Why can't alzeihmers be diagnosed with an mri scan?

Alzheimers: Alzheimers can not be definitively diagnosed without a brain biopsy, and this is not done till post mortem. Many people have muliinfarct dimensia which is causing there probelm. This can be seen on CT or MRI. Often the conditions overlap and both can cause the dementia symptoms. Alzheimers shows nerofibrillary tangles on microscopic exam.

MRI images: MRI studies by themselves are simply computer generated pictures of what the body parts could look like when you scan and reconstruct emphasizing different physical characteristics of the molecules in the field of view. With experience, the Doctor can tell normal from abnormal tissues. If the abnormal tissues are too similar to normal tissues, then they all look the same, and you cannot tell.