Positron Emission Tomography (PET) is a technique for measuring the concentrations of positron-emitting radioisotopes within the tissue of living subjects.  PET allows us to look at organ functions by measuring levels of energy  (or activity) in specific areas of the organ.  For example, patients may be injected with a radioisotope of glucose.  The glucose finds its way to the brain and other organs through the bloodstream. Since glucose is normally the fuel for organ tissue and cells, the more active an organ is, the more glucose it uses.  PET uses this technology to produce 3-D color images on the computer, where brighter colors indicate more activity.  Identifying organ functions is key in diagnosis of diseases such as the detection of cancer tumor in its early stages, where effective treatment can still be prescribed.