Positron Emission Tomography (PET) Imaging

Last Update: 4 March 2026

This is part of the HSC Physics course under the topic Applications of the Motor Effect.

HSC Physics Syllabus

• Describe the properties of a positron

• Explain how positron emission tomography (PET) uses positron–electron annihilation to produce a medical scan

Positron Emission Tomography (PET) Scan Explained (Video)

This video focuses on the HSC Physics syllabus and explains how PET imaging works in terms of the underlying subatomic physics.

What is a Positron?

In the early 20th century, physicists discovered that every particle of matter has an "antimatter" counterpart.

The positron is the antimatter version of the electron. It has:

• The exact same mass as an electron (`9.109 \times 10^{-31}\text{ kg}`).

• The exact same magnitude of charge, but it is positive (`+1.602 \times 10^{-19}\text{ C}`) instead of negative.

Positrons are produced naturally through a process called plus beta decay. In an unstable nucleus with too many protons, a proton transforms into a neutron, emitting a positron and a neutrino in the process.

Positrons are not commonly found in nature as upon emission they are quickly annihilated with nearby electrons to release energy.

What is PET?

Positron Emission Tomography (PET) is a nuclear medicine imaging technique. It works by injecting a patient with a biological molecule (like glucose) tagged with a positron-emitting radioisotope (most commonly Fluorine-18).

 

 

Cells that are highly active, such as cancer cells, consume more glucose than healthy cells. They pull the radioactive tracer inside them, effectively "labeling" themselves for the scanner to identify.

 

Example of PET imaging

 

In the world of medical imaging, most techniques like X-rays or CT scans show us what the body looks like (structure). However, Positron Emission Tomography (PET) is different, it shows us how the body is functioning (metabolism). 

The Physics Behind PET Imaging

The magic of PET happens through three distinct stages of physics:

1. Beta-plus Decay

Once the tracer is inside the body, the radioisotope decays and releases a positron. This positron travels a very short distance (usually a few millimetres) before it bumps into an electron from a nearby atom.

For example, fluorodeoxyglucose is a common radioactively labelled molecule used in PET imaging. The radioisotope fluorine-18 undergoes beta plus decay to produce oxygen-18, a positron and a neutrino:

 

 

Fluorine-18 has a half-life of 110 minutes which is relatively short for radioisotopes. This duration makes it suitable for medical use as it has both quick onset and decay so it washes out of patients' body in a timely manner.

2. Electron-positron Annihilation

The collision between a positron and an electron causes annihilation. Their entire mass is converted into energy according to Einstein’s mass-energy equivalence equation:

 

$$E = mc^2$$

$$E = (2 \times 9.109 \times 10^{-31})(3 \times 10^8)^2$$

$$E = 1.64 \times 10^{-13} \text{J}$$

This energy is released in the form of two high-energy gamma-ray photons. Each gamma photon has the same amount of energy:

$$E = \frac{1.64 \times 10^{-13}}{2}$$

$$E = 8.2 \times 10^{-14} \text{J or } 0.511 \text{ MeV}$$

 

To satisfy the Law of Conservation of Momentum, these two photons must travel in exactly opposite directions (180° apart). This is because the total momentum before the annihilation was effectively zero due to the relatively negligible velocities of the particles, so the total momentum after must also sum to zero.

3. Coincidence Detection

The patient is surrounded by a ring of gamma-ray detectors. The scanner is programmed to look for coincidences – two gamma rays hitting opposite sides of the ring at the exact same time.

 

Diagram adapted from Research Gate

 

By drawing a straight line (called a Line of Response) between those two detectors, the computer knows the annihilation event happened somewhere along that path. After detecting millions of these lines, a 3D image is reconstructed, showing exactly where the "hot spots" of activity are located.

Advantages/Benefits of PET Imaging

• Functional Imaging: Unlike CT or MRI, which show the anatomy, PET shows the biological activity. This allows for the detection of diseases based on changes in metabolism, which often occur long before physical changes to the organs are visible.

• Early Cancer Detection: PET is highly sensitive to the increased glucose consumption of malignant tumors, making it one of the most effective tools for early-stage cancer diagnosis and staging.

• Monitoring Treatment: Because it measures activity, PET can show if a chemotherapy treatment is working within days of starting, as the "hot spots" will dim if the cancer cells begin to die.

Disadvantages/Limitations of PET Imaging

1. Ionising Radiation Exposure

Because the procedure requires the injection of a radioactive tracer, the patient is exposed to ionising radiation.

• The Dose: A typical PET scan delivers a radiation dose roughly equivalent to several years of natural background radiation. While this is generally considered safe for diagnostic purposes, it is managed carefully to minimise long-term risks. The dose is further minimised by using the minimal effective dose possible and promoting clearance from body by encouraging hydration following PET.

• Gamma Radiation: The $511$ keV gamma photons produced by annihilation have high penetrating power and can cause cellular damage, though the short half-life of the tracers (like $110$ minutes for F-18) ensures the radiation clears the body quickly.

2. Challenges in Interpretation

Interpreting a PET scan is not always straightforward. "Hot spots" do not always equal cancer.

• False Positives: Areas of infection or inflammation also consume high amounts of glucose. A healing wound or an active infection can "light up" on a scan, potentially being mistaken for a tumor.

• Physiological Uptake: Certain organs, like the brain and heart, naturally use huge amounts of glucose. The bladder also appears bright because the tracer is excreted through urine. Radiologists must distinguish this "normal" brightness from disease.

• Small Lesions: PET has lower spatial resolution than CT or MRI. If a tumor is very small, it may not produce enough signal to be detected.

3. Logistical and Financial Costs

• Cyclotron Proximity: Because the isotopes have such short half-lives, they must be produced in a particle accelerator (cyclotron) located near the hospital. If the transport takes too long, the tracer becomes useless.

• Cost: PET scanners and the production of radioisotopes are extremely expensive, making the scans significantly more costly than traditional X-rays or Ultrasounds.