Intermolecular Double-quantum Coherences Research Articles

Various facets of intermolecular transfer of phase coherence by nuclear dipolar fields.

It has long been recognized that dipolar fields can mediate intermolecular transfer of phase coherence from abundant solvent to sparse solute spins. Generally, the dipolar field has been considered while acting during prolonged free-precession delays. Recently, we have shown that transfer can also occur during suitable uninterrupted radio frequency pulse trains that are used for total correlation spectroscopy. Here, we will expand upon the latter work. First, analytical expressions for the evolution of the solvent magnetization under continuous irradiation and the influence of the dipolar field are derived. These expressions facilitate the simulations of the transfer process. Then, a pulse sequence for the retrieval of high-resolution spectra in inhomogeneous magnetic fields is described, along with another sequence to detect a transfer from an intermolecular double-quantum coherence. Finally, various schemes are discussed where the magnetization is modulated by a combination of multiple selective radio frequency pulses and pulsed field gradients in different directions. In these schemes, the magnetization is manipulated in such a way that the dipolar field, which originates from a single-spin species, can be decomposed into two components. Each component originates from a part of the magnetization that is modulated in a different direction. Both can independently, but simultaneously, mediate an intermolecular transfer of phase coherence.

In situ enzymatic hydrolysis characterisation of phospholipid using 1H NMR in a heterogeneous environment

Enzymatic hydrolysis of phospholipids is an important biological event in organisms that is also widely applied in the food industry. In situ nuclear magnetic resonance (NMR) is a powerful non-invasive technique for studying the mechanism of enzymatic reactions. However, their application in phospholipid hydrolysis is limited. In this study, we demonstrated that in a phospholipase A1 (PLA1)-catalysed 1,2-diacyl-sn-glycero-3-phosphocholine (POPC) hydrolysis system in an aqueous emulsion, the accumulation of POPC hydrolysates caused severe heterogeneity, which significantly reduced the 1H NMR spectral intensity and resolution. To overcome the sample heterogeneity, we used the intermolecular dipolar-interaction enhanced all lines-II (IDEAL-II) sequence to obtain one-dimensional (1D) high-resolution 1H intermolecular double quantum coherence (iDQCs) NMR spectra through two-dimensional (2D) acquisition. These spectra enabled us to assign and trace the characteristic peaks of POPC and its hydrolysates in real time. In particular, we performed quantitative estimation of the reaction kinetics based on the g2 and g3 protons and provided direct evidence of the enzymatic hydrolysis process. This study not only fills the gap in 1D 1H spectrum data of phospholipids and their mixtures with hydrolysates, but also provides an effective method for a comprehensive and rapid analysis of fatty acids and even more complex heterogeneous systems.

Cover Image

The cover image is based on the Research Article Evaluation of brown adipose tissue with intermolecular double-quantum coherence magnetic resonance spectroscopy at 3.0 T by Liangjie Lin et al., https://doi.org/10.1002/nbm.4676.

Evaluation of brown adipose tissue with intermolecular double-quantum coherence magnetic resonance spectroscopy at 3.0T.

