Indirect detection of nuclear magnetic resonance via geometrically induced long-range dipolar fields

We report the indirect detection of the magnetization of one spin species via the NMR signal of a second species. Our method relies on the control of long-range dipolar fields between two separate objects, in this case, a water droplet (sensor) immersed in a tube containing mineral oil (sample). Unlike prior experiments, no gradient pulses are used; rather, the setup geometry is exploited to select the part of the sample to be probed and modulate the spin alignment in the sensor. Our results are discussed in the context of Dipolar Field Microscopy, a proposed strategy in which the detector is a hyperpolarized tip.     

Effects of residual single-quantum coherences in intermolecular multiple-quantum coherence studies

Intermolecular multiple-quantum coherences (iMQCs) have been reported to offer a sensitivity to sample structure at a specific user-defined length scale down to the order of 10 microm. When assessing this novel contrast mechanism in controlled phantom experiments, we have observed three different mechanisms whereby residual single-quantum coherences (SQCs) arising from intense high spatial frequencies, stimulated echoes and strong spatially encoding gradients can produce significant changes in signal contrast at particular length scales. These changes which only appear when components arising from SQCs and iMQCs are both present in the detected signal, are similar to changes previously attributed to iMQCs alone. We demonstrate each mechanism by which these residual SQCs arise and describe methods for their suppression.     

Flow effects in long-range dipolar field MRI

Incoherent spin motion, such as diffusion, can lead to significant signal loss in multiple spin echoes (MSE) experiments, sometimes to its complete extinction. Coherent spin motion, such as laminar flow, can also modify the magnetization in MSE imaging and yield additional contrast. Our experimental results indicate that MSE is flow-sensitive. Our theoretical analysis and experimental results show how the effect of the distant dipolar field can be annihilated by flow. This effect can be quantified by directly solving the nonlinear Bloch equation, taking into account the deformation of the dipolar field by motion. Unexpected results have been observed, such as a recovery of the dipolar interaction due to flow in the "magic angle" condition.     

Optically detected nuclear magnetic resonance at the sub-micron scale

Nuclear magnetic resonance is arguably one of the most powerful techniques available today to characterize diverse systems. However, the low sensitivity of the standard detection method constrains the applicability of this technique to samples having effective dimensions not less than a few microns. Here, we propose a novel scheme and device for the indirect detection of the nuclear spin signal at a submicroscopic scale. This approach--for which the name Dipolar Field Microscopy is suggested--is based on the manipulation of the long-range nuclear dipolar interaction created between the sample and a semiconductor tip located close to its surface. After a preparation interval, the local magnetization of the sample is used to modulate the nuclear magnetization in the semiconductor tip, which, in turn is determined by an optical inspection. Based on results previously reported, it is shown that, in principle, images and/or localized high-resolution spectra of the sample can be retrieved with spatial resolution proportional to the size of the tip.     

Theoretical studies of the effect of the dipolar field in multiple spin-echo sequences with refocusing pulses of finite duration

It has been observed recently that the finite duration of refocusing rf pulses in a multiecho acquisition of the signal formed under the influence of the dipolar field leads to significant signal attenuation [S. Kennedy, Z. Chen, C.K. Wong, E.W.-C. Kwok, J. Zhong, Investigation of multiple-echo spin-echo signal acquisition under distant dipole-dipole interactions, Proc. Int. Soc. Magn. Reson. Med. 13 (2005) 2288]. Hereto, we quantify the phenomenon by evaluating analytically the influences of both the distant dipolar field (DDF) and transverse relaxation T2 on the magnetization in a multiecho pulse sequence based on correlation spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED). Analytic expressions for the magnetization were obtained, which demonstrate explicitly the origin of rephased signal in the presence of the finite pi pulses in the multiecho train. The expressions also explain the effects of the DDF and T2 during the refocusing pulses on the signal strength, and show the substantial signal dependence on the phase of the rf pulses. We show that when the DDF effect during the pulse is canceled, the signal rises primarily during the free evolution time in the acquisition period. This elucidates the signal attenuation when the rf pulses cover a significant proportion of time in the sequence. In addition, we performed an optimization on the number of refocusing pulses that maximizes the total acquired signal using parameters for water, brain white matter, and muscle. We found that maximal signal-to-noise ratio is obtained when the pulse duration approximately equals the free evolution time in the samples with a wide range of T2.     

