Which statement about typical impedance values in biologic media is true?

Which statement about typical impedance values in biologic media is true?

• A. They are expressed in rayls or mrayls
• B. They are expressed in ohms
• C. They are dimensionless
• D. They are expressed in pascals

Answer: (A)

Acoustic impedance in biologic media is the ratio of acoustic pressure to particle velocity, so its units come from combining pressure and velocity. This yields Pa·s/m, which are defined as rayls. In biology and ultrasound, impedance is often expressed as Z = ρc, with ρ the density and c the speed of sound; for soft tissues this lands around about 1.5 million rayls (MRayls). That’s why the standard way to express typical impedance values in biologic media is in rayls or MRayls. The other options don’t fit: ohms are the unit for electrical impedance, pascals measure pressure rather than impedance, and impedance is not dimensionless because it carries a specific unit (Pa·s/m).

Acoustic impedance in biologic media is the ratio of acoustic pressure to particle velocity, so its units come from combining pressure and velocity. This yields Pa·s/m, which are defined as rayls. In biology and ultrasound, impedance is often expressed as Z = ρc, with ρ the density and c the speed of sound; for soft tissues this lands around about 1.5 million rayls (MRayls). That’s why the standard way to express typical impedance values in biologic media is in rayls or MRayls.

The other options don’t fit: ohms are the unit for electrical impedance, pascals measure pressure rather than impedance, and impedance is not dimensionless because it carries a specific unit (Pa·s/m).