title: Real-time feedback for spatiotemporal field stabilization in MR systems.
abstract: OBJECTIVE
  MR imaging and spectroscopy require a highly stable, uniform background field. The field stability is typically limited by hardware imperfections, external perturbations, or field fluctuations of physiological origin. The purpose of the present work is to address these issues by introducing spatiotemporal field stabilization based on real-time sensing and feedback control.
  
  
  METHODS
  An array of NMR field probes is used to sense the field evolution in a whole-body MR system concurrently with regular system operation. The field observations serve as inputs to a proportional-integral controller that governs correction currents in gradient and higher-order shim coils such as to keep the field stable in a volume of interest.
  
  
  RESULTS
  The feedback system was successfully set up, currently reaching a minimum latency of 20 ms. Its utility is first demonstrated by countering thermal field drift during an EPI protocol. It is then used to address respiratory field fluctuations in a T2 *-weighted brain exam, resulting in substantially improved image quality.
  
  
  CONCLUSIONS
  Feedback field control is an effective means of eliminating dynamic field distortions in MR systems. Third-order spatial control at an update time of 100 ms has proven sufficient to largely eliminate thermal and breathing effects in brain imaging at 7 Tesla.
authors.affiliation: Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Switzerland.
authors.forename: Yolanda
authors.initials: Y
authors.lastname: Duerst
authors.forename: Bertram J
authors.initials: BJ
authors.lastname: Wilm
authors.forename: Benjamin E
authors.initials: BE
authors.lastname: Dietrich
authors.forename: S Johanna
authors.initials: SJ
authors.lastname: Vannesjo
authors.forename: Christoph
authors.initials: C
authors.lastname: Barmet
authors.forename: Thomas
authors.initials: T
authors.lastname: Schmid
authors.forename: David O
authors.initials: DO
authors.lastname: Brunner
authors.forename: Klaas P
authors.initials: KP
authors.lastname: Pruessmann
mesh_terms: D001921:Brain / Q000033:anatomy & histology*; D003199:Computer Systems; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005246:Feedback; D007089:Image Enhancement / Q000295:instrumentation* / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D062211:Spatio-Temporal Analysis
keywords: 7 Tesla; NMR field probes; T2*-weighted imaging; dynamic shimming; field camera; field control