​What is the gel applied for ultrasound examination?

Health care workers who perform ultrasound examinations always apply a layer of clear gel before touching the probe with the examiner. You might think it’s a lubricant, but that’s just a secondary role.

The official name of this gel is Ultrasonic Coupling Agent, and its main purpose, as its name suggests, is as a medium for “coupling”. Ultrasonography scans the human body with ultrasound waves (frequency greater than 20 kHz) that are inaudible to the human ear, and converts the echo signals into image information inside the body. B-ultrasound presents black and white images with different brightness (Brightness), while color Doppler ultrasound presents the blood flow of tissues in color on this basis.

However, as a mechanical wave, the propagation of sound waves is affected by changes in the medium. Although the probe of the ultrasound equipment is next to the skin, the dry contact is inevitably separated by air. Due to the large difference in acoustic impedance between the air and the human body, two completely different media, sound waves will be reflected a lot at the interface between the two, and a lot of sound energy has been lost before entering the human body.

In order for sound waves to reach the body smoothly, a medium that can squeeze out the air and has an acoustic impedance similar to that of the human body is required. This is the role of ultrasonic couplants. The gel-like substance can fully fill the gap between the probe and the skin, and the acoustic impedance similar to that of the human body reduces the loss of acoustic energy. It is a similar principle to hold back urine when doing gynecological B-ultrasound.