The transducer you are using transmits wide-bandwidth pulses whose frequency content is 2-5MHz. Which of the following most correctly describes the reflected echo signals after they have traversed the tissue?

The transducer you are using transmits wide-bandwidth pulses whose frequency content is 2-5MHz. Which of the following most correctly describes the reflected echo signals after they have traversed the tissue?

• A. The echo signals will be shifted down in frequency due to the increased attenuation of higher frequencies.
• B. The echo signals will be of reduced intensity but will have the same frequency content as the transmitted beam.
• C. The echo signals will be shifted upward in frequency due to the increased absorption of the lower frequencies.
• D. Only the center frequency component of the bandwidth will be reflected back to the transducer.

Answer: (A)

Frequency-dependent attenuation in tissue shapes the spectrum of echoes. Higher-frequency components within a broadband pulse are damped more than lower-frequency components as the wave travels through tissue. When the pulse returns, the spectrum is shifted toward lower frequencies and the overall bandwidth may narrow, with a reduction in amplitude due to attenuation. So the reflected echo signals end up with lower frequency content than the transmitted pulse. This is why the echo isn’t identical to the transmitted spectrum, and it isn’t restricted to just the center frequency. It’s also not shifted upward, since lower frequencies suffer less attenuation. The echo contains multiple frequency components (albeit attenuated), not only a single center frequency.

Frequency-dependent attenuation in tissue shapes the spectrum of echoes. Higher-frequency components within a broadband pulse are damped more than lower-frequency components as the wave travels through tissue. When the pulse returns, the spectrum is shifted toward lower frequencies and the overall bandwidth may narrow, with a reduction in amplitude due to attenuation. So the reflected echo signals end up with lower frequency content than the transmitted pulse.

This is why the echo isn’t identical to the transmitted spectrum, and it isn’t restricted to just the center frequency. It’s also not shifted upward, since lower frequencies suffer less attenuation. The echo contains multiple frequency components (albeit attenuated), not only a single center frequency.