Alzheimer's is associated with tissue loss

The University of California, Los Angeles, team hopes its work could lead to earlier diagnosis - possibly even before symptoms become apparent.

Their technique combines use of a chemical marker with sophisticated scanning technology.

The study is published in Proceedings of the National Academy of Sciences.

This new marker will offer a useful strategy for early detection and more effective treatment Professor Gary Small

It was already known that Alzheimer's causes the death of cells in the brain's key memory centre, the hippocampus, causing the structure eventually to wither away.

Using their new method, the UCLA team were able to track tissue loss from the earliest stages.

Mood chemical

They focused on measuring the density of a specific type of receptor that responds to the mood chemical serotonin.

These receptors are usually found in particularly high density in the cells of the hippocampus that are most vulnerable to the effects of Alzheimer's disease.

The researchers recorded a drop in receptor density in the hippocampus - and other key memory centres - of 49% of Alzheimer's patients.

But they also recorded a similar fall in 24% of patients with more mild symptoms of impaired brain function, who might have been in the very earliest stages of Alzheimer's.

Researcher Professor Jorge Barrio said: "We hope this new method will lead us to a better understanding of Alzheimer's disease as well as a new strategy for early detection.

His colleague Professor Gary Small said: "A shrinking hippocampus is a hallmark sign of Alzheimer's disease and this new marker will offer a useful strategy for early detection and more effective treatment."

Harriet Millward, of the Alzheimer's Research Trust, said a way to identify people with Alzheimer's before they develop clinical signs of the disease would be a significant breakthrough.

However, she said the research had not proved that loss of serotonin receptors was necessarily a sign that brain cells were being killed off. She said it was possible that there were other reasons.

"There is still a lot of work to be done before this promising method could be widely used, but these results could help scientists to better understand how the disease progresses and develop new methods of early diagnosis.

"When we want to try out new treatments in the future these scanning techniques could also help to assess if any new drugs are having an effect."

The Alzheimer's Research Trust is investigating other ways of using sophisticated scans to speed up diagnosis of the disease.