The role of emission tomography in dementia

Positron emission tomography (PET) and single photon emission tomography (SPET) offer the opportunity to improve a diagnosis of dementia by providing regional functional measurements, which can be used to substantiate the clinical judgement. Further progress in the differential diagnosis among degenerative dementias is expected from pathological confirmation in the follow-up of patients evaluated with neuroimaging methods. A prospective multi-center cohort study of patients with possible or probable Alzheimer's disease mostly with presenile onset, showed impairment of brain glucose metabolism in temporoparietal or frontal association areas, as measured with PET. This was associated significantly with dementia severity, clinical classification, presence of multiple cognitive deficits, and history of progression. In addition, prospective longitudinal analysis showed a significant association between initial metabolic impairment (metabolic ratio = 0.80) and subsequent clinical deterioration. In patients with mild cognitive deficits at entry, the risk of deterioration was up to 4.7-times higher if metabolism was severely impaired than with mild or absent metabolic impairment. In the future, it might be possible to use different tracers to measure neurotransmitter release or receptor function. It may also be possible to scan the patient while performing cognitive tasks to examine changes in functional brain activity during pharmacological treatments.