In the current study, we propose a single-voxel (SV) magnetic resonance spectroscopy (MRS) pulse sequence, based on intermolecular double-quantum coherence (iDQC), for in vivo specific assessment of brown adipose tissue (BAT) at 3T. The multilocular adipocyte, present in BAT, typically contains a large number of small lipid droplets surrounded by abundant intracellular water, while the monolocular adipocyte, present in white adipose tissue (WAT), accommodates only a single large lipid droplet with much less water content. The SV-iDQC sequence probes the spatial correlation between water and fat spins at a distance of about the size of an adipocyte, thus can be used for assessment of BAT, even when mixed with WAT and/or muscle tissues. This sequence for measurement of water-to-fat (water-fat) iDQC signals was tested on phantoms and mouse BAT and WAT tissues. It was then used to differentiate adipose tissues in the supraclavicular and subcutaneous regions of healthy youth human volunteers (n =6). Phantom results with water-fat emulsions demonstrated enhanced water-fat iDQC signal with increased voxel size, increased energy level of emulsification, or increased distribution balance of water and fat spins. The animal tissue experiments resulted in obvious water-fat iDQC signal in mouse BAT, while this signal was almost absent in the WAT spectrum. The optimal choice of the dipolar coupling distance for the observation was approximately 100 μm, as tested on both emulsion phantom and animal tissue. The water-fat iDQC signals observed in the supraclavicular adipose tissues were higher than in the subcutaneous adipose tissues in healthy young volunteers (0.43 ±0.36 vs. 0.10±0.06, p=0.06). It was concluded that the iDQC-based sequence has potential for assessment of mouse and human BAT at 3T, which is of interest for clinical research and the diagnosis of obesity and associated diseases.

Detection of fatty acid composition of trabecular bone marrow by localized iDQC MRS at 3 T: A pilot study in healthy volunteers

BackgroundAlthough a growing body of research shows that the bone marrow adipose tissue (BMAT) may play an essential role in bone inflammation and energy metabolism, available noninvasive methods for distinguishing different fatty acids in BMAT are still limited, in spite of their potential to provide novel biomarkers for bone related diseases. PurposeTo assess the ability of a localized intermolecular double quantum coherence (iDQC) spectroscopy sequence to resolve more fatty acid peaks than conventional MR spectroscopy (MRS), like polyunsaturated fatty acids (PUFA), from the human BMAT in the presence of trabecular bone; To preliminarily investigate whether the fatty acids composition is different between different regions and groups. ResultsCompared with conventional MRS results, additional four fatty acids peaks were well resolved using the proposed method in human BMAT in the presence of trabecular bone. In addition, a different fat composition was found between distal femur and proximal tibia: fat was more unsaturated (vinyl, *p < 0.01; diallylic, *p < 0.01) in distal femur bone marrow than in proximal tibia, and this higher unsaturation level was caused by PUFA (r = 0.67, diallylic, *p < 0.01). No significant difference in fatty acid composition were found either between left and right legs, or between female and male in the healthy young subjects studied. ConclusionThis study demonstrated that the unsaturated fatty acids information of human BMAT in the presence of trabecular bone can be clearly identified with the localized iDQC at 3 T. The resolved peaks, especially PUFA, may serve as additional diagnostic biomarkers for BMAT related diseases in the future.

High-resolution two-dimensional 1H J-resolved MRS measurements on in vivo samples.

Magnetic resonance spectroscopy (MRS) provides a noninvasive tool for metabolite characterization of in vivo biological samples. Conventional MRS measurements on biological samples generally suffer from field inhomogeneity caused by intrinsic magnetic susceptibility variations inside samples. Compared to one-dimensional MRS, two-dimensional (2D) J-resolved spectroscopy enables resolving J couplings along one of the spectral dimension and benefits to metabolite identification and analyses. Intermolecular double-quantum coherences (iDQC) has been proven to be insensitive to magnetic field inhomogeneity, herein we propose a MRS approach based on iDQC evolution and optimal echo sampling scheme to achieve high-resolution 2D J-resolved measurements on biological samples. The applicability of the proposed method is evaluated with experiments on an ex vivo pig brain tissue and an in vivo rat brain tissue. Compared to conventional MRS method which is sensitive to field inhomogeneity inside investigated biological tissues, the proposed method holds immunity to this field inhomogeneity and the quality of resulting spectra may not be influenced by localized voxel size variation. The signal to noise ratio enhancement of the proposed method benefitting from the optimal echo signal sampling is verified with a solution experiment. The new method provides a promising way for high-resolution MRS measurements on biological samples. In combination with fast acquisition strategy, it may find some promising biomedical applications.

