Optimal multichannel transmission for improved cr-MREPT

Magnetic resonance electrical properties tomography (MR-EPT), aiming at reconstructing the EP's at radio frequencies, uses the H+ field (both magnitude and phase) distribution within the object. One of the MR-EPT algorithms, cr-MREPT, accurately reconstructs the internal tissue boundaries, however, it faces an artifact which occurs at the regions where the convective field, [partial derivative H+/partial derivative x-i partial derivative H+/partial derivative y, i(partial derivative H+/partial derivative x-i partial derivative H+/partial derivative y)](T), has a low magnitude (at the noise level). This study aims to develop an artifact-free conductivity reconstruction by modifying the H+ field inside the region of interest (ROI), using multiple RF transmission techniques in MRI. An eight channel multi-transmit transverse electromagnetic array is used in two different drive configurations. The first drive is the standard volume excitation configuration where all ports are driven with the same magnitude and with 45 degrees phase increment between adjacent channels. In the second drive, the drive voltage magnitude and phases for each of the eight drive ports are modified to generate a desired H+ distribution such that the low convective field region moves to another non-overlapping position. Finally, data from both drive experiments are simultaneously used to reconstruct EP's. Computer simulations using cylindrical phantoms and a brain model are conducted and it is shown that the low convective field artifact can be eliminated. It is further shown that it is not necessary to re-calculate the port drive RF voltage magnitude and phases for each patient. The implementation issues of this method are briefly discussed.