title: Effect of magnetic field fluctuation on ultra-low field MRI measurements in the unshielded laboratory environment.
abstract: Magnetic field fluctuations in our unshielded urban laboratory can reach hundreds of nT in the noisy daytime and is only a few nT in the quiet midnight. The field fluctuation causes the Larmor frequency fL to drift randomly for several Hz during the unshielded ultra-low field (ULF) nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) measurements, thus seriously spoiling the averaging effect and causing imaging artifacts. By using an active compensation (AC) technique based on the spatial correlation of the low-frequency magnetic field fluctuation, the field fluctuation can be suppressed to tens of nT, which is a moderate situation between the noisy daytime and the quiet midnight. In this paper, the effect of the field fluctuation on ULF MRI measurements was investigated. The 1D and 2D MRI signals of a water phantom were measured using a second-order low-Tc superconducting quantum interference device (SQUID) in three fluctuation cases: severe fluctuation (noisy daytime), moderate fluctuation (daytime with AC) and minute fluctuation (quiet midnight) when different gradient fields were applied. When the active compensation is applied or when the frequency encoding gradient field Gx reaches a sufficiently strong value in our measurements, the image artifacts become invisible in all three fluctuation cases. Therefore it is feasible to perform ULF-MRI measurements in unshielded urban environment without imaging artifacts originating from magnetic fluctuations by using the active compensation technique and/or strong gradient fields.
authors.affiliation: State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), Shanghai 200050, China; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany.
authors.forename: Chao
authors.initials: C
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), Shanghai 200050, China; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany.
authors.forename: Baolin
authors.initials: B
authors.lastname: Chang
authors.affiliation: State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), Shanghai 200050, China; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany.
authors.forename: Longqing
authors.initials: L
authors.lastname: Qiu
authors.affiliation: State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), Shanghai 200050, China; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany. Electronic address: donghui@mail.sim.ac.cn.
authors.forename: Hui
authors.initials: H
authors.lastname: Dong
authors.affiliation: State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), Shanghai 200050, China; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany.
authors.forename: Yang
authors.initials: Y
authors.lastname: Qiu
authors.affiliation: Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich (FZJ), D-52425 Jülich, Germany; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany.
authors.forename: Yi
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich (FZJ), D-52425 Jülich, Germany; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany.
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Krause
authors.affiliation: Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich (FZJ), D-52425 Jülich, Germany; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Offenhäusser
authors.affiliation: State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), Shanghai 200050, China; Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai, Shanghai 200050, China and FZJ, D-52425 Jülich, Germany.
authors.forename: Xiaoming
authors.initials: X
authors.lastname: Xie
keywords: Magnetic field fluctuation; Ultra-low field MRI; dc SQUID