Did carotid ultrasound imaging and came out normal. Possible CABG short term. Is cta of the neck needed, how much radiation it adds or risk.?

Shouldn't. If u/s clear, should be good to go. If they still want a cta, is fair to ask & get rationale, as aren't free--$/dye/rad--it's a reasonable question. If do have to get it, not a huge radiation load as mostly soft tissues.