They have different densities and different ultrasound propagation speed according to different tissues and organs of animals, that is, different acoustic impedance characteristics to produce a certain frequency ultrasound, and with this kind of ultrasound enter the body of animal, we can almost see that, different organs of the body through the interface is generated reflection echo; echo reflected different sizes, so that after the turn received echo, the detector and digital processing such as scan conversion, form a standard video signal, showing the organ sectional images on a monitor screen.