HOW DOES PET WORK?

A positron is a positively charged particle with the same mass as an electron. After being emitted from the nucleus of an atom, it travels for a short distance - in the case of PET, through surrounding tissue - losing energy as it collides with other molecules.

As the positron comes close to a stop, it combines with an electron, and the mass of both particles is converted into energy. This is called an annihilation. The resulting energy is dispersed in the form of two high-energy gamma rays or photons, traveling outward and in opposite directions from each other.

PET imaging systems detect these events with several rings of gamma-ray detectors that surround the patient. When a detector "sees" a photon from an annihilation event, the detector opposite it looks for a matching photon; and if two matching photons are recorded within nanoseconds of each other, the detectors register a coincidence along the line between the detectors - a line of response.

The PET system then draws lines of responses between each detector pair registering a coincidence event during a scan. When it's finished, there will be areas of overlapping lines which indicate more highly concentrated areas of radioactivity. The higher the number of coincidences detected, the more precise this calculation will be.

The system then uses this information to construct an image, using algorithms similar to those applied in CT, MR and nuclear medicine.