Vascular ultrasound uses sound waves to evaluate the body's circulatory system and help identify blockages in the arteries and veins and detect blood clots. A Doppler ultrasound study – a technique that evaluates blood flow through a blood vessel – is usually part of this exam. Ultrasound is a non-invasive exam that does not use ionizing radiation, has no known harmful effects, and provides images of soft tissues that don't show up on X-ray images.