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. 2021 Jan;85(1):182-196.
doi: 10.1002/mrm.28423. Epub 2020 Jul 23.

B0 shimming of the human heart at 7T

Michael Hock  1 , Maxim Terekhov  1 , Maria Roxana Stefanescu  1 , David Lohr  1 , Stefan Herz  1   2 , Theresa Reiter  1   3 , Markus Ankenbrand  1 , Aleksander Kosmala  1   2 , Tobias Gassenmaier  1   2 , Christoph Juchem  4   5 , Laura Maria Schreiber  1
Affiliations

B0 shimming of the human heart at 7T

Michael Hock et al. Magn Reson Med. 2021 Jan.

Abstract

Purpose: Inhomogeneities of the static magnetic B0 field are a major limiting factor in cardiac MRI at ultrahigh field (≥ 7T), as they result in signal loss and image distortions. Different magnetic susceptibilities of the myocardium and surrounding tissue in combination with cardiac motion lead to strong spatio-temporal B0 -field inhomogeneities, and their homogenization (B0 shimming) is a prerequisite. Limitations of state-of-the-art shimming are described, regional B0 variations are measured, and a methodology for spherical harmonics shimming of the B0 field within the human myocardium is proposed.

Methods: The spatial B0 -field distribution in the heart was analyzed as well as temporal B0 -field variations in the myocardium over the cardiac cycle. Different shim region-of-interest selections were compared, and hardware limitations of spherical harmonics B0 shimming were evaluated by calibration-based B0 -field modeling. The role of third-order spherical harmonics terms was analyzed as well as potential benefits from cardiac phase-specific shimming.

Results: The strongest B0 -field inhomogeneities were observed in localized spots within the left-ventricular and right-ventricular myocardium and varied between systolic and diastolic cardiac phases. An anatomy-driven shim region-of-interest selection allowed for improved B0 -field homogeneity compared with a standard shim region-of-interest cuboid. Third-order spherical harmonics terms were demonstrated to be beneficial for shimming of these myocardial B0 -field inhomogeneities. Initial results from the in vivo implementation of a potential shim strategy were obtained. Simulated cardiac phase-specific shimming was performed, and a shim term-by-term analysis revealed periodic variations of required currents.

Conclusion: Challenges in state-of-the-art B0 shimming of the human heart at 7 T were described. Cardiac phase-specific shimming strategies were found to be superior to vendor-supplied shimming.

Keywords: 7 T; B0; cardiac MRI; shimming; ultrahigh field.

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References

REFERENCES

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