Design and Fabrication of Micro-scaled Spiral Surface RF coils for 3T MRI

因著醫學影像的需求,空間解析度相較於時間解析度在磁共振造影(Magnetic Resonant Imaging, MRI)的演進中較不被重視。高空間解析度的MRI成像將是生物研究及其他應用的一大利器，至今有相當多研究探討如何提高射頻線圈的靈敏度使其在高時間空間解析度底下維持足夠的信雜比（Signal-to-Noise Ratio, SNR）。其中線圈的微小化似乎是磁共振顯微造影的最佳解答。除了線圈大小、磁場均勻度以及線圈品質因素，調整匹配以及其他種種考量將會決定射頻線圈的最終形態。平面螺旋的設計將有可能是平衡各項參數的理想結果。 本論文中將會研究印刷電路板上雕刻出來的平面螺旋線圈的特性。等效電路以及各項公式被逐一檢視且另外模擬線圈的B1磁場。射頻線圈電路將被調整至3T底下氫質子的共振頻率－125.3MHz。利用此一線圈我們取得了高解析度的魚肝油及鼠腦截面影像. 結果顯示即便線圈體積小至將近1/3,相較於慣用的微系統成像表面射頻線圈仍有良好的信雜比。 接著，一個經過初步設計用來作為生物樣本造影的990微米見方的方形表面螺旋線圈以微製程技術的方法製造出來。這一個45圈的銅製程線圈我們打以鋁線的放法串接到電路板上並調整匹配至125.3MHz。連接完成的線圈電路有著相當良好的S11曲線。更多有關此線圈的特性可由毛細管成像實驗中測得。我們成功的取得了外徑78μm 作為一個先遣實驗及首次使用微製程的高圈數磁共振表面射頻線圈，本研究已經踏出邁向磁共振顯微成像成功的第一步了。尚有許多研究能延續著本題目而發揮出微型化線圈的最大潛能。Spatial resolution of Magnetic Resonance Imaging (MRI) has been relatively left out in the advancement of MRI compared to temporal resolution due to medical needs. High spatial resolution MR imaging might become powerful tools for imaging of biological studies and other applications. Plenty of researches have been done to increase the sensitivity of the radiofrequency (RF) surface coils to maintain acceptable SNR of images with higher spatial and temporal resolution. Among all, downsizing of the coil size seems to be the most suitable answer for magnetic resonance microscopy. Other than the size of the coil, magnetic field homogeneity along with Q value, tuning and matching of the coil and other considerations determines the final construction of the RF surface coil. Spiral-shaped design for micro RF coils might be an ideal solution that balances most of the factors. In this thesis we first studied the characteristics of spiral surface coils manufactured by numerical controlled milling machine on printed circuit (PC) boards. Equivalent models and equations of spiral coils are thoroughly inspected and B1 magnetic field simulated. The RF circuit is then tuned and matched to 125.3MHz, the magnetic resonant frequency of H1 under 3T. High-resolution fish oil and rat brain images taken by the PC board spiral coil. The result displays the appreciable coil sensitivity of small sample MR imaging. Next, after for establishing design criteria for MR imaging of biological samples, a 990μmm square rectangular spiral surface coil is produced with microfabrication process. The 45-turn micro-scaled copper coil was then wire-bonded onto a circuit board and tuned/matched to 125.3MHz. S11 of the circuit shows great frequency response. More of the MEMS coil properties were observed throughout the capillary imaging experiment. Image of a 700µm outer diameter, 400μm inner diameter, 20mM CuSO4 solution filled capillary was taken. Another structural phantom consists of a smaller capillary tube inside a larger tube with fish oil filled between is also constructed and image taken. Both with isotropic spatial resolutions of 78μm, images were acquired with the microcoil. As a preliminary study and the first CMOS fabricated high number-of-turns MRI RF surface coil, this study has made a successful step towards magnetic resonance microscopy and its vast applications. Much more will be done to induce the full potential of micro-scaled MR coil and to verify its limits.Table of Contents Ch. 1 Introduction 1 1.1 Motivation 1 1.2 Goals of This Study 3 1.3 History of MRI Spatial resolution improvements 3 1.4 Surface Spiral coil development (NMR,MRI) 4 1.5 Thesis Structure 4 Ch. 2 Principles of MRI surface coils 6 2.1 MRI Introduction 6 2.1.1 Introduction to MRI Hardware 6 2.1.2 Radiofrequency Coils 6 2.2 Signal-To-Noise Ratio of MRI 8 2.2.1 Magnetic Resonance Signal & noise source 8 2.2.2 Total SNR 10 2.2.3 Difficulties for High-Resolution Imaging 11 2.3 Radiofrequency Coils Properties 13 2.3.1 Equivalent Models 13 2.3.2 Tune & Match 15 2.3.3 Other Coil Performance Indicators 19 2.4 Discussion 20 Ch. 3 Design and Fabrication of PCB Surface Spiral Coils 21 3.1 Introduction 21 3.2 Spiral Coil Theories 21 3.3 Material & Methods 26 3.3.1 Spiral coil design 26 3.3.2 Hardware 28 3.3.3 Software 28 3.3.4 Coil Production 29 3.3.5 Experiment Setup 30 3.4 Results & Discussion 31 3.4.1 Coil Performance 31 3.4.2 Imaging Performance 32 3.5 Discussion 34 Ch. 4 Design & Fabrication of Micro Spiral Coil 35 4.1 Introduction 35 4.2 Micro Spiral Coil Theories 35 4.2.1 Planar Inductor Perspective 35 4.3 Material & Methods 39 4.3.1 Micro Coil Design 39 4.3.2 Microfabrication Process 42 4.4 Computer Simulation 43 4.4.1 Numerical Methods 44 4.4.2 Simulation Software 44 4.4.3 Simulation Results 45 4.5 Results & Discussion 46 4.5.1 Micro Coil Properties 46 4.5.2 Experiment Setup 48 4.5.3 Imaging Results 50 4.6 Discussion 54 Ch. 5 Discussion & Conclusion 56 5.1 Discussion 56 5.2 Conclusion 60 5.3 Future Works 61 5.3.1 Hardware Issues 61 5.3.2 Low Noise Amplifier (LNA) and Integrated Tuning/Matching62 5.3.3 Pulse Sequence Design for MRM 63 5.3.4 Other Possible Applications of Downsized Coils 63 Appendix 65 A Specification Sheets of American Technical Ceramics Non-Magnetic Capacitors 65 B Micro RF Coil Layout 68 References 7