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Cited by 128Atomic and Subatomic Physics ResearchElectromagnetic Compatibility and MeasurementsCharacterization and Applications of Magnetic Nanoparticles

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A lightweight magnetically shielded room with active shielding

Open access
Holmes, Niall, Rea, Molly, Chalmers, James, Leggett, James, Edwards, Lucy J., Nell, Paul, Pink, Stephen, Patel, Prashant, Wood, Jack, Murby, Nick, Woolger, David, Dawson, Eliot, Mariani, Christopher, Tierney, Tim M., Mellor, Stephanie, O’Neill, George C., Boto, Elena, Hill, Ryan M., Shah, Vishal, Osborne, James, Pardington, Rosemarie, Fierlinger, Peter, Barnes, Gareth R., Glover, Paul, Brookes, Matthew J., Bowtell, Richard·2022·Scientific Reports
DOI 10.1038/s41598-022-17346-1PMC PMC9363499
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Abstract

Magnetically shielded rooms (MSRs) use multiple layers of materials such as MuMetal to screen external magnetic fields that would otherwise interfere with high precision magnetic field measurements such as magnetoencephalography (MEG). Optically pumped magnetometers (OPMs) have enabled the development of wearable MEG systems which have the potential to provide a motion tolerant functional brain imaging system with high spatiotemporal resolution. Despite significant promise, OPMs impose stringent magnetic shielding requirements, operating around a zero magnetic field resonance within a dynamic range of ± 5 nT. MSRs developed for OPM-MEG must therefore effectively shield external sources and provide a low remnant magnetic field inside the enclosure. Existing MSRs optimised for OPM-MEG are expensive, heavy, and difficult to site. Electromagnetic coils are used to further cancel the remnant field inside the MSR enabling participant movements during OPM-MEG, but present coil systems are challenging to engineer and occupy space in the MSR limiting participant movements and negatively impacting patient experience. Here we present a lightweight MSR design (30% reduction in weight and 40–60% reduction in external dimensions compared to a standard OPM-optimised MSR) which takes significant steps towards addressing these barriers. We also designed a ‘window coil’ active shielding system, featuring a series of simple rectangular coils placed directly onto the walls of the MSR. By mapping the remnant magnetic field inside the MSR, and the magnetic field produced by the coils, we can identify optimal coil currents and cancel the remnant magnetic field over the central cubic metre to just |B|= 670 ± 160 pT. These advances reduce the cost, installation time and siting restrictions of MSRs which will be essential for the widespread deployment of OPM-MEG.

