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

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Medical Physics

Background:

• Ultra-high field (UHF) MRI, particularly at 7 Tesla (7T), offers enhanced signal-to-noise ratio and spectral resolution.
• However, UHF MRI is prone to artifacts like chemical shift, complicating accurate fat/water separation (FWS).
• Robust FWS is essential for quantitative MRI and accurate tissue characterization.

Purpose of the Study:

• To develop and validate a novel 3D radial MRI sequence with quasi-continuous echo time (TE) sampling for reliable FWS at 7T.
• To implement a reconstruction workflow incorporating off-resonance correction and graph cut segmentation for improved fat and water signal differentiation.
• To assess the sequence's performance in phantom and in vivo measurements, focusing on artifact reduction and prevention of fat/water swaps.

Main Methods:

• A 3D radial density-adapted sequence with quasi-continuous TE sampling was implemented on a 7T whole-body MRI system.
• A reconstruction workflow featuring off-resonance correction to mitigate chemical shift artifacts was developed.
• Fat and water signals were separated using a graph cut algorithm, generating proton density fat fraction maps. Validation was performed using phantoms and in vivo lower leg scans.

Main Results:

• The sequence allowed sampling of the fat/water oscillation curve with a minimal mean TE of 0.27 ms and a maximal mean TE of 10.13 ms, featuring an effective TE increment of 85 μs.
• Off-resonance correction effectively reduced chemical shift artifacts in fat signals.
• Phantom fat quantification showed high accuracy (1.5% mean absolute error), and the pipeline demonstrated consistent fat/water signal interpretation without swaps in both phantom and in vivo data.

Conclusions:

• The 3D radial sequence with quasi-continuous TEs is effective for FWS at 7T.
• The high sampling rate (effective TE increment < 100 μs) provides robustness against fat/water swaps, a common challenge at ultra-high field strengths.
• This workflow enables reliable quantitative fat fraction mapping at 7T.