Single-Scan High-Resolution 2-D $J$ -Resolved Spectroscopy in Inhomogeneous Magnetic Fields.

A method is proposed to obtain high-resolution 2-D -resolved nuclear magnetic resonance (NMR) spectra in inhomogeneous magnetic fields. The proposed experiment enables the acquisition of an entire 2-D spectrum in a single scan by utilizing intermolecular double-quantum coherences and the spatial encoding of NMR observables. Chemical shifts, coupling constants, and multiplet patterns are recovered even when field inhomogeneities are severe enough to completely obscure conventional NMR spectra. After intentional deshimming to yield inhomogeneous magnetic fields, the method was demonstrated on ethyl 3-bromoproprionate in acetone and on a complex mixture of organic compounds. To illustrate the technique's applicability to biological samples with intrinsic magnetic field inhomogeneities arising from macroscopic magnetic susceptibility variations, we performed the experiment on a pig bone marrow sample. Our results show that the new method is a fast and effective tool for studying complex chemical mixtures and biological tissues. The method could potentially be useful for real-time in vivo NMR studies.

A method for longitudinal relaxation time measurement in inhomogeneous fields

The spin-lattice relaxation time (T1) plays a crucial role in the study of spin dynamics, signal optimization and data quantification. However, the measurement of chemical shift-specific T1 constants is hampered by the magnetic field inhomogeneity due to poorly shimmed external magnetic fields or intrinsic magnetic susceptibility heterogeneity in samples. In this study, we present a new protocol to determine chemical shift-specific T1 constants in inhomogeneous fields. Based on intermolecular double-quantum coherences, the new method can resolve overlapped peaks in inhomogeneous fields. The measurement results are in consistent with the measurements in homogeneous fields using the conventional method. Since spatial encoding technique is involved, the experimental time for the new method is very close to that for the conventional method. With the aid of T1 knowledge, some concealed information can be exploited by T1 weighting experiments.

Fast quantification of fatty acid profile of intact fish by intermolecular double‐quantum coherence 1H‐NMR spectroscopy

Intermolecular double quantum coherence (iDQC) 1H‐nuclear magnetic resonance spectroscopy (1H NMR), which serves as a complementary method in the analyses of fatty acids, is used to investigate the intact salmon muscle and the whole zebra fish. The spectra of fatty acids of the intact salmon muscle, whose resonances are overlapped with metabolites peaks in the conventional NMR spectra, can be resolved in the presence of severe intrinsic structural inhomogeneity without sample pretreatments and special NMR accessories, such as those for high‐speed sample spinning. For improving the practicability of the iDQC method, a localized module is combined with the iDQC method so that the fatty acids of the whole zebra fish can be detected non‐invasively. All the iDQC results are verified by the extraction NMR spectra. In addition, fatty acid composition of the salmon muscle is quantitatively analyzed based on the iDQC and extraction NMR spectra. The calculated results from these two methods are in good agreement. Therefore, the iDQC method may serve as a feasible one‐step and fast screening method for fish quality analyses and lipid inspections of other biological tissues.Practical applications: The intermolecular doublequantum coherence (iDQC) can be an appropriate lipid detecting method of tissues when the samples are not suitable for chemical extraction. The iDQC with a localized module may be more practical than magic angle spinning for applications in in vivo and in situ NMR experiments, in which samples are not allowed to spin.Fatty acids of the whole zebra fish can be identified by iDQC method when PRESS with water suppression fails to produce high‐resolution spectrum.

