radio frequency antenna designs and methodologies for human brain computer interface and ...

191 Pages·2015·6.07 MB·English

RADIO FREQUENCY ANTENNA DESIGNS AND METHODOLOGIES
FOR HUMAN BRAIN COMPUTER INTERFACE
AND ULTRAHIGH FIELD MAGNETIC RESONANCE IMAGING

by
Yujuan Zhao
B.Eng., Hefei University of Technology, 2006
M.S., Shanghai Jiao Tong University, 2009

Submitted to the Graduate Faculty of
Swanson School of Engineering in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy

University of Pittsburgh
2015
UNIVERSITY OF PITTSBURGH
SWANSON SCHOOL OF ENGINEERING

This dissertation was presented

by

Yujuan Zhao

It was defended on
February 10, 2015
and approved by
George D. Stetten, M.D., Ph.D., Professor, Department of Bioengineering
Howard J. Aizenstein, M.D., Ph.D., Associate Professor, Psychiatry
Zhi-Hong Mao, Ph. D. Associate Professor, Departments of Electrical and Computer
Engineering
Dissertation Director: Tamer S. Ibrahim, Ph.D., Associate Professor, Department of
Bioengineering and Radiology

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Copyright © by Yujuan Zhao
2015
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RADIO FREQUENCY ANTENNA DESIGNS AND METHODOLOGIES
FOR HUMAN BRAIN COMPUTER INTERFACE
AND ULTRAHIGH FIELD MAGNETIC RESONANCE IMAGING
Yujuan Zhao, Ph.D.
University of Pittsburgh, 2015

Brain Computer Interface (BCI) and Magnetic Resonance Imaging (MRI) are two powerful
medical diagnostic techniques used for human brain studies. However, wired power connection
is a huge impediment for the clinical application of BCI, and most current BCIs have only been
designed for immobile users in a carefully controlled environment. For the ultrahigh field (≥7T)
MRI, limitations such as inhomogeneous distribution of the transmit field (B +) and potential
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high power deposition inside the human tissues have not yet been fully combated by existing
methods and are central in making ultrahigh field MRI practical for clinical use. In this
dissertation, radio frequency (RF) methods are applied and RF antennas/coils are designed and
optimized in order to overcome these barriers. These methods include: 1) designing implanted
miniature antennas to transmit power wirelessly for implanted BCIs; 2) optimizing a new 20-
channel transmit array design for 7 Tesla MRI neuroimaging applications; and 3) developing and
implementing a dual-optimization method to design the RF shielding for fast MRI imaging
methods.
First, three miniaturized implanted antennas are designed and results obtained using finite
difference time domain (FDTD) simulations demonstrate that a maximum RF power of up to 1.8
miliwatts can be received at 2 GHz when the antennas are implanted at the dura, without
violating the government safety regulations. Second, Eigenmode arrangement of the 20-channel
transmit coil allows control of RF excitation not only at the XY plane but also along the Z
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direction. The presented results show the optimized eigenmode could generate 3D uniform
transmit B + excitations. The optimization results have been verified by in-vivo experiments, and
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they are applied with different protocol sequences on a Siemens 7 Tesla MRI human whole body
scanner equipped with 8 parallel transmit channels. Third, echo planar imaging (EPI), B + maps
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and S matrix measurements are used to verify that the proposed RF shielding can suppress the
eddy currents while maintaining the RF characteristics of the transmit coil.
The contributions presented here will provide a long-term and safer power transmission
path compared to the wire-connected implanted BCIs and will bring ultrahigh field MRI
technology closer to clinical applications.

