This is the seventh of eight stories in our Web feature on self-experimenters.

Most adolescents' fond remembrances of childhood would not include lying motionless for spans of five to 10 minutes in the narrow confines of a giant, clanging machine. For Alexandra Giedd, known in her family as Sasha, it was an eagerly anticipated ritual.

Once every three months from the age of four, she and her father would set out for his lab at the National Institute of Mental Health (NIMH) in Bethesda, Md. There, child psychiatrist Jay Giedd would give his daughter blankets and earplugs before sliding her into the magnetic resonance imaging (MRI) machine for the latest installment of what they called "brain pictures." The scientific justification wasn't entirely clear to her at first. Mostly, she says, "I got to spend time with my dad."

But for him, it was all part of his project to trace the growth and development of the human brain from infancy to adolescence. "It's always been puzzling," Giedd says, "that so many things happen during adolescence." Rates of suicide and fatal car crashes peak during the teenage years. Adolescence is also when many psychological problems may first strike, particularly anxiety, attention deficit / hyperactivity disorder (ADHD) and substance abuse.

Giedd, 47, hopes to link such behaviors to changes in the developing brain. "No one had just mapped out the typical path, and without that it was very hard to know about illnesses," he says. In his ongoing work at the NIMH, he has collected some 6,000 scans from 2,214 children, teens and adults, most between the ages of three and 30. Half are healthy individuals and twins; the rest suffer from schizophrenia or other psychological disorders.

When Sasha was born in 1993, the researcher in Giedd could not help but see a unique opportunity. Most of his subjects came to the lab for scanning biennially, which meant he might misjudge how rapidly their brains evolved between visits. With a budding brain on hand, he could take more frequent peeks at its development. So once she was old enough to express an interest, he invited her to take part. "I knew he took pictures of brains," says Sasha, now 15. "I thought that would be cool, I guess."

The scans revealed rapid changes in her brain even after it reached nearly full size at age six. Between ages six and seven, her corpus callosum, which links the brain's left and right hemispheres, grew 80 percent in size.

The project was cut short, however, after only four years. The NIMH Institutional Review Board (IRB), which oversees human research, flagged the child's inclusion in her father's study as a problem, because she might feel coerced into participating. By then Giedd had taken 83 scans of her brain and had also scanned the brains of two of his three sons—Jordan, 13, and Bryce, 10.

Including family members in a research study is highly unusual and goes against the policies of many review boards, says Norman Fost, professor of pediatrics and director of the medical ethics program at the University of Wisconsin–Madison. "Our guidelines routinely say that recruitment should exclude people in status relationships"—people over whom the researcher has authority, such as employees—he says.

An MRI machine applies radio pulses that cause hydrogen atoms to wobble in a detectable way. The process also deposits heat to tissues. Fost says that MRI scans are generally considered safe, but they can trigger claustrophobia and may cause needless anxiety if they turn up an unexpected quirk that proved benign.

Giedd acknowledges the potential for abuse but sees nothing wrong in his particular case. "If I wouldn't feel comfortable doing something with my own child, then I wouldn't feel comfortable doing it to someone else's child," he says.

The subject herself says she was disappointed to learn there would be no more brain pictures. Now when she visits her father's office, she can only watch. "Sometimes I wish I could do it again to see how I've changed since I was little," she says. But even if the IRB changed its stance, there's a new hitch: Small pieces of metal interfere with the imaging process, and this past November, Sasha got braces.