Magnetically Shielded Room Research Articles

Real-time, model-based magnetic field correction for moving, wearable MEG

Most neuroimaging techniques require the participant to remain still for reliable recordings to be made. Optically pumped magnetometer (OPM) based magnetoencephalography (OP-MEG) however, is a neuroimaging technique which can be used to measure neural signals during large participant movement (approximately 1 m) within a magnetically shielded room (MSR) (Boto et al., 2018; Seymour et al., 2021). Nevertheless, environmental magnetic fields vary both spatially and temporally and OPMs can only operate within a limited magnetic field range, which constrains participant movement. Here we implement real-time updates to electromagnetic coils mounted on-board of the OPMs, to cancel out the changing background magnetic fields. The coil currents were chosen based on a continually updating harmonic model of the background magnetic field, effectively implementing homogeneous field correction (HFC) in real-time (Tierney et al., 2021). During a stationary, empty room recording, we show an improvement in very low frequency noise of 24 dB. In an auditory paradigm, during participant movement of up to 2 m within a magnetically shielded room, introduction of the real-time correction more than doubled the proportion of trials in which no sensor saturated recorded outside of a 50 cm radius from the optimally-shielded centre of the room. The main advantage of such model-based (rather than direct) feedback is that it could allow one to correct field components along unmeasured OPM axes, potentially mitigating sensor gain and calibration issues (Borna et al., 2022).

Enabling ambulatory movement in wearable magnetoencephalography with matrix coil active magnetic shielding

The ability to collect high-quality neuroimaging data during ambulatory participant movement would enable a wealth of neuroscientific paradigms. Wearable magnetoencephalography (MEG) based on optically pumped magnetometers (OPMs) has the potential to allow participant movement during a scan. However, the strict zero magnetic field requirement of OPMs means that systems must be operated inside a magnetically shielded room (MSR) and also require active shielding using electromagnetic coils to cancel residual fields and field changes (due to external sources and sensor movements) that would otherwise prevent accurate neuronal source reconstructions. Existing active shielding systems only compensate fields over small, fixed regions and do not allow ambulatory movement. Here we describe the matrix coil, a new type of active shielding system for OPM-MEG which is formed from 48 square unit coils arranged on two planes which can compensate magnetic fields in regions that can be flexibly placed between the planes. Through the integration of optical tracking with OPM data acquisition, field changes induced by participant movement are cancelled with low latency (25 ms). High-quality MEG source data were collected despite the presence of large (65 cm translations and 270° rotations) ambulatory participant movements.

A Magnetic Compensation System Composed of Biplanar Coils Avoiding Coupling Effect of Magnetic Shielding

Magnetic compensation coils are often used to improve the effectiveness of a magnetically shielded room (MSR), but a distortion of the field is caused by the shielding layer consisting of high-permeability material, which leads to a reduction in compensation capacity. To solve this problem, a new method is presented in this article based on a target-field method, an image method, and particle swarm optimization. By this way, a magnetic compensation system composed of biplanar coils is designed and fabricated to compensate the background field for a wearable magnetoencephalography device inside the MSR, and the coupling effect between the magnetic shielding material and the compensation field is effectively avoided. Experimental data show that these compensation coils produce uniform fields and field gradients with the inhomogeneity of less than 3.8%, consistent with the simulation results. In addition, this system realizes a real-time compensation to the residual field and field gradient in the MSR.

Iterative Optimization Algorithm to Design Biplanar Coils for Dynamic Magnetoencephalography

In this article, an iterative optimization algorithm is proposed to design biplanar coils, which is used for dynamic magnetoencephalography to compensate for residual fields in the magnetic shielding room. The effects of magnetic shielding layers and plane’s side length on the uniformity are both considered for designing coils. The iterative calculation is used to minimize the side length of the coil plane. The biplanar coils with 1.3-m side length are designed, which consist of three homogeneous-field coils ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$B_{x}, B_{y},\; { \rm and }\; B_{z}\; { \rm coils }$</tex-math></inline-formula> ) and five gradient-field coils ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dB_{x}/dy, dB_{x}/dz, dB_{y}/dy, dB_{y}/dz,\; { \rm and }\; dB_{z}/dz\; { \rm coils }$</tex-math></inline-formula> ). The coil system can produce homogeneous and gradient fields within 1% error over the volume of 40 cm × 40 cm × 40 cm. Through active magnetic shielding, the central field inside the magnetic room is reduced from 7.56 to 0.17 nT, and standard deviation from the mean value in the target area falls from 1.366 to 0.177 nT. The dynamic auditory stimulation experiment proves that the biplanar coil system will improve the quality of the evoked signals.

