Does a Positron Emission Tomography (PET) scan require the use of gamma radiation as input?

From the Guidelines

A PET scan does not require the use of gamma radiation as input, but rather uses a radioactive tracer that emits positrons, which then produce gamma rays upon annihilation with electrons in the body. The process of PET image acquisition involves the detection of these gamma rays, which are produced after a positron emitted from the PET isotope collides with a neighboring electron, resulting in the generation of two gamma photons that travel in opposite directions 1. The PET scanner detects these gamma rays, which are then used to create detailed images of the body. The use of gamma radiation as input is not necessary, as the gamma rays are generated internally through the annihilation of positrons and electrons.

Key Points to Consider

• The basis of PET signal generation lies in the production of gamma photons after positron annihilation, not the input of gamma radiation 1.
• The detection of gamma rays by the PET scanner allows for the creation of detailed images showing metabolic activity in different parts of the body.
• The most common tracer used in PET scans is FDG (fluorodeoxyglucose), a glucose analog that concentrates in tissues with high metabolic activity.
• PET scans are particularly useful for detecting cancer, heart problems, brain disorders, and other conditions where cellular function may be altered before structural changes occur.

Technical Considerations

• PET image acquisition involves a series of technical processes, including the detection of gamma rays, correction for signal loss or attenuation, and reconstruction of the image using algorithms such as filtered back projection (FBP) or iterative algorithms like ordered-subsets expectation maximization (OSEM) 1.
• The accuracy of image reconstruction is critically important for quantification purposes, and different assumptions and algorithms may be used depending on the scanner type.

From the Research

Positron Emission Tomography (PET) Scan Requirements

• A PET scan does not directly require the use of gamma radiation as input, but rather it detects the gamma rays emitted indirectly by a positron-emitting radionuclide (tracer) introduced into the body 2.
• The tracer is typically a biologically active molecule, such as 18F-fluorodeoxyglucose (FDG), which is an analogue of glucose, and its concentrations in the body give tissue metabolic activity in terms of regional glucose uptake 2, 3.
• The PET camera detects pairs of gamma rays emitted by the positron-emitting radionuclide, and images of tracer concentration in 3-dimensional space within the body are then reconstructed by computer analysis 2, 4.
• The use of gamma radiation is inherent in the detection process, as the gamma rays are emitted by the tracer and detected by the PET camera, but the input to the PET scan is the tracer itself, not gamma radiation 5, 6.

Radiation Exposure and Safety Considerations

• The radiation exposure of patients and personnel during a PET/CT procedure is a concern, and special radiation protection routines are required due to the high energy of the radiation emitted by the tracer 5.
• The total effective dose to the patient from a PET/CT procedure is approximately 10 mSv, with the major part coming from internal irradiation due to radiopharmaceuticals within the patients, and a minor part due to the CT scan 5.
• Close contact between the patient and a small child should be avoided for a couple of hours after the administration of the radiopharmaceutical to minimize radiation exposure 5.