title: Effects of gradient encoding and number of signal averages on fractional anisotropy and fiber density index in vivo at 1.5 tesla.
abstract: BACKGROUND
  Tensor estimation can be improved by increasing the number of gradient directions (NGD) or increasing the number of signal averages (NSA), but at a cost of increased scan time.
  
  
  OBJECTIVE
  To evaluate the effects of NGD and NSA on fractional anisotropy (FA) and fiber density index (FDI) in vivo.
  
  
  METHODS
  Ten healthy adults were scanned on a 1.5T system using nine different diffusion tensor sequences. Combinations of 7 NGD, 15 NGD, and 25 NGD with 1 NSA, 2 NSA, and 3 NSA were used, with scan times varying from 2 to 18 min. Regions of interest (ROIs) were placed in the internal capsules, middle cerebellar peduncles, and splenium of the corpus callosum, and FA and FDI were calculated. Analysis of variance was used to assess whether there was a difference in FA and FDI of different combinations of NGD and NSA.
  
  
  RESULTS
  There was no significant difference in FA of different combinations of NGD and NSA of the ROIs (P>0.005). There was a significant difference in FDI between 7 NGD/1 NSA and 25 NGD/3 NSA in all three ROIs (P<0.005). There were no significant differences in FDI between 15 NGD/3 NSA, 25 NGD/1 NSA, and 25 NGD/2 NSA and 25 NGD/3 NSA in all ROIs (P>0.005).
  
  
  CONCLUSIONS
  We have not found any significant difference in FA with varying NGD and NSA in vivo in areas with relatively high anisotropy. However, lower NGD resulted in reduced FDI in vivo. With larger NGD, NSA has less influence on FDI. The optimal sequence among the nine sequences tested with the shortest scan time was 25 NGD/1 NSA.
authors.affiliation: Diagnostic Imaging, Hospital for Sick Children, Toronto, Ontario, Canada. elysa.widjaja@sickkids.ca
authors.forename: E
authors.initials: E
authors.lastname: Widjaja
authors.forename: S Z
authors.initials: SZ
authors.lastname: Mahmoodabadi
authors.forename: D
authors.initials: D
authors.lastname: Rea
authors.forename: R
authors.initials: R
authors.lastname: Moineddin
authors.forename: L
authors.initials: L
authors.lastname: Vidarsson
authors.forename: D
authors.initials: D
authors.lastname: Nilsson
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D016880:Anisotropy; D001931:Brain Mapping / Q000379:methods*; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D009412:Nerve Fibers / Q000648:ultrastructure*