[Contribution of various MRI techniques to the characterization of Mild cognitive impairment]

Mild cognitive impairment (MCI) is considered as a strong risk factor for Alzheimer's disease (AD) or other dementia syndromes. Since the last decade, numerous publications have been aimed to characterize early detectable brain changes in vivo, using more and more efficient neuroimaging techniques. This review is devoted to the brain damages detectable in MCI patients according to the MRI techniques available to date. The greatest number of studies, using structural and functional imaging, report many abnormalities principally located in the medial temporal lobe. They show, especially in this region, cortical atrophy, reduction of glucose metabolism, decrease in regional cerebral blood flew and biochemical changes. Moreover, progresses in the functional methods allow to detect brain activity during memory tasks, trying to specify whether this activity increases or decreases according to the task and the severity of cognitive impairment. The contribution of each RMI technique (morphologic, metabolic, and functional) is addressed in order to reveal how relevant they are to distinguish subjects with MCI from patients with early AD. Furthermore, their relevance to discriminate between MCI with different degrees of cognitive deficit and their power to predict the risk of conversion to AD is discussed. Finally, we review the main assumptions made to explain the underlying mechanisms of cognitive decline and present evidences in favor of dynamic compensatory processes, existence of cognitive reserve and disconnection processes. Many neuroimaging data support the pattern of installation and evolution of brain damages found in AD, reinforcing the idea that a part of amnesic MCI is probably a prodromal state of AD.