优化重聚脉冲提高梯度场核磁共振信号强度  

Optimization of nuclear magnetic resonance refocusing pulses to enhance signal intensity in gradient B_0 field

作　　者：李新[1,2] 肖立志[1] 刘化冰[1] 张宗富[1] 郭葆鑫[1] 于慧俊[1] 宗芳荣[1] 

机构地区：[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

出　　处：《物理学报》2013年第14期453-459,共7页Acta Physica Sinica

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

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.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.

关 键 词：核磁共振 信号强度 重聚脉冲角度 BLOCH方程 

分 类 号：O482.532[理学—固体物理]