A Study on the Design and Optimization of RF Coils for Ultra-High Field MRI System = 초고자장 자기공명영상장치 고주파 코일의 최적화에 관한 연구

다국어 초록 (Multilingual Abstract)

I. Introduction
The ultimate advantage of the ultra-high field (UHF) magnetic resonance imaging (MRI) with increased static magnetic field, B0, intensity is the increase in signal-to-noise ratio (SNR). However, as B0 intensity increases, the non-uniformity of the electromagnetic field passing through the dielectric sample increases, resulting in degraded uniformity and SNR of the acquired image, and increasing the localized specific absorption rate (SAR). In this thesis, the following studies were conducted to improve these problems in UHF radio-frequency (RF) coils.

II. High-pass Spoke Coil for Parallel Imaging
The proposed study reports the design and testing of spoke coil (SC), that compensates for the degradation in the g-factor that is caused by the insufficient sensitivity of the coil array along the main magnetic field direction. EM simulations were performed to evaluate the performance of the proposed coil in a 7 T MRI system. 25-channel surface loop array and short dipole array containing SC or circular loop coil (LC) on the top of the phantom are 3D modeled. The SENSE g-factor and SNR of coil arrays are calculated and evaluated, and the quality factor (Q), scaling factor (α), and noise-corelated resistance (R) are considered. The directed effective magnetic field (B1−) generated by the SC improves the g-factor by compensating for the insufficient sensitivity information gathered by the array. However, this arrangement should be optimized to increase the available sensitivity information, depending on the type and structure of the coil elements in the array.

III. Bilateral Monopole Antenna for Improving RF Transmission
Inhomogeneous magnetic flux density under UHF MRI is mainly caused by a shortened RF wavelength propagating through a dielectric medium. Numerous studies on several types of traveling-wave antenna for UHF MRI are underway because of the potential alternative strategy of a large field of view (FOV) with deeper penetration depth. In this study, a bilateral monopole antenna is proposed to improve RF transmission field (|B1+|) uniformity at 297.2 MHz and 7 T. To investigate |B1+| uniformity in this scenario, a bilateral monopole antenna is optimized by performing an electromagnetic (EM) simulation using a finite-difference time-domain (FDTD) method. Improvements achieved using the optimized bilateral monopole antenna structure are discussed. The arranged two monopole antennas in opposite directions form a uniform |B1+| field distribution compared to the dipole and loop array.

IV. Coaxial Cable Loop Coil for Self-Decoupling
Array coils have been used in MRI as receive coils to achieve large FOV and reduce the image encoding time by implementing parallel imaging techniques. In RF coil array, the elements such as resonant loop coil have coupled each other due to magnetic and electric coupling. The common methods such as overlap, capacitive, and inductive decoupling have drawbacks that are applicable only adjacent coils. And the powerful preamplifier decoupling method is can be applied only to receive array. In this study, the self-decoupled loop coil composed of coaxial cable is proposed and evaluated. The self-decoupled coaxial cable loop coil compared with capacitive-decoupled and non-decoupled surface loop coil by calculating and measuring the loaded and unloaded quality factor ratio, transmit field simulation, and image acquisition. The proposed coaxial cable loop coil has lower efficiency compared to the surface loop coil, but has high isolation performance and can be applied to non-adjacent coil elements.

목차 (Table of Contents)

• ABSTRACT (IN ENGLISH) 1
• CONTENTS 4
• TABLE CONTENTS 6
• FIGURE CONTENTS 7
• I. Introduction 12
• 1. Ultra-High Field MRI 12
• 2. Ultra-High Field RF Coils 14
• 3. Objectives 17
• II. High-Pass Spoke Coil for Parallel Imaging 18
• 1. Introduction 18
• 2. Methods 20
• 3. Results and Discussion 29
• III. Bilateral Monopole Antenna for Improved RF Transmission 37
• 1. Introduction 37
• 2. Methods 39
• 3. Results and Discussion 43
• IV. Coaxial Cable Loop Coil for Self-Decoupling 50
• 1. Introduction 50
• 2. Methods 52
• 3. Results and Discussion 58
• V. Conclusion 68
• 1. High-Pass Spoke Coil for Parallel Imaging 69
• 2. Bilateral Monopole Antenna for Improved RF Transmission 70
• 3. Coaxial Cable Loop Coil for Self-Decoupling 71
• APPENDIX A. COAXIAL CABLE MODELING 72
• APPENDIX B. T/R SWITCH AND BALUN IN RF COILS 74
• ACKNOWLEDGEMENTS 79
• REFERENCES 80
• ABSTRACT (IN KOREAN) 89