陳志宏臺灣大學：電機工程學研究所林胤藏Lin, In-TsangIn-TsangLin2007-11-262018-07-062007-11-262018-07-062005http://ntur.lib.ntu.edu.tw//handle/246246/53199磁共振影像（Magnetic Resonance Imaging, MRI）技術的發展主要有兩大目標，一是加快掃描速度，一是增強影像解析度。一個影響掃描速度和解析度極限的重要條件是影像的訊雜比。MRI 中雜訊的來源主要有受測物體和線圈，其中降低線圈雜訊的方法之一就是使用電阻極低的高溫超導線圈。 目前已有的高溫超導線圈大致上可分為兩類：帶狀超導線材及薄膜線圈。其中帶狀超導線較適合直徑大於六公分之表面線圈及體線圈，薄膜線圈則較適合製作小尺度的表面線圈。本論文從適用於 3T MRI 之帶狀超導表面線圈開始進行製作和測試，並設計出薄膜線圈幾何結構和配合之恆溫器系統。 本論文中以 Bi2Sr2Ca2Cu3Ox 超導帶製作直徑七公分的表面線圈，以不同導電度的假體得到其對相同結構銅線圈的訊雜比增益大致符合理論計算結果。此超導線圈在水果及鼠腦影像中獲得的訊雜比分別是傳統表面線圈的二點四倍與二點七倍，估算約可將取像時間縮短為原來的五分之一，這在需要較長掃描時間的實驗例如擴散影像中將可節省數十分鐘甚至數小時的可觀時間。 在較小尺度的線圈我們選擇在 LaAlO3 基版上鍍 YBa2Cu3O7 薄膜為材料。我們以軟體成功模擬在 3T MRI 中心頻率 125.3MHz 共振之螺旋線圈型式及磁場分佈，並針對 Bruker S116 mini 梯度線圈系統設計一套以 G10 玻璃纖維材質之低溫恆溫器，此恆溫器約可將線圈維持在 84 K 之工作溫度 52 分鐘。 採用超導線圈所能得到的訊雜比增益隨受測物的尺度下降而上升，因此可預期若能將超導線圈應用在本實驗室進行中的分子影像上，大幅改善的訊雜比十分有利於提供分子影像所需的小範圍的高解析度影像。我們也預備製作高溫超導多通道陣列線圈，再配合平行影像方法和其他提高訊號之技術如氙對比影像，以期達成快速掃描、影像高解析度及高訊雜比的目標。Abstract There are two main purposes of the development of Magnetic Resonance Imaging (MRI) techniques. One is to increase scanning speed; another is to increase image resolution. An important factor that affects the limitation of acquisition time and resolution is the signal-to-noise ratio (SNR). There are two main sources of MRI noises: sample and receiving coil. Thus one way to reduce the coil noise is using non-resistive high-temperature superconducting (HTS) coils. Previous works on HTS coils can be put into two categories: tape coils and thin-film coils. Tape coils are suitable for surface coils with diameters larger than 6 cm or volume coils; thin-film coils are more suitable for a smaller planar surface coil. In this work we began from making tape coils to be used on Bruker 3T MRI system, to the design of thin-film surface coil pattern and the cryostat. Commercialized Bi2Sr2Ca2Cu3Ox tape was used to fabricate a HTS tape coil with a diameter of 7 cm. Tested by phantoms of different conductivity, we got a plot of HTS SNR gain over an equivalent copper coil that was in agreement with theoretical prediction. A SNR gain of about 2.4 can be obtained from the HTS tape coil over a conventional copper surface coil in kiwi fruit images and the acquisition time was expected to reduce to one-fifth the original time when keeping the same SNR. This is very beneficial when doing experiments that usually takes a long time (as conventional diffusion imaging); the saved time length can be up to several hours. YBa2Cu3O7 (YBCO) coated on LaAlO3 substrate was chosen to make the small scale surface coil. Successful simulations of double-sided YBCO coils on a 2cm*2cm LaAlO3 substrate were done by Sonnet 6.0a. A spiral pattern of the HTS film that resonates at 125.3 MHz was decided, and its B1 field was also found out. We also designed a G10 glass fiber cryostat to be fit in the Bruker S116 mini gradient system. This cryostat can keep the HTS coil at 84 K for about 52 minutes. The SNR gain of HTS coils increases as the sample size goes down, so we can expect it to improve the SNR significantly when implemented in molecular imaging that needs very small FOV and high resolution. We also aim on developing multi-channel HTS phased arrays, together with other techniques that can raise signal voltage (for example, xenon imaging), to achieve the goal of ultra fast, high resolution and high SNR imaging.Contents 1 Introduction.…………………………………………………………………………………1-1 1.1 Motivation.…………………………………………………………....…………………….1-2 1.2 Reviews on MR Superconducting RF Coil Development…...……………………………..1-2 1.2.1 Brief history of superconductors………………………….……………………..…..1-2 1.2.2 Previous MR superconducting RF coils………………………...…………..……….1-6 1.3 Limitations of HTS thin-film RF receiver coil…...………………………...……………….1-7 1.4 Outline of the thesis………………..…………….…………………………...……………..1-7 2 Literature review on HTS MRI RF Coils ………………………...……………………….2-1 2.1 SNR of a MRI System………………………………………………………………………2-1 2.1.1 Signal detected in the RF coil ………………………………………………...