How common is short-term memory after a stroke?

Is it possible that these kinds of strokes do not show up on an MRI?

I think it is very common, it's called vascular dementia. Mom's stroke was pretty mild (no paralysis), it was in the thalamus of her brain where everything connects, so she has a lot of disconnect (hard time remembering and making good decisions, also finding the words she needs). Web MD says about 1 in 4 people who have a stroke develop signs of dementia within 1 year. Hope this helps.