Materials chemistry of triplet dynamic nuclear polarization.

A nuclear magnetic resonance advance. Imaging fluorinated anesthetics in the brain.

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While dynamic nuclear polarization using photo-excited triplet electrons (triplet-DNP) can improve the sensitivity of nuclear magnetic resonance at room temperature, it has not been carried out in water. Here, we report the first example of triplet-DNP in water by downsizing the conventional bulk crystals to nanocrystals.

Paul Christian Lauterbur

Appropriate drilling fluid additives which can build effective filter cakes are key components to minimize formation damage. Today there exists no effective and quantitative technique which can assess the formation damage minimization potential of any novel drilling fluid. This study provides the methodology for such a technique, by integrating HP/HT fluid loss data, derived from standard API HP/HT filter press, with filter cake quality and filtrate penetration depth, derived by Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) techniques. Novel nano-based drilling fluidwas made of de-ionized water, 7 wt% commercial Na-bentonite, and addition of custom made iron oxide (CM Fe3O4) nanoparticles (NP) at 0.5 wt%. Its excellent filtration characteristics were determined at elevated pressures and temperatures using a HP/HT filter press (500 psi/350°F) with ceramic discs, mimicking the formation, as filter media of k=775 mD. NMR T2 relaxation time measurements provided the pore size distribution of the ceramic discs, and a newly developed MRI technique by our team allowed direct visualization on the core-ceramic disc of the produced filter cake and filtrate penetration depth, providing insights on filtration characteristics. Addition of 0.5 wt% Fe3O4 NP resulted in a decrease of fluid loss by 47% compared to that obtained from the base fluid (BF) without the nanoparticles. The excellent filtration characteristics were attributed to the synergistic interaction of NP with bentonite platelets producing a strong microstructure network leading to rigid and very low permeability filter cakes. Spin-spin relaxation time (T2) NMR measurements revealed different pore size distribution of the nano-enhanced drilling fluid showing its tendency to occupy the smallest pores of the filter medium (ceramic disc). MRI T2 measurements revealed the saturation profiles of the drilling fluids inside the ceramic discs, in 3-D space. This allowed the visual representation of the penetration depth of the filtrate produced in the ceramic discs of the different tested drilling fluids. Our integrated characterization approach, gives us tools to analyze inter-particle interactions in the filter cakes at HP/HT conditions in order to explain the significant improvement in fluid loss performance of novel drilling fluids. The information gained in this study is considered to be extremely significant for deriving an effective methodology for assessing formation damage potential of any newly developed drilling fluid which can lead to more efficient and cost-effective drilling activities.

In the last few years there has been rapid progress in the application of nuclear magnetic resonance clinical imaging as a diagnostic aid (Partain et al, 1983; Steiner, 1983; Worthington, 1983). Records kept by the operators of NMR imaging equipment indicate that about 1800 individuals have now been examined without incident within the United Kingdom. In view of this progress, an advisory group (Appendix) appointed by the National Radiological Protection Board recommends some modifications to the original advice given in 1981 (NRPB, 1981). The principal modifications are that: (1) It is no longer considered necessary to exclude volunteers with a history of epilepsy or cardiac disease. (2) It is not obligatory that a registered medical practitioner trained in resuscitation techniques be present during an exposure. (3) Higher limits are recommended for rates of change of magnetic flux density when the period of change is less than 10 ms. (4) The radiofrequency limit of 15 MHz has been removed. Theoretical considerations indicate that the interaction of an applied magnetic field with the intrinsic magnetic moments of protons and electrons within a molecule could alter its rate of chemical reaction (McLauchlan, 1981). Chemical reactions involving radical pair intermediates are expected to be sensitive to magnetic fields for this reason and a number of enzyme reactions that might be expected to be affected have been identified (A. Cass, personal communication). However, there is no experimental confirmation of these theoretical possibilities at present.

The drive to improve the sensitivity of nuclear magnetic resonance (NMR) to smaller and smaller sample volumes has led to the development of a variety of techniques distinct from conventional inductive detection. In this chapter, we focus on the technique of force-detected NMR as one of the most successful in yielding sensitivity improvements. We review the rationale for the technique, its basic principles, and give a brief history of its most important results. We then cover in greater detail its application in the first demonstration of three-dimensional (3D) nuclear magnetic resonance imaging (MRI) with nanometer-scale resolution. Next we present recent developments and likely paths for improvement. Finally, the technique and its potential are discussed in the context of competing and complementary technologies.

Dynamic nuclear polarization (DNP) has become a very important hyperpolarization method because it can dramatically increase the sensitivity of nuclear magnetic resonance (NMR) of various molecules. Liquid-state DNP based on Overhauser effect is capable of directly enhancing polarization of all kinds of nuclei in the system. The combination of simultaneous Overhauser multi-nuclei enhancements with the multi-nuclei parallel acquisitions provides a variety of important applications in both MR spectroscopy (MRS) and image (MRI). Here we present two simple illustrative examples for simultaneously enhanced multi-nuclear spectra and images to demonstrate the principle and superiority. We have observed very large simultaneous DNP enhancements for different nuclei, such as 1H and 23Na, 1H and 31P, 19F and 31P, especially for the first time to report sodium ion enhancement in liquid. We have also obtained the simultaneous images of 19H and 31P, 19F and 31P at low field by solution-state DNP for the first time.