……..2-1 2.1.2 Noise component in the RF coil……………………………………………………..2-2 2.1.3 RF coil quality factor and SNR……………………………………………………...2-5 2.2 Review on the Development of HTS RF Coil in MRI…………………………..………….2-7 2.2.1 HTS thin-film RF coil……………………………………………………...………..2-7 2.3 Inherent Obstacles of HTS Thin-Film Surface Coil………………………………….…….2-8 2.3.1 Limited field of view (FOV) and imaging depth…………………………...……….2-8 2.3.2 Filling factor…………………………………………………………………...…….2-9 2.3.3 High cost and complicated fabrication process……………………………...………2-9 2.4 Advantage of HTS Tape MRI Receiver Coil………………………………………...……..2-9 2.4.1 Flexible size and configuration……………………………………….……...…….2-10 2.4.2 Low cost and simple fabrication……………………………..…………………….2-10 2.5 Design of HTS Tape Coils…………………………………………………..…………….2-10 2.5.1 Impedance of high-temperature superconductors………………………………….2-10 2.5.2 Tuning and matching……………………………………………………….….......2-12 2.6 Research Aim……………………………………………………………………....….......2-12 3 Design and Fabrication of HTS Tape Coil..………………………………………………..3-1 3.1 Introduction…………………………………..……………………………………………..3-1 3.2 Theory…………………………………………..…………………………………………..3-3 3.3 Material and Methods……………………………..……………………………………......3-3 3.3.1 Bi-2223 HTS Wire…………………………..…………………………………...….3-3 3.3.2 Experiment Setup……………………………..………………………………...…...3-5 3.4 Single-turn HTS tape RF coil……………………………………………………………….3-7 3.4.1 Coil design and fabrication process……………………………………………...….3-7 3.5 Results……………………………………………………………………………………....3-8 3.5.1 Frequency responses and Q factors………………………………………………….3-8 3.5.2 Stimulation results…………………………………………………………………..3-11 3.5.3 Imaging results……………………………………………………………………..3-11 3.5.4 SNR profile………………………………………………………...………………3-22 3.5.5 Theoretical and experimental SNR gain comparison………………………………3-23 3.5.6 Dewar Fabrication…....................................................……………………………3-23 3.6 Discussion…..……………………………………………………………………………..3-24 3.7 Conclusion…..…………………………………………….……………………………….3-25 4 Diffusion Tensor MRI Using High-Temperature Superconducting Tape RF coil………...4-1 4.1 Introduction……………………………………………………………………………..........4-1 4.2 Diffusion Measurement by NMR………………….……………………………………......4-2 4.2.1 NMR pulsed gradient diffusion measurement……………………………………....4-3 4.2.2 Diffusion tensor magnetic resonance imaging………………………………………4-4 4.3 Materials and Methods………………………………………………………………..….....4-5 4.3.1 Rat model…………………………………………………………………………............4-5 4.3.2 Imaging techniques…………………………………………………………….........4-6 4.3.3 Diffusion tensor reconstruction................................................................................4-8 4.4 Results……………………………………………………………………………………....4-8 4.4.1 DTI of rat brain……………………………………………………………………...4-8 4.4.2 Compare the accuracy of the DTI using HTS coil and copper coil………………..4-11 4.5 Conclusion………………………………………………………………………………...4-14 5 Conclusion and Future Works……………………………………………………………...5-1 5.1 Discussion………………………………………………………………………………....5-1 5.1.1 Effect of MRI system loss on RF coil……………………………………………….5-2 5.1.2 Discrepancy between theoretical and experimental SNR gain……………………...5-2 5.1.3 Potential applications………………………………………………………………..5-3 5.1.4 Advantage………………………………………………….………………………...5-3 5.1.5 Limitations and future development………………………………………………...5-4 5.2 Conclusion…………………………………………………………………………………5-4 5.3 Future Works………………………………………………………………………………5-5 5.3.1 Bi-2223 tape coils……………………………………………………………………5-5 5.3.2 Phased array HTSC….………………………………………………………...5-5 5.3.3 YBCO thin-film planar coils………………………………………………………...5-5 List of Figures 1.1 Year of discovery of superconductors with their transition temperature…………...………..1-6 2.1 Equivalent Circuit of the LC loop and a Matching Coil: ..….2-12 3.1 Bi-2223 HTS Tape………………..………………………..……….………………………..3-2 3.2 BSCCO Structure …………………………………………………………………….……...3-4 3.3 Experiment Setup………………………………………………………………………….