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An MRI-based switched gradient impulse response characterization method with uniform eigenmode excitation.

Kulam Najmudeen Magdoom1, Malisa Sarntinoranont2, Thomas H Mareci3

  • 1Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, United States.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|March 29, 2020
PubMed
Summary

This study introduces a new MRI method using a Tukey windowed Shifted Sine-Integral (Tw-SSI) pulse to accurately measure the switched gradient impulse response function (sGIRF). This technique minimizes image distortions and improves accuracy in quantitative MRI.

Keywords:
Eddy currentsGradient pre-emphasisMRI based field monitoringMechanical resonance

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Gradient Coil Engineering
  • Quantitative Imaging

Background:

  • Switching gradients in MRI induce eddy currents and vibrations, leading to gradient time integral errors.
  • These errors cause k-space distortions and q-space imaging inaccuracies, compromising image quality and quantitative analysis.

Purpose of the Study:

  • To develop an unbiased, MRI-based method for measuring the switched gradient impulse response function (sGIRF).
  • To introduce a novel gradient excitation pulse, the Tukey windowed Shifted Sine-Integral (Tw-SSI) pulse, for uniform excitation of gradient eigenmodes.

Main Methods:

  • Developed a 3D MRI-based field mapping technique utilizing Hadamard encoding.
  • Introduced the Tw-SSI pulse to uniformly excite gradient eigenmodes over a defined bandwidth.
  • Simultaneously acquired sGIRF for all three gradient axes.

Main Results:

  • The Tw-SSI pulse uniformly distributes energy across eigenmodes, exciting weak bandlimited cross-terms more effectively than traditional trapezoidal pulses.
  • The MRI field mapping method captured both direct and cross-terms of the sGIRF, revealing mechanical resonant modes of gradient coils.
  • Mechanical modes persisted longer than eddy currents in the studied shielded gradient coil.

Conclusions:

  • The developed method provides an unbiased measurement of sGIRF, crucial for accurate quantitative MRI.
  • The tunable Tw-SSI pulse allows flexible, bandwidth-specific sGIRF measurements.
  • This rapid MRI field mapping technique can be implemented on any MRI system for gradient pre-emphasis, coil design evaluation, and higher-order shim characterization.