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Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease

Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease

Published on: December 18, 2016

Dynamic B₀ shimming for multiband imaging using high order spherical harmonic shims.

Hoby P Hetherington¹, Chan Hong Moon¹, Michael Schwerter²

¹Department of Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.

Magnetic Resonance in Medicine

|August 29, 2020

Summary

This summary is machine-generated.

Dynamic multiband shimming using high-order spherical harmonics significantly improves B0 field uniformity at 7T. This advanced technique offers performance comparable to slice-by-slice methods, reducing echo planar imaging distortions.

Keywords:

constrained optimization echo planar imaging image distortion multiband shimming spherical harmonics

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Area of Science:

Magnetic Resonance Imaging (MRI)
Neuroimaging
Biophysics

Background:

B0 field inhomogeneity is a major challenge in high-field MRI, particularly at 7T.
Spherical harmonic shims offer a powerful tool for correcting magnetic field distortions.
Dynamic shimming strategies are needed to adapt to rapid physiological changes and improve image quality.

Purpose of the Study:

To implement and evaluate a dynamic shimming strategy using spherical harmonic shims at 7T.
To compare the effectiveness of dynamic slice-by-slice and multiband shimming techniques.
To reduce B0 field inhomogeneity for improved MRI data acquisition.

Main Methods:

Utilized a 7T MRI scanner with a spherical harmonic shim insert (1st-4th+ order).
Implemented dynamic shim updating with fast ramp times (2 ms) and constrained optimization to minimize eddy currents.
Acquired data from eight subjects to assess shimming performance.

Main Results:

Dynamic shimming with 1st-4th+ order shims reduced B0 field standard deviation from 32.4 ± 2.6 Hz to 15.1 ± 1.7 Hz.
Multiband shimming (MBs=2) achieved near-equivalent performance (14.4 ± 2.0 Hz) to slice-by-slice methods.
Dynamic updating of a subset of shims (non-z-degenerate) also yielded comparable results (14.3 ± 1.5 Hz).

Conclusions:

High-order spherical harmonic shimming significantly improves B0 field uniformity at 7T compared to static global shimming.
Dynamic multiband shimming provides a viable and efficient alternative to dynamic slice-by-slice shimming.
These advanced shimming techniques effectively reduce distortions in echo planar imaging, enhancing neuroimaging quality.