What is the approximate speed of sound in soft tissue, and what does this value influence in ultrasound imaging?

• A. About 1540 m/s; it sets wavelength, time-of-flight calculations, and depth calibration.
• B. About 1540 m/s; it determines imaging color scale.
• C. About 340 m/s; it sets time-of-flight.
• D. About 1000 m/s; it sets wavelength.

Answer: (A)

In soft tissue, sound travels at about 1540 meters per second. This speed matters because it sets the wavelength for a given transmit frequency (λ = c/f), so the tissue speed defines how finely structures can be resolved for a given frequency. It also governs how we convert the ultrasound round-trip travel time into depth: depth is estimated from the travel time using depth ≈ (c × round-trip time) / 2. If we assume the wrong speed, the depth calibration will be off, leading to inaccurate measurements of how deep structures are. The imaging color scale, on the other hand, is determined by factors like dynamic range and display settings, not by the speed of sound.