Improving the accuracy of pulsed field gradient NMR diffusion experiments: Correction for gradient non-uniformity
Improving the accuracy of pulsed field gradient NMR diffusion experiments: Correction for gradient non-uniformity
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DOI：
10.1016/j.jmr.2009.01.025
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发表时间：
2009-05-01
影响因子：
2.2
通讯作者：
Morris, Gareth A.
中科院分区：
文献类型：
--
作者：
Connell, Mark A.;Bowyer, Paul J.;Morris, Gareth A.
脉冲场梯度核磁共振是一种确定自扩散系数的成熟技术。然而，系统误差的一个重要来源存在于所施加的脉冲场梯度的空间变化中。不均匀的脉冲场梯度导致峰值振幅的衰减偏离对梯度平方的预期指数依赖性。这会产生两个不良影响：表观扩散系数将偏离真实值，其程度取决于实验参数的选择，并且非线性最小二乘拟合估计的误差将包含显着的系统贡献。特别是，通过 NMR 信号扩散衰减的指数拟合确定的表观扩散系数将取决于所使用的精确脉冲宽度和所选择的梯度幅度的范围。如果将实验衰减数据拟合到针对测量的梯度和信号强度的空间依赖性进行校正的函数，则可以部分补偿这些问题。本研究描述了用于校准 NMR 扩散测量的现有方法的一般替代方法。通过在存在弱读取梯度的情况下测量脉冲场梯度回波来映射脉冲场梯度幅度和信号强度的主要纵向变化。然后使用这些数据构建整个样本的预测信号衰减函数，该函数被参数化为幂级数的指数。结果将使用新校准方法获得的扩散系数与以前的文献值进行了比较。 (C) 2009 年，爱思唯尔公司出版。
Pulsed field gradient NMR is a well-established technique for the determination of self-diffusion coefficients. However, a significant source of systematic error exists in the spatial variation of the applied pulsed field gradient. Non-uniform pulsed field gradients cause the decay of peak amplitudes to deviate from the expected exponential dependence on gradient squared. This has two undesirable effects: the apparent diffusion coefficient will deviate from the true value to an extent determined by the choice of experimental parameters, and the error estimated by the nonlinear least squares fitting will contain a significant systematic contribution. In particular, the apparent diffusion coefficient determined by exponential fitting of the diffusional attenuation of NMR signals will depend both oil the exact pulse widths used and on the range of gradient amplitudes chosen. These problems can be partially compensated for if experimental attenuation data are fitted to a function corrected for the measured spatial dependence of the gradient and signal strength. This study describes a general alternative to existing methods for the calibration of NMR diffusion measurements. The dominant longitudinal variation of the pulsed field gradient amplitude and the signal strength are mapped by measuring pulsed field gradient echoes in the presence of a weak read gradient. These data are then used to construct a predicted signal decay function for the whole sample, which is parameterised as the exponential of a power series. Results are presented which compare diffusion coefficients obtained using the new calibration method with previous literature values. (C) 2009 Published by Elsevier Inc.