Imaging Techniques for Vascular Structures of Head and Neck

Which imaging technique is considered the gold standard for imaging vascular structures of the brain?

Which imaging technique is preferred for imaging the aorta and coronaries despite its main drawbacks of contrast use and radiation exposure?

Which imaging technique is the least invasive, can be done bedside, and is cost-effective for imaging vessels close to the skin surface?

Which imaging technique is non-invasive, involves no radiation exposure, and is preferred for carotids and intracranial vessels?

Which component forms the circle of Willis by extending from the distal dural ring at the superior clinoid to just below the origin of the posterior communicating artery (PCOM)?