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  • Workshop presentation
  • Open Access

Importance of k-space trajectory on Off resonance artifact in echo-planar velocity imaging

  • 1, 2 and
  • 1, 2
Journal of Cardiovascular Magnetic Resonance201214 (Suppl 1) :W68

https://doi.org/10.1186/1532-429X-14-S1-W68

  • Published:

Keywords

  • Point Spread Function
  • Echo Planar Imaging
  • Resonance Artifact
  • Velocity Encode
  • Shift Artifact

Summary

Top-down and center-out echo planar imaging (EPI) trajectories were thoroughly studied in theory, phantom scans, and volunteer scans to establish a clear understanding of the manifestation of off-resonance artifacts.

Background

EPI is a highly efficient data acquisition technique, but is sensitive to off-resonance. In cardiac and flow imaging, field inhomogeneity is typically 70Hz in the myocardium and 100+ Hz in the blood pool at 1.5T(1). Choice of k-space trajectories is important; the center-out trajectory is often recommended over top-down to minimize TE and thereby maximize signal and minimize flow and motion error accumulation. Previous work has noted higher artifact with the center-out trajectory (2) although a comprehensive and systematic description is lacking.

Methods

Theoretical point spread function (PSF) calculations and computer simulations were performed to compare the center-out and top-down EPI trajectories. A gradient echo planar sequence (GRE-EPI) was developed with through plane two-sided (symmetric) velocity encoding and an echo time of 2.2ms (center-out) and 6.3ms (top-down). Shared velocity encoding (SVE) was used to reconstruct flow images (3). A constant flow phantom was imaged matching clinical image parameters. Demonstrative scans at the aortic valve in a single volunteer were preformed. In both phantom and volunteer scans, a frequency offset applied to investigate off-resonance effects.

Results

PSF analysis and computer simulations revealed that off-resonance causes a simple positional shift with top-down trajectory while the center-out trajectory leads to a more severe and complex artifact comprised of a positional shift, splitting, and blurring (see Figure). The distance of the shift artifact is twice as great with the center-out trajectory compared to top-down.
Figure 1
Figure 1

Magnitude and phase images for both trajectories for various off resonances. Phase images are magnitude threshold masked. Shift, splitting, and blurring artifacts were easily seen with the center-out trajectory and were not appreciable for the top-down trajectory in all experiments.

The top-down trajectory does not modulate the phase of the signal whereas the center-out trajectory does. This in combination with the phase effects from velocity encoding leads to complex artifacts affecting both the magnitude and phase image.

For the center-out trajectory, artifact phase modulation and velocity encoding leads to differences in magnitude images from the positive and negative velocity encoded k-spaces. This can cause a severe flickering the in the magnitude cines in the presence of flow and off-resonance.

The center-out trajectory provided a 15.6% higher signal than the top-down trajectory attributable to the shorter TE.

Flow quantification is overestimated and peak velocity suprizingly well maintained (Table 1).
Table 1

Peak velocity and flow quantification.

 

Trajectory

Top-Down

Top-Down

Center-Out

Center-Out

 

Off Resonance

0 Hz

100Hz

0 Hz

100Hz

Peak Velocity (cm/s)

Simulation

135.7

135.7 (no change)

135.7

129.8 (+11.7%)

 

Phantom

137.5

138.8 (+0.9%)

138.5

133.2 (-3.8%)

 

Volunteer

140.4

146.3 (+4.2%)

134.3

147.6 (+9.9%)

Flow (ml/s)

Phantom

484.7

487.1 (+0.5%)

496.3

838.7 (+69%)

Flow quantification for the off resonant center-out trajectory due to the spreading of the velocity over a larger ROI. The peak velocity is surprisingly only slightly underestimated with off-resonant center-out trajectory. Phantom experiments with an ideally homogenous signal intensity showed the magnitude signal intensity standard deviation didn&#8217t increase greatly with off-resonance top-down trajectory (5.2@0Hz to 8.3@100Hz) while it did for the center-out trajectory (13.0@0Hz to 39.3@100Hz).

Conclusions

A center-out EPI trajectory produces a more complex, severe, and variable artifact than a top-down trajectory with only a moderate improvement in the signal level.

Authors’ Affiliations

(1)
Department of Biomedical Engineering, The Ohio State University, Columbus, OH, USA
(2)
Dorothy M. Davis Heart & Lung Research Institute, The Ohio State University, Columbus, OH, USA

References

  1. Reeder : MRM. 1998, 39: 988-998.Google Scholar
  2. Luk-Pat : MRM. 1997, 37: 3,436-447.Google Scholar
  3. Lin , Hung-Yu : Shared Velocity Encoding (SVE): A method to improve the temporal resolution of phase contrast velocity measurements. MRM.Google Scholar

Copyright

© Bender and Simonetti; licensee BioMed Central Ltd. 2012

This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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