@ellafarris - Everyone loses some white matter in the brain over the course of their lifetime. This is a normal aging process and what causes our cognitive skills to weaken as we get older.
White brain matter diseases vary widely with over one hundred different causes. Whether or not there's any cause for your daughter-in-laws concern depends on the location and development of brain lesions in the white matter.
This is more than likely why her doctor has ordered the MRI. The migraines may have caused some damage that might lead to high blood pressure, high cholesterol or an increased risk of stroke.