Wellcome Centre for Human Neuroimaging, UCL Queen Square Institute of Neurology, University College London, London, UK.
Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA.
Magn Reson Med. 2022 Jul;88(1):280-291. doi: 10.1002/mrm.29216. Epub 2022 Mar 21.
Inter-scan motion is a substantial source of error in estimation methods based on multiple volumes, for example, variable flip angle (VFA), and can be expected to increase at 7T where fields are more inhomogeneous. The established correction scheme does not translate to 7T since it requires a body coil reference. Here we introduce two alternatives that outperform the established method. Since they compute relative sensitivities they do not require body coil images.
The proposed methods use coil-combined magnitude images to obtain the relative coil sensitivities. The first method efficiently computes the relative sensitivities via a simple ratio; the second by fitting a more sophisticated generative model.
maps were computed using the VFA approach. Multiple datasets were acquired at 3T and 7T, with and without motion between the acquisition of the VFA volumes. maps were constructed without correction, with the proposed corrections, and (at 3T) with the previously established correction scheme. The effect of the greater inhomogeneity in the transmit field at 7T was also explored by acquiring maps at each position.
At 3T, the proposed methods outperform the baseline method. Inter-scan motion artifacts were also reduced at 7T. However, at 7T reproducibility only converged on that of the no motion condition if position-specific transmit field effects were also incorporated.
The proposed methods simplify inter-scan motion correction of maps and are applicable at both 3T and 7T, where a body coil is typically not available. The open-source code for all methods is made publicly available.
在基于多个容积的估计方法中，例如可变翻转角（VFA），扫描间运动是一个很大的误差源，并且可以预期在磁场不均匀性更高的 7T 中会增加。已建立的校正方案不适用于 7T，因为它需要体线圈参考。在这里，我们介绍两种优于已建立方法的替代方法。由于它们计算相对灵敏度，因此不需要体线圈图像。
所提出的方法使用线圈组合的幅度图像来获得相对线圈灵敏度。第一种方法通过简单的比率有效地计算相对灵敏度；第二种方法通过拟合更复杂的生成模型来计算。
使用 VFA 方法计算图。在 3T 和 7T 采集了多个数据集，在采集 VFA 容积之间存在和不存在运动。在没有校正、使用提出的校正和（在 3T 时）使用以前建立的校正方案的情况下构建图。还通过在每个位置采集图来探索在传输场中更大的不均匀性对 7T 的影响。
在 3T 时，所提出的方法优于基线方法。在 7T 时，扫描间运动伪影也减少了。然而，在 7T 时，仅在包含位置特定的传输场效应时，可重复性才收敛于无运动条件。
所提出的方法简化了 图的扫描间运动校正，并且适用于 3T 和 7T，在这些场强下通常没有体线圈。所有方法的开源代码都公开提供。
