Design Methods of Bi-Planar Gradient Coils to Suppress the Ferromagnetic Boundary Coupling Inside the Magnetically Shielded Booth

The magnetically shielded booth (MSB), noted for its compactness and high-cost performance, offers a near-zero magnetic environment for magnetoencephalography (MEG) and magnetocardiography (MCG). However, the limited internal space inside the MSB, with large gradient distributions, is not conducive to enhance the performance of optically pumped atomic magnetometers (OPMs). Meanwhile, the actual MSB is a complex structure with ventilation and waveguide holes, such that the shielding layer is not equivalent to an ideal plane. The traditional image method (IM) for designing coils aiming to suppress the ferromagnetic boundary coupling is limited to eliminate the partial gradient field, which limits the accuracy of the active magnetic compensation system (AMCS) and is unable to achieve high signal-to-noise ratio (SNR). To mitigate these issues, the bi-planar gradient coil (BGC) has been introduced to eliminate the larger gradients. In this study, dB_x/dz and dB_z/dz coils were designed using the optimized multiple IM (O-MIM). The thickness of the magnetic shielding layer was considered to reduce the field gradient, which has the most influence on OPMs’ sensitivity. Experiments showed that the maximum gradient inhomogeneity error in the target region is less than 1.16%, which is consistent with simulation predictions. When active magnetic compensation system-bi-planar gradient coils (AMCS-BGCs) were operating, the maximum residual gradient ΔG values of the dB_x/dz and dB_z/dz coils designed by the O-MIM and IM were 0.8 and 12.1 nT/m and 0.7 and 16.2 nT/m, respectively, whereby a more uniform gradient and larger region of gradient field were obtained during MCG and MEG tests.