title: Single-shot imaging with higher-dimensional encoding using magnetic field monitoring and concomitant field correction.
abstract: OBJECTIVE
  PatLoc (Parallel Imaging Technique using Localized Gradients) accelerates imaging and introduces a resolution variation across the field-of-view. Higher-dimensional encoding employs more spatial encoding magnetic fields (SEMs) than the corresponding image dimensionality requires, e.g. by applying two quadratic and two linear spatial encoding magnetic fields to reconstruct a 2D image. Images acquired with higher-dimensional single-shot trajectories can exhibit strong artifacts and geometric distortions. In this work, the source of these artifacts is analyzed and a reliable correction strategy is derived.
  
  
  METHODS
  A dynamic field camera was built for encoding field calibration. Concomitant fields of linear and nonlinear spatial encoding magnetic fields were analyzed. A combined basis consisting of spherical harmonics and concomitant terms was proposed and used for encoding field calibration and image reconstruction.
  
  
  RESULTS
  A good agreement between the analytical solution for the concomitant fields and the magnetic field simulations of the custom-built PatLoc SEM coil was observed. Substantial image quality improvements were obtained using a dynamic field camera for encoding field calibration combined with the proposed combined basis.
  
  
  CONCLUSIONS
  The importance of trajectory calibration for single-shot higher-dimensional encoding is demonstrated using the combined basis including spherical harmonics and concomitant terms, which treats the concomitant fields as an integral part of the encoding.
authors.affiliation: Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany.
authors.forename: Frederik
authors.initials: F
authors.lastname: Testud
authors.forename: Daniel
authors.initials: D
authors.lastname: Gallichan
authors.forename: Kelvin J
authors.initials: KJ
authors.lastname: Layton
authors.forename: Christoph
authors.initials: C
authors.lastname: Barmet
authors.forename: Anna M
authors.initials: AM
authors.lastname: Welz
authors.forename: Andrew
authors.initials: A
authors.lastname: Dewdney
authors.forename: Chris A
authors.initials: CA
authors.lastname: Cocosco
authors.forename: Klaas P
authors.initials: KP
authors.lastname: Pruessmann
authors.forename: Jürgen
authors.initials: J
authors.lastname: Hennig
authors.forename: Maxim
authors.initials: M
authors.lastname: Zaitsev
mesh_terms: D000465:Algorithms; D016477:Artifacts; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000379:methods*; D011829:Radiation Dosage; D011874:Radiometry; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
keywords: Maxwell terms; concomitant fields; dynamic field camera; field probes; higher-dimensional trajectories; local k-space, PatLoc; magnetic field monitoring; non-Fourier encoding; nonlinear spatial encoding; quadratic fields; rapid imaging; single-shot imaging