What exactly is a carotid ultrasound and doppler ultrasound?

Noninvasive image: A noninvasive painless image of the carotid artery.

Image/sound: Doppler test depends on particles shift or movement and the result will be different sound intensities, ultrasound is a visual/image test of particles depending on densities. When you combine these two test we get the duplex ultrasound. For carotid duplex we evaluate picture or image of the artery with ultrasound and identify velocities or particles shift( blood on this case) to quantify stenosis.

Ultrasound: A carotid ultrasound is a study of your carotid arteries which supply blood to your brain. It looks for narrowings and plaques in these vessels. Doppler looks at blood flow in these vessels and the ratio of flow in the internal carotid and common carotid. Doppler also measures the rate of flow in cm/sec to help determine if there is stenosis which can be dangerous and increase risk of stroke.