It could lead to new ways of diagnosing the condition and of testing the effectiveness of new drugs.
The technology, reported in the journal Neuron, can identify inside a living brain clumps of a protein called tau that is closely linked to the disease.
Alzheimer's Research UK said it was promising work.
Alzheimer's disease is a problem for researchers trying to come up with a cure. The brain starts to die years before any symptoms are detected, which means drugs are probably given too late.
A diagnosis of Alzheimer's cannot be made with absolute certainty until a patient has died and their brain is examined. It is also not 100% clear what is the cause of the dementia and what are just symptoms.
One protein, called tau, is very closely linked to the disease, with tangles of tau thought to be one way in which brain cells are killed.
The team, lead by the National Institute of Radiological Sciences in Chiba, used positron emission tomography to build a 3D picture of tau in the brain.
They developed a chemical that could bind to tau and then be detected during a brain scan.
Tests on mice and people with suspected Alzheimer's showed the technology could detect tau.
Dr Makoto Higuchi, from the National Institute of Radiological Sciences in Japan, said: "Positron emission tomography images of tau accumulation... provide robust information on brain regions developing or at risk for tau-induced neuronal death."
The research is at an early stage, but it could eventually lead to an actual test for Alzheimer's disease.
It might also allow researchers to closely follow the impact drugs that affect tau have on the brain.
Another protein - beta amyloid - is also linked to Alzheimer's and can be detected in similar tests.
Dr Eric Karran, director of research at Alzheimer's Research UK, said: "This promising early study highlights a potential new method for detecting tau - a key player in both Alzheimer's and frontotemporal dementia - in the living brain.
"With new drugs in development designed to target tau, scans capable of visualising the protein inside the brain could be important for assessing whether treatments in clinical trials are hitting their target.
"If this method is shown to be effective, such a scan could also be a useful aid for providing people with an accurate diagnosis, as well as for monitoring disease progression."

The Anthony's chain of food outlets included restaurants in the city's Corn Exchange building and a patisserie in the Victoria Quarter.
The firm was founded in 2004 by chef Anthony Flinn and his father, also called Anthony Flinn.
Lines Henry Ltd have been appointed as administrators and said there was not enough money for the firm to continue trading.
Michael Simister, of Lines Henry, said: "It is sad when such a well-known business runs into difficulties.
"Unfortunately, the business has had to close, with the loss of 70 jobs, because there is not enough money available to continue trading."
He added despite good reviews the business had struggled with increased competition and difficult economic conditions.

West Midlands Police said the man had been detained in Reading and the woman in Wokingham by counter-terrorism officers at about 19:30 BST.
The pair, believed to be friends, were taken to a police station in the West Midlands for questioning.
The force said the arrests had been intelligence-led.
The suspects did not pose any imminent threat, a spokesperson added.
Supt Robin Rickard from Thames Valley Police said the arrests "were part of an ongoing investigation by West Midlands Counter Terrorism Unit and were pre-planned".
"Officers from Reading and Wokingham are providing support and there are searches taking place at the suspects' home addresses," he said.
Pioneering brain imaging that can detect the build-up of destructive proteins linked to Alzheimer's has been developed by Japanese scientists.
[NEXT_CONCEPT]
The failure of a restaurant business in Leeds has led to the loss of 70 jobs.
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A 24-year-old man and a 23-year-old woman have been arrested in Berkshire on suspicion of preparing for acts of terrorism in the UK.