The ______ is used to eliminate frequency shifts below a set threshold from the display.

• A. Wall filter
• B. Time gain compensation
• C. Pulse repetition frequency
• D. Gain

Answer: (A)

In Doppler ultrasound, a wall filter is used to suppress signals that come from tissue and wall motion, which produce very low-frequency Doppler shifts. By applying a high-pass filter with a set threshold, these slow or stationary signals are removed from the display, leaving the higher-frequency shifts that correspond to true blood flow. This helps prevent clutter from tissue motion from obscuring the flow signal. The threshold can be adjusted to strike a balance: set it high enough to remove clutter but not so high that it eliminates legitimately slow flow.

The other controls affect brightness or sampling differently: time gain compensation adjusts signal strength with depth, pulse repetition frequency relates to how frequently the system samples Doppler signals (and helps with aliasing), and gain amplifies all signals. None of them specifically remove low-frequency Doppler components like a wall filter does.