A scanning technique similar to Magnetic Resonance Imaging (MRI) and Computer axial tomography (CT or CAT) which works in one fundamentally different way than said scanning techniques. Instead of mapping brain structure, the PET scan maps the rate of at which the brain metabolizes sugar. Radioactive sugar is injected into the blood stream, allowed to be metabolized for a time. The subject is then placed under the a unit which can detect radiation and interpret it visually, creating a map of the brain's functional areas. MRI and CT scans only observe the structural differences in parts of the brain whereas the PET can detect areas that are not functioning or functioning too extensively.

As an example of how PET scans are beneficial, let us consider attention deficit hyperactivity disorder (ADHD). Comparing PET scan images obtained from "normal" individuals against those with ADHD, we see that indiviuals with ADHD actually have less brain activity than they should. Although the reason less activity causes ADHD is unclear, it introduced new ideas about how to treat the disease. It turns out that ADHD individuals respond to stimulants, such as methylphenidate (ritalin), which increase their level of neural activity.

In addition to identifying traits of psychological disorders, PET scanning has revealed things about the way normal people function in every day life. For example, PET images have shown that slower readers have lots of activity in speech regions and then in regions where meaning is added. Fast readers have activity in these regions simultaneously.