Reconstruction of porous material geometry by stochastic optimization based on bulk NMR measurements of the dipolar field

The dependence of the bulk signal intensity from a CRAZED NMR pulse sequence on magnetic field gradient strength and direction as a method to probe the geometry of porous materials is investigated. In this article, we report on the reconstruction of three-dimensional media consisting of a void phase and an NMR-observable liquid phase using the bulk intensity of the distant dipolar field. The correlation gradient strength and direction provide the spatial encoding of the material geometry. An integral equation for the total signal intensity is then solved numerically by a simulated annealing algorithm to recover the indicator function of the fluid phase. Results show that cylindrical and spherical structures smaller than the volume contributing to the NMR signal can be resolved using three values of the correlation distance and three orthogonal gradient directions. This is done by minimizing a cost function which measures the distance between the bulk signal dependence on gradient parameters for the simulated configuration and the signal dependence for the target configuration. The algorithm can reconstruct and differentiate their spherical and cylindrical phase-inverted equivalents. It can also differentiate horizontal from vertical cylinders, demonstrating the potential for assessing structural anisotropy and other coarse geometric quantifiers in a porous material.     

2D ESR image reconstruction from 1D projections using the modulated field gradient method

A method for the reconstruction of 2D ESR images from 1 D projections which is based on the modulated field gradient method has been explored. The 2D distribution of spin-labeled stearic acid in oriented and unoriented dimyristoyl phosphatidylcholine multilayers on a flat quartz support was determined. Such samples are potentially useful for the determination of lipid lateral diffusion in oriented multilayers by monitoring the spreading of a sharp concentration profile in one or two dimensions. The limitations of the method are discussed and the improvements which are needed for dynamic measurements are outlined.     

Sensitivity-enhanced 2D IPAP,TROSY-anti-TROSY,and E.COSY experiments: alternatives for measuring dipolar 15N-1HN couplings

Sensitivity-enhanced versions of the IPAP, TROSY-anti-TROSY, and E.COSY experiments for measuring one-bond 15N-1HN couplings are presented. Together with the previously developed sensitivity-enhanced E.COSY-type HSQC experiment they comprise a suite of sensitivity-enhanced experiments that allows one to chose the optimal spectrum for accurate measurement of one-bond 15N-1HN residual dipolar couplings in proteins. Since one-bond 15N-1HN residual dipolar couplings play uniquely important roles in structural NMR, these additional methods provide further tools for improving structure determination of proteins and other biological macromolecules.     

Through-space R3-HETCOR experiments between spin-1/2 and half-integer quadrupolar nuclei in solid-state NMR

We present several new methods that allow to obtain through-space 2D HETCOR spectra between spin-1/2 and half-integer quadrupolar nuclei in the solid state. These methods use the rotary-resonance concept to create hetero-nuclear coherences through the dipolar interaction instead of scalar coupling into the HMQC and refocused INEPT experiments for spin n/2 (n>1). In opposite to those based on the cross-polarization transfer to quadrupolar nuclei, the methods are very robust and easy to set-up.     

High-resolution NMR spectra in inhomogeneous fields utilizing the CRAZED sequence without coherence selection gradients

Coherence selection gradients have been considered as indispensable for high-resolution NMR spectroscopy in inhomogeneous fields utilizing the CRAZED-type sequences. However, our experimental results demonstrate that these gradients can be omitted if an appropriate phase cycling is applied. The measured linewidth of reconstructing 1D high-resolution spectral peaks does not depend on the dipolar correlation distance determined by the coherence selection gradients, but is only affected by diffusion and T(2) relaxation. This finding suggests the need to reconsider the mechanism for the iMQC-based high-resolution spectroscopy.     

Regularized resolvent transform for direct calculation of 45 degrees projections of 2D J spectra

The regularized resolvent transform (RRT) has been applied in a novel way to J-resolved spectra. This involves the direct calculation of the 45 degrees projection without constructing the 2D spectrum. The results show a significant resolution enhancement over that obtained by the 45 degrees projection of a 2D Fourier spectrum, even for much larger signals. In particular, RRT is able to resolve peaks that belong to different overlapping multiplets in a very crowded spectral region, where the conventional technique fails for any signal size. The resolving power of this method along with the significantly shorter signals required, make this method a powerful tool in spectral assignment.     

Intermolecular double-quantum coherence imaging without coherence selection gradients and its application in in vivo MRI

In the COSY Revamped with Asymmetric Z-gradient Echo Detection (CRAZED) experiments, magnetization is modulated by the distant dipolar field (DDF) generated by coherence selection gradient (CSG) commonly in sinusoidal wave-form and results in detectable intermolecular multiple-quantum coherence (iMQC) signal. IMQCs have some attractive features, but their intrinsic weak signal intensity prevents their widespread applications. In this paper, a new phase cycling scheme was applied to obtain intermolecular double-quantum coherence (iDQC) signal. It is found that DDF can arise from nonspherical sample geometry or background inhomogeneous field in the absence of CSGs, which is more efficient than that created from CSGs. The experimental results show that the resulting DDF can refocus the ± iDQC signals simultaneously and thus enhance the signal intensity to about two folds of that from the conventional CRAZED sequence. Theoretical prediction and experiments give coincident results.     

