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A comparison of phase encoding ordering schemes in T2-weighted GRASE imaging
G Johnson1, D A Feinberg, V Venkataraman
1Department of Radiology, New York University Medical Center, New York 10016, USA.
Magnetic Resonance in Medicine
|September 1, 1996
Summary
Optimizing gradient and spin echo (GRASE) imaging involves changing the phase encoding order to reduce ghosting artifacts. The k-banded (kb) GRASE ordering generally shows the lowest artifact levels, offering a good balance between different methods.
Area of Science:
- Magnetic Resonance Imaging
- Medical Physics
- Biomedical Engineering
Background:
- Gradient and spin echo (GRASE) imaging is an advanced MRI technique combining gradient and radiofrequency (RF) refocusing.
- This combination can lead to phase modulations within the echo train, potentially causing significant ghosting artifacts, especially with linear phase encoding orders.
- Artifacts are often exacerbated by factors like chemical shift, susceptibility differences, and static field inhomogeneities.
Purpose of the Study:
- To compare the effectiveness of different phase encoding orders in reducing artifacts in T2-weighted GRASE imaging.
- To evaluate various implementations of standard GRASE and k-banded (kb) GRASE sequences.
- To assess the impact of phase encoding order on image resolution and contrast.
Main Methods:
- Comparison of linear, partially randomized, standard GRASE, and k-banded (kb) GRASE phase encoding orders.
- Utilizing computer simulations to quantify resolution and artifact levels.
- Acquisition and analysis of phantom and volunteer images to validate simulation results.
Main Results:
- Linear phase encoding is highly susceptible to ghosting artifacts from chemical shift, susceptibility, and field inhomogeneities.
- Standard GRASE ordering is robust against these but vulnerable to short T2 signal artifacts.
- k-banded (kb) GRASE offers an intermediate solution with generally lower artifact levels; partially randomized ordering results in diffuse artifacts.
- Computer simulations indicate similar spatial resolution and contrast across all tested phase encoding orders.
Conclusions:
- Phase encoding order significantly impacts artifact levels in GRASE imaging.
- k-banded (kb) GRASE provides a favorable balance, minimizing artifacts while maintaining image quality.
- The choice of phase encoding strategy is crucial for optimizing artifact reduction in T2-weighted GRASE MRI.