Doppler Instrumentation

12 cards

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Cards

basicI.H-001
How many crystals does a CW Doppler transducer require, minimum?
→ Two — one continuously transmits, the other continuously receives.
basicI.H-002
How many crystals does a PW Doppler transducer require, minimum?
→ One. It alternates between sending and receiving.
basicI.H-003
What is 'range ambiguity' and which Doppler mode has it?
→ Range ambiguity = inability to localize the depth of the reflected signal along the beam. CW Doppler has range ambiguity (echoes come from the entire beam overlap).
basicI.H-004
What is the main advantage of PW Doppler over CW?
→ Range resolution — the ability to sample velocity from a specific depth (the sample volume), free of range ambiguity.
basicI.H-005
What is the main disadvantage of PW Doppler?
→ Aliasing — cannot accurately measure velocities that produce Doppler shifts exceeding the Nyquist limit (½ PRF).
clozeI.H-006
The Nyquist limit equals ½ × PRF.
basicI.H-007
List five ways to reduce or eliminate PW-Doppler aliasing.
→ 1) Switch to CW Doppler. 2) Use a lower-frequency transducer. 3) Move the sample volume shallower (raises PRF). 4) Increase the velocity scale. 5) Baseline shift (cosmetic — extends display range).
basicI.H-008
Why does aliasing never occur with continuous-wave Doppler?
→ CW has no PRF — there is no sampling step to violate the Nyquist criterion.
basicI.H-009
On a spectral Doppler display, what does the x-axis represent? The y-axis?
→ X-axis: time. Y-axis: Doppler shift or velocity (positive = toward transducer, above baseline).
basicI.H-010
Small vs large sample-volume length: what happens to the spectral display?
→ Small SV → clean spectral window (few RBCs sampled). Large SV → spectral broadening / fill-in (wider range of velocities included).
basicI.H-011
What is high-PRF Doppler and what does it trade off?
→ Uses multiple simultaneous sample gates so signals from 2× (or more) the primary depth also register. Extends measurable velocity beyond the Nyquist limit but reintroduces range ambiguity.
basicI.H-012
CW transducers use damped/undamped, high-Q/low-Q, wide/narrow bandwidth crystals?
→ Undamped, high-Q, low (narrow) bandwidth — for high sensitivity to small Doppler shifts. PW/imaging transducers use damped, low-Q, wide bandwidth for short pulses.