Addressing concomitant gradient phase errors in time‐interleaved chemical shift‐encoded MRI fat fraction and R2 mapping with a pass‐specific phase fitting method

• Published in: Magnetic resonance in medicine 2022-06, Vol.87 (6), p.2826-2838
• Main Authors: Roberts, Nathan T., Hernando, Diego, Panagiotopoulos, Nikolaos, Reeder, Scott B.
• Format: Article
• Language: English
• Subjects: Bias; Chemical equilibrium; chemical shift encoded MRI; Coefficient of variation; concomitant gradients; Errors; In vivo methods and tests; iron quantification; Lower bounds; Magnetic resonance imaging; PDFF estimation; Proton density (concentration); R2 estimation; Waveforms
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.29175

Purpose Concomitant gradients induce phase errors that increase quadratically with distance from isocenter. This work proposes a complex‐based fitting method that addresses concomitant gradient phase errors in chemical shift encoded (CSE) MRI estimation of proton density fat fraction (PDFF) and R2* through joint estimation of pass‐specific phase terms. This method is applicable to time‐interleaved multi‐echo gradient‐echo acquisitions (i.e., multi‐pass acquisitions) and does not require prior knowledge of gradient waveforms typically needed to address concomitant gradient phase errors. Theory and methods A CSE‐MRI spoiled gradient echo signal model, with pass‐specific phase terms, is introduced for non‐linear least squares estimation of PDFF and R2* in the presence of concomitant gradient phase errors. Cramér‐Rao lower bound analysis was used to determine noise performance tradeoffs of the proposed fitting method, which was then validated in both phantom and in vivo experiments. Results The proposed fitting method removed PDFF and R2* estimation errors up to 12% and 10 s–1, respectively, at ±12 cm off isocenter (S/I) in a water phantom. In healthy volunteers, PDFF and R2* bias was reduced by ~10% (12 cm off‐isocenter) and ~30 s–1 (16 cm off‐isocenter), respectively. An evaluation in 29 clinical liver datasets demonstrated reduced PDFF bias and variability (8.4% improvement in the coefficient of variation), even with the imaging volume centered at isocenter. Conclusion Concomitant gradient induced phase errors in multi‐pass CSE‐MRI acquisitions can result in PDFF and R2* estimation biases away from isocenter. The proposed fitting method enables accurate PDFF and R2* quantification in the presence of concomitant gradient phase errors without knowledge of imaging gradient waveforms.