Free-breathing R2∗ mapping of hepatic iron overload in children using 3D multi-echo UTE cones MRI

Abstract

Purpose: To enable motion-robust, ungated, free-breathing $R_2^{\ast }$ mapping of hepatic iron overload in children with 3D multi-echo UTE cones MRI.

Methods: A golden-ratio re-ordered 3D multi-echo UTE cones acquisition was developed with chemical-shift encoding (CSE). Multi-echo complex-valued source images were reconstructed via gridding and coil combination, followed by confounder-corrected $R_2^{\ast }$ (=1/ $T_2^{\ast }$ ) mapping. A phantom containing 15 different concentrations of gadolinium solution (0-300 mM) was imaged at 3T. 3D multi-echo UTE cones with an initial TE of 0.036 ms and Cartesian CSE-MRI (IDEAL-IQ) sequences were performed. With institutional review board approval, 85 subjects (81 pediatric patients with iron overload + 4 healthy volunteers) were imaged at 3T using 3D multi-echo UTE cones with free breathing (FB cones), IDEAL-IQ with breath holding (BH Cartesian), and free breathing (FB Cartesian). Overall image quality of $R_2^{\ast }$ maps was scored by 2 blinded experts and compared by a Wilcoxon rank-sum test. For each pediatric subject, the paired $R_2^{\ast }$ maps were assessed to determine if a corresponding artifact-free 15 mm region-of-interest (ROI) could be identified at a mid-liver level on both images. Agreement between resulting $R_2^{\ast }$ quantification from FB cones and BH/FB Cartesian was assessed with Bland-Altman and linear correlation analyses.

Results: ROI-based regression analysis showed a linear relationship between gadolinium concentration and $R_2^{\ast }$ in IDEAL-IQ (y = 8.83x - 52.10, R² = 0.995) as well as in cones (y = 9.19x - 64.16, R² = 0.992). ROI-based Bland-Altman analysis showed that the mean difference (MD) was 0.15% and the SD was 5.78%. However, IDEAL-IQ $R_2^{\ast }$ measurements beyond 200 mM substantially deviated from a linear relationship for IDEAL-IQ (y = 5.85x + 127.61, R² = 0.827), as opposed to cones (y = 10.87x - 166.96, R² = 0.984). In vivo, FB cones $R_2^{\ast }$ had similar image quality with BH and FB Cartesian in 15 and 42 cases, respectively. FB cones $R_2^{\ast }$ had better image quality scores than BH and FB Cartesian in 3 and 21 cases, respectively, where BH/FB Cartesian exhibited severe ghosting artifacts. ROI-based Bland-Altman analyses were 2.23% (MD) and 6.59% (SD) between FB cones and BH Cartesian and were 0.21% (MD) and 7.02% (SD) between FB cones and FB Cartesian, suggesting a good agreement between FB cones and BH (FB) Cartesian $R_2^{\ast }$ . Strong linear relationships were observed between BH Cartesian and FB cones (y = 1.00x + 1.07, R² = 0.996) and FB Cartesian and FB cones (y = 0.98x + 1.68, R² = 0.999).

Conclusion: Golden-ratio re-ordered 3D multi-echo UTE Cones MRI enabled motion-robust, ungated, and free-breathing $R_2^{\ast }$ mapping of hepatic iron overload, with comparable $R_2^{\ast }$ measurements and image quality to BH Cartesian, and better image quality than FB Cartesian.

Keywords: 3D multi-echo UTE cones k-space sampling trajectory; chemical-shift-encoded MRI; confounder-corrected ${\mathrm{R}}_2^{\ast }$; free-breathing liver ${\mathrm{R}}_2^{\ast }$ mapping; hepatic iron overload.