高溫超導線圈於3T磁振造影系統之研究:小動物平臺之建置與應用

High Temperature Superconducting RF Coil on 3T MRI System: Animal Platform Development and Applications

陳凱淵(Kai-Yuan Chen)

指導教授 ： 陳志宏

國立臺灣大學/電機資訊學院/電機工程學研究所/碩士(2007年)

https://doi.org/10.6342/NTU.2007.02642

摘要

在磁振造影系統中，信雜比是是決定影像品質一個很重要的因素。為了提升影像品質,可使用高溫超導材料低溫線圈降低由阻抗所引起之雜訊。本實驗室在之前研究之高溫超導線圈系統中，將高溫超導線圈在臨界溫度下的信雜比與傳統銅線圈在室溫下的信雜比比較，於人體系統上可以得到二點六倍的信雜比增益，以及在動物系統上三點八倍的增益。然而先前的研究當中所使用之高溫超導線圈保溫系統難以被應用於進一步的活體實驗上。本實驗的目的在於高溫超導線圈系統的建立，並設計適用於小動物活體實驗的高溫超導線圈平台。
在本實驗中，利用玻璃材質設計高溫超導線圈保溫系統，並於保溫系統中利用真空層使受測物不受低溫的影響以利於進行小動物活體實驗。於系統中並將高溫超導保溫系統與液態氮冷卻系統結合，以提供高溫超導線圈穩定之低溫環境。於本研究中，以食鹽水以及大鼠為受測物來驗證使用高溫超導線圈所改善之信雜比。於食鹽水實驗中，高溫超導線圈於臨界溫度下相對於銅線圈於室溫中可以得到平均三倍的信雜比增益。於大鼠實驗中，比較兩線圈系統可以得到三點五倍信雜比的改善。藉由使用高溫超導線圈所提升的影像品質，可以將鼠腦影像之空間解析度提升到65μm x 84μm。
此外，此高溫超導線圈系統也進一步應用於活體Dynamic Contrast Enhancement (DCE) MR imaging中。藉由觀察對比顯影劑(Gd-DTPA)於小鼠腎臟的代謝作用來比較由高溫超導以及銅線圈於相同實驗架構下的造影結果。在量化評估上，先由T2權重之解剖影像來比較兩線圈系統之信雜比差異，藉由高溫超導線圈於小鼠腎臟影像上的信雜比可提升約三點二倍。於動態影像中，信雜比平均提升三倍，對比劑注入後之對比雜訊比提升約三點八倍，信號提升率可以增加四倍。
本實驗中所建構之高溫超導線圈活體小動物平台，可以成功的提昇影像信雜比，並可以將系統應用在活體小動物實驗中。並且於DCE造影中可以得到顯著的信號提升。未來可以將此系統應用在更廣泛的小動物實驗中，例如動物的功能性磁振造影，以及擴散性影像中。

關鍵字

磁振造影 ； 射頻線圈 ； 高溫超導體 ； DCE MR Imaging

並列摘要

Signal-to-noise ratio (SNR) is a critical factor to image quality of Magnetic Resonance Imaging (MRI). To boost SNR, cryogenic radio-frequency (RF) coils using high-temperature superconducting (HTS) material can be used to reduce the noise from coil resistance. In our previous studies, using human HTS RF coil system with diameter of 7 cm at critical temperature can achieve 2.6 times SNR gain compared with conventional copper coil at room temperature. For animal HTS RF coil system with diameter of 4 cm, SNR can be achieved 3.8 times higher. However, owing to the limitation of cryostat design and thermal insulation, in-vivo animal studies have not been completely established. Therefore, the goal of this study was to build a HTS RF coil platform for in-vivo rat experiments and verification study of the capability was performed.

In this study, cooling the coils was accomplished with liquid nitrogen immersion. The cryostat was fabricated using glass material with vacuum layer for thermal insulation, which was approximately 20-mm thick. An animal holder was used for rat experiments, including anesthesia transportation system and warming bed to preserve the physiological condition of rats. To verify the capability, the experiments of phantom and rat brain were performed. Compared with HTS coil, a comparable copper coil was used with the same configuration. To implement on in-vivo experiment, dynamic contrast-enhanced (DCE) MRI on mice kidney was studied.

In results, the SNR gain of saline phantom was 3 folds higher using HTS coil and in anatomical image comparison of rat brain, 3.5-fold SNR gain was achieved. Consistent with previous studies, SNR gain in the same level was accomplished. Furthermore, improved SNR can be translated to increase the spatial resolution of MR images, which was verified in the high-resolution imaging study with 65x 84 μm2 in-plane resolution. The feasibility of the HTS coil platform was verified in DCE experiments, which was used to observe the metabolism of mice kidney by Gd-DTPA injection. Coronal sections of mice kidney were acquired to evaluate the SNR gain over the kidney and temporal change of signal intensity. For anatomical images of kidney, SNR gain was 3.2 folds higher of using HTS coil, and similarly, for DCE images, the average SNR gain was 3 folds.
In this study, we successfully developed a HTS RF coil platform for in-vivo rat experiment and demonstrated its capability of improving SNR and sensitivity to DCE change. With higher SNR, better temporal and spatial resolutions can be achieved to gain more insights to pathophysiological changes of tissues. The future works of our study are to optimize the HTS coil platform and implementation on various applications, including high-resolution imaging, fiber tracking of diffusion imaging and molecular imaging.

並列關鍵字

MRI ； RF coil ； High Temperature Superconductor ； DCE MR Imaging

參考文獻

[2] L. Darrasse and J. C. Ginefri, "Perspectives with cryogenic RF probes in biomedical MRI," Biochimie, vol. 85, pp. 915-37, Sep 2003.

[3] H. K. Onnes, "Further experiments with liquid helium D - On the change of the electrical resistance of pure metals at very low temperatures, etc V The disappearance of the resistance of mercury," Proceedings of the Koninklijke Akademie Van Wetenschappen Te Amsterdam, vol. 14, pp. 113-115, 1911.

[6] C. E. Hayes, W. A. Edelstein, J. F. Schenck, O. M. Mueller, and M. Eash, "An Efficient, Highly Homogeneous Radiofrequency Coil for Whole-Body Nmr Imaging at 1.5-T," Journal of Magnetic Resonance, vol. 63, pp. 622-628, 1985.

[7] M. D. Harpen, "Sample Noise with Circular Surface Coils," Medical Physics, vol. 14, pp. 616-618, Jul-Aug 1987.

[8] J. M. Wang, A. Reykowski, and J. Dickas, "Calculation of the Signal-to-Noise Ratio for Simple Surface Coils and Arrays of Coils," Ieee Transactions on Biomedical Engineering, vol. 42, pp. 908-917, Sep 1995.