P 208 A K-Rb hybrid Optically Pumped Atomic Magnetometer toward Ultra-low field Multimodal MRI Systems

Abstract

To contribute for improvement of longevity and quality of human life, we have been developing optically pumped atomic magnetometers (OPAMs) to measure tiny biomagnetic fields and MRIs. In recent years, OPAMs have reached sensitivities comparable to and even surpassing those of magnetometers based on super-conducting quantum interference devices (SQUIDs) [1] . In addition, OPAMs have the intrinsic advantage of not requiring cryogenic cooling. Meanwhile, MRI is one of the most useful diagnostic imaging modalities, which enables visualization of the anatomy and function of the human. However, the conventional high magnetic field scanner has some limitations such as high cost and risk for patients with metal implants. To acquire MRIs without these limitations, ultra-low filed (ULF) MRI systems attracted attention in recent years. We have developed an OPAM using a hybrid cell of K and Rb atoms [2] toward ULF multimodal MRI systems. The objective of this study is to investigate the optimal properties of the OPAM theoretically comparing with experiment results. The sensor head of the OPAM used in this study was a cubic Pyrex glass cell, whose size was 3 x 3 x 3 cm 3 and within which K and Rb atoms were enclosed with He and N 2 as buffer gases at a ratio of 10 to 1 and at a total pressure of 150 kPa at room temperature. The Rb atoms were spin-polarized by a circularly-polarized pump beam and the spin polarization was transferred to the K atoms by spin exchange collisions. The spin polarization rotated around the external magnetic field orthogonal to both the pump and probe beams. Here, the plane of a linearly-polarized probe beam penetrating the group of spin-polarized atoms is rotated by Faraday effect. We applied a 100 Hz sinusoidal magnetic field of 48 pT as a test signal and examined sensitivities of the OPAM We theoretically investigated the properties of the hybrid OPAM and considered the adequacy of the properties comparing with experiment results. We have found that the experimental results agree well with the theoretical values. The optimum density ratio of K and Rb atoms was expected to be 10-100 for more sensitive magnetometers. We could measure of human magnetocardiograms (MCGs) and plan to measure MR signals using the hybrid OPAM. We developed a K-Rb hybrid OPAM and examined its sensitivities. Results shown in this study demonstrate the feasibility of the OPAM as a magnetic sensor toward ULF multimodal MRI systems. 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.