Simultaneous acquisition of multiple orders of intermolecular multiple-quantum coherence images

Recent studies have demonstrated the ability to detect images based on intermolecular multiple-quantum coherences (iMQCs) that correspond to flipping of two or more separated spins simultaneously, as opposed to conventional magnetic resonance where only one spin is flipped at a time. Until now, iMQC imaging has only acquired one coherence signal per pulse sequence. Here we report a new sequence that successfully detects five orders of coherence (2, 1, 0, −1, and −2-quantum coherence images) in one pulse sequence, with each signal having its full intensity. The simultaneous acquisition highlights substantial contrast differences between conventional and iMQC images, and between the different types of iMQC images.     

Hyperpolarized carbon-carbon intermolecular multiple quantum coherences

Intermolecular multiple quantum coherences (iMQCs) can provide unique contrast with sub-voxel resolution. However, the characteristic growth rate of iMQCs mostly limits these effects to either hydrogen or hydrogen-coupled systems for thermally polarized samples. Hyperpolarization techniques such as dynamic nuclear polarization (DNP) allow for significant increases in the carbon signal (even more signal than that from hydrogen), making carbon iMQCs achievable. We present the first intermolecular multiple quantum signal between two carbon nuclei.     

分子间多量子相干横向弛豫时间(和自扩散系数)测量的参数优化和数据处理方法

针对测量横向弛豫时间T₂的CPMG脉冲序列和我们所设计的可同时测量高极化单组份单 峰核自旋体系n阶分子间多量子相干横向弛豫时间T_2,n和自扩散系数D_n的改进的CRAZED脉冲序列,分析了影响测量T₂、T_2,n（或D_n）的各种因素,并着重从技术方面讨论了准确测量T_2,n和D_n的实验参数优化和实验数据处理方法.     

Magnetic resonance microscopic imaging based on high-order intermolecular multiple-quantum coherences

Most imaging studies using intermolecular multiple-quantum coherences (iMQCs) have focused on the two-spin dipolar interactions--zero and double quantum coherences. Here, we report the results of various experimental studies to assess the feasibility of magnetic resonance microscopy with high-order iMQCs in model systems at 7 and 14 T. Experimental results demonstrated that the iMQC microscopic images with high coherence orders are readily observable at high field and have unique contrast depending on the sample microstructure and coherence order.     

液体NMR中平板间多量子相干受限扩散行为的有限差分模拟

将平板间单量子相干受限扩散理论表述推广到多量子相干,并结合积算符矩阵、Bloch方程和有限差分方法进行模拟. 通过模拟找出平板间受限扩散信号衰减随平板间距变化的规律,并与实际体系比较. 结果表明：平板间n量子相干的自旋回波信号衰减曲线与单量子类似,且其产生同样衍射图样所需的脉冲梯度场强度仅为单量子的1/|n|,可用于测量微小的平板间距. 本文的模拟方法可进一步推广到复杂体系的研究.     

Signal enhancement in CRAZED experiments

Many of the promising applications of the CRAZED (COSY Revamped with Asymmetric Z-gradient Echo Detection) experiments are in biomedical and clinical technologies. In tissue, however, signal from the typical CRAZED experiment is largely limited by transverse relaxation. When relaxation is included, the maximum achievable signal from a prototypical CRAZED sequence, in the linear regime, is proportional to T(2)/tau(d). This means that for samples with a short T(2), as encountered in vivo, signals from intermolecular multiple-quantum coherences (iMQCs) reach very diminished signal intensities. While relaxation is generally regarded as a fundamental constraint, we show here that when T(2) is short but T(1) is long, as in tissue, there are simple sequence modifications that can increase signal beyond the T(2) limit. To better utilize the available signal intensity from iMQCs we propose a method to substitute part of the transverse magnetization with the longitudinally modulated magnetization. In this paper we show, with both simulations and experimental results, that in the presence of strong transverse relaxation the standard CRAZED scheme is not the optimal method for observing iMQCs, and can be improved upon with simple modifications.     

