Doctors can clearly see and photograph how Alzheimer's disease progresses in living brains for the first time.
Until now, scientists have been largely limited to examining the disease after a patient dies.
But new technology provides a way to highlight the proteins that distort communication between brain cells in Alzheimer's patients.
The researchers, at Sweden's Uppsala University, hope it will aid earlier diagnosis of the disease.
The technology works by attaching a radioactive marker, called thioflavin, to the tangles of protein, known as amyloid plaques, that are found in the brains of patients with Alzheimer's.
Once these proteins have been highlighted, they can be seen using a sophisticated positron emission tomography (PET) scans.
The researchers say their technology should enable doctors to monitor closely just how Alzheimer's attacks the brain.
It should also make it possible to identify the disease in its earliest stages, and to start treatment quickly. Research has shown that the earlier Alzheimer's drugs are taken, the more effective they are likely to be.
In addition, it should make it easier to assess the impact of new drugs on the progression of disease.
Researcher Dr Henry Engler said: "This will improve our knowledge about Alzheimer's disease and will give new possibilities in the battle against dementia diseases."
Other forms of dementia
The Swedish team used the technology to compare the brains of 16 people with Alzheimer's with those of nine healthy volunteers.
The fluorescent marker they used in their study was first identified by a team from the University of Pittsburgh.
The researchers are now using the technology to study patients with other forms of dementia to pinpoint differences in the progression of the different forms of the disease.
According to Alzheimer's Disease International, there are currently about 18 million people in the world suffering from dementia.
Professor Clive Ballard, director of research at the Alzheimer's Society, said recent developments had greatly improved understanding of the mechanisms leading to Alzheimer's disease.
"Until now, it has only been possible to measure amyloid in the brain after people die, or make indirect estimates by measuring the levels in spinal fluid.
"This new method potentially offers a tremendous opportunity to measure this protein during life.
"If confirmed, this will be extremely valuable in assessing the effectiveness of new treatments.
"In addition, as better treatments become available, it will be essential to identify people with Alzheimer's disease as early as possible in the disease process.
"This technique offers a potentially valuable tool to achieve this goal."
However, Professor Ballard warned that the research was still at an early stage, and significant investment in scanners would be required if it was ever to be made widely available in the UK.
Harriet Millward, of the Alzheimer's Research Trust, said: "It is not yet clear from the research whether this technique will be able to accurately diagnose Alzheimer's early on in the disease process, something which could be vital for the effectiveness of future treatments.
"PET scans are also currently very expensive and primarily only available in specialist research units as a means of learning more about Alzheimer's disease.
"It is early days for this technique and the research so far has only involved a small number of patients, but these preliminary results are encouraging."