优化重聚脉冲提高梯度场核磁共振信号强度

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： 核磁共振 信号强度 重聚脉冲角度 Bloch方程     

D-T2二维核磁共振脉冲序列及反演方法改进设计

面对日益复杂的勘探对象, D-T₂二维核磁共振技术在实际应用中面临无法兼顾扩散系数测量范围和横向弛豫分辨率的困境. 脉冲序列作为D-T₂二维核磁共振数据采集的核心技术, 其性能优劣直接影响应用效果, 在综合对比PFG, STE-PFG, BP-PFG、改良式CPMG, 扩散编程, 多回波间隔CPMG脉冲序列性能的基础上, 有效融合脉冲梯度场、恒定梯度场D-T₂脉冲序列的优点, 本文提出一种基于脉冲梯度场的双变量、两窗口D-T₂脉冲序列改进设计. 针对两个窗口的D-T₂二维核磁共振数据反演, 为突破现有反演方法无法兼顾反演精度和解谱效率的瓶颈, 本着第二个窗口回波信号为主、第一个窗口回波信号为辅的原则, 本文提出一种同时使用两个窗口数据参与解谱的联合TSVD反演方法. 气水、油水、稠油、油气水模型不同信噪比条件下的数值模拟结果表明, 本文提供的D-T₂改进脉冲序列达到了平衡扩散系数测量范围和横向弛豫分辨率的设计要求, 本文提供的联合TSVD反演方法也有效平衡了反演精度要求和解谱效率. 文中的D-T₂改进脉冲序列及联合TSVD反演方法在复杂油气藏流体识别和产能预测中具有广泛的应用前景, 可为促进国内D-T₂二维核磁共振岩心分析技术的发展提供有利条件.     

三种测量橡胶交联密度的核磁共振方法比较

核磁共振（NMR）技术已经成为测量橡胶交联密度的一种常用技术手段．该文探讨了用于测量橡胶交联密度的三种NMR方法：¹H双量子（DQ）NMR、¹H Hahn回波和¹H CPMG回波,讨论了这些方法测量值差异的来源．在此基础上进一步研究了¹H CPMG回波方法中回波时间对测量值的影响,探讨了改善¹H CPMG回波方法的方案．     

铁磁合金中核磁共振自旋回波的形状

本文报导了对非晶态铁磁合金F₇₉B_15.5S_3.5C₂样品的¹¹B核磁共振自旋回波的测量结果。所得回波的形状既与射频场强度有关也与射频脉冲宽度有关。从理论上计算了几种条件下的回波波形。实验结果与计算结果基本符合。     

基于改进非局部均值的CPMG信号去噪算法

采用低场核磁共振技术进行检测时,接收到的回波信号微弱且信噪比低,真实的信号容易淹没在背景噪声中,严重影响到后续的反演等操作的准确性．针对这一问题,提出利用非局部均值滤波算法对CPMG(Carr Purcell Meiboom Gill)回波信号进行降噪的方法．首先,对算法中至关重要的参数选择的方法进行分析,提出了利用Stein无偏风险估计的自适应参数选取方法;然后,根据回波信号的特性对算法进行改进,即利用信号点数据方差的不同,自适应地求取各点进行非局部均值滤波时的相似窗宽度;最后,求取利用最优参数进行降噪后的CPMG回波信号．对仿真数据和真实数据的反演结果对比分析表明,该改进的非局部均值滤波算法能够取得更好的滤波效果,能够获得较优的反演谱．     

基于FPGA的核磁共振测井仪控制逻辑设计

核磁共振（NMR）测井仪以CPMG脉冲序列为测量基础,按照CPMG脉冲序列的时序要求完成大功率射频脉冲的发射和微弱回波信号的接收, 在脉冲发射完成后快速泄放天线中储存的能量. 该文介绍一种基于现场可编程门阵列(FPGA)的NMR测井仪控制逻辑和典型脉冲序列,详细说明在CPMG脉冲序列下FPGA的工作时序和流程;讨论NMR测井仪发射电路、Q-转换电路和隔离电路的控制原理和时序要求,给出各电路的控制逻辑仿真结果;最后,利用所设计的控制逻辑和自制NMR测井仪探头,在实验室条件下对水溶液进行了测量,得到满意的结果.     

正交检波系统的校正

现有核磁共振波谱仪器大都采用正交检波系统. 这样可以避免谱线折叠,降低对射频功率的要求,提高信噪比. 正交检波系统如果两通道的增益不完全相同或相位不是严格相差900,会导致产生镜像峰,这种情况可以用相位循环来改善. 如果增益或相位差偏离较大,则需要手工校正. 本文通过对单峰样品实验数据进行拟合的方法计算出增益和相位调节的幅度.     

