title: Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry.
abstract: OBJECTIVE
  MR thermometry (MRT) is a noninvasive method for measuring temperature that can potentially be used for radio frequency (RF) safety monitoring. This application requires measuring absolute temperature. In this study, a multigradient-echo (mGE) MRT sequence was used for that purpose. A drawback of this sequence, however, is that its accuracy is affected by background gradients. In this article, we present a method to minimize this effect and to improve absolute temperature measurements using MRI.
  
  
  BACKGROUND
  By determining background gradients using a B0 map or by combining data acquired with two opposing readout directions, the error can be removed in a homogenous phantom, thus improving temperature maps.
  
  
  METHODS
  All scans were performed on a 3T system using ethylene glycol-filled phantoms. Background gradients were varied, and one phantom was uniformly heated to validate both compensation approaches. Independent temperature recordings were made with optical probes.
  
  
  RESULTS
  Errors correlated closely to the background gradients in all experiments. Temperature distributions showed a much smaller standard deviation when the corrections were applied (0.21°C vs. 0.45°C) and correlated well with thermo-optical probes.
  
  
  CONCLUSIONS
  The corrections offer the possibility to measure RF heating in phantoms more precisely. This allows mGE MRT to become a valuable tool in RF safety assessment.
authors.affiliation: Department of Radiotherapy, Imaging Division, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Frank F J
authors.initials: FF
authors.lastname: Simonis
authors.forename: Esben T
authors.initials: ET
authors.lastname: Petersen
authors.forename: Lambertus W
authors.initials: LW
authors.lastname: Bartels
authors.forename: Jan J W
authors.initials: JJ
authors.lastname: Lagendijk
authors.forename: Cornelis A T
authors.initials: CA
authors.lastname: van den Berg
mesh_terms: D000465:Algorithms; D016477:Artifacts; D017352:Echo-Planar Imaging / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D060526:Magnetic Fields; D017711:Nonlinear Dynamics; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D013817:Thermography / Q000379:methods*
keywords: MR thermometry; RF safety monitoring; absolute temperature; background gradient; multigradient-echo