ARDMS SPI - ARDMS Sonography Principles and Instrumentation Pulsed-Wave and CW Doppler Questions and Answers — Questions and Answers
Question 1: Which of the following is a primary advantage of Continuous-Wave (CW) Doppler over Pulsed-Wave (PW) Doppler?
- It provides superior range resolution.
- It uses a single crystal for both transmission and reception.
- It is not limited by aliasing at high velocities. (Correct answer)
- It offers better axial resolution in the image.
Correct answer: It is not limited by aliasing at high velocities.
Continuous-Wave (CW) Doppler continuously transmits and receives ultrasound, so it does not have a sampling rate (PRF) that limits velocity measurement. This allows it to accurately measure very high velocities without the aliasing artifact that affects Pulsed-Wave (PW) Doppler.
Question 2: A sonographer observes that the spectral waveform on a Pulsed-Wave Doppler display appears to "wrap around" from the top of the scale to the bottom. This artifact is known as:
- Aliasing (Correct answer)
- Range ambiguity
- Spectral broadening
- Crosstalk
Correct answer: Aliasing
Aliasing occurs in PW Doppler when the measured Doppler shift exceeds the Nyquist limit (half the PRF). The system cannot sample the signal fast enough, causing the highest velocities to be incorrectly displayed on the opposite side of the baseline.
Question 3: A sonographer is performing a PW Doppler examination and notes significant aliasing. Which of the following adjustments is LEAST likely to correct this artifact?
- Increasing the Pulse Repetition Frequency (PRF).
- Shifting the spectral baseline.
- Switching to a lower frequency transducer.
- Increasing the overall Doppler gain. (Correct answer)
Correct answer: Increasing the overall Doppler gain.
Increasing the Doppler gain amplifies the entire signal, including the aliased components, making the display brighter but does not correct the aliasing itself. Increasing the PRF raises the Nyquist limit, shifting the baseline provides more room for the waveform, and using a lower frequency transducer reduces the Doppler shift for a given velocity; all are valid methods to reduce or eliminate aliasing.
Question 4: Which of the following statements accurately describes a key characteristic of Continuous-Wave (CW) Doppler?
- It utilizes a single piezoelectric element that alternates between sending and receiving.
- It cannot determine the exact depth from which a velocity signal originates. (Correct answer)
- It is limited by the Nyquist criterion for high-velocity measurements.
- It provides excellent range resolution by using a small sample volume.
Correct answer: It cannot determine the exact depth from which a velocity signal originates.
Because a CW Doppler system transmits and receives sound continuously along the entire beam, it detects all velocities in its path. It cannot isolate a specific depth, a characteristic known as range ambiguity.
Question 5: A Pulsed-Wave Doppler system is operating with a Pulse Repetition Frequency (PRF) of 12 kHz. What is the maximum Doppler shift that can be displayed without aliasing?
- 6 kHz (Correct answer)
- 12 kHz
- 24 kHz
- 3 kHz
Correct answer: 6 kHz
The Nyquist limit states that the highest Doppler frequency that can be measured without aliasing is one-half of the Pulse Repetition Frequency (PRF). Therefore, with a PRF of 12 kHz, the Nyquist limit is 6 kHz (12,000 Hz / 2).
Question 6: A sonographer is tasked with measuring the peak velocity of a severe aortic stenosis jet, which is expected to be approximately 4 m/s. Which Doppler modality is most appropriate for this measurement and why?
- Pulsed-Wave Doppler, because its specific sample volume can be placed precisely in the jet.
- Power Doppler, because it is most sensitive to the presence of flow.
- Continuous-Wave Doppler, because it is not subject to aliasing and can measure very high velocities. (Correct answer)
- Color Flow Doppler, because it can visualize the turbulent flow pattern.
Correct answer: Continuous-Wave Doppler, because it is not subject to aliasing and can measure very high velocities.
A velocity of 4 m/s will create a Doppler shift that far exceeds the Nyquist limit of Pulsed-Wave Doppler, resulting in severe aliasing. Continuous-Wave (CW) Doppler is the correct choice because it does not alias and is designed to accurately measure such high velocities. While PW Doppler is precise in location, it cannot handle the speed. Color and Power Doppler are for qualitative assessment, not peak velocity quantification.
ARDMS SPI - ARDMS Sonography Principles and Instrumentation Pulsed-Wave and CW Doppler Questions and Answers
Which of the following is a primary advantage of Continuous-Wave (CW) Doppler over Pulsed-Wave (PW) Doppler?