Rapid, High-resolution and Distortion-free R 2 * Mapping of Fetal Brain using Multi-echo Radial FLASH and Model-based Reconstruction.

Purpose: To develop a rapid, high-resolution and distortion-free quantitative $R_2^{*}$ mapping technique for fetal brain at 3 T.

Methods: A 2D multi-echo radial FLASH sequence with blip gradients is adapted for fetal brain data acquisition during maternal free breathing at 3 T. A calibrationless model-based reconstruction with sparsity constraints is developed to jointly estimate water, fat, $R_2^{*}$ and $B_0$ field maps directly from the acquired k-space data. Validations have been performed on numerical and NIST phantoms and five fetal subjects ranging from 27 weeks to 36 weeks gestation age.

Results: Both numerical and experimental phantom studies confirm good accuracy and precision of the proposed method. In fetal studies, both the parallel imaging compressed sensing (PICS) technique with a Graph Cut algorithm and the model-based approach proved effective for parameter quantification, with the latter providing enhanced image details. Compared to commonly used multi-echo EPI approaches, the proposed radial technique shows improved spatial resolution (1.1 × 1.1 × 3 mm³ vs. 2-3 × 2-3 × 3 mm³) and reduced distortion. Quantitative $R_2^{*}$ results confirm good agreement between the two acquisition strategies. Additionally, high-resolution, distortion-free $R_2^{*}$ -weighted images can be synthesized, offering complementary information to HASTE.

Conclusion: This work demonstrates the feasibility of radial acquisition for motion-robust quantitative $R_2^{*}$ mapping of the fetal brain. This proposed multi-echo radial FLASH, combined with calibrationless model-based reconstruction, achieves accurate, distortion-free fetal brain $R_2^{*}$ mapping at a nominal resolution of 1.1 × 1.1 × 3 mm³ within 2 seconds.