@article {271, title = {{\textquoteright}Ex situ{\textquoteright} magnetic resonance volume imaging}, journal = {Chemical Physics Letters}, volume = {467}, year = {2009}, note = {Chem Phys Lett386VCTimes Cited:2Cited References Count:25}, month = {Jan 5}, pages = {398-401}, abstract = {
The portable NMR community has introduced advances that have allowed for a variety of studies. Imaging of static and moving objects has almost become standardized. The inherent static field gradients of portable systems have, however, limited such studies to imaging of slices perpendicular to the main gradient; full volume imaging in transportable, open systems has not been actively pursued. We present a true three-dimensional image of a phantom in an ex situ, electromagnet-based system. The basic concepts and designs put forth here extend in a straightforward fashion to higher fields and imaging of larger samples by ex situ methodologies. (C) 2008 Elsevier B.V. All rights reserved.
}, keywords = {array}, isbn = {0009-2614}, doi = {Doi 10.1016/J.Cplett.2008.11.069}, url = {Shimming systems are required to provide sufficient field homogeneity for high resolution nuclear magnetic resonance (NMR). In certain specialized applications, such as rotating-field NMR and mobile ex situ NMR, permanent magnet-based shimming systems can provide considerable advantages. We present a simple two-dimensional shimming method based on harmonic corrector rings which can provide arbitrary multipole order shimming corrections. Results demonstrate, for example, that quadrupolar order shimming improves the linewidth by up to an order of magnitude. An additional order of magnitude reduction is in principle achievable by utilizing this shimming method for z-gradient correction and higher order xy gradients. (c) 2007 American Institute of Physics.
}, keywords = {field}, isbn = {0034-6748}, doi = {Doi 10.1063/1.2713438}, url = {Spectroscopic imaging of a sample placed outside of both the radio frequency and the imaging gradient coils is presented. The sample is placed in a field with a permanent one-dimensional inhomogeneity. The imaging gradients used for phase encoding are designed to produce a static field that depends only on the transverse direction, uncoupling the effects associated with the single-sided nature of these coils. Two-dimensional imaging coupled with chemical shift information is obtained via the ex situ matching technique. Open-saddle geometry is used to match the static field profile for chemical shift information recovery. (C) 2006 Wiley Periodicals, Inc.
}, keywords = {probe}, isbn = {1552-5031}, doi = {Doi 10.1002/Cmr.B.20069}, url = {High-resolution NMR spectra of materials subject to anisotropic broadening are usually obtained by rotating the sample about the magic angle, which is 54.7 degrees to the static magnetic field. In projected magic angle spinning (p-MAS), the sample is spun about two angles, neither of which is the magic angle. This provides a method of obtaining isotropic spectra while spinning at shallow angles. The p-MAS experiment may be used in situations where spinning the sample at the magic angle is not possible due to geometric or other constraints, allowing the choice of spinning angle to be determined by factors such as the shape of the sample, rather than by the spin physics. The application of this technique to bovine tissue samples is demonstrated as a proof of principle for future biological or medical applications.
}, keywords = {field}, isbn = {0740-3194}, doi = {Doi 10.1002/Mrm.20585}, url = {Magic-angle sample spinning is one of the cornerstones in high-resolution NMR of solid and semisolid materials. The technique enhances spectral resolution by averaging away rank 2 anisotropic spin interactions, thereby producing isotropic-like spectra with resolved chemical shifts and scalar couplings. In principle, it should be possible to induce similar effects in a static sample if the direction of the magnetic field is varied (e.g., magic-angle rotation of the B-0 field). Here we will review some recent experimental results that show progress toward this goal. Also, we will explore some alternative approaches that may enable the recovery of spectral resolution in cases where the field is rotating off the magic angle. Such a possibility could help mitigate the technical problems that render difficult the practical implementation of this method at moderately strong magnetic fields. (c) 2005 Elsevier Inc. All rights reserved.
}, keywords = {samples}, isbn = {0730-725X}, doi = {Doi 10.1016/J.Mri.2004.11.067}, url = {Resolved NMR spectra from samples in inhomogeneous B-0 and B-1 fields can be obtained with the so-called \"ex situ\" methodology, employing a train of composite or adiabatic z-rotation RF pulses to periodically refocus the inhomogeneous broadening during the detection of the time-domain signal. Earlier schemes relied on a linear correlation between the inhomogeneous B0 and B, fields. Here the pulse length, bandwidth, and amplitude of the adiabatic pulses of the hyperbolic secant type are adjusted to improve the refocusing for a setup with non-linear correlation. The field correlation is measured using a two-dimensional nutation experiment augmented with a third dimension with varying RF carrier frequency accounting for off-resonance effects. The pulse optimization is performed with a computer algorithm using the experimentally determined field correlation and a standard adiabatic z-rotation pulse as a starting point for the iterative optimization procedure. The shape of the z-rotation RF pulse is manipulated to provide refocusing for the conditions given by the sample-, magnet-, and RF-coil geometry. (c) 2005 Elsevier Inc. All rights reserved.
