MRI Physics 3 — Questions and Answers
Question 1: In k-space, what does the center of k-space primarily determine?
- Image contrast (Correct answer)
- Image resolution
- Slice position
- Echo time
Correct answer: Image contrast
The center of k-space contains low spatial frequencies that carry the bulk of image contrast and overall signal intensity.
Question 2: Which MRI sequence is most sensitive to hemorrhage detection because of its sensitivity to blood products?
- Gradient echo (GRE) (Correct answer)
- Inversion recovery (IR)
- Fast spin echo (FSE)
- Diffusion-weighted imaging (DWI)
Correct answer: Gradient echo (GRE)
Gradient echo sequences are sensitive to magnetic susceptibility differences caused by hemosiderin and deoxyhemoglobin, making them ideal for detecting hemorrhage.
Question 3: What is the effect of increasing the echo time (TE) on T2-weighted images?
- Increases T2 weighting by allowing more transverse relaxation (Correct answer)
- Decreases T2 weighting by increasing signal
- Has no effect on tissue contrast
- Increases T1 weighting
Correct answer: Increases T2 weighting by allowing more transverse relaxation
A longer TE allows more time for T2 decay, so tissues with short T2 lose more signal, increasing contrast between tissues with different T2 values.
Question 4: During inversion recovery, the null point (TI) for fat suppression (STIR) is set to:
- TI = T1(fat) × ln(2) (Correct answer)
- TI = T2(fat) × 0.693
- TI = TR / 2
- TI = TE × T1(fat)
Correct answer: TI = T1(fat) × ln(2)
The null point occurs when the recovering longitudinal magnetization of fat passes through zero, which is at TI ≈ T1(fat) × ln(2) ≈ 0.693 × T1(fat).
Question 5: Which gradient is primarily responsible for frequency encoding in MRI?
- Readout gradient (Correct answer)
- Phase-encoding gradient
- Slice-select gradient
- Spoiler gradient
Correct answer: Readout gradient
The readout (frequency-encoding) gradient is applied during signal acquisition, causing spins at different positions to precess at different frequencies.
Question 6: Gibbs ringing artifact is caused by:
- Truncation of k-space data at high spatial frequencies (Correct answer)
- Motion during phase encoding
- Chemical shift between fat and water
- RF pulse imperfections
Correct answer: Truncation of k-space data at high spatial frequencies
Gibbs ringing (truncation artifact) occurs when k-space is sampled with insufficient high-frequency data, causing oscillating bands near sharp tissue interfaces.
Question 7: What is the main advantage of parallel imaging techniques (e.g., GRAPPA, SENSE) in MRI?
- Reduced acquisition time by undersampling k-space using multiple coil elements (Correct answer)
- Improved SNR by adding signals from multiple coils
- Elimination of susceptibility artifacts
- Increased spatial resolution without penalty
Correct answer: Reduced acquisition time by undersampling k-space using multiple coil elements
Parallel imaging uses the spatial sensitivity profiles of phased-array coils to reconstruct missing k-space lines, allowing faster acquisitions at the cost of some SNR.
MRI Physics 3
In k-space, what does the center of k-space primarily determine?