title: A geometrical shift results in erroneous appearance of low frequency tissue eddy current induced phase maps.
abstract: Knowledge on low frequency (LF) tissue conductivity is relevant for various biomedical purposes. To obtain this information, LF phase maps arising from time-varying imaging gradients have been demonstrated to create a LF conductivity contrast. Essential in this methodology is the subtraction of phase images acquired with opposite gradient polarities to separate LF and RF phase effects. Here we demonstrate how sensitive these subtractions are with respect to geometrical distortions.
  
  
  
  The effect of geometrical distortions on LF phase maps is mathematically defined. After quantifying typical geometrical distortions, their effects on LF phase maps are evaluated using conductive phantoms. For validation, electromagnetic simulations of LF phase maps were performed.
  
  
  
  Even sub-voxel distortions of 10% of the voxel size, measured for a typical LF MR sequence, cause leakage of RF phase into LF phase of several milli-radians, leading to a misleading pattern of LF phase maps. This leakage is mathematically confirmed, while simulations indicate that the expected LF phase should be in order of micro-radians.
  
  
  
  The conductivity scaling of LF phase maps is attributable to the RF phase leakage, thus dependent on the RF conductivity. In fact, simulations show that the LF phase is not measurable. Magn Reson Med 76:905-912, 2016. © 2015 Wiley Periodicals, Inc.
authors.affiliation: Center for Image Sciences, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
authors.forename: Stefano
authors.initials: S
authors.lastname: Mandija
authors.affiliation: Department of Radiotheraphy, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
authors.forename: Astrid L H M W
authors.initials: AL
authors.lastname: van Lier
authors.affiliation: Philips Research Europe-Hamburg, Roentgenstr 24-26, 22335, Hamburg, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Katscher
authors.affiliation: Department of Psychiatry, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
authors.forename: Petar I
authors.initials: PI
authors.lastname: Petrov
authors.affiliation: Department of Psychiatry, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
authors.forename: Sebastian F W
authors.initials: SF
authors.lastname: Neggers
authors.affiliation: Center for Image Sciences, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.|Department of Radiology, Imaging Division, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
authors.forename: Peter R
authors.initials: PR
authors.lastname: Luijten
authors.affiliation: Center for Image Sciences, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.|Department of Radiotheraphy, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
authors.forename: Cornelis A T
authors.initials: CA
authors.lastname: van den Berg
mesh_terms: D000818:Animals; D016477:Artifacts; D003198:Computer Simulation; D004553:Electric Conductivity; D004574:Electromagnetic Fields; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008954:Models, Biological; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods*; D012542:Scattering, Radiation
keywords: EPT; conductivity; eddy currents; geometrical distortions; low frequency