Age-Specific Contrast Optimization of bSSFP in Fetal Brain

Balanced steady-state free-precession (bSSFP) enables rapid fetal brain acquisition but suffers from inherently low tissue contrast. This study aims to develop an age-specific optimization strategy for bSSFP to enhance gray matter-white matter (GM-WM) contrast in the developing fetal brain.

An optimization pipeline was designed for 2D and 3D bSSFP sequences, integrating the Transition Into Driven Equilibrium (TIDE) model with the extended phase graph (EPG) algorithm. Using T1 and T2 relaxation times measured from fetal brains, the pipeline derived age-specific variable flip angle (VFA) trains to maximize GM-WM contrast while constraining signal-to-noise ratio (SNR) and point-spread function. Performance was benchmarked against default bSSFP and T2-weighted half-Fourier acquisition single-shot turbo spin echo (HASTE) sequences based on relative contrast and segmentation accuracy.

The contrast enhancement was evaluated in a cohort of 18 fetal brains from 24 to 37 gestational weeks. The optimized 2D and 3D bSSFP sequences yielded significantly higher relative cortical contrast compared with the default bSSFP (p < 0.001 for 2D and p < 0.002 for 3D), achieving average improvements of 4.13-fold and 4.06-fold, respectively. Furthermore, in the segmentation tasks, the optimized images yielded substantially higher segmentation accuracy compared with HASTE images (DICE = 0.93 for optimized 2D bSSFP and 0.89 for optimized 3D bSSFP, vs. 0.79 for HASTE).

The proposed age-specific bSSFP optimization strategy enhanced the characterization of GM-WM contrast in fetuses of different gestational ages. This readily translatable method holds potential to advance both clinical assessments and neurodevelopmental studies of the fetal brain.