A:  Christian Doppler was a 19th century physicist who determined a way to measure the speed of a moving object using sound waves.  Today, this principle is used for a variety of uses including catching speeders and predicting weather.  Advanced ultrasound machines use Doppler-based technology to allow us to actually see blood flowing through arteries, veins, or the heart.  The movement of blood is shown as colors that represent various speeds and directions of flow.

As a practical matter, this technology allows us to better diagnose heart diseases and liver shunts.  It can also help us determine the blood supply to an organ or limb if compromise is suspected or an operation is being considered.