Apparent diffusion behaviors of spins in the presence of distant dipolar field in two-component solution NMR

The diffusion behaviors of spins in the presence of distant dipolar field in two-component spin systems during the second evolution period of a modified CRAZED sequence before acquisition were investigated. Theoretical formulas were deduced based on the distant dipolar field model. The simulation results and experimental observations are consistent with the theoretical predictions. This study shows that the relative intensities of signals from intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) have the same diffusion attenuation characteristic under the combined effect of diffusion weighting gradients and distant dipolar field during the second evolution period. This diffusion attenuation may be different from that of conventional single-quantum coherence signal, depending on the relative orientation of the diffusion weighting gradients to the coherence selection gradients. The results presented herein are helpful for understanding the effect of distant dipolar field from a spin system on the diffusion behavior of other spin system and the signal properties in the iZQC or iDQC magnetic resonance imaging.     

Methylene spectral editing in solid-state 13C NMR by three-spin coherence selection

A robust new solid-state nuclear magnetic resonance (NMR) method for selecting CH2 signals in magic-angle spinning (MAS) 13C NMR spectra is presented. Heteronuclear dipolar evolution for a duration of 0.043 ms, under MREV-8 homonuclear proton decoupling, converts 13C magnetization of CH2 groups into two- and three-spin coherences. The CH2 selection in the SIJ (C H H) spin system is based on the three-spin coherence S(x)I(z)J(z), which is distinguished from 13C magnetization (S(x)) by a 1H 0 degrees/90 degrees pulse consisting of two 45 degrees pulses. The two-spin coherences of the type S(y)I(z) are removed by a 13C 90 degrees x-pulse. The three-spin coherence is reconverted into magnetization during the remainder of the rotation period, still under MREV-8 decoupling. The required elimination of 13C chemical-shift precession is achieved by a prefocusing 180 degrees pulse bracketed by two rotation periods. The selection of the desired three-spin coherence has an efficiency of 13% theoretically and of 8% experimentally relative to the standard CP/MAS spectrum. However, long-range couplings also produce some three-spin coherences of methine (CH) carbons. Therefore, the length of the 13C pulse flipping the two-spin coherences is increased by 12% to slightly invert the CH signals arising from two-spin coherences and thus cancel the signal from long-range three-spin coherences. The signal intensity in this cleaner spectrum is 6% relative to the regular CP/TOSS spectrum. The only residual signal is from methyl groups, which are suppressed at least sixfold relative to the CH2 peaks. The experiment is demonstrated on cholesteryl acetate and applied to two humic acids.     

Accurate measurements of small J coupling constants under inhomogeneous fields via intermolecular multiple-quantum coherences

Two new NMR pulse sequences, based on intermolecular multiple-quantum coherences (iMQCs), were developed to obtain apparent J coupling constants with a scaling factor from one to infinity relative to the conventional J coupling constants. Here the apparent J coupling constants were defined as apparent peak separations in unit of Hz in a reconstructed spectrum for a coupled spin system. Except for the adjustable scaling factor for apparent J coupling constants, the sequences hold the advantage of high acquisition efficiency, and retain the spectral information such as chemical shifts, multiplet patterns, and relative peak areas under inhomogeneous fields. For spin systems with small scalar coupling constants, well-resolved J-spectra can be achieved by selecting a proper scaling factor. Theoretical predictions are in good agreement with simulation results and experimental observations.     

Functional magnetic resonance imaging with intermolecular multiple-quantum coherences

For the first time, we demonstrate here functional magnetic resonance imaging (fMRI) using intermolecular multiple-quantum coherences (iMQCs). iMQCs are normally not observed in liquid-state NMR because dipolar interactions between spins average to zero. If the magnetic isotropy of the sample is broken through the use of magnetic field gradients, dipolar couplings can reappear, and hence iMQCs can be observed. Conventional (BOLD) fMRI measures susceptibility variations averaged over each voxel. In the experiment performed here, the sensitivity of iMQCs to frequency variations over mesoscopic and well-defined distances is exploited. We show that iMQC contrast is qualitatively and quantitatively different from BOLD contrast in a visual stimulation task. While the number of activated pixels is smaller in iMQC contrast, the intensity change in some pixels exceeds that of BOLD contrast severalfold.     

