Improvement and comparison of complex B₁ mapping techniques for use in MREPT

Abstract

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.