Ultrasound is a kind of sound wave with a vibration frequency of more than 20,000 Hz per second, which exceeds the hearing range of the human ear. There are two characteristics of ultrasonic waves: one can be directed to propagate, the frequency of ultrasonic waves is high, and the wavelength is very short. Except vacuum, it cannot concentrate in one direction in gas, liquid and solid. Second, it can produce interface reflection, which can produce reflection when it encounters an acoustic interface composed of two substances with different acoustic impedances during propagation. Therefore, it can detect the directional ultrasound of a certain part or organ of the body, and based on the reflected signal (ie echo), the state of the detected part is used as a diagnosis. This technique is an ultrasonic diagnostic technique.