MRI Physics 4 — Questions and Answers
Question 1: Which phenomenon causes the chemical shift artifact in MRI, and in which direction does it occur?
- Resonance frequency difference between fat and water; frequency-encode direction (Correct answer)
- Susceptibility differences between tissues; phase-encode direction
- B1 field inhomogeneity; slice-select direction
- T1 differences between tissues; readout direction
Correct answer: Resonance frequency difference between fat and water; frequency-encode direction
Fat protons resonate at ~3.5 ppm lower frequency than water, causing spatial misregistration along the frequency-encoding direction.
Question 2: In MRI, the term 'flip angle' refers to:
- The angle by which the net magnetization is rotated from B0 by the RF pulse (Correct answer)
- The angle of the gradient coils relative to the main magnet
- The phase angle accumulated during readout
- The angular difference between excitation and reception coils
Correct answer: The angle by which the net magnetization is rotated from B0 by the RF pulse
The flip angle is the degree of rotation of the net magnetization vector away from the longitudinal axis produced by an RF pulse.
Question 3: What is the Ernst angle in MRI?
- The flip angle that maximizes signal for a given TR and T1 (Correct answer)
- The angle at which the slice becomes isotropic
- The phase angle at which fat and water are in phase
- The gradient angle that minimizes eddy currents
Correct answer: The flip angle that maximizes signal for a given TR and T1
The Ernst angle (cos θ = e^(−TR/T1)) is the optimal flip angle for steady-state gradient echo imaging that maximizes SNR efficiency.
Question 4: Susceptibility-weighted imaging (SWI) exploits differences in which magnetic property between tissues?
- Magnetic susceptibility (Correct answer)
- T1 relaxation time
- Proton density
- Chemical shift
Correct answer: Magnetic susceptibility
SWI uses the phase and magnitude information from a long-TE GRE sequence to detect subtle differences in magnetic susceptibility (e.g., deoxyhemoglobin, iron, calcium).
Question 5: In diffusion-weighted imaging (DWI), a high b-value results in:
- Greater diffusion weighting and lower overall signal (Correct answer)
- Less diffusion weighting and higher signal
- Reduced T2 contribution to signal
- Increased signal from rapidly diffusing molecules
Correct answer: Greater diffusion weighting and lower overall signal
Higher b-values apply stronger diffusion-sensitizing gradients, so freely diffusing water loses more signal, increasing lesion conspicuity for restricted diffusion.
Question 6: The apparent diffusion coefficient (ADC) map is calculated to eliminate which confounding effect in DWI?
- T2 shine-through (Correct answer)
- T1 recovery
- Gibbs ringing
- Chemical shift misregistration
Correct answer: T2 shine-through
ADC maps remove T2 shine-through by computing the diffusion coefficient from multiple b-values, isolating true diffusion restriction from long-T2 tissue signal.
Question 7: What is the purpose of crusher (spoiler) gradients in a spin echo sequence?
- To dephase residual transverse magnetization before the next RF pulse (Correct answer)
- To refocus magnetization after the 180° pulse
- To select the imaging slice thickness
- To encode spatial position in the frequency direction
Correct answer: To dephase residual transverse magnetization before the next RF pulse
Crusher gradients surround the 180° refocusing pulse to eliminate stimulated echoes from imperfect flip angles that could contaminate image quality.
MRI Physics 4
Which phenomenon causes the chemical shift artifact in MRI, and in which direction does it occur?