Two-Dimensional J-Resolved NMR Analyses of Fish and Its Products via Spatially Encoded Intermolecular Double-Quantum Coherences

Nuclear magnetic resonance (NMR) spectroscopy provides a powerful tool for food analyses due to its high-throughput information related to component analyses and its non-invasive property without altering detected samples. Current liquid NMR approaches, however, are subject to the influence of field inhomogeneity in direct measurements of biological foods, generally resorting to techniques of tissue extractions or magic-angle spin to eliminate the inhomogeneity. Therefore, we propose an NMR approach based on intermolecular double-quantum coherences (iDQCs) and spatial encoding technique to fast recover high-resolution two-dimensional (2D) J-resolved spectra in inhomogeneous fields. Inheriting from the immunity to field inhomogeneity of iDQCs and the enhancement in acquisition efficiency of the spatial encoding technique, the proposed method can resist the field inhomogeneity in biological foods without superfluous pretreatments and time-consuming shimming procedures, and yield satisfactory 2D J-resolved information for analyses within minutes. Fish and its products, which are closely related to human daily life, capture extensive attentions with the rapid development of the aquaculture. At first, experiments on cod liver oils are implemented in a purposely deshimmed magnetic field to demonstrate the feasibility of the proposed approach. Then, caviars and an intact fish are tested to further show the applicability of the proposed approach for direct measurements on intact biological food samples. Herein, the proposed method may constitute an effective technique in analyzing fish and its products.

Resolution enhancement in MR spectroscopy of red bone marrow fat via intermolecular double-quantum coherences

High-resolution 1H magnetic resonance spectroscopy (MRS) is generally inaccessible in red bone marrow (RBM) tissues using conventional MRS techniques. This is because signal from these tissues suffers from severe inhomogeneity in the main static B0 field originated from the intrinsic honeycomb structures in trabecular bone. One way to reduce effects of B0 field inhomogeneity is by using the intermolecular double quantum coherence (iDQC) technique, which has been shown in other systems to obtain signals insensitive to B0 field inhomogeneity. In the present study, we employed an iDQC approach to enhance the spectral resolution of RBM. The feasibility and performance of this method for achieving high resolution MRS was verified by experiments on phantoms and pig vertebral bone samples. Unsaturated fatty acid peaks which overlap in the conventional MRS were well resolved and identified in the iDQC spectrum. Quantitative comparison of fractions of three types of fatty acids was performed between iDQC spectra on the in situ RMB and conventional MRS on the extracted fat from the same RBM. Observations of unsaturated fatty acids with iDQC MRS may provide valuable information and may hold potential in diagnosis of diseases such as obesity, diabetes, and leukemia.

Spatially Localized Two-Dimensional J-Resolved NMR Spectroscopy via Intermolecular Double-Quantum Coherences for Biological Samples at 7 T.

Background and PurposeMagnetic resonance spectroscopy (MRS) constitutes a mainstream technique for characterizing biological samples. Benefiting from the separation of chemical shifts and J couplings, spatially localized two-dimensional (2D) J-resolved spectroscopy (JPRESS) shows better identification of complex metabolite resonances than one-dimensional MRS does and facilitates the extraction of J coupling information. However, due to variations of macroscopic magnetic susceptibility in biological samples, conventional JPRESS spectra generally suffer from the influence of field inhomogeneity. In this paper, we investigated the implementation of the localized 2D J-resolved spectroscopy based on intermolecular double-quantum coherences (iDQCs) on a 7 T MRI scanner.Materials and MethodsA γ-aminobutyric acid (GABA) aqueous solution, an intact pig brain tissue, and a whole fish (Harpadon nehereus) were explored by using the localized iDQC J-resolved spectroscopy (iDQCJRES) method, and the results were compared to those obtained by using the conventional 2D JPRESS method.ResultsInhomogeneous line broadening, caused by the variations of macroscopic magnetic susceptibility in the detected biological samples (the intact pig brain tissue and the whole fish), degrades the quality of 2D JPRESS spectra, particularly when a large voxel is selected and some strongly structured components are included (such as the fish spinal cord). By contrast, high-resolution 2D J-resolved information satisfactory for metabolite analyses can be obtained from localized 2D iDQCJRES spectra without voxel size limitation and field shimming. From the contrastive experiments, it is obvious that the spectral information observed in the localized iDQCJRES spectra acquired from large voxels without field shimming procedure (i.e. in inhomogeneous fields) is similar to that provided by the JPRESS spectra acquired from small voxels after field shimming procedure (i.e. in relatively homogeneous fields).ConclusionThe localized iDQCJRES method holds advantage for recovering high-resolution 2D J-resolved information from inhomogeneous fields caused by external non-ideal field condition or internal macroscopic magnetic susceptibility variations in biological samples, and it is free of voxel size limitation and time-consuming field shimming procedure. This method presents a complementary way to the conventional JPRESS method for MRS measurements on MRI systems equipped with broad inner bores, and may provide a promising tool for in vivo MRS applications.

