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41.
NMR波谱学中的高温近似的有效性及其相关问题 总被引:1,自引:1,他引:0
高温近似(HTA)是NMR波谱学和MRI技术中久经考虑且应用最广的一个近似. 最近几年有一大批引人注目的文章断言对高场下的浓溶液,如水的质子COSY谱,该近似会 失效. 与声称的HTA失效同样令人惊讶的是其副产品之一,即认为在低粘度液体如水里,存 在分子间的多量子相干(MQC). 根据理论分析及实验结果,本文将对液体中HTA和分子间MQC 作一仔细考查. 结论是:(1)室温下,常规NMR样品如水,不是一个宏观量子系统;(2)室温下,HTA仍然是NMR里最值得信赖的近似;(3)质子COSY谱(如水)中所观察到的多重自旋回波( MSE)峰并不意味存在分子间的量子力学自旋相互作用. 相似文献
42.
Analytical calculations using the Bloch formalism were performed to assess the dependence on T1 of the echo amplitudes for the Phase-Alternating Phase-Shift (PHAPS) multiple spin-echo protocol. Measurements in a 0.5 T MR imaging unit were performed to ratify the analytical results. especially for low T2 values, the echo amplitudes were erroneous, with an increasing contribution from stimulated echo components with increasing T1. Apart from affecting T2 estimates, stimulated echoes generated a non-monoexponential signal decay of the echo trains. The results confirmed previous simulation studies as regards the dependence on T1 of T2 estimates from PHAPS. 相似文献
43.
Karlsson T Hughes CE Schmedt auf der Günne J Levitt MH 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2001,148(2):238-247
We describe a new technique for double-quantum excitation in magic-angle-spinning NMR of powdered solids. The technique is designed to efficiently excite double-quantum coherence in the vicinity of a rotational resonance condition. The offset from rotational resonance allows the double-quantum filtered signals to be observed with high resolution and sensitivity. The method uses rotational excitation of zero-quantum coherence, assisted by radiofrequency pulse cycles. The zero-quantum coherence is converted into double-quantum coherence by a frequency-selective inversion sequence. Experiments on [(13)C(2), (15)N]-glycine demonstrate a double-quantum filtering efficiency of approximately 41% at a sample rotation frequency of 8.300 kHz, which is 1.600 kHz away from the n = 1 rotational resonance. We achieve 32% double-quantum filtering efficiency at a spinning frequency of 9.250 kHz, which is 2.550 kHz away from rotational resonance. 相似文献
44.
Trains of spin echoes are normally modulated by homonuclear scalar couplings. It has long been known that echo modulations are quenched when the pulse-repetition rates are much larger than the offsets of the coupling partners, because the spin systems behave as if they consisted of magnetically equivalent spins when the offsets are suppressed. This type of quenching of the echo modulations can occur when the radio-frequency (RF) pulses are ideal, that is, when they are perfectly homogeneous, properly calibrated to induce rotations through an angle, pi, and have an RF amplitude, omega(1)=-gammaB(1), that is strong compared to the largest offset, Omega(S)=omega(0S)-omega(RF), with respect to the carrier frequency. Recently, it was discovered that echo modulations can also be quenched when the RF pulses are nonideal, that is, when they are too weak to bring about an ideal rotation of the magnetization of the coupling partners, so that the effective fields associated with the RF pulses are tilted in the rotating frame. This phenomenon typically occurs when the pulse-repetition rates are much slower than the offset of the coupling partner. Under such conditions, it turns out, however, that for certain offsets, when the phase, Phi(S) (which arises from a free precession of the magnetization of the coupling partner, S, in the pulse interval, 2tau, and the pulse length, tau(pi)), approaches a multiple of 2pi, the echo modulations are restored. However, the frequencies of these echo modulations are not simply determined by the homonuclear scalar coupling, J(IS). The Fourier transforms of the echo trains (the so-called "J spectra") reveal surprising multiplet patterns, and the amplitudes of the echo modulations depend on the offsets of the coupling partners. Herein, we present a unified theory, based on an average-Hamiltonian approach, to describe these effects for two-spin systems. Experimental evidence of echo modulations in a system of two spins is presented. Experiments with three and more spins, backed up by extensive numerical simulations, will be presented elsewhere. 相似文献
45.
Clark CA Barker GJ Tofts PS 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,142(2):358-363
Navigator echoes provide a means with which to remove motion artifacts from diffusion-weighted images obtained using any multishot imaging technique. However, residual motion artifact is often present in the corrected images rendering the technique unreliable. It is shown that velocity-compensated diffusion sensitization when used in tandem with a navigator echo further reduces the degree of residual motion artifacts present in the corrected images and improves the reliability and clinical utility of the technique. This is demonstrated by applying a method for quantification of motion artifact to brain images of healthy volunteers scanned using both conventional (Stejskal-Tanner) and velocity-compensated gradient sensitization. Other factors affecting the efficacy of the navigator echo technique, such as brain pulsatile motion, gradient b factor, and navigator echo signal-to-noise ratio, are also discussed. 相似文献
46.
47.
利用声音反射信号重建室内空间结构,实现声源室内定位和室内建图,越来越受到业界的重视。本文针对现有基于image模型重构室内空间结构的方法得到的结果误差较大的问题,提出一种利用声音信号到达时间差TDOA(Time Difference of Arrival)最小二乘误差的方法来提取一阶反射信号,利用一阶反射信号到达时间实现对墙壁、天花板、地板等室内主要反射面位置的探测,可以实现在收发无法精准同步状态下空间结构的重建。实测结果表明,在2.3 m宽*2.37 m高的走廊中得到的建图结果平均误差为0.05 m,该方法能够满足对空间结构的重建的要求,为辅助室内声源定位和室内建图提供一种更加实用的解决方案。 相似文献
48.
本文通过活化能渐近分析方法研究了非绝热正向阴燃波的两种典型结构,即反应滞后结构和反应前导结构,比较了当来流空气流量发生变化时两种结构的主要特征参数的变化规律。研究结果表明,相比较于绝热情况,当考虑阴燃向外界的对流热损失效应时,两种正向阴燃结构随来流空气流量变化时的一维响应均表现为两个解分支,相应地存在一个熄灭极限,对应... 相似文献
49.
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. 相似文献
50.