Mapping magnetic susceptibility anisotropies of white matter in vivo in the human brain at 7T

Xu Li, Deepti S. Vikram, Issel Anne L. Lim, Craig K. Jones, Jonathan A.D. Farrell, Peter C.M. van Zijl

Research output: Contribution to journalArticlepeer-review

Abstract

High-resolution magnetic resonance phase- or frequency-shift images acquired at high field show contrast related to magnetic susceptibility differences between tissues. Such contrast varies with the orientation of the organ in the field, but the development of quantitative susceptibility mapping (QSM) has made it possible to reproducibly image the intrinsic tissue susceptibility contrast. However, recent studies indicate that magnetic susceptibility is anisotropic in brain white matter and, as such, needs to be described by a symmetric second-rank tensor (χ--). To fully determine the elements of this tensor, it would be necessary to acquire frequency data at six or more orientations. Assuming cylindrical symmetry of the susceptibility tensor in myelinated white matter fibers, we propose a simplified method to reconstruct the susceptibility tensor in terms of a mean magnetic susceptibility, MMS=(χ//+2χ⊥)/3 and a magnetic susceptibility anisotropy, MSA=χ //, where χ // and χ are susceptibility parallel and perpendicular to the white matter fiber direction, respectively. Computer simulations show that with a practical head rotation angle of around 20°-30°, four head orientations suffice to reproducibly reconstruct the tensor with good accuracy. We tested this approach on whole brain 1×1×1mm 3 frequency data acquired from five healthy subjects at 7T. The frequency information from phase images collected at four head orientations was combined with the fiber direction information extracted from diffusion tensor imaging (DTI) to map the white matter susceptibility tensor. The MMS and MSA were quantified for regions in several large white matter fiber structures, including the corona radiata, posterior thalamic radiation and corpus callosum. MMS ranged from -0.037 to -0.053ppm (referenced to CSF being about zero). MSA values could be quantified without the need for a reference and ranged between 0.004 and 0.029ppm, in line with the expectation that the susceptibility perpendicular to the fiber is more diamagnetic than the one parallel to it.

Original languageEnglish (US)
Pages (from-to)314-330
Number of pages17
JournalNeuroImage
Volume62
Issue number1
DOIs
StatePublished - Aug 1 2012

Keywords

  • Diffusion tensor imaging
  • High field
  • Magnetic susceptibility anisotropy
  • Mean magnetic susceptibility
  • Susceptibility tensor imaging
  • White matter

ASJC Scopus subject areas

  • Neurology
  • Cognitive Neuroscience

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