This is the ultrasonic beam emitted by the probe affixed to the skin, which is pointed towards an organ, reflected by the latter, and recaptured by the same probe. The latter thus produces an image of the structure investigated. The ultrasonic beam only circulates well in water (and bounces in the air). This is why a gel must is applied between the probe and the skin. In this way, artifacts are avoided and a better quality image is obtained.