The UK government is to double funding for dementia research by 2025 and make available on the NHS a scan that can rule out Alzheimer's disease, David Cameron announced as a G8 summit began in London.

The prime minister has invited health chiefs from the world's richest countries to the UK to discuss what to do about dementia, with the number of sufferers predicted to treble to 135 million globally by 2050.

Cameron said he would double funding research from £66m in 2015 to £122m in 2025, although that is still well below government funding for cancer, which stood at £267m in 2007-08.

The first scan on the NHS to rule out Alzheimer's, which works by identifying one of the two proteins that builds in the brain during the disease, will take place at Charing Cross hospital, part of Imperial College NHS trust, in London on Thursday and many more patients will benefit from the new technology.

The health secretary, Jeremy Hunt, said he hoped the dementia summit would have the same effect as the G8 summit in Gleneagles on HIV/Aids in 2005.

"Today should be an optimistic day," he told BBC Breakfast. "Tony Blair had the G8 summit in Gleneagles in 2005 on HIV/Aids and actually that did turn out in retrospect to be a turning point in the battle against Aids.

"I think if you bring the world's leaders together, health ministers from across the world, and we are all resolved that we really are going to do something about this as we face up to an ageing society."

Cameron said the government was "throwing everything we have at making the UK the place to invest and locate and work in life sciences".

He added: "If we are to beat dementia, we must also work globally, with nations, business and scientists from all over the world working together as we did with cancer, and with HIV and Aids."

Alzheimer's disease, the most common form of dementia, affects around 500,000 people in the UK but it is believed that only 45% of people with dementia in the UK have a diagnosis.

The Pet (positron emission tomography) scan being used at Charing Cross hospital produces detailed three-dimensional images of the inside of the body and can be used to visualise amyloid plaques in the brains of adult patients, with cognitive impairment being evaluated for Alzheimer's disease and other potential causes.

Lilly, the US company that created the technology, said that in a study of patients in the United States diagnosed with Alzheimer's disease, its Amyvid scan found that 20% did not have the amyloid plaques and therefore did not have the disease, a figure it says mirrors independent estimates of the level of misdiagnosis of the disease.

Dr Richard Perry, a consultant neurologist at Imperial College Healthcare NHS trust, said: "For people who have memory problems and who are concerned about them, knowing the cause, whether it is Alzheimer's disease or not, is the first step in getting the right sort of treatment."

The scan cannot be used to make a positive diagnosis of Alzheimer's as it cannot identify tau, a protein that accumulates inside nerve cells as the disease progresses. But as amyloid can build in the brain 10 to 15 years before Alzheimer's symptoms – such as memory loss – show, it can also be a powerful tool for researchers.

Dr Eric Karran, director of research for Alzheimer's Research UK, said the tool may "be used to ensure that patients entering clinical trials for amyloid-targeting drugs are appropriately selected. Amyvid also provides an important approach, which may be used as a research tool in clinical trials to monitor the biological signs of Alzheimer's in the living brain and increase our understanding of how the disease progresses."