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Figures

Figure 1 The Light MuRoom magnetically shielded room. (a) Computer model of the Light MuRoom, cross-section revealing framework and layer structure. (b) Photograph of the exterior of the participant facing side of the MSR. (c) Photograph of the interior of the MSR with wearable OPM-MEG system mounted on a plastic mannequin.
Figure 1 The Light MuRoom magnetically shielded room. (a) Computer model of the Light MuRoom, cross-section revealing framework and layer structure. (b) Photograph of the exterior of the participant facing side of the MSR. (c) Photograph of the interior of the MSR with wearable OPM-MEG system mounted on a plastic mannequin.CC BY · PubMed Central
Figure 2 The window coil active magnetic shielding system. (a) Parameterisation of the window coil. Four square coils are arranged with four-fold symmetry on each face. The coil square side length, offset from the centre of the window and offset in the vertical axis are shared between all faces. Coil parameters are optimised to produce known components of magnetic field over the central cubic metre of the MSR. (b) Final optimised window coil featuring 24 identical square coils. This structure is challenging to engineer due to the need to incorporate the access door and projection ports in 3/6 walls. (c) Adapted design accounts for geometry of the MSR and features 27 coils. (d) Drawing of the corner of a single coil panel. To accommodate 20 turns of wire in each coil, a series of grooves are arranged in a spiral pattern into which copper wire is placed. The conductor return path is also shown. (e) Final model of the window coil featuring all panels. (f) Photograph of installed coil panels (floor coils hidden beneath the flooring and door coils hidden by angle of photograph) taken prior to cladding.
Figure 2 The window coil active magnetic shielding system. (a) Parameterisation of the window coil. Four square coils are arranged with four-fold symmetry on each face. The coil square side length, offset from the centre of the window and offset in the vertical axis are shared between all faces. Coil parameters are optimised to produce known components of magnetic field over the central cubic metre of the MSR. (b) Final optimised window coil featuring 24 identical square coils. This structure is challenging to engineer due to the need to incorporate the access door and projection ports in 3/6 walls. (c) Adapted design accounts for geometry of the MSR and features 27 coils. (d) Drawing of the corner of a single coil panel. To accommodate 20 turns of wire in each coil, a series of grooves are arranged in a spiral pattern into which copper wire is placed. The conductor return path is also shown. (e) Final model of the window coil featuring all panels. (f) Photograph of installed coil panels (floor coils hidden beneath the flooring and door coils hidden by angle of photograph) taken prior to cladding.CC BY · PubMed Central
Figure 3 Simulated magnetic field components produced by the window coil active magnetic shielding system. Contours of the magnetic field variation for (a) a uniform magnetic field Bz, (b) a longitudinal field gradient dBx/dx=-dBy/dy and (c) a transverse field gradient dBx/dz=dBz/dx. All contours are shown in three planes (arranged from left to right): z = 0 m |x|, |y|< 0.5 m, y = 0 m |x|, |z|< 0.5 m and x = 0 m |z|, |y|< 0.5 m respectively. The field values at each target point are normalised to the target field or field gradient strength (of 5 nT or 5 nT/m) to show deviation from uniformity. Contours of the remaining magnetic field components are shown in Online Appendix 1.
Figure 3 Simulated magnetic field components produced by the window coil active magnetic shielding system. Contours of the magnetic field variation for (a) a uniform magnetic field Bz, (b) a longitudinal field gradient dBx/dx=-dBy/dy and (c) a transverse field gradient dBx/dz=dBz/dx. All contours are shown in three planes (arranged from left to right): z = 0 m |x|, |y|< 0.5 m, y = 0 m |x|, |z|< 0.5 m and x = 0 m |z|, |y|< 0.5 m respectively. The field values at each target point are normalised to the target field or field gradient strength (of 5 nT or 5 nT/m) to show deviation from uniformity. Contours of the remaining magnetic field components are shown in Online Appendix 1.CC BY · PubMed Central
Figure 4 Active compensation of the remnant magnetic field using a field mapping method. (a) Two tri-axial fluxgate magnetometers attached to a plastic stick. A series of five infrared reflective markers are also attached to the stick allowing optical tracking of the position and orientation of the sensors within the MSR. (b) Schematic of the field mapping setup. The tracking cameras are mounted in the corners of the MSR and highlighted in blue. The dashed black volume shows the central cubic metre volume within which the stick was moved. The green highlighted marks show the path which the fluxgate magnetometers followed during the field mapping process, covering most of the central cubic metre of the MSR. (c) Magnetometer data from a single component of one triaxial sensor measured when a single coil was activated. By combining the data from all magnetometers with the optical tracking data a spherical harmonic model can be used to approximate the strength and spatial variation of the field produced by each coil. (d) The red trace shows the magnetic field measured by one magnetometer in the MSR with all coils switched off. The magnetic field model of each coil was used to calculate coil voltages which produce the required nulling field. Once voltages had been applied, the mapping was performed again. The blue trace shows the magnetometer data after nulling where similar sensor translations and rotations produce little to no change in the measured field. (e) Field mapping and nulling was repeated 8 times. The bar chart shows a consistent remnant field following demagnetisation and a consistent reduction in the RMS magnitude of the three uniform field components found by the model when the compensation is applied. (f) A similar reduction is seen in the RMS magnitude of the five field gradient components.
Figure 4 Active compensation of the remnant magnetic field using a field mapping method. (a) Two tri-axial fluxgate magnetometers attached to a plastic stick. A series of five infrared reflective markers are also attached to the stick allowing optical tracking of the position and orientation of the sensors within the MSR. (b) Schematic of the field mapping setup. The tracking cameras are mounted in the corners of the MSR and highlighted in blue. The dashed black volume shows the central cubic metre volume within which the stick was moved. The green highlighted marks show the path which the fluxgate magnetometers followed during the field mapping process, covering most of the central cubic metre of the MSR. (c) Magnetometer data from a single component of one triaxial sensor measured when a single coil was activated. By combining the data from all magnetometers with the optical tracking data a spherical harmonic model can be used to approximate the strength and spatial variation of the field produced by each coil. (d) The red trace shows the magnetic field measured by one magnetometer in the MSR with all coils switched off. The magnetic field model of each coil was used to calculate coil voltages which produce the required nulling field. Once voltages had been applied, the mapping was performed again. The blue trace shows the magnetometer data after nulling where similar sensor translations and rotations produce little to no change in the measured field. (e) Field mapping and nulling was repeated 8 times. The bar chart shows a consistent remnant field following demagnetisation and a consistent reduction in the RMS magnitude of the three uniform field components found by the model when the compensation is applied. (f) A similar reduction is seen in the RMS magnitude of the five field gradient components.CC BY · PubMed Central
Figure 5 OPM data taken in the Light MuRoom. (a) Timecourses of the change in magnetic field experienced by eight dual-axis OPMs (16 total channels) which were placed at the centre of the empty MSR with the window coil system switched on. Each OPM was configured to measure two components of magnetic field. Over 10 min the change in field is ~ 300 pT indicating good shielding performance and a quiet magnetic environment. (b) Mean and range (shaded) of the power spectral density of the data collected from the OPMs with and without the window coil system active. The black dashed line indicates 15 fT/√Hz. Data suggests that the MSR is a suitable environment for OPM-MEG, and that the coil system does not add additional magnetic field noise.
Figure 5 OPM data taken in the Light MuRoom. (a) Timecourses of the change in magnetic field experienced by eight dual-axis OPMs (16 total channels) which were placed at the centre of the empty MSR with the window coil system switched on. Each OPM was configured to measure two components of magnetic field. Over 10 min the change in field is ~ 300 pT indicating good shielding performance and a quiet magnetic environment. (b) Mean and range (shaded) of the power spectral density of the data collected from the OPMs with and without the window coil system active. The black dashed line indicates 15 fT/√Hz. Data suggests that the MSR is a suitable environment for OPM-MEG, and that the coil system does not add additional magnetic field noise.CC BY · PubMed Central

