Optimized motion-insensitive PDFF mapping of the liver.

PURPOSE: To implement, optimize, and validate parallel imaging (PI)-accelerated, 2D, flip angle modulated (FAM) chemical shift-encoded quantification of liver proton-density fat fraction (PDFF), with motion insensitivity.

METHODS

The optimization cost function that determines flip angles in FAM was generalized for PI. Phantom studies and prospective studies in volunteers with varying liver fat levels were performed. Free-breathing FAM was acquired in the axial, sagittal, and coronal planes, with varying nominal PI acceleration factors (R) of 1.0 to 3.0. A breath-held, commercially available 3D chemical shift-encoded method was acquired as reference for PDFF. Overall image quality, qualitative SNR, and motion artifacts for all methods were Likert-scale rated. PDFF measured by FAM was compared to reference to assess bias. Test-retest repeatability was assessed for all methods by repeating acquisitions after volunteer repositioning. Noise performance was assessed with standard deviation of PDFF maps as R increased.

RESULTS

The reader study (N = 3 readers/10 subjects) demonstrated excellent image quality for FAM during free-breathing, with reduced motion artifacts compared to breath-held reference (p < 0.01). PI-accelerated FAM shows fewer motion artifacts than unaccelerated FAM (p < 0.01). In all planes and accelerations, PDFF measured by FAM showed good agreement with reference PDFF measurements (mean bias: -0.4% to 2.0% PDFF; 95% limits of agreement: 2.8% to 4.0% PDFF). FAM in axial and coronal planes showed similar or improved repeatability (repeatability coefficient = 1.7% to 2.6% PDFF) compared to the reference (2.7%). Sagittal FAM shows similar or worse repeatability (repeatability coefficient = 3.0% to 3.6%). FAM with R = 2.0 has good noise performance and high SNR efficiency.

CONCLUSION

FAM, in axial or coronal planes with R = 2.0, is optimal for motion-insensitive liver PDFF quantification.