Fat/Water Separation at 7 T Using a 3D Radial Sequence With Quasi‐Continuous Echo Times

PURPOSE: To achieve a reliable fat/water separation (FWS) at 7 T using a new 3D radial sequence and reconstruction workflow with quasi-continuous TE sampling. METHODS: A 3D radial density-adapted sequence with quasi-continuous TE sampling was developed for a 7 T whole-body MRI system. The reconstruction workflow included an off-resonance correction to reduce chemical shift artifacts. Fat and water signals were separated using a graph cut algorithm, yielding proton density fat fraction maps. The sequence and reconstruction pipeline were tested and validated using phantom measurements and in vivo acquisitions of the lower leg of healthy volunteers. RESULTS: The proposed sequence and reconstruction pipeline enabled sampling of the fat/water oscillation curve over a range from a minimal mean TE of 0.27 ms to a maximum mean TE of 10.13 ms, with an effective TE increment of 85 μs. The off-resonant correction significantly reduced chemical shift artifacts in the fat signal. The data points of the fat/water oscillation showed good agreement with a multi-peak fit function. The quantification of phantom fat percentages was verified with a mean absolute error of 1.5%. The proposed pipeline resulted in consistent interpretation of fat and water signals without any swaps, neither in phantom nor in in vivo measurements. CONCLUSION: This study successfully demonstrated the application of a 3D radial sequence with quasi-continuous TEs for FWS at 7 T. Due to the high sampling rate (effective TE increment below 100 μs), the presented FWS workflow demonstrated high robustness against fat/water swaps, which are typical at ultra-high field strengths.