Reference-Based Integral MR-EPT: Simulation and Experiment Studies at 9.4 T MRI

Current integral-equation (IE)-based magnetic resonance (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 (${{\boldsymbol B}}_1^ + $ 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 (3-D) integral-based MR-EPT method. Numerical simulation results indicated that the incident RF fields obtained from the reference subject provide accurate 3-D 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 9.4 T 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.