Is there a dementia rating chart?

My mother In Law went in for a MRI and of course was the one to answer the phone when the tech called the day of the results. All we know from her was he said she was normal. If she is normal, grapefruit a grow on pineapple trees! Anyway, from what I read, there is a 1 to 5 rating depending on the severity of the dementia, 5 being not recognizing family or places and not able to care for ones self at all.
Is this the information that comes from the MRI we would get when the family doctor recieces the MRI info, or does this test only show that she's not bleeding or having strokes?
I also didn't know if a CAT scan would be a test that would give added info. I am certain myself she would be in the #4 rating- recognizes family but can't remember her address and information. There's good and bad days of course, it just seems we jumped from 2-4 rather quickly in a year or so. Or she hid it VERY well.