Department of Radiology Medical Physics, University Hospital Freiburg, Freiburg, Germany.
MAGMA. 2010 Sep;23(4):263-73. doi: 10.1007/s10334-010-0225-8. Epub 2010 Aug 6.
State-of-the-art MR techniques that rely on echo planar imaging (EPI), such as real-time fMRI, are limited in their applicability by both subject motion and B(0) field inhomogeneities. The goal of this work is to demonstrate that in principle it is possible to accurately predict the B(0) field inhomogeneities that occur during echo planar imaging in the presence of large scale head motion and apply this knowledge for distortion correction.
In this work, prospective motion correction is combined with a field-prediction method and a method for correcting geometric distortions in EPI. To validate the methods, echo planar images were acquired of a custom-made phantom rotated to different angles relative to the B(0) field. For each orientation, field maps were acquired for comparison with the field predictions.
The calculated field maps are very similar to the measured field maps for all orientations used in the experiments. The root mean squared error (RMSE) of the difference maps was between 15 to 20 Hz. The quality of distortion correction using calculated field maps is comparable to distortion correction done with measured field maps.
The results suggest that distortion-free echo planar imaging of moving objects may be feasible if prospective motion correction is combined with a field inhomogeneity estimation approach.
依赖于 echo 平面成像 (EPI) 的最先进的磁共振技术，如实时 fMRI，由于受试者运动和 B(0) 场不均匀性而在适用性方面受到限制。这项工作的目的是证明在存在大尺度头部运动的情况下，准确预测 echo 平面成像过程中发生的 B(0) 场不均匀性是有可能的，并将这一知识应用于失真校正。
在这项工作中，前瞻性运动校正与场预测方法和 EPI 中的几何失真校正方法相结合。为了验证这些方法，对相对于 B(0) 场旋转到不同角度的定制化 phantom 进行了 echo 平面成像采集。对于每个方向，都采集了场图以便与场预测进行比较。
计算出的场图与实验中使用的所有方向的测量场图非常相似。差值图的均方根误差 (RMSE) 在 15 到 20 Hz 之间。使用计算出的场图进行失真校正的质量与使用测量场图进行失真校正的质量相当。
结果表明，如果将前瞻性运动校正与场不均匀性估计方法相结合，可能实现对运动物体的无失真 echo 平面成像。
