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  • 1
    Language: English
    In: The Journal of chemical physics, 14 April 2012, Vol.136(14), pp.144106
    Description: We present an analytical solution of the Bloch-McConnell equations for the case of chemical exchange saturation transfer between hyperpolarized nuclei in cavities and in solvent (HyperCEST experiment). This allows quantitative investigation of host-guest interactions by means of nuclear magnetic resonance spectroscopy and, due to the strong HyperCEST signal enhancement, even NMR imaging. Hosts of interest can be hydrophobic cavities in macromolecules or artificial cages like cryptophane-A which was proposed as a targeted biosensor. Relevant system parameters as exchange rate and host concentration can be obtained from the monoexponential depolarization process which is shown to be governed by the smallest eigenvalue in modulus. For this dominant eigenvalue we present a useful approximation leading to the depolarization rate for the case of on- and off-resonant irradiation. It is shown that this rate is a generalization of the longitudinal relaxation rate in the rotating frame. We demonstrate for the free and cryptophane-A-encapsulated xenon system, by comparison with numerical simulations, that HyperCEST experiments are precisely described in the valid range of this widely applicable analytical approximation. Altogether, the proposed analytical solution allows optimization and quantitative analysis of HyperCEST experiments but also characterization and optimal design of possible biosensors.
    Keywords: Xenon -- Chemistry
    ISSN: 00219606
    E-ISSN: 1089-7690
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  • 2
    In: NMR in Biomedicine, May 2013, Vol.26(5), pp.507-518
    Description: Chemical exchange observed by NMR saturation transfer (CEST) and spin‐lock (SL) experiments provide an MRI contrast by indirect detection of exchanging protons. The determination of the relative concentrations and exchange rates is commonly achieved by numerical integration of the Bloch–McConnell equations. We derive an analytical solution of the Bloch–McConnell equations that describes the magnetization of coupled spin populations under radiofrequency irradiation. As CEST and off‐resonant SL are equivalent, their steady‐state magnetization and dynamics can be predicted by the same single eigenvalue: the longitudinal relaxation rate in the rotating frame . For the case of slowly exchanging systems, e.g. amide protons, the saturation of the small proton pool is affected by transverse relaxation (R). It turns out, that R is also significant for intermediate exchange, such as amine‐ or hydroxyl‐exchange or paramagnetic CEST agents, if pools are only partially saturated. We propose a solution for that includes of the exchanging pool by extending existing approaches, and verify it by numerical simulations. With the appropriate projection factors, we obtain an analytical solution for CEST and SL for nonzero of the exchanging pool, exchange rates in the range 1–10 Hz, from 0.1 to 20 μT and arbitrary chemical shift differences between the exchanging pools, whilst considering the dilution by direct water saturation across the entire Z‐spectra. This allows the optimization of irradiation parameters and the quantification of pH‐dependent exchange rates and metabolite concentrations. In addition, we propose evaluation methods that correct for concomitant direct saturation effects. It is shown that existing theoretical treatments for CEST are special cases of this approach. Copyright © 2012 John Wiley & Sons, Ltd. We provide a solution of the Bloch–McConnell equations for the case of spin‐lock (SL) and chemical exchange saturation transfer (CEST). Both transient and steady‐state spectra are dominated by . We extend previous approximations for by the transverse relaxation in the rare spin population , which is important for the case of slowly exchanging or partially saturated systems.
    Keywords: Spin‐Lock ; Magnetization Transfer ; Bloch–Mcconnell Equations ; Chemical Exchange Saturation Transfer ; Paracest ; Hypercest
    ISSN: 0952-3480
    E-ISSN: 1099-1492
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  • 3
    Language: English
    In: NeuroImage, 01 October 2018, Vol.179, pp.144-155
    Description: The high chemical shift separation at 9.4 T allows for selective saturation of proton pools in exchange with water protons. For the first time, highly selective and comprehensive chemical exchange saturation transfer (CEST) experiments were performed in the human brain at 9.4 T. This work provides insight into CEST signals in the human brain in comparison with existing animal studies, as well as with CEST effects in vivo at lower field strengths. A novel snapshot-CEST method for human brain scans at 9.4 T was optimized and employed for highly-spectrally-resolved (95 offsets) CEST measurements in healthy subjects and one brain tumor patient. Reproducibility and stability between scans was verified in grey and white matter after B , B , and motion correction of the acquired 3D CEST volumes. Two-step Lorentzian fitting was used to further improve separation of spectrally discernible signals to create known and novel CEST contrast maps at 9.4 T. At a saturation power of B  = 0.5 μT most selective CEST effects could be obtained in the human brain with high inter-scan reproducibility. While contrast behavior of previously measured signals at lower field, namely amide-, guanidyl- and NOE-CEST effects, could be reproduced, novel signals at 2.7 ppm, and −1.6 ppm could be verified in healthy subjects and in a brain tumor patient for the first time. High spectral resolution chemical exchange saturation transfer at 9.4 T allows deeper insights into the Z-spectrum structure of the human brain, and provides many different contrasts showing different correlations in healthy tissue and in tumor-affected areas of the brain, generating hypotheses for future investigations of in-vivo-CEST at UHF.
