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Fig. 9A. —78-year-old man with retrograde flow in left external carotid artery caused by complete occlusion of left common carotid artery. Spectral Doppler sonography tracing from left external carotid artery shows blood flow directed toward transducer (note positive velocities on scale). Therefore, flow in external carotid artery is retrograde toward carotid bulb and then antegrade in internal carotid artery.