Importance of k-space trajectory on Off resonance artifact in echo-planar velocity imaging
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Importance of k-space trajectory on Off resonance artifact in echo-planar velocity imaging Jacob A Bender1,2*, Orlando P Simonetti1,2 From 15th Annual SCMR Scientific Sessions Orlando, FL, USA. 2-5 February 2012 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 kspace 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 (topdown). 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 offresonance effects.
Results PSF analysis and computer simulations revealed that offresonance causes a simple positional shift with topdown 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. 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 offresonance. 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). 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. Author details 1 Department of Biomedical Engineering, The Ohio State University
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