Positron emission tomography: Unlike CT or MRI, which look at anatomy or body form, PET studies metabolic activity or body function. As a result, PET can detect tumors in lymph nodes, for example, even before they enlarge and are detectable with MRI or CT. In PET imaging, the patient receives a small intravenous injection of a radio-active medication (a form of sugar). The images show areas of abnormal metabolism, helping to detect tumors and other diseases that are often not detectable by other means. Although PET is most often used in the detection and staging of cancer, there is increasing use of PET in the brain for the diagnosis of certain dementias like Alzheimer's disease and in the heart for evaluation of coronary artery disease.