PURPOSE: To develop a rapid, high-resolution and distortion-free technique for simultaneous water-fat separation, R 2 * and B 0 mapping of the fetal brain at 3T.

METHODS: A 2D multi-echo radial FLASH sequence with blip gradients is adapted for data acquisition during maternal free breathing. A calibrationless model-based reconstruction with sparsity constraints is developed to jointly estimate water, fat, R 2 * and B 0 field maps directly from k-space. This approach was validated and compared to reference methods using numerical and NIST phantoms and data from nine fetuses between 26 and 36 weeks of gestation age.

RESULTS: Both numerical and experimental phantom studies confirm good accuracy and precision. In fetal studies, model-based reconstruction yields quantitative R 2 * values in close agreement with those from a parallel imaging compressed sensing (PICS) technique using Graph Cut (intra-class correlation coefficient [ICC] = 0.9601), while providing enhanced image detail. Repeated scans confirm good reproducibility (ICC = 0.9213). Compared to multi-echo EPI, the proposed radial technique produces higher-resolution (1.1 × 1.1 × 3 mm3 vs. 2-3 × 2-3 × 3 mm3) R 2 * maps with reduced distortion. Despite of differences in motion, resolution and distortion, R 2 * values are comparable between the two acquisition strategies (ICC = 0.8049). Additionally, the proposed approach enables synthesis of high-resolution and distortion-free R 2 * -weighted images.

CONCLUSION: This study demonstrates the feasibility of using multi-echo radial FLASH combined with calibrationless model-based reconstruction for motion-robust, distortion-free R 2 * mapping of the fetal brain at 3T, achieving a nominal resolution of 1.1 × 1.1 × 3 mm3 within 2 seconds per slice.