TEMPERATURE AND DUAL-FREQUENCY CONDUCTIVITY MAPPING USING MR-BASED ELECTRICAL TISSUE PROPERTY IMAGING
Kyung Hee University Graduate School
Eung Je Woo
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Magnetic resonance electrical impedance tomography (MREIT) is being developed to produce high-resolution cross-sectional conductivity images of an object. Injecting an external current to an electrically conducting object produces internal distributions of voltage, current density and change in the phase of precessing protons which is reflected in the MRI phase image. Using an MRI scanner, we can measure the z component of induced magnetic flux density (Bz) inside the object which in turn used to produce conductivity image. Magnetic resonance electrical property tomography (MREPT) derives the electric conductivity and permittivity from the spatially sensitive B1 distributions arising from the applied RF coils, which can be obtained by mapping the transmit RF field. MREPT does not use any additional devices such as external electrodes, currents, or RF probes to enhance the feasibility of the approach. In addition, MREPT does not require the solution of an inverse problem that could compromise spatial image resolution. Conductivity distributions can be reconstructed to a certain accuracy from phase images while permittivity distributions can be reconstructed approximately from magnitude images of the RF transmit field. Since the biological tissues show frequency-dependent conductivity spectra, their values at different frequencies may provide valuable diagnostic information. Several studies have reported unique conductivity distributions in various phantom, animal, and human experiments. MREIT and MREPT techniques are, therefore, complementary and can provide new information when combined. In first part of thesis, we will then present a method for low- and high-frequency conductivity images using a single scan. Presenting results of various phantom imaging experiments and animal disease models, we will show that MREIT and MREPT are complementary and suggest future experimental studies on animals and human subjects. Temperatures mapping of core body or local region are used in wide clinical applications. We focus on the fact that there is approximatively linear relationship between tissue conductivity and temperature, even though it is likely confounded by changes in cellular morphology induced by the heat. Since the electrical property of tissue becomes more conductive due to increasing ion mobility as it is heated, monitoring the internal conductivity distribution is an alternative to detect the temperature distribution in the region of interest (ROI). Using the ability that the MREIT technique acquires high-resolution internal conductivity distribution, in this thesis, we propose a method to provide a potential non-invasive alternative to monitor temperature distribution from recovering the electrical conductivity distribution by measuring one component of the magnetic flux density vector subjected to the injected current. Since MREIT uses externally injected current to measure internal conductivity variation, the proposed method provides the internal conductivity information and it can measure the internal signal variations simultaneously from MR phase information by removing the magnetic flux density signal by the externally injected current. We tried to find the relation between temperature and conductivity contrast from phantom experiment results. We also develop a continuous monitoring method for temperature distribution and tissue property changes during RF ablation via recovering the electrical conductivity using MREIT. To enhance the quality of the measured Bz data, we used the injected current nonlinear encoding (ICNE) method based on a fast interleaved multi-echo gradient recalled echo (MGRE) pulse sequence with multi-channel coil and optimized the multiple eight echo responses using the Bz optimization method. We evaluate the proposed MREIT continuous monitoring method for RF ablation on a bovine phantom with high purified bipolar silver electrodes and a filter stage to apply RF energy without artifacts from the ablator. To verify the temporal and regional relation between the temperature and the electrical conductivity variation, we made chronological maps from the time-series of the conductivity images during ablation. We calculated the relative conductivity contrast ratio (rCCR) between the ablated region and the control region which was far from the ablation area in the reconstructed images. This feasibility study demonstrated that the conductivity images produced by MREIT can monitor the RF ablation procedure continuously. In addition, we analyzed the physiological status of tissue to characterize the ablation lesion using conductivity changes.