Toward ultra-low field multimodal MRI with atomic magnetometer

In recent years, optically pumped atomic magnetometers (OPAMs) operating under spin-exchange relaxation-free conditions have reached sensitivities comparable to and even surpassing those of magnetometers based on superconducting quantum interference devices (SQUIDs). We have been developing a high-sensitivity OPAM as a magnetic sensor to measure biomagnetic fields and magnetic resonance (MR) signals. Recently, an ultra-low field (ULF) MRI system with an OPAM has been proposed. Since OPAM does not require cryogenic cooling, it allows easily to measure extremely small magnetic fields. In this paper, we describe principles of OPAM and results of biomagnetic field measurements with it. To test the performance of our newly developed OPAM, we made a phantom that models neuronal currents in the brain and measured tiny magnetic field distributions. The results of magnetic field distributions with the phantom scanning two-dimensionally above the magnetometer showed good agreement with theoretical calculations. In addition, we demonstrated measurements of human magnetocardiograms with our OPAM. Finally, we describe feasibility of OPAMs as magnetic sensors for measuring magnetoencephalograms and MRI signals simultaneously toward ULF multimodal MRI systems.