How is a PET scanner able to locate a tumour in the patient's body?

The patient is injected with a radioactive isotope with short half-life, for eg Fluorine-18. This isotope decays into positrons. The area damaged with tumour requires more blood supply compared to the healthy cells. Therefore higher concentration of the isotope is transported to the damaged area where the positrons from the isotope interact with naturally occurring electrons in the body to cause annihilation. This electron-positron annihilation leads to production of two gamma rays due to conservation of energy, which travel in opposite direction. These gamma rays are detected by the detectors in a ring around the patient. The computer connected to these detectors calculate the location of the origin of the gamma ray pairs by triangulation.