用于土壤水分测量的磁共振射频线圈宽频匹配方法

准确检测土壤水分对于改善农业用水管理至关重要.为了实现土壤水分的准确测量,本文设计了一种微型inside-out核磁共振（NMR）传感器（测量区域位于传感器周围）.其中磁体由三个轴向磁化的圆柱磁体组成,射频线圈由四个相同的弧面螺旋线圈串联组成.通过调整圆柱体之间的半径和高度,获得了一个圆周对称恒定梯度场.并利用宽频带匹配方法,增加射频线圈的激励区域,从而提高整体信噪比.将本文设计的传感器用于测量含水量不同的小颗粒沙土和大颗粒石英砂土壤模型样品的CPMG（Carr-Purcell-Meiboom-Gill）信号,通过逆拉普拉斯变换获得了T₂分布谱,并将T₂谱峰面积与称重法获得的含水量进行了相关性分析,探讨了该传感器用于测量土壤水分的可行性.     

基于FPGA的低场核磁共振谱仪脉冲编程器设计与实现

脉冲编程器在核磁共振(NMR) 谱仪中非常重要,具有产生脉冲序列所需的射频脉冲,控制回波信号的同步接收等功能. 基于FPGA的同步性能和时序控制功能,设计了低场NMR谱仪的脉冲编程器,可产生任意脉冲序列,实现脉冲频率、相位、幅度的灵活调节. 利用设计的脉冲编程器在0.540 Tesla均匀磁场下进行了CuSO₄溶液的T₁和T₂测量,得到高信噪比的NMR回波串.     

柴油-水混合物成分的核磁共振分析

采用CPMG自旋回波序列方法研究了柴油-水混合物的核磁共振横向弛豫特性,获得了具有双主峰特征的横向弛豫时间谱,分析了柴油体积分数对双峰位置与强度的影响.结果表明:峰强与柴油体积分数呈现良好的线性相关,利用这种关系可检测柴油-水混合物的成分.     

Categories of coherence pathways for the CPMG sequence

The CPMG sequence has been extremely useful for efficient measurements of NMR signal, spin-spin relaxation, and diffusion, particularly in inhomogeneous magnetic fields, such as when samples are outside the magnet and RF coil. Due to the inaccuracy of the pulses and the off-resonance effects, the CPMG echoes have contributions from the Hahn echo as well as signals that are similar to stimulated echoes. The systematic understanding of the CPMG pulse sequence requires decomposing the magnetization dynamics into different coherence pathways. In this paper, we describe a method to classify the CPMG coherence pathways and illustrate the nature of these types of pathways. This classification shows that direct echo and stimulated echoes are the major contribution to the CPMG signal. It also provides a clear understanding of the effect of restricted diffusion in porous media.     

一种降低井下核磁共振振铃的新方法

核磁共振（NMR）测井仪在测量过程中,振铃噪声幅度一般很高,会影响回波信号的检测. 常用的交叉相位对脉冲序列（PAPS）虽然能有效地降低振铃影响,但由于振铃与频率有关,使PAPS叠加只能在同频率之间展开,大大降低了NMR测井的纵向分辨率. 利用回波信号和振铃噪声的相位特征,设计了一种脉冲序列,采用PAPS和回波间叠加相结合的方式降低振铃噪声. 研究表明,相对于PAPS技术,提高了测量效率,同时提高了回波串的信噪比和NMR测井的纵向分辨率.     

一种改进的核磁共振测井测量脉冲序列

提出了一种可以提高核磁共振测井测量速度的新型测量脉冲序列,它基于对原有CPMG测量脉冲序列的改进.在原有的测量序列中,在一个CPMG脉冲序列结束之后,直接进行等待(T₁延时),以使磁化强度矢量能够完全恢复到平衡位置,然后再进行下一个CPMG脉冲序列的测量.由于测井仪器的连续运动,在测量含有长T₁成分的地层时,等待时间需要取得较长,使得测井速度很低.但在实际地层中,恰恰又是油气的T₁较长,这无疑给核磁共振测井带来了麻烦.作者发现,如果在一个CPMG脉冲序列结束之后在-x方向先施加一个90°脉冲(或在x方向施加一个270°脉冲),再进行等待,就能更好地使长T₁成分得到极化,从而缩短等待时间、提高测量速度.文中对这种改进后的脉冲序列进行了计算和分析,证明它确实具有一定的优越性.     

