title: Efficient concomitant and remanence field artifact reduction in ultra-low-field MRI using a frequency-space formulation.
abstract: OBJECTIVE
  For ultra-low-field MRI, the spatial-encoding magnetic fields generated by gradient coils can have strong concomitant fields leading to prominent image distortion. Additionally, using superconducting magnet to pre-polarize magnetization can improve the signal-to-noise ratio of ultra-low-field MRI. Yet the spatially inhomogeneous remanence field due to the permanently trapped flux inside a superconducting pre-polarizing coil modulates magnetization and causes further image distortion.
  
  
  METHODS
  We propose a two-stage frequency-space (f-x) formulation to accurately describe the dynamics of spatially-encoded magnetization under the influence of concomitant and remanence fields, which allows for correcting image distortion due to concomitant and remanence fields.
  
  
  RESULTS
  Our method is computationally efficient as it uses a combination of the fast Fourier transform algorithm and a linear equation solver. With sufficiently dense discretization in solving the linear equation, the performance of this f-x method was found to be stable among different choices of the regularization parameter and the regularization matrix.
  
  
  CONCLUSIONS
  We present this method together with numerical simulations and experimental data to demonstrate how concomitant and remanence field artifacts in ultra-low-field MRI can be corrected efficiently.
authors.affiliation: Department of Mathematics, National Taiwan University, Taipei, Taiwan; Department of Biomedical Engineering and Computational Science, Aalto University School of Science, Espoo, Finland.
authors.forename: Yi-Cheng
authors.initials: YC
authors.lastname: Hsu
authors.forename: Panu T
authors.initials: PT
authors.lastname: Vesanen
authors.forename: Jaakko O
authors.initials: JO
authors.lastname: Nieminen
authors.forename: Koos C J
authors.initials: KC
authors.lastname: Zevenhoven
authors.forename: Juhani
authors.initials: J
authors.lastname: Dabek
authors.forename: Lauri
authors.initials: L
authors.lastname: Parkkonen
authors.forename: I-Liang
authors.initials: IL
authors.lastname: Chern
authors.forename: Risto J
authors.initials: RJ
authors.lastname: Ilmoniemi
authors.forename: Fa-Hsuan
authors.initials: FH
authors.lastname: Lin
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001921:Brain / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted