What is the speed of sound in soft tissue?

• A. 1,450 m/s
• B. 1,560 m/s
• C. 1,540 m/s
• D. 500 m/s

Answer: (C)

Speed in soft tissue is a property set by the tissue’s density and elasticity. For most soft tissues encountered in ultrasound, the commonly used reference value is 1,540 meters per second (about 1.54 km/s). This is the standard speed used to convert the time it takes for ultrasound pulses to travel to a structure and back into depth, with depth roughly equal to half the round-trip time multiplied by that speed. The number reflects typical soft-tissue properties: it’s fast enough to transmit waves efficiently, but not so fast as in rigid solids.

The other numbers correspond to different media. About 1,450 m/s is more representative of fat, which is less stiff. Around 1,560 m/s is within the general tissue range but is not the standard reference used in most imaging systems. A value of 500 m/s is far too low for soft tissue and would indicate something much less stiff, not what ultrasound encounters in typical soft tissues.