Brain 18F-FDG PET in the diagnosis of neurodegenerative dementias: comparison with perfusion SPECT and with clinical evaluations lacking nuclear imaging.

The clinical identification and differential diagnosis of dementias is especially challenging in the early stages, but the need for early, accurate diagnosis has become more important, now that several medications for the treatment of mild to moderate Alzheimer's disease (AD) are available. Many neurodegenerative diseases produce significant brain-function alterations detectable with PET or SPECT even when structural images with CT or MRI reveal no specific abnormalities. (18)F-FDG PET images of AD demonstrate focally decreased cerebral metabolism involving especially the posterior cingulate and neocortical association cortices, while largely sparing the basal ganglia, thalamus, cerebellum, and cortex mediating primary sensory and motor functions. Assessment of the precise diagnostic accuracy of PET had until recently been hindered by the paucity of data on diagnoses made using PET and confirmed by definitive histopathologic examination. In the past few years, however, studies comparing neuropathologic examination with PET have established reliable and consistent accuracy for diagnostic evaluations using PET-accuracies substantially exceeding those of comparable studies of the diagnostic value of SPECT or of both modalities assessed side by side, or of clinical evaluations done without nuclear imaging. Similar data are emerging concerning the prognostic value of (18)F-FDG PET. Improvements in the ability of PET to identify very early changes associated with AD and other neurodegenerative dementias are currently outpacing improvements in therapeutic options, but with advances in potential preventive and disease-modifying treatments appearing imminent, early detection and diagnosis will play an increasing role in the management of dementing illness.