The application of electromagnetic theory in microwave therapy and magnetic resonance imaging

THE APPLICATION OF ELECTROMAGNETIC THEORY IN MICROWAVE THERAPY AND MAGNETIC RESONANCE IMAGING LIANG DANDAN (B ENG., XIDIAN UNIVERSITY) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 Acknowledgements My deepest gratitude goes first and foremost to Dr Hui Hon Tat, my main supervisor, for his professional guidance and sharp insights into my research work Without his illuminating instruction and constant encouragement, this thesis could not have reached its present form I am deeply grateful to him for his warmhearted help and great support to my job hunting I am also indebted to my previous main supervisor, Prof Joshua LeWei Li, who left for UESTC, for initiating the interesting project and giving valuable directions on my work Many thanks also go to my co-supervisor, Prof Yeo Tat Soon, for his precious discussions and suggestions on revising the research papers I would like to thank the National University of Singapore for providing scholarship to support me to pursue my doctoral degree I would like to thank our lab technologist, Mr Jack Ng, for his help in providing me with the facilities to carry out my research I would also like to thank my labmates and friends for their support and friendship Last but not the least, I would like to express my deep appreciation to my family for their love i Contents Acknowledgements i Contents ii Summary vi List of Tables viii List of Figures ix Chapter Introduction 1.1 Background of Microwave Cancer Therapy 1.2 Background of Magnetic Resonance Imaging 1.2.1 MRI Operation Principle 1.2.2 MRI Hardware 1.3 Thesis Organization 1.4 Publications Chapter Application of Microwave in Cancer Therapy 12 2.1 Introduction 12 2.2 Microwave Dielectric Heating Principle 12 ii 2.3 Microwave Thermotherapy for Breast Cancer 14 2.3.1 Treatment Setup and Numerical Modeling 15 2.3.2 Numerical Calculation of SAR 18 2.3.2.1 SAR Expression 18 2.3.2.2 FEKO Calculation 19 2.3.3 Simulation Results and Discussions 20 2.3.3.1 Polarization Direction of the Incident Plane Wave 21 2.3.3.2 Needle Insertion Direction 22 2.3.4 Conclusion 26 2.4 Shielding Effects of Radially Distributed Needles 27 2.4.1 Formulation of the Problem 28 2.4.2 Basis Functions 30 2.4.3 Testing Procedure 31 2.4.4 Matrix Equation Derivation 33 2.4.5 Calculation of the Near Field and the Poynting Vector 34 2.4.6 Numerical Results and Shielding Effect 35 2.5 Chapter Summary 37 Chapter Design of the Vertical Phased Coil Array for Increasing the SNR of MRI 39 3.1 Introduction 39 3.2 Motivation of the Design 39 3.3 Theoretical Analysis of the SNR Increase 41 3.4 Chapter Summary 48 iii Chapter Experimental Study of the Vertical Phased Coil Array 50 4.1 Introduction 50 4.2 Experimental Setup for a Simulated MRI Environment 52 4.2.1 Construction of the Receiving Phased Array Coils 53 4.2.2 Construction of the Source Coil 54 4.2.3 The Phantom Loading 55 4.2.4 VNA 56 4.3 Measurement Results and Discussions 57 4.4 Chapter Summary 64 Chapter The Increase of SNR by Using Vertical Phased Coil Arrays in MRI - Numerical Experiments Demonstration 65 5.1 Introduction 65 5.2 Simulation of the Signal and Noise in the Numerical Experiments 66 5.3 Determination of the Combiner Coefficients 70 5.4 SNR Calculation and Discussion 72 5.5 Chapter Summary 76 Chapter Design of a Multi-layered Surface Coil Array for Enlarged FOV and Increased SNR Performance 77 6.1 Introduction 77 6.2 Derivation of the SNR for the Multi-Layered Surface Coil Array 78 iv 6.3 Numerical Experiments and Results 84 6.3.1 Simulation of the Signal and Noise in the Numerical Experiments 84 6.3.2 SNR Performance of the Multi-Layered Surface Coil Array 88 6.4 Chapter Summary 93 Chapter Conclusion and Discussions 94 7.1 Conclusion 94 7.2 Limitations and Future Work 96 Bibliography 98 Appendix The Square Strip Coil with Distributed Capacitors and Matching Network 111 v Summary This thesis studies the application of electromagnetic theory in two aspects: characterizing the microwave thermal effect in cancer therapy and solving the low signalto-noise ratio (SNR) issue in magnetic resonance imaging (MRI) In the first part of the thesis (Chapter 2), an invasive microwave breast cancer therapy which uses the needle insertion to guide microwave power into the tumor region is investigated through the calculation of specific absorption rate (SAR) in a simulated breast model in FEKO It is shown by the simulation results that the heating effect can be adjusted by the direction of incident wave and the needle insertion direction, and the best heating and focusing effect in tumor region is obtained Then a shielding method which consists of radially distributed needles is discussed, and the shielding effect is shown by the smaller Poynting vector values in the protected region The second part of the thesis (Chapter to Chapter 6) is to deal with the problem of low SNR in an MRI system A vertical phased coil array which consists of a number of vertically stacked surface coils is proposed The SNR increase is firstly explained in theory with the conclusion that SNR can be increased by increasing the number of coils in the array provided that the mutual coupling can be removed Then the decoupling method