In an hour it can read your mind as no machine has done before. Give it a little bit more time, and it will reveal the good and bad in your heart.

The technology that does it--a Positron Emission Tomography scanner--is medicine's latest electronic detective."You could do an autopsy virtually without putting a blade to the body," said Dr. Edward Buonocore, chairman of the radiology department at the University of Tennessee Hospital in Knoxville.

PET is the first "imaging" technology that shows live metabolic activity in the brain and internal organs. Because of that, it can detect disorders much earlier than X-rays and other types of computerized scanning techniques.

The images made from a PET scanner give physicians a better idea of a patient's internal condition. It detects and evaluates tumors, neurological disorders and the conditions of organs. In many caes, it can keep patients from undergoing exploratory surgery.

The University of Tennessee Hospital is the first in the nation to use its PET scanner for clinical purposes. Other U.S. hospitals are using their scanners for research.

PET technology was developed in the 1970s. Its forerunner was computerized tomography, a technique that is widely used to detect tumors. The CT can detect dead and malignant areas of the brain. PET, however, is used to study the living brain.

A PET scanner makes images of the brain or an internal organ by tracking radioactive substances in the body and detecting how they interact with the body's chemistry.

The procedure begins by injecting a patient with radiopharmaceutical "tracers." In some cases, the tracers can be inhaled. Once in the body the tracers attach themselves to certain chemical compounds such as glucose. The tracers have a short life and pose less danger than a normal X-ray, experts say.

Examinations are painless. The patient needs to be awake. However, his eyes should be closed so visual stimuli do not activate portions of the brain, said Karl Hubner, the hospital's director of nuclear medicine. A PET scan takes about an hour. The hospital's institute is equipped to serve eight to 10 patients a day.

PET technology can be used for a variety of clinical and research purposes.

--Strokes: the scan is used to identify changes in metabolism and blood flow in the heart. This can allow physicians to evanluate which patients will benefit the most from bypass surgery. "It is an excellent tool to learn the true viability of the heart," Buonocore said. --Epilepsy: the scan is used to detect abnormal brain metabolism in people who have partial complex seizures. These patients may not respond to medicine, but their epilepsy can be treated by surgery, Hubner said.

--Malignancies: PET is used to develop "profiles" of a tumor's characteristics by comparing the metabolic rates of glucose and oxygen in the brain. Physicians use PET to determine the best treatment for a tumor.

--Alzheimer's disease: Researchers are using PET to study the characteristics of the disease, which leaves elderly people incapacitated. According to a report from the U.S. Department of Health and Human Services, PET may be able to identity the first stages of Alzheimer's disease. If so, treatment with drugs may be more helpful.

--Psychiatric disorders: Researchers using PET are trying to learn more about the brain's chemistry. Studies have suggested that some pyschoses--such as schizophrenia--involve abnormal chemistry in the brain.

--Brain research: Scientists are using the technology to determine with more exactness which parts of the brain process sounds, smells, sight and motor activity.

Physicians say PET is rapidly changing the way in which patients are diagnosed. But the technology has its drawbacks.

One problem is expense. A complete scan of an organ can cost $1,000.