Ultrasound imaging is a widely used medical technique that allows healthcare professionals to visualize internal body structures without invasive procedures. It functions by sending high-frequency sound waves into the body and then interpreting the echoes that return. During an ultrasound examination, a clear, gel-like substance is always applied to the skin. This raises a common question: why is this gel consistently used for these imaging procedures?
Overcoming the Air Barrier
The primary reason for using gel during an ultrasound is to overcome the significant barrier that air presents to sound waves. Sound waves do not travel effectively through air. When sound waves encounter a boundary between two different materials, like air and skin, a portion of the wave is reflected. Air has a vastly different “acoustic impedance”—a material’s resistance to sound passage—compared to both the ultrasound transducer and human skin.
If tiny air pockets existed between the transducer—the handheld device that emits and receives sound waves—and the patient’s skin, nearly all ultrasound waves would reflect. This reflection would prevent sound waves from entering the body efficiently, resulting in a poor or absent image. The gel fills this air gap, creating a continuous medium that allows sound waves to pass seamlessly from the transducer into the body. This continuous pathway is essential for obtaining clear and accurate diagnostic images.
Essential Characteristics of the Gel
Ultrasound gel possesses specific properties that make it highly effective. It is typically water-based, which is crucial because water has an acoustic impedance similar to soft tissues in the body. This similarity allows for efficient transmission of sound waves into the body, minimizing reflection at the skin surface. The gel’s high viscosity, or thickness, prevents it from running off the skin and helps maintain consistent contact between the transducer and the body throughout the examination.
Manufacturers formulate ultrasound gel to be free of air bubbles. Air bubbles within the gel would create interfaces that scatter or reflect sound waves, leading to image artifacts or distortions. Eliminating these bubbles ensures a smooth and continuous pathway for sound transmission, contributing to optimal image quality. The gel’s composition is also designed to be non-staining and easily spreadable.
Practical Use and Safety
During an ultrasound procedure, the gel is applied generously to the skin area being examined, or sometimes directly to the transducer. The transducer is then pressed gently against the gel-covered skin, allowing the gel to spread evenly and eliminate any air. This creates the necessary acoustic coupling for sound waves to travel into the body and for echoes to return to the transducer.
Ultrasound gel is formulated with patient safety in mind. Most gels are non-toxic, non-irritating, and hypoallergenic, reducing the risk of adverse skin reactions. Standard ultrasound gels are often bacteriostatic, meaning they inhibit bacterial growth, which helps prevent infection during routine examinations on intact skin. For procedures involving non-intact skin or invasive techniques, sterile, single-use gel is typically recommended. The water-soluble nature of the gel makes it easy to clean off the skin and clothing after the procedure.