Ultra-low field magnetic resonance imaging (ULF-MRI) using an optically pumped magnetometer (OPM) as a receiver is expected to lower the cost of implementation and maintenance of MRI scanner1. It is also promising to integrate with Magnetoencephalography (MEG) and ultra-low field functional MRI (ULF-fMRI) for multimodal brain function measurements. Assuming integration with MEG, the static magnetic field of ultra-low-field fMRI should be generated by a resistive electromagnet that can be turned on and off.
In this study, we carried out numerical simulations and optimization of magnetic fields generated by a magnet consisting of multiple circular coil pairs for the OPM-based ULF-MRI scanner. In order to achieve a wide field uniformity (homogeneity) at the center of the magnet, the number of turns in each coil was set to the optimal value by solving linear equations, while the radius and position in each coil were searched based on Bayesian optimization. Without reducing the size of the internal space of the multi-coil magnet, the size of the uniform region defined as the variation of less than 100 ppm from the center of the magnet could be enlarged about 2 to 3 times in both axial and radial directions for those in the Helmholtz coil. In addition, the spread of the uniform region was the same in the axial and radial directions, making the diameter spherical volume (DSV) more spherical than that of the Helmholtz coil.
These results suggest that the newly designed magnet and optimization method are promising for the development of the OPM-based compact MRI scanner.