Dr Wise or Nurse:

I had a follow up visit to the Dr to discuss the MRI and she said that there is no tumors/masses or lesions. But, she said that my brain is shrinking quicker than average my age. She said that that could be attributed to my head injury with the dent that I have in my head, but it could mean something else.
Can you tell me what else that could mean? I have read that early alzheimers is a possibility. But, what else?
She recommended another MRI in three to six months to track the progress, if any.