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DURHAM, N.C. -- Three-year-old Sarah Geib of Greensboro has become the 10,000th patient to undergo a PET scan at Duke University Medical Center.

Positron Emission Tomography -- or PET -- scanning is an imaging technology that allows physicians and technologists to diagnose and direct treatment of some of the most serious health problems. Cancer applications are the most common, followed by neurological and cardiac.

"We treated 2,510 patients -- one-fourth of the 10,000-patient total -- last year alone," says Scott Welpe, facility supervisor and assistant chief technologist in the Department of Nuclear Medicine. "In addition to doing PET imaging for most of the Southeast, we get referrals from all over the country for people who actually have PET facilities closer to their home."

"Ours was one of the first programs to bring PET to the patient population," Welpe says. "Although the technology has existed since the 1970s, PET imaging is just beginning to catch on as a valuable clinical modality, and most clinical programs are just now being started."

Sarah underwent a PET scan on March 13 to help determine the cause of a seizure she had on Jan. 29 -- her third birthday. Her PET image allowed her pediatric neurosurgeon, Dr. Timothy George of Duke, to eliminate infection or an aggressive malignancy as the cause of her seizures, and she has been tentatively diagnosed with focal cortical dysgenesis (FCD), a rare anomaly that causes the brain to be "mis-wired" at conception, altering its subsequent development.

"PET is useful in diagnosing and treating seizure disorders because it tells us which areas of the brain are being affected by the seizures and allows us to evaluate seizure severity and range based on the chemical activity of the affected areas," George says, adding that periodic PET scans will supplement other types of imaging to monitor and evaluate Sarah's treatment.

Positron emission tomography exposes the chemical processes taking place in the body. While magnetic resonance imaging (MRI) and computerized axial tomography (CT or CAT) scans yield detailed images of tissue structure, PET reveals how tissue is functioning at a cellular level.

"PET is a valuable diagnostic and disease management tool because, by revealing the chemistry behind many disease processes, it clarifies abnormalities shown by other imaging modalities," says Dr. Edward Coleman, director of the Duke department of nuclear medicine.

"For that reason, a PET scan can often help us detect disease that can't be detected using other modalities."

For a PET scan, patients are injected with a minuscule amount of a radioactive version of a substance the body uses regularly -- typically glucose, a sugar. Following the same pathway in the body as the regular molecule, the radioactive tracer accumulates in tissues that need a lot of energy, including heart muscle, cancers and active areas of the brain.

As it sits in an active cell, the radioactive material decays by emitting a tiny particle called a positron. The positron reacts with an electron in the tissue to produce two photons, energy that is detected by PET. Because of unique aspects of positrons, PET can determine to within a few millimeters where the radioactive material was when it decayed. Collectively, the information is used to evaluate tissues' metabolism: The more photons in a given area, the more cellular activity.

"Metabolic uptake tells us a lot about what's going on with tissue," says Welpe. "For instance, a high level of cellular activity may indicate cancer or tumor cells, while no activity may mean scar tissue."

For nearly all patients, PET means more comprehensive imaging data and fewer invasive procedures. In Sarah's case, MRI and CT imaging revealed an area of thickened brain tissue. Fortunately, Sarah could have a PET scan to help determine the exact cause of the problem rather than a biopsy, which would have been the case in the past.

"Instead of supplementing what we learn from other imaging modalities with biopsy results, we're now supplementing it with PET data in many cases to answer questions about diagnosis and therapy effectiveness," Coleman says. "Because PET imaging tells us whether the area in question is functioning at all, too much or too little, it's helping us avoid unnecessary -- and often costly -- procedures and therapies."