Diagnostic Imaging
September 2004

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PET, SPECT, tracers help characterize dementias

Dementia with Lewy bodies shares clinical and pathological features with Alzheimer's disease, and distinguishing the two poses diagnostic difficulties. But dementia with Lewy bodies manifests a considerable degeneration of nigral neurons as well as cortical neuronal loss. Therefore, Dr. Angela Park and colleagues at Seoul National University College of Medicine in Korea compared the regional cerebral metabolic impairment and the dopaminergic neuronal integrity between patients with dementia with Lewy bodies and AD. They used FDG-PET and iodine-123-fluoropropyl-beta-CIT (FP-CIT) SPECT as measures for the diagnosis of the diseases.

FP-CIT is a relatively new ligand, which the researchers chose because it has faster kinetic properties than beta-CIT. Patients can be scanned three to six hours after injection.

The investigators found different biochemical features, in terms of regional brain metabolism and dopaminergic neuronal integrity, be-tween dementia with Lewy bodies and Alzheimer's. PET images of patients with dementia with Lewy bodies revealed hypometabolism bilaterally in the occipital cortices, lateral occipitotemporal gyri, cunei, caudate nucleus, and thalami, compared with controls. Compared with Alzheimer's patients, differences were most pronounced in the occipital cortex.

The caudate nucleus and putamen were significantly reduced in comparison with Alzheimer's subjects and controls. The ratio of putamen-to-caudate nucleus V3'' was not significantly different between dementia with Lewy bodies patients and controls, indicating that dopamine transporter density in the caudate nucleus and putamen is evenly affected in dementia with Lewy bodies. Park concluded that a measure of glucose in the occipital cortex and striatal dopamine transporter density may help differentiate dementia with Lewy bodies from Alzheimer's.

In another SNM study, Dr. Karl Herholz, a neurologist at Max-Planck Institute and University in Cologne, Germany, and colleagues from multiple centers found that FDG-PET metabolic patterns suggest a classification of Alzheimer's disease versus frontotemporal dementia that strongly differs from clinical criteria in 10% to 15% of patients.

Researchers prospectively examined 248 patients with clinical Alzheimer's disease and 32 with frontotemporal dementia. PET misclassified 14 clinically diagnosed frontotemporal dementia patients and 23 Alzheimer's patients, for a 15% misclassification rate. PET was less discriminating when patients experienced disease onset at a later age. This makes sense because multifactorial disease may have features of both, Herholz said.

The analysis used in this study allowed the researchers to determine how certain the discrimination was. A closer look revealed a significant correlation between the certainty of discrimination and the age of onset. When they limited the analysis to 91 subjects in whom the error probability was less than 5%, they found that PET misclassified seven subjects with clinical Alzheimer's and two with frontotemporal dementia.

The problem was not the discrimination algorithm, Herholz said. The cases probably would have been misclassified on visual analysis. The frontotemporal images showed more diffuse frontal impairment, while the Alzheimer's cases had the typical temporoparietal uptake.