Hadamard-encoded localized high-resolution NMR spectroscopy via intermolecular double-quantum coherences

A scheme based on Hadamard encoding and intermolecular double-quantum coherences is designed to obtain localized one-dimensional high-resolution NMR spectra in inhomogeneous fields. Brief theoretical derivation was performed to illuminate its principle. Experiments were carried out on phantom solution and biological tissues to verify its effectiveness in yielding useful spectral information and efficiency in suppressing solvent signal even when the field inhomogeneity is sufficiently severe to erase almost all spectral information. This sequence may provide a promising way for analyzing heterogeneous biological tissues and chemical systems.

High-Resolution Two-Dimensional J-Resolved NMR Spectroscopy for Biological Systems

NMR spectroscopy is a principal tool in metabolomic studies and can, in theory, yield atom-level information critical for understanding biological systems. Nevertheless, NMR investigations on biological tissues generally have to contend with field inhomogeneities originating from variations in macroscopic magnetic susceptibility; these field inhomogeneities broaden spectral lines and thereby obscure metabolite signals. The congestion in one-dimensional NMR spectra of biological tissues often leads to ambiguities in metabolite identification and quantification. We propose an NMR approach based on intermolecular double-quantum coherences to recover high-resolution two-dimensional (2D) J-resolved spectra from inhomogeneous magnetic fields, such as those created by susceptibility variations in intact biological tissues. The proposed method makes it possible to acquire high-resolution 2D J-resolved spectra on intact biological samples without recourse to time-consuming shimming procedures or the use of specialized hardware, such as magic-angle-spinning probes. Separation of chemical shifts and J couplings along two distinct dimensions is achieved, which reduces spectral crowding and increases metabolite specificity. Moreover, the apparent J coupling constants observed are magnified by a factor of 3, facilitating the accurate measurement of small J couplings, which is useful in metabolic analyses. Dramatically improved spectral resolution is demonstrated in our applications of the technique on pig brain tissues. The resulting spectra contain a wealth of chemical shift and J-coupling information that is invaluable for metabolite analyses. A spatially localized experiment applied on an intact fish (Crossocheilus siamensis) reveals the promise of the proposed method in in vivo metabolite studies. Moreover, the proposed method makes few demands on spectrometer hardware and therefore constitutes a convenient and effective manner for metabonomics study of biological systems.

Chemical exchange saturation transfer MRI using intermolecular double-quantum coherences with multiple refocusing pulses

Chemical exchange saturation transfer (CEST) provides a new type of image contrast in MRI. Due to the intrinsically low CEST effect, new and improved experimental techniques are required to achieve reliable and quantitative CEST images. In the present work, we proposed a novel and more sensitive CEST acquisition approach, based on the intermolecular double-quantum coherence with a module of multiple refocusing pulses (iDQC-MRP). Experiments were performed on creatine and egg white phantoms using a Varian 7T animal MRI scanner. The iDQC-MRP CEST technique showed a substantial enhancement in CEST and nuclear Overhauser enhancement (NOE) signal intensities, compared to the standard single-quantum coherence approach. In addition, the iDQC-MRP approach increased the signal-to-noise ratio of acquired saturation images, compared to the conventional iDQC approach. The new iDQC-MRP CEST sequence provides a promising way for exploiting in vivo CEST and NOE imaging applications.

