In recent years, it was demonstrated that optically pumped magnetometers (OPMs) using alkali-metal-vapors operating under spin-exchange relaxation-free (SERF) condition have reached sensitivities comparable to and even surpassing those of SQUIDs. The OPM is suitable to be used as a receiving sensor for NMR signals at ultra-low field (ULF) of 10 mT or less.
In 2017, we succeeded to reconstruct the first MR images obtained at a Larmor frequency of 5 kHz by using a prototype of ULF-MRI scanner with a pump-probe-type OPM [1]. In 2022, for the first time, we reported a magnetic shieldless ULF-MRI scanner with the OPM combined with a flux transformer operating at a Larmor frequency of 300 kHz (B0 = 7.05 mT) [2].
Meanwhile, we also reported the possibility to detect neural magnetic field dependent (NMFD) changes in MR signals as a new fMRI principle toward direct measurements of neural activities and functional connectivity as well [3]. The measurement principle of the NMFD-fMRI might be applicable not only conventional MRI scanners but also the OPM-based ULF-MRI scanner.
After describing principles of the pump-probe-type OPM, we introduce the newly developed OPM-based ULF-MRI scanner. Finally, we discuss how much B0 can be lowered to reconstruct MR images with sufficient S/N. We believe that the applicability of an affordable OPM-based ULF-MRI scanner might provide important advancements in neuroscience and improve the clinical diagnosis of neurological and psychiatric disorders as well.
[1] I. Hilschenz, T. Kobayashi, et al., JMR, 274, 89-94 (2017)
[2] S. Hori, T. Kobayashi, et al., JMR, 343, 107280, (2022)
[3] Y. Ito, M. Ueno and T. Kobayashi, Scientific Reports, 10, 5463 (2020)