A wall filter in Doppler ultrasound is designed to remove low-frequency signals that are often caused by tissue motion, vessel wall movement, or other forms of motion artifact. These low-frequency signals can clutter the Doppler spectrum, making it difficult to accurately interpret blood flow velocities. By filtering out these unwanted low-frequency signals, the wall filter helps to enhance the clarity of the Doppler signal, allowing for more accurate measurements of blood flow velocities.
Reference:
American Registry for Diagnostic Medical Sonography (ARDMS). Sonography Principles and Instrumentation (SPI) Examination Review Guide.