Which statement is true about wall filters in Doppler ultrasound?

• A. They are low-pass filters removing high-frequency signals
• B. They are high-pass filters that pass higher-frequency signals and eliminate lower-frequency signals
• C. They equalize all frequencies
• D. They do not affect velocity measurements

Answer: (B)

Wall filters remove tissue-motion clutter by acting as a high-pass filter. In Doppler signals, motion from the vessel walls or surrounding tissue produces low-frequency components, while blood flow at practical velocities generates higher-frequency Doppler shifts. The wall filter attenuates the low-frequency part and preserves the higher-frequency signals from flowing blood, helping to separate true flow from clutter. This improves the accuracy of flow visualization, but if the cutoff is too high, very slow flow can be underestimated or clipped. So the wall filter is best described as a high-pass filter that passes higher-frequency flow signals and eliminates lower-frequency signals, and it does affect velocity measurements, especially for slow flow.