A Three-Component Model for Magnetization Transfer. Solution by Projection-Operator Technique,and Application to Cartilage

A projection-operator technique is applied to a general three-component model for magnetization transfer, extending our previous two-component model [R. S. Adler and H. N. Yeung,J. Magn. Reson. A104,321 (1993), and H. N. Yeung, R. S. Adler, and S. D. Swanson,J. Magn. Reson. A106,37 (1994)]. The PO technique provides an elegant means of deriving a simple, effective rate equation in which there is natural separation of relaxation and source terms and allows incorporation of Redfield–Provotorov theory without any additional assumptions or restrictive conditions. The PO technique is extended to incorporate more general, multicomponent models. The three-component model is used to fit experimental data from samples of human hyaline cartilage and fibrocartilage. The fits of the three-component model are compared to the fits of the two-component model.     

Projection Presaturation. III. Accurate Selective Excitation or Presaturation of the Regions of Tailored Shape in the Presence of Short-T1 Species

A new method is described which further increases the accuracy of localization by projection presaturation (PP), a technique for multidimensional spatial localization of contoured regions of interest (ROIs), whose shape, size, position, and number can be chosen arbitrarily (J. Magn. Reson.90, 313, 1990). By applying a nonselective radiofrequency inversion pulse after the PP saturation cycle and nulling the residual longitudinal magnetization of the region outside the ROI, the outer-volume signal is suppressed to the "noise" level, thus improving the accuracy of the basic PP localization, even when saturating a large exterior volume of short-T₁ species. The ability of the method to perform spatial and spectral localization simultaneously is also described. Alternatively, the single-shot methods for selective saturation (instead of selective excitation) of single or multiple contoured regions at arbitrary positions in an extended volume are presented. In particular, the results of selective excitation and saturation of a region of conformal geometric shape in two and three dimensions in a phantom as well as in vivo (healthy volunteers) are presented to demonstrate the efficacy of the method. Applications of these methods include flow- and motion-artifact suppression in body MRI, accurate conformal localization for in vivo spectroscopy in the presence of chemical shift, outer-volume suppression for resolution or speed enhancement in ultrafast imaging (Magn. Reson. Med.13, 77, 1990; J. Phys. C10, L55, 1977), and spin tagging by selective excitation/saturation of flowing spins for flow studies and angiography.     

Iterative analysis of strong coupling effects in semiselective J spectroscopy

A computer program for the iterative analysis of semiselective J spectra (A. Bax, J. Magn. Reson.52, 330 (1983)) is described, and experimental and calculated f₁ cross sections through semiselective 2D J spectra for the three-spin system of fumaric acid monoethyl ester and the five-spin system of thiophene are compared.     

Reference Deconvolution in the Frequency Domain

Reference deconvolution, i.e., using the lineshape distortions of a reference signal with known ideal shape to deduce a correction function for the whole spectrum, is normally performed in the time domain. As a disadvantage, reference signals of higher multiplicity cannot be employed because of mathematical instabilities. In this work we show that these difficulties can be circumvented by carrying out reference deconvolution in the frequency domain. The computational demands of this approach are higher, but not prohibitive, because the width of the correction function is only a fraction of that of the whole spectrum. An iterative algorithm was implemented that yields the optimum widths of the correction function and of the ideal reference signal. Singular value decomposition was found to produce better results thanLUdecomposition of the design matrix. The feasibility of the deconvolution method and of the algorithm are demonstrated using both synthetic and experimental data.     

Application to Rat Lung of the Extended Rorschach–Hazlewood Model of Spin–Lattice Relaxation

The spin–lattice relaxation timeT₁was measured in excised degassed (airless) rat lungs over the frequency range 6.7 to 80.5 MHz. The observed frequency dependence was fitted successfully to the water–biopolymer cross-relaxation theory proposed by H. E. Rorschach and C. F. Hazlewood (RH) [J. Magn. Reson.70,79 (1986)]. The rotating frame spin–lattice relaxation timeT_1ρwas also measured in rat lung fragments over the frequency range 0.56 to 5.6 kHz, and the observed frequency dependence was explained with an extension of the RH model. The agreement between the theory and the experimental data in both cases is good.     

