An ultrasound is a medical diagnostic tool that uses high-frequency sound waves to create images of structures inside the body. A device called a transducer sends these waves into the body and captures the returning echoes to form a picture. Applying a clear gel to the skin is a necessity for the procedure to function effectively. Without this gel, the sound waves would be unable to enter the body, making the scan impossible.
Why Sound Waves Cannot Travel Through Air
The need for the gel stems from a fundamental principle of physics known as acoustic impedance, which is a material’s resistance to the transmission of sound energy. When a sound wave encounters a boundary between two materials with vastly different acoustic impedances, nearly all the sound energy is reflected. Air has a very low acoustic impedance, while soft tissue in the human body has a much higher impedance.
The difference in acoustic impedance between air and skin is enormous, creating a severe mismatch. If the transducer were placed directly on the skin, the microscopic layer of air trapped between the device and the body would cause more than 99% of the sound waves to reflect immediately. This extreme reflection means almost no sound energy would enter the body to collect diagnostic information. The procedure would only produce a poor-quality, mostly blank image, requiring a solution to bridge this air gap.
The Gel’s Role as an Acoustic Coupling Medium
The gel acts as an acoustic coupling medium, a liquid bridge designed to eliminate the microscopic air barrier and facilitate the transfer of sound energy. The gel’s formula is engineered to have an acoustic impedance that closely matches that of human skin and soft tissues. This matching allows the sound waves to pass from the transducer, through the gel, and into the body with minimal reflection.
By displacing the air, the gel ensures that the sound waves travel in a continuous medium, maximizing the proportion of energy that penetrates the skin. This transmission efficiency is necessary for both parts of the process: sending the waves into the body and receiving the returning echoes used to generate the image. The coupling medium transforms an interface that would otherwise reflect virtually all the sound into one that transmits nearly all of it, enabling visualization of internal anatomy.
What Is Ultrasound Gel Made Of and Is It Safe
The composition of standard ultrasound gel is straightforward, built primarily around a water base, which is an excellent conductor of sound waves. To achieve its viscous consistency, the formula includes a thickening agent, such as carbomer or cellulose, preventing it from running off the skin. Humectants like propylene glycol or glycerin are also added to prevent the gel from drying out and maintain its texture.
The gel is designed to be hypoallergenic, non-toxic, and water-soluble, making it easy to wipe off the skin after the procedure. Although individuals with highly sensitive skin may experience mild, localized irritation from certain ingredients, the gel is broadly considered safe for all skin types. Specialized versions, such as sterile packets or heated gels for comfort, exist, but they all share the fundamental function of providing acoustic coupling.