Positron Emission Tomography

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.

Positron Emission Tomography (PET) is not in fact limited to brain metabolism studies, but is a powerful and general imaging technique. PET works because of the fact that when a positron and an electron mutually anihillate, two gamma rays are emitted with a specific energy (about 511KeV each) and in precisely opposite directions. By detecting these two gammas it is possible to localize that anihillation event, which can be safely assumed to be in the near vinicinity of a positron emitter.

Any element which decays by the positron (also known as beta plus) pathway can be used as a PET tracer; commonly used are fluorine-18 and oxygen-15, which are chosen for their short half-lives, ease of production (via proton bombarbment at 20-40MeV in a reasonably inexpensive cyclotron) and chemical utility.

These elements are then chemically incorporated into biologically useful molecules; in the case of F18, usually fluorodeoxyglucose, a useful label for sugar metabolisis imaging, and sometimes (as in my lab) fluorestradiol, used for breast cancer studies. O15 is usually used as water to do blood flow/volume studies.

PET tracer production is one of the few places outside of a reactor facility one is likely to encounter Curie-quantity radioisotopes. A Curie, like a Farad, is an extremely large unit; microcuries or picocuries are more commonly encountered.