CABT Imaging Systems 5 — Questions and Answers
Question 1: In MRI, which parameter is adjusted to change T1 contrast between tissues?
- Echo time (TE)
- Repetition time (TR) (Correct answer)
- Flip angle in gradient echo sequences only
- Bandwidth
Correct answer: Repetition time (TR)
Repetition time (TR) controls how much T1 relaxation occurs between pulses; a short TR emphasizes T1 differences between tissues, producing T1-weighted contrast.
Question 2: Which ionizing radiation interaction with matter is the dominant process for diagnostic X-ray energies (20–150 keV) in soft tissue?
- Photoelectric effect
- Compton scattering (Correct answer)
- Pair production
- Coherent (Rayleigh) scattering
Correct answer: Compton scattering
Compton scattering dominates in soft tissue over the diagnostic energy range because photon energy greatly exceeds electron binding energies, making interaction probability nearly independent of atomic number.
Question 3: A technologist notices vertical line artifacts on a flat-panel X-ray detector image. The most likely cause is:
- Patient motion during exposure
- Defective column(s) of detector elements (Correct answer)
- Grid line aliasing
- Incorrect AEC chamber selection
Correct answer: Defective column(s) of detector elements
Vertical line artifacts on a flat-panel detector typically indicate one or more dead or malfunctioning columns of detector pixels that produce no signal or a constant incorrect value.
Question 4: The primary advantage of iterative reconstruction over filtered back projection (FBP) in CT is:
- Faster processing speed
- Reduction of image noise at the same or lower dose (Correct answer)
- Improved contrast resolution only at high kVp
- Elimination of beam hardening artifacts
Correct answer: Reduction of image noise at the same or lower dose
Iterative reconstruction algorithms model the acquisition physics and reduce noise more effectively than FBP, allowing diagnostic images at lower tube current and reduced patient dose.
Question 5: In nuclear medicine, the physical half-life of Technetium-99m (Tc-99m) is approximately:
- 2.4 hours
- 6 hours (Correct answer)
- 13 hours
- 24 hours
Correct answer: 6 hours
Tc-99m has a physical half-life of approximately 6 hours, which is short enough to limit patient dose while long enough to complete most nuclear medicine procedures.
Question 6: Which ultrasound imaging mode displays the amplitude of returning echoes as vertical spikes plotted against depth?
- B-mode
- M-mode
- A-mode (Correct answer)
- Doppler mode
Correct answer: A-mode
A-mode (amplitude mode) is the simplest ultrasound display, showing echo amplitude on the Y-axis versus depth (time of flight) on the X-axis as a series of spikes.
Question 7: When performing quality assurance on a mammography unit, the mean glandular dose (MGD) is used rather than skin dose because:
- MGD is easier to measure directly
- MGD better reflects radiation risk to the radiosensitive glandular breast tissue (Correct answer)
- Skin dose is always higher and less clinically relevant
- MGD is required by the FDA but not internationally
Correct answer: MGD better reflects radiation risk to the radiosensitive glandular breast tissue
Glandular tissue is the most radiobiologically sensitive component of the breast and the site of radiation-induced carcinogenesis, making MGD the clinically meaningful dose metric for mammography.
CABT Imaging Systems 5
In MRI, which parameter is adjusted to change T1 contrast between tissues?