Over 35 million people worldwide are affected by dementia, a figure expected to double by 2030 Getty

A new magnetic resonance imaging technique can detect signs of cognitive decline in the brain before symptoms are evident, researchers have found.

Arterial spin labeling is a promising method that has the potential to serve as a biomarker in very early diagnosis of preclinical dementia.

Over 35 million people are affected by the condition worldwide, a number expected to double by 2030.

Problems in the brain related to dementia, such as reduced blood flow, might be present for years but are not evident because of cognitive reserve, a phenomenon where other parts of the brain compensate for deficits in one area.

Early detection of cognitive decline is critical because treatments for Alzheimer's disease, the most common type of dementia, are most effective in this early phase.

Researchers have been studying the benefits of ASL, which measures brain perfusion, or penetration of blood into the tissue.

"ASL MRI is simple to perform, doesn't require special equipment and only adds a few minutes to the exam," said study author Sven Haller, of the University of Geneva, Switzerland.

For the research, the group examined 148 healthy elderly participants and 65 people with mild cognitive impairment.

The participants underwent brain MRIs and a neuropsychological assessment, a common battery of tests used to determine cognitive ability.

Of the 148 healthy individuals, 75 remained stable, while 73 deteriorated cognitively, results showed at a clinical follow up 18 months later. Those who deteriorated showed reduced perfusion at their baseline ASL exams.

The posterior cingulate cortex, an area in the middle of the brain associated with the default mode network, was specifically affected. It is a neural network that is active when the brain is not concentrating on a specific task, and declines in this network are more pronounced in those with Alzheimer's disease.

"There is a known close link between neural activity and brain perfusion in the posterior cingulate cortex," Dr Haller added. "Less perfusion indicates decreased neural activity."

Previous research using positron emission tomography (PET), the current gold standard for brain metabolism imaging, found patients with Alzheimer's disease had reduced metabolism in the same area of the brain where the perfusion abnormalities were found using ASL.

This suggests a close link between brain metabolism and perfusion, according to Dr Haller.

However, although PET can identify markers of Alzheimer's disease in the brain and cerebrospinal fluid, it exposes the patient to radiation, whereas ASL does not.

As the number of dementia patients increases, researchers continue to make breakthroughs to detect the disease earlier. In July, King's College London scientists developed a blood test that can predict the onset of dementia with 87% accuracy. Speaking to BBC News, the researchers said the "major step forward" will help with the development of new drugs to treat the condition.

The research was published online in the journal Radiology.