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

PurposeTo 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). Materials and MethodsPDFF was estimated at 3T in 30 subjects using two sets of five MRI sequences with FA from 1 degrees to 5 degrees 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. ResultsMost 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 degrees and 3 degrees provided the highest accuracy, while FAs of 1 degrees and 5 degrees 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). ConclusionMRI-PDFF was repeatable and, for most FAs, accurate in both sequence sets. In the TR7/E6 sequence set, FAs of 2 degrees and 3 degrees informally provided the highest accuracy. In the TR14/E12 sequence set, all FAs provided similar accuracy. J. Magn. Reson. Imaging 2014;39:440-447. (c) 2013 Wiley Periodicals, Inc.