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Correction of
Shear Artifact Caused by Unbalanced Alternating Readout Gradient in
Echo-Planar Imaging |
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Jr-yuan Chiou1,2,
C. Beom Ahn2,3, L. Tugan Muftuler2, O. Nalcioglu1,2 |
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1. Department of
Electrical and Computer Engineering, 2. John Tu and
Thomas Yuen Center for Functional Onco-Imaging, College of Medicine,
University of California, Irvine, CA-92697-5020 (USA) 3. |
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Manuscript received: Accepted for publication: Abstract The shear artifact in echo planar
images arises from the imperfections in the readout gradient. One reason is
due to the improper linear shim by the readout gradient, and another is
caused from the imbalance of the alternating readout gradient. The
mis-shimmed readout gradient produces a static field error, and the corresponding
distortion can be removed by .eld mapping techniques. However, since the .eld error caused by the unbalanced
alternating readout gradient is not static, it cannot be measured by the regular .eld mapping techniques. A technique,
based on a gradient waveform measurement, is described for the correction of the
distortion resulting from the unbalanced alternating readout gradient. The
unbalanced portion of the alternating gradient can be determined by a linear .tting of the phase accumulated from
the unbalanced alternating gradient versus time, 2Ti (Ti is interecho spacing), and converted
to an ‘equivalent static .eld error’. Then, the distortions resulting from both the of-resonance
related factors and the unbalanced
alternating gradient could be removed simultaneously by un-warping algorithms
according to the modified field maps. This technique requires
only a minor pulse sequence modi.cation and a few additional scans to collect the necessary data on the unbalanced
alternating gradient. Consequently, the image quality can be further
improved. KEYWORDS: Shear artifact, unbalanced
alternating gradient, EPI, MRI. |
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