    Keywords: Chemical Exchange Saturation Transfer ; 9.4 t ; Ultra-High Field ; Human Brain ; Apt ; Noe ; Medicine
    ISSN: 1053-8119
    E-ISSN: 1095-9572
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  • 4
    In: NMR in Biomedicine, December 2013, Vol.26(12), pp.1815-1822
    Description: MR Z‐spectroscopy allows enhanced imaging contrast on the basis of saturation transfer between the proton pools of cellular compounds and water, occurring via chemical exchange (chemical exchange saturation transfer, CEST) or dipole–dipole coupling (nuclear Overhauser effect, NOE). In previous studies, signals observed in the aliphatic proton region of Z‐spectra have been assigned to NOEs between protons in water molecules and protons at the surface of proteins. We investigated a possible relationship between the signal strength of NOE peaks in Z‐spectra obtained at  = 7 T and protein structure. Here, we report a correlation of NOE‐mediated saturation transfer with the structural state of bovine serum albumin (BSA), which was monitored by fluorescence spectroscopy. Encouraged by CEST signal changes observed in tumor tissue, our observation also points to a possible contrast mechanism for MRI sensitive to the structural integrity of proteins in cells. Therefore, protein folding should be considered as an additional property affecting saturation transfer between water and proteins, in combination with the microenvironment and physiological quantities, such as metabolite concentration, temperature and pH. Copyright © 2013 John Wiley & Sons, Ltd. Urea‐dependent unfolding of bovine serum albumin (BSA), which can be monitored by fluorescence spectroscopy, affects saturation transfer between water and aliphatic protons of the protein mediated by dipole–dipole couplings (nuclear Overhauser effect, NOE). We show at 7 T that the NOE imaging contrast of the BSA solution is a function of protein structure and propose that, besides concentration, temperature and pH, protein folding/unfolding generates an additional contrast mechanism of CEST MRI.
    Keywords: Mri ; Chemical Exchange Saturation Transfer Cest ; Nuclear Overhauser Effect Noe ; Protein Folding ; Bovine Serum Albumin Bsa ; Cancer ; Brain Tumors
    ISSN: 0952-3480
    E-ISSN: 1099-1492
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  • 5
    In: NMR in Biomedicine, April 2018, Vol.31(4), pp.n/a-n/a
    Description: Gradient echo (GRE)‐based acquisition provides a robust readout method for chemical exchange saturation transfer (CEST) at ultrahigh field (UHF). To develop a snapshot‐CEST approach, the transient GRE signal and point spread function were investigated in detail, leading to optimized measurement parameters and reordering schemes for fast and robust volumetric CEST imaging. Simulation of the transient GRE signal was used to determine the optimal sequence parameters and the maximum feasible number of ‐space lines. Point spread function analysis provided an insight into the induced ‐space filtering and the performance of different rectangular reordering schemes in terms of blurring, signal‐to‐noise ratio (SNR) and relaxation dependence. Simulation results were confirmed in magnetic resonance imaging (MRI) measurements of healthy subjects. Minimal repetition time (TR) is beneficial for snapshot‐GRE readout. At 9.4 T, for TR = 4 ms and optimal flip angle close to the Ernst angle, a maximum of 562 ‐space lines can be acquired after a single presaturation, providing decent SNR with high image quality. For spiral‐centric reordered ‐space acquisition, the image quality can be further improved using a rectangular spiral reordering scheme adjusted to the field of view. Application of the derived snapshot‐CEST sequence for fast imaging acquisition in the human brain at 9.4 T shows excellent image quality in amide and nuclear Overhauser enhancement (NOE), and enables guanidyl CEST detection. The proposed snapshot‐CEST establishes a fast and robust volumetric CEST approach ready for the imaging of known and novel exchange‐weighted contrasts at UHF. A snapshot gradient echo chemical exchange saturation transfer magnetic resonance imaging (GRE CEST MRI) sequence for fast and robust volumetric imaging at ultrahigh field is proposed. After Bloch equation and point spread function investigation, optimal settings were derived to be: minimal repetition time (TR), flip angle close to the Ernst angle, a maximum of 562 acquired ‐space lines and a rectangular spiral reordering scheme adjusted to the field of view. Application of snapshot‐CEST in the human brain at 9.4 T shows excellent image quality in amide, nuclear Overhauser enhancement (NOE) and guanidyl CEST MRI.
