Efficient Water-Fat Separation for Extremity MRI at Ultra-Low-Field (0.05 T)

Ultra-low-field magnetic resonance imaging (ULF-MRI) is highly promising for extremity musculoskeletal (MSK) imaging due to its portability, cost-effectiveness, and rapid deployability. Nevertheless, fat signal hyperintensity in ULF-MRI images often obscures pathological information, and the Dixon method used in our prior study has limitations in ULF settings, including reliance on a priori phase information and increased scan duration.

To address these challenges, we propose an improved two-point Dixon method for efficient water-fat separation in extremity imaging at 0.05 T. The optimizations include incorporation of T₂* correction to compensate for signal decay induced by long echo time and the use of a region-growing-based algorithm to eliminate dependence on prior information. Additionally, both multi-echo spin-echo and gradient-echo sequences were implemented to enable single-scan data acquisition. Experiments were conducted on a home-built 0.05 T MRI scanner, including phantom evaluations and in vivo extremity imaging.

For phantom evaluation, the scan time was approximately 3 min, and the fat fraction values for Phantom 1 and Phantom 2 were 0.94 ± 0.02 and 0.91 ± 0.02, respectively. For in vivo imaging, the scan time was about 13 min, and clear water-only and fat-only images were successfully generated, which effectively distinguished muscle and fat tissues. Comparative results at 3 T demonstrated close agreement, supporting the validity of the proposed approach at 0.05 T.

These findings suggest a feasible solution for robust fat suppression in ULF-MRI, extend the application potential of ULF systems for quantitative diagnosis of skeletal muscle injury and bedside follow-up of osteoarticular diseases, and lay the groundwork for future clinical studies on MSK disease diagnosis.