The Design and Application of Micro RF Coil

磁振造影為一非侵入式的影像系統，在造影技術上的改善主要著重在加速造 影時間以及提高影像的解析度。雖然在臨床上，加速造影較影像解析度來的重 要；但是，高空間解析度的影像在生物研究和其他研究的應用是相當重要的一 環。目前有許多研究在探討如何提升線圈的靈敏度使得在高空間解析度的影像有 良好的訊雜比；其中，將線圈微小化可以達到我們需要的目標。 本研究會從基本的磁共振訊雜比理論來說明在提高空間解析度下，磁共振訊 號的損失。並且利用數值模擬分析來了解在線圈尺寸縮小下，射頻磁場的分佈變 化。相較過去的準靜磁場模擬方法，我們使用有限元素法，建立一近於真實的三 維模型並將其調諧和阻抗匹配至3T下的拉莫頻率和50 ohm來觀察射頻磁場的分 佈和最佳化相位陣列線圈去耦合。撘配電路設計使激發和接收分開達到高品質的 磁共振影像。 斑馬魚為一重要的神經科學研究的模型，而利用光學方法來造影斑馬魚都在 早期胚胎發育的時期，由於成魚是不透明。本論文利用自行架設的非侵入式之磁 共振顯微影像系統取得斑馬魚(zebrafish, Danio)成魚魚腦和鼠腦海馬迴 (hippocampus)兩種不同的小動物高解析度磁振造影；相較於現有的商用線圈做比 較確實在訊雜比和影像對比有明顯的提升，約3~4倍。証明了利用自製的射頻線 圈組成的磁共振影像系統確實能夠獲得高品質的小動物之高解析度影像。Magnetic resonance image (MRI) is a non-invasive imaging system. The two main streams of technique improvement are decreasing scan time and increasing spatial resolution. In clinical, temporal resolution is more important than spatial resolution. But high spatial resolution MR imaging might become powerful tool for biological studies and other applications. At the present time, there are many researches have been done to increase the sensitivity of the radiofrequency (RF) coils to increase signal-to-noise ratio (SNR) with higher spatial and temporal resolution. Among all, decreasing coil size seems to be the suitable way for magnetic resonance microscopy (MRM). In this thesis, we will probe into the effect between high spatial resolution and the loss of magnetic resonance signal from the basic MR theory. We also simulate the magnetic field distribution of RF coil by downsizing coil size using numerical analysis. Instead of conventional quasi-static magnetic field simulation, we construct a 3D model approached real structure and tune/match to 125.3 MHz/ 50 ohm to obtain magnetic filed distribution and optimize decouple of micro-fabricated phase array coil. iii Then we match up circuit design and transmit-only and receive-only (TORO) mode to obtain high quality MR image. Zebrafish is an important model of neuroscience development and optical studies in zebrafish are restricted to very early developmental stages due to the opacity of the juvenile and adult stages. In the study, we used non-invasive MRI system obtain two different kinds of small animal high resolution MR images: zebrafish brain and hippocampus of rat. Compared with commercial micro volume coil, the SNR and CNR (contrast-to-noise) can be improved clearly by using our RF coils, about 3~4 times. It can be validated that our RF coils indeed can obtain high quality small animal magnetic resonance microscope.中文摘要.......................................................................................................................i Abstract.........................................................................................................................ii 圖目錄...........................................................................................................................iv 表目錄..........................................................................................................................vii 第一章 緒論 1.1 研究背景.…………………………………………………………. 1 1.2 研究動機………………………………………………………….. 2 1.3 論文架構………………………………………………………….. 2 第二章 磁振造影之射頻線圈理論 2.1 磁振造影硬體介紹…………………………………………………. 4 2.1.1 主磁場……………………………………………………… 4 2.1.2 梯度線圈…………………………………………………… 5 2.1.3 射頻線圈…………………………………………………… 5 2.2 相位陣列線圈................................................................................... 6 2.2.1 互感........................................................................................ 7 2.2.2 重疊去耦合............................................................................ 7 2.2.3 低輸入阻抗放大器去耦合.................................................... 8 2.2 射頻線圈的性質............................................................................... 9 2.2.1 等效電路................................................................................ 9 2.2.2 調諧........................................................................................ 10 2.2.3 阻抗匹配................................................................................ 11 2.4 激發與接收分開模式......................................................................... 12 2.5 磁共振顯微影像................................................................................. 13 2.6 磁振造影之訊雜比........................................................................... 14 2.6.1 磁共振訊號及雜訊................................................................ 14 2.6.2 總訊雜比................................................................................ 15 2.6.3 高解析度影像之困難............................................................ 15 第三章 數值分析及射頻線圈製作 3.1 單圈射頻表面線圈之電磁分析........................................................ 17 3.2 馬鞍線圈之電磁分析........................................................................ 19 3.3 微型相位陣列線圈之電磁分析........................................................ 20 3.3.1 相位陣列線圈之電磁分析................................................... 20 3.3.2 線圈中心距離與穿透係數的關係....................................... 21 3.3.3 穿透係數與磁場分佈的關係............................................... 27 3.4 射頻線圈的電路設計........................................................................ 29 3.4.1 馬鞍線圈之電路設計........................................................... 29 3.4.2 微型表面線圈之電路設計................................................... 30 3.4.3 微型相位陣列線圈之電路設計........................................... 30 3.5 射頻線圈的製作................................................................................ 31 3.5.1 馬鞍線圈製作....................................................................... 31 3.5.2 微型表面線圈製作............................................................... 32 3.5.3 微型相位陣列線圈製作....................................................... 33 3.5.4 低輸入阻抗前置放大器....................................................... 34 3.6 射頻線圈的量測................................................................................ 35 3.7 高解析度磁共振影像之實驗架設.................................................... 37 第四章 結果 4.1 小動物之高解析度磁共振影像........................................................ 39 4.1.1 斑馬魚成魚之腦部解剖影像.................................................. 39 4.1.2 實驗設計................................................................................. 41 4.1.3 實驗結果................................................................................. 41 4.1.4 體線圈之斑馬魚腦部影像..................................................... 43 4.2 鼠腦的海馬迴之高解析度解剖影像................................................ 45 4.2.1 實驗設計................................................................................. 45 4.2.2 實驗結果................................................................................. 46 4.3 討論.....................................................................................................49 第五章 討論、結論及未來工作 5.1 討論.................................................................................................... 52 5.1.1 射頻線圈...............................................................................53 5.1.2 實驗設計...............................................................................53 5.1 結論.................................................................................................... 53 5.2 未來工作............................................................................................ 54 5.2.1 硬體改良..................................................................................54 5.2.2 應用..........................................................................................55 參考文獻....................................................................................................5