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

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.

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.

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.

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.