Ultrasound produces sound waves that are beamed into the body causing return echoes that are recorded to “visualize” structures beneath the skin. Most ultrasound examinations are done using a sonar device outside your body, though some ultrasound examinations involve placing a device inside your body. The ability to measure different echoes reflected from a variety of tissues allows a shadow picture to be constructed. The technology is especially accurate at seeing the interface between solid and fluid filled spaces. These are actually the same principles that allow SONAR on boats to see the bottom of the ocean.