RVT Hemodynamics and Doppler Principles 3 — Questions and Answers
Question 1: The Nyquist limit in pulsed wave Doppler determines the maximum detectable Doppler shift without aliasing, which equals:
- The pulse repetition frequency (PRF)
- Half the pulse repetition frequency (PRF/2) (Correct answer)
- Twice the pulse repetition frequency (2×PRF)
- The transducer frequency
Correct answer: Half the pulse repetition frequency (PRF/2)
Aliasing occurs when the Doppler shift exceeds the Nyquist limit, which is one-half of the pulse repetition frequency.
Question 2: Which factor does NOT affect the Doppler frequency shift?
- Velocity of the moving reflector
- Transmitted frequency
- Angle between beam and flow
- Depth of the vessel (Correct answer)
Correct answer: Depth of the vessel
Depth affects PRF and time-gain compensation but is not a variable in the Doppler equation (fd = 2ft·v·cosθ / c).
Question 3: A peak systolic velocity ratio (stenotic/prestenotic) greater than 4.0 in the internal carotid artery typically indicates stenosis of:
- Less than 50%
- 50–69%
- 70–99% (Correct answer)
- Near occlusion only
Correct answer: 70–99%
A velocity ratio greater than 4.0 is a validated criterion for ≥70% ICA stenosis, corresponding to hemodynamically significant disease.
Question 4: In continuous wave (CW) Doppler, the major advantage over pulsed wave Doppler is:
- Range resolution
- Ability to measure very high velocities without aliasing (Correct answer)
- Color flow mapping capability
- Better lateral resolution
Correct answer: Ability to measure very high velocities without aliasing
CW Doppler has no PRF limitation and can accurately measure very high velocities, making it ideal for severe stenoses or cardiac jets.
Question 5: The spectral broadening seen in disturbed flow distal to a stenosis is caused by:
- Increased laminar flow organization
- A wide range of velocities across the vessel lumen (Correct answer)
- Elevated peak systolic velocity only
- Reduced wall filter settings
Correct answer: A wide range of velocities across the vessel lumen
Turbulent and disturbed flow creates multiple velocity vectors simultaneously, filling in the normally clear spectral window below the waveform.
Question 6: Which statement correctly describes the relationship between vessel radius and flow resistance according to Poiseuille's law?
- Resistance is directly proportional to radius
- Resistance is inversely proportional to the square of radius
- Resistance is inversely proportional to the fourth power of radius (Correct answer)
- Resistance is proportional to the cube of radius
Correct answer: Resistance is inversely proportional to the fourth power of radius
Poiseuille's law states R = 8ηL / πr⁴, meaning a 50% reduction in radius increases resistance 16-fold.
Question 7: Color Doppler aliasing at a vessel with high velocity can be corrected by:
- Decreasing the color gain
- Increasing the pulse repetition frequency (Correct answer)
- Increasing the Doppler angle to 90°
- Decreasing transmitted frequency
Correct answer: Increasing the pulse repetition frequency
Increasing the PRF raises the Nyquist limit, allowing higher velocities to be displayed without aliasing in color or spectral Doppler.
RVT Hemodynamics and Doppler Principles 3
The Nyquist limit in pulsed wave Doppler determines the maximum detectable Doppler shift without aliasing, which equals: