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

Tan C, Cai S, Huang Y - PLoS ONE (2015)

Bottom Line: 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).The 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, China.

ABSTRACT

Background and purpose: Magnetic 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 methods: A 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.

Results: Inhomogeneous 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).

Conclusion: The 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.

No MeSH data available.


Related in: MedlinePlus

Pulse sequence diagram of the localized iDQCJRES.Full vertical bar is the non-selective RF pulse, Gauss-shaped pulse is solvent-selective RF pulse, Sinc-shaped pulses stand for three slice-selective refocusing π RF pulses, trapezoids along three orthogonal directions are slice-selective gradients, vertical-line represent “W5” binomial π pulses. G and -2G are coherence selection gradients, G1 and G2 are crasher gradients for the water suppression. The coherence transfer pathway is presented and the product operators are applied to show the coherence states of solvent I and solute S spins.
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pone.0134109.g001: Pulse sequence diagram of the localized iDQCJRES.Full vertical bar is the non-selective RF pulse, Gauss-shaped pulse is solvent-selective RF pulse, Sinc-shaped pulses stand for three slice-selective refocusing π RF pulses, trapezoids along three orthogonal directions are slice-selective gradients, vertical-line represent “W5” binomial π pulses. G and -2G are coherence selection gradients, G1 and G2 are crasher gradients for the water suppression. The coherence transfer pathway is presented and the product operators are applied to show the coherence states of solvent I and solute S spins.

Mentions: A PRESS like module [25], consisting of three slice-selective refocusing π RF pulses and corresponding slice-selective gradients along orthogonal directions, is integrated into the iDQCJRES sequence for spatial localization (Fig 1). In this localized iDQCJRES sequence, the PRESS like module can not only select the region of interest in the detected sample, but also refocus the resulting iDQC signals. The last slice-selective refocusing π RF pulse in the PRESS like module is inserted into the middle of the delay interval (2Δ) to preserve the desired signals before the distant dipolar interaction takes effect. Therefore, the non-selective π RF pulse used in the non-localized iDQCJRES can be omitted in the localized iDQCJRES sequence. Water suppression is a prerequisite for measurements of biological samples. Different from water suppression modules used in the original PRESS and JPRESS sequences, the double gradient echo W5 module implemented right before acquisition period is used in the localized iDQCJRES sequence to suppress the water signal [26, 27]. In this water suppression module, the crusher gradients are applied along the x, y and z directions. The first π/2 RF pulse is non-selective, and the second RF pulse (π/2)I is selective for the water proton. A pair of linear coherence selection gradients (CSGs) with an area ratio of 1:-2 are employed along the z direction to select the desired coherence transfer pathway 0 → +2 → +1 → −1. Two indirect evolution periods, t1 and t2, are used for the desired signal evolution. Consider a homogeneous solution consisting of I (corresponding to solvent) and S (corresponding to solute) components, where I is an isolated single spin-1/2 system and S is an AX spin-1/2 system that includes Sk and Sl spins coupled by a Jkl scalar interaction. The evolution of two-spin order term for the desired signal from the localized iDQCJRES sequence can be understood intuitively by the product operator analysis as following,IzSz→(π/2)14I+S+(t1/2)→(π/2)I18IzS+(t2/2)→[(π)−(π)−(π)],DISIZSz18S−(t1/2+t2/2+t3),(1)where DISIzSz represents the distant dipolar interaction for iDQC between solvent and solute spins. According to the iMQC treatment [28], high temperature approximation is abandoned and the two spin term IzSz is the start point for the signal evolution. The localized iDQCJRES sequence starts with a non-selective π/2 RF pulse, and the iDQC term I+S+ is selected by the CSGs and evolved during the first evolution periods t1/2. After that, the second (π/2)I RF pulse selective for I spin transforms I+ into (0.5I+ − 0.5I− − Iz) and only the term IzS+ is persevered by the CSGs. Then the PRESS like module localizes the region of interest in the sample and overturns the coherence order from IzS+ to IzS−. Finally, the spin term IzS− evolves into observable signal by the distant dipolar interaction during the evolution period t1/2+t2/2+t3. In this sequence, two indirect evolution periods, t1 and t2, are used and each is divided into two equal parts for the desired signal evolution. For the indirect evolution period t1, I+S+ (iDQC term) evolution is involved in the first t1/2 and S¯ evolution is involved in the second t1/2, thus only the field inhomogeneity and J coupling are preserved in the F1 dimension. For the indirect evolution period t2, IzS+ evolution is involved in the first t2/2 and S¯ evolution is involved in the second t2/2, thus only the J coupling is observed in the F2 dimension. Before the acquisition period t3, the distant dipolar interaction takes effects and transfers IzS¯ into observable S¯ for signal acquisition. The field inhomogeneity effect remains in the F3 dimension. Since the double gradient echo W5 module right before acquisition only acts to suppress the water signal and does not influence the desired coherence transfer pathway and the desired solute signals, we ignore it in the theoretical analysis.


