Speed of sound

The speed of sound in soft tissue is approximately 1540 m/s.  Since diagnostic ultrasound primarily images soft tissues, 1540 m/s is the value that is programmed into the ultrasound machine as the calibration speed used in the range equation. 

However, from time to time the tissue being scanned may not be soft tissue.  For example, sound travels more slowly through fat at a speed closer to 1440 m/s.  As a result, if the sound is traveling more slowly than the expected calibration speed, the echoes will take longer to return to the transducer and the range equation will misposition the interfaces farther from the transducer than they should be.  This results in a speed of sound artifact, also called propagation speed error.

A nice example of this is the more distant mispositioning of the right hemidiaphragm in a patient with a large fatty adrenal tumor.  The sound travels more slowly through the fatty tumor which causes the echoes from the diaphragm deep to the tumor to be mispositioned farther from the transducer, producing an artifactual "break" in the diaphragm.  Well-educated sonographers will recognize this artifact and even find it helpful in identifying the fatty composition of the tumor.  See the example in the adjacent image.      

Historically, 1540 m/s was the only calibration speed programmed into the machine.  However, in newer machines, the operator is given the option of slightly increasing or decreasing the system's calibration speed.

For example, if the machine is primarily being used to image fatty tissue like breast tissue, then the operator can reduce the calibration speed from 1540 m/s to 1440 m/s to more closely match the actual propagation speed of sound in fat.  This would minimize the chance of speed of sound artifacts and mispositioning of interfaces.

For a demonstration of this option on the GE Logiq E9 system, watch the short video below: