For accurate distance measurements in an ultrasound tissue phantom, the phantom sound propagation speed should be closest to which value?

• A. 13 us/cm
• B. 1.54 m/s
• C. 1540 m/s
• D. 1/2 the speed of sound in tissue

Answer: (C)

The essential idea is that ultrasound depth measurements come from time-of-flight, which relies on the speed of sound in the medium. To get accurate distances in a tissue phantom, the phantom’s sound speed should match the speed of sound in real soft tissue, so the depth scales line up with true distances. The widely used value for soft tissue is about 1540 m/s, so having the phantom close to this speed ensures that measured depths correspond correctly to actual depths.

Options that imply slower speeds, like roughly 770 m/s (which corresponds to about 13 microseconds per centimeter), would bias depths by nearly a factor of two. A value as slow as 1.54 m/s is orders of magnitude too slow for sound in tissue. Half the tissue speed would also be around 770 m/s, not matching true tissue propagation. Therefore, 1540 m/s is the best match for accurate distance measurements in a tissue phantom.