Magn Reson Med. 2019;00:1–15. wileyonlinelibrary.com/journal/mrm | 1 © 2019 International Society for Magnetic Resonance in Medicine Received: 24 March 2019 | Revised: 12 September 2019 | Accepted: 5 October 2019 DOI: 10.1002/mrm.28052 FULL PAPER Effect of respiratory motion on free‐breathing 3D stack‐of‐radial liver R ∗ 2 relaxometry and improved quantification accuracy using self‐gating Xiaodong Zhong 1 | Tess Armstrong 2,3 | Marcel D. Nickel 4 | Stephan A.R. Kannengiesser 4 | Li Pan 5 | Brian M. Dale 6 | Vibhas Deshpande 7 | Berthold Kiefer 4 | Holden H. Wu 2,3 1 MR R&D Collaborations, Siemens Healthcare, Los Angeles, California 2 Department of Radiological Sciences, David Geffen School of Medicine, University of California Los Angeles, Los Angeles, California 3 Physics and Biology in Medicine Interdepartmental Program, David Geffen School of Medicine, University of California Los Angeles, Los Angeles, California 4 MR Application Development, Siemens Healthcare GmbH, Erlangen, Germany 5 MR R&D Collaborations, Siemens Healthcare, Baltimore, Maryland 6 MR R&D Collaborations, Siemens Healthcare, Cary, North Carolina 7 MR R&D Collaborations, Siemens Healthcare, Austin, Texas Xiaodong Zhong and Tess Armstrong contributed equally to this work. Correspondence Xiaodong Zhong, PhD, MR R&D Collaborations, Siemens Healthcare, 10945 Le Conte Ave, Suite 3371J, Los Angeles, CA 90095. Email: xiaodong.zhong@siemens- healthineers.com Funding information This study was supported in part by Siemens Medical Solutions USA, Inc. and the Department of Radiological Sciences at the University of California, Los Angeles. Purpose: To develop an accurate free‐breathing 3D liver R ∗ 2 mapping approach and to evaluate it in vivo. Methods: A free‐breathing multi‐echo stack‐of‐radial sequence was applied in 5 normal subjects and 6 patients at 3 Tesla. Respiratory motion compensation was implemented using the inherent self‐gating signal. A breath‐hold Cartesian acquisi- tion was the reference standard. Proton density fat fraction and R ∗ 2 were measured and compared between radial and Cartesian methods using Bland‐Altman plots. The normal subject results were fitted to a linear mixed model (P < .05 considered significant). Results: Free‐breathing stack‐of‐radial without self‐gating exhibited signal attenu- ation in echo images and artifactually elevated apparent R ∗ 2 values. In the Bland‐ Altman plots of normal subjects, compared to breath‐hold Cartesian, free‐breathing stack‐of‐radial acquisitions of 22, 30, 36, and 44 slices, had mean R ∗ 2 differences of 27.4, 19.4, 10.9, and 14.7 s −1 with 800 radial views, and they had 18.4, 11.9, 9.7, and 27.7 s −1 with 404 views, which were reduced to 0.4, 0.9, −0.2, and −0.7 s −1 and to −1.7, −1.9, −2.1, and 0.5 s −1 with self‐gating, respectively. No substantial proton density fat fraction differences were found. The linear mixed model showed free‐breathing radial R ∗ 2 results without self‐gating were significantly biased by 17.2 s −1 averagely (P = .002), which was eliminated with self‐gating (P = .930). Proton density fat fraction results were not different (P > .234). For patients,