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Jacques BITTOUN, Andre CAPDEROU, Genevieve GUILLOT, Ernst OTTEN, Herve SAINT-JALMES, Anne Ziegler
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Measurements in the Earth's magnetic field offer a reasonable alternative to strong field NMR. A weak B0 provides unique experimental conditions comparable only with NMR in the aimless field of a superconducting magnet where the effects of strong concomitant gradient components can be studied. A non-uniform magnetic field in NMR can be described by a magnetic field gradient only when it is weaker than the main magnetic field, B0. According to Maxwell's equations, the direction of a magnetic field is changing if the magnetic field is not uniform. This means that there is always more than one component of the field different from zero. The magnetic field gradient caused by a susceptibility mismatch is much smaller in a weak field, which results in a longer signal and helps to increase the S/N. Also the longitudinal and transversal relaxations are different and measurements of relaxation give information on molecular dynamics outside the reach of strong field NMR.
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We report new phenomena in low-field ^{1}H nuclear magnetic resonance (NMR) spectroscopy using parahydrogen induced polarization (PHIP), enabling determination of chemical shift differences, δν, and the scalar coupling constant J. NMR experiments performed with thermal polarization in millitesla magnetic fields do not allow the determination of scalar coupling constants for homonuclear coupled spins in the inverse weak coupling regime (δν<J). We show here that low-field PHIP experiments in the inverse weak coupling regime enable the precise determination of δν and J. Furthermore we experimentally prove that observed splittings are related to δν in a nonlinear way. Naturally abundant ^{13}C and ^{29}Si isotopes lead to heteronuclear J-coupled ^{1}H-multiplet lines with amplitudes significantly enhanced compared to the amplitudes for thermally prepolarized spins. PHIP-enhanced NMR in the millitesla regime allows us to measure characteristic NMR parameters in a single scan using samples containing rare spins in natural abundance.
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Magnetic resonance often relies on a semi-classical picture in which the spin particles are submitted to quantum theory and the electromagnetic field is treated as a classical field. Although in many applications there are very good reasons to work within this theoretical framework, it appears worthwhile either for educational purposes, or for studies in magnetic resonance with microscopically small samples or very weak rf fields as well as for other applications that may seem exotic today, to ask how to gain a unified view when comparing the concepts and methods of quantum electrodynamics (QED) with those of classical electrodynamics commonly used in magnetic resonance. The present article attempts to develop such a unified view for electromagnetic interactions in magnetic resonance by focusing on the concept of virtual photon exchange based on the Feynman propagator technique and by exploring the cross links between basic aspects of ‘‘semi-classical magnetic resonance’’ and the same basic aspects of magnetic resonance as seen through the frame of QED.
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High-precision nuclear magnetic resonance spectroscopy generally requires the use of powerful magnets. But using Earth's magnetic field allows us to gain some of the same information on the cheap.
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Details are given of a simple apparatus for the observation of nuclear precession in the Earth's magnetic field. Spin echos are generated using a pulse of oscillatory magnetic field resonant at the Larmor frequency. Students can use the system to measure a nuclear magnetic moment, nuclear relaxation times, and the local strength and orientation of the Earth's magnetic field.
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Transient nutations of the resultant nuclear magnetic moment vector are set up by applying radiofrequency power in the form of pulses in the neighborhood of resonance (omega=gammaH0). The nutations have an initial amplitude depending on the state of magnetization at the start of a pulse and on the proximity to resonance, and are damped by spin-spin and spin-lattice interaction. The thermal relaxation time can be directly found by observing the dependence of initial amplitude on the time between pulses. The spin-spin time constant T2 can be found from the rate of decay even in the presence of normally disturbing inhomogeneity in magnetic field. Sensitivity is in many cases comparable to that obtained in the modulation method with narrow band amplifiers. The fast response due to the relatively wide band widths used can be applied to a rapid search for unknown resonances. The effects observed are in qualitative accord with predictions based on the Bloch theory.
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A demonstration experiment is described, which illustrates fundamental physical processes with NMR and MRI. The terms precession, Larmor frequency, gyromagnetic ratio, 90 degrees pulse, relaxation time, and echo signal are explained with it. The experiment can be set up with the usual laboratory means, at low cost.
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Beware of the ideas hidden behind the words: quantons (quantum things) are neither waves, nor particles. A gastronomical metaphor and a geometrical one are proposed to clarify the relationship between quantum and classical concepts.
