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Co-analysis of Brain Structure and Function using fMRI and Diffusion-weighted Imaging
Published on: November 8, 2012
Diffusion anisotropy indexes are sensitive to selecting the EPI readout-encoding bandwidth at high-field MRI
Geon-Ho Jahng1, Michael W Weiner, Norbert Schuff
1Department of Radiology, East-West Neo Medical Center, College of Medicine, Kyung Hee University, Seoul 134-090, South Korea. ghjahng@gmail.com
Magnetic Resonance Imaging
|April 19, 2008
Summary
Diffusion tensor magnetic resonance imaging (DT-MRI) accuracy is affected by echo planar imaging (EPI) bandwidth due to Nyquist ghosting. Tuning EPI readout gradient modulation frequency can minimize these artifacts, improving DT-MRI measurements.
Area of Science:
- Magnetic Resonance Imaging
- Biophysics
- Medical Physics
Background:
- Diffusion tensor magnetic resonance imaging (DT-MRI) relies on echo planar imaging (EPI) for water diffusion mapping.
- EPI acquisitions can induce mechanical vibrations, leading to magnetic field fluctuations and Nyquist ghosting artifacts in DT-MRI data.
- Understanding the impact of EPI parameters on DT-MRI accuracy is crucial for reliable measurements.
Purpose of the Study:
- To investigate the effects of EPI readout gradient modulation frequency, linked to EPI readout bandwidth (BW), on DT-MRI measurement accuracy.
- To assess the relationship between EPI BW and Nyquist ghosting in a high magnetic field system.
- To determine if EPI BW variations impact diffusion anisotropy indexes in human brain imaging.
Main Methods:
- A spherical water phantom and nine human volunteers were used for the study.
- Spin-echo EPI acquisition with a 128x128 matrix and diffusion sensitization gradients (b=800 s/mm²) were employed.
- EPI was performed at various bandwidths, and analysis of variance (ANOVA) was used to assess BW effects and Nyquist ghost intensities.
Main Results:
- Phantom studies revealed a direct correlation between EPI bandwidths and Nyquist ghost intensities.
- EPI bandwidth variations significantly corrupted diffusion anisotropy indexes (fractional anisotropy, relative anisotropy) in human brain studies (F=10.5, P=.0001).
- Minimizing bandwidth dependence was achieved by adjusting the EPI readout gradient modulation frequency (F=1.48, P=.25).
Conclusions:
- Diffusion anisotropy indexes derived from DT-MRI are sensitive to EPI readout bandwidth due to Nyquist ghosting artifacts.
- The impact of EPI bandwidth on DT-MRI measurements can be mitigated by optimizing the readout gradient modulation frequency.
- Tuning the EPI bandwidth to avoid harmonics of mechanical gradient vibrations is key to improving DT-MRI accuracy.
