The primary role of MRI (and CT for that matter) in the diagnosis of Alzheimer disease is the assessment of volume change in characteristic locations which can yield a diagnostic accuracy of up to '7%. Unfortunately, such volume loss is not apparent early in the course of the disease.
The diagnosis should be made on the basis of two features:
1. mesial temporal lobe atrophy (particularly hippocampus and entorhinal cortex)
2. temporoparietal cortical atrophy
Mesial temporal lobe atrophy can be assessed directly or indirectly. Directly assessment is of hippocampal or parahippocampal volume loss while indirect assessment relies on an enlargement of the parahippocampal fissures. The former is more sensitive and specific but ideally requires actual volumetric calculations rather than 'eye-balling' the scan. These measures have been combined in the medial temporal atrophy score which has been shown to be predictive of progression from mild cognitive impairment (MCI) to dementia.
Additionally, and particularly relevant to posterior cortical atrophy or early onset Alzheimer disease, is the presence of parietal atrophy. This is often best seen on the interhemispheric surface of the parietal lobe (see neurodegenerative MRI brain: an approach) by examining the posterior cingulate sulcal and parietooccipital sulcal size and degree of atrophy of the precuneus and cortical surface of the parietal lobe. This has also been combined into a scoring system (see posterior atrophy score of parietal atrophy a.k.a. Koedam score).
Brain volume measurements, assessed with segmentation, demonstrates that patients with Alzheimer's disease have accelerated rates of brain volume loss, typically around twice normal (1% vs. ~0.5% per year). This is even more marked in the hippocampi with affected individuals exhibiting three times the volume loss per year (~-.5% vs. ~1.5% per year).