ABR ABR Imaging Physics and Equipment 2 — Questions and Answers
Question 1: In MRI, what phenomenon causes signal loss at tissue interfaces with different magnetic susceptibilities?
- Susceptibility-induced T2* dephasing (Correct answer)
- T1 relaxation differences
- Chemical shift artifact
- Gibbs ringing artifact
Correct answer: Susceptibility-induced T2* dephasing
Differences in magnetic susceptibility at tissue interfaces cause local field inhomogeneities that lead to rapid T2* dephasing and signal loss on gradient-echo sequences.
Question 2: What is the purpose of the slice-select gradient in MRI pulse sequences?
- Excites only the spins within a chosen slice by matching their resonant frequency (Correct answer)
- Encodes spatial information in the frequency direction
- Encodes spatial information in the phase direction
- Refocuses spin dephasing
Correct answer: Excites only the spins within a chosen slice by matching their resonant frequency
The slice-select gradient creates a linear variation in magnetic field strength so that only protons within a specific slice have the Larmor frequency matching the applied RF pulse.
Question 3: In fluoroscopy, what is the automatic brightness control (ABC) system designed to do?
- Maintain constant image brightness by adjusting kVp and/or mA as patient thickness changes (Correct answer)
- Reduce patient entrance dose automatically
- Filter scattered radiation
- Improve spatial resolution
Correct answer: Maintain constant image brightness by adjusting kVp and/or mA as patient thickness changes
The ABC system automatically adjusts the x-ray generator's kVp and mA to compensate for changes in patient attenuation, keeping the image receptor input dose (and thus brightness) constant.
Question 4: What is the primary advantage of a flat-panel detector over an image intensifier in fluoroscopy?
- Larger field of view with less geometric distortion and better DQE uniformity (Correct answer)
- Lower patient dose in all clinical scenarios
- Higher frame rate capability
- Lower cost of manufacture
Correct answer: Larger field of view with less geometric distortion and better DQE uniformity
Flat-panel detectors provide a larger, geometrically undistorted field of view and more uniform DQE across the image compared to the pincushion distortion and veiling glare of image intensifiers.
Question 5: In nuclear medicine, what is the purpose of a parallel-hole collimator on a gamma camera?
- To accept only photons traveling perpendicular to the detector, providing spatial localization (Correct answer)
- To focus photons from a point source onto the detector
- To amplify photon signal
- To filter out scattered photons by energy
Correct answer: To accept only photons traveling perpendicular to the detector, providing spatial localization
The parallel-hole collimator accepts only photons traveling roughly perpendicular to its face, rejecting angled photons to provide spatial information about the source distribution.
Question 6: What is the Nyquist theorem's implication for digital image sampling in radiology?
- The sampling frequency must be at least twice the highest spatial frequency present to avoid aliasing (Correct answer)
- Images must be sampled at equal intervals in both axes
- Bit depth must exceed 12 bits for diagnostic quality
- Pixel size equals the detector element size
Correct answer: The sampling frequency must be at least twice the highest spatial frequency present to avoid aliasing
The Nyquist theorem states that to accurately digitize a signal, the sampling rate must be at least twice the highest frequency component, otherwise aliasing artifacts appear.
ABR ABR Imaging Physics and Equipment 2
In MRI, what phenomenon causes signal loss at tissue interfaces with different magnetic susceptibilities?