High pass filter is also called a _______.

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

Answer: (B)

In Doppler ultrasound, the slow, low-frequency signals from tissue and vessel walls create clutter that can hide the blood flow shifts. A wall filter is the high-pass filter used to suppress that clutter by attenuating near-zero frequencies, so the higher-frequency Doppler shifts from moving blood pass through clearly. The name comes from its purpose of removing wall motion artifacts, not from the physical walls themselves. If you used a low-pass approach, you’d let the slow tissue signals through and lose the blood flow information; a baseline or generic noise filter doesn’t specifically target wall motion, and it wouldn’t reliably preserve the useful higher-frequency flow signals.