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

Magnetic Resonance in Medicine, vol. 85, pp. 531–543

Abstract

Purpose To describe and implement a strategy for dynamic slice‐by‐slice and multiband B0 shimming using spherical harmonic shims in the human brain at 7T. Theory For thin axial slices, spherical harmonic shims can be divided into pairs of shims (z‐degenerate and non‐z‐degenerate) that are spatially degenerate, such that only ½ of the shims (non‐z‐degenerate) are required for single slice optimizations. However, when combined, the pairs of shims can be used to simultaneously generate the same in‐plane symmetries but with different amplitudes as a function of their z location. This enables multiband shimming equivalent to that achievable by single slice‐by‐slice optimization. Methods All data were acquired at 7T using a spherical harmonic shim insert enabling shimming up through 4th order with two additional 5th order shims (1st‐4th+). Dynamic shim updating was achieved using a 10A shim power supply with 2 ms ramps and constrained optimizations to minimize eddy currents. Results In groups of eight subjects, we demonstrated that: 1) dynamic updating using 1st‐4th+ order shims reduced the SD of the B0 field over the whole brain from 32.4 ± 2.6 and 24.9 ± 2 Hz with 1st‐2nd and 1st‐4th+ static global shimming to 15.1 ± 1.7 Hz; 2) near equivalent performance was achieved when dynamically updating only the non‐z‐degenerate shims (14.3 ± 1.5 Hz), or when a using multiband shim factor of 2, MBs = 2, and all shims (14.4 ± 2.0 Hz). Conclusion High order spherical harmonics provide substantial improvements over static global shimming and enable dynamic multiband shimming with near equivalent performance to that of dynamic slice‐by‐slice shimming. This reduces distortion in echo planar imaging.

Authors 5

1. Hoby P. Hetherington corresponding

   University of Pittsburgh

   Affiliation as printed

   Department of Radiology University of Pittsburgh Pittsburgh Pennsylvania USA

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

2. Chan‐Hong Moon

   University of Pittsburgh

   Affiliation as printed

   Department of Radiology University of Pittsburgh Pittsburgh Pennsylvania USA

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

3. Michael Schwerter

   Forschungszentrum Jülich

   Affiliation as printed

   Institute of Neuroscience and Medicine 4 Forschungszentrum INM‐4 Jülich North Rhine‐Westphalia Germany

4. Nadim Jon Shah Aachen Department of Neurology

   Forschungszentrum Jülich · Jülich Aachen Research Alliance · RWTH Aachen University

   Affiliation as printed

   Department of Neurology RWTH Aachen University Aachen North Rhine‐Westphalia Germany

   Institute of Neuroscience and Medicine 11 JARA Forschungszentrum INM‐11 Jülich North Rhine‐Westphalia Germany

   Institute of Neuroscience and Medicine 4 Forschungszentrum INM‐4 Jülich North Rhine‐Westphalia Germany

   JARA ‐ BRAIN ‐ Translational Medicine Aachen North Rhine‐Westphalia Germany

5. Jullie W. Pan

   University of Pittsburgh

   Affiliation as printed

   Department of Radiology University of Pittsburgh Pittsburgh Pennsylvania USA

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