T1 bias in chemical shift‐encoded liver fat‐fraction: Role of the flip angle

PURPOSE: To investigate flip angle (FA)-dependent T1 bias in chemical shift-encoded fat-fraction (FF) and to evaluate a strategy for correcting this bias to achieve accurate MRI-based estimates of liver fat with optimized signal-to-noise ratio (SNR). MATERIALS AND METHODS: Thirty-three obese patients, 14 men/19 women, aged 57.3 ± 13.9 years underwent 3 Tesla (T) liver MRI including MR-spectroscopy and four three-echo-complex chemical shift-encoded MRI sequences using different FAs (1°/3°/10°/20°). FF was estimated with R2* correction and multi-peak fat spectral modeling. The FF for each FA with and without T1 correction was compared with spectroscopy as a reference standard, using linear regression. Relative SNR of the magnitude data were assessed for each flip angle. RESULTS: The correlation between chemical shift-encoded MRI and spectroscopy was high (R(2) ≈ 0.9). Without T1 correction, the agreement of both techniques showed no significant differences in slope (PFlipAngle1 ° = 0.385/PFlipAngle3 ° = 0.289) using low FA. High FA resulted in significant different slopes (PFlipAngle10 ° = 0.016/PFlipAngle20 ° = 0.014. T1 bias was successfully corrected using the T1 correction strategy (slope:PFlipAngle10 ° = 0.387/PFlipAngle20 ° = 0.440). Additionally, the use of high FA (near the Ernst angle) improved the SNR of the magnitude data (FA1 vs. FA3; respectively FA1 vs. FA10 P ≤ 0.001). CONCLUSION: T1 bias is a strong confounder in the assessment of liver fat using chemical shift imaging with high FA. However, using a larger flip angle with T1 correction leads to higher SNR, and residual error after T1 correction is very small.