Other than finding tumors or very recent bleeds, MRI's do not have the fine resolution needed to find most neurological damage associated with concussion, etc.

Only research institutes have the high resolution MRI's, 5 or 6 Tesla or above, to see microscopic damage. An fMRI (functional MRI) can find finer damage by imaging the blood flow and metabolism processes in a small area of the brain. fMRI's are not yet in common usage.