dc.contributor.advisor | İder, Yusuf Ziya | |
dc.contributor.author | Yıldız, Gülşah | |
dc.date.accessioned | 2018-08-29T13:40:40Z | |
dc.date.available | 2018-08-29T13:40:40Z | |
dc.date.copyright | 2018-08 | |
dc.date.issued | 2018-08 | |
dc.date.submitted | 2018-08-28 | |
dc.identifier.uri | http://hdl.handle.net/11693/47755 | |
dc.description | Cataloged from PDF version of article. | en_US |
dc.description | Thesis (M.S.): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2018. | en_US |
dc.description | Includes bibliographical references (leaves 54-60). | en_US |
dc.description.abstract | 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. | en_US |
dc.description.statementofresponsibility | by Gülşah Yıldız. | en_US |
dc.format.extent | xvi, 62 leaves : illustrations, charts (some color) ; 30 cm. | en_US |
dc.language.iso | English | en_US |
dc.rights | info:eu-repo/semantics/openAccess | en_US |
dc.subject | Magnetic Resonance Electrical Properties Tomography (MREPT) | en_US |
dc.subject | Convection Reaction Equation Based MREPT (cr-MREPT) | en_US |
dc.subject | Electrical Property Imaging | en_US |
dc.subject | Conductivity Imaging | en_US |
dc.subject | Padding | en_US |
dc.subject | LCF | en_US |
dc.subject | Multichannel Electrical Property Imaging | en_US |
dc.title | Low convective field artifact elimination using dielectric padding and multichannel receive in cr-MREPT conductivity images | en_US |
dc.title.alternative | Dielektrik yastık ve çok kanallı alıcı kullanımı ile kr-MREÖT iletkenlik görüntülerinde düşük konvektif bölge artefaktlarının giderimi | en_US |
dc.type | Thesis | en_US |
dc.department | Department of Electrical and Electronics Engineering | en_US |
dc.publisher | Bilkent University | en_US |
dc.description.degree | M.S. | en_US |
dc.identifier.itemid | B158922 | |
dc.embargo.release | 2019-05-01 | |