}, keywords = {spectra}, isbn = {1090-7807}, doi = {Doi 10.1016/J.Jmr.2005.03.006}, url = {Magnetic resonance imaging (MRI) encounters fundamental limits in circumstances in which the static magnetic field is not sufficiently strong to truncate unwanted, so-called concomitant components of the gradient field. This limitation affects the attainable optimal image fidelity and resolution most prominently in low-field imaging. in this article, we introduce the use of pulsed magnetic-field averaging toward relaxing these constraints. It is found that the image of an object can be retrieved by pulsed low fields in the presence of the full spatial variation of the imaging encoding gradient field even in the absence of the typical uniform high-field time-independent contribution. In addition, error-compensation schemes can be introduced through the application of symmetrized pulse sequences. Such schemes substantially mitigate artifacts related to evolution in strong magnetic-field gradients, magnetic fields that vary in direction and orientation, and imperfections of the applied field pulses.
}, keywords = {selection}, isbn = {0027-8424}, doi = {Doi 10.1073/Pnas.0409115102}, url = {Nuclear Magnetic Resonance has revolutionized modern science by its precision, selectivity and non-envasiveness. From complicated biomolecules to materials, from living organisms to nanometric particles, Magnetic Resonance Imaging and Spectroscopy have provided a wealth of invaluable information. Those studies take place in the laboratory, since they require strong and extremely homogeneous superconducting magnets and this represents a major limitation for the technique. Furthermore, the size of the object or subject to study is limited since it has to fit inside the bore of the magnet. Efforts to alleviate those problems lead to the recent development of portable magnetic resonance systems. Their use remained, however, mainly qualitative, since spectroscopic information could not be recovered. We have introduced recently an approach to regain this lost spectral information even in the presence of inhomogeneous magnetic fields. Our approach is based on the matching between the effect of the radio-frequency field and the effect of the static magnetic field. Several practical implementations will be reviewed and put in perspective for their applicability and efficiency in ex-situ NMR. (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.
}, keywords = {in-vivo}, isbn = {1631-0705}, doi = {Doi 10.1016/J.Crhy.2004.03.016}, url = {Mechanical rotation of a sample at 54.7degrees with respect to the static magnetic field, so-called magic-angle spinning (MAS), is currently a routine procedure in nuclear magnetic resonance (NMR). The technique enhances the spectral resolution by averaging away anisotropic spin interactions thereby producing isotropic-like spectra with resolved chemical shifts and scalar Couplings. It should be possible to induce similar effects in a static sample if the direction of the magnetic field is varied, e.g., magic-angle rotation of the B-0 field (B-0-MAS). Here, this principle is experimentally demonstrated in a static sample of solid hyperpolarized xenon at similar to3.4mT. By extension to moderately high fields, it is possible to foresee interesting applications in situations where physical manipulation of the sample is inconvenient or impossible. Such situations are expected to arise in many cases from materials to biomedicine and are particularly relevant to the novel approach of ex situ NMR spectroscopy and imaging. (C) 2004 Elsevier Inc. All rights reserved.
}, keywords = {hz}, isbn = {1090-7807}, doi = {Doi 10.1016/J.Jmr.2004.03.023}, url = {A way to use adiabatic radiofrequency pulses and modulated magnetic-field gradient pulses, together constituting a \"shim pulse,\" for NMR spectroscopy and imaging is demonstrated. These pulses capitalize on phase shifts derived from probe gradient coils to compensate for nonlinear intrinsic main magnetic field homogeneity for spectroscopy, as well as for deviations from linear gradients for imaging. This approach opens up the possibility of exploiting cheaper, less-than-perfect magnets and gradient coils for NMR applications.
}, keywords = {acquisition}, isbn = {0027-8424}, doi = {Doi 10.1073/Pnas.0408296102}, url = {A pulse sequence consisting of an excitation pulse and two adiabatic full-passage pulses with scaled relative peak amplitudes is combined with phase encoding to recover chemical shift information within 3D images in a 1D inhomogeneous static magnetic field with a matched rf field gradient. The results are discussed in the context of ex situ magnetic resonance and imaging. The future directions of our research in implementing the ex situ technique in a real one-sided system are also discussed.
