title: Monitoring, analysis, and correction of magnetic field fluctuations in echo planar imaging time series.
abstract: OBJECTIVE
  To assess the utility of concurrent magnetic field monitoring for observing and correcting for variations in k-space trajectories and global background fields that occur in single-shot echo planar imaging (EPI) time series as typically used in functional MRI (fMRI).
  
  
  METHODS
  Field monitoring was performed using an array of NMR field probes operated concurrently with series of single-shot EPI acquisitions from a static phantom. The observed fluctuations in field evolution were analyzed in terms of their temporal and spatial behavior at the field level as well as at the level of reconstructed image series. The potential to correct for such fluctuations was assessed by accounting for them upon image reconstruction. An indication of the number and relative magnitude of underlying effects was obtained via principal component analysis.
  
  
  RESULTS
  Trajectory and global field variations were found to induce substantial image fluctuations. Global field fluctuations induced standard deviations in image intensity up to 31%. Fluctuations in the trajectory induced ghosting artifacts with standard deviations up to 2%. Concurrent magnetic field monitoring reduced the fluctuations in the EPI time series to a maximum of 1.2%.
  
  
  CONCLUSIONS
  Concurrent magnetic field monitoring holds the potential to improve the net sensitivity of fMRI by reducing signal fluctuations unrelated to brain activity.
authors.affiliation: Institute of Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland.|Translational Neuromodeling Unit, Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland.
authors.forename: Lars
authors.initials: L
authors.lastname: Kasper
authors.affiliation: Institute of Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland.
authors.forename: Saskia
authors.identifier: http://orcid.org/0000-0001-8242-8008
authors.initials: S
authors.lastname: Bollmann
authors.affiliation: Institute of Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland.
authors.forename: S Johanna
authors.initials: SJ
authors.lastname: Vannesjo
authors.affiliation: Institute of Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland.
authors.forename: Simon
authors.initials: S
authors.lastname: Gross
authors.affiliation: Institute of Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland.
authors.forename: Maximilian
authors.identifier: http://orcid.org/0000-0002-2211-5568
authors.initials: M
authors.lastname: Haeberlin
authors.affiliation: Institute of Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland.
authors.forename: Benjamin E
authors.initials: BE
authors.lastname: Dietrich
authors.affiliation: Institute of Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland.
authors.forename: Klaas P
authors.initials: KP
authors.lastname: Pruessmann
mesh_terms: D016477:Artifacts; D001931:Brain Mapping / Q000295:instrumentation* / Q000379:methods*; D017352:Echo-Planar Imaging / Q000295:instrumentation* / Q000379:methods*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005246:Feedback; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D060526:Magnetic Fields; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013382:Subtraction Technique
keywords: PCA; SFNR; fMRI; gradients; noise; thermal drifts