On The Potential For Passive Dielectric Materials To Improve Safety And Performance Of Magnetic Resonance Imaging

Open Access
Author:
Luo, Wei
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 25, 2014
Committee Members:
  • Qing X Yang, Dissertation Advisor
  • Qing X Yang, Committee Chair
  • Akhlesh Lakhtakia, Committee Member
  • Michael T Lanagan, Committee Member
  • Osama O Awadelkarim, Committee Member
  • Thomas Neuberger, Committee Member
  • Christopher Collins, Special Member
Keywords:
  • MRI
  • FDTD
  • Dielectric Material
Abstract:
High dielectric materials (HDM) have been used to improve aspects of MRI such as decreasing specific absorption rate (SAR), increasing local signal-to-noise ratio (SNR), and enhancing the excitation homogeneity, resulting in an improvement of MRI safety and an increase in imaging quality. The use of HDM comes with a relatively low cost and the HDM could be directly incorporated into the existing clinical practices without additional changes in MRI sequence and patient care. In the current research and development of HDM in MRI, there are several challenges. For example, (I) limited material has been introduced or created as HDM in MRI. (II) New measurement technique may be required to measure the electrical properties of a new HDM in the frequency range that is relevant to MRI. (III) Advanced optimization method is demanded to combine with numerical simulations to design the MRI coil incorporating HDM. (IV) The lack of a fair comparison between the coil design incorporating HDM and the coil design without HDM is one of the reasons that the coil engineers hesitate to use HDM in their design. (V) There is no general guideline on the usage of HDM for clinical practice and this is another reason that slows the development of HDM. Two different studies were discussed in this dissertation to advance the research and development of HDM in MRI. In the first study, a new HDM consisting of sintered BaTiO3 beads in deuterated water was created for MRI applications. This material has a much higher relative permittivity compared to all of the dielectric materials developed before. The relative permittivity and electrical conductivity of this new HDM were characterized at 3T (at approximately 125 MHz) and 7T (at approximately 300 MHz) using resonant cavities. Its superior performance in MRI was demonstrated by comparing the transmit efficiency, receive sensitivity, and SNR to previously used dielectric materials in simulation and experiment at 3T. This new HDM was later used to improve MRI performance in a spine imaging. In the second study, an attempt was made (I) to answer the question that whether an optimal MRI coil incorporated with HDM would outperform an optimal MRI coil without HDM and (II) to explore the improvement in coil efficiency brought by the effect of HDM. A simple hybrid design consisting of a loop coil and an HDM disk was optimized to image a lossy sphere at 123 MHz in electromagnetic simulation using particle swarm optimization. The coil location, coil radius, HDM location, HDM geometry, and HDM material properties were considered together as parameters in the optimization. The performance of the optimal hybrid design for imaging at any depth in the lossy sphere along the coil axis turned out to be better than the corresponding optimal Coil-only design, with an average of 87% and 23% improvement in coil efficiency for locations near the center and periphery of the sphere, respectively. This result showed, for the first time, that an optimal MRI coil incorporated with HDM would outperform an optimal MRI coil without HDM. It also showed, for the first time, that the HDM can improve the coil efficiency significantly in a location deep in the imaging sample. The optimization used in here demonstrated a possible method to design an hybrid MRI coil. This study provided valuable information and opened new possibilities for further HDM development in MRI applications.