MRI MRI Image Quality and Optimization 1 — Questions and Answers
Question 1: Which parameter, when increased, will most directly improve the signal-to-noise ratio (SNR) in an MRI image?
- Increasing the number of excitations (NEX/NSA) (Correct answer)
- Increasing the echo train length (ETL)
- Decreasing the field of view (FOV)
- Increasing the bandwidth
Correct answer: Increasing the number of excitations (NEX/NSA)
Increasing NEX/NSA averages more signal acquisitions, which reduces noise and improves SNR proportional to the square root of the number of averages.
Question 2: What is the relationship between voxel size and SNR in MRI?
- Larger voxels produce higher SNR (Correct answer)
- Smaller voxels produce higher SNR
- Voxel size has no effect on SNR
- SNR decreases as voxel volume doubles
Correct answer: Larger voxels produce higher SNR
Larger voxels contain more protons contributing signal, increasing SNR, though at the cost of spatial resolution.
Question 3: Which of the following changes will increase spatial resolution without changing the FOV?
- Increasing the matrix size (Correct answer)
- Decreasing the matrix size
- Increasing slice thickness
- Increasing NEX
Correct answer: Increasing the matrix size
Increasing matrix size means more phase and frequency encoding steps, reducing voxel size and improving spatial resolution within the same FOV.
Question 4: A technologist wants to reduce scan time while maintaining similar SNR. Which strategy is most effective?
- Use parallel imaging (SENSE/GRAPPA) (Correct answer)
- Increase slice thickness
- Decrease the matrix size
- Increase the repetition time (TR)
Correct answer: Use parallel imaging (SENSE/GRAPPA)
Parallel imaging techniques use multiple coil elements to under-sample k-space, reducing scan time while maintaining acceptable SNR through reconstruction algorithms.
Question 5: What happens to image contrast when the echo time (TE) is increased in a T2-weighted sequence?
- T2 contrast increases as tissues with short T2 decay faster (Correct answer)
- T1 contrast increases
- SNR improves due to longer signal collection
- Tissues appear uniformly bright
Correct answer: T2 contrast increases as tissues with short T2 decay faster
Longer TE allows more T2 decay to occur, so tissues with short T2 lose signal faster and appear darker, enhancing T2 contrast between tissues.
Question 6: Which of the following parameters most directly controls T1 contrast in a spin echo sequence?
- Repetition time (TR) (Correct answer)
- Echo time (TE)
- Flip angle in gradient echo
- Bandwidth
Correct answer: Repetition time (TR)
TR determines how much longitudinal relaxation occurs between excitations; a short TR emphasizes T1 differences because tissues with shorter T1 recover faster.
Question 7: What is the primary trade-off when reducing the field of view (FOV) in MRI?
- Improved spatial resolution but increased risk of aliasing artifacts (Correct answer)
- Decreased scan time and improved SNR
- Improved SNR with no change in resolution
- Increased slice coverage with same resolution
Correct answer: Improved spatial resolution but increased risk of aliasing artifacts
Reducing FOV increases spatial resolution by making voxels smaller, but if anatomy extends beyond the reduced FOV, aliasing (wrap-around) artifacts occur.
MRI MRI Image Quality and Optimization 1
Which parameter, when increased, will most directly improve the signal-to-noise ratio (SNR) in an MRI image?