T1 bias in chemical shift-encoded liver fat-fraction: role of the flip angle.
OBJECTIVE
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).


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) &asymp; 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).


CONCLUSIONS
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.
Department of Radiology and Neuroradiology, University Greifswald, Greifswald, Germany.
Jens-Peter
JP
Kühn
Christina
C
Jahn
Diego
D
Hernando
Werner
W
Siegmund
Stefan
S
Hadlich
Julia
J
Mayerle
Jörg
J
Pfannmöller
Sonke
S
Langner
Scott
S
Reeder
D000273:Adipose Tissue; D000465:Algorithms; D016477:Artifacts; D005234:Fatty Liver / Q000150:complications / Q000473:pathology*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D050152:Intra-Abdominal Fat / Q000473:pathology*; D008099:Liver; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000150:complications / Q000473:pathology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical shift imaging; fatty liver; liver; magnetic resonance imaging; magnetic resonance spectroscopy