Comparative Studies of Phase-Cycling Schemes for Multiple π-Pulse Sequences

Recently, a new phase cycling scheme was introduced by this laboratory for use in biological solid-state NMR experiments involving multiple π-pulses with characteristics that suggested it may enhance the sensitivity of these kind of experiments (Y. Li and J. N. S. Evans, 1995,Chem. Phys. Lett.241,79 and Erratum, 1995,ibid.246,527; Y. Li and J. N. S. Evans, 1996,J. Magn. Reson. B111,296). The new sequence followed the supercycled concept proposed a decade ago for heteronuclear decoupling experiments. In this paper, more detailed experiments demonstrate that the claim of enhanced sensitivity was unfounded, and in fact the supercycle proposed differs little from the established XY-8 and XY-16 based supercycles.     

MAG-PGSTE: a new STE-based PGSE NMR sequence for the determination of diffusion in magnetically inhomogeneous samples

A new stimulated echo based pulsed gradient spin-echo sequence, MAG-PGSTE, has been developed for the determination of self-diffusion in magnetically inhomogeneous samples. The sequence was tested on two glass bead samples (i.e., 212-300 and <106 microm glass bead packs). The MAG-PGSTE sequence was compared to the MAGSTE (or MPFG) (P.Z. Sun, J.G. Seland, D. Cory, Background gradient suppression in pulsed gradient stimulated echo measurements, J. Magn. Reson. 161 (2003) 168-173; P.Z. Sun, S.A. Smith, J. Zhou, Analysis of the magic asymmetric gradient stimulated echo sequence with shaped gradients, J. Magn. Reson. 171 (2004) 324-329; P.Z. Sun, Improved diffusion measurement in heterogeneous systems using the magic asymmetric gradient stimulated echo (MAGSTE) technique, J. Magn. Reson. 187 (2007) 177-183; P. Galvosas, F. Stallmach, J. K?rger, Background gradient suppression in stimulated echo NMR diffusion studies using magic pulsed field gradient ratios, J. Magn. Reson. 166 (2004) 164-173, P. Galvosas, PFG NMR-Diffusionsuntersuchungen mit ultra-hohen gepulsten magnetischen Feldgradienten an mikropor?sen Materialien, Ph.D. Thesis, Universit?t Leipzig, 2003, P.Z. Sun, Nuclear Magnetic Resonance Microscopy and Diffusion, Ph.D. Thesis, Massachusetts Institute of Technology, 2003] sequence and Cotts 13-interval [R.M. Cotts, M.J.R. Hoch, T. Sun, J.T. Marker, Pulsed field gradient stimulated echo methods for improved NMR diffusion measurements in heterogeneous systems, J. Magn. Reson. 83 (1989) 252-266] sequence using both glass bead samples. The MAG-PGSTE and MAGSTE (or MPFG) sequences outperformed the Cotts 13-interval sequence in the measurement of diffusion coefficients; more interestingly, for the sample with higher background gradients (i.e., the <106 microm glass bead sample), the MAG-PGSTE sequence provided higher signal-to-noise ratios and thus better diffusion measurements than the MAGSTE and Cotts 13-interval sequences. In addition, the MAG-PGSTE sequence provided good characterization of the surface-to-volume ratio for the glass bead samples.     

Measurement of the principal values of the anisotropic diffusion tensor in an unoriented sample by exploiting the chemical shift anisotropy: (19)F PGSE NMR with homonuclear decoupling

NMR methods (S. V. Dvinskikh et al., J. Magn. Reson. 142, 102-110 (2000) and S. V. Dvinskikh and I. Furó, J. Magn. Reson. 144, 142-149 (2000)) that combine PGSE with dipolar decoupling are extended to polycrystalline solids and unoriented liquid crystals. Decoupling suppresses dipolar dephasing not only during the gradient pulses but also under signal acquisition so that the detected spectral shape is dominated by the chemical shift tensor of the selected nucleus. The decay of the spectral intensity at different positions in the powder spectrum provides the diffusion coefficient in sample regions with their crystal axes oriented differently with respect to the direction of the field gradient. Hence, one can obtain the principal values of the diffusion tensor. The method is demonstrated by (19)F PGSE NMR with homonuclear decoupling in a lyotropic lamellar liquid crystal.     