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  1. Juhas, A., Pekari, N. & Toepfer, H. Magnetic field of rectangular current loop with sides parallel and perpendicular to the surface of high-permeability material. SJEE 11, 701–717 (2014).

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Metadata

Authors
Holmes, N., Rea, M., Chalmers, J., Leggett, J., Edwards, L. J., Nell, P., Pink, S., Patel, P., Wood, J., Murby, N., Woolger, D., Dawson, E., Mariani, C., Tierney, T. M., Mellor, S., O’Neill, G. C., Boto, E., Hill, R. M., Shah, V., Osborne, J., Pardington, R., Fierlinger, P., Barnes, G. R., Glover, P., Brookes, M. J., & Bowtell, R.
Affiliations
  • University of Nottingham (GB)
  • Wellcome Centre for Human Neuroimaging (GB)
  • QuSpin (United States) (US)
  • Technical University of Munich (DE)
Corresponding author
Holmes, Niall
ORCID
Venue
Scientific Reports
Publisher
Springer Science and Business Media LLC
Published
2022-08-09
Volume
12
Issue
1
Pages
13561-13561
ISSN
2045-2322 (ISSN-L)
DOI
10.1038/s41598-022-17346-1
PMC
PMC9363499
OpenAlex
W4291020559
Cited by
121
References
31
Citations by year
2026: 24 · 2025: 39 · 2024: 34 · 2023: 22 · 2022: 2
FWCI
9.95
Type
journal article
Language
en
Open access
Licence
Locations
  • Scientific Reports (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • Repository@Nottingham (University of Nottingham) (publishedVersion, other-oa)
  • Repository@Nottingham (University of Nottingham) (submittedVersion, cc-by)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
  • UCL Discovery (University College London) (submittedVersion)
  • mediaTUM – the media and publications repository of the Technical University Munich (Technical University Munich) (submittedVersion, other-oa)
Funders
  • Engineering and Physical Sciences Research Council (EP/T001046/1)
  • Innovate UK (104604)
  • Wellcome Trust (203257/Z/16/Z)
Topics
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Atomic and Subatomic Physics Research
  • Physical Sciences › Engineering › Electrical and Electronic Engineering › Electromagnetic Compatibility and Measurements
  • Physical Sciences › Engineering › Biomedical Engineering › Characterization and Applications of Magnetic Nanoparticles
Keywords
magnetically shielded room, active magnetic shielding, optically pumped magnetometers, mu-metal, electromagnetic coils
MeSH terms
Brain, Humans, Magnetic Resonance Imaging, Magnetic Resonance Spectroscopy, Magnetoencephalography, Functional Neuroimaging, Magnetic Fields

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