What is PET imaging?

PET (Positron Emission Tomography) is a tomographic nuclear medicine imaging technique that detects pairs of gamma photons produced by positron annihilation and reconstructs their origin in three dimensions.

PET imaging detects pairs of 511 keV photons produced by positron annihilation and reconstructs their origin using coincidence detection.

Unlike SPECT, which detects single gamma photons using mechanical collimation, PET detects two 511 keV photons emitted simultaneously in opposite directions following positron annihilation. By detecting these photon pairs in coincidence, PET determines that the annihilation event occurred somewhere along a straight line between the detectors.

PET therefore uses electronic collimation, which dramatically improves sensitivity compared with SPECT and allows more accurate three-dimensional imaging of radiotracer distribution.

Understanding the physics

PET imaging begins with administration of a positron-emitting radionuclide, such as fluorine-18. After emission, the positron travels a short distance in tissue before interacting with an electron. When they meet, they annihilate.

Annihilation converts the mass of the positron–electron pair into energy in the form of two gamma photons, each with an energy of 511 keV. These photons are emitted approximately 180° apart.

A PET scanner consists of a ring of detectors surrounding the patient. If two detectors register photons simultaneously (within a very short timing window), the system assumes they originated from the same annihilation event. This is called coincidence detection.

The system does not know the exact location of the event, but it knows it occurred somewhere along the line connecting the two detectors. This line is called a line of response (LOR).

By collecting millions of such coincidence events, the system reconstructs the three-dimensional distribution of tracer activity.

Because PET does not rely on mechanical collimators, it achieves much higher sensitivity than SPECT. However, spatial resolution is limited by factors such as positron range, non-collinearity of annihilation photons, and detector resolution.

Where this matters clinically

PET provides superior sensitivity and quantitative accuracy compared with SPECT. It is widely used in oncology, neurology, and cardiology, particularly with F-18 FDG. Its higher sensitivity enables shorter acquisition times or lower administered activity.