.
Follow the sequence from tracer injection to beta-plus decay to annihilation.
Calculate the energy of each gamma-ray photon from the electron and positron masses.
Explain how opposite photon directions and arrival times allow the scanner to reconstruct tracer distribution.

Check Your Understanding

Why must the tracer be absorbed by the tissue being studied?
Why does PET use beta-plus emitting nuclei?
Why are two gamma-ray photons produced in opposite directions?
How does detector timing help locate where annihilation occurred?

Common Mistakes

Saying the scanner detects positrons directly. The detected radiation is the gamma-ray photons from annihilation.
Forgetting that the two photons share the released energy.
Describing the tracer as simply highlighting an organ without linking it to tissue absorption and radioactive decay.
Mentioning mass-energy conservation but ignoring momentum conservation when explaining opposite photon directions.

Next Steps

Practise the annihilation photon energy calculation and the sequence of PET image formation.
Compare ultrasound, X-ray, CT, and PET methods in the revision lesson.