Structural and functional MRI in mild cognitive impairment.

Mild cognitive impairment (MCI), and the amnestic subtype of MCI in particular, is the most recent concept used to describe the intermediary state between healthy aging and Alzheimer's disease (AD). It is hoped that research focusing on MCI would yield markers for early identification of individuals with prodromal AD at such a pre-dementia stage when potential disease modifying therapies would be most efficacious. Magnetic resonance imaging (MRI) combined with various data analysis methods provides tools to investigate alterations in brain structure and function in vivo. Structurally, MCI is characterized by atrophy of the medial temporal lobe (MTL) structures such as the hippocampus and entorhinal cortex, and the amount of atrophy in MCI is intermediate between healthy aging and AD. Additionally, atrophy of the posteromedial cortices such as the posterior cingulum and precuneus as well as of the lateral temporal cortices has been reported. The pattern of atrophy appears to vary according to the subtype of MCI. Functional MRI studies in MCI, compared to healthy aging and AD, have demonstrated both increased and decreased MTL activity during encoding novel visually presented material. Differences in the MTL activation pattern in MCI subjects may relate to differences in the severity of cognitive decline. There is some evidence that increased MTL activity observed during encoding may be compensatory due to incipient atrophy in the MTL structures. The resting state (or, "default mode") network, and the posteromedial cortical regions in particular, appear to malfunction in MCI. It is suggested that both altered MTL and posteromedial cortical function may be indicative of future cognitive decline from MCI to clinical AD.