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．     

核磁共振测井受激回波校正方法研究

核磁共振测井中,受激回波导致CPMG序列中前2个回波与期望值存在较大差异,严重影响仪器信噪比和储层评价效果. 为进一步改善核磁共振测井仪性能,通过探究受激回波产生机理,揭示其对CPMG回波串作用规律,提出了受激回波的校正方法. 同时针对仪器负载不同,相同发射功率产生射频场强度各异的特性,建立受激回波校正系数与射频场强的数学关系,在不同应用环境中实现对CPMG测量结果的校正. 实验表明,在信噪比较低的应用环境中,采用传统的舍去前2个回波的方法误差达24.58%,而该文设计的校正算法误差仅为4.9%.     

WATR法测量稀水溶液的1H-NMR谱

本文讨论了用WATR法消除稀水溶液中的水共振峰,进行¹H-NMR谱测定的原理、影响T₂的因素及应用实例。WATR法是在水溶液中加入氯化胺、盐酸羟胺和盐酸胍等水质子交换试剂以降低水质子的T₂,然后用CPMG脉冲系列来测定¹H-NMB谱。本法最适用于稀水溶液中处于水共振峰内或附近的样品的共振峰的测定,还可测定稀水溶液中¹⁵N-¹H的化学位移及耦合常数,到目前为止尚未见有这方面的报导。     

Deuteron Solid-State NMR Relaxation Measurements Reveal Two Distinct Conformational Exchange Processes in the Disordered N-Terminal Domain of Amyloid-β Fibrils

We employed deuterium solid-state NMR techniques under static conditions to discern the details of the μs–ms timescale motions in the flexible N-terminal subdomain of Aβ_1–40 amyloid fibrils, which spans residues 1–16. In particular, we utilized a rotating frame (R_1ρ) and the newly developed time domain quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) relaxation measurements at the selectively deuterated side chains of A2, H6, and G9. The two experiments are complementary in terms of probing somewhat different timescales of motions, governed by the tensor parameters and the sampling window of the magnetization decay curves. The results indicated two mobile “free” states of the N-terminal domain undergoing global diffusive motions, with isotropic diffusion coefficients of 0.7−1 ⋅ 10⁸ and 0.3−3 ⋅ 10⁶ad² s⁻¹. The free states are also involved in the conformational exchange with a single bound state, in which the diffusive motions are quenched, likely due to transient interactions with the structured hydrophobic core. The conformational exchange rate constants are 2−3 ⋅ 10⁵ s⁻¹ and 2−3 ⋅ 10⁴ s⁻¹ for the fast and slow diffusion free states, respectively.     

Behavior of Two Almost Identical Spins during the CPMG Pulse Sequence

Multiple‐spin‐echo experiments have found wide use in nuclear magnetic resonance spectroscopy. In particular, the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence is used to determine transverse relaxation times T₂. Herein it is demonstrated, both theoretically and experimentally, that for a pair of almost identical spins‐1/2 the experimental setup can have a profound effect on the observed spin dynamics. It is shown that, in the case of dipolar relaxation, the measured T₂ values can roughly vary between the limits of identical and unlike spins, just depending on the repetition rate of π pulses with respect to chemical shift separation. Such an effect can, in the extreme narrowing regime, amount to a 50 % difference.     

通过高斯加权的1H CPMG弛豫曲线测定橡胶交联密度

CPMG（Carr-Purcell-Meiboom-Gill）回波法是测量橡胶交联密度[常用交联点之间的分子量（M_c）表示]的一种常用核磁共振（NMR）技术,但实验发现通过该技术获得的M_c对于CPMG序列中脉冲间隔时间具有较强的依赖性,导致交联密度NMR测量值与橡胶材料硬度的相关性低.为了克服这一缺点,本文对不同脉冲间隔时间下CPMG实验测得的质子横向驰豫曲线进行高斯加权.通过对高斯加权求和后的质子横向驰豫曲线进行处理分析,实现了对橡胶交联密度更加准确地测量,大幅提升了天然橡胶交联密度NMR测量值与材料硬度的相关性.本文方案测量能获得与¹H DQ NMR方法相当,或比之更佳的交联密度-硬度相关性.同时,本文方案比¹H DQ NMR方法更为高效,整体测量时间缩短为¹H DQ NMR实验时间的1/10.     

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.     

Exact solution of the CPMG pulse sequence with phase variation down the echo train: application to R₂ measurements

An implicit exact algebraic solution of CPMG experiments is presented and applied to fit experiments. Approximate solutions are also employed to explore oscillations and effective decay rates of CPMG experiments. The simplest algebraic approximate solution has illustrated that measured intensities will oscillate in the conventional CPMG experiments and that using even echoes can suppress errors of measurements of R? due to the imperfection of high-power pulses. To deal with low-power pulses with finite width, we adapt the effective field to calculate oscillations. An optimization model with the effective field approximation and dimensionless variables is proposed to quantify oscillations of measured intensities of CPMG experiments of different phases of the π pulses. We show, as was known using other methods, that repeating one group of four pulses with different phases in CPMG experiments, which we call phase variation, but others call phase alternation or phase cycling, can significantly smooth the dependence of measured intensities on frequency offset in the range of ±?γB?. In this paper, a second-order expression with respect to the ratio of frequency offset to π-pulse amplitude is developed to describe the effective R? of CPMG experiments when using a group phase variation scheme. Experiments demonstrate that (1) the exact calculation of CPMG experiments can remarkably eliminate systematic errors in measured R?s due to the effects of frequency offset, even in the absence of phase variation; (2) CPMG experiments with group phase variation can substantially remove oscillations and effects of the field inhomogeneity; (3) the second-order expression of the effective decay rate with phase variation is able to provide reliable estimates of R? when offsets are roughly within ±?γB?; and, most significantly, (4) the more sophisticated optimization model using an exact solution of the discretized CPMG experiment extends, to ±γB?, the range of offsets for which reliable estimates of R? can be obtained when using the preferred phase variation scheme.     

77Se solid-state NMR investigations on As(x)Se(1-x) glasses using CPMG acquisition under MAS

⁷⁷Se (I=1/2) solid-state NMR is a very sensitive probe of the local structure of selenide glasses, which themselves are promising for optical applications. In this work, we show that although ⁷⁷Se has a low natural abundance (7.58%) and a wide spectral range, the sensitivity can be dramatically increased using Carr–Purcell–Meiboom–Gill (CPMG) trains of rotor-synchronized π pulses during the detection of ⁷⁷Se magnetization but may be affected by chemical shift anisotropy when the Magic Angle Spinning rate is not fast enough and by offset effects. The indirect dimension of the T₂^CPMG-resolved spectrum shows a strong influence of the J-couplings between naturally occurring ⁷⁷Se pairs. The resulting spectra show that the structural model known as “chains crossing model” is not entirely suitable to describe the glassy network of the Se-rich compositions.