If you are imaging with a 5 MHz linear array transducer, which sound parameter stays constant as the beam propagates through tissue?

• A. Frequency
• B. Intensity
• C. Amplitude
• D. None of the above

Answer: (D)

The key idea is that the frequency of the ultrasound is set by the transducer and does not change as the wave travels through tissue in a linear medium. A 5 MHz transducer sends out waves at that frequency, and while the wave loses energy as it propagates—so its amplitude and thus its intensity decrease due to attenuation, absorption, scattering, and geometric spreading—the central frequency remains essentially the same.

Amplitude decreases with depth because some energy is absorbed or scattered by the tissue and the wave spreads out, reducing the peak pressure amplitude. Intensity, which is the power per unit area, also drops as the beam attenuates.

So the parameter that stays constant is the frequency; the others vary with depth.