Free-Breathing Volumetric Liver $R_2^{*}$ and Proton Density Fat Fraction Quantification in Pediatric Patients Using Stack-of-Radial MRI With Self-Gating Motion Compensation

Background: Stack-of-radial multiecho gradient-echo MRI is promising for free-breathing liver $R_2^{*}$ quantification and may benefit children.

Purpose: To validate stack-of-radial MRI with self-gating motion compensation in phantoms, and to evaluate it in children.

Study type: Prospective.

Phantoms: Four vials with different $R_2^{*}$ driven by a motion stage.

Subjects: Sixteen pediatric patients with suspected nonalcoholic fatty liver disease or steatohepatitis (five females, 13 ± 4 years, body mass index 29.2 ± 8.6 kg/m² ).

Field strength/sequences: Stack-of-radial, and 2D and 3D Cartesian multiecho gradient-echo sequences at 3T.

Assessment: Ungated and gated stack-of-radial proton density fat fraction (PDFF) and $R_2^{*}$ maps were reconstructed without and with self-gating motion compensation. Stack-of-radial $R_2^{*}$ measurements of phantoms without and with motion were validated against reference 2D Cartesian results of phantoms without motion. In subjects, free-breathing stack-of-radial and reference breath-hold 3D Cartesian were acquired. Subject inclusion for statistical analysis and region of interest placement were determined independently by two observers.

Statistical tests: Phantom results were fitted with a weighted linear model. Demographic differences between excluded and included subjects were tested by multivariate analysis of variance. PDFF and $R_2^{*}$ measurements were compared using Bland-Altman analysis. Interobserver agreement was assessed by the intraclass correlation coefficient (ICC).

Results: Ungated stack-of-radial $R_2^{*}$ inside moving phantom vials showed a significant positive bias of 64.3 s^-1 (P < 0.00001), unlike gated results (P > 0.31). Subject inclusion decisions for statistical analysis from two observers were consistent. No significant differences were found between four excluded and 12 included subjects (P = 0.14). Compared to breath-hold Cartesian, ungated and gated free-breathing stack-of-radial exhibited mean $R_2^{*}$ differences of 18.5 s^-1 and 3.6 s^-1 . Mean PDFF differences were 1.1% and 1.0% for ungated and gated measurements, respectively. Interobserver agreement was excellent (ICC for PDFF = 0.99, ICC for $R_2^{*}$ = 0.90; P < 0.0003).

Data conclusion: Stack-of-radial MRI with self-gating motion compensation seems to allow free-breathing liver $R_2^{*}$ and PDFF quantification in children.

Level of evidence: 2 TECHNICAL EFFICACY STAGE: 2.