PET scan refers to positron emission tomography, which has many applications. Two of them are described below.

Neurology
The PET scan measures blood flow and glucose metabolism throughout the brain. When nerve cells in a region of the brain become active, blood flow and metabolism in that region increase. These increases are usually shown as red and yellow colors on a PET scan. Shades of blue and black indicate decreased or no activity within a brain region. In essence, a PET scan produces a "map" of the active brain.

Oncology
The PET scan is used to find malignant tumor cells in the body. A small amount of radioactive glucose (sugar) is injected into a vein. The PET scanner rotates around the body and makes a picture of where glucose is being used in the body. Malignant tumor cells show up brighter in the picture because they are more active and take up more glucose than normal cells do.