The cold sensation of ultrasound gel is a common experience during medical imaging, yet the gel is an indispensable part of the procedure. Ultrasound imaging uses high-frequency sound waves to create pictures of internal body structures, such as organs, muscles, and blood vessels. For these sound waves to travel effectively, the gel acts as a necessary intermediary between the imaging device, called a transducer, and the patient’s skin.
The Essential Function of Ultrasound Gel
The primary purpose of ultrasound gel is to facilitate acoustic coupling, which is the efficient transfer of sound energy between two materials. Sound waves travel poorly through air due to acoustic impedance mismatch. Air has a vastly different acoustic impedance—a measure of a material’s resistance to sound wave propagation—compared to the human body’s soft tissues.
If the transducer were placed directly on the skin, microscopic air pockets would reflect almost all sound waves. Without the gel, image formation would be virtually impossible. The gel eliminates these tiny air gaps and provides a continuous, fluid pathway for the sound waves.
The gel is specifically formulated to have an acoustic impedance value very similar to that of human soft tissue. This matching reduces sound wave reflection at the skin surface, allowing a high percentage of the waves to travel into the body and return to the transducer. By creating a seamless acoustic connection, the gel ensures the resulting internal images are clear and diagnostically useful.
Chemical Composition and Thermal Properties
Ultrasound gel is classified as a hydrogel, meaning its primary component is water, typically well over 90% purified water. The water is combined with gelling or thickening agents, like polymers such as carbopol or propylene glycol, to create the necessary viscosity and non-drip consistency. Water has a high specific heat capacity, causing the gel to retain the ambient room temperature effectively.
The cold shock experienced upon application is mainly due to the gel’s high thermal conductivity. When the gel touches the warm skin, it quickly draws heat away from the body to equalize the temperature difference. This rapid transfer of thermal energy creates the immediate, noticeable sensation of cold.
The remaining chemical components are usually non-staining, hypoallergenic, and include a preservative to prevent microbial growth. This formulation ensures the gel is safe for contact with the skin and does not interfere with the delicate components of the transducer. The composition is a balance of acoustic efficiency, appropriate texture, and hygienic stability.
Clinical Storage and Temperature Management
The most straightforward reason the gel feels cold is its typical storage at standard room temperature, often between 15°C and 25°C. Since the average human body temperature is around 37°C, any substance stored below this naturally feels cool upon contact. Clinics generally store the gel at room temperature to maintain its chemical stability and prevent component separation, which can affect its acoustic conductivity.
While the cold sensation is uncomfortable for patients, heating the gel introduces infection control concerns. Warm temperatures can encourage the multiplication of bacteria, especially if the gel bottle tip becomes contaminated. Although some facilities use electric gel warmers for patient comfort, these devices must be meticulously maintained and their use is often limited to specific procedures, such as obstetrics or pediatrics.
The preference for room-temperature storage prioritizes the product’s long-term stability and patient safety over comfort. Standard practice favors storage that minimizes the risk of microbial contamination and degradation of the gel’s acoustic properties.