What Doppler control removes frequencies below a given threshold, resulting in a clean zero baseline?

• A. Low-pass filter
• B. Baseline removal
• C. High pass filter
• D. Noise filter

Answer: (C)

Removing frequencies below a threshold is achieved with a high-pass filter. This type of filter attenuates slow, low-frequency components—such as tissue motion and baseline drift—while allowing the higher-frequency Doppler shifts from blood flow to pass. By cutting out those slow variations, the signal’s baseline is stabilized toward zero, giving a clean zero baseline. A low-pass filter would do the opposite by preserving low frequencies and suppressing the Doppler frequencies, which wouldn’t produce the desired clean baseline. Baseline removal and a general noise filter don’t specifically implement the frequency-threshold approach needed here.