Guo Lei, Jin Jin, Li Mingyan, Wang Yaohui, Liu Chun Yi, Liu Feng, Crozier Stuart
IEEE Trans Biomed Eng. 2018 Nov 5. doi: 10.1109/TBME.2018.2879667.
Current integral-equation (IE) based MR electrical properties tomography (EPT) methods utilize simulated incident radio-frequency (RF) fields, which are inaccurate and lead to reconstruction errors. To improve the accuracy and practicability of IE-based MR-EPT methods, a new approach is presented that obtains the incident fields using reference subjects and RF field mapping techniques. The Incident field approximation (IFA) is first demonstrated in this paper. This approximation assumes that two imaged subjects with similar coil/subject interactions will have similar incident RF fields, thus one can feed the estimation of the incident fields within the imaged subject into the calculation of those within a homogeneous subject (reference subject). This is done by measuring the total RF fields ( field) of the reference using field mapping techniques, using the known EPs of the reference subject and by rearranging Ampere's Law and the integral equations. The calculated incident RF fields are then used to reconstruct the EPs' distribution with a three-dimensional (3D) integral-based MR-EPT method. Numerical simulation results indicated that the incident RF fields obtained from the reference subject provide accurate 3D reconstruction of EPs with less than 16% root mean square error (RMSE) in noise-free scenario while the conventional IE method had more than 28% RMSE. The phantom-based experiments at 9.4T MRI system have also been conducted to evaluate the performance of the proposed method and the results indicated that the proposed method achieved desirable robustness against the noise in practical scenario with less than 21% RMSE while the conventional differential equation-based method showed worse than 37% RMSE.
当前基于积分方程（IE）的磁共振电阻抗断层成像（EPT）方法利用模拟的入射射频（RF）场，这些场不准确并导致重建误差。为了提高基于IE的MR-EPT方法的准确性和实用性，提出了一种新方法，该方法使用参考对象和RF场映射技术来获取入射场。本文首次论证了入射场近似（IFA）。这种近似假设两个具有相似线圈/对象相互作用的成像对象将具有相似的入射RF场，因此可以将成像对象内入射场的估计值输入到均匀对象（参考对象）内入射场的计算中。这是通过使用场映射技术测量参考对象的总RF场（场）、使用参考对象的已知电特性（EPs）并重新排列安培定律和积分方程来实现的。然后，使用计算得到的入射RF场，通过基于三维（3D）积分的MR-EPT方法重建EPs的分布。数值模拟结果表明，在无噪声情况下，从参考对象获得的入射RF场能够以小于16%的均方根误差（RMSE）对EPs进行准确的3D重建，而传统的IE方法的RMSE超过28%。还在9.4T MRI系统上进行了基于模型的实验，以评估所提出方法的性能，结果表明，在所提出的方法在实际场景中对噪声具有理想的鲁棒性，RMSE小于21%，而传统的基于微分方程的方法的RMSE则超过37%。
