A Brainy Marvel Called PET

By Anastasia Toufexis

New scanner, a cousin of the CAT, unlocks metabolic secrets

The latest pictures of the human brain come in electric blues and glowing yellows. They are produced by the PET scanner, one of a series of machines that are helping make diagnosis less of an art and more of a science. The PET scanner looks rather like a sophisticated airplane engine, with a hollow core. It is a cousin of the CAT scanner that nearly a decade ago wedded the technique of X rays with computer technology to give cross-sectional views of internal body structures, not just bones but soft tissues as well. But scanning by CAT (for computerized axial tomography) is limited to anatomy. It lets doctors see an organ's shape and form, but cannot tell how it is functioning. PET (for positron emission tomography) allows the physician to examine the brain and body in ways never before possible, providing metabolic portraits, and revealing the rate at which sick and healthy tissues consume biochemicals.

In PET studies, an individual either inhales or is injected with a biochemical, for example glucose, which is the brain's main source of energy, tagged with a telltale radioactive substance that emits positively charged particles. These positrons, when they combine with negatively charged electrons normally found in the body's cells, emit gamma rays that can be detected by a scanning device. Collected and translated into color-coded images, the resulting patterns indicate the intensity of metabolic activity. Because the radioactive substances are so short-lived, anyone undergoing a PET scan is exposed to very little radiation.

PET scans promise to revolutionize certain kinds of diagnosis. Researchers are already using the technique to study blood flow and metabolism in the heart and blood vessels, in hopes of better understanding the mechanisms of heart attacks and strokes and choosing therapy. There is a potential application in cancer as well. Since many malignant tumors consume glucose at much higher rates than surrounding tissue, the efficacy of drug therapy may be measured by the drugs' ability to alter glucose consumption as shown through PET scans.

Still, the most dramatic impact of PET scanning so far has been in studies of the brain. The technique is painlessly providing detailed information about how a normal brain reacts biochemically to such stimuli as the eyes seeing light, the ears hearing a story and even the movement of an arm or a leg. For example, when a subject moves his right hand, the PET scan indicates increased glucose use by the region of the left side of the brain controlling the action. Physicians have begun to use PET scanning in determining therapy for people who have had strokes or epileptic seizures. Measuring metabolic activity in the brains of stroke victims or those with occluded arteries can aid doctors in deciding whether surgery would be beneficial. Scans can also help locate areas of the brain suspected of inducing epileptic seizures.

Some researchers are using PET scans to explore the brains of people suffering from schizophrenia, manic-depressive illness and senile dementia. Their hope is that by scanning hundreds, even thousands, of patients with such conditions, distinctive patterns of biochemical activity will emerge, making diagnosis easier and more precise. Says Chemist Alfred Wolf of Brookhaven National Laboratory on Long Island: "A diagnosis with cognitive tests, for example memory quizzes, takes days. The whole PET procedure takes under 90 minutes."

Preliminary evidence from PET scans suggests that in schizophrenics the frontal part of the brain consumes glucose at a very low rate. In manic-depressives, glucose seems to burn at a very high rate during the manic phase. (No pattern has been found for the depressive phase.) People with senile dementia show decreased glucose metabolism; the more advanced the case, the lower the activity. Researchers also plan to use PET for biochemical brain portraits of patients with multiple sclerosis, Huntington's chorea and possibly alcoholism.

Presently PET scanning requires highly skilled specialists, including chemists, physicists, mathematicians, computer scientists and physicians. The initial costs are also very high. A cyclotron to make radioactive compounds, PET equipment, and a facility to house the operation can run into millions of dollars. Even so, PET scanners are being set up all over the world. Six years ago, there were only four medical centers in the U.S. where teams of scientists were actively engaged in developing PET. Today there are about ten, including Massachusetts General Hospital in Boston, Washington University in St. Louis, the University of California in Los Angeles and Donner Laboratory at Berkeley. There should be at least 15 different centers involved in PET scanning by 1983. By then Europe will have more than a dozen centers with PET equipment and Japan possibly ten. Says Wolf: "The field is just beginning to take off." --By Anastasia Toufexis

This file is automatically generated by a robot program, so viewer discretion is required.