Carotid ultrasound is a painless and harmless test that uses high-frequency sound waves to create pictures of the insides of the two large arteries in your neck. These arteries, called carotid arteries, supply your brain with oxygen-rich blood. You have one carotid artery on each side of your neck. Carotid ultrasound shows whether a substance called plaque has narrowed your carotid arteries. Plaque is made up of fat, cholesterol, calcium, and other substances found in the blood. Plaque builds up on the insides of your arteries as you age. This condition is called carotid artery disease. You have two common carotid arteries—one on each side of your neck—that divide into internal and external carotid arteries. Too much plaque in a carotid artery can cause a stroke. The plaque can slow down or block the flow of blood through the artery, allowing a blood clot to form. A piece of the blood clot can break off and get stuck in the artery, blocking blood flow to the brain. This is what causes a stroke. A standard carotid ultrasound shows the structure of your carotid arteries. Your carotid ultrasound test may include a Doppler ultrasound. Doppler ultrasound is a special test that shows the movement of blood through your blood vessels. Your doctor often will need results from both types of ultrasound to fully assess whether there’s a problem with blood flow through your carotid arteries.