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

Abstract

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 (R2 ≍ 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.

Measurement of fat content in vertebral marrow using a modified dixon sequence to differentiate benign from malignant processes

“…We speculated that the FF and FF ratio rather than the LDR could help to differentiate benign from malignant lesions in those difficult situations. Second, they used a large flip angle (25°), which led to an overestimation of fat due to the shorter T 1 values as compared with water . In fact, they reported higher values of FF in both benign and malignant groups compared to our results (37.3% [Kim et al] vs. 23.3% [ours] for focal red marrow deposition and 12.8% [Kim et al] vs. 2.8% [ours] for malignancy).…”

Proton density fat fraction MRI of vertebral bone marrow: Accuracy, repeatability, and reproducibility among readers, field strengths, and imaging platforms

“…This may induce either an overestimation or an underestimation of the measured fat content, depending on the fat distribution in the region surrounding the spectroscopy voxel. Although numerous factors that can hamper adequate PDFF estimation in vertebral bone marrow (T 1 , T 2 , T 2 *, and multipeak spectral complexity of fat) were accounted for in this study, overestimation by imaging‐based PDFF could reflect residual T 1 ‐bias, especially at flip angles ≥5° . This is because the T 1 relaxation times are widely divergent among the water and fat compartments in vertebral bone marrow, potentially leading to significant quantification errors .…”

Cross‐sectional correlation between hepatic R2* and proton density fat fraction (PDFF) in children with hepatic steatosis

“…A 3D multiecho SPGR sequence was acquired with full liver coverage in a single 20‐second breath‐hold with 2D parallel imaging and an effective net acceleration factor of 2.2 using Auto‐calibrating Reconstruction for Cartesian imaging. To minimize T 1 weighting, a low flip angle (3°) was used with 6.9 msec TR . Six echoes were obtained per TR at TEs, as these are appropriate for fat–water separation using complex fitting.…”