    Keywords: Apt ; Cest ; Chemical Exchange Saturation Transfer ; Human Brain ; Noe ; Snapshot ; Uhf ; 9.4 T
    ISSN: 0952-3480
    E-ISSN: 1099-1492
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  • 6
    Language: English
    In: PLoS ONE, 01 January 2015, Vol.10(3), p.e0121220
    Description: To explore the correlation between Nuclear Overhauser Enhancement (NOE)-mediated signals and tumor cellularity in glioblastoma utilizing the apparent diffusion coefficient (ADC) and cell density from histologic specimens. NOE is one type of chemical exchange saturation transfer (CEST) that originates from mobile macromolecules such as proteins and might be associated with tumor cellularity via altered protein synthesis in proliferating cells.For 15 patients with newly diagnosed glioblastoma, NOE-mediated CEST-contrast was acquired at 7 Tesla (asymmetric magnetization transfer ratio (MTRasym) at 3.3ppm, B1 = 0.7 μT). Contrast enhanced T1 (CE-T1), T2 and diffusion-weighted MRI (DWI) were acquired at 3 Tesla and coregistered. The T2 edema and the CE-T1 tumor were segmented. ADC and MTRasym values within both regions of interest were correlated voxelwise yielding the correlation coefficient rSpearman (rSp). In three patients who underwent stereotactic biopsy, cell density of 12 specimens per patient was correlated with corresponding MTRasym and ADC values of the biopsy site.Eight of 15 patients showed a weak or moderate positive correlation of MTRasym and ADC within the T2 edema (0.16≤rSp≤0.53, p〈0.05). Seven correlations were statistically insignificant (p〉0.05, n = 4) or yielded rSp≈0 (p〈0.05, n = 3). No trend towards a correlation between MTRasym and ADC was found in CE-T1 tumor (-0.31〈rSp〈0.28, p〈0.05, n = 9; p〉0.05, n = 6). The biopsy-analysis within CE-T1 tumor revealed a strong positive correlation between tumor cellularity and MTRasym values in two of the three patients (rSppatient3 = 0.69 and rSppatient15 = 0.87, p〈0.05), while the correlation of ADC and cellularity was heterogeneous (rSppatient3 = 0.545 (p = 0.067), rSppatient4 = -0.021 (p = 0.948), rSppatient15 = -0.755 (p = 0.005)).NOE-imaging is a new contrast promising insight into pathophysiologic processes in glioblastoma regarding cell density and protein content, setting itself apart from DWI. Future studies might be based on the assumption that NOE-mediated CEST visualizes cellularity more accurately than ADC, especially in the CE-T1 tumor region.
    Keywords: Sciences (General)
    E-ISSN: 1932-6203
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  • 7
    Language: English
    In: Inorganic chemistry, 17 July 2017, Vol.56(14), pp.7737-7745
    Description: We report a detailed study of the solution structure and water exchange rate of a Eu(III) complex with the cyclen-based ligand L, containing (S)-2-(2-acetamido)-3-(4-(trifluoromethyl)phenyl)propanoate pendant arms at positions 1 and 7 of the cyclen ring and acetylglycinate pendants at positions 4 and 10. The EuL complex was characterized by a combination of NMR and luminescence spectroscopy and density functional theory (DFT) calculations. The chemical exchange saturation transfer (CEST) spectra obtained at different temperatures and saturation powers present a CEST signal attributed to the coordinated water molecule. However, the spectra recorded at low temperatures (10 °C) and low saturation powers revealed the presence of two different species with coordinated water molecules having very similar chemical shifts. Determination of the water exchange rates of the coordinated water molecules was carried out by using the Bloch four-pool model that accounts for the presence of these isomers, and this model was compared to conventional methods for CEST quantification, namely the Omega plot and QUESP (quantification of exchange rate as a function of saturation power), which assume the presence of a single CEST active species. The results indicated that only the four-pool Bloch equations provide reasonable water exchange rates and activation parameters. Solution NMR studies and DFT calculations indicated that the two isomers present in solution correspond to the SS-Δ(λλλλ) and SS-Λ(δδδδ) isomers, which present capped square-antiprismatic (SAP) coordination environments. Additional NMR studies on the EuL and EuL complexes, which present four (S)-2-(2-acetamido)-3-(4-(trifluoromethyl)phenyl)propanoate or acetylglycinate pendant arms, respectively, confirm the results obtained for EuL.
