Journal Article

FZJ-2020-02995

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Dynamic B 0 shimming for multiband imaging using high order spherical harmonic shims

Hetherington, H. P. (Corresponding author) ; Moon, C. H. ; Schwerter, M.FZJ* ; Shah, N. J.FZJ* ; Pan, J. W.

2021
Wiley-Liss New York, NY [u.a.]

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Please use a persistent id in citations: http://hdl.handle.net/2128/26312  doi:10.1002/mrm.28438

Abstract: PurposeTo describe and implement a strategy for dynamic slice‐by‐slice and multiband B0 shimming using spherical harmonic shims in the human brain at 7T.TheoryFor 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.MethodsAll 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.ResultsIn 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).ConclusionHigh 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.

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Contributing Institute(s):

1. Physik der Medizinischen Bildgebung (INM-4)
2. Jara-Institut Quantum Information (INM-11)
3. Jülich-Aachen Research Alliance - Translational Brain Medicine (JARA-BRAIN)

Research Program(s):

1. 573 - Neuroimaging (POF3-573) (POF3-573)

Appears in the scientific report 2020

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Database coverage:
; ; BIOSIS Previews ; Biological Abstracts ; Clarivate Analytics Master Journal List ; Current Contents - Clinical Medicine ; Current Contents - Life Sciences ; DEAL Wiley ; Essential Science Indicators ; IF < 5 ; JCR ; Nationallizenz ; SCOPUS ; Science Citation Index ; Science Citation Index Expanded ; Web of Science Core Collection

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