is introduced through a simulated MRI system in a laboratory experiment, and good decoupling results are obtained, thus validating the feasibility of the proposed vertical phased coil array The SNR variation with the number vi of coils in the array is shown through a series of rigorous numerical experiments, and it is found that in the situation of decoupling, the SNR of the system is significantly increased by the vertical phased coil array Subsequently a multi-layered surface coil array which consists of multiple surface coils in both the vertical and horizontal directions is developed to increase the SNR of MRI with large field of view (FOV) for scanning large samples The SNR performance of the multi-layered surface coil array is investigated through numerical experiments, and improved SNR performance is obtained Original contributions: Investigation on the heating effect of a novel invasive microwave breast cancer therapeutic method Design of a vertical phased coil array for increasing SNR performance of MRI Successful application of a new decoupling method to efficiently remove the coupling effect in vertical phased coil arrays Design of a multi-layered surface coil array for MRI with both a large FOV and improved SNR performance vii List of Tables Table 2.1: The dimensions of the different parts of the breast model 17 Table 2.2: The dielectric properties of each medium in the breast model at a frequency of 1GHz 17 Table 2.3: Volume-average SARs in each medium of the plane wave incidence directions labeled as Case and Case in Fig 2.4 22 Table 2.4: Volume-average SARs in each medium for the needle insertion directions shown in Fig 2.5 24 Table 2.5: Volume-average SARs in each medium for the needle insertion directions shown in Fig 2.6 24 Table 4.1: The receiving mutual impedances of the two stacked array coils with separation 59 Table 4.2: The performance of the combiner coefficients with respect to coil separation 62 Table 6.1: The SNRs of a single layered surface coil array under the decoupling matrix method 89 Table 6.2: The SNRs of a single layered surface coil array under the overlapping decoupling method 89 viii List of Figures Figure 1.1: The simplified flowchart of MRI operation Figure 1.2: Block diagram of an MRI system [40] Figure 2.1: Parallel-plate applicator 13 Figure 2.2: A sketch of the treatment setup for a microwave invasive method, modified from [26] 16 Figure 2.3: The YOZ cutting plane of the model in FEKO 18 Figure 2.4: The incident directions of the plane wave in the two cases The blue arrow represents the incident direction while the red arrow represents the polarization direction 22 Figure 2.5: The method of horizontally 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according to the optimization results of FEKO In this appendix, we introduce the procedures for designing the coil in Fig 5.2 by FEKO 111 Figure A1.1: The equivalent circuit of a square surface coil Figure A1.2: The coil model in FEKO Step 1: Build the strip coil model without adding distributed capacitors and matching network in FEKO as in Fig A1.2, excite the coil with a voltage source, and evaluate the input impedance by FEKO We obtain the input resistance and reactance as R c  1.2122 m  and X c  70.53  112 Step 2: Add distributed capacitors as shown in Fig A1.3 to make the coil resonant at 38.3 MHz (corresponds to a 0.9 T MRI system) Using (A1.2), we calculate the total capacitance of the distributed capacitors as C total  58.918 pF Based on the positioning of the capacitors in Fig A1.3, we calculate the value of C as C  5C total  294.59 pF Evaluate the input impedance of the coil with the distributed capacitors, and we have R c  1.1831 m  and X c  13.3569  L=12cm C_top L=12cm 1cm C C 2C 2C Za ZL CT CM CM To LNA with reference impedance of 50 Ω Figure A1.3: The schematic diagram of the square strip coil with distributed capacitors and matching network shown in the dashed box Step 3: 113 The coil is connected to LNA with reference impedance of 50 We calculate the matching network by [101] CT   Rc R in  R c R in 0  Rc  X c 2  Rc  X c Rc R in  R c  R c R in  X c 2   Xc R in  R c  R c R in  X c 0  Rc  X c 2   R in  R c  R c R in  X c 2  (A1.3) CM  2 Rc R in  R c  X c  R c R in  2 (A1.4) where R in  50  , and we have C T  309.6 pF and C M  3.03 pF Figure A1.4: The optimization result of the reflection coefficient to make the coil resonance at 38.3 MHz Step 4: 114 Add the determined matching network to the coil model in FEKO, and tune the value of the capacitor Ctop to optimize the reflection coefficient at the coil terminal to make it resonant at 38.3 MHz, and we have the optimization result as in Fig A1.4 115 ... application of electromagnetic theory in two aspects: characterizing the microwave thermal effect in cancer therapy and solving the low signalto-noise ratio (SNR) issue in magnetic resonance imaging. .. with the application of electromagnetic theory in two aspects: microwave cancer therapy and magnetic resonance imaging (MRI) In this chapter, the research background of the two aspects is introduced,... Cancer Therapy 2.1 Introduction In this chapter, the application of microwave thermal effect in cancer therapy is discussed First, the heating principle of microwave to human body tissue is explained