[Dementias and memory impairment of degenerative origin].

In dementias and memory impairments of neurodegenerative origin, imaging of brain metabolism and perfusion by means of positron or single-photon emission tomography or functional MRI has utility in the following domains: 1) pathophysiology, by revealing the topography and severity of synaptic damage even at the earliest stages of the disease, at a time when structural imaging is of little or no help; 2) diagnosis, by showing profiles of abnormalities characteristic to each disorder, even though such profiles are neither specific to, nor constantly present in each disorder; 3) pharmacology, by providing a surrogate marker to evaluate objectively the effect of the drug being tested; and 4) neuropsychology, by mapping both the brain structures that underlie each subtype of memory impairment, and the reorganisation of large-scale networks that subserve task performance despite established synaptic lesions.