}, keywords = {pulses}, isbn = {0027-8424}, doi = {Doi 10.1073/Pnas.0403016101}, url = {NMR spectra of C-13-iodomethane oriented in three different liquid crystalline solvents have been collected and analyzed under spinning at various angles with respect to the static magnetic field. For each sample the ratio of homonuclear (H-1-H-1) to heteronuclear (C-13-H-1) dipolar couplings, which is a function of the geometry of the solute molecule, does not change significantly with the scaling of the dipolar couplings due to spinning at different angles. This result implies that the \&$\#$39;apparent bond angle deviations\&$\#$39; (Deltatheta(a)), previously calculated from thermotropic liquid crystals, arise from a solvent effect and are not an artifact from scaling the anisotropic interactions. (C) 2004 Elsevier B.V. All rights reserved.
}, keywords = {phases}, isbn = {0009-2614}, doi = {Doi 10.1016/J.Cplett.2004.10.009}, url = {The use of inhomogeneous but spatially correlated static and radiofrequency (RF) magnetic fields offers a potential methodology for performing magnetic resonance spectroscopy of samples placed outside the bore of the magnet. However, its practical implementation still presents challenging problems, among them the control of nuclear spins over broad frequency offset intervals. The present study introduces an efficient method of encoding the phase of the magnetization when the variation of the static field along the sample is much larger than the RF amplitude. The procedure is based on the use of consecutively applied full-passage adiabatic pulses. The induced phase modulation is broadband and selective because it does not depend on the offset relative to the central frequency and the limits can be sharply defined. Finally, the encoded phase depends almost linearly on the local RF amplitude. All these features enable the recovery of an inhomogeneity-free spectrum with amplitudes close to the theoretically attainable maximum. Published by Elsevier Science (USA).
}, keywords = {fields}, isbn = {1090-7807}, doi = {Doi 10.1016/S1090-7807(03)00157-5}, url = {We show how high-resolution NMR spectra can be obtained for solids for which the spectra are normally broadened due to structural disorder. The method relies on correlations in the chemical shifts between pairs of coupled spins. It is found experimentally that there are strong correlations in the chemical shifts between neighboring spins in both phosphorus-31 and carbon-13 spectra. These correlations can be exploited not only to provide resolution in two-dimensional spectra, but also to yield \"chains\" of correlated chemical shifts, constituting a valuable new source of structural information for disordered materials.
}, keywords = {SPECTROSCOPY}, isbn = {0002-7863}, doi = {Doi 10.1021/Ja0292389}, url = {High-resolution NMR of anisotropic samples is typically performed by spinning the sample around an axis at the magic angle of 54.7degrees with the static magnetic field. Geometric and engineering constraints often prevent spinning at this specific angle. Implementations of magic angle field rotation are extremely demanding due to power requirements or an inaccessible geometry. We present a methodology for taking the magic out of MAS while still obtaining both isotropic and anisotropic spectral information during sample spinning or field rotation at arbitrary angles. Using projected-MAS, we obtained resolved scaled isotropic chemical shifts in inhomogeneously broadened spinning samples. Published by Elsevier B.V.
}, keywords = {axis}, isbn = {0009-2614}, doi = {Doi 10.1016/S0009-2614(03)01149-7}, url = {The chemical shift spectrum of a liquid embedded in a porous sample spinning at the magic angle has been recovered in the presence of static field and rf gradients. Field inhomogeneity and susceptibility broadening are averaged by a procedure that combines magic-angle turning with a train of z-rotation pulses. The experiment emulates the situation encountered in \&$\#$39;ex situ\&$\#$39; NMR in which the sample is located away from the field sources. Given the equivalence of field and sample spinning, the results suggest that the use of a rotating magnetic field and refocusing pulses might enable the study of samples such as solids or fluids in porous materials external to the magnet. (C) 2002 Elsevier Science B.V. All rights reserved.
}, keywords = {pulses}, isbn = {0009-2614}, doi = {Doi 10.1016/S0009-2614(02)00642-5}, url = {Investigations were made of rotary resonance recouplings (R-3) of chemical shift anisotropy (CSA), heteronuclear dipolar (HTD), and homonuclear dipolar (HMD) couplings involving half-integer quadrupolar nuclei under magic-angle sample spinning condition. Under rotary resonance conditions provided by a low amplitude rf field and a high spinning speed, the spectrum of the central transition coherence of half-integer quadrupolar nuclei shows recouplings of CSA, HTD, and HMD interactions that depend on the ratio of the rf field to the spinning speed. These new properties can be used to extract electronic and structural information about the sample that are otherwise difficult to extract in the presence of a dominant quadrupolar interaction. An average Hamiltonian theory is used to explain the recoupling properties of various interactions. Experimental implementations of the R-3 are demonstrated on model compounds with spin-3/2 systems. (C) 2002 American Institute of Physics.
