Research indicates that MRI does not significantly improve the prediction of dementia. It does have some value in differential diagnosis.
Stephan, BC. Tzourio, C. [and] Auriacombe, S. [et al] (2015). Usefulness of data from magnetic resonance imaging to improve prediction of dementia: population based cohort study. British Medical Journal (BMJ). June 22nd 2015; 350: h2863.