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Acoustic impedance refers to the reflection and transmission characteristics of a substance. It is a measure of absorption of sound and the ratio of sound pressure at a boundary surface to the sound flux. Sound flux is flow velocity multiplied by area. If we draw an analogy to electronic circuits, acoustic impedance is like electrical resistance through a wire, sound pressure is like voltage, and flow velocity is like current. The equation that brings it all together is:

where Z = acoustic impedance, p = sound pressure (or tissue density), and v = velocity (Nyland 2002).

The amplitude of a reflected sound wave is proportional to the difference in acoustic impedance between two different tissues. Air has a low impedance and bone has a high impedance when compared to soft tissue (Reef 1998) (Figure 2.2). Therefore, when a sound wave comes across a soft tissue–bone or a soft tissue–air interface (large difference in acoustic impedance), nearly all of the sound waves are strongly reflected (and a bright white echogenic line is formed at either interface). Reflection is why the sonographer cannot image through bone (solid) or lung (air) and this points up one of the most common misnomers in clinical ultrasonography: when imaging through the liver into the thorax, we term the bright, curved cranial border as the diaphragm. In reality, the diaphragm is uncommonly imaged except in bicavitary effusions. The bright white (hyperechoic), curved line is actually the strongly reflective surface of the lung (air) at the soft tissue–air boundary or interface serving as a strong reflector.

By comparing the acoustic impedance of most tissues in the body other than bone (solid) and lung (air), we see that they are very similar (there is little difference in acoustic impedance among them). This similarity makes ultrasound a great imaging tool for examining into and through soft tissues (their parenchyma). On the other hand, due to the large difference in acoustic impedance between soft tissue–air and soft tissue–bone interfaces, ultrasound is not an effective tool for examination beyond the surfaces of either aerated lung, gas‐containing hollow viscus or bone (Reef 1998).