title: Multi-directional anisotropy from diffusion orientation distribution functions.
abstract: OBJECTIVE
  To evaluate a model-independent, multi-directional anisotropy (MDA) metric that is analytically and experimentally equivalent to fractional anisotropy (FA) in single-direction diffusivity, but potentially superior to FA in its sensitivity to the underlying anisotropy of multi-directional diffusivity.
  
  
  METHODS
  An expression for MDA was defined from the orientation distribution function (ODF) and its analytical relation to FA was derived. Simulations of single and crossed double-fibers were performed using a compressed-sensing-accelerated diffusion-spectrum-imaging (CS-DSI) scheme. In vivo brain imaging using CS-DSI was performed on eight healthy subjects. MDA was compared with FA and with another ODF-based metric known as generalized FA (GFA).
  
  
  RESULTS
  In simulated single-direction fibers, MDA was shown to be equivalent to FA (from FA = 0.2 to 0.8). In crossed fibers, MDA provided superior differentiation of the underlying anisotropy as compared to FA and GFA. In vivo analysis shows that the MDA was superior to both FA (P = 0.015) and GFA (P = 0.021) in terms of its relative accuracy in crossed fiber regions.
  
  
  CONCLUSIONS
  MDA provides a potentially superior measure of fiber anisotropy relative to conventional FA or GFA, and may be used to improve the assessment of disease in regions with multi-directional brain fibers.
authors.affiliation: GE Global Research, Niskayuna, New York, USA.
authors.forename: Ek T
authors.initials: ET
authors.lastname: Tan
authors.forename: Luca
authors.initials: L
authors.lastname: Marinelli
authors.forename: Jonathan I
authors.initials: JI
authors.lastname: Sperl
authors.forename: Marion I
authors.initials: MI
authors.lastname: Menzel
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Hardy
mesh_terms: D000328:Adult; D000368:Aged; D016880:Anisotropy; D001921:Brain / Q000502:physiology*; D056324:Diffusion Tensor Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009413:Nerve Fibers, Myelinated / Q000502:physiology; D012016:Reference Values; D015203:Reproducibility of Results
keywords: diffusion imaging; diffusion spectrum imaging; diffusion tensor imaging; fractional anisotropy; multi-directional anisotropy