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:

Material	Density (kgm-3)	Speed of sound (ms-1)	Acoustic impedance (kgm-2s-1x106)
Air	1.3	330	0.000429
Water	1000	1450	1.50
Bone (average)	1500	4000	6.00
Blood	1060	1570	1.59
Muscle (average)	1075	1590	1.70
Soft tissue (average)	1050	1500	1.58
Fat	925	1450	1.38

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.