Transceive phase corrected 2D contrast source inversion-electrical properties tomography.

Peter R S Stijnman,Cornelis A T Van Den Berg,Rob F Remis,Stefano Mandija Stefano Mandija,Patrick S Fuchs

doi:10.1002/mrm.28619

Abstract

PurposeTo remove the necessity of the tranceive phase assumption for CSI‐EPT and show electrical properties maps reconstructed from measured data obtained using a standard 3T birdcage body coil setup.MethodsThe existing CSI‐EPT algorithm is reformulated to use the transceive phase rather than relying on the transceive phase assumption. Furthermore, the radio frequency (RF)‐shield is numerically implemented to accurately model the RF fields inside the MRI scanner. We verify that the reformulated two‐dimensional (2D) CSI‐EPT algorithm can reconstruct electrical properties maps given 2D electromagnetic simulations. Afterward, the algorithm is tested with three‐dimensional (3D) FDTD simulations to investigate if the 2D CSI‐EPT can retrieve the electrical properties for 3D RF fields. Finally, an MR experiment at 3T with a phantom is performed.ResultsFrom the results of the 2D simulations, it is seen that CSI‐EPT can reconstruct the electrical properties using MRI accessible quantities. For 3D simulations, it is observed that the electrical properties are underestimated, nonetheless, CSI‐EPT has a lower standard deviation than the standard Helmholtz based methods. Finally, the first CSI‐EPT reconstructions based on measured data are presented showing comparable accuracy and precision to reconstructions based on simulated data, and demonstrating the feasibility of CSI‐EPT.ConclusionsThe CSI‐EPT algorithm was rewritten to use MRI accessible quantities. This allows for CSI‐EPT to fully exploit the benefits of the higher static magnetic field strengths with a standard quadrature birdcage coil setup.

Highlights

Electrical properties tomography (EPT) is an MR-based technique aiming at measuring the electrical properties of tissues
The inclusion of the radio frequency (RF)-shield, even with a numerical approximation, results in a more realistic model of the incident RF fields which helps with the practical implementation of the contrast source inversion (CSI)-EPT method
The reconstruction for the transceive phase corrected CSI-EPT algorithm improves due to the higher field strength with its inherently increased sensitivity

Summary

Introduction

Electrical properties tomography (EPT) is an MR-based technique aiming at measuring the electrical properties (conductivity and permittivity) of tissues This is achieved in a non-invasive manner through MRI-based mapping of the circularly polarized magnetic component (B1+, the transmit efficiency) of the transmit radio frequency (RF) field. There is a variety of different EPT approaches that have been recently published as shown in the review work.[1] A large group of these approaches are derivative based and stem from the Helmholtz equation for magnetic fields.[13,14,15,16,17] In these approaches, a second-order derivative using finite difference kernels needs to be computed on the measured B1+ fields This leads to noise amplification in the reconstructed EPT maps and introduces errors in the reconstruction of the electrical properties most notably at tissue boundaries.[18,19]

Methods

Results

Discussion

Conclusion