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Confusion exists over the application of the Principle of Reciprocity to NMR signal strength calculations when wavelength in a sample is comparable to the latter's size. A simple and easily reproduced bench experiment that validates the principle is therefore described. Building on the experimental results, elementary mathematics are employed to derive simple equations for the B1 fields in both the negatively and the positively rotating frames. Using these equations, the reader is then guided through the steps needed to deduce correctly the signal received in an NMR experiment. The article should serve as a resource for those attempting signal strength calculations. © 2000 John Wiley & Sons, Inc. Concepts Magn Reson 12: 173–187, 2000
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NMR relaxation parameters are usually derived from series of 2D experiments. The whole procedure can be very time consuming, especially for the study of the relaxation of nuclei at natural abundance. Palmer and Mandel have proposed the use of accordion spectroscopy to determine one relaxation parameter using two experiments only. In this paper, we show that the experimental time can be further reduced, by recording only three experiments for the determination of both the longitudinal and transverse relaxation rates. The analysis of the experiments is performed in the frequency domain, the relaxation rates being deduced from the linewidth of the peaks of interest. A detailed statistical analysis of errors introduced by the line fitting procedure on derived relaxation parameters was used to derive guidelines for the choice of experimental parameters. This procedure was applied to the study of the Cα relaxation parameters of a six-residue unlabeled peptide. The results were compared with those obtained by classical accordion spectroscopy. Copyright © 2001 John Wiley & Sons, Ltd.
Article
We have measured the diffusive motion of water molecules in the brine inclusions of Antarctic sea ice using a specially constructed nuclear magnetic resonance (NMR) apparatus. The method relies on the use of pulsed magnetic field gradients in precise analogy to well established laboratory procedures. One version of the apparatus utilised core samples extracted from the ice sheet which were subsequently analysed on site while a later version utilised a probehead which was inserted into the ice sheet, thus minimising any sample perturbation. The diffusive motion of water molecules in the brine inclusions is found to be strongly anisotropic, and, over short length scales, exhibits a rapidity greatly in excess of that expected for thermal equilibrium Brownian behaviour, an effect which we attribute to convective transport.
Article
A fresh approach to the calculation of signal-to-noise ratio, using the Principle of Reciprocity, is formulated. The method is shown, for a solenoidal receiving coil, to give the same results as the traditional method of calculation, but its advantage lies in its ability to predict the ratio for other coil configurations. Particular attention is paid to the poor performance of a saddle-shaped (or Helmholtz) coil. Some of the practical problems involved are also discussed, including the error of matching the probe to the input impedance of the preamplifier.
Article
In this thesis we investigate the ways in which the sensitivity, resolution and overall performance of an Earth's field NMR system can be improved without significantly compromising its simplicity, portability or affordability. We investigate the limits of the information obtainable using this device and present a range of methods for calculating and analyzing NMR spectroscopy experiments detected in the Earth's magnetic field. We demonstrate significant improvements in the performance of a commercial Earth's field NMR device, the Terranova-MRI, through several apparatus developments. First-order shimming is added to the system in order to counter any local inhomogeneity of the Earth's field. The spectral resolution of the instrument is further improved through the introduction of a field locking system to counter the natural temporal drift in the magnitude of the Earth's magnetic field. External noise interference is reduced through the use of Faraday screening, effectively increasing the signal-to-noise ratio (SNR) performance of the device. We explore three signal enhancement methodologies for optimizing the SNR performance of the system. Prepolarization, with an electromagnet as well as a permanent magnet array, is considered and compared to dynamic nuclear polarization (DNP) and hyperpolarization via optical pumping. We present a detailed theoretical discussion of DNP in low-fields and demonstrate the application of this technique for signal enhancement in EFNMR. An apparatus for performing DNP in the Earth's field is presented and optimized. A density matrix approach to simulating one- and two-dimensional Earth's field NMR experiments is presented. These numerical simulations, along with a perturbation theory approach to calculating one-dimensional EFNMR spectra of tightly coupled heteronuclear systems, are explored and compared to experimental spectra of the tetrahydroborate and ammonium ions. These systems are of particular interest for NMR detected in the Earth's field because they contain strongly coupled nuclei of differing spin, a situation previously unexplored in the literature. Multi-dimensional Earth's field NMR spectroscopy methods, in particular the correlation spectroscopy (COSY) experiment, are implemented and optimized through the use of shimming, field stabilization and noise screening. The 2D COSY spectrum of monofluorobenzene is analyzed and compared to calculated spectra in order to determine the indirect spin-spin coupling constants of this molecule in the Earth's magnetic field. A 2D COSY spectrum of 1,4-difluorobenzene is also presented and compared to simulation. The SNR performance of COSY in the Earth's field is greatly improved through the use of DNP for signal enhancement. A high-quality, 2D COSY EFNMR spectrum with DNP acquired from 2,2,2- trifluoroethanol is presented and compared to simulation. The particular features of this spectrum, which result from the use of DNP for signal enhancement, are discussed with reference to a density matrix simulation and to a one-dimensional spectrum calculated using perturbation theory. The strong indirect spin-spin coupling regime in fields weaker than the Earth's magnetic field is explored through exact calculations and density matrix simulations of a 13C-enriched methyl group. A novel multi-dimensional EFNMR method for observing such spectra is discussed. This experiment allows for the resolution of strongly coupled NMR spectra both in the Earth's magnetic field, in the directly detected domain, and in weaker fields, in the indirectly detected domain. In the final section of this thesis, residual dipolar coupling is observed by EFNMR for the first time in a system of poly-[gamma]-benzyl-L-glutamate (PBLG) in dichloromethane. The form of the EFNMR spectrum of this liquid crystalline system is discussed and compared to equivalent high-field (9.4T) spectra.
