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


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.


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.


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.


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.


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.


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  3. Lin , Hung-Yu : Shared Velocity Encoding (SVE): A method to improve the temporal resolution of phase contrast velocity measurements. MRM.

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Bender, J.A., Simonetti, O.P. Importance of k-space trajectory on Off resonance artifact in echo-planar velocity imaging. J Cardiovasc Magn Reson 14 (Suppl 1), W68 (2012).

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