TIA or Vertigo, are ultrasounds reliable

Hello,
A couple weeks ago, i all of a sudden could not walk a straight line, so I woke my husband up and he took me to the hospital, they did a ekg, chest xray, blood work and a ct scan and checked my cartoid arteries on the side of my neck. Everything looked ok, but they saw a lacunar infarct of uncertain age. They said it wasn't what was causing my problems, since it was older. They figured maybe inner ear. I then went for a second opinion to a neurologist who said it may not of been a old lacunar stroke and may be nothing. He was thinking inner ear also, by my symptoms I described to him. He sent me for a ultrasound on the arteries in the back of my neck which showed that the right vertebral artery may be narrowed. Now they are sending me for a mri/mra with contrast to check. He said that odds are that i don't have a narrowing but the ultrasound suggested it was, but the ultrasound is often wrong. Has anyone ever heard the ultrasound is not that reliable? I have searched on line and can't find anything like that. And has anyone had the mra with contrast and how was it, and how long did it take? Any similar stories?
Thanks so much for your help. Im also getting checked for Meneire's disease (inner ear).