    Keywords: Water – Chemical Properties ; Chemical Reactions – Usage ; Isomers (Chemistry) – Research;
    ISSN: 00201669
    E-ISSN: 1520-510X
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  • 8
    Language: English
    In: Chemistry - A European Journal, Feb 1, 2019, Vol.25(7), p.1687(4)
    Description: Byline: Liat Avram, Vaclav Havel, Ronit Shusterman-Krush,Mark A. Iron,Moritz Zaiss,Vladimir A indelaA,Amnon Bar-Shir Keywords: binding dynamic; GEST; host-guest; NMR spectroscopy; supramolecular chemistry Abstract The accumulated knowledge regarding molecular architectures is based on established, reliable, and accessible analytical tools that provide robust structural and functional information on assemblies. However, both the dynamicity and low population of noncovalently interacting moieties within studied molecular systems limit the efficiency and accuracy of traditional methods. Herein, the use of a saturation transfer-based NMR approach to study the dynamic binding characteristics of an anion to a series of synthetic receptors derived from bambusuril macrocycles is demonstrated. The exchange rates of BF.sub.4.sup.- are mediated by the side chains on the receptor (100 s.sup.-1〈k.sub.ex〈5000 s.sup.-1), which play a critical role in receptor-anion binding dynamics. The signal amplification obtained with this approach allows for the identification of different types of intermolecular interactions between the receptor and the anion, something that could not have been detected by techniques hitherto used to study molecular assemblies. These findings, which are supported by a computational molecular dynamic study, demonstrate the uniqueness and added value of this NMR method. Supporting information: Additional Supporting Information may be found in the online version of this article As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. CAPTION(S): Supplementary
    Keywords: Molecular Dynamics ; Nuclear Magnetic Resonance Spectroscopy ; Money
    ISSN: 0947-6539
    E-ISSN: 15213765
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  • 9
    Language: English
    In: NMR in biomedicine, March 2014, Vol.27(3), pp.240-52
    Description: Endogenous chemical exchange saturation transfer (CEST) effects are always diluted by competing effects, such as direct water proton saturation (spillover) and semi-solid macromolecular magnetization transfer (MT). This leads to unwanted T2 and MT signal contributions that lessen the CEST signal specificity to the underlying biochemical exchange processes. A spillover correction is of special interest for clinical static field strengths and protons resonating near the water peak. This is the case for all endogenous CEST agents, such as amide proton transfer, -OH-CEST of glycosaminoglycans, glucose or myo-inositol, and amine exchange of creatine or glutamate. All CEST effects also appear to be scaled by the T1 relaxation time of water, as they are mediated by the water pool. This forms the motivation for simple metrics that correct the CEST signal. Based on eigenspace theory, we propose a novel magnetization transfer ratio (MTRRex ), employing the inverse Z-spectrum, which eliminates spillover and semi-solid MT effects. This metric can be simply related to Rex , the exchange-dependent relaxation rate in the rotating frame, and ka , the inherent exchange rate. Furthermore, it can be scaled by the duty cycle, allowing for simple translation to clinical protocols. For verification, the amine proton exchange of creatine in solutions with different agar concentrations was studied experimentally at a clinical field strength of 3 T, where spillover effects are large. We demonstrate that spillover can be properly corrected and that quantitative evaluation of pH and creatine concentration is possible. This proves that MTRRex is a quantitative and biophysically specific CEST-MRI metric. Applied to acute stroke induced in rat brain, the corrected CEST signal shows significantly higher contrast between the stroke area and normal tissue, as well as less B1 dependence, than conventional approaches.
    Keywords: Cest ; Mt ; Creatine ; Ph-Weighted Imaging ; Spillover ; Spin-Lock ; Stroke ; Magnetic Resonance Imaging ; Spectrum Analysis ; Stroke -- Diagnosis
    ISSN: 09523480
    E-ISSN: 1099-1492
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  • 10
    Language: English
    In: The Journal of Chemical Physics, 14 April 2012, Vol.136(14)
    Description: We present an analytical solution of the Bloch–McConnell equations for the case of chemical exchange saturation transfer between hyperpolarized nuclei in cavities and in solvent (HyperCEST experiment). This allows quantitative investigation of host–guest interactions by means of nuclear magnetic resonance spectroscopy and, due to the strong HyperCEST signal enhancement, even NMR imaging. Hosts of interest can be hydrophobic cavities in macromolecules or artificial cages like cryptophane-A which was proposed as a targeted biosensor. Relevant system parameters as exchange rate and host concentration can be obtained from the monoexponential depolarization process which is shown to be governed by the smallest eigenvalue in modulus. For this dominant eigenvalue we present a useful approximation leading to the depolarization rate for the case of on- and off-resonant irradiation. It is shown that this rate is a generalization of the longitudinal relaxation rate in the rotating frame. We demonstrate for the free and cryptophane-A-encapsulated xenon system, by comparison with numerical simulations, that HyperCEST experiments are precisely described in the valid range of this widely applicable analytical approximation. Altogether, the proposed analytical solution allows optimization and quantitative analysis of HyperCEST experiments but also characterization and optimal design of possible biosensors.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
    Source: © 2012 American Institute of Physics (AIP)〈img src=http://exlibris-pub.s3.amazonaws.com/AIP_edited.gif style="vertical-align:middle;margin-left:7px"〉
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