Question

110 Q1. 1. Explain the term acoustic impedance and state the tissue properties which determine its value.

Answer 1

Solution:-

Acoustic impedance (Z) 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

If the density of a tissue increases, impedance increases. Similarly, but less intuitively, if the speed of sound increases, then impedance also increases because z is directly proportional to both .

The effect of acoustic impedance in medical ultrasound becomes noticeable at interfaces between different tissue types. The ability of an ultrasound wave to transfer from one tissue type to another depends on the difference in impedance of the two tissues. If the difference is large, then the sound is reflected. We grasp this intuitively at a macroscopic level. If you were to yell into a canyon, you would expect an echo to return to you. The sound wave in air meets the dense rocky canyon wall and reverberates off it back to you; the sound wave does not just pass into the rock. This is due to the difference in impedance.

The SI unit for acoustic impedance is the Rayl, kg/(m2s), after J W Strutt, 3rd Baron Rayleigh.