What does the term "acoustic impedance" refer to in ultrasound?

Acoustic impedance is defined as the resistance that a material offers to the passage of sound waves. It is a crucial concept in ultrasound because it determines how much of the sound wave is transmitted through a medium versus how much is reflected back when the sound wave encounters different tissue interfaces or boundaries. Acoustic impedance is a product of the density of the material and the speed of sound within that material.

In clinical ultrasound, different tissues in the body—like muscle, fat, and bone—have varying acoustic impedances. When ultrasound waves travel through these tissues, they may encounter boundaries with differing acoustic impedances, leading to reflection and transmission of the sound waves. A high degree of impedance mismatch can lead to more reflection, which is essential for creating images.

While factors like sound absorption, speed of sound, and reflection are relevant in the context of ultrasound, they do not adequately capture the essence of acoustic impedance as the specific resistance to sound transmission through materials. Thus, the definition of acoustic impedance as the resistance of a material to the passage of sound waves aligns accurately with the principles of ultrasound physics.