Doppler is the tool that measures the speed of the blood flow in cm. per second.

Speed of the blood flow through the artery leading to the brain is valuable information because it helps define exactly how narrow, or occluded the artery may be due to the plaque.

The most common type of stroke is ischemic stroke (lack of oxygen/blood to the brain) due to blocked carotid arteries, so ultrasound of the carotid arteries is the ideal test in the attempt to prevent ischemic stroke.