dc.description.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. | en_US |