A positron emission tomography (PET) scan is an imaging test that allows your doctor to check for diseases in your body.

The scan uses a special dye that has radioactive tracers. These tracers are injected into a vein in your arm. Your organs and tissues then absorb the tracer. When highlighted under a PET scanner, the tracers help your doctor to see how well your organs and tissues are working. The PET scan can measure blood flow, oxygen use, glucose metabolism (how your body uses sugar), and much more.

For more information and a video the Cancer Research website is very interesting or shall I say informative.