Alzheimer's disease could be definitively diagnosed for the first time after scientists proved brain scans can pick up the condition in its earliest stages. Currently the only way to check if Alzheimer's is present is to look at the brain of a patient after death.

For patients who are still alive, doctors usually use special cognitive tests which monitor memory and everyday skills such as washing and dressing.

But now researchers at the University of California have proven that it is possible to spot the sticky amyloid plaques and tau protein tangles which cause the disease and pinpoint the moment that they trigger Alzheimer's.

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The US scientists were able to track the progressive stages of Alzheimer's, even in adults who showed no symptoms. It means that people at risk from the condition - such as one in five of the population who carry the APOE gene variant - could be regularly screened, while it could also reassure people suffering mild memory problems that they do not have the disease.

The distribution of tau and amyloid plaques in the brain is called "Braak staging" as it was discovered by Heiko and Eva Braak, who studied the brains of Alzheimer's patients after death.

"Our study is the first to show the staging in people who are not only alive, but who have no signs of cognitive impairment," said the study's lead investigator Dr William Jagust, a professor at UC Berkeley's School of Public Health. "This opens the door to the use of scans as a diagnostic and staging tool."

There are currently 850,000 people living with dementia in Britain, which is due to rise to one million by 2025 and two million by 2050. Although there are no drugs to treat the disease, it is hoped that within five years treatments may be available which can delay the onset. It is, therefore, crucial to be able to pick up the disease early enough for drugs to be effective.

The new brain imaging is carried out using positron emission tomography scanners, which look at cellular-level changes in organs and tissue. Current scans only look for a decrease in brain cells or check that symptoms are not caused by another condition, such as a brain tumour.

The new technology was tested on 53 adults, five of whom were young adults, 33 who were pensioners without any neurodegeneration and 15 who had suspected Alzheimer's disease. The scientists were able to prove definitively which were clear, at risk and those who had the condition.

The findings also shed new light on how tau protein and amyloid plaques build up as the brain ages. For many years, the accumulation of amyloid plaques was considered the main culprit in Alzheimer's. But now the scientists believe that both tau and amyloid work together to cause the disease.The research was published in the journal Neuron.