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

Tan C, Cai S, Huang Y - PLoS ONE (2015)

Pulse sequence diagram of the localized iDQCJRES.Full vertical bar is the non-selective RF pulse, Gauss-shaped pulse is solvent-selective RF pulse, Sinc-shaped pulses stand for three slice-selective refocusing π RF pulses, trapezoids along three orthogonal directions are slice-selective gradients, vertical-line represent “W5” binomial π pulses. G and -2G are coherence selection gradients, G1 and G2 are crasher gradients for the water suppression. The coherence transfer pathway is presented and the product operators are applied to show the coherence states of solvent I and solute S spins.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4514627&req=5

pone.0134109.g001: Pulse sequence diagram of the localized iDQCJRES.Full vertical bar is the non-selective RF pulse, Gauss-shaped pulse is solvent-selective RF pulse, Sinc-shaped pulses stand for three slice-selective refocusing π RF pulses, trapezoids along three orthogonal directions are slice-selective gradients, vertical-line represent “W5” binomial π pulses. G and -2G are coherence selection gradients, G1 and G2 are crasher gradients for the water suppression. The coherence transfer pathway is presented and the product operators are applied to show the coherence states of solvent I and solute S spins.
Mentions: A PRESS like module [25], consisting of three slice-selective refocusing π RF pulses and corresponding slice-selective gradients along orthogonal directions, is integrated into the iDQCJRES sequence for spatial localization (Fig 1). In this localized iDQCJRES sequence, the PRESS like module can not only select the region of interest in the detected sample, but also refocus the resulting iDQC signals. The last slice-selective refocusing π RF pulse in the PRESS like module is inserted into the middle of the delay interval (2Δ) to preserve the desired signals before the distant dipolar interaction takes effect. Therefore, the non-selective π RF pulse used in the non-localized iDQCJRES can be omitted in the localized iDQCJRES sequence. Water suppression is a prerequisite for measurements of biological samples. Different from water suppression modules used in the original PRESS and JPRESS sequences, the double gradient echo W5 module implemented right before acquisition period is used in the localized iDQCJRES sequence to suppress the water signal [26, 27]. In this water suppression module, the crusher gradients are applied along the x, y and z directions. The first π/2 RF pulse is non-selective, and the second RF pulse (π/2)I is selective for the water proton. A pair of linear coherence selection gradients (CSGs) with an area ratio of 1:-2 are employed along the z direction to select the desired coherence transfer pathway 0 → +2 → +1 → −1. Two indirect evolution periods, t1 and t2, are used for the desired signal evolution. Consider a homogeneous solution consisting of I (corresponding to solvent) and S (corresponding to solute) components, where I is an isolated single spin-1/2 system and S is an AX spin-1/2 system that includes Sk and Sl spins coupled by a Jkl scalar interaction. The evolution of two-spin order term for the desired signal from the localized iDQCJRES sequence can be understood intuitively by the product operator analysis as following,IzSz→(π/2)14I+S+(t1/2)→(π/2)I18IzS+(t2/2)→[(π)−(π)−(π)],DISIZSz18S−(t1/2+t2/2+t3),(1)where DISIzSz represents the distant dipolar interaction for iDQC between solvent and solute spins. According to the iMQC treatment [28], high temperature approximation is abandoned and the two spin term IzSz is the start point for the signal evolution. The localized iDQCJRES sequence starts with a non-selective π/2 RF pulse, and the iDQC term I+S+ is selected by the CSGs and evolved during the first evolution periods t1/2. After that, the second (π/2)I RF pulse selective for I spin transforms I+ into (0.5I+ − 0.5I− − Iz) and only the term IzS+ is persevered by the CSGs. Then the PRESS like module localizes the region of interest in the sample and overturns the coherence order from IzS+ to IzS−. Finally, the spin term IzS− evolves into observable signal by the distant dipolar interaction during the evolution period t1/2+t2/2+t3. In this sequence, two indirect evolution periods, t1 and t2, are used and each is divided into two equal parts for the desired signal evolution. For the indirect evolution period t1, I+S+ (iDQC term) evolution is involved in the first t1/2 and S¯ evolution is involved in the second t1/2, thus only the field inhomogeneity and J coupling are preserved in the F1 dimension. For the indirect evolution period t2, IzS+ evolution is involved in the first t2/2 and S¯ evolution is involved in the second t2/2, thus only the J coupling is observed in the F2 dimension. Before the acquisition period t3, the distant dipolar interaction takes effects and transfers IzS¯ into observable S¯ for signal acquisition. The field inhomogeneity effect remains in the F3 dimension. Since the double gradient echo W5 module right before acquisition only acts to suppress the water signal and does not influence the desired coherence transfer pathway and the desired solute signals, we ignore it in the theoretical analysis.

Bottom Line: 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).The 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, China.

ABSTRACT

Background and purpose: Magnetic 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 methods: A 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.

Results: Inhomogeneous 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).

Conclusion: The 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.

No MeSH data available.


Related in: MedlinePlus