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Sound travels at specific known velocities through various materials. Remember from physics that sound travels faster though solids than through liquid or gas, and its velocity through various body tissues is known (Figure 2.1). Notice that velocity is similar through most of the soft tissues; however, current ultrasound machines cannot determine what tissues are being penetrated. Therefore, most ultrasound machines use an average velocity of 1540 m/sec for their imaging algorithms, averaging the speed of sound through fat, liver, kidney, blood, and muscle (Coltera 2010). Some newer machines allow the user to select a different constant for the specific tissue or structure they are imaging.

The first and last columns in Figure 2.1 illustrate that sound passes relatively slowly through air and relatively quickly through bone. Anyone who has picked up an ultrasound probe knows that bone (solid) or lung (air) cannot be adequately penetrated using ultrasound, thus only the surfaces of each are visible. To address this issue, the sonographer must understand the principle of acoustic impedance.

Figure 2.1. Velocity (m/sec) of sound through common body tissues or substances. Note the similar velocity through most soft tissues. This is the basis for using 1540 m/sec as the number in depth calculations by the ultrasound processor (Coltrera 2010).

Pearl: Remember the saying: Ultrasound hates bone or stone and is not too fair with air.