* Color Doppler imaging is most effective when the angle between the ultrasound beam and the flow of blood is small.
* At an angle of 88 degrees, the flow of blood is nearly perpendicular to the ultrasound beam.
* When the angle is close to 90 degrees, the Doppler shift (frequency change) approaches zero, resulting in little to no color being visualized on the Doppler image.
* Thus, to obtain a color signal, the angle should be optimized to be as close to 0 degrees as possible, with 60 degrees being the practical limit for accurate Doppler measurements.References:
* ARDMS Sonography Principles and Instrumentation guidelines on Doppler angle and its effect on Doppler imaging.

The wall filter is used in color Doppler and spectral Doppler imaging to eliminate low-frequency Doppler shifts caused by tissue motion or vessel wall movement. Adjusting the wall filter removes these low-frequency signals without affecting higher-frequency Doppler shifts that represent blood flow. Other settings like gain, scale, and persistence do not selectively filter out low-frequency shifts in the same manner.
Reference:
American Registry for Diagnostic Medical Sonography (ARDMS) Sonography Principles and Instrumentation guidelines.