Ultrasound is a spectrum of sound frequencies above the human hearing range. We need to change that oversight. Nothing could be farther from the truth. As stated above, ultrasound is simply rarefactions and compression of molecules above the human hearing range. Sorry, I lost my train of thought.
Every substance has an echogenicity, that is, an echo signature.
When sound waves strike an object, some of the waves are transmitted through, and some are reflected back. Every substance will response differently to the sound striking it's surface. This is dependent on the physical characteristics of the substance. An image is formed from the reflection of the sound waves. The density of skin differs from that of liver.