Radiation damping and reciprocity in nuclear magnetic resonance: The replacement of the filling factor
Radiation damping and reciprocity in nuclear magnetic resonance: The replacement of the filling factor
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DOI：
10.1016/j.jmr.2010.06.001
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发表时间：
2010-09-01
影响因子：
2.2
通讯作者：
Van Criekinge, Mark
中科院分区：
文献类型：
--
作者：
Tropp, James;Van Criekinge, Mark
描述核磁共振（NMR）中辐射阻尼的基本方程式是通过互惠原理重写的，以消除阻尼常数对填充因子的依赖性 - 既不是独特地定义的参数，以易于测量。新方程使用的是，实验室坐标中射频B场的峰值峰峰幅度除以检测线圈电阻的平方根，为此，简单而直接的测量手段存在。我们使用效率来定义固有阻尼常数，即探针和前置放大器都与系统阻抗完全匹配时获得的效率。对于前置放大器的不完美匹配，显示阻尼常数随着探测器的电距离而变化，并给出了方程并执行模拟，以预测距离依赖性，而距离依赖性（对于无损线）是周期性模量的半波长。纯净水中质子的辐射阻尼游离诱导NMR信号的实验测量在14.1 T的静态B场强度下进行；并使用可变线方法测量了内在阻尼常数。对于直径为5 mm的样品，在反检测探针中，我们测量的固有阻尼常数为204 s（-1），对应于小尖端角度的65 Hz的阻尼线宽。基于效率的三个单独测量值，预测的内在线宽为52.3 Hz，或测量值的80％。 （c）2010 Elsevier Inc.保留所有权利。
The basic equation describing radiation damping in nuclear magnetic resonance (NMR) is rewritten by means of the reciprocity principle, to remove the dependence of the damping constant upon filling factor - a parameter which is neither uniquely defined for easily measured. The new equation uses instead the transceive efficiency, i.e. the peak amplitude of the radiofrequency B field in laboratory coordinates, divided by the square root of the resistance of the detection coil, for which a simple and direct means of measurement exists. We use the efficiency to define the intrinsic damping constant, i.e. that which obtains when both probe and preamplifier are perfectly matched to the system impedance. For imperfect matching of the preamp, it is shown that the damping constant varies with electrical distance to the probe, and equations are given and simulations performed, to predict the distance dependence, which (for lossless lines) is periodic modulo a half wavelength. Experimental measurements of the radiation-damped free induction NMR signal of protons in neat water are performed at a static B field strength of 14.1 T; and an intrinsic damping constant measured using the variable line method. For a sample of 5 mm diameter, in an inverse detection probe we measure an intrinsic damping constant of 204 s(-1), corresponding to a damping linewidth of 65 Hz for small tip angles. The predicted intrinsic linewidth, based upon three separate measurements of the efficiency, is 52.3 Hz, or 80% of the measured value. (C) 2010 Elsevier Inc. All rights reserved.