Acoustic impedance

When an ultrasonic pulse enters the body it is reflected from the boundary between different types of tissue. The ease with which an ultrasonic pulse can travel through a material depends on a property of the material called acoustic impedance (Z). This is defined as:

The units for acoustic impedance are kgm^-2s^-1 and values for the acoustic impedance of some materials are given in the following table:

The greater the difference between the acoustic impedances of the two materials at a boundary in the body the greater the amount of reflection – two materials with the same acoustic impedance would give no reflection (or refraction) while two with widely separated values would give much larger reflections.

The ratio of the reflected intensity I_r to the incident intensity (I_o) is given by:

I_r/I_o = (Z₂ – Z₁)²/(Z₂+Z₁)²
For example a boundary between fat and muscle would give 1% reflection while that between fat and air would give almost 100% reflection. Hence the need for a coupling gel between the transducer and the skin.