}, keywords = {polarization}, isbn = {0021-9606}, doi = {Doi 10.1063/1.1506907}, url = {In this Letter the possibility of selective excitation in coupled multispin systems is studied theoretically. A general method of transforming any selective pulse developed for uncoupled systems into a form that is selective in coupled systems is presented. This is accomplished by adding a small perturbation to a decoupling radiofrequency (RF) field. When viewed in an interaction frame given by the decoupling RF field, this method generates, in an averaged sense, a propagator similar to the propagator of uncoupled spins under a shaped RF pulse. Preliminary experimental results are presented for the case of selective excitation in proton nuclear magnetic resonance in liquid crystals. (C) 2002 Elsevier Science B.V. All rights reserved.
}, keywords = {spin-diffusion}, isbn = {0009-2614}, doi = {Doi 10.1016/S0009-2614(02)00493-1}, url = {In a recent publication we presented a method to obtain highly resolved NMR spectra in the presence of an inhomogeneous B-0 field with the help of a matched RF gradient. If RF gradient pulses are combined with \"ideal\" 90degrees pulses to form inhomogeneous z rotation pulses, the line broadening caused by the B-0 gradient can be refocused, while the full chemical shift information is maintained. This approach is of potential use for NMR spectroscopy in an inhomogeneous magnetic field produced by an \"ex-situ\" surface spectrometer. In this contribution, we extend this method toward two-dimensional spectroscopy with high resolution in one or both dimensions. Line narrowing in the indirect dimension can be achieved by two types of nutation echoes, thus leading to depth-sensitive NMR spectra with full chemical shift information. If the nutation echo in the indirect dimension is combined with a stroboscopic acquisition using inhomogeneous z-rotation pulses, highly resolved two-dimensional correlation spectra can be obtained in matched field gradients. Finally, we demonstrate that an INEPT coherence transfer from proton to carbon spins is possible in inhomogeneous B-0 fields. Thus, it is possible to obtain one-dimensional C-13 NMR spectra with increased sensitivity and two-dimensional HETCOR spectra in the presence of B-0 gradients of 0.4 mT/cm. These schemes may be of some value for ex-situ NMR analysis of materials and biological systems. (C) 2002 Elsevier Science (USA).
}, keywords = {echoes}, isbn = {1090-7807}, doi = {Doi 10.1006/Jmre.2002.2545}, url = {An approach toward high-resolution NMR spectroscopy on samples located outside the physical confines of a magnet (ex situ NMR), has recently been described [C.A. Meriles, D. Sakellariou, H. Heise, A.J. Moule, A. Pines, Science 293 (2001) 82]. Nutation echoes are generated by a train of z-rotation pulses in the presence of spatially matched static and rf field gradients. These pulses were based on a combination of \&$\#$39;perfect\&$\#$39; pi/2 constant rotation composite pulses and \&$\#$39;imperfect\&$\#$39; ordinary pulses of variable length. Here we introduce a new class of \&$\#$39;self-compensated\&$\#$39; z-rotation composite pulses based only on variable rotation inversion pulses that lead to an rf dependent phase shift. Experiments and simulations show that these new pulses perform well at high B-0 gradients and require less rf power than previous schemes. (C) 2002 Elsevier Science B.V. All rights reserved.
}, keywords = {echoes}, isbn = {0009-2614}, doi = {Doi 10.1016/S0009-2614(02)01116-8}, url = {Nuclear magnetic resonance (NMR) experiments are typically performed with samples immersed in a magnet shimmed to high homogeneity. However, there are many circumstances in which it is impractical or undesirable to insert objects or subjects into the bore of a high-field magnet. Here we present a methodology based on an adaptation of nutation echoes that provides resolved spectra in the presence of matched inhomogeneous static and radiofrequency fields, thereby opening the way to high-resolution exsitu NMR, The observation of chemical shifts is regained through the use of multiple-pulse sequences of correlated, composite z-rotation pulses, producing resolved NMR spectra of liquid samples.
}, keywords = {echoes}, isbn = {0036-8075}, doi = {Doi 10.1126/Science.1061498}, url = {