Researchers hope the technique can be used to predict the likelihood of developing diseases such as Alzheimer's.
Previous studies have developed live brain imaging for amyloid and tau proteins, implicated in cognitive diseases, but tests have been limited to a handful of patients.
Researchers from the University of California in Los Angeles tested a radioisotope label on 43 middle-aged and elderly volunteers. Half of the participants were ageing normally and half had mild cognitive impairment.
Participants were injected with the label and PET was used to assess its binding to plaques within the brain.
After two years, patients in the cognitive decline group had increased levels of label binding across several areas of the brain, indicating increased plaque deposition. The normally ageing group showed no such increases.
In the cognitive decline group, binding at the frontal and parietal lobes provided the most accurate identification for patients who were at the highest risk of developing Alzheimer's disease.
Higher levels of binding at the start of the study were also associated with future decline in most cognitive domains.
Across all participants, binding of the label at certain sites was linked to progression of memory decline.
The researchers said the test could help to identify those at risk of cognitive decline and those likely to benefit from treatment, and could also be used to track the effectiveness of interventions.