PET scanning for the detection of Alzheimer’s disease

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 to the investigation of 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 slightly decreases with age to a regionally different degree, in most types of dementia severe changes in glucose metabolism are observed. Degenerative dementia of the Alzheimer type is characterized by a metabolic disturbance most prominent in the parieto-occipitotemporal association cortex and later in the frontal lobe, while primary cortical areas, basal ganglia, thalamus, brainstem and cerebellum are not affected. Thanks to this typical pattern Alzheimer’s disease can be differentiated from other dementia syndromes, such as Pick’s disease (with the metabolic depression mostly prominent in the frontal and temporal lobe), multi-infarct dementia (with multiple focal metabolic defects), Huntington’s chorea (with metabolic disturbances in the neostriatum) and other diseases leading to cognitive impairment with more or less typical metabolic patterns. A ratio calculated form CMRGl of affected (temporo-parieto-occipital and frontal association cortex) and non-affected brain regions (primary cortical areas, brainstem, cerebellum) enabled us to separate clearly AD patients from agematched controls and to discriminate those patients suffering from cognitive impairment of other origin in 82% of the cases. The discrimination power can be further improved by specific activation studies. In demented patients PET can also be used to assess the effects of treatment on disturbed metabolism. Such studies demonstrated an equalization of metabolic heterogeneities in patients responding to muscarinic cholinergic agonists, as well as a diffuse increase of metabolism during treatment with piracetam and phosphatidylserine. The therapeutic relevance of such metabolic effects, however, remains to be proved in controlled clinical trials.