Pilot tone-guided focused navigation for free-breathing whole-liver fat-water and T2* quantification

Purpose To achieve whole-liver motion-corrected fat fraction (FF) and R2* quantification with a 3-minute free-breathing (FB) 3D radial isotropic acquisition, for increased organ coverage, ease-of-use, and patient comfort. Methods A FB 3D radial multiecho gradient-echo liver acquisition with integrated Pilot Tone (PT) navigation and NTE=8 echoes was reconstructed with a motion-correction algorithm based on focused navigation and guided by PT signals (PT-fNAV), with and without a denoising step. Fat fraction (FF) and R2* quantification using a graph cut algorithm was performed on the motion-corrected whole-liver multiecho volumes. Volunteer experiments (n=10) at 1.5T included reference 3D and 2D Cartesian breath-hold (BH) acquisitions. Image sharpness was assessed to evaluate the quality of motion correction with PT-fNAV, compared to a motion-resolved reconstruction. Fat-water images and parametric maps were compared to BH reference acquisitions following Cartesian trajectories, and to a routinely used clinical software (MRQuantiF). Results The image sharpness provided by PT-fNAV (with and without denoising) was similar in end-expiratory motion-resolved reconstructions. The 3D radial FB FF maps compared well with reference BH 3D Cartesian maps (bias +0.7%, limits of agreement (LOA) [-2.5; 4.0]%) and with 2D quantification with MRQuantiF (-0.2%, LOA [-1.1; 0.6]%). While expected visual deviations between proposed FB and reference BH R2* maps were observed, no significant differences were found in quantitative analyses. Conclusion A 3D radial technique with retrospective motion correction by PT-fNAV enabled FF and R2* quantification of the whole-liver at 1.5T. The FB whole-liver acquisition at isotropic spatial resolution compared in accuracy with BH techniques, enabling 3D assessment of steatosis in individuals with limited respiratory capabilities.