In ultrasound, a beam of sound at a very high frequency (that cannot be heard) is sent into the body from a small vibrating crystal in a hand-held scanner head. When the beam meets a surface between tissues of different density, echoes of the sound beam are sent back into the scanner head. The time between sending the sound and receiving the echo back is fed into a computer, which in turn creates an image that is projected on a television screen. Ultrasound is a very safe type of imaging; this is why it is so widely used during pregnancy. Doppler ultrasound A Doppler study is a noninvasive test that can be used to evaluate blood flow by bouncing high-frequency sound waves (ultrasound) off red blood cells. The Doppler Effect is a change in the frequency of sound waves caused by moving objects. A Doppler study can estimate how fast blood flows by measuring the rate of change in its pitch (frequency). A Doppler study can help diagnose bloody clots, heart and leg valve problems and blocked or narrowed arteries. What to expect? After lying down, the area to be examined will be exposed. Generally a contact gel will be used between the scanner head and skin. The scanner head is then pressed against your skin and moved around and over the area to be examined. At the same time the internal images will appear onto a screen.