Article
Ce travail présente une étude expérimentale sur l'effet de la diffusion restreinte de l'hélium-3 hyperpolarisé dans l'acinus pulmonaire effectuée à bas champ magnétique 0,1 T. Plusieurs fantômes avec différentes tailles et connections modélisant l'acinus humain sain et à un stade précoce de l'emphysème ont été réalisés selon le modèle de Kitaoka. L'atténuation du signal dévie par rapport au comportement prévu de décroissance exponentielle en G2, G étant l'intensité de gradient. Cette observation indique une certaine ambiguïté sur la possibilité de quantifier de façon absolue le coefficient de diffusion apparent (ADC), sauf dans la limite G faible. Des simulations Monte-Carlo sont en bon accord avec les mesures. Des séquences originales rapides basées sur le principe des échos de spin multiples ont été développées, pour accéder à une valeur globale d'ADC à des temps longs permettant l'exploration du gaz dans toute la structure de branchement de l'acinus. Des mesures sur un modèle animal d'emphysème (rat) ont été comparées à des cartes obtenues à partir d'acquisitions standard avec petits angles de basculement, elles indiquent une augmentation systématique et toujours significative des ADC par rapport au contrôle sain, pour plusieurs protocoles de mesure. La méthode globale a une meilleure sensibilité que la cartographie standard, en outre elle donne un plus fort contraste d'ADC entre animaux sains et avec emphysème du fait de la possibilité d'employer des valeurs de G plus faibles. Ces outils de mesure de diffusion par IRM et RMN des gaz hyperpolarisés ouvrent des voies prometteuses aussi bien pour la physique de la diffusion que pour les applications médicales.
Article
NMR relaxometry in the Earth's magnetic field is used to characterize porous systems and polymer gels. The used instrument is a home built Earth's field NMR (EFNMR) device equipped with some unique features such as first order gradiometer polarization and detection coils, shimming coils and a shielding box that make it possible to directly derive the relaxation times from the free induction decay signal inside a laboratory building with a signal to noise ratio of about 100. Different kinds of experiments show the dexterity of the EFNMR device in a broad range of applications. By analyzing the signal amplitude, information such as water content, water distribution or drying behavior of different kind of samples is obtained. From the analysis of the relaxation times, information about the environment of the protons is derived. In this way, the pore size distribution of porous materials and the polymerization characteristics of polymer gels containing water is obtained.
Article
Dynamic nuclear polarization (DNP) is introduced as a powerful tool for polarization enhancement in multi-dimensional Earth's field NMR spectroscopy. Maximum polarization enhancements, relative to thermal equilibrium in the Earth's magnetic field, are calculated theoretically and compared to the more traditional prepolarization approach for NMR sensitivity enhancement at ultra-low fields. Signal enhancement factors on the order of 3000 are demonstrated experimentally using DNP with a nitroxide free radical, TEMPO, which contains an unpaired electron which is strongly coupled to a neighboring (14)N nucleus via the hyperfine interaction. A high-quality 2D (19)F-(1)H COSY spectrum acquired in the Earth's magnetic field with DNP enhancement is presented and compared to simulation.
Article
The internal dynamics of a cyclic peptide which was designed to mimic an antigenic loop of the haemagglutinin, is studied through heteronuclear relaxation along the 13C alpha-1H alpha vectors and through homonuclear relaxation along the 1H alpha-1HN and 1H beta-1H beta' vectors. Order parameters are extracted from the longitudinal and cross-relaxation data. Molecular dynamics simulations are performed and the order parameters are calculated in different ways from the trajectories. The simulation, which is performed in vacuo, gives smaller order parameters (vector motions of larger amplitude) than the experimental results. However, the general features of the experimental order parameters are reproduced by the molecular dynamics simulation. The flexibility of the molecule can then be investigated from the results of the molecular dynamics. It shows that the mobility observed through the order parameters is due to motions in flanking regions, remote from the observed vectors.
