Zur Y, Stokar S
Elscint MRI Center, Haifa, Israel.
Magn Reson Med. 1996 Feb;35(2):252-60. doi: 10.1002/mrm.1910350218.
Eddy currents, which are induced in the magnet cryostat by pulsed magnetic field gradients in MRI, generate undesired eddy fields within the imaging volume. In this work, an automated and computerized algorithm to compensate these eddy currents is presented. The compensation is done in two steps: (i) Eddy fields are symmetrized electronically with an R-C filter. (ii) The symmetric eddy fields are compensated by another R-C filter. The compensation algorithm is iterative; therefore, errors that remain from one iteration are eliminated in the next iteration. Hence, the compensation process is very robust and accurate. It is shown that all the even harmonics of the eddy fields are eliminated by the symmetrization process, but the odd field harmonics remain. The amplitude of these odd harmonics can be significantly reduced if the gradient coils are designed so that the field they generate is spatially similar to the eddy fields.
在磁共振成像（MRI）中，脉冲磁场梯度会在磁体低温恒温器中感应出涡流，这些涡流会在成像区域内产生不希望有的涡旋场。在这项工作中，提出了一种自动且计算机化的算法来补偿这些涡流。补偿分两步进行：（i）使用R - C滤波器对涡旋场进行电子对称化处理。（ii）通过另一个R - C滤波器对对称化后的涡旋场进行补偿。补偿算法是迭代的；因此，一次迭代中残留的误差会在下一次迭代中被消除。所以，补偿过程非常稳健且准确。结果表明，通过对称化处理可消除涡旋场的所有偶次谐波，但奇次场谐波仍然存在。如果梯度线圈设计成使其产生的场在空间上与涡旋场相似，那么这些奇次谐波的幅度可以显著降低。
