title: Effect of flip angle on the accuracy and repeatability of hepatic proton density fat fraction estimation by complex data-based, T1-independent, T2*-corrected, spectrum-modeled MRI.
abstract: OBJECTIVE
  To evaluate the effect of flip angle (FA) on accuracy and within-examination repeatability of hepatic proton-density fat fraction (PDFF) estimation with complex data-based magnetic resonance imaging (MRI).
  
  
  METHODS
  PDFF was estimated at 3T in 30 subjects using two sets of five MRI sequences with FA from 1° to 5° in each set. One set used 7 msec repetition time and acquired 6 echoes (TR7/E6); the other used 14 msec and acquired 12 echoes (TR14/E12). For each FA in both sets the accuracy of MRI-PDFF was assessed relative to MR spectroscopy (MRS)-PDFF using four regression parameters (slope, intercept, average bias, R(2) ). Each subject had four random sequences repeated; within-examination repeatability of MRI-PDFF for each FA was assessed with intraclass correlation coefficient (ICC). Pairwise comparisons were made using bootstrap-based tests.
  
  
  RESULTS
  Most FAs provided high MRI-PDFF estimation accuracy (intercept range -1.25 to 0.84, slope 0.89-1.06, average bias 0.24-1.65, R(2) 0.85-0.97). Most comparisons of regression parameters between FAs were not significant. Informally, in the TR7/E6 set, FAs of 2° and 3° provided the highest accuracy, while FAs of 1° and 5° provided the lowest. In the TR14/E12 set, accuracy parameters did not differ consistently between FAs. FAs in both sets provided high within-examination repeatability (ICC range 0.981-0.998).
  
  
  CONCLUSIONS
  MRI-PDFF was repeatable and, for most FAs, accurate in both sequence sets. In the TR7/E6 sequence set, FAs of 2° and 3° informally provided the highest accuracy. In the TR14/E12 sequence set, all FAs provided similar accuracy.
authors.affiliation: Liver Imaging Group (LIG), Department of Radiology, University of California at San Diego, San Diego, California, USA.
authors.forename: Benjamin L
authors.initials: BL
authors.lastname: Johnson
authors.forename: Michael E
authors.initials: ME
authors.lastname: Schroeder
authors.forename: Tanya
authors.initials: T
authors.lastname: Wolfson
authors.forename: Anthony C
authors.initials: AC
authors.lastname: Gamst
authors.forename: Gavin
authors.initials: G
authors.lastname: Hamilton
authors.forename: Masoud
authors.initials: M
authors.lastname: Shiehmorteza
authors.forename: Rohit
authors.initials: R
authors.lastname: Loomba
authors.forename: Jeffrey B
authors.initials: JB
authors.lastname: Schwimmer
authors.forename: Scott
authors.initials: S
authors.lastname: Reeder
authors.forename: Michael S
authors.initials: MS
authors.lastname: Middleton
authors.forename: Claude B
authors.initials: CB
authors.lastname: Sirlin
mesh_terms: D050154:Adiposity / Q000502:physiology*; D000293:Adolescent; D000328:Adult; D000368:Aged; D000465:Algorithms; D016477:Artifacts; D005260:Female; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008099:Liver / Q000502:physiology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D011522:Protons; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D055815:Young Adult
chemical_list: D011522:Protons
keywords: accuracy; flip angle; magnetic resonance imaging; proton-density fat fraction; repeatability