Nested Magnetic Field Compensation With Regulated Coefficients for Bio-Magnetic Field Measurement

For a spin-exchange relaxation free (SERF) optically pumped magnetometer (OPM) that obtains its best performance in a zero magnetic field, an operation environment with approximately zero magnetic field is desired. This article proposes a nested magnetic field compensation system for a magnetic shielding room (MSR) to suppress the magnetic field. Two sets of coils, one set located outside of the MSR and the other one placed inside of the MSR, are combined in the system. The magnetic fields outside of the MSR are measured by tunneling magnetoresistance (TMR) sensors, while the fields inside the MSR are monitored by SERF OPMs. The two kinds of sensors complement each other in terms of measurement range and sensitivity. A nested two-stage proportional-integral (PI) closed-loop control strategy is employed to compensate for the magnetic field inside the MSR. The control parameters are regulated by the magnitude of the magnetic field to take advantage of the two kinds of sensors. The effect of the proposed system is experimentally tested. The three-axis field fluctuations are reduced by 97.52%, 98%, and 98.64% by the proposed method, respectively. The proposed method is compared with a conventional two-stage series PI control system. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> -axis magnetic field fluctuations of the conventional PI control system and the proposed system are 75.81 and 11.71 pT, respectively. The magnetic fluctuation of the proposed system is about 15.5% of the fluctuation of the conventional PI system. To validate the feasibility of the proposed magnetic field compensation system for ultra-weak bio-magnetic field measurement, magnetocardiography (MCG) and magnetoencephalography (MEG) signals are measured.

Design of Biplane Coils Based on Funnel Algorithm for Generating a Near-Zero Magnetic Environment in MSR

The biplane coils (BCs) based on the inverse-design method are crucial for creating the near-zero magnetic environment in magnetic shielding room (MSR), but multi-parameters optimization remains a key factor that limits the performance of the BCs. This study proposes a funnel algorithm (FA) to solve the issue of multi-parameter optimization in the design. The upper and lower limits of the parameters are dynamically adjusted based on the previous calculating step. By gradually reducing the upper and lower limits, we achieve the BCs with optimal performance. This method has been verified by both the theoretical analysis and experimental test. Compared with other methods, the designed coils show a higher space utilization ratio (SUR) and smaller relative errors, with the SUR improved from 1.6% to 3.75% and the relative errors all less than 2%. Based on the designed BCs, the total residual magnetic field is stabilized below 40.4 pT at the central point of the MSR, with the noise reaching the intrinsic noise of the flux-gate magnetometer in the frequency range of interest. The proposed method can significantly improve the performance of the BCs, thus reducing the space and construct cost of the MSR and promoting the magnetoencephalogram (MEG) measurement based on the quantum technology.

Research on the Design Method of Gradient Magnetic Field Coil Based on the Magnetic Shielding Room

Research on the Design Method of Gradient Magnetic Field Coil Based on the Magnetic Shielding Room

Design of Biplanar Nested Square Uniform Coil in Lightweight Magnetically Shielded Room for Magnetocardiography System

Design of Biplanar Nested Square Uniform Coil in Lightweight Magnetically Shielded Room for Magnetocardiography System

Simulation of Large-scale Magnetic Shielding Room for High-precision Vector Magnetometer Ground Test and Calibration System

Simulation of Large-scale Magnetic Shielding Room for High-precision Vector Magnetometer Ground Test and Calibration System

Simulation of Large-scale Magnetic Shielding Room for High-precision Vector Magnetometer Ground Test and Calibration System

Simulation of Large-scale Magnetic Shielding Room for High-precision Vector Magnetometer Ground Test and Calibration System

The very large n2EDM magnetically shielded room with an exceptional performance for fundamental physics measurements.

