Actively Shielded Bias Field Tuning Coil for Optically Pumped Atomic Magnetometer Toward Ultralow Field MRI

In this paper, we describe a method for directly detecting magnetic resonance (MR) signals using optically pumped atomic magnetometers (OPAMs) in ultra-low-field MR imaging (ULF-MRI). For direct detection with OPAMs in ULF-MRI, we propose the use of an actively shielded bias field tuning coil to match the resonant frequencies between MR signals and OPAM. Design of the tuning coil was based on both a target field approach and a stream function method. The results of field distribution analyses demonstrated that the resonant frequency of an OPAM could be tuned by the actively shielded bias field tuning coil. In addition, the actively shielded bias field tuning coil minimized the distortion of an external magnetic field in the sample region. These results indicate that actively shielded bias field tuning coil can be designed to match the resonant frequencies of MR signal and an OPAM with the inhomogeneity of sample region <;0.25%. However, the actively shielded bias field tuning coil must be fabricated with an error of 0.5 mm or less between the centers of the primary and shield coils.