@adah - I don't happen to be familiar with small-caliber basilar artery, but I'm guessing that perhaps @jenniferhunter or @johnbishop may know something about it or be able to point to some research about it.

Will you be meeting with your doctor, during which you might ask about what this is and any implications for you? If you have no appointment set up, can you message your doctor or call his or her office to enquire? How did you come to be having an MRI?