Which vessel is described as having retrograde flow on the transtemporal study?

• A. Anterior cerebral artery
• B. Middle cerebral artery
• C. Posterior cerebral artery
• D. Terminal internal carotid artery

Answer: (A)

Retrograde flow on a transtemporal Doppler study points to collateral circulation through the circle of Willis. The anterior cerebral artery is the vessel most likely to show this reversed flow pattern when there is compromise of the ipsilateral carotid system, because blood can reach the affected hemisphere via the contralateral side through the anterior communicating artery. In that collateral setup, flow in the ipsilateral anterior cerebral artery can appear retrograde relative to the usual antegrade direction, making the ACA the vessel described. The other vessels (MCA, PCA, and terminal internal carotid) are less characteristically the site of retrograde flow under typical collateral scenarios.