Passive shimming of magnetically shielded room using ferromagnetic plates

Shixu Jin, Akihiro Kuwahata, Shinichi Chikaki, Mutsuko Hatano, Masaki Sekino

Research output: Contribution to journalArticlepeer-review

Abstract

Although recent wearable devices for magnetoencephalography (MEG) facilitate measurements of unrestrained subjects, a significant challenge has arisen owing to large signal perturbations, caused by human movements under a non-uniform magnetic field inside large magnetically shielded rooms (MSRs). This study proposes and demonstrates a novel method for improving the magnetic field uniformity of an MSR by placing ferromagnetic plates on its surfaces. We solved an inverse problem to identify the appropriate locations of shimming plates for a given target distribution of a magnetic field. We used Tikhonov regularization and L-curve method to determine the optimal parameters. By solving the inverse problem, we obtained the locations and quantities of steel plates that could reduce the magnetic field non-uniformity relative to the original one without shimming. In a two-dimensional numerical simulation, we evaluated the uniformity by calculating the standard deviation of the magnetic field at nine measurement points inside an MSR. The standard deviations were 0.21 and 0.38 with and without shimming, indicating that the magnetic field uniformity has improved by 44% after shimming. An experimental study on a prototype MSR also indicated a 9% improvement. These results suggest that the proposed shimming method has potential for improving the magnetic field uniformity inside large MSRs for the MEG of freely moving subjects and for fundamental physical measurements.

Original languageEnglish
JournalIEEE Transactions on Magnetics
DOIs
Publication statusAccepted/In press - 2021

Keywords

  • Inverse problem
  • Inverse problems
  • Magnetic field measurement
  • Magnetic field shielding
  • Magnetic field uniformity
  • Magnetic fields
  • Magnetic noise
  • Magnetic resonance imaging
  • Magnetic shielding
  • Shimming design
  • Standards

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Electrical and Electronic Engineering

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