What is the commonly cited average speed of sound in soft tissue in millimeters per microsecond?

• A. 1.54 mm/µs
• B. 0.77 mm/µs
• C. 2.00 mm/µs
• D. 3.00 mm/µs

Answer: (A)

In ultrasound imaging, depth is calculated from the time it takes for a sound pulse to travel to a reflector and back, so the depth depends on how fast sound moves through tissue. Soft tissue is commonly modeled with a speed of about 1540 meters per second, which is 1.54 millimeters per microsecond. That conversion is straightforward: 1540 m/s equals 1.54 mm/µs because 1540 m/s is 1,540,000 mm/s, and dividing by 1,000,000 µs gives 1.54 mm per µs.

This speed is chosen as the standard because most soft tissues in the body have speeds near this value, making depth estimates accurate across typical imaging scenarios. Using the two-way travel time, depth is calculated as speed multiplied by time and then divided by two, so with 1.54 mm/µs, the depth in millimeters is about 0.77 times the two-way time in microseconds (depth ≈ 1.54 × time / 2 = 0.77 × time). The other options correspond to speeds that are not characteristic of soft tissue, which is why they aren’t used for standard tissue depth calculations.