The National Institute on Aging hopes the study will encourage drug companies to make new drugs to treat Alzheimer's, which affects an estimated 4.5 million Americans but which is projected to strike many millions more as the baby boomers age.

The $60 million, 5-year public-private partnership will include 800 adults across the United States and Canada, and experts in neurology, imaging, Alzheimer's and other areas.

It will use two types of scans -- magnetic resonance imaging, or MRI and positron emission tomography, or PET -- as well as biological markers in the blood, urine and spinal fluid to see which give the best indication of how a patient's disease is progressing.

"This is an extraordinary pooling of talent and resources toward a common goal -- delaying or preventing Alzheimer's disease," said NIA director Dr. Richard Hodes.

"The initiative should become a landmark study ... helping us to find biological changes early so that we can identify the people at highest risk of the disease and test the effectiveness of new therapies more quickly and efficiently."

Hodes said companies are already testing drugs aimed at slowing Alzheimer's, which steadily destroys the brain with a buildup of proteins and tangled nerve fibers. There is no cure and current treatments only temporarily help symptoms.

Knowing how to chart progress could shave years off a clinical trial, saving millions, Hodes told reporters at an American Medical Association briefing.

Dr. Michael Weiner of the University of California San Francisco, who led the study, said normal healthy people and those with mild cognitive impairment will be included in the trial for comparison.

"In normal subjects there is a little bit of shrinkage but it is only 1 percent to 2 percent per year," Weiner told the briefing.

In Alzheimer's the rate is much greater, he said.

"As ... those nerve cells are lost the brain shrinks, and especially in certain regions like the hippocampus," Weiner said. "If you had a drug that slowed the rate of nerve loss, it would be reasonable to think it would slow the rate of brain shrinkage."

The study will also seek to develop a standard for using imaging and biological markers to diagnose and track Alzheimer's.

Alzheimer's starts out with mild memory loss but can progress quickly to confusion and disorientation. Patients lose the ability to care for themselves and often fail to recognize loved ones.

Last month Medicare agreed to pay for the use of PET scans to diagnose Alzheimer's, an important step in validating the test and setting a standard for private insurers to follow.