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TABLE OF CONTENTS
PREFACE ................................................................................................................................. XVI
1.0 INTRODUCTION ................................................................................................................. 1
1.1 MOTIVATION ........................................................................................................... 2
1.1.1 Challenges of Brain Computer interfaces: direct wire connections ........... 2
1.1.2 Challenges of Ultrahigh-Field MRI: Transmit Fields Inhomogeneity and
Specific Absorption Rate ................................................................................ 3
1.1.3 Challenges of Ultrahigh Field MRI: Eddy Currents .................................... 3
1.2 OBJECTIVES OF THIS DISSERTATION ............................................................. 4
1.3 THE STRUCTURE OF THIS DISSERTATION .................................................... 6
2.0 BACKGROUND .................................................................................................................. 11
2.1 BRAIN COMPUTER INTERFACE ....................................................................... 11
2.1.1 Brain Computer Interfaces Review ............................................................. 11
2.1.2 Brain Computer Interface Architecture ...................................................... 12
2.2 IMPLANTED ANTENNA WITHIN THE HUMAN ENVIRONMENT ............. 13
2.2.1 Antenna Geometry and Miniaturization Techniques ................................ 13
2.2.2 Wireless Data Transmission and Power Transmission .............................. 14
2.2.3 RF Safety ........................................................................................................ 15
2.3 MAGNETIC RESONANCE IMAGING ................................................................ 15
2.3.1 Static Magnetic Field ..................................................................................... 16
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2.3.2 RF Excitation ................................................................................................. 17
2.3.3 RF Coils .......................................................................................................... 18
2.3.4 Spatial Encoding ............................................................................................ 19
2.3.5 Pulse and Sequence Design ........................................................................... 20
2.4 FINITE DIFFERENCE TIME DOMAIN METHOD ........................................... 21
2.4.1 The Finite-Difference Time-Domain formulation ...................................... 21
2.4.2 One Dimensional Transmission Line Excitation ........................................ 26
2.5 FINITE ELEMENT METHOD............................................................................... 28
2.5.1 Finite Elements in Electromagnetics ............................................................ 29
3.0 IMPLANTED MINIATURIZED ANTENNA FOR BRAIN COMPUTER
INTERFACE APPLICATIONS ........................................................................................ 31
3.1 INTRODUCTION..................................................................................................... 31
3.2 MATERIALS AND METHODS ............................................................................. 33
3.2.1 FDTD Simulation and the Transmission Line Feed Model ....................... 33
3.2.2 Antenna Geometry and Antenna Performance Parameters ..................... 35
3.2.3 Human Head Model ...................................................................................... 36
3.2.4 Antenna Measurement Set-up ...................................................................... 37
3.3 RESULTS .................................................................................................................. 38
3.3.1 Measurement Validation of the FDTD Simulations ................................... 38
3.3.2 Effects of Ultra-thin Insulating Layers ........................................................ 39
3.3.3 Effects of the Insulating Layer Dielectric Properties ................................. 42
3.3.4 Effects of the Head Tissues Properties ........................................................ 43
3.3.5 Effects of the Human Head Phantom Shape and Dielectric Properties ... 46
3.3.6 Designs of the Implanted Antennas ............................................................. 48
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3.3.7 Maximum Power Reception without SAR Violations ................................ 51
3.4 CONCLUSION ......................................................................................................... 53
4.0 IN-DEPTH ANALYSIS OF THE ELECTROMAGNETIC PSEUDO MODES
PREDUCED BY A 20 CHANNEL TIC-TAC-TOE TRANSMIT ARRAY ................... 55
4.1 INTRODUCTION..................................................................................................... 55
4.2 MATERIAL AND METHODS ............................................................................... 57
4.2.1 The RF Coil .................................................................................................... 57
4.2.2 Determination of Eigenmodes ...................................................................... 59
4.2.3 Helmholtz Equation and Current Requirements ....................................... 60
4.2.4 Simulations and Experiments ....................................................................... 62
4.3 RESULTS .................................................................................................................. 63
4.3.1 Eigenmodes inside the Phantom and the Head Model ............................... 63
4.3.1.1 B + Field and SAR Comparison for Mode 1 at Different Levels ........ 63
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4.3.1.2 B + Field and SAR Comparison for Other Modes at Different Levels66
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4.3.1.3 Peak Local SAR....................................................................................... 69
4.3.2 Experimental Demonstration ....................................................................... 70
4.4 DISCUSSION AND CONCLUSION ...................................................................... 72
5.0 TRANSMIT ARRAY EIGENMODES OPTIMIZATION .............................................. 75
5.1 INTRODUCTION..................................................................................................... 75
5.2 MATERIALS AND METHODS ............................................................................. 77
5.2.1 RF Coil and Eigenmodes ............................................................................... 77
5.2.2 FDTD Simulations and Field Optimization ................................................ 78
5.2.3 Experiments ................................................................................................... 80
5.3 RESULTS .................................................................................................................. 81
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5.3.1 Slice Excitation Verification and Limits of Homogenous Slice via RF
Shimming ....................................................................................................... 81
5.3.2 3D Field Simulation Verification .................................................................. 84
5.3.3 Transmit Field and Absorbed Power Efficiency ........................................ 85
5.3.4 Optimization Criteria Comparison .............................................................. 87
5.3.5 Comparison between Different Coils and Applications ............................. 89
5.3.6 Experimental Verifications ........................................................................... 91
5.3.7 20 Modes and B + shimming Optimizations Comparison .......................... 92
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5.3.8 Virtual Observation Points Applications and Verification........................ 93
5.4 DISCUSSION AND CONCLUSION ...................................................................... 96
6.0 DUAL OPTIMIZATION METHOD OF RF AND QUASI-STATIC FIELD
SIMULATIONS FOR REDUCTION OF EDDY CURRENTS GENERATED ON 7T
RF COIL SHIELDING ....................................................................................................... 98
6.1 INTRODUCTION..................................................................................................... 98
6.2 METHODS .............................................................................................................. 100
6.2.1 The RF Coil .................................................................................................. 100
6.2.2 Gradient Field Induced Eddy Current Simulations (FEM) .................... 102
6.2.3 Full Wave RF Field Simulations (FDTD) .................................................. 105
6.2.4 RF Testing and 7T Experiments ................................................................ 106
6.3 RESULTS ................................................................................................................ 107
6.3.1 Eddy Current Simulation Verification and Z Gradient Field Behavior
along the Magnet Axis ................................................................................ 107
6.3.2 Comparison of Rectangular and Circular Shielding................................ 109
6.3.3 Effects of Thickness of Copper Layers and the Top Panel ...................... 111
6.3.4 Effects of Simple-Structured Slots ............................................................. 114
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6.3.5 Dual Optimization Approach ..................................................................... 115
6.3.6 In-Vivo Demonstration ................................................................................ 117
6.4 DISCUSSION .......................................................................................................... 118
7.0 CONCLUSIONS AND FUTURE WORK ...................................................................... 123
7.1 SUMMARY AND FINDINGS ............................................................................... 123
7.2 CONTRIBUTION OF THIS DISSERTATION .................................................. 124
7.2.1 Non 50 Ohm Antenna and SAR Regulation Considerations ................... 124
7.2.2 A New RF coil Mode Excitation Paradigm ............................................... 126
7.2.3 New Eddy Currents Calculation and Shielding Slot Methods ................ 127
7.3 FUTURE WORKS .................................................................................................. 128
7.3.1 Implanted Antenna Designed for Wireless Power Transmission ........... 128
7.3.2 RF Coil Designed for 7 Tesla MRI ............................................................. 129
APPENDIX A ............................................................................................................................ 130
APPENDIX B ............................................................................................................................ 135
BIBLIOGRAPHY ..................................................................................................................... 157
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