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Pulsed-Wave and CW Doppler Flashcards

7 cards from real ARDMS SPI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Pulsed-Wave and CW Doppler flashcards as text

  1. What is the primary disadvantage of continuous-wave (CW) Doppler compared to pulsed-wave (PW) Doppler?

    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.

  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?

    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.

  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?

    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.

  4. The Doppler equation states that the Doppler shift is directly proportional to which of the following?

    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.

  5. When the Doppler angle approaches 90°, what happens to the measured Doppler shift?

    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.

  6. Which of the following best describes the concept of the Nyquist limit in pulsed-wave Doppler?

    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).

  7. A CW Doppler device is ideally suited for measuring flow in which clinical scenario?

    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).