bilateral carotid artery stenosiscarotid bifurcation atherosclerosisus carotid doppler bilateralus carotid duplex bilateralcarotid siphon atherosclerosisbilateral carotid occlusionbilateral carotid artery diseasebilateral carotid artery occlusioncarotid arteriosclerosisbilateral ica stenosisextracranial atherosclerosiscarotid artery plaque