Dynamic nuclear polarisation (DNP) is a process that transfers electron spin polarisation to nuclei by applying resonant microwave radiation, and has been widely used to improve the sensitivity of nuclear magnetic resonance (NMR). Here we demonstrate new levels of performance for static cross-effect proton DNP using high peak power chirped inversion pulses at 94 GHz to create a strong polarisation gradient across the inhomogeneously broadened line of the mono-radical 4-amino TEMPO. Enhancements of up to 340 are achieved at an average power of a few hundred mW, with fast build-up times (3 s). Experiments are performed using a home-built wideband kW pulsed electron paramagnetic resonance (EPR) spectrometer operating at 94 GHz, integrated with an NMR detection system. Simultaneous DNP and EPR characterisation of other mono-radicals and biradicals, as a function of temperature, leads to additional insights into limiting relaxation mechanisms and give further motivation for the development of wideband pulsed amplifiers for DNP at higher frequencies.

Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) techniques are widely used in food science, mainly because they are non-invasive techniques. MRI, as a non-destructive technique, allows the study of intact samples and without any preparation of the samples before analysis. In food processing, the monitoring of distribution and water content is a consolidated analysis technique, frequently used on the market in order to preserve appropriate nutritional and health characteristics of food according to quality standards. In a food matrix, the variation of the water content is related to the changes in the internal structure and in the physico-chemical properties that occur during the transformation process. In this study MRI technique is used to evaluate the variation of the water content as a function of the drying time. Pumpkin samples are analyzed at four different drying temperatures of 50, 60, 65, and 70°C. The transverse relaxation time, T2, is used to assess the hydration level of the samples by comparing the information extracted from MR images with the drying kinetics measured by gravimetric method. Moreover, T2 maps are used to correlate the change in water distribution with the change in T2 values. The results show that the global weight loss curves obtained with the standard gravimetric method and with the MRI data are in excellent agreement. This work indicates that monitoring changes in the T2 profile of food (i.e., pumpkin) is a useful method for evaluating moisture profiles and changes induced on the sample during the drying process.

Nuclear magnetic resonance imaging in central nervous system disease

Various hyperpolarization methods are able to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) by several orders of magnitude. Among these methods are para-hydrogen-induced polarization (PHIP) and signal amplification by reversible exchange (SABRE), which exploit the strong nuclear alignment of para-hydrogen. Several SABRE experiments have been reported but, so far, it has not been possible to account for the experimentally observed sign and magnetic-field dependence of substrate polarization. Herein, we present an analysis based on level anti-crossings (LACs), which provides a complete understanding of the SABRE effect. The field-dependence of both net and anti-phase polarization is measured for several ligands, which can be reproduced by the theory. The similar SABRE field-dependence for different ligands is also explained. In general, the LAC concept allows complex spin dynamics to be unraveled, and is crucial for optimizing the performance of novel hyperpolarization methods in NMR and MRI techniques.

Dynamic nuclear polarization (DNP) has significantly enhanced the sensitivity of nuclear magnetic resonance (NMR) experiments. While Overhauser DNP has a well-established nuclear counterpart, the nuclear Overhauser effect (NOE), the cross-effect DNP has traditionally been considered as an exclusively electron-mediated process. Here, we present the first evidence of a nuclear counterpart to the cross effect, termed the nuclear cross effect, which mediates polarization transfer in a three-spin nuclear system. The effect was experimentally verified using single-crystal 13C-15N-labeled glycine, with results consistent with both analytical theory and numerical simulations. The experimental realization of the pulsed nuclear cross-effect in this work provides new insights into its theoretical foundations, addressing challenges posed by hardware limitations and the absence of a suitable model system for pulsed cross-effect DNP. Furthermore, the pulsed cross-effect mechanism has the potential to advance enhanced NMR applications in various fields.

