3.2.4. Positron Emission Tomography (1990’s). The patient is injected with radioactive
isotopes that emit particles called positrons (anti-electrons). When a
positron meets an electron, the collision produces a pair of gamma ray photons
having the same energy but moving in opposite directions. From the position and
delay between the photon pair on a receptor, the origin of the photons can be
determined. While MRI and CT can only detect anatomical changes, PET is a
functional modality that can be used to visualize pathologies at the much finer
molecular level. This is achieved by employing radioisotopes that have different
rates of intake for different tissues. For example, the change of regional blood flow