Double-quantum-filtered NMR signals in inhomogeneous magnetic fields

The possibility of exciting and detecting proton NMR double-quantum coherences in inhomogeneous static and radiofrequency magnetic fields was investigated. For this purpose specialized pulse sequences which partially refocus the strongly inhomogeneous evolution of the spin system and generate double-quantum buildup and decay curves were implemented on the NMR MOUSE (mobile universal surface explorer). The theoretical justification of the method was developed for the simple two-spin-1/2 system. The performances of the same pulse sequences were also tested on a solid-state high-field NMR spectrometer. It was shown that DQ decay curves have a better signal-to-noise ratio in the initial time regime than DQ buildup curves. The double-quantum buildup and decay curves were recorded for a series of cross-linked natural rubber samples. These curves give access to quantitative values of the ratio of proton total residual dipolar couplings which are in good agreement with those measured in homogeneous fields. A linear dependence of these ratios on the sulfur-accelerator content was found.     

High-resolution absorptive intermolecular multiple-quantum coherence NMR spectroscopy under inhomogeneous fields

Intermolecular multiple-quantum coherence (iMQC) is capable of improving NMR spectral resolution using a 2D shearing manipulation method. A pulse sequence termed CT-iDH, which combines intermolecular double-quantum filter (iDQF) with a modified constant-time (CT) scheme, is designed to achieve fast acquisition of high-resolution intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) spectra without strong coupling artifacts. Furthermore, double-absorption lineshapes are first realized in 2D intermolecular multi-quantum coherences (iMQCs) spectra under inhomogeneous fields through a combination of iZQC and iDQC signals to double the resolution without loss of sensitivity. Theoretically the spectral linewidth can be further reduced by half compared to original iMQC high-resolution spectra. Several experiments were performed to test the feasibility of the new method and the improvements are evaluated quantitatively. The study suggests potential applications for in vivo spectroscopy.     

Heteronuclear intermolecular single-quantum coherences in liquid nuclear magnetic resonance

This paper analyses the heteronuclear Cosy Revamped by Asymmetric Z-gradient Echo Detection pulse sequence. General theoretical expressions of the pulse sequence with arbitrary flip angles were derived by using dipolar field treatment and signals originating from heteronuclear intermolecular single-quantum coherences （iSQCs） in highly-polarized two spin-1/2 systems were mainly discussed in order to find the optimal flip angles. The results show that signals from heteronuclear iSQCs decay slower than those from intermolecular double-quantum coherences or intermolecular zero-quantum coherences. Magical angle experiments validate that the signals are from heteronuclear iSQCs and insensitive to the imperfection of radio-frequency flip angles. All experimental observations are in excellent agreement with theoretical predictions. The quantum-mechanical treatment leads to similar predictions to the dipolar field treatment.     

双Z脉冲实现多量子相干选择检测

本文提出一种改进的多量子相干选择检测(滤波)方法。滤波Z脉冲分别在准备期和混合期末。它能有效地防止单量子相干的"漏进"并避免使用数字相移器。在一般谱仪上容易实现,用密度算符理论对AMX体系进行理论计算,实验结果与理论预期相符。     

液体核磁共振中由分子间偶极耦合和分子内标量耦合共同作用后的谱图裂分模式

在高极化多自旋液体样品中,同时存在着分子间偶极（D）耦合和分子内标量（J）耦合,它们的共同作用产生了一些原来观测不到的分子间多量子相干信号。而且,信号的裂分模式与只存在J耦合的多自旋体系中观测到的多量子相干信号的裂分模式不同。本文从理论和实验上研究了这些禁阻的共振峰及其独特的裂分模式。为了比较验证,我们以I2S3+X自旋体系为例,结合使用选择和非选择性的射频脉冲序列来获得分子间双量子相干信号的五种裂分模式。进而归纳出对IpSq+Xk (p, q, k = 1, 2, 3,…)自旋体系普适的裂分模式规则。并指出,它们中如(1:0:-1)的裂分模式会放大J耦合裂分,使得J耦合常数的测量更精确,特别在J耦合常数很小或不均匀场中的J耦合常数的测量中具有诱人的应用前景。结果表明理论预测,计算机模拟和实验观测结果三者吻合的很好。     