We present the magnetically shielded room (MSR) for the n2EDM experiment at the Paul Scherrer Institute, which features an interior cubic volume with each side of length 2.92m, thus providing an accessible space of 25 m3. The MSR has 87 openings of diameter up to 220mm for operating the experimental apparatus inside and an intermediate space between the layers for housing sensitive signal processing electronics. The characterization measurements show a remanent magnetic field in the central 1 m3 below 100 pT and a field below 600 pT in the entire inner volume, up to 4cm to the walls. The quasi-static shielding factor at 0.01Hz measured with a sinusoidal 2 μT peak-to-peak signal is about 100 000 in all three spatial directions and increases rapidly with frequency to reach 108 above 1Hz.

Accuracy Assessment of Simplified Computation of Active and Passive Magnetic Shielding for Optically Pumped Magnetometers

A low residual environmental magnetic field is required for the proper operation of most optically pumped magnetometers (OPMs). This is achieved using a combination of passive and active magnetic shielding. Passive magnetic shielding often uses multiple layers of highly permeable materials. A realistic two-layer magnetically shielded room (MSR) for biomagnetic measurements is numerically studied using the finite element method. A measured B-H characteristic of Mumetal is used, which is linearly extrapolated from the Rayleigh region (RR) into the low magnetic field range. This extension of the magnetization curve yields no significant differences compared to using a constant permeability value for the low magnetic field range. Furthermore, we model the MSR with only one shield and apply a simple analytical method of images (MOI). The MOI yields on average a 100-fold reduction in computation time. The relative difference of the magnetic flux density computed with MOI and finite element method (FEM) is smaller than < 1% at the center of a spherical region of interest (ROI) with a radius of 0.3 m desired from the point of magnetoencephalography (MEG) using OPMs. The achieved accuracy of the MOI makes it suitable for the optimization of active shielding coils.

A lightweight magnetically shielded room with active shielding

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.

FEA simulations for low-frequency multi-layer magnetically shielded rooms

This paper proposes using Finite Element Analysis (FEA) simulations to optimize the design structure for low-frequency Magnetically Shielded Rooms (MSRs). In constructing a multi-layer MSR, the different characteristics of the material and laminated structure will bring different levels of magnetic Shielding Effectiveness (SE). The theoretical SE of an MSR can be determined quickly. By using the method used in this paper, the ideal laminated material structure can be found without increasing the MSR construction cost. According to the simulation results and the actual MSR measurement results we built, the optimized MSR design structure can improve the SE by 13 dB. In the area where the external measurement magnetic field is 37 820 nT, the magnetic field in the MSR is as low as 28 nT, and the SE of the MSR is higher than 57.3 dB. The method proposed by this research can provide the theoretical basis for optimal design structure and the FEA simulation method for engineering practice, which can effectively improve the SE of shielded rooms and save the construction cost. The FEA simulations used in this paper can be obtained from the following URL: https://github.com/YuukiAsuna/-Finite-element-simulation-of-material-lamination-sequence.

Magnetic Enhancement-Based Multi-Objective Optimization Design of the Large-Scale High-Intensity Homogeneous Magnetic Field Coil System

The large-scale high-intensity homogeneous magnetic field is an important resource to perform magnetic immunity tests for large scientific facilities. Generally, a multi-coil system with large cross-sectional area is used to generate the required test field, which has high power loss and conductor mass. In this article, we propose a novel method that introduces magnetically shielded room (MSR) to enhance the coil magnetic field and reduce power loss. The analytical model of the coil system based on MSR is established through the multi-reflection image method. In addition, the multi-objective optimization model is brought forward based on the non-dominated sorting genetic algorithm (NSGA-II). Theoretical results indicate that the magnetic field can be significantly enhanced under the presence of MSR. Consequently, prominent reductions in total current by about 61.4% and in total power loss by 75.5%–81.7% are achieved. Finally, the effectiveness of the theoretical result is numerically verified by the finite-element analysis (FEA). The proposed method provides a novel and promising way to reduce the power loss and conductor mass of such high-intensity magnet systems and other similar applications.

