IEEE Trans Med Imaging. 2020 Jan;39(1):175-187. doi: 10.1109/TMI.2019.2922318. Epub 2019 Jun 13.
Many integral equation (IE)-based magnetic resonance electrical property tomography (MR-EPT) methods use unloaded incident radio-frequency (RF) fields from simulations that may not fully reflect the real situation and thus lead to reconstruction errors. To improve the accuracy of IE-based MR-EPT methods, a novel approach that enables the calculation of loaded coil current distributions and avoids the explicit use of incident RF fields is presented in this paper. In the proposed method, a hybrid source composed of the current source from the coil and the contrast source from the subject are introduced in the integral equations. Because the loaded coil current distributions can be extracted from the reconstructed hybrid source, the simulated incident RF fields are eliminated from the problem formulations. To improve the convergence performance, a modified conjugate gradient (CG) scheme was used where the gradients of the current source and contrast source were balanced through using different weighting parameters. The proposed method was verified through full-wave simulations at 9.4 and 7 T involving a heterogeneous ball and an anatomical head phantom. The numerical results indicated that by using the proposed method, an accurate coil current distributions and EPs profiles can be reconstructed and the desirable robustness against noise can also be achieved.
许多基于积分方程 (IE) 的磁共振电阻抗断层成像 (MR-EPT) 方法使用来自模拟的未加载的入射射频 (RF) 场，这些场可能无法完全反映实际情况，从而导致重建误差。为了提高基于 IE 的 MR-EPT 方法的准确性，本文提出了一种新的方法，该方法能够计算加载线圈电流分布，并且避免显式使用入射 RF 场。在提出的方法中，在积分方程中引入了由线圈电流源和对比物源组成的混合源。由于可以从重建的混合源中提取加载线圈电流分布，因此从问题公式中消除了模拟入射 RF 场。为了提高收敛性能，使用了修正的共轭梯度 (CG) 方案，其中通过使用不同的加权参数来平衡电流源和对比物源的梯度。该方法在 9.4 和 7 T 下的全波模拟中进行了验证，涉及不均匀球和解剖头模型。数值结果表明，通过使用所提出的方法，可以重建准确的线圈电流分布和 EP 谱，并实现对噪声的理想鲁棒性。
