Positron emission tomography (PET) scanning deals with positrons instead of gamma rays detected by gamma cameras. The positrons annihilate to produce two opposite traveling gamma rays to be detected coincidentally, thus improving resolution. In PET scanning, a radioactive, biologically active substance, most often 18F-FDG, is injected into a patient and the radiation emitted by the patient is detected to produce multiplanar images of the body. Metabolically more active tissues, such as cancer, concentrate the active substance more than normal tissues. PET images can be combined (or "fused") with anatomic (CT) imaging, to more accurately localize PET findings and thereby improve diagnostic accuracy.