What determines how much of the sound wave is reflected at the interface between two tissues?

The correct choice is determined by acoustic impedance, which is a crucial concept in ultrasound physics. Acoustic impedance is defined as the product of the density of a tissue and the speed of sound within that tissue. When an ultrasound wave encounters the boundary between two different tissues, such as muscle and fat, a portion of the wave is reflected back to the transducer while the remainder continues into the second medium.

The amount of sound wave reflection at an interface is dictated by the difference in acoustic impedances of the two tissues involved. If the two tissues have significantly different acoustic impedances, a larger proportion of the sound wave will be reflected back. In contrast, if the acoustic impedances are similar, less reflection occurs, and more sound energy is transmitted into the second tissue.

Other factors, such as frequency of the sound wave, sound wave intensity, and transducer position, do play roles in ultrasound imaging; however, they do not fundamentally determine the reflection at a tissue interface. For example, while frequency can affect resolution and penetration, and intensity can impact the amount of energy transmitted, it is the difference in acoustic impedance that is the key factor in determining how much of the sound wave is reflected at such interfaces.