Ultrasound Physics – Propagation of Ultrasound through Tissues

Propagation Speed speed of sound when it travels through a medium

The speed of sound is determined by the medium through which the sound travels, for example speed of sound traveling through soft tissue will be different than speed of sound in the air.

Density and Stiffness of a particular medium determines the speed of sound in that particular medium, for example density and stiffness of gas, liquid and solid is different therefore the speed of sound is different in these three mediums.

The speed of sound is faster in solids and slower in gas.

Gas   <   Liquid   <   Solid

Higher the density of the medium, slower the speed of sound

Higher the stiffness of the medium, faster the speed of sound

Average speed of sound in soft tissue is:

1.54 km/s, or 1,540 m/s, or 1.54 mm/µs

The sound waves, regardless of the frequency travel at the same speed through any specific medium. For example sound with frequency of 7 MHz and sound with frequency of 2.5 MHz travel at the same speed through the same medium.

 

Propagation Speed of Sound in different Tissues

Air

330 m/s

Lung

300 –   1,200 m/s

Fat

1,450   m/s

Soft   Tissue

1,540   m/s

Bone

2,000 –   4,000 m/s

 

The propagation speed of sound is only determined by the medium and cannot be changed by the sonographer.

Speed = frequency × wavelength

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