During Doppler interrogation of the carotid artery, spectral mirroring is most commonly caused by which condition?

• A. Doppler angle near 0 degrees
• B. Doppler angle near 90 degrees
• C. Wall filter set to high
• D. PRF set too low

Answer: (B)

Spectral mirroring happens when the Doppler beam is almost perpendicular to the blood flow. When the angle is near 90 degrees, the Doppler frequency shift becomes tiny (cosine of 90° is zero), so the signal is dominated by noise and stationary tissue signals. The system can produce a ghost spectrum that appears on the opposite side of the baseline because the very small frequency shifts are easily misinterpreted, creating a mirror image of the waveform. This artifact is common in carotid studies because it’s hard to keep the beam precisely parallel to the vessel when scanning at that location.

If the angle were near 0 degrees, you’d see a strong, unidirectional Doppler shift rather than a mirrored copy. A high wall filter mainly suppresses low-frequency clutter and isn’t the cause of mirroring. A PRF set too low leads to aliasing, not a mirror image, so that wouldn’t be the typical explanation for spectral mirroring.