Scientists have developed a technique that allows them to watch the spread of Alzheimer's disease through the brains of living patients "like a flow of lava," as one researcher put it.

The technique, described today in The Journal of Neuroscience, may help pharmaceutical companies evaluate the effectiveness of Alzheimer's drugs and aid in the early identification of people who are at highest risk for developing the disease.

In the technique, a computer analyzes single brain scans taken over time and generates three-dimensional videos.

"People have used imaging before, but the studies have really been like taking Polaroid pictures at the ballet," said Dr. Paul Thompson, assistant professor of neurology at the University of California at Los Angeles and the lead author of the report.

"This is the first study to chart the dynamic spread of Alzheimer's in the brain."

The videos were based on the analysis of subtle changes in the MRI scans of 12 patients with Alzheimer's, compared with those of 14 elderly people without the disease. The videos depict the average loss of brain cells in different brain areas for the Alzheimer's patients.

Researchers have long known through autopsy studies that Alzheimer's patients show the progressive death of nerve cells in many areas of the brain.

But in the videos, the damage can be seen moving from structures involved with memory to brain areas involved in emotion and in the control of behavior.

Other regions of the brain -- sensory centers responsible for vision and touch, for example -- remained untouched, Thompson said, calling them "islands of cells in all this devastating sequence of loss."

The researchers found the loss of brain tissue progressed at a rate of 4 percent to 5 percent each year in Alzheimer's patients. In healthy brains, about 0.5 percent is lost each year in aging.

Dr. Thomas Chase, chief of experimental therapeutics at the National Institute of Neurological Disorders and Stroke and an expert on Alzheimer's treatment, said the new technique "adds to our understanding of the time course that unfolds in an Alzheimer's brain."

The ability to see the disease progressing over time in the whole brain might in the future help scientists separate subtypes of Alzheimer's, Chase said.

But he added, "The real question, is, `Does this advance our ability to deal with Alzheimer's, to understand precisely what the disease is and to discover more effective therapies?' "

Dr. Antonio Convit, a research scientist at the Nathan S. Kline Institute for Psychiatric Research, said the technique might be used to tell who, among people who show on psychological tests early signs of mental impairment characteristic of Alzheimer's, will develop the disease.