Functional imaging patterns in Alzheimer's disease. Relationships to neurobiology

Brain imaging with functional techniques, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) have been widely applied to the study of dementia. While the pattern of temporal and parietal hypometabolism and hypoperfusion have often been suggested to be of diagnostic utility in ascertaining that a dementia is due to Alzheimer's disease (AD), the exact sensitivity and specificity of this pattern in clinically important situations is unclear. These imaging findings have been of considerable interest, however, in describing the regional patterns of predilection in the disease. Evidence supports the contention that the earliest sites of functional impairment in AD are in the temporal lobes. Surprisingly, however, mesial temporal lobe hypometabolism was difficult to detect in a group of mildly demented AD patients in comparison to a group of healthy older subjects. These results suggest that simple use of mesial temporal lobe metabolic rates as a diagnostic for AD may not be fruitful, and that evaluation of the earliest stages of AD can be most productively studied by investigating healthy older individuals.