Ultrasound technology uses high-frequency sound waves to create real-time images of the body’s internal structures. This non-invasive diagnostic tool relies on the transmission of acoustic energy from a handheld probe, called a transducer, into the patient’s tissues. For the sound waves to successfully travel into the body and return as echoes to form a clear image, there must be a completely uninterrupted path. The application of a specialized gel is necessary to maintain this continuous medium between the electronic device and the skin surface.
Matching Acoustic Impedance
The primary function of ultrasound gel is to facilitate the efficient transfer of sound waves by matching a property known as acoustic impedance. Acoustic impedance is a physical characteristic of a medium, defined by the product of its density and the speed of sound traveling through it. When an acoustic wave encounters a boundary between two materials with significantly different acoustic impedances, most of the wave’s energy is reflected, not transmitted. The acoustic impedance of the soft tissues inside the human body is very similar to that of the gel, which is predominantly water-based. In contrast, the impedance of air is extremely low, differing from the skin’s impedance by a factor of thousands, which would cause nearly all sound energy to bounce back immediately. Therefore, the gel acts as a perfect coupler, bridging the physical gap between the transducer and the skin to create a continuous, impedance-matched pathway that allows acoustic energy to enter the body with minimal reflection.
The Consequences of Air Pockets
If the coupling gel is not used, or if air pockets are trapped beneath the probe, the sound waves are unable to penetrate the skin’s surface. The tiny air gaps created by skin pores, fine hair, and microscopic surface irregularities are enough to cause a near-complete reflection of the transmitted acoustic energy. Because the acoustic impedance difference between air and soft tissue is profound, up to 99% of the sound waves would be reflected away before even entering the body. This immediate and total reflection prevents the sound waves from reaching the deeper structures that need to be imaged. Since the transducer relies on returning echoes to build an image, a lack of penetrating waves means no echoes are generated from the internal organs, and the resulting ultrasound image would appear black or scattered.
Composition and Safety of Ultrasound Gel
Ultrasound gel is formulated to be a conductive, viscous substance that is safe for direct contact with the skin. The composition is typically water-based, which gives it the necessary acoustic properties to match the impedance of soft tissue. To give the gel its characteristic thickness and prevent it from running off the skin, it includes thickening agents such as propylene glycol, glycerin, or carbomer. These ingredients also function as humectants to maintain the gel’s consistency and prevent it from drying out too quickly. The medical-grade gel is designed to be hypoallergenic, non-staining, and non-irritating to the skin. Specialized sterile gels are used for invasive procedures, such as those involving non-intact skin or mucous membranes, to eliminate any risk of microbial contamination.