Conformational Equilibria in Polypeptides. II. Dihedral-Angle Distribution in Antamanide Based on Three-Bond Coupling Information

The use of³Jcoupling information in deriving dihedral-angle restraints for polypeptide-structure determination in the presence of conformational equilibria is illustrated withantamanide,cyclo(–Val¹–Pro²– Pro³– Ala⁴– Phe⁵– Phe⁶– Pro⁷– Pro⁸– Phe⁹– Phe¹⁰–). The experimental basis comprises accurate three-bond coupling constants as obtained from both homonuclear [C. Griesinger, O. W. Sørensen, and R. R. Ernst,J. Magn. Reson.75,474 (1987)] and heteronuclear [J. M. Schmidt,J. Magn. Reson.124,298 (1997)] exclusive correlation spectroscopy (E.COSY). For the backbone and side-chain dihedral angles in the nonproline residues, φ and χ₁, respectively, probability-distribution functions are derived and evaluated on the basis of χ²statistics and significance estimates. Various motional models are considered in the quantitative compilation of molecular-geometry parameters from spin-system parameters. From the³Jcoupling analysis, antamanide is found to possess a very flexible structure which is consistent with the results previously obtained in homonuclear NOE and¹³C–T₁relaxation studies. To fully agree with experiment, rotamer equilibria must be assumed for almost all of the torsions investigated in the peptide.     

The accuracy of whole brain N-acetylaspartate quantification

A non-localizing pulse sequence to quantify the total amount of N-acetylaspartate (NAA) in the whole brain (WBNAA) was introduced recently [Magn. Reson. Med. 40, 684–689 (1998)]. However, it is known that regional magnetic field inhomogeneities, ΔB₀s, arising from susceptibility differences at tissue interfaces, shift and broaden local resonances to outside the integration window, leading to an underestimation of the true amount of NAA in the entire brain. To quantify the upper limit of this loss, the whole-head proton MR spectrum (¹H-MRS) of the water was integrated over the same frequency width as the NAA. The ratio of this area/total-water-line was 75 ± 5% in 5 volunteers. The procedure was repeated with the brain-only water peak, obtained by summing signals only from voxels within that organ from a three-dimensional chemical-shift-imaging (3D CSI) set. It indicated that <10% of the water signal loss occurred in the brain. Therefore, by analogy, WBNAA accounts for >90% of that metabolite.     

Broad-band NMR with a high spectral and spatial resolution

Spatially selective excitation sequence CARVE (completely arbitrary regional volume excitation) excites signal from an arbitrarily shaped profile (I. Sersa, S. Macura: Magn. Reson. Med.37, 920–931, 1997) by an interleaved sequence of precalculated small tip angle radio-frequency pulses and gradient pulses. Here we propose a spatially selective observation method based on the CARVE principles which is insensitive to the relaxation and the off-resonance effects. The method, CARVED (CARVE detection), excites spins uniformly across the sample and across the spectrum but achieves spatial selectivity by weighted coaddition of the signals after the data acquisition. CARVE-D is suitable for spatially selective high-resolution nuclear magnetic resonance spectroscopy in chemically and geometrically complex systems. The method is analyzed theoretically and demonstrated experimentally on model systems.     

Accurate intensities of broad NMR lines from composite pulse experiments

Accurate determination of integral intensities of broad lines is difficult when spin relaxation during the applied pulses cannot be neglected and/or when ringing of the tank circuit interferes with the signal. Here we present an extension of the analytical solution of the generalized Bloch equations (G. A. Morris and P. B. Chilvers, J. Magn. Reson. A 107, 236 (1994)), which is then used to evaluate the signal intensity obtained in a composite pulse experiment designed to cancel ringing effects. Comparing intensities of broad and narrow (81)Br spectral lines tests and proves the accuracy of this approach.     

The carbon dioxide molecule: A new derivation of the potential,spectroscopic, and molecular constants

A new determination of the potential energy function of the carbon dioxide molecule from its vibrorotational spectrum is presented. Starting from the previous determination made by A. Chédin [J. Mol. Spectrosc.76, 430–491 (1979)] and from a significantly larger set of updated experimental data, the new potential is shown to provide a better agreement between theoretical and experimental eigenenergies. A similar improvement in the prediction of eigenstates is expected.     

