"The annihilation of positrons with electrons from biological tissues constitutes the basis of Positron Emission Tomography (PET)... widely used in ... [S]ubstances called radiotracers and radiopharmaceuticals are injected into the patient. These are chemical compounds in which one or more atoms have been replaced by short-lived, positron-emitting, radioisotope of elements that are abundant in the body, like Carbon-11... ... Oxygen-15... and Fluor-18... the latter... for the localization and monitoring of tumors... Since these isotopes are short-lived... they must be produced just before being injected... To do this, the corresponding [common] elements are bombarded with protons... from a small accelerator. ...[I]nside the PET scanner ...a series of detector rings ...record the gamma radiation emitted when the positrons are annihilated inside the body. ...[T]he recorded signals are used to make a series of slices that combine to for a 3-D image. ...[T]hey allow doctors to assess the condition of organs and tissues as they can monitor blood flow and many bodily and metabolic processes, including neuronal transmission."