IEEE Trans Med Imaging. 2018 Jan;37(1):12-19. doi: 10.1109/TMI.2017.2667227. Epub 2017 Feb 9.
Geometric distortion induced by the main B0 field disrupts the consistency of fetal echo planar imaging (EPI) data, on which diffusion and functional magnetic resonance imaging is based. In this paper, we present a novel data-driven method for simultaneous motion and distortion correction of fetal EPI. A motion-corrected and reconstructed T2 weighted single shot fast spin echo (ssFSE) volume is used as a model of undistorted fetal brain anatomy. Our algorithm interleaves two registration steps: estimation of fetal motion parameters by aligning EPI slices to the model; and deformable registration of EPI slices to slices simulated from the undistorted model to estimate the distortion field. The deformable registration is regularized by a physically inspired Laplacian constraint, to model distortion induced by a source-free background B0 field. Our experiments show that distortion correction significantly improves consistency of reconstructed EPI volumes with ssFSE volumes. In addition, the estimated distortion fields are consistent with fields calculated from acquired field maps, and the Laplacian constraint is essential for estimation of plausible distortion fields. The EPI volumes reconstructed from different scans of the same subject were more consistent when the proposed method was used in comparison with EPI volumes reconstructed from data distortion corrected using a separately acquired B0 field map.
主磁场 B0 引起的几何变形会破坏基于扩散和功能磁共振成像的胎儿回波平面成像（EPI）数据的一致性。在本文中，我们提出了一种新的基于数据驱动的方法，用于对胎儿 EPI 进行运动和变形校正。运动校正和重建的 T2 加权单次快速自旋回波（ssFSE）体积被用作未变形胎儿大脑解剖结构的模型。我们的算法交错两个配准步骤：通过将 EPI 切片与模型对齐来估计胎儿运动参数；以及对 EPI 切片到从未变形模型模拟的切片进行可变形配准，以估计变形场。可变形配准通过物理启发的拉普拉斯约束进行正则化，以模拟由无源性背景 B0 场引起的变形。我们的实验表明，变形校正显著提高了重建的 EPI 体积与 ssFSE 体积的一致性。此外，所估计的变形场与从采集的场图计算出的场一致，并且拉普拉斯约束对于估计合理的变形场至关重要。与使用单独采集的 B0 场图校正数据变形相比，使用所提出的方法从同一受试者的不同扫描中重建的 EPI 体积更一致。
