We have been developing optically pumped atomic magnetometers (OPAMs) for the measurements of biomagnetic and NMR/MRI signals. In recent years, OPAMs using alkali metal vapors have reached sensitivities comparable to and even surpassing those of superconducting quantum interference devices (SQUIDs). The most sensitive atomic magnetometer has sensitivity in the sub-femtotesla range. Meanwhile, ultra-low filed (ULF) MRI has attracted attention because of its low running cost, smaller size, lighter weight, portability, and minimum patient exposure. In ULF-MRI, because of the low frequency of the MR signals, it is difficult to detect the MR signals with conventional pickup coils used in high field MRI. In contrast, the OPAMs are highly sensitive in low frequency range. In recent years, the studies on ULF-MRI with OPAMs or SQUIDs have progressed1 . We developed and reported a portable potassium OPAM module using an electrically heated oven and orthogonal pump and probe beams coupled from polarization-maintaining optical fibers2 . In the module, the optical beam path was folded along the magnetic field to be measured. In this study, we tried to measure free induction decay (FID) NMR signal with the combination of the described portable potassium OPAM module and a flux transformer (FT). We could successfully detect proton FID NMR signal under the static magnetic field (B0) of 234.874 microtesla (Larmor frequency of 10 kHz). This is the first report of NMR signal detection with a portable OPAM module. These results demonstrate the feasibility of the OPAM module as a magnetic sensor toward ULF multimodal MRI systems.
Acknowledgements: This work is partly supported by Grant-in-Aid for Researches (No. 24240081 & 24650221) and the Innovative Techno-Hub for Integrated Medical Bio-imaging of the Project for Developing Innovation Systems, from MEXT, Japan.