…3-6 3.4 Bi-2223 Tape Coils…………………………………………………………………………..3-9 3.5 Equivalent Copper Coil and Conventional Copper Coil (Diameter=4cm)…………………..3-9 3.6 Unloaded Q of the 4 cm HTS tape coil (red line) and copper coil (blue line)………...........3-10 3.7Coil configuration…………………...………………………………………………….…...3-12 3.8(a) is perpendicular B1 Field of the copper coil and (b) is perpendicular B1 Field of the HTS coil resonated at 125.3 MHz, respectively.…………………………………..........3-12 3.9 A 10cm diameter spherical phantom filled with 10mM CuSO4 solution…………………3-13 3.10 The system setup of the phantom experiment at 3T…………….………………………..3-14 3.11 Coronal phantom images, contour images comparison, and mesh images comparison by Bruker fast spin-echo sequence. The background noise of mesh image acquired from HTS coil is lower than copper coil significantly…….………………………………………...3-14 3.12 Imaging Geometry (Rat Brain)……………………………………………………..……..3-15 3.13 Images of the brain of a rat with (a) HTS tape coil in 77K with SNR was 120, and (b) the copper coil in the room temperature with SNR was 30. The SNR gain of 4 by using the HTS surface coil with the same acquisition time of using the copper coil………………..3-16 3.14 (a) Rat brain images acquired 4 cm HTS tape coil, (b) True rat brain anatomy images…..3-16 3.15 Images of the brain of a rat with (a) the image matrix size 256 x 256 acquired from the copper coil in the room temperature with SNR was 17.44, (b) image matrix size 512 x 512 acquired from copper coil in the room temperature with SNR was 4.347, (b) image matrix size 256 x 256 acquired from HTS tape coil in 77k with SNR was 66.81, and (c) image matrix size 512 x 512 acquired from HTS tape coil in 77k with SNR was 14.059…….....3-17 3.16 Images of the wrist of human with (a) the copper coil in the room temperature with SNR was 169, and (b) HTS tape coil in 77K with SNR was 331…….…………………….......3-19 3.17 Image histogram and after histogram square root processing with (a) the copper coil in the room temperature, and (b) HTS tape coil in 77K………………………………..……3-20 3.18 Comparison of the copper coil histogram and HTSC histogram. It shows the noise level decreases about two gain after using HTSC………………………………………………3-20 3.19 Plots of intensities along the horizontal axis extracted from phantom images, in which the red, blue curves denote the HTS and copper coil at room temperature, respectively....3-21 3.18: Predicted and measured SNR advantage of 4 cm and 20 cm HTS coil over equivalent copper coil…………………………………………………………………………………3-22 4.1 Diagram of spin-echo diffusion sequence……………...………………………….……...….4-7 4.2 DTI using HTS coil (a) One unattenuated reference image, (b)~(g) Six diffusion-attenuated images by applying six noncollinear diffusion-sensitizing gradients…………….........…...4-9 4.3 (a) DTI of the rat brain using HTS coil, where the organized cerebral cortex structures, corpus callosum and the hippocampus were shown in the enlarged image (b)……….....…4-10 4.4 DTI of the brain of a rat with (a) the copper coil in the room temperature. (b) The copper coil in the room temperature with the same slice as (a). (c) HTS tape coil in 77K. (d) HTS tape coil in 77K with the same slice as (c)…………………………………………………4-13 5.1 Transverse field image in the central xy plane. The surface coil is 10 cm in diameter, the images commence 1 cm above the coils’ center line, and the field plots pertain to the bottoms of the images…………………………………………………………….…..…...…5-6 5.2 Images of the rat with two HTS coils in 77k…………………………………………….…..5-7 List of Tables 3.1 Bi-2223 Wire Properties…………………………………………………………………..…3-5 3.2 HTS Coils of Different Diameters………………………………………………………….3-10 3.3 Quality Factors for HTS at 77K and Copper Coils at room temperature (Cu), ul: unloaded, ul: loaded(outside magnet)……………………….……………………...………….………3-112619672 bytesapplication/pdfen-US鉍鍶鈣銅氧Bi-2223thesishttp://ntur.lib.ntu.edu.tw/bitstream/246246/53199/1/ntu-94-R92921112-1.pdf