I lost my beloved father three weeks ago and my 91 year old mother says, she talks to him well actually to his urn that has his picture and sits above the mantle every day. She has been diagnosed with vascular dementia and his death has plunged her to state of grief. She says, she feels his presence all the time it is like a white vapor. Her neurologist's wants to order an MRI to confirm the dementia but we in the family have seen signs of it for the last three months. I tell her that it is good that she talks to her husband / my dear father but for how long
That said, ok, your mom may have some dementia. And l say "some" because MOST dementia comes with some memory deficits, and she seems to me to be remembering your father far too well to have sunk very deeply into the dementia abyss as yet.
MRI's with their clanging and banging, not to mention the need to be still for long periods of time, are scary and trying. My mom had to be virtually put out with 4 mg of Valium in order to tolerate the process and the cap on her head. I wouldn't put your mom through that at this point in time with everything else she's going through. Maybe never unless there is a definitive reason why you need to.
Whether it's structured religiosity, new age spirituality or personal philosophy, more people believe in the afterlife then don't. It always amazes me that people who have faith in that concept, are surprised that there may from time to time be some visual and/or physical manifestation. Who's to say that the white vapor she mentiones ISN'T your dad coming through from the non-physical? What's the harm in letting her believe if it comforts her?
It sounds to me like your mom's doing a great job of grieving and I wouldn't worry so much about the dementia unless unmanageable behaviors become a problem. MRIs can be really difficult and create anxiety; I'm not sure I'd put her through that right now. You're both grieving; express it, honor it and take care of one another... Your obviously doing a great job!
The MRI might be helpful as a guide, but I shouldn't rush it unless there's a pressing clinical need to do so. In addition to your mother's current grief (which the whole family must be feeling, too), don't forget that during the last three months, when you've been noticing signs of dementia, she will have been under enormous stress.
With my mother, I find it very difficult to know what is causing her concentration, memory and mental processes to be poor on a given day: it could be one kind of dementia or another, but it could also be fatigue, depression, illness, deafness, loss of vision… In the end, does it matter? Your mother isn't delusional - she doesn't actually think your father is physically present in the ordinary meaning of it. Anything else, it's all right to play it by ear and do/say whatever makes her most comfortable.
And please do take a little time and space for your own grief. My sympathies x