Hennel Franciszek, Wilm Bertram, Roesler Manuela B, Weiger Markus, Dietrich Benjamin, Pruessmann Klaas P
Institute for Biomedical Engineering, ETH Zurich, and University of Zurich, Zurich, Switzerland.
Skope Magnetic Resonance Technologies Inc., Zurich, Switzerland.
Magn Reson Med. 2020 Aug;84(2):751-761. doi: 10.1002/mrm.28168. Epub 2020 Jan 21.
To demonstrate the utility of a high-performance gradient insert for ultrafast MRI of the human head.
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.
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.
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.
展示一种高性能梯度插入件在人体头部超快磁共振成像中的应用价值。
首次将回波平面成像（EPI）用于人体头部扫描，读出梯度幅度为100 mT/m， slew率为1200 T/m/s，信号带宽近1 MHz。为避免涡流引起的伪影，用核磁共振探头在多个点动态监测磁场，用高达五阶的实心谐波进行建模，并将其纳入图像重建。对涡流的回波内效应可忽略不计进行近似处理，以加速高阶重建。将基于场监测的方法与最近提出的通过偶数和奇数回波的单独重建进行相位误差估计的方法进行比较。
使用梯度插入件获得的图像失真明显低于同一系统的全身30 mT/m、200 T/m/s梯度的情况。然而，必须对高空间阶次的涡流进行适当表征和校正，以避免持续出现二维奈奎斯特鬼影。多位置监测被证明是一种测量涡流的稳健方法，并且比基于图像的方法允许更高的欠采样率。所提出的涡流效应近似方法通过对每个空间维度进行单独处理，可显著加速高阶重建。
只要采取稳健措施来考虑涡流对图像编码的贡献，具有足够切换速率的强梯度对EPI的质量非常有益。
