Positron emission tomography Positron emission tomography (PET)is one of the nuclear medicines and functional imaging technique which is used to observe metabolic processes in the body. This system mainly detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the human body on a biologically active molecule. Apart from this three-dimensional images of tracer concentration within the body are then constructed by computer analysis. In modern positron emission tomography-Ct scanners, three dimensional imaging is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine. Biologically active molecules is generally chosen for the positron emission tomography. Use of this tracer to explore the possibility of cancer metastasis. However on a minority basis, many of the other radioactive tracers are used in positron emission tomography to image the tissue concentration of other types of molecules of interest.