Advancements in MR hardware systems and magnetic field control: B₀ shimming, RF coils, and gradient techniques for enhancing magnetic resonance imaging and spectroscopy

Abstract

High magnetic field homogeneity is critical for magnetic resonance imaging (MRI), functional MRI, and magnetic resonance spectroscopy (MRS) applications. B₀ inhomogeneity during MR scans is a long-standing problem resulting from magnet imperfections and site conditions, with the main issue being the inhomogeneity across the human body caused by differences in magnetic susceptibilities between tissues, resulting in signal loss, image distortion, and poor spectral resolution. Through a combination of passive and active shim techniques, as well as technological advances employing multi-coil techniques, optimal coil design, motion tracking, and real-time modifications, improved field homogeneity and image quality have been achieved in MRI/MRS. The integration of RF and shim coils brings a high shim efficiency due to the proximity of participants. This technique will potentially be applied to high-density RF coils with a high-density shim array for improved B₀ homogeneity. Simultaneous shimming and image encoding can be achieved using multi-coil array, which also enables the development of novel encoding methods using advanced magnetic field control. Field monitoring enables the capture and real-time compensation for dynamic field perturbance beyond the static background inhomogeneity. These advancements have the potential to better use the scanner performance to enhance diagnostic capabilities and broaden applications of MRI/MRS in a variety of clinical and research settings. The purpose of this paper is to provide an overview of the latest advances in B₀ magnetic field shimming and magnetic field control techniques as well as MR hardware, and to emphasize their significance and potential impact on improving the data quality of MRI/MRS.

Keywords

B₀ shim / fMRI / passive shim / spherical harmonic shim / multi-coil shim / iPRES / shim-RF coil / gradient

Author contributions

Yun Shang (conceptualization, formal analysis, investigation, methodology, resources, writing - original draft), Gizeaddis Lamesgin Simegn (investigation, methodology, writing - review and editing), Kelly Gillen (writing - review and editing), Hsin-Jung Yang (writing - review and editing), and Hui Han (conceptualization, supervision, writing - review and editing)

Conflict of interest

One of the authors, Hui Han, is also the editor board member of Psychoradiology. He was blinded from reviewing or making decisions on the manuscript.

Acknowledgements and disclosure

This study was supported by the National Institute of Neurological Disorders and Stroke (NINDS) and the National Institutes of Health (NIH) grants R01NS121544 and R01HL156818. Additionally, support was provided by NIH SBIR grant R43NS120795. We acknowledge data collection and sharing from the project funded by the Frontotemporal Lobar Degeneration Neuroimaging Initiative (National Institutes of Health Grant R01 AG032306). Hsin-Jung Yang and Hui Han hold equity in Lucidity Medical LLC.

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