What determines the acoustic impedance of a material and why is it useful in understanding ultrasound imaging?

Acoustic impedance is the density of a material multiplied by the speed of sound in the material.
Knowing these two values allows use to calculate how much of the sound energy is reflected at an interface between two materials, or two different types of tissue in the body. The greater the difference, the more reflection. This is the reason for the use of coupling gel between the scanner probe and the skin, removing trapped air, to reduce reflection of ultrasound at the skin, by reducing the acoustic impedance difference.