A heteronuclear intermolecular single-quantum coherence scheme for high-resolution 2D J-resolved 1H NMR spectra in inhomogeneous magnetic fields

Based on heteronuclear intermolecular single-quantum coherences between proton (¹H) and quadrupolar nuclei (i.e. deuterium ²H), a three-dimensional nuclear magnetic resonance (NMR) pulse sequence is proposed for recovering high-resolution two-dimensional J-resolved NMR spectra from samples mixed with a deuterated solvent in the presence of large magnetic field inhomogeneities. Benefitting from excitation of spins via two different radio frequency (RF) transmit channels, this sequence is suitable for applications in randomly large inhomogeneous fields and the solvent suppression generally required in homonuclear intermolecular multiple-quantum coherence approaches is no longer necessary. Systematic theoretical analyses are given based on the distant dipolar field treatment. Experiment on a sample of corn oil in deuterated acetone and ethyl 3-bromopropionate and acetone dissolved in DMSO-d₆ in a deshimmed field with severe inhomogeneous broadening is performed to show the feasibility and applicability of this sequence.     

High pressure 1D and 2D NMR experiments on model biomembranes

Abstract

We developed a high-pressure high-resolution probe for NMR experiments on biological systems which enabled us to study the phase behaviour of model biomembranes by 1D and 2D NMR experiments at temperatures up to 65°C and at pressures up to 3000 bar.     

用光谱和EPR谱确定CsMgBr3:Ni2+的局域结构

本文采用半自洽场(semi-SCF)自由Ni2 的3d轨道波函数、点电荷—偶极子模型和Ni2 -6X-(X=F,Cl,Br,I)络合物的μ-κ-α模型,建立了结构参数与光谱、EPR谱之间的定量关系,利用完全对角化方法,由光谱和电子顺磁共振(EPR)谱,确定了CsMgBr3:Ni2 晶体在77K温度时的局域结构参数,统一解释了CsMgBr3:Ni2 晶体的局域结构、光谱和EPR谱。所得理论结果与实验值符合得很好。此外,还讨论了晶体局域结构发生畸变的原因。     

Dependence of residual dipolar couplings on foot angle in H MR spectra from skeletal muscle

Foot dorsi and plantar flexion affects the pennation angle of skeletal muscle fibers and changes the fiber direction with respect to the main magnetic field, thereby affecting MR spectrum of the muscle. In order to analyze the effect that foot flexion has on the MR spectrum, tibialis anterior (TA) and soleus muscles were studied in humans and rats. Localized MRS was performed at different foot angles in clinical and pre-clinical settings using a 3 T MRI/MRS GE Excite HD and 7 T Bruker Clinscan scanner, respectively. In this study we show the effect of foot angle variation on total Creatine (tCr) resonance of ¹H spectrum at 3.03 and 3.93 ppm for TA and soleus muscles. In addition to this, we observe a 4-line splitting pattern for methylene resonance of tCr in the rat TA spectrum for a specific foot angle. This observation is attributed to the individual splitting of creatine and phosphocreatine of the tCr signal. Novel hydrogel application is demonstrated and used to support our in vivo observations and for the first time splitting of individual resonances of Cr and PCr has been shown in an in vitro set-up.     

Probing of susceptibility structures through the distant dipolar field effect

In this paper, the utilization of the distant dipolar field (DDF) signal to extract the properties of susceptibility structures over a subvoxel length scale is investigated. Numerical simulations are performed to study a system of randomly distributed blood vessels with a susceptibility offset inside a voxel. It is shown that the DDF signal of the system as a function of the strength of the correlation gradient field manifests a peak that depends on the volume ratio, size, and susceptibility offset of the blood vessels. In particular, the location of the signal peak is found to vary as powers of these parameters. As a result, by varying the strength of the correlation gradient field, the characteristic properties of the blood vessels can be extracted from the peak position of the DDF signal. It is also found that, for a given volume ratio of the blood vessels, a smaller size of the blood vessels can be probed when the susceptibility offset is increased. Nevertheless, it is demonstrated that, owing to the broad width of the signal peak, the DDF effect generally cannot be used for the preferential selection of the signal arising from the blood vessels on the length scale determined by the correlation length.     

Measurement of homonuclear proton couplings from regular 2D COSY spectra

An interactive computer procedure is described which determines (1)H--(1)H couplings from fitting the cross-peak multiplets in regular phase-sensitive COSY spectra. The robustness and simplicity of the method rely on the fact that a given cross-peak intensity is not an independent variable in the fitting procedure, making it possible to measure couplings accurately even from individual cross peaks with unresolved multiplet structure.