Storage and retrieval of coherent optical information in atomic populations

We propose a simple and powerful protocol to map an arbitrary atomic coherence between two quantum states into a population distribution of three metastable states, and later to retrieve the atomic coherence from the population distribution. The protocol applies simple sequences of radiation pulses with arbitrary temporal profile, either as coincident or as consecutive pulses. Mapping of rather short-lived atomic coherences into very long-lived atomic populations permits the prolongation of storage times (e.g. of optical information encoded in atomic coherences) by many orders of magnitude — without the need for complicated techniques to reduce homogeneous broadenings.     

核磁共振中多量子相干扩散行为的理论表述和计算机模拟

改进了Warren所提出的CRAZED脉冲序列以研究分子间多量子相干的扩散过程 ,讨论了利用核磁共振测量分子内和分子间多量子相干表观自扩散系数的理论表述 ,采用粒子的随机行走模型模拟其扩散行为 .在短脉冲近似和长脉冲梯度场两种实验条件下 ,分别获得了因扩散引起的不同相干阶数的相对信号衰减强度随梯度场脉冲间隔时间的变化曲线 ,由此得到分子内多量子相干和分子间多量子相干的表观扩散率与溶液分子扩散系数的关系 .还将计算机模拟结果与理论预测进行分析和比较 ,发现二者能很好地吻合 .研究结果表明 ,分子间多量子相干的表观扩散率与常规的分子内多量子相干的表观扩散率明显不同 ,因此 ,分子间多量子相干的表观扩散率可能提供一种新的核磁共振成像的对比度机理     

Enhanced sensitivity to residual dipolar couplings of elastomers by higher-order multiple-quantum NMR

The homonuclear and heteronuclear residual dipolar couplings in elastomers reflect changes in the cross-link density, temperature, the uniaxial and biaxial extension or compression as well as the presence of penetrant molecules. It is shown theoretically that for an isolated methyl group the relative changes in the intensity of the homonuclear double-quantum buildup curves in the initial time regime due to variation of the residual dipolar coupling strength is less sensitive than the changes in the triple-quantum filtered NMR signal when considering the same excitation/reconversion time. For a quadrupolar nucleus with spin I=2 the sensitivity enhancement was simulated for four-quantum, triple-quantum, and double-quantum buildup curves. In this case the four-quantum build-up curve shows the highest sensitivity to changes of spin couplings. This enhanced sensitivity to the residual dipolar couplings was tested experimentally by measuring 1H double-quantum, triple-quantum, and four-quantum buildup curves of differently cross-linked natural rubber samples. In the initial excitation/reconversion time regime, where the residual dipolar couplings can be measured model free, the relative changes in the intensity of the four-quantum buildup curves are about five times higher than those of the double-quantum coherences. For the first time proton four-quantum coherences were recorded for cross-linked elastomers.     

NMR波谱学中的高温近似的有效性及其相关问题

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.     

Intermolecular double quantum coherences (iDQc) and diffusion-weighted imaging (DWI) imaging of the human brain at 1.5 T

To study the sensitivity of intermolecular double quantum coherences (iDQc) imaging contrast to brain microstructure and brain anisotropy, we investigated the iDQC contrast between differently structured areas of the brain according to the strength and the direction of the applied correlation gradient. Thus diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) maps have been obtained. This procedure, which consists of analyzing both iDQc and DWI images at different gradient strength and gradient direction, could be a promising tool for clinical brain investigations performed with higher than 1.5 T magnetic fields.