Fast high-resolution 2D NMR spectroscopy in inhomogeneous fields via Hadamard frequency encoding and spatial encoding

Two new pulse sequences via Hadamard frequency encoding and spatial encoding are proposed to achieve high-resolution intermolecular double-quantum coherence correlation spectroscopy (iDQC-COSY) in inhomogeneous fields within a few scans. Hadamard encoding and spatial encoding are utilized to reduce acquisition time, while iDQC is used to remove the influence of inhomogeneous fields. Two Hadamard encoded schemes, namely, solute Hadamard encoding and solvent Hadamard encoding, are different in the two sequences. Detail analyses of the advantages and disadvantages of the two sequences are carried out. Acquisition times are both shortened by orders of magnitude. Experiments are performed to show their efficiencies.

Brown adipose tissue mapping in rats with combined intermolecular double‐quantum coherence and Dixon water–fat MRI

Brown adipose tissue (BAT) is a promising therapeutic target in obesity studies. Recently, MRI has been proposed for the mapping of BAT. However, because of the limitation of spatial resolution, similar to the existing positron emission tomography and computed tomography techniques for BAT detection, it fails to distinguish BAT cells when they are mixed with other cells. In this work, a new MRI method is proposed, combining intermolecular double-quantum coherence and the chemical shift-encoded Dixon method. Its contrast depends on the water to fat ratio at the cellular scale, which is smaller than the imaging voxel size. The feasibility of this MRI method was shown with computer simulations and phantoms, and preliminary imaging of BAT of rats at 7 T. Both computer simulations and experimental results are consistent with theoretical predictions. The method provides a novel contrast mechanism and can map BAT distribution exclusively. In particular, a mixture of BAT cells and white adipose tissue cells was detected in an older rat, which was undetectable by other noninvasive methods. This method may be applicable to a wide range of uses in BAT-related studies, including the formation and variation of BAT.

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.

Hadamard encoded 2D correlation spectroscopy in inhomogeneous fields

In this Letter, we present a new approach to obtain two-dimensional high-resolution COSY spectra in inhomogeneous fields based on Hadamard encoding. It is an extension of one-dimensional high-resolution intermolecular double-quantum coherence Hadamard encoding approach. The approach is time-efficient and can recover useful structural information even when the inhomogeneous line broadening leads to overlap of neighboring resonances in the conventional COSY spectrum. Theoretical expression of the resulting signals was deduced based on intermolecular multiple-quantum coherence treatment in combination with distant dipolar field treatment. Experiments were performed to verify its feasibility and advantages over previous methods.

Characterization of restricted diffusion in uni- and multi-lamellar vesicles using short distance iMQCs

Improved understanding of the entrapment, transport, and release of drugs and small molecules within vesicles is important for drug delivery. Most methods rely on contrast agents or probe molecules; here, we propose a new MRI method to detect signal from water spins with restricted diffusion. This method is based on intermolecular double quantum coherences (iDQCs), which can probe the restricted diffusion characteristics at well-defined and tunable microscopic distance scales. By using an exceedingly short (and previously inaccessible) distance, the iDQC signal arises only from restricted diffusion spins and thereby provides a mechanism to directly image vesicle entrapment, transport, and release. Using uni- and multi-lamellar liposomes and polymersomes, we show how the composition, lamellar structure, vesicle size, and concentration affects the iDQC signal between coupled water spins at very short separation distances. The iDQC signal correlates well with conventional diffusion MRI and a proposed biexponential (multicompartmental) diffusion model. Finally, the iDQC signal was used to monitor dynamic changes in the lamellar structure as temperature-sensitive liposomes released their contents. These short distance iDQCs can probe the amount and diffusion of water entrapped in vesicles, which may be useful to further understand vesicle properties in materials science and drug delivery applications.