Doppler ultrasound scanning measures blood flowing through the carotid arteries or the arteries at the base of the brain. This test assesses the risk of stroke.

This painless technique shows different speeds of blood flow in different colors on a computer screen. It also can show blocked or reduced blood flow in the arteries of the neck that could cause a stroke. Additionally, it can show blood clots in leg veins that could break loose and block blood flow to the lungs.

During duplex Doppler ultrasound, a handheld instrument (transducer) is passed lightly over the skin above a blood vessel. The transducer sends and receives sound waves that are amplified through a microphone. The sound waves bounce off solid objects, including blood cells. The movement of blood cells causes a change in pitch of the reflected sound waves (called the Doppler effect). If there is no blood flow, the pitch does not change. A computer processes the information from the reflected sound waves to provide graphs or pictures that represent the flow of blood through the blood vessels. These can be saved for future review or evaluation.

There are four types of Doppler ultrasound, including:

• Continuous wave Doppler. This measures how continuous sound waves change in pitch as they encounter blood flow blockages or narrowed blood vessels. This type of ultrasound can be done at a person's hospital bedside to provide a quick estimate of the damage or disease.
• Duplex Doppler. This produces a picture of a blood vessel and the organs that surround it. A computer converts the Doppler sounds into a graph that provides information about the speed and direction of blood flow through the blood vessel being examined.
• Color Doppler. In this procedure, a computer converts the Doppler sounds into colors that are overlaid on the image of a blood vessel. The colors represent the speed and direction of flow through the vessel.
• Power Doppler. This is a new technique being developed that is up to five times more sensitive than color Doppler. Power Doppler can get pictures that are difficult or impossible from standard color Doppler. Power Doppler is most commonly used to evaluate blood flow through vessels within solid organs. Blood flow in individual blood vessels is most commonly evaluated by duplex Doppler.