A waveform from the internal carotid artery at a 45-degree Doppler angle shows aliasing. Which option will help unwrap this waveform?

• A. Increase the pulse repetition frequency
• B. Increase the Doppler frequency
• C. Decrease the angle of incidence
• D. Decrease the high pass filter

Answer: (A)

Aliasing in Doppler ultrasound happens when the velocity-induced Doppler shift exceeds half the pulse repetition frequency (the Nyquist limit). When the waveform shows aliasing at a 45-degree angle, the measured velocity is higher than what the current PRF can unambiguously sample. Increasing the pulse repetition frequency raises that Nyquist limit, allowing higher velocities to be measured without wrap and effectively unwraps the spectrum.

Context helps: the angle affects the calculated velocity through the cos(theta) term, but changing the angle won’t fix aliasing by itself; it can even worsen the Doppler shift if you angle the beam so that the cos theta increases. Increasing Doppler frequency or decreasing the high-pass filter won’t address the fundamental sampling limit either.