Bertram Schwarzschild described the important contributions made by the 2003 Nobel laureates Paul Lauterbur and Peter Mansfield during the 1970s (Physics Today, December 2003, page 24). They extended magnetic resonance imaging for applications in medicine into two and three dimensions and to faster speeds. Schwarzschild refers to an article by Felix Wehrli (Physics Today, Physics Today 0031-9228 45 6 1992 34 https://doi.org/10.1063/1.881310. June 1992, page 34 ), but Wehrli makes no mention of the earlier use of magnetic field gradients and MRI during the 1950s and 1960s (see Physics Today, January 1993, page 94).Schwarzschild does mention the early use of magnetic field gradients to correlate the nuclear signal (usually proton) with spatial coordinates. He cites work by Erwin Harm and by Edward Purcell and me, 1 1. E. L. Hahn, Phys. Rev. 80, 580 (1950); https://doi.org/10.1103/PhysRev.80.580 H. Y. Carr, E. M. Purcell, Phys Rev. 94, 630 (1954). https://doi.org/10.1103/PhysRev.94.630 but that work is not directly related to internal macroscopic imaging. Hahn used existing gradients to observe diffusion, while Purcell and I intentionally applied additional gradients to study diffusion. For modern internal MRI, another component in addition to the applied gradient is necessary. Unlike for diffusion or flow studies, the object being observed must have spatially inhomogeneous internal structure. To the best of my knowledge, the first time such a field gradient was applied to a nuclear magnetic resonance (NMR) sample having an inhomogeneously distributed internal spin system was during a demonstration I conducted for my PhD thesis. 2 2. H. Y. Carr, “Free Precession Techniques in Nuclear Magnetic Resonance,” PhD thesis, Harvard U., Cambridge, MA (1952); Encyclopedia of Nuclear Magnetic Resonance, vol. 1, Wiley and Sons, Hoboken, NJ (1996), p. 253. My motivation in that demonstration was to help faculty and other graduate students understand the recently discovered and rather complicated NMR fine structure in ethyl alcohol. A special object, built for the demonstration, consisted initially of two and later three small separated groups of protons having numbers of protons in the ratios of 3:2:1. The groups were placed in a straight line parallel to the direction of the magnetic field gradient. Thus that first MRI image or spin map was one-dimensional. Although medical applications for 1D imaging are very few, in physics there are several; the most notable was in the discovery of superfluid helium-3. The 1952 demonstration used the new and faster pulsed NMR technique 3 3. D. D. Stark, W. G. Bradley, Magnetic Resonance Imaging, C. V. Mosby, St. Louis, MO (1988), p. 9. rather than the simpler and slower continuous-wave frequency (or field) sweep technique. 4 4. C. Leon Partain et al. , Nuclear Magnetic Resonance (NMR) Imaging, W. B. Saunders, Philadelphia (1983), chap. 17. The visual observation of the three separated groups of protons provided confidence in the pulsed MRI technique with its spatial image coming from the Fourier transform of the complicated time-dependent free-induction decay. That first MRI demonstration was not reported in the 1954 paper, which concentrated on the basic techniques of the new pulsed NMR used with internally homogeneous samples. Other topics, including gradient echoes, electric shims, and the spinning of the samples to obtain narrower lines for observing very fine structure, were left for later papers.My thesis received significant distribution. In particular, Nuclear Magnetic Resonance Specialties near Pittsburgh, Pennsylvania, where Lauterbur and Raymond Damadian did their initial MRI work, received a copy in 1967. Shortly thereafter, the president of the company requested additional copies for some of the purchasers of the company’s pulsed NMR spectrometers.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. E. L. Hahn, Phys. Rev. 80, 580 (1950); https://doi.org/10.1103/PhysRev.80.580 , Google ScholarCrossref, ISI H. Y. Carr, E. M. Purcell, Phys Rev. 94, 630 (1954). https://doi.org/10.1103/PhysRev.94.630 , , Google ScholarCrossref, ISI2. H. Y. Carr, “Free Precession Techniques in Nuclear Magnetic Resonance,” PhD thesis, Harvard U., Cambridge, MA (1952); Google Scholar Encyclopedia of Nuclear Magnetic Resonance, vol. 1, Wiley and Sons, Hoboken, NJ (1996), p. 253. Google Scholar3. D. D. Stark, W. G. Bradley, Magnetic Resonance Imaging, C. V. Mosby, St. Louis, MO (1988), p. 9. Google Scholar4. C. Leon Partain et al. , Nuclear Magnetic Resonance (NMR) Imaging, W. B. Saunders, Philadelphia (1983), chap. 17. Google Scholar© 2004 American Institute of Physics.

Nuclear magnetic resonance (NMR) has been broadly investigated and applied in many industries (e.g., medical treatment, food safety and materials) as a powerful approach to explore the internal structure and composition of substances. However, the need for more sensitivity has been one of the most severe problems hindering the practical application of NMR. This paper suggests various approaches to deal with this defect, from adjusting factors based on the theoretical principle to the progressive hyperpolarization method. Classified by polarization sources, several representative sub-fields of hyperpolarization are further interpreted and analyzed, including Dynamic Nuclear Polarization (DNP), Optical Pumping (OP) and Parahydrogen Induced Polarization (PHIP). Based on the analysis and prospect of the aforementioned feasible methods, the main factors that restrict the sensitivity of NMR will be identified. Moreover, the prospects and new technologies brought by the methods based on different theoretical foundations have been considered to improve these factors and make NMR apply to a broader range of fields. Overall, these results not only indicate the current research status but also shed light on guiding further exploration of methods of enhancing NMR sensitivity and usability.