Echo-planar imaging of the human head with 100 mT/m gradients and high-order modeling of eddy current fields
Echo-planar imaging of the human head with 100 mT/m gradients and high-order modeling of eddy current fields
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DOI：
10.1002/mrm.28168
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发表时间：
2020-01-21
影响因子：
3.3
通讯作者：
Pruessmann, Klaas P.
中科院分区：
文献类型：
--
作者：
Hennel, Franciszek;Wilm, Bertram;Pruessmann, Klaas P.
目的证明高性能梯度衬垫在人体头部超快MRI中的实用性。方法首次使用EPI，其读出梯度幅度为100 mT/m，转换速率为1200 T/m/s，信号带宽接近1 MHz，用于人体头部扫描。为了避免涡流引起的伪影，在多个点用NMR探头动态监测磁场，通过高达五阶的固体谐波建模，并包括在图像重建中。为了加速高阶重建，对涡流的内部回波效应进行了近似，可以忽略不计。场监测为基础的方法进行了比较，最近提出的相位误差估计从单独的重建的偶数和奇数回波。结果梯度内插法获得的图像畸变明显低于同系统全身30 mT/m、200 T/m/s梯度下的图像畸变。然而，为了避免持续的2D奈奎斯特重影，必须适当地表征和校正高空间阶的涡流。多位置监测被证明是一个强大的方法来测量涡流，并允许更高的欠采样率比基于图像的方法。所提出的涡流效应的近似允许通过对每个空间维度的单独处理来显著加速高阶重构。结论强梯度与适当的切换率是非常有益的EPI的质量提供了鲁棒的措施，包括涡流的图像编码的贡献。
Purpose To demonstrate the utility of a high-performance gradient insert for ultrafast MRI of the human head. Methods EPI was used for the first time with a readout gradient amplitude of 100 mT/m, 1200 T/m/s slew rate, and nearly 1 MHz signal bandwidth for human head scanning. To avoid artefacts due to eddy currents, the magnetic field was dynamically monitored with NMR probes at multiple points, modeled by solid harmonics up to fifth order, and included in the image reconstruction. An approximation of a negligible intra-echo effect of the eddy currents was made to accelerate the high-order reconstruction. The field monitoring-based approach was compared with a recently proposed phase error estimation from separate reconstructions of even and odd echoes. Results Images obtained with the gradient insert have significantly lower distortions than it is the case with the whole body 30 mT/m, 200 T/m/s gradients of the same system. However, eddy currents of high spatial order must be properly characterized and corrected for in order to avoid a persistent 2D Nyquist ghost. Multi-position monitoring proves to be a robust method to measure the eddy currents and allows higher undersampling rates than the image-based approach. The proposed approximation of the eddy currents effect allows a significant acceleration of the high-order reconstruction by a separate processing of each spatial dimension. Conclusion Strong gradients with adequate switching rates are highly beneficial for the quality of EPI provided that robust measures are taken to include the contribution of eddy currents to the image encoding.