Medical Ultrasound relies on the reflection, refraction and scattering of ultrasound waves by the body structures to produce an image. If it did not occur, none of the Ultrasound energy would return to the ultrasound transducer to be converted in to an image. The strength of reflection of the Ultrasound wave at an interface is partially dependent upon the difference in acoustic impedance of the two structures making up the interface. The greater this difference in impedance, the more of the ultrasound wave is reflected.
At the skin / transducer interface there is a very significant difference in the acoustic impedance of the two structures and therefore virtually all of the Ultrasound energy is reflected. Couple this fact with the observation that the Ultrasound has to pass through this interface twice, once on the way into the body, and once on the way back out to the transducer, and you can easily see that no significant ultrasound energy is going to return to generate an image.
Furthermore, Ultrasound passes through soft tissue at an average speed of 1540 meters per second. In air it is approximately 331meters per second. An Ultrasound machine is unable to correct for this difference, so any air between transducer and body would cause additional issues. This situation is easily alleviated by the use of Ultrasound gel between the transducer probe and the body. This effectively removes the air/skin interface and provides a transition between the transducer and skin which extensively reduces the reflection of the sound waves. Its success in doing so is very easily demonstrated by simply trying to scan with a dry transducer probe on dry skin.
So, what makes a good gel?
A good Ultrasound gel may be blue tinted or clear, but will be non-staining. It will be acoustically correct for the broad range of frequencies used in Diagnostic and Therapeutic ultrasound procedures and in Aqueous suspension. It is also important that it is Hypoallergenic, bacteriostatic, non-sensitizing and non-irritating. There are many medical ultrasound gels on the market and there is very little difference in their constitution except some are runnier than others.
Sonographers vary in whether they prefer a runny variety easily manipulated around the body during the scan, but also tending to spread outside the field of interest and wetting patients clothes; or the less runny variety, which can dry off during an extended scan (requiring additional gel to be applied) but is more easily controlled. A further difference in gel is the choice of clear or blue tinted varieties. The latter does not stain clothes but can make it easier to see where the gel is after the examination, allowing the patient to dry it off more thoroughly.