Low convective field artifact elimination using dielectric padding and multichannel receive in cr-MREPT conductivity images
Imaging the electrical conductivity of the tissues in RF frequencies is an important tool for medical diagnostic purposes along with the local specific absorption rate estimation that is closely related to MR safety aspects. Magnetic Resonance Electrical Properties Tomography (MREPT) algorithms use the fact that the electrical properties of the object of interest perturb the B1 field and that they can be reconstructed by solving an inverse problem that requires the measured B1 field. Convection-reaction-equation based magnetic resonance electrical properties tomography (cr-MREPT) provides conductivity images that are boundary artifact free and robust against noise in contrast to conventional MREPT algorithms. However, these images suffer from the Low Convective Field (LCF) artifact. This thesis propose two methods to eliminate the LCF artifact. One of which is to use dielectric pads in alternating positions to modify the transmit magnetic field and shift the LCF region from each other in different excitation data. Within an electromagnetic model, pads with different parameters (electrical properties, pad thickness, pad height, arc angle, and thickness of the pad-object gap) are simulated. First, the effect of high dielectric and high conductive pads onto the B1 field is analyzed. Then, two data sets with the pad located on various locations of the object (phantom) are acquired, and the corresponding linear system of equations are simultaneously solved (combined) to get LCF artifact free conductivity images. In experimental studies, water pads and BaTiO3 pads are used with agar-saline phantoms. In general, a pad should have 180ffi arc angle and the same height with the phantom for maximum benefit. Also, the closer the pad is to the phantom, the more pronounced is its effect. Increasing the pad thickness and/or the relative permittivity of the pad increases the LCF shift while excessive amounts of these parameters cause errors in conductivity reconstructions because of the failure in the assumption made such that the z-component of the magnetic field (HZ) is neglected in the solution. Conductivity of the pad, on the other hand, has minimal effect on elimination of the LCF artifact. Using the proposed technique, LCF artifact is removed and also the reconstructed conductivity values are improved. Thick water pads are proved to be better than the thin ones whereas high dielectric pads must be preferred as thin. The drawbacks of this method are that the acquisition time increases with the multiples of the excitation number and that the HZ assumption may fail to validate significantly with the choice of pad parameters. The second method proposes a solution that requires 1 excitation only and circumvents the LCF artifact. It uses the difference between the receive sensitivities of a multichannel receive coil as a means to alter the LCF regions in each channel data. Although it loses its accuracy for a non-quadrature coil, transceive phase assumption, which approximates the transmit phase as the half of the transcieve phase, is utilized and the data formed from different channels are combined to reconstruct LCF-free conductivity images. Comparing the results, this latter technique is superior to the original method as LCF artifact is eliminated and is superior to the padding technique as it requires at least half the time required for padding. However, the multichannel receive method lacks accuracy due to the incorrect phase, whereas it can be a valuable tool for non-quantitative conductivity imaging that only the contrast between the neighboring tissues is sufficient.
Thesis (M.S.): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2018.
Includes bibliographical references (leaves 54-60).