The device shoots tiny magnetic particles into the nose which enter the bloodstream and are carried to the brain.

Each particle is fused to an antibody that targets and binds to rogue molecules believed to play an early role in the disease.

Magnetic resonance imaging (MRI) can detect particles and molecules.

To date scientists have only tested the technique in the laboratory on human brain tissue cultures. But if it can be shown to work in human patients it could lead to a major leap forward in managing Alzheimer's.

Scientists believe the changes that lead to Alzheimer's begin decades before the first symptoms appear.

By the time a patient is diagnosed the disease is already far advanced, and experts suspect that is the main reason why a number of promising drugs have failed in patient trials.

Details of the research were presented at a meeting of the Society for Neuroscience in New Orleans.

Lead scientist Dr William Klein, from Northwestern University, Chicago, said: "We have created a probe that targets a unique marker of Alzheimer's disease. This technology is a promising tool for early AD diagnosis."

The antibodies developed by Dr Klein's team are attached to magnetic nanoparticles which allow them to be tracked by an MRI scan. The scientists are now working on incorporating the particles into a spray.