The reason you need a gel between the USG head and the body is to bridge the gap with something of a similar speed of sound. Barriers between materials of signi

The reason you need a gel between the USG head and the body is to bridge the gap with something of a similar speed of sound. Barriers between materials of significantly different speed of sound reflect large part of sound waves; having such a strong barrier at the skin would make the SNR much worse. Speed of sound in metals is much larger than speed of sound in water or air. I'm unaware of liquids that could be used in place of USG gel for examining metal objects (maybe mercury? but that would be bad from the environmental and health/safety POV).