VT Ultrasound Physics & Doppler Principles 3 — Questions and Answers
Question 1: What is the approximate speed of sound in soft tissue used as the standard for ultrasound imaging?
- 1,480 m/s
- 1,540 m/s (Correct answer)
- 1,600 m/s
- 340 m/s
Correct answer: 1,540 m/s
Most diagnostic ultrasound systems assume a propagation speed of 1,540 m/s in soft tissue.
Question 2: The term 'wall filter' in Doppler ultrasound is used to eliminate signals from:
- High-velocity arterial flow
- Low-frequency venous signals
- Slow-moving vessel walls and tissue (Correct answer)
- Noise from electronic interference
Correct answer: Slow-moving vessel walls and tissue
Wall filters (high-pass filters) remove low-frequency Doppler shifts caused by slow-moving vessel walls and surrounding tissue clutter.
Question 3: When the angle of insonation is 0° in the Doppler equation, the cosine value is:
- 0
- 0.5
- 1.0 (Correct answer)
- Undefined
Correct answer: 1.0
cos(0°) = 1.0, meaning the beam is perfectly parallel to flow, yielding maximum Doppler frequency shift.
Question 4: Which transducer design provides the best near-field resolution for superficial vascular structures like the carotid artery?
- Curved array (convex)
- Phased array (sector)
- Linear array (Correct answer)
- Mechanical sector
Correct answer: Linear array
Linear array transducers produce a rectangular field of view and superior near-field resolution ideal for superficial vessels.
Question 5: Increasing the pulse repetition frequency (PRF) in pulsed Doppler will:
- Decrease the maximum detectable velocity
- Increase the maximum detectable velocity (Correct answer)
- Increase image depth
- Reduce spatial resolution
Correct answer: Increase the maximum detectable velocity
A higher PRF raises the Nyquist limit (PRF/2), allowing detection of higher velocities before aliasing occurs.
Question 6: Acoustic shadowing behind a calcified plaque is caused by:
- Increased transmission through the plaque
- High acoustic impedance mismatch and attenuation by calcium (Correct answer)
- Refraction of the beam around the plaque
- Side lobe energy hitting the plaque
Correct answer: High acoustic impedance mismatch and attenuation by calcium
Calcium has very high acoustic impedance, causing strong reflection and absorption that prevents sound from reaching structures beyond it.
Question 7: In spectral Doppler analysis, the spectral broadening index reflects:
- Peak systolic velocity alone
- The range of velocities present within the sample volume (Correct answer)
- The ratio of systolic to diastolic velocity
- Color saturation of the Doppler signal
Correct answer: The range of velocities present within the sample volume
Spectral broadening indicates a wide distribution of velocities within the sample volume, often associated with disturbed or turbulent flow.
VT Ultrasound Physics & Doppler Principles 3
What is the approximate speed of sound in soft tissue used as the standard for ultrasound imaging?