Which medium yields the fastest propagation speed for ultrasound?

Which medium yields the fastest propagation speed for ultrasound?

• A. Soft tissue
• B. Lung (air)
• C. Fat
• D. Bone

Answer: (D)

In ultrasound, speed depends mainly on how stiff a material is and how dense it is. The stiffer the medium, the faster the wave travels; the denser the medium, the slower it travels. For fluids, speed roughly follows the square root of the bulk modulus divided by density, while for solids the stiffness plus density still drives the same trend. Air is very compressible and light, so its sound speed is about 343 m/s, which is slow. Lungs contain a lot of air, so the effective propagation speed through lung tissue is much slower and highly affected by interfaces and scattering. Fat is less stiff than soft tissue, so its speed is a bit slower than soft tissue. Soft tissue itself conducts ultrasound around 1540 m/s, which is much faster than air-containing media or fat. Bone, on the other hand, is extremely stiff (high elastic modulus), so the speed of sound is the highest among these tissues, typically in the range of 3000–4000 m/s. That combination of high stiffness and the resulting high wave speed makes bone the fastest propagation medium in this set.

In ultrasound, speed depends mainly on how stiff a material is and how dense it is. The stiffer the medium, the faster the wave travels; the denser the medium, the slower it travels. For fluids, speed roughly follows the square root of the bulk modulus divided by density, while for solids the stiffness plus density still drives the same trend.

Air is very compressible and light, so its sound speed is about 343 m/s, which is slow. Lungs contain a lot of air, so the effective propagation speed through lung tissue is much slower and highly affected by interfaces and scattering. Fat is less stiff than soft tissue, so its speed is a bit slower than soft tissue. Soft tissue itself conducts ultrasound around 1540 m/s, which is much faster than air-containing media or fat.

Bone, on the other hand, is extremely stiff (high elastic modulus), so the speed of sound is the highest among these tissues, typically in the range of 3000–4000 m/s. That combination of high stiffness and the resulting high wave speed makes bone the fastest propagation medium in this set.