title: Effects of white matter microstructure on phase and susceptibility maps.
abstract: OBJECTIVE
  To investigate the effects on quantitative susceptibility mapping (QSM) and susceptibility tensor imaging (STI) of the frequency variation produced by the microstructure of white matter (WM).
  
  
  METHODS
  The frequency offsets in a WM tissue sample that are not explained by the effect of bulk isotropic or anisotropic magnetic susceptibility, but rather result from the local microstructure, were characterized for the first time. QSM and STI were then applied to simulated frequency maps that were calculated using a digitized whole-brain, WM model formed from anatomical and diffusion tensor imaging data acquired from a volunteer. In this model, the magnitudes of the frequency contributions due to anisotropy and microstructure were derived from the results of the tissue experiments.
  
  
  RESULTS
  The simulations suggest that the frequency contribution of microstructure is much larger than that due to bulk effects of anisotropic magnetic susceptibility. In QSM, the microstructure contribution introduced artificial WM heterogeneity. For the STI processing, the microstructure contribution caused the susceptibility anisotropy to be significantly overestimated.
  
  
  CONCLUSIONS
  Microstructure-related phase offsets in WM yield artifacts in the calculated susceptibility maps. If susceptibility mapping is to become a robust MRI technique, further research should be carried out to reduce the confounding effects of microstructure-related frequency contributions.
authors.affiliation: Sir Peter Mansfield Magnetic Resonance Centre, School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, United Kingdom.
authors.forename: Samuel
authors.initials: S
authors.lastname: Wharton
authors.forename: Richard
authors.initials: R
authors.lastname: Bowtell
mesh_terms: D016477:Artifacts; D056324:Diffusion Tensor Imaging / Q000379:methods*; D017097:Electric Impedance; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D066298:In Vitro Techniques; D060526:Magnetic Fields; D009900:Optic Nerve / Q000166:cytology* / Q000502:physiology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D066127:White Matter / Q000166:cytology* / Q000502:physiology*
keywords: Quantitative susceptibility mapping; anisotropy; gradient echo MRI; microstructure; phase imaging; white matter