What is PET scanning and how is it used in Medical Physics?

PET scanning, or Positron Emission Tomography, is a medical imaging technique used in Medical Physics to observe metabolic processes in the body. It involves the use of radioactive tracers that emit positrons, which interact with electrons in the body to produce gamma rays. These gamma rays are detected by the scanner to create detailed images of the body's internal structures and functions. PET scans are particularly useful in diagnosing and monitoring conditions like cancer, heart disease, and brain disorders.

How does a PET scan differ from other imaging techniques like MRI or CT scans?

PET scans differ from MRI and CT scans in that they provide functional imaging rather than just structural images. While MRI and CT scans show the anatomy of the body, PET scans reveal how tissues and organs are functioning by tracking the metabolic activity. This makes PET scans particularly useful for detecting cancerous tissues, as they often have higher metabolic rates than normal tissues. In the context of Cambridge International A Levels, understanding these differences is crucial for students studying Medical Physics.

What role do radioactive tracers play in PET scanning?

Radioactive tracers are essential in PET scanning as they emit positrons, which are crucial for the imaging process. These tracers are typically compounds like glucose labeled with a radioactive isotope, such as fluorine-18. When introduced into the body, the tracer accumulates in areas with high metabolic activity. The emitted positrons interact with electrons, resulting in the emission of gamma rays that are detected by the PET scanner to produce images. This process is a key topic in Medical Physics for Cambridge International A Levels students.

What are the safety considerations associated with PET scanning?

PET scanning involves exposure to a small amount of radiation due to the use of radioactive tracers. However, the levels are generally considered safe and are carefully controlled to minimize risk. The benefits of accurate diagnosis and monitoring often outweigh the risks. It is important for students studying Medical Physics at the Cambridge International A Levels to understand the safety protocols and the importance of minimizing radiation exposure in medical imaging.

Why is PET scanning important in the diagnosis and treatment of cancer?

PET scanning is crucial in the diagnosis and treatment of cancer because it provides detailed images of metabolic activity, allowing for the detection of cancerous cells that often have higher metabolic rates than normal cells. This helps in identifying the presence and spread of cancer, assessing the effectiveness of treatment, and planning further interventions. For Cambridge International A Levels students, understanding the application of PET scans in oncology is an important aspect of Medical Physics.

Why is "Forgetting back-to-back direction" a common mistake in PET scanning?

Why it happens: Don't apply momentum conservation. How to avoid it: Photons emitted at 180° to conserve linear momentum (initial system at rest). Source: 9702 Examiner Reports 2022-2024

Why is "Choosing wrong tracer half-life" a common mistake in PET scanning?

Why it happens: Confusing with diagnostic dose constraints. How to avoid it: Short half-life balances signal vs patient dose. Minutes-to-hours typical.

Cambridge International A Levels Physics (9702)

PET scanning

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Short Notes - PET scanning

Detailed Study Notes

Detailed notes on Medical Physics for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

PET Scanning Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Tracer + positron-electron annihilation + coincidence detection.

At a glance

Positron-emitting tracer (e.g. F-18 FDG).
Annihilation: $e^+ + e^- \to 2\gamma$ .
Each $\gamma$ = 0.511 MeV.
Back-to-back (momentum conservation).
Coincidence detection → 3D image.

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

24.3.1 — Describe positron emission and annihilation.
24.3.2 — Explain coincidence detection in PET.
24.3.3 — Compare PET to CAT/MRI.

PET principles

Annihilation + coincidence.

Step 1 — Tracer. Short-half-life $\beta^+$ emitter (F-18 in fluorodeoxyglucose, $t_{1/2} \approx 110$ min) injected. Accumulates where metabolism is high (e.g. tumour, active brain).

Step 2 — Decay. Nucleus emits POSITRON. Positron travels few mm, slows, annihilates with nearby ELECTRON.

Step 3 — Annihilation. $e^+ + e^- \to 2\gamma$ . Each photon has $E = m_e c^2 = 0.511$ MeV. By momentum conservation (initial system $\approx$ at rest), photons emitted at 180° (back-to-back).

Figure 1 — Inside the annihilation event

The 0.511 MeV comes from the rest energy of each particle, and the 180° comes from conservation of momentum. Both are marking points.

Step 4 — Detection. Ring of detectors around patient. Pairs of detectors in COINCIDENCE — both photons arriving within a few ns → annihilation happened on the line connecting them.

Figure 2 — PET: coincidence detection fixes a line of response

Two detectors firing in coincidence define a line of response. One event gives a line, not a point — the point comes from combining many lines.

Step 5 — Reconstruction. Many lines of response → 3D image of tracer concentration.

Figure 3 — Many lines of response locate the tracer

Each coincidence narrows the event down to a line. Thousands of lines intersect where the tracer has concentrated, which is what the scanner reconstructs.

Functional imaging. Unlike CAT (structure), PET shows METABOLIC ACTIVITY → tumours, brain activity.

Figure 4 — A PET scan in five stages

Work through the five stages in order. Missing the coincidence step, the photon energy or the 180° is what loses the marks.

PET at a glance.