Stray field measurements of flow displacement distributions without pulsed field gradients

The probability distribution P(zeta) of diffusive and advective molecular displacements is determined using a fixed field gradient (FFG) pulse sequence, on fluid flow through a Bentheimer sandstone, in the grossly inhomogeneous stray field of a super-conducting magnet. Two decades of q-space are scanned with stimulated echoes, using the gradient of the stray field and variable encoding times delta. The strength of the gradient permits the use of short encoding times, which is desirable for limiting the distorting effects produced by flow displacements through susceptibility induced field inhomogeneities. CPMG and CP echo trains are used to refocus separately the real and imaginary parts of the stimulated echo, for experimental efficiency.     

Fast measurements of average flow velocity by Low-Field ¹H NMR

In this paper, we describe a method for measuring the average flow velocity of a sample by means of Nuclear Magnetic Resonance. This method is based on the Carr-Purcell-Meiboom-Gill (CPMG) sequence and does not require the application of any additional static or pulsed magnetic field gradients to the background magnetic field. The technique is based on analyzing the early-time behavior of the echo amplitudes of the CPMG sequence. Measurements of average flow velocity of water are presented. The experimental results show a linear relationship between the slope/y-intercept ratio of a linear fit of the first echoes in the CPMG sequence, and the average flow velocity of the flowing fluid. The proposed method can be implemented in low-cost Low-Field NMR spectrometers allowing a continuous monitoring of the average velocity of a fluid in almost real-time, even if the flow velocity changes rapidly.     

2D NMR技术在石油测井中的应用

近几年,2D NMR技术得到迅速发展,特别是在核磁共振测井领域. 该文将主要介绍2D NMR技术的脉冲序列、弛豫原理以及2D NMR技术在石油测井中应用. 2D NMR技术是在梯度场的作用下,利用一系列回波时间间隔不同的CPMG脉冲进行测量,利用二维的数学反演得到2D NMR. 2D NMR技术可以直接测量自扩散系数、弛豫时间、原油粘度、含油饱和度、可动水饱和度、孔隙度、渗透率等地层流体性质和岩石物性参数. 从2D NMR谱上,可以直观的区分油、气、水,判断储层润湿性,确定内部磁场梯度等. 2D NMR技术为识别流体类型提供了新方法.     

Restricted diffusion in grossly inhomogeneous fields

We analyze the effects of geometrical restriction on the nuclear magnetization of spins diffusing in grossly inhomogeneous fields where radio-frequency (RF) pulses are weak relative to the total field inhomogeneity, making the rotation angle space-dependent and thus exciting multiple coherence pathways. We show how to separate the effects of restricted diffusion from the effects of the pulses in the case when the change in the field experienced by a diffusing spin in the course of the experiment is small compared to the RF magnitude. We then derive explicit formulas for the contribution of individual coherence pathways to the total magnetization in arbitrary pulse sequences. We find that, for long diffusion times, restriction can dramatically alter the spectrum and the shape of a particular echo, while for short times, the correction will be proportional to the pore space surface-to-volume ratio. We demonstrate these results on the example of the early echoes of the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence.     

Spin-lock techniques and CPMG imaging sequences: a critical appraisal of T1p contrast at 0.15 T

The addition of a spin-lock preparatory sequence to a Carr-Purcell-Meiboom-Gill (CPMG) imaging sequence provides a method which allows an accurate and simple comparison of T1p and T2 contrast. Sagittal and axial brain images, produced with the application of a three pulse preparatory spin-lock sequence prior to a sixteen-echo CPMG imaging sequence, are compared with images acquired without the spin-lock sequence. The CPMG sequence uses non-selective refocusing pulses. Therefore, observed echo signals accurately reflect T2 relaxation. This allows a convenient method for assessing the degree to which T1p and T2 contrast differ. The spin-lock CPMG (SL-CPMG) images were acquired with a spin-locking field amplitude of 0.4 G and resemble heavily T2-weighted images at 0.15 T. Quantitative analyses of signal intensities from edema and normal brain tissue confirm the qualitative observations. This in vivo method should prove useful for determining when the additional RF power deposition associated with spin-locking techniques will provide an alternate form of tissue contrast than that available from additional echo collection.     

Stimulated Anti-Echo Selection in Spatially Localized NMR Spectroscopy

Spectral localization using the stimulated-echo acquisition mode (STEAM) is one of the most popular methods in volume-localizedin vivoNMR spectroscopy. The localized volume signal is generated via stimulated echoes from spins excited by three 90° RF pulses, and the conventional STEAM sequence detects the stimulated-echo signal. From an analysis of the STEAM pulse sequence using the coherence transfer pathway formalism, stimulated anti-echoes are also formed by the same pulse sequence, which constitute the other half of the localized signal in the STEAM experiment. A new scheme of pulsed field gradients for the selection of stimulated anti-echoes was proposed, and localized spectroscopy in the stimulated anti-echo selection mode was achieved on a phantom and fromin vivorat brain.