How definitive is an mri as a screening device to rule out dementia, either alzheimer's or other types?

Not great at all: Mri scans of the brain are usually done as part of a dementia workup, but if within normal limits it doesn't rule out dementia. Cognitive and memory changes can be happening long before physical changes show up on an mri. The more definitive findings are on clinical interviews -- with specialized neuropsychological testing may be helpful also.

It's not: Mri is a useful test to look for strokes, masses and shrinkage of brain tissue. It can assist in diagnosis- particularly in frontotemporal dementia(ftd), as there is characteristic atrophy of regions in the brain. Generally, DX is a clinical determination, although there are newer scans becoming available to diagnose ad. Pet scans maybe useful as well for DX ftd.