Image Formation and Processing Flashcards
7 cards from real ARDMS SPI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Image Formation and Processing flashcards as text
Demodulation in ultrasound signal processing refers to:
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.
What is the purpose of time-gain compensation (TGC) in ultrasound imaging?
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.
In a phased-array transducer, electronic focusing is achieved by:
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.
Dynamic receive focusing (dynamic aperture) in ultrasound systems:
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.
Grating lobes in array transducers are caused by:
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.
Which statement correctly describes the relationship between transmit frequency and axial resolution?
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).
The aliasing artifact seen in pulsed-wave Doppler when the Nyquist limit is exceeded manifests on the spectral display as:
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.