Hello & welcome to Just Answer! If you have any questions after reading my answer, be sure to let me know.
No, you cannot diagnose MS in this situation of brain atrophy alone,
There are two basic rules for diagnosing MS:
Multiple sclerosis means just that -- multiple (more than one), sclerosis (areas of damage; scarring or hardening).
Hence without the characteristic plaques on an MRI a diagnosis can not be made,
Please remember to accept the answer by pressing the GREEN accept button, do get back for any clarifications