The positron, as opposed to the photon, is a particle that doesn’t exist naturally in nature. All positron emitting radio-isotopes are created artificially. In the body these particles interact with naturally occurring electrons in the tissues, which causes an annihilation reaction i.e. both destroy each other and cease to exist as a positron and electron respectively. Instead they form two new photons (similar to the photons used in conventional nuclear medicine imaging).
These two new photons are repelled in opposite directions at 180° at an incredible speed. Using specialized computer timing-circuitry, a PET scanner can determine which pair of photons (that strike opposite sides of the ring) came from which annihilation reaction—which is also the place the radio-isotope is bound e.g. the cancer. A straight line can then be drawn on a three-dimensional plane connecting these photons. As hundreds of thousands of these lines overlap, an image is created showing all the “spots” where the radioactive isotope is most concentrated (i.e. at cancer sites, infection sites) or not concentrated at all (e.g. cardiac death, Alzheimer’s disease and epileptogenic foci).
The work-horse radioactive isotope injected in PET imaging is Fluoride-18 which can be coupled to a number of different chemicals, drugs or molecules to image various processes. But the number of radioactive isotopes being developed for PET—and the chemicals that bind to them—is rapidly expanding.
By fusing the PET data with CT-scanner data, we are better able to localize these pathological processes from the underlying normal tissue. This hybrid imaging is referred to as PET-CT.
At TheraMed Nuclear we make use of the Mediso Anyscan® system for PET-CT. We are also able to fuse the PET data to MRI data for even better localization, which is of use especially in children where the risks from radiation exposure needs to be limited.