The PET Tracers

Obtaining images of the body in a PET machine requires the injection into the bloodstream of the patient or inhalation of radioactively-labelled chemical substances which are equivalent or closely analogs to naturally ocurring substances in the body. Radioactive labelling means attaching by chemical synthesis one or more atoms of the positron-emitting radioactive isotopes produced by a nuclear cyclotron or generator. These labelled substances are called radiopharmaceuticals or tracers.

The following are some of the tracers used for studies of the brain:

FDG or Fluorodeoxyglucose: it similar in structure to glucose, so it can be used to trace parts of the brain which are using more glucose than others (i.e., they are more active, thus requiring more energy, which comes from burning glucose). It is used to carry out physiological studies of memory, cognition, etc., as well as to detect and diagnose several kinds of tumors and other diseases of the human brain. Deoxyglucose (marked with Carbon-11) is also used for the same purpose.

O-15 Water: this tracer is a molecule of common water with radioactive oxygen in the place of the non-radioactive isotope. Thus it distributes mainly in the blood and can be used to measure blood flow and in functional brain studies.

C-11 or O15 Carbon Monoxide: used to quantify and make images of cerebral blood volume

In addition, there are hundreds of other specific radiopharmaceuticals which can be synthesized and used to study function in the brain, such as C-11 or F-18 N-methylspiperone, for mapping dopamine (DOPA) and serotonine, C-11 flumazenil for benzodiazepine (GABA) and C-11 carfentanil, for opiate receptors.

Image credit: CTI Corporation.

From: The PET Scan: A New Window Into the Brain
By: Renato M.E. Sabbatini, PhD
In: Brain & Mind Magazine, March 1997.