Zhou X J, Du Y P, Bernstein M A, Reynolds H G, Maier J K, Polzin J A
General Electric Medical Systems, Milwaukee, Wisconsin 53201, USA.
Magn Reson Med. 1998 Apr;39(4):596-605. doi: 10.1002/mrm.1910390413.
When a linear magnetic field gradient is used, spatially higher-order magnetic fields are produced to satisfy the Maxwell equations. It has been observed that the higher-order magnetic field produced by the readout gradient causes axial echo planar images acquired with a horizontal solenoid magnet to shift along the phase-encoding direction and lose image intensities. Both the shift and intensity reduction become increasingly severe as the slice offset from the isocenter increases. These phenomena are quantitatively analyzed, and good correlation between experiments and theory has been established. The analysis also predicts a previously unreported Nyquist ghost on images with very large slice offsets. This ghost has been verified with computer simulations. Based on the analysis, several methods have been developed to eliminate the image shift, the intensity reduction, and the ghost. Selected methods have been implemented on a commercial scanner and proved effective in removing these image artifacts.
当使用线性磁场梯度时，会产生空间高阶磁场以满足麦克斯韦方程组。据观察，读出梯度产生的高阶磁场会使使用水平螺线管磁体采集的轴向回波平面图像沿相位编码方向偏移并损失图像强度。随着切片与等中心的偏移增加，偏移和强度降低都变得越来越严重。对这些现象进行了定量分析，并在实验与理论之间建立了良好的相关性。该分析还预测了在具有非常大切片偏移的图像上会出现以前未报道过的奈奎斯特伪影。这种伪影已通过计算机模拟得到验证。基于该分析，已开发出几种方法来消除图像偏移、强度降低和伪影。已在商业扫描仪上实施了选定的方法，并证明在去除这些图像伪影方面是有效的。
