RVT Ultrasound Physics and Instrumentation 2 — Questions and Answers
Question 1: The Doppler equation used to calculate flow velocity is: v = (f_d × c) / (2 × f_0 × cos θ). What does 'θ' represent?
- The angle between the ultrasound beam and the direction of blood flow (Correct answer)
- The angle between the transducer face and the vessel wall
- The angle of incidence at the tissue boundary
- The beam divergence angle beyond the focal zone
Correct answer: The angle between the ultrasound beam and the direction of blood flow
θ is the Doppler angle—the angle between the insonating beam and the axis of blood flow; as θ approaches 90°, cos θ approaches 0 and velocity calculation becomes unreliable.
Question 2: Aliasing in color flow Doppler imaging appears as:
- An abrupt color reversal in the center of a high-velocity jet (Correct answer)
- A complete absence of color within the vessel lumen
- A uniform increase in red signal intensity throughout the vessel
- Acoustic shadowing distal to a calcified plaque
Correct answer: An abrupt color reversal in the center of a high-velocity jet
When peak Doppler shift exceeds the Nyquist limit, aliasing causes the color to wrap around and display the opposite color (e.g., red becomes blue) at the site of high velocity.
Question 3: Which technique is most effective for eliminating aliasing in pulsed-wave Doppler without switching to continuous-wave mode?
- Increasing the pulse repetition frequency (PRF) (Correct answer)
- Decreasing the transducer operating frequency
- Reducing the Doppler gain
- Widening the sample volume gate
Correct answer: Increasing the pulse repetition frequency (PRF)
Increasing PRF raises the Nyquist limit (PRF/2), allowing higher Doppler shift frequencies to be sampled without aliasing.
Question 4: Power Doppler differs from conventional color flow Doppler primarily in that it displays:
- The amplitude (power) of the Doppler signal rather than frequency shift (Correct answer)
- Bidirectional flow with higher velocity sensitivity
- Mean velocity values averaged across the vessel diameter
- Pulsatility index mapped in color across the image
Correct answer: The amplitude (power) of the Doppler signal rather than frequency shift
Power Doppler maps the intensity (energy) of the returning Doppler signal, making it more sensitive to slow flow and small vessels but providing no directional information.
Question 5: The range resolution (axial resolution) of a pulsed ultrasound system is primarily determined by:
- Spatial pulse length (Correct answer)
- Transducer aperture size
- Pulse repetition period
- Focal zone depth
Correct answer: Spatial pulse length
Axial resolution ≈ spatial pulse length / 2; shorter pulses (higher frequency, fewer cycles) improve the ability to distinguish two closely spaced reflectors along the beam axis.
Question 6: A duplex ultrasound system combines which two imaging modalities?
- B-mode (gray-scale) imaging and pulsed-wave Doppler (Correct answer)
- Continuous-wave Doppler and M-mode imaging
- Color Doppler and CT angiography
- Power Doppler and contrast-enhanced ultrasound
Correct answer: B-mode (gray-scale) imaging and pulsed-wave Doppler
Duplex ultrasound integrates real-time B-mode imaging for anatomical visualization with pulsed-wave Doppler spectral analysis for hemodynamic assessment.
Question 7: Time gain compensation (TGC) in ultrasound imaging is used to:
- Compensate for the increasing attenuation of sound with depth (Correct answer)
- Increase the pulse repetition frequency for deeper structures
- Reduce side-lobe artifacts from high-amplitude reflectors
- Adjust the Doppler angle automatically for angle correction
Correct answer: Compensate for the increasing attenuation of sound with depth
TGC applies progressively increasing amplification to returning echoes from greater depths to counteract attenuation, producing a uniform brightness display regardless of depth.
RVT Ultrasound Physics and Instrumentation 2
The Doppler equation used to calculate flow velocity is: v = (f_d × c) / (2 × f_0 × cos θ).
What does 'θ' represent?