Residual field suppression for magnetocardiography measurement inside a thin magnetically shielded room using bi-planar coil

For a magnetocardiography (MCG) system inside a magnetically shielded room (MSR), the residual field should be further suppressed to obtain high-quality MCG signals. In this paper, a compensation system has been developed based on the bi-planar coil and the proportional-integral-derivative (PID) controller. The bi-planar coil, derived from the target-field theory and the Tikhonov regularization method, is utilized to generate magnetic field with high uniformity in the pre-defined target region. And the PID controller is introduced to provide dynamic compensation current for the coil, according to the residual field change monitored by a reference SQUID magnetometer. The measurement results show that the noise suppression ratio (NSR) can reach above 20 dB in the low-frequency range from 0.1 Hz to 50 Hz. The DC component and the fluctuation of the residual field in time-domain can be respectively suppressed to 0 pT and 4 pT, indicating that this proposed compensation method is useful for the MCG measurement.

Prediction of Cylindrical Magnetic Shielding Performance by Considering the Magnetic Field Strength Inside the Material

Magnetoencephalography (MEG) is highly sensitive to even a small amount of magnetic noise, and thus, a high-performance magnetically shielded room (MSR) is essential. The shielding effect of a novel MSR should be predicted to verify whether it satisfies the required specifications of the MEG at the design stage. Therefore, an accurate prediction method based on the properties of shielding materials and the shape of the MSR is necessary. In this article, we proposed a prediction method using a simple formula and permeability measured against very small magnetic field fluctuations. Using a ring-shaped test piece, permeability was measured against a very small magnetic field corresponding to the Rayleigh region, and the initial permeability and Rayleigh constant were estimated to determine the permeability calculated by the formula. The magnetic field generated inside the material was estimated by magnetic analysis, and it was confirmed that the permeability of the Rayleigh region should be used in the formula. To verify the prediction accuracy, we examined a simple example of a cylindrical shell by comparing the calculated results with the experimental results. It was confirmed that the proposed method is

Calibration of SQUID Magnetometers in Multichannel MCG System Based on Bi-Planar Coil

In a practical multichannel magnetocardiography (MCG) system based on superconducting quantum interference device (SQUID) magnetometers, the Tesla/Volt coefficient must be precisely calibrated for accurate analysis of heart diseases. In this article, we utilize the biplanar coil to calibrate the SQUID magnetometers in an MCG system for the first time. The coil design theory is discussed in detail, including the target-field theory and the Tikhonov regularization method. Also, the coil winding patterns can be obtained after discretization of the stream function, which is related to the current density. By defining the parameter of calibration uncertainty, the performance of the biplanar coil is analyzed and compared with a rectangular Helmholtz coil based on the COMSOL simulation method and experimental measurement. The measurement results are well in coincidence with the simulation ones, indicating that the biplanar coil has remarkable advantages over the Helmholtz coil. Also, Tesla/Volt coefficient calibrations of four SQUID magnetometers in an MCG system are conducted inside a magnetically shielded room (MSR) depending on these two coils. The final results show the superiority of the biplanar coil method in MCG system calibration.

Magnetic field compensation coil design for magnetoencephalography

While optically pumped magnetometers (OPMs) can be attached to the head of a person and allow for highly sensitive recordings of the human magnetoencephalogram (MEG), they are mostly limited to an operational range of approximately 5 nT. Consequently, even inside a magnetically shielded room (MSR), movements in the remnant magnetic field disable the OPMs. Active suppression of the remnant field utilizing compensation coils is therefore essential. We propose 8 compensation coils on 5 sides of a cube with a side length of approximately 2 m which were optimized for operation inside an MSR. Compared to previously built bi-planar compensation coils, the coils proposed in this report are more complex in geometry and achieved smaller errors for simulated compensation fields. The proposed coils will allow for larger head movements or smaller movement artifacts in future MEG experiments compared to existing coils.

A Study of Enclosed Magnetic Shielding Room By Simulation

The magnetic shielding room (MSR), which is mainly composed of permalloy and aluminum layer, is widely used in the measurement of near-zero magnetic environment. This paper mainly uses ANSYS Maxwell software to simulate MSR and improve its design method. The shielding effectiveness (SE) is introduced to evaluate its performance. The experimental data show that the shielding parameter SE has a best value under certain conditions. SE is inversely proportional to the size of the shielding space, and is proportional to the thickness of each layer, and there is an optimal value for the gap between the two layers. This equivalent circuit to the MSR can be explained well.