Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies from person to person and is approximately 20 kilohertz (20,000 hertz) in healthy young adults. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz. ultrasound is a safe and painless test that uses sound waves to make images of the abdomen (belly).
During the examination, an ultrasound machine sends sound waves into the abdominal area and images are recorded on a computer. The black-and-white images show the internal structures of the abdomen, such as the appendix, intestines, liver, gall bladder, pancreas, spleen, kidneys, and urinary bladder. The gel is applied to the scan area, before contacting the ultrasound probe to your Ultrasonic waves generated from the probe are reflected from the boundary of the tissue, and the probe receive reflected waves to make an ultrasound image. However, the larger the difference in acoustic characteristics between the two tissues, the stronger the reflection and the stronger the echo (white on the screen) appears. The acoustic properties (resistance) of each tissue are related to their density, which is greater in order of bone> soft tissue> fat> air. Therefore, if you apply ultrasonic probe without gel, 99.9% of ultrasonic waves are reflected at the boundary of air - soft tissue (body). Ultrasound image should show the difference between the tissues in body (normal-lesion, etc.), not the difference between the air and body.
To solve this situation, the gel is used. In other words, when the acoustic resistance of the two media is made equal by using gel, ultrasonic waves are transmitted to body without reflection.
The other purpose of the gel is the role of lubrication, which moves the probe smoothly when performing a ultrasound scan. If the gel is not used, it may be stiff and the patient may feel pain.