The propagation speed of sound waves through tissue is an important element of ultrasound scans. Ultrasound machines assume sound waves travel at a speed of 1540 m/sec through tissue 1. In reality, the speed of sound is affected by the density and elasticity of the medium through which it is traveling and these factors are not constant for human tissues. The propagation speed of sound is higher in tissues with increased stiffness and reduced density2.
Examples of propagation velocities in different tissues are given below 2 :
- air: 330 m/sec
- fat: 1450 m/sec
- water: 1480 m/sec
- liver: 1550 m/sec
- kidney: 1560 m/sec
- blood: 1570 m/sec
- muscle: 1580 m/sec
- bone: 4080 m/sec
Ultrasound machines use echo-ranging to determine the distance between the transducer and reflective interfaces 2. As the machine assumes a constant propagation speed of 1540 m/sec, speed displacement artifact can occur if the sound wave traverses tissues of differing propagation speeds 1.
Differences in propagation speeds are also relevant in causing refraction and the formation of refraction artifact 1.