Article
We present NMR measurements of the diffusion of hyperpolarized 3He in the human lung performed at fields much lower than those of conventional MRI scanners. The measurements were made on standing subjects using homebuilt apparatus operating at 3mT. O(2)-limited transverse relaxation (T(2) up to 15-35s) could be measured in vivo. Accurate global diffusion measurements have been performed in vivo and in a plastic bag; the average apparent diffusion coefficient (ADC) in vivo was 14.2+/-0.6mm(2)/s, whereas the diffusion coefficient in the bag (3He diluted in N(2)) was 79.5+/-1mm(2)/s. 1D ADC mapping with high SNR ( approximately 200-300) demonstrates the real possibility of performing quality lung imaging at extremely low fields.
Article
MRI has been proposed for non-invasive detection and quantification of liver iron content, but has not been validated as a reproducible and sensitive method, especially in patients with mild iron overload. We aimed to assess the accuracy of a simple, rapid, and easy to implement MRI procedure to detect and quantify hepatic iron stores. Of 191 patients recruited, 17 were excluded and 174 studied, 139 in a study group and 35 in a validation group. All patients underwent both percutaneous liver biopsy with biochemical assessment of hepatic iron concentration (B-HIC) and MRI of the liver with various gradient-recalled-echo (GRE) sequences obtained with a 1.5 T magnet. Correlation between liver to muscle (L/M) signal intensity ratio and liver iron concentration was calculated. An algorithm to calculate magnetic resonance hepatic iron concentration (MR-HIC) was developed with data from the study group and then applied to the validation group. A highly T2-weighted GRE sequence was most sensitive, with 89% sensitivity and 80% specificity in the validation group, with an L/M ratio below 0.88. This threshold allowed us to detect all clinically relevant liver iron overload greater than 60 micromol/g (normal value <36 micromol/g). With other sequences, an L/M ratio less than 1 was highly specific (>87%) for raised hepatic iron concentration. With respect to B-HIC range analysed (3-375 micromol/g), mean difference and 95% CI between B-HIC and MR-HIC were quite similar for study and validation groups (0.8 micromol/g [-6.3 to 7.9] and -2.1 micromol/g [-12.9 to 8.9], respectively). MRI is a rapid, non-invasive, and cost effective technique that could limit use of liver biopsy to assess liver iron content. Our MR-HIC algorithm is designed to be used on various magnetic resonance machines.
Article
The human lung and its functions are extremely sensitive to gravity; however, the conventional high-field magnets used for most laser-polarized (3)He MRI of the human lung restrict subjects to lying horizontally. Imaging of human lungs using inhaled laser-polarized (3)He gas is demonstrated in an open-access very-low-magnetic-field (<5 mT) MRI instrument. This prototype device employs a simple, low-cost electromagnet, with an open geometry that allows variation of the orientation of the imaging subject in a two-dimensional plane. As a demonstration, two-dimensional lung images were acquired with 4-mm in-plane resolution from a subject in two orientations: lying supine and sitting in a vertical position with one arm raised. Experience with this prototype device will guide optimization of a second-generation very-low-field imager to enable studies of human pulmonary physiology as a function of subject orientation.
Article
To evaluate the usefulness of a time-efficient MRI method for the quantitative determination of tissue iron in the liver and heart of beta-thalassemic patients using spin-spin relaxation rate, R2, measurements. Images were obtained at 1.5 T from aqueous Gd-DTPA solutions (0.106-8 mM) and from the liver and heart of 46 beta-thalassemic patients and 10 controls. The imaging sequence used was a respiratory-triggered 16-echo Carr-Purcell-Meiboom-Gill (CPMG) spin-echo (SE) pulse sequence (TR = 2000 msec, TE(min) = 5 msec, echo spacing (ES) = 5 msec, matrix = 192 x 256, slice thickness = 10 mm). Liver iron concentration (LIC) measurements were obtained for 22 patients through biopsy specimens excised from the relevant liver segment. Biopsy specimens were also evaluated regarding iron grade and fibrosis. Serum ferritin (SF) measurements were obtained in all patients. A statistically significant difference was found between patients and healthy controls in mean liver (P < 0.004) and myocardium (P < 0.004) R2 values. The R2 values correlated well with Gd DTPA concentration (r = 0.996, P < 0.0001) and LIC (r = 0.874, P < 0.0001). A less significant relationship (r = 0.791, P < 0.0001) was found between LIC measurements and SF levels. R2 measurements appear to be significantly affected (P = 0.04) by different degrees of hepatic fibrosis. The patients' liver R2 values did not correlate with myocardial R2 values (r = 0.038, P < 0.21). Tissue iron deposition in beta-thalassemic patients may be adequately quantified using R2 measurements obtained with a 16-echo MRI sequence with short ES (5 msec), even in patients with a relatively increased iron burden.
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