Browsing by Subject "Magnetic Resonance Electrical Properties Tomography (MREPT)"

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Open Access
Improvement and comparison of complex B₁ mapping techniques for use in MREPT
(2018-09) Özdemir, Safa
Impedance imaging, (i.e., conductivity, , and permittivity, ) provides helpful information about contrast between healthy and malignant tissues. As one of the impedance imaging techniques, Magnetic Resonance Electrical Properties Tomography (MREPT) uses the perturbation on B1 caused by electrical properties, and via solving the inverse problem with the help of measured B1 field, electrical properties are obtained. Therefore, to obtain conductivity using MREPT, the knowledge of B1 phase and magnitude is required. This thesis focuses on improvement and comparison of complex B1 mapping techniques for use in MREPT. In this manner, balanced steady-state free precession (bSSFP) imaging, which is one of the best candidates to obtain B1 phase, is investigated. bSSFP imaging has high speed, high signal-to-noise ratio (SNR), motion insensitivity and automatic eddy current compensation. On the other hand, it suffers greatly from B0 inhomogeneity and the concomitant "banding artifact". In regions of banding artifact, MR signal reduces significantly in magnitude, and also phase errors occur. The correction of phase errors is conducted by using three different techniques: Inserting B0 and T2 information, linearization for off-resonance estimation (LORE) algorithm, and PLANET method. In the next step, 2D version of phase-based convection-reaction equation based MREPT (phase-based cr-MREPT) technique is utilized to obtain conductivity maps from corrected phase images that are acquired from three aforementioned techniques. In order to verify the effects of correction techniques, an experimental agar-saline phantom with conductivity contrasts is constructed. It is shown that, for all phase correcting techniques, banding artifact is removed from phase images and accurate conductivity maps are obtained. Yet, inserting B0 and T2 information results in lengthy scanning time if both B0 and T2 information is acquired via traditional, reliable methods which are widely considered as golden truth. On the other hand, PLANET method suffers from B0 drift and propagation of error. Therefore, LORE algorithm is considered as the best candidate to obtain B1 phase images which is required to find conductivity maps. Besides phase-based MREPT methods, there also exists MREPT methods that requires both B1 phase and magnitude information. In the purpose of acquiring B1 magnitude images, three different methods are investigated, namely double angle (DA) method, actual ip-angle imaging (AFI) method, and Bloch-Siegert shift (BSS) based method. To analyze B1 magnitude mapping qualities of these methods, theoretical SNR calculations and phantom experiments are conducted. Both theoretical and experimental studies reveal that, based on SNR results, BSS based method is advantageous over AFI method and DA method. For each of B1 magnitude mapping methods, conductivity maps are obtained. It is found that, although standard MREPT method is indifferent to the choice of B1 magnitude mapping methods, high-SNR B1 magnitude maps provide better conductivity results for standard cr-MREPT method.
Open Access
Iterative fitting approach to CR-MREPT
(2019-06) Boğa, Çelik
Electrical properties (conductivity, and permittivity, ) imaging, reveals information about the contrast between tissues. Magnetic Resonance Electrical Properties Tomography (MREPT) is one of the electrical properties imaging techniques, which provides conductivity and permittivity images at Larmor frequency using the perturbations in the transmit magnetic eld, B+ 1 . Standard-MREPT (std-MREPT) method is the simplest method for obtaining electrical properties from the B+ 1 eld distribution, however it su ers from the boundary artifacts between tissue transitions. In order to eliminate this artifact, many methods are proposed. One such method is the Convection Reaction equation based MREPT (cr-MREPT). cr-MREPT method solves the boundary artifact problem, however Low Convective Field (LCF) artifact occurs in the resulting electrical property images. In this thesis, Iterative Fitting Approach to cr-MREPT is developed for investigating the possibility of elimination of LCF artifact. In this method, forward problem of obtaining magnetic eld with the given electrical properties inside the region of interest is solved iteratively and electrical properties are updated at each iteration until the di erence between the solution of the forward problem and the measured magnetic eld is small. Forward problem is a di usion convection reaction partial di erential equation and the solution for the magnetic eld is obtained by the Finite Di erence Method. By using the norm of the difference between the solution of the forward problem and the measured magnetic eld, electrical properties are obtained via Gauss-Newton method. Obtaining electrical property updates at each iteration, is not a well conditioned problem therefore Tikhonov and Total Variation regularizations are implemented to solve this problem. For the realization of the Total Variation regularization, Primal Dual Interior Point Method (PDIPM) is used. Using the COMSOL Multiphysics, simulation phantoms are modeled and B+ 1 data for each phantom is generated for electrical property reconstructions. 2D simulation phantom, modeled as an in- nitely long cylindrical object, is assumed to be under the e ect of the clockwise rotating radio-frequency (RF) eld. Second phantom modeled, is a cylindrical object with nite length and z- independent electrical properties, that is placed in a Quadrature Birdcage Coil (QBC). Third phantom modeled is a cylindrical object placed in a QBC, with z- dependent electrical properties. In addition to the simulation phantoms, z- independent experimental phantoms are also created for MRI experiments. Conductivity reconstructions of 2D simulation phantom, do not su er from LCF artifact and have accurate conductivity values for both Tikhonov and Total Variation regularizations. While, 2D center slice reconstructions of the zindependent simulation and experimental phantoms do not have LCF artifact, resulting conductivity values are lower than the expected conductivity values. These low conductivity values are obtained because of the inaccurate solution of the forward problem in 2D for 3D phantoms. When Iterative Fitting Approach is extended to 3D, such that solution of the forward problem is also obtained in 3D, resulting electrical property reconstruction does not have LCF artifact and obtained conductivity values are as expected for both z- independent simulation and experimental phantom. Reconstructions obtained for the z- dependent simulation phantom shows that electrical properties varying all 3 directions can be accurately reconstructed using Iterative Fitting Approach. For Iterative Fitting Approach reconstructions, voxel size of 2 mm is used for the 3D experimental phantom and voxel size of 1.5 mm is used for all simulation phantoms and 2D experimental phantom. Reconstructions obtained for all phantom with Iterative Fitting Approach are LCF artifact free. Conductivity reconstructions obtained using Tikhonov and Total Variation regularizations have similar resolutions (1-2 pixels) but Total Variation regularization results in smoother conductivity values inside the tissues compared to the Tikhonov regularization.
Open Access
Low convective field artifact elimination using dielectric padding and multichannel receive in cr-MREPT conductivity images
(2018-08) Yıldız, Gülşah
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.