Positron Emission Tomography (PET)
Basic Principles for Positron Emission Tomography (PET)
The most common route to introduce a radioactive tracer in vivo is through an intravenous (i.v.) injection. The radioligand accumulates with time in the target tissue and through decay it emits a positron (e+) which travels only a short distance (few millimetres, depending on the type of radionuclide) before losing its energy to the surrounding electron rich tissue. Through annihilation, the mass of an electron and the positron is converted into electromagnetic energy, which is released as two high-energy photons (511 keV) in a random direction 180º±0.25 º (non-collinearity) apart. These high energy photons have a high chance to leave the body and therefore can be detected during PET measurements (see figure).
The facts that photons travel in a line, creating a so called line of response, and that each photons detection time can be individually recorded, make it possible to separate random detection events from the hundreds of thousands coincidence events per second. It is the reconstruction algorithm’s task to translate this enormous dataset of coincidence records into meaningful three-dimensional images (x, y, z spatial dimensions, 3D).
The inherent limitation of spatial resolution in PET data collected from a PET measurement could lack the information for an accurate regional identification and quantification. To circumvent these issues, PET imaging is usually coupled with some type of anatomical imaging.