Computational Radiology Laboratory, Department of Radiology, Boston Children's Hospital, Boston, Massachussetts.
Harvard Medical School, Boston, Massachussetts.
Magn Reson Med. 2020 Feb;83(2):575-589. doi: 10.1002/mrm.27957. Epub 2019 Aug 29.
To measure spatiotemporal B field changes in real time using FID navigators (FIDnavs) and to demonstrate the efficacy of retrospectively correcting high-resolution -weighted images using a novel FIDnav framework.
A forward model of the complex FIDnav signals was generated by simulating the effect of changes in the underlying B inhomogeneity coefficients, with spatial encoding provided by a multi-channel reference image. Experiments were performed at 3T to assess the accuracy of B field estimates from FIDnavs acquired from a 64-channel head coil under different shim settings and in 5 volunteers performing deep-breathing and nose-touching tasks designed to modulate the B field. Second-order, in-plane spherical harmonic (SH) inhomogeneity coefficients estimated from FIDnavs were incorporated into an iterative reconstruction to retrospectively correct 2D gradient-echo images acquired in both axial and sagittal planes.
Spatiotemporal B field changes measured from rapidly acquired FIDnavs were in good agreement with the results of second-order SH fitting to the measured field maps. FIDnav field estimates accounted for a significant proportion of the ΔB variance induced by deep breathing (64 ± 21%) and nose touching (67 ± 34%) across all volunteers. Ghosting, blurring, and intensity modulation artifacts in -weighted images, induced by spatiotemporal field changes, were visibly reduced following retrospective correction with FIDnav inhomogeneity coefficients.
Spatially resolved B inhomogeneity changes up to second order can be characterized in real time using the proposed approach. Retrospective FIDnav correction substantially improves -weighted image quality in the presence of strong B field modulations, with potential for real-time shimming.
使用 FID 导航仪（FIDnav）实时测量时空 B 场变化，并展示使用新型 FIDnav 框架对高分辨率 -加权图像进行回顾性校正的效果。
通过模拟基础 B 不均匀性系数变化的影响，生成复杂 FIDnav 信号的正向模型，空间编码由多通道参考图像提供。在 3T 下进行实验，以评估在不同匀场设置下从 64 通道头部线圈采集的 FIDnav 进行 B 场估计的准确性，并在 5 名志愿者中进行深呼吸和触摸鼻子的任务，以调制 B 场。从 FIDnav 中估计的二阶、平面内球谐（SH）不均匀性系数被纳入迭代重建中，以对在轴向和矢状面采集的 2D 梯度回波图像进行回顾性校正。
从快速采集的 FIDnav 中测量的时空 B 场变化与测量磁场图的二阶 SH 拟合结果吻合良好。FIDnav 场估计解释了所有志愿者深呼吸（64±21%）和触摸鼻子（67±34%）引起的 ΔB 方差的重要部分。在存在时空场变化的情况下，使用 FIDnav 不均匀性系数进行回顾性校正后， -加权图像中的鬼影、模糊和强度调制伪影明显减少。
使用所提出的方法可以实时表征高达二阶的空间分辨 B 不均匀性变化。在存在强 B 场调制的情况下，回顾性 FIDnav 校正可显著改善 -加权图像质量，具有实时匀场的潜力。
