PET criteria for diagnosis of Alzheimer’s disease and other dementias

Summary

At present, PET is the only technology affording the quantitative, three-dimensional imaging of various aspects of brain function. Since function and metabolism are coupled, and since glucose is the dominant substrate of the brain’s energy metabolism, studies of glucose metabolism by PET of 2(¹⁸F)-fluoro-2-deoxy-D-glucose (FDG) are widely applied for investigating the participation of various brain systems in simple or complex stimulations and tasks. In focal or diffuse disorders of the brain, functional impairment of affected or inactivated brain regions is a reproducible finding.

While glucose metabolism is decreased slightly with age in a regionally different degree, in most types of dementia severe changes of glucose metabolism are observed. Degenerative dementia of the Alzheimer type is characterized by a metabolic disturbance most prominent in the parieto-occipito-temporal association cortex and later in the frontal lobe, while primary cortical areas, basal ganglia, thalamus, brainstem and cerebellum are not affected. By this typical pattern Alzheimer disease can be differentiated from other dementia syndromes, as e. g., Pick’s disease (with the metabolic depression most prominent in the frontal and temporal lobe), multi infarct dementia (with multiple focal metabolic defects), Huntington’s chorea (with metabolic disturbance in the neostriatum) and other diseases leading to cognitive impairment with more or less typical metabolic patterns. A ratio calculated from CMRG1 of affected (temporo-parieto-occipital and frontal association cortex) and non-affected brain regions (primary cortical areas, brainstem, cerebellum) was able to separate clearly AD patients from age-matched controls and permitted discrimination of patients suffering from cognitive impairment of other origin in 82%. The discrimination power can be further improved by specific activitation studies. In demented patients PET can also be used to assess treatment effects on disturbed metabolism. Such studies demonstrated an equalization of metabolic heterogeneities in patients responding to muscarinergic cholinagonists and diffuse increase of metabolism during treatment with piracetam and phosphatidylserine. The therapeutic relevance of such metabolic effects, however, must be proved in controlled clinical trials.