I am sorry that your question was unanswered for very long.
Following are the common causes of brain atrophy;
1) Vitamin B 12 deficiency.
2) Excessive alcohol
4) Injury to brain
5) Infectious diseases
6) Huntington’s disease,
7) Corticobasal degeneration,
8) Posterior cortical atrophy,
9) Multiple system atrophy
10) Progressive supranuclear palsy.
11) Hypothyroidism or hyperthyroidism
12) Hypernatremia (increased sodium)
13) Cerebrovascular disease
Apart from MRI, a proton MR spectroscopy would be able to ascertain the cause.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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