PET scanning
Cambridge A-Level Physics (9702) · Unit 24: Medical physics · 7 flashcards
PET scanning is topic 24.3 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 24 — Medical physics , alongside Production and use of ultrasound and Production and use of X-rays. In one line: A radioactive tracer is a substance containing radioactive nuclei that is introduced into the body and absorbed by the tissue being studied. In medical imaging, it allows the detection and visualisation of specific tissues or processes.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 1 definition, 5 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the definition card to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Radioactive tracer, and how is it used in medical imaging
A radioactive tracer is a substance containing radioactive nuclei that is introduced into the body and absorbed by the tissue being studied. In medical imaging, it allows the detection and visualisation of specific tissues or processes.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand what a tracer is: something put into the body that contains some radioactive nuclei and is taken up by the tissue under investigation.
- Recall that PET scanning (positron emission tomography) uses a tracer that undergoes β⁺ decay.
- Understand that a particle and its antiparticle annihilate when they interact, conserving both mass-energy and momentum.
- Explain that, during a PET scan, positrons released as the tracer decays meet electrons in the tissue and annihilate, creating two gamma-ray photons that move off in opposite directions.
- Calculate the energy of each gamma-ray photon produced when an electron and a positron annihilate.
- Understand that annihilation gamma-ray photons escape from the body to be detected, and that their arrival times can be processed to produce an image showing the tracer's concentration in the tissue.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in PET scanning
- › Define annihilation as a process where a particle and its antiparticle interact and their entire mass is converted into energy (gamma photons).
- › In annihilation calculations, always divide the total energy (2mc²) by two if you are asked for the energy of a single photon.
What is a radioactive tracer, and how is it used in medical imaging?
A radioactive tracer is a substance containing radioactive nuclei that is introduced into the body and absorbed by the tissue being studied. In medical imaging, it allows the detection and visualisation of specific tissues or processes.
Why is a β+ (positron) emitting tracer used in PET scanning?
PET scanning relies on the annihilation of positrons with electrons. β+ emitting tracers allow positrons to be released within the body, leading to annihilation events and the production of detectable gamma rays.
Describe the process of annihilation in the context of PET scanning.
Annihilation occurs when a positron (emitted by the tracer) interacts with an electron in the tissue. This interaction results in the complete conversion of their mass into energy, producing two gamma-ray photons.
What is produced by the annihilation of a positron and an electron in PET scanning, and in what direction do they travel?
The annihilation produces two gamma-ray photons. These photons travel in approximately opposite directions (180° apart) to conserve momentum.
Explain how the arrival times of gamma-ray photons are used to create an image in PET scanning.
Detectors around the patient register the arrival times of the gamma-ray photons. By identifying coincident detections (photons arriving at opposite detectors simultaneously), the location of the annihilation event can be pinpointed, allowing the construction of an image representing tracer concentration.
How do the laws of conservation of mass-energy and momentum apply in the process of electron-positron annihilation?
The total mass-energy before annihilation (electron + positron) equals the total energy of the photons produced. Momentum is conserved because the photons are emitted in opposite directions with equal momentum magnitudes, resulting in a net momentum of zero (same as initial state).
Calculate the energy of a gamma-ray photon produced during positron-electron annihilation. (Electron/positron mass = 9.11 × 10⁻³¹ kg, c = 3.0 × 10⁸ m/s)
The total energy released is 2mc² (electron + positron, kinetic energy negligible), shared equally between the two photons, so each photon has E = mc² = (9.11 × 10⁻³¹ kg)(3.0 × 10⁸ m/s)² = 8.2 × 10⁻¹⁴ J, which is about 0.51 MeV.
Review the material
Read full revision notes on PET scanning — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 24 — Medical physics
PET scanning sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this PET scanning deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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