Trakic A, Liu F, Lopez H Sanchez, Wang H, Crozier S
School of Information Technology and Electric Engineering, University of Queensland, Brisbane, Australia.
Magn Reson Med. 2007 Jun;57(6):1119-30. doi: 10.1002/mrm.21243.
The switching of magnetic field gradient coils in magnetic resonance imaging (MRI) inevitably induces transient eddy currents in conducting system components, such as the cryostat vessel. These secondary currents degrade the spatial and temporal performance of the gradient coils, and compensation methods are commonly employed to correct for these distortions. This theoretical study shows that by incorporating the eddy currents into the coil optimization process, it is possible to modify a gradient coil design so that the fields created by the coil and the eddy currents combine together to generate a spatially homogeneous gradient that follows the input pulse. Shielded and unshielded longitudinal gradient coils are used to exemplify this novel approach. To assist in the evaluation of transient eddy currents induced within a realistic cryostat vessel, a low-frequency finite-difference time-domain (FDTD) method using the total-field scattered-field (TFSF) scheme was performed. The simulations demonstrate the effectiveness of the proposed method for optimizing longitudinal gradient fields while taking into account the spatial and temporal behavior of the eddy currents.
在磁共振成像（MRI）中，磁场梯度线圈的切换不可避免地会在诸如低温容器等导电系统部件中感应出瞬态涡流。这些二次电流会降低梯度线圈的空间和时间性能，通常采用补偿方法来校正这些失真。这项理论研究表明，通过将涡流纳入线圈优化过程，可以修改梯度线圈设计，使线圈产生的场与涡流共同作用，产生一个跟随输入脉冲的空间均匀梯度。屏蔽和非屏蔽纵向梯度线圈被用来例证这种新方法。为了协助评估实际低温容器内感应出的瞬态涡流，采用了使用总场散射场（TFSF）方案的低频时域有限差分（FDTD）方法。模拟结果证明了所提出的方法在考虑涡流的空间和时间行为的同时优化纵向梯度场的有效性。
