RVT Ultrasound Physics and Instrumentation 4 — Questions and Answers
Question 1: Which artifact appears as a bright echogenic line with posterior shadowing caused by total reflection at a gas interface?
- Comet-tail artifact
- Dirty shadowing (Correct answer)
- Clean shadowing
- Mirror image artifact
Correct answer: Dirty shadowing
Dirty shadowing (reverberation shadowing) occurs at gas interfaces where multiple reflections create a heterogeneous shadow with internal echoes.
Question 2: The time-gain compensation (TGC) control is used primarily to:
- Increase frame rate
- Compensate for attenuation of sound with depth (Correct answer)
- Reduce side lobe artifacts
- Improve axial resolution
Correct answer: Compensate for attenuation of sound with depth
TGC amplifies returning echoes from deeper structures to compensate for the progressive attenuation of ultrasound as it travels through tissue.
Question 3: In pulsed-wave Doppler, aliasing occurs when the Doppler shift frequency exceeds:
- The transducer frequency
- Half the pulse repetition frequency (Nyquist limit) (Correct answer)
- The wall filter setting
- The sample volume depth
Correct answer: Half the pulse repetition frequency (Nyquist limit)
Aliasing occurs when the Doppler shift exceeds the Nyquist limit (PRF/2), causing the waveform to wrap around and appear on the opposite side of the baseline.
Question 4: Which transducer design uses multiple elements fired in sequence to create a rectangular field of view?
- Phased array
- Linear array (Correct answer)
- Curvilinear array
- Annular array
Correct answer: Linear array
Linear array transducers fire elements sequentially across the footprint to produce a rectangular image with consistent width at all depths.
Question 5: The acoustic impedance of a medium is determined by:
- Frequency and wavelength
- Density and propagation speed (Correct answer)
- Attenuation coefficient and depth
- Pulse duration and duty factor
Correct answer: Density and propagation speed
Acoustic impedance (Z) equals the product of the medium's density (ρ) and the speed of sound (c) in that medium: Z = ρc.
Question 6: Color flow mapping assigns color based on:
- Amplitude of the returning echo
- Direction and mean velocity of blood flow relative to the transducer (Correct answer)
- Depth of the vessel being interrogated
- Absolute flow volume in milliliters per minute
Correct answer: Direction and mean velocity of blood flow relative to the transducer
Color Doppler maps the mean frequency shift (proportional to velocity) of moving blood onto a color scale, with direction indicated by hue (typically red toward, blue away).
Question 7: Lateral resolution in ultrasound is best improved by:
- Increasing pulse duration
- Focusing the beam at the depth of interest (Correct answer)
- Lowering the transducer frequency
- Increasing the pulse repetition frequency
Correct answer: Focusing the beam at the depth of interest
Lateral resolution equals beam width, which is minimized at the focal zone; therefore focusing the beam at the target depth optimizes lateral resolution.
RVT Ultrasound Physics and Instrumentation 4
Which artifact appears as a bright echogenic line with posterior shadowing caused by total reflection at a gas interface?