People volunteering for studies that require a brain scan are well advised to be prepared for the unexpected. Scientists are finding that the normal brain isn't necessarily normal.

Judy Illes, a senior research scholar at the Stanford Center for Biomedical Ethics, said that she and others have found that 18 percent of healthy volunteers had some kind of brain anomaly. While only 2 percent to 8 percent have required clinical follow-up, these incidental findings have raised concerns among scientists who are using the newest technology to unravel the mysteries of the brain.

Illes recently organized a meeting at the National Institutes of Health in Bethesda, Md., to discuss the potential problems.

Thousands of healthy volunteers have signed on to such studies for a variety of reasons, she said. Some have family members with brain illnesses; others are simply interested in basic research.

But, she said, no one signs up expecting that something abnormal will be found.

Unexpected findings, including aneurysms, cysts and cancer, have sparked a debate as researchers grapple with questions ranging from how to interpret a normal scan to figuring out what to do when they discover anything unusual.

"We are finding a lot of things that we never anticipated," Illes said.

B.J. Casey, a psychologist at Weill Cornell Medical Center in Manhattan, remembers the day that she stared at a white mass on an MRI scan of a volunteer. "We all realized that it was something that shouldn't be there," Casey said. A neuroradiologist was called in and determined that the mass was a brain tumor.

"We saved (the volunteer's) life," Casey said.

But other times the mass turns out to be a cyst — which may represent events during the brain's development but no longer is cause for concern. As such, it doesn't threaten medical well-being — and researchers are debating whether telling the volunteer would become a cause of anxiety.

Casey, who conducts pediatric scanning studies, said any finding that appears abnormal should be confirmed before the volunteer is informed. "You don't want to tell parents something is wrong with their child when it isn't," she said.

Peering inside the nondiseased human brain is a relatively new phenomenon and raises the question: What is normal?

"It's a judgment call," said Dr. David Eidelberg, director of neuroscience at the Institute for Medical Research at North Shore-Long Island Jewish Health System in Manhasset, N.Y. "There are lots of variants of normal. Do you tell a person that they have a cyst in their brain that will never alter the course of their life? I'm not sure."

The busiest research-scanning machine these days is an MRI, which provides a picture of the brain's anatomy. A PET scan takes snapshots of brain function.

By the end of the two-day meeting, a consensus had emerged that people who sign on for these studies should be given consent forms stating that the normal brain may not always be "normal."