A lot. Mri is superior. Utrasound reliable in total block or excellent flow but unable to determine degrees of stenosis.
Mostly technical. The carotid arteries are well seen on Carotid duplex scanning. The problems are whether we can get good images of the arteries due to the patients anatomy. Sometimes thick necks and relative positon of the vessels make it hard to see them. The other issue is that when the carotid arteries enter the skull, they can no longer be seen on ultrasound.