优化重聚脉冲提高梯度场核磁共振信号强度

李新; 肖立志; 刘化冰; 张宗富; 郭葆鑫; 于慧俊; 宗芳荣

doi:10.7498/aps.62.147602

摘要

缩短射频脉冲宽度, 有助于解决脉冲电力消耗大、样品吸收率高、信噪比低等极端条件核磁共振探测的关键问题. 本文首先分析射频脉冲角度对核磁共振自旋回波信号强度的影响机理, 基于Bloch方程推导了回波信号幅度与扳转角、重聚角的关系. 在特制核磁共振分析仪上采用变脉冲角度技术, 分别在均匀磁场和梯度磁场条件下实现对扳转角和重聚角与回波信号强度关系的数值模拟和实验测量. 结果表明, 梯度场中, 扳转角为90°、重聚角为140°的射频脉冲组合获得最大首波信号强度, 比180°脉冲对应的回波幅值提高13%, 能耗降低至78%. 选用该重聚角(140°) 优化设计饱和恢复脉冲序列探测流体的纵向弛豫时间T1特性, 准确获得 T1分布.该结果对于低电力供应、且对信噪比有较高要求的核磁共振测量, 如随钻核磁共振测井和在线核磁共振快速检测等, 具有重要意义.

关键词:

Abstract

It is an efficient protocol to use the refocusing flip angle pulse optimization technique to solve special engineering technical problems in nuclear magnetic resonance (NMR) measurements. By reducing RF pulse duration, the low refocusing flip angle pulses can consume lower power, satisfy specific absorption rate of samples, and improve signal-to-noise ratio as well. To further analyze the function mechanism of pulse angles, the dependence of signal intensity on RF pulse is studied in homogenous magnetic field and constant gradient magnetic field respectively. Afterwards, echo amplitudes with various tip angles and flip angles ranging from 0° to 180° are compared with conventional sequence of 90° pulse followed by 180° pulses theoretically and experimentally. For the constant gradient field, the refocusing pulse of flip angle can be as low as 140°, defined as the optimum herein, to obtain the strongest signal intensity, enhanced by 13% compared with that of 180°. Moreover, T1 distributions measured by the conventional and optimal sequences for distilled water at room temperature are compared, and good conformances of T1 between the two pulse sequences are obtained, which demonstrates the optimal refocusing pulse can be directly applied to T1 measurement. The results provide constructive suggestion for designing pulse sequences for signal intensity enhancement in NMR logging while drilling and NMR online quick analysis.

Keywords:

作者及机构信息

1.
中国石油大学, 油气资源与探测国家重点实验室, 北京 102249;

2.
中国石油化工股份有限公司石油工程技术研究院测录井研究所, 北京 100101

基金项目: 国家自然科学基金(批准号: 41074102, 41130417);国家111计划(批准号: B13010)和教育部长江学者和创新团队发展计划资助的课题.

Authors and contacts

1.
State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China;

2.
Sinopec Research Institute of Petroleum Engineering, Beijing 100101, China

Funds: Project supported by the National Natural Science Foundation of China (Grant Nos. 41074102, 41130417), “111 Program” of China (Grant No. B13010) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China.

参考文献

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[2]	Alsop D C 1997 Magn. Reson. Med. 37 176
[3]	Hennig J, Nauerth A, Friedburg H 1986 Magn. Reson. Med. 3 823
[4]	Haase A, Frahm J, Matthaei D, Hänicke W, Merboldt K D 1986 J. Magn. Reson. 67 258
[5]	McIntyre D J O, Hennel F, Morris P G 1998 J. Magn. Reson. 130 58
[6]	Andrade F D, Netto A M, Colnago L A 2011 Talanta 84 84
[7]	Andrade F D, Netto A M, Colnago L A 2012 J. Magn. Reson. 214 184
[8]	Hrlimann M D, Griffin D D 2000 J. Magn. Reson. 143 120
[9]	Song Y Q 2002 J. Magn. Reson. 157 82
[10]	Reiderman A, Itskovich G, Krugliak Z, Beard D R 2001 Magn. Reson. Imaging 19 569
[11]	Li X, Xiao L Z, Liu H B 2011 Well Logging Technol. 35 200 (in Chinese) [李新, 肖立志, 刘化冰 2011测井技术 35 200]
[12]	Coates G R, Xiao L Z, Prammer M G 1999 NMR Logging Principles and Applications (Houston: Halliburton Energy Services) p101
[13]	Zu D L 2004 Magnetic Resonance Imaging (Beijing: Higher Education Press) p83 (in Chinese) [俎栋林 2004 核磁共振成像学(北京: 高等教育出版社) 第83页]
[14]	Bloch F 1946 Phys. Rev. 70 460
[15]	Chen J F, Liu W Q, Zhong W X 2006 Acta Phys. Sin. 55 884 [陈杰夫, 刘婉秋, 钟万勰 2006 55 884]
[16]	Casanova F, Perlo J, Blmich B 2011 Single-Sided NMR (Berlin Heidelberg: Springer-Verlag) p12
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[18]	Anferova S, Anferov V, Arnold J, Talnishnikh E, Voda M A, Kupferschlager K, Blmler P, Clauser C, Blmich B 2007 Magn. Reson. Imaging 25 474

施引文献

[1]	Hennig J, Friedburg H 1988 Magn. Reson. Imaging 6 391
[2]	Alsop D C 1997 Magn. Reson. Med. 37 176
[3]	Hennig J, Nauerth A, Friedburg H 1986 Magn. Reson. Med. 3 823
[4]	Haase A, Frahm J, Matthaei D, Hänicke W, Merboldt K D 1986 J. Magn. Reson. 67 258
[5]	McIntyre D J O, Hennel F, Morris P G 1998 J. Magn. Reson. 130 58
[6]	Andrade F D, Netto A M, Colnago L A 2011 Talanta 84 84
[7]	Andrade F D, Netto A M, Colnago L A 2012 J. Magn. Reson. 214 184
[8]	Hrlimann M D, Griffin D D 2000 J. Magn. Reson. 143 120
[9]	Song Y Q 2002 J. Magn. Reson. 157 82
[10]	Reiderman A, Itskovich G, Krugliak Z, Beard D R 2001 Magn. Reson. Imaging 19 569
[11]	Li X, Xiao L Z, Liu H B 2011 Well Logging Technol. 35 200 (in Chinese) [李新, 肖立志, 刘化冰 2011测井技术 35 200]
[12]	Coates G R, Xiao L Z, Prammer M G 1999 NMR Logging Principles and Applications (Houston: Halliburton Energy Services) p101
[13]	Zu D L 2004 Magnetic Resonance Imaging (Beijing: Higher Education Press) p83 (in Chinese) [俎栋林 2004 核磁共振成像学(北京: 高等教育出版社) 第83页]
[14]	Bloch F 1946 Phys. Rev. 70 460
[15]	Chen J F, Liu W Q, Zhong W X 2006 Acta Phys. Sin. 55 884 [陈杰夫, 刘婉秋, 钟万勰 2006 55 884]
[16]	Casanova F, Perlo J, Blmich B 2011 Single-Sided NMR (Berlin Heidelberg: Springer-Verlag) p12
[17]	Anferova S, Anferov V, Rata D G, Blmich B, Arnold J, Clauser C, Blmler P, Raich H 2004 Concepts Magn. Reson. B 23B 26
[18]	Anferova S, Anferov V, Arnold J, Talnishnikh E, Voda M A, Kupferschlager K, Blmler P, Clauser C, Blmich B 2007 Magn. Reson. Imaging 25 474

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