PET (positron emission tomography) uses positrons to produce high-resolution images of the body. Positron-emitting radioactive isotopes (e.g. Fluor-18) are attached to chemical substances such as glucose. These are injected into the bloodstream, where they release positrons that meet electrons in the body and annihilate. The annihilations produce gamma rays that are used to construct images, showing where the glucose has accumulated.