Before the advent of high resolution functional neuroimaging techniques during the last several years, it has not been possible to identify the areas within the living human brain that are associated with specific functions of the central nervous system, such as vision, hearing, regulation of motor activity or perception of pain. All information up to that date were derived either from studies using electrical stimulation of the brain in conscious patients being evaluated for neurosurgeries, or from studying defects in patients with known structural lesions of the brain related to strokes or surgeries. Over the past decade, several novel imaging techniques have been developed that have enabled researchers to study the activity of nerve cells within the brain, and to identify specific receptors that mediate these neuronal activities. These imaging techniques include positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and magneto- encephalograpy.

In PET neuroimaging, a tracer amount of a very short-lived radioactive isotope is injected intravenously, which emits positively charged particles, so called positrons. When these positrons collide with one of their negatively charged counterparts, the electrons, a small amount of radiation is emitted which can be detected by the scanner. The emitted radiation, which is nearly identical to X-rays, is monitored over the patient’s head and allows the investigator to determine how much of the isotope is present at any given time within a particular region of the brain. Usually, the isotope used to measure changes in regional brain activity is radioactive water (O15-water). The O15-water mixes with blood and the amount of radioactivity in a given region of the brain is directly related to the amount of blood flow in this part of the brain. Since nerve cells receive an increased amount of blood flow when they are active, an increased amount of blood flow indicates an increased activity of nerve cells in this region.

Within the last few years, PET neuroimaging has revolutionized our understanding of how the brain works; regions and networks have been identified which are responsible for our ability to see, generate words, experience emotions and visualize images of the mind. Receptors for a number of chemical messengers within the brain have been identified, including those for the so-called endorphins, the body’s own pain killers. Specific alterations of brain activity have been identified in depression, anxiety disorders, sleep disorders and pain perception (see article about altered perception of visceral pain in IBS patients). Due to the enormous costs of maintaining a PET imaging facility, there are only a few research centers in the world that can perform these studies. UCLA researchers have been the pioneers in developing and applying these techniques to a broad range of research areas, including the study of brain areas involved in the perception and modulation of abdominal pain.

How safe is it to undergo such an imaging study? The total amount of radiation received during a typical study is less than that received during a computerized tomography (CT) scan of the body. Due to its very short half-life (less than 2 minutes), the amount of active isotope in the body decreases to non-detectable levels within minutes of injection. None of the radioactivity is incorporated into body tissues.