Question

Explain the term acoustic impedance and state the tissue properties which determine its value

Answer 1

Acoustic impedance (Z) --- It is a physical property of tissue. It describes how much resistance an ultrasound beam encounters as it passes through a tissue.

Acoustic impedance depends on:

• the density of the tissue (d, in kg/m3)
• the speed of the sound wave (c, in m/s)

and they are related by:  Z = d x c

So, if the density of a tissue increases, impedance increases. Similarly, but less intuitively, if the speed of sound increases, then impedance also increases.