Bernstein M A, Zhou X J, Polzin J A, King K F, Ganin A, Pelc N J, Glover G H
GE Medical Systems, Waukesha, Wisconsin 53188, USA.
Magn Reson Med. 1998 Feb;39(2):300-8. doi: 10.1002/mrm.1910390218.
Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.
每当激活线性梯度时，就会产生具有非线性空间依赖性的伴随磁场。这是麦克斯韦方程组的结果，即在成像体积内磁场的散度必须为零，且旋度可忽略不计。伴随场或麦克斯韦场在MRI文献中已有超过10年的描述。在本文中，我们通过理论和实验证明了伴随场中另外两个最低阶项的存在，我们将其称为交叉项。伴随梯度交叉项仅在纵向梯度Gz与横向梯度（Gx或Gy）同时激活时出现。详细研究了所有伴随梯度项对相位对比成像的影响。描述了几种减少或消除由伴随磁场引起的相位误差的方法。证明了一种无需增加最小回波时间（TE）的联合脉冲序列 - 重建方法的可行性。由于伴随场的最低阶项与G²/B₀成正比，鉴于当前对具有更强梯度和/或更弱主磁场的系统的兴趣，伴随梯度项的重要性预计会增加。