Feature	Detail
Tracer	A short half-life $\beta^+$ emitter — F-18 in fluorodeoxyglucose, $t_{1/2} \approx 110$ min
Where it goes	Wherever metabolism is high — a tumour, or active brain tissue
The event	$e^+ + e^- \to 2\gamma$ , after the positron has travelled a few mm
Photon energy	0.511 MeV each, equal to $m_e c^2$
Directions	180° apart, forced by conservation of momentum
Detection	A ring of detectors; a pair firing within a few ns counts as a coincidence
What the image shows	Tracer concentration, so metabolic activity — not structure

This is the contrast with CAT: CAT maps structure, PET maps what the tissue is actually doing.

Cambridge tip. Always include 'coincidence' AND '0.511 MeV' AND '180°' in your description.

Tracer → $\beta^+$ → annihilation.
Each photon 0.511 MeV.
Coincidence pairs → image.

See the full worked example for pet scanning →

Quick recap

Positron-emitting tracer accumulates at target.
Annihilation gives back-to-back 0.511 MeV photons.
Coincidence detection localises events.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

$e^+ + e^- \to 2\gamma$ .
0.511 MeV each, 180°.
Coincidence detection.

How it’s examined

PET on Paper 4 typically 6-8 marks. Most-tested: annihilation energy (4 marks), coincidence principle (5 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/42 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

Take this whole topic with you

Step-by-step worked examples — PET scanning

Step-by-step solutions to past-paper-style questions on pet scanning, written exactly the way a tutor would explain them at the board.

1Annihilation photon energy (4 marks)
Extended• Adapted from 9702/42 May/Jun 2024• annihilation
Question
Show that each photon from $e^+ e^-$ annihilation has energy $\approx 0.511$ MeV. (4 marks)
Step-by-step solution
1. Step 1
  Total rest energy: $2m_e c^2 = 2 \times 0.511 = 1.022$ MeV.
2. Step 2
  Conservation of momentum → two photons back-to-back, equal energy: $E = 0.511$ MeV each.
Answer
$E \approx 0.511$ MeV.
2Tracer activity (5 marks)
Extended• radioactive-decay
Question
F-18 tracer half-life 110 min. After 220 min what fraction remains? (5 marks)
Step-by-step solution
1. Step 1
  Two half-lives → fraction = $(1/2)^2 = 0.25$ .
Answer
25% remaining.

Key Formulae — PET scanning

The formulae you need to memorise for pet scanning on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

Annihilation energy
$E_{\text{photon}} = m_e c^2 \approx 0.511 \text{ MeV}$
When to use
Each photon from electron-positron annihilation has rest-mass energy.

Key Definitions and Keywords — PET scanning

Definitions to memorise and the exact keywords mark schemes credit for pet scanning answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

PET (Positron Emission Tomography)
Examiner keyword
Imaging technique using positron-emitting tracer; back-to-back 511 keV annihilation photons detected in coincidence.
Radiotracer
Examiner keyword
Short half-life positron emitter (e.g. F-18 with $t_{1/2}$ = 110 min) attached to biologically active molecule (FDG glucose analogue).
Coincidence detection
Examiner keyword
Two opposing detectors record both photons within nanoseconds → annihilation point lies on line between them.

Common Mistakes and Misconceptions — PET scanning

The traps other students keep falling into on pet scanning questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Forgetting back-to-back direction
9702 Examiner Reports 2022-2024
Why it happens
Don't apply momentum conservation.
How to avoid it
Photons emitted at 180° to conserve linear momentum (initial system at rest).
✕Choosing wrong tracer half-life
Why it happens
Confusing with diagnostic dose constraints.
How to avoid it
Short half-life balances signal vs patient dose. Minutes-to-hours typical.

PET scanning — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Feature

Detail

Tracer

A short half-life

\beta^+

emitter — F-18 in fluorodeoxyglucose,

t_{1/2} \approx 110

min

Where it goes

Wherever metabolism is high — a tumour, or active brain tissue

The event

e^+ + e^- \to 2\gamma

, after the positron has travelled a few mm

Photon energy

0.511 MeV each, equal to

m_e c^2

Directions

180° apart, forced by conservation of momentum

Detection

A ring of detectors; a pair firing within a few ns counts as a coincidence

What the image shows

Tracer concentration, so metabolic activity — not structure

PET scanning

Short Notes - PET scanning

Detailed Study Notes

At a glance

What you’ll learn

PET principles

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Annihilation photon energy (4 marks)

2Tracer activity (5 marks)

Annihilation energy

PET (Positron Emission Tomography)

Radiotracer

Coincidence detection

✕Forgetting back-to-back direction

✕Choosing wrong tracer half-life

PET scanning — frequently asked questions

PET scanning

Short Notes - PET scanning

Detailed Study Notes

At a glance

What you’ll learn

PET principles

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Annihilation photon energy (4 marks)

2Tracer activity (5 marks)

Annihilation energy

PET (Positron Emission Tomography)

Radiotracer

Coincidence detection

✕Forgetting back-to-back direction

✕Choosing wrong tracer half-life

PET scanning — frequently asked questions