ARDMS SPI Image Formation and Processing 5 — Questions and Answers
Question 1: Demodulation in ultrasound signal processing refers to:
- Removing the radiofrequency carrier to extract the envelope of the echo signal (Correct answer)
- Digitizing the analog echo signal
- Applying time-gain compensation to raw RF data
- Steering the beam electronically to different angles
Correct answer: Removing the radiofrequency carrier to extract the envelope of the echo signal
Demodulation (envelope detection) extracts the amplitude envelope from the high-frequency RF echo signal, which represents tissue reflectivity and is used to build the gray-scale image.
Question 2: What is the purpose of time-gain compensation (TGC) in ultrasound imaging?
- To increase the transmit frequency with depth
- To amplify echoes from deeper structures to compensate for attenuation (Correct answer)
- To reduce the pulse repetition frequency at greater depths
- To adjust the scan line density across the image
Correct answer: To amplify echoes from deeper structures to compensate for attenuation
TGC applies progressively increasing gain with depth to offset the depth-dependent attenuation of sound in tissue, making echoes from similar structures appear equally bright regardless of depth.
Question 3: In a phased-array transducer, electronic focusing is achieved by:
- Using a physical acoustic lens in front of the elements
- Applying time delays to the firing sequence of individual elements (Correct answer)
- Varying the transmit frequency element by element
- Mechanically rotating the transducer array
Correct answer: Applying time delays to the firing sequence of individual elements
Precise time delays introduced across phased-array elements cause the wavefronts to converge at a desired focal depth, creating electronic focusing without moving parts.
Question 4: Dynamic receive focusing (dynamic aperture) in ultrasound systems:
- Changes transmit focus depth during a single pulse cycle
- Continuously adjusts receive delays to focus at the depth from which echoes are returning (Correct answer)
- Increases the number of transmit pulses to improve lateral resolution
- Widens the beam on receive to improve elevational resolution
Correct answer: Continuously adjusts receive delays to focus at the depth from which echoes are returning
As echoes return from progressively greater depths, the system continuously updates the receive time delays so the beam is always focused at the current echo depth, improving lateral resolution throughout the image.
Question 5: Grating lobes in array transducers are caused by:
- Excessive acoustic power causing nonlinear propagation
- Constructive interference from equally spaced element spacing greater than half a wavelength (Correct answer)
- Reflection of side lobes off the transducer housing
- Refraction of the main beam at tissue interfaces
Correct answer: Constructive interference from equally spaced element spacing greater than half a wavelength
When element spacing exceeds half a wavelength, spatial aliasing causes grating lobes — additional beams fired at off-axis angles that can produce spurious echoes in the image.
Question 6: Which statement correctly describes the relationship between transmit frequency and axial resolution?
- Lower frequency yields better axial resolution due to longer wavelengths
- Higher frequency yields better axial resolution because spatial pulse length decreases (Correct answer)
- Frequency has no effect on axial resolution; only pulse duration matters
- Axial resolution improves as frequency decreases because penetration increases
Correct answer: Higher frequency yields better axial resolution because spatial pulse length decreases
Higher frequency produces shorter wavelengths and shorter spatial pulse lengths, directly improving axial resolution (minimum resolvable depth separation = SPL/2).
Question 7: The aliasing artifact seen in pulsed-wave Doppler when the Nyquist limit is exceeded manifests on the spectral display as:
- A mirror image of the vessel on the opposite side of the baseline
- Flow velocities wrapping around and appearing on the opposite side of the baseline (Correct answer)
- Complete loss of the Doppler signal at high velocities
- A broadened spectral envelope with increased gain
Correct answer: Flow velocities wrapping around and appearing on the opposite side of the baseline
When the Doppler shift frequency exceeds half the PRF (Nyquist limit), the sampled signal wraps around, causing high-velocity flow to alias and appear on the opposite side of the zero-baseline.
ARDMS SPI Image Formation and Processing 5
Demodulation in ultrasound signal processing refers to: