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

Purpose To enable motion‐robust, ungated, free‐breathing R2∗ 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 R2∗ (=1/T2∗) 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 R2∗ maps was scored by 2 blinded experts and compared by a Wilcoxon rank‐sum test. For each pediatric subject, the paired R2∗ 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 R2∗ 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 R2∗ 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 R2∗ 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 R2∗ had similar image quality with BH and FB Cartesian in 15 and 42 cases, respectively. FB cones R2∗ 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 R2∗. Strong linear relationships were observed between BH Cartesian and FB cones (y = 1.00x + 1.07, R2 = 0.996) and FB Cartesian and FB c