Welcome to the forum. It sounds like you are just in the beginning stages of being worked up. Are you seeing both a Neurosurgeon and a Cardiologist?
I strongly recommend that the 2 MD's communicate with each other and keep you in the loop...even if you have to tell them to talk to each other. Yes, Hypoplastic vessels in the brain is not normal nor is having a TIA. A TIA is a precursor to a stroke. Besides taking aspirin, what else did the doctor tell you? If you are unsure, call him and ask him.
Keep us informed. Also get copies of your diagnostic tests for your records (tell the doctors you want a copy sent or faxed to you).