This study was undertaken to determine if measurements of absolute regional cerebral metabolic rate for glucose (rCMRglc) and variance of rCMRglc are independent of time between 30 and 45 min following injection of the radiotracer. Sixteen subjects received two sets of 18FDG PET scans commencing 30 and 45 min following intravenous injection of 18FDG. No statistically significant differences were detected in either absolute rCMRglc or rCMRglc variance between the two sets of scans. These data demonstrate that for most FDG PET studies, scanning can commence 30 min after injection of the radiotracer without compromising the metabolic data.

Regional cerebral metabolic rates for glucose (rCMRglc), as measured with positron emission tomography, and neuropsychological function were studied longitudinally (range, 15 to 48 months) in 11 mildly impaired patients with dementia of the Alzheimer type (DAT) and compared with results from patients with moderate and severe DAT and from controls. At initial evaluation, association cortex metabolic asymmetries were greater in patients with DAT than in controls for all dementia severities and correlated significantly with neuropsychological discrepancies between visuospatial and language abilities in patients with moderate dementia. In mildly impaired patients, right-left metabolic asymmetries in the association cortices were directionally stable and became more pronounced over time. At initial evaluation, these patients had significant impairment, relative to controls, on tests of memory and attention to complex tasks but not on tests of language and visuospatial function. Memory, attention, language, and visuospatial impairments, however, all worsened significantly over time. In mildly impaired patients, correlations between right-left metabolic asymmetries and neuropsychological discrepancies were insignificant initially but were significant at last evaluation. These results demonstrate that heterogeneous nonmemory language and visuospatial impairments in early DAT are related to and predicted by the earlier-appearing distribution of metabolic reductions in the association neocortex.

The parietal-frontal distribution of reductions of regional cerebral metabolic rates for glucose (rCMRglc) was studied in 32 patients with mild to severe dementia of the Alzheimer type (DAT), using positron emission tomography and fluorodeoxyglucose, and was related to patterns of neuropsychological impairment. In moderate and severe DAT patients, one frontal association region, the premotor cortex, demonstrated significant metabolic reductions equivalent to reductions in the parietal association cortex, and the ratio of parietal to premotor rCMRglc had significantly greater variance than in controls. In moderately demented patients, parietal-premotor and parietal-prefrontal metabolic ratios correlated significantly with neuropsychological impairments. Disproportionate parietal hypometabolism was associated with more impairment of verbal comprehension, calculations, visuospatial construction, and immediate visuospatial memory span. Disproportionate frontal hypometabolism was associated with more impaired verbal fluency and attention. Longitudinal follow-up of 20 of the patients showed that parietal/frontal metabolic ratios and their correlated neuropsychological patterns were stable over time, as dementia severity worsened. These results indicate that in moderate to severe DAT patients, metabolic reductions in the premotor cortex are as severe as the reductions in the parietal association cortex. Moreover, the parietal-premotor distribution of metabolic reductions is variable and related to variable patterns of cognitive impairment.

To examine the progression of neuropsychologic and metabolic changes in the early stages of dementia of the Alzheimer type (DAT), we studied 11 midly demented patients longitudinally. Three aspects of neuropsychological function were measured: memory, attention to complex sets and abstract reasoning, and lateralized functions, i.e., language and visuoconstruction. Regional cerebral metabolic rates for glucose were measured in frontal, parietal, and temporal association cortices. Our results show that, in general, memory deficits are the first neuropsychological impairments to occur in DAT, followed by problems with attention to complex cognitive sets and abstract reasoning, which are followed in turn by deficits in language and visuospatial abilities. In addition, neocortical metabolic abnormalities usually precede impairment of neocortically mediated attention and abstract reasoning by 8 to 16 months, and precede impairment of neocortically mediated language and visuospatial function by 12 to 37 months. These findings suggest that the first nonmnestic neuropsychological consequence of neocortical physiological dysfunction in DAT is a loss of attentional capacity. Since neocortical metabolic changes generally precede the appearance of neocortically mediated neuropsychological dysfunction, physiologic dysfunction may exist for some time before cognition is affected.