B1 + phase retrieval for non-quadrature radio frequency excitation and its preliminary application in MR-EPT

Abstract

Non-quadrature radio frequency (RF) excitation has been widely studied in the fields of RF shimming, local SAR estimation, and MR-EPT with the use of multi-channel transceiver arrays. These studies generally require the retrieval of the complex transmit field ( ), which can be accomplished by acquiring its magnitude and phase in different steps. Magnitude of the transmit field is acquired with the conventional methods which give accurate results for both quadrature and non-quadrature excitations. On the other hand, there is no straightforward method to acquire the absolute phase of the transmit field and generally approximations in MRI experiments are made in order to get it. However, many of these approximations fail in non-quadrature excitation and/or in ultra high fields. In this study, we propose a simple method to acquire the absolute transmit phase in non-quadrature excitation with an eight channel transceiver TEM array for 3 T. The proposed method requires the application of a single additional quadrature drive in order to get the receive phases of the individual channels of the transceiver coil. These receive phases are then subtracted from the transceive phase of the non-quadrature drive experiment to acquire its transmit phase. The developed ideas are tested in the framework of simulations and MRI experiments with the use of four different non-quadrature drive configurations. It has been observed that the simulated and experimentally acquired transmit phase distributions tend to have a strong consensus which supports the validity of the proposed method. Finally, the estimated transmit phase distribution of non-quadrature drive is used in the standard MR-EPT study to get the conductivity reconstructions in order for the validation of its eligibility in MR-EPT studies.