Adult Echocardiography Practice Exam

What happens to the Doppler signal when the Nyquist limit is reached?

1. A. mirror image

2. B. alias

3. C. increase in frequency

4. D. decrease in frequency

The correct answer is: B. alias

When the Nyquist limit is reached in Doppler ultrasound, aliasing occurs. This phenomenon arises from insufficient sampling of the Doppler signal, leading to misrepresentation of the actual velocity of blood flow. The Nyquist limit is defined as half of the pulse repetition frequency (PRF); if the blood flow velocity exceeds this limit, the system cannot accurately represent the signal's true frequency and instead produces a lower frequency, resulting in an inversion or "wrap-around" of the waveform. This aliasing can appear as a mirrored or additional spectral waveform on the display, confusing the interpretation of blood flow velocities. In clinical practice, recognizing aliasing is crucial for optimizing Doppler measurements, which may involve adjusting the PRF, using different sample volumes, or selecting alternative angles to diminish the impact of aliasing on the assessment of cardiac and vascular function.