Institute of Neuroscience and Medicine 4, INM-4, Forschungszentrum Jülich, Jülich, Germany.
RWTH Aachen University, Aachen, Germany.
Magn Reson Med. 2021 May;85(5):2892-2903. doi: 10.1002/mrm.28597. Epub 2020 Nov 17.
To propose two innovations to existing eddy current characterization techniques, which include (1) an efficient spatio-temporal sampling scheme and (2) a model-based fitting of spherical harmonic eddy current components.
This work introduces a three-plane 2D image-based acquisition scheme to efficiently sample eddy current fields. Additionally, a model-based spherical harmonic decomposition is presented, which reduces fitting noise using a rank minimization to impose an exponential decay on the eddy current amplitude evolution. Both techniques are applied in combination and analyzed in simulations for their applicability in reconstructing suitable pre-emphasis parameters. In a proof-of-concept measurement, the routine is tested for its propriety to correctly quantify user-defined field dynamics. Furthermore, based on acquired precompensation and postcompensation eddy current data, the suitability of pre-emphasis parameters calculated based on the proposed technique is evaluated.
Simulation results derived from 500 data sets demonstrate the applicability of the acquisition scheme for the spatio-temporal sampling of eddy current fields. Compared with a conventional data processing strategy, the proposed model-based approach yields pre-emphasis parameters that reduce the average maximum residual field offset within a 10-cm-diameter spherical volume from 3.17 Hz to 0.58 Hz. Experimental data prove the proposed routine to be suitable to measure and effectively compensate for eddy currents within 10 minutes of acquisition time.
The proposed framework was found to be well-suited to efficiently characterize and compensate for eddy current fields in a one-time calibration effort. It can be applied to facilitate pre-emphasis implementations, such as for dynamic B shimming applications.
提出两种创新的涡流特性分析技术，包括（1）高效的时空采样方案和（2）基于模型的球谐涡流分量拟合。
本工作引入了一种基于三平面 2D 图像的采集方案，以有效地采样涡流场。此外，还提出了一种基于模型的球谐分解方法，该方法通过秩最小化减少拟合噪声，对涡流幅度演化施加指数衰减。这两种技术结合使用，并在模拟中进行了分析，以评估其在重构合适的预强调参数方面的适用性。在概念验证测量中，测试了该方法是否适合正确量化用户定义的场动力学。此外，基于获取的预补偿和后补偿涡流数据，评估了基于所提出技术计算的预强调参数的适用性。
从 500 个数据集得出的模拟结果表明，该采集方案适用于涡流场的时空采样。与传统的数据处理策略相比，所提出的基于模型的方法可以将 10cm 直径球体内平均最大残余场偏移的预强调参数从 3.17Hz 降低到 0.58Hz。实验数据证明了所提出的方案适合在 10 分钟的采集时间内测量和有效地补偿涡流。
所提出的框架被发现非常适合在一次校准中高效地描述和补偿涡流场。它可用于促进预强调的实施，例如用于动态 B 匀场应用。
