The liquid applied when doing ultrasound examination is medical coupling agent, medical coupling agent is composed of ingredients such as carbomer, glycerin and purified water, medical coupling agent is a water-based polymer gel preparation, its acoustic impedance is similar to that of human soft tissue, and it is widely used in clinical operations. Coupling agent it is characterized by neutral pH, non-toxic, tasteless, non-irritating, non-greasy, non-toxic and harmless to human body, non-corrosive and non-damaging to the ultrasound probe, nowadays, there are also ultrasound coupling agents with disinfection and sterilization function in the market. Why is it necessary to apply couplant for ultrasound examination? The main thing is that it can eliminate the air between the probe and the skin as a way to improve the clarity of the image display. If there is air between the probe and the skin, the ultrasound will encounter the air and total reflection will occur, resulting in the attenuation of the sound energy, the ultrasound cannot enter the interior of the body, and it will not play a role in the examination. In addition, it also plays the role of lubrication, reduce the friction between the probe surface and the skin, so that the probe can be flexible sliding probe.