Decreases in brain volume seen via MRI are considered to be signs of decreased synapse density, neuron loss, and cell shrinkage. Tondelli et al. hypothesized that healthy subjects who would develop AD by up to 10 years later would show reduced brain volume in certain brain areas in an MRI at baseline.
Specifically, via voxel-based morphometry, they found greater brain volume in certain regions in individuals who did not develop AD, with the largest differences seen in the medial temporal lobe (hippocampus and amydgala). They also report a significant correlation between the degree of bilaterial temporal lobe atrophy and a measure of cognitive impairment, although it would have been nice to see the actual coefficient.
Perhaps the most interesting part of their study is when the authors used image segmentation of subcortical regions with standardized vertices to compare across subjects. This allowed them to find shape differences in the right hippocampus that could distinguish (> 93% classification accuracy) between subjects who converted to AD from those that remained healthy. This kind of measure could allow for greater insight into the mechanisms of deterioration at play.
Tondelli M, et al. 2011 Structural MRI changes detectable up to ten years before clinical Alzheimer’s disease. Neurobio Aging. PubMed.