title: Localization errors in MR spectroscopic imaging due to the drift of the main magnetic field and their correction.
abstract: OBJECTIVE
  To analyze the effect of B0 field drift on multivoxel MR spectroscopic imaging and to propose an approach for its correction.
  
  
  METHODS
  It is shown, both theoretically and in a phantom, that for ∼30 min acquisitions a linear B0 drift (∼0.1 ppm/h) will cause localization errors that can reach several voxels (centimeters) in the slower varying phase encoding directions. An efficient and unbiased estimator is proposed for tracking the drift by interleaving short (∼ T2*), nonlocalized acquisitions on the nonsuppressed water each pulse repetition time, as shown in 10 volunteers at 1.5 and 3 T.
  
  
  RESULTS
  The drift is shown to be predominantly linear in both the phantom and volunteers at both fields. The localization errors are observed and quantified in both phantom and volunteers. The unbiased estimator is shown to reliably track the instantaneous frequency in vivo despite only using a small portion of the FID.
  
  
  CONCLUSIONS
  Contrary to single-voxel MR spectroscopy, where it leads to line broadening, field drift can lead to localization errors in the longer chemical shift imaging experiments. Fortunately, this drift can be obtained at a negligible cost to sequence timing, and corrected for in post processing.
authors.affiliation: Department of Radiology, New York University School of Medicine, 660 First Avenue, New York, New York, USA.
authors.forename: Assaf
authors.initials: A
authors.lastname: Tal
authors.forename: Oded
authors.initials: O
authors.lastname: Gonen
mesh_terms: D000465:Algorithms; D016477:Artifacts; D003198:Computer Simulation; D004867:Equipment Design; D019544:Equipment Failure Analysis; D060526:Magnetic Fields; D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation* / Q000379:methods*; D008962:Models, Theoretical; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*