ARDMS SPI Pulsed-Wave and CW Doppler 2 — Questions and Answers
Question 1: What is the primary disadvantage of continuous-wave (CW) Doppler compared to pulsed-wave (PW) Doppler?
- It cannot detect high-velocity flow
- It has no range resolution and cannot localize the depth of flow (Correct answer)
- It produces more noise artifacts
- It requires a higher PRF setting
Correct answer: It has no range resolution and cannot localize the depth of flow
CW Doppler uses two separate crystals (one transmitting, one receiving continuously), so it cannot determine the depth of origin of Doppler signals — it samples all velocities along the entire beam path.
Question 2: In pulsed-wave Doppler, the sample volume (gate) is positioned within the vessel of interest. What determines the axial length of the sample volume?
- The pulse repetition frequency
- The gate length (time duration of the listening window) (Correct answer)
- The Doppler angle
- The transducer frequency
Correct answer: The gate length (time duration of the listening window)
The axial length of the PW Doppler sample volume is determined by the gate length — the longer the listening window (in time), the larger the axial sample volume.
Question 3: A sonographer is scanning a patient and detects aliasing on PW Doppler. Which of the following actions would BEST eliminate aliasing without switching to CW Doppler?
- Increase the Doppler angle to >60°
- Decrease the PRF
- Increase the baseline shift and/or increase the PRF (Correct answer)
- Increase the wall filter
Correct answer: Increase the baseline shift and/or increase the PRF
Aliasing occurs when the Doppler shift exceeds the Nyquist limit (½ PRF); increasing the PRF raises the Nyquist limit, and shifting the baseline allows display of higher unidirectional velocities.
Question 4: The Doppler equation states that the Doppler shift is directly proportional to which of the following?
- The cosine of the Doppler angle only
- The transmitted frequency, flow velocity, and cosine of the Doppler angle (Correct answer)
- The pulse repetition period
- The sample volume depth
Correct answer: The transmitted frequency, flow velocity, and cosine of the Doppler angle
The Doppler shift (Δf) = 2 × f₀ × v × cos θ / c, meaning it is directly proportional to the transmitted frequency, flow velocity, and the cosine of the angle between the beam and flow direction.
Question 5: When the Doppler angle approaches 90°, what happens to the measured Doppler shift?
- It doubles
- It reaches its maximum value
- It approaches zero, making velocity measurement unreliable (Correct answer)
- It equals the transmitted frequency
Correct answer: It approaches zero, making velocity measurement unreliable
At 90°, cos 90° = 0, so the calculated Doppler shift approaches zero regardless of actual flow velocity, making angle correction and velocity measurement unreliable.
Question 6: Which of the following best describes the concept of the Nyquist limit in pulsed-wave Doppler?
- The maximum depth from which a signal can be returned
- The minimum detectable velocity
- The maximum Doppler shift frequency that can be unambiguously detected, equal to one-half the PRF (Correct answer)
- The maximum wall filter setting
Correct answer: The maximum Doppler shift frequency that can be unambiguously detected, equal to one-half the PRF
The Nyquist limit is PRF/2; Doppler shifts exceeding this value cannot be correctly displayed and appear aliased (wrapped around the baseline).
Question 7: A CW Doppler device is ideally suited for measuring flow in which clinical scenario?
- Determining flow at a specific depth in the portal vein
- Measuring high-velocity stenotic jets such as aortic stenosis where aliasing is a problem with PW (Correct answer)
- Evaluating slow venous flow near the skin surface
- Mapping color flow in the superficial femoral artery
Correct answer: Measuring high-velocity stenotic jets such as aortic stenosis where aliasing is a problem with PW
CW Doppler has no PRF limitation and therefore no aliasing, making it ideal for measuring very high velocities such as those in severe aortic stenosis (often >4 m/s).
ARDMS SPI Pulsed-Wave and CW Doppler 2
What is the primary disadvantage of continuous-wave (CW) Doppler compared to pulsed-wave (PW) Doppler?