On Neglecting Chemical Exchange Effects When Correcting in VivoP MRS Data for Partial Saturation

This article replies to Spencer et al. (J. Magn. Reson.149, 251–257, 2001) concerning the degree to which chemical exchange affects partial saturation corrections using saturation factors. Considering the important case of in vivo³¹P NMR, we employ differential analysis to demonstrate a broad range of experimental conditions over which chemical exchange minimally affects saturation factors, and near-optimum signal-to-noise ratio is preserved. The analysis contradicts Spencer et al.'s broad claim that chemical exchange results in a strong dependence of saturation factors upon M₀'s and T₁ and exchange parameters. For Spencer et al.'s example of a dynamic ³¹P NMR experiment in which phosphocreatine varies 20-fold, we show that our strategy of measuring saturation factors at the start and end of the study reduces errors in saturation corrections to 2% for the high-energy phosphates.     

Efficient 5QMAS NMR of spin-5/2 nuclei: use of fast amplitude-modulated radio-frequency pulses and cogwheel phase cycling

We report here an efficient multiple-quantum magic-angle spinning (MQMAS) pulse sequence involving fast amplitude-modulated (FAM) radio-frequency pulses for excitation and conversion of five-quantum (5Q) coherences of spin-5/2 nuclei. The use of a FAM-I type pulse train for the conversion of 5Q into 1Q coherences proves to be easier to implement experimentally than the earlier suggested use of a FAM-II type sequence [J. Magn. Reson. 154 (2002) 280], while delivering at least equal signal enhancement. Results of numerical simulations and experimental 27Al 5QMAS spectra of aluminium acetylacetonate for different excitation and conversion schemes are compared to substantiate these claims. We also demonstrate the feasibility of acquiring 5QMAS spectra of spin-5/2 systems using cogwheel phase cycling [J. Magn. Reson. 155 (2002) 300] to select the desired coherence pathways. A cogwheel phase cycle of only 57 steps is shown to be as effective as the minimum conventional nested 77-step phase cycle.     

Analysis of the ν3 band of 12CH3D at 7.6 μm

The previously reported (J. Mol. Spectrosc.68, 195–222 (1977)) study of the CH₃D spectrum occurring at 1033–1270 cm^?1 which was mainly concerned with the ν₆ fundamental has now been extended to cover the region 1270–1420 cm^?1. In all, 342 transitions belonging to the ν₃ band are now assigned. Both the ν₃ and ν₆ bands are processed simultaneously taking into account of the Coriolis interaction between them, and the fitting of all the experimental data led to 21 significant spectroscopic constants for the states v₆= 1 and v₃ = 1 of CH₃D.     

Assessing signal enhancement in distant dipolar field-based sequences

The possibility of improving the signal-to-noise efficiency of NMR signal refocused by long-range dipolar interactions has been discussed recently [R.T. Branca, G. Galiana, W.S. Warren, Signal enhancement in CRAZED experiments, J. Magn. Reson. 187 (2007) 38-43]. For systems where T(1)>T(2), by including an extra radio-frequency pulse in a standard CRAZED sequence, it is possible to increase the available signal by exploiting its sensitivity to T(1) relaxation. Here, we use analytical calculations to investigate the source of this improved signal and determine the maximum enhancement provided by the method.     

Single crystal neutron inelastic scattering measurement of stiffness constant in disordered f.c.c. Ni-Mn alloys

Inelastic neutron scattering measurement of stiffness constant D on simple crystals of disordered f.c.c. Ni-Mn alloys has been performed at differen Mn concentrations (1.5%, 6%, 12%, 18% at.Mn). Stiffness constant D show similar behaviour vs concentration as observed in Ni-Fe alloys by Hennion et al. [Solid State Commun.17, 899 (1975)]. This behaviour does not agree with that observed by inelastic small-angle magnetic scattering measurements by Menshikov et al. [Int. J. Magn.1, 91 (1975)]. A noticeable enhancement of the spin-wave width is also observed at higher Mn content and higher spin wave energies. The values of pair exchange integrals are also given.     

Nuclear longitudinal relaxation time images by radiofrequency field gradients.

Combination of the Super Fast Inversion Recovery (SUFIR) method (D. Canet, J. Brondeau, and K. Elbayed, J. Magn. Reson. 77, 483 (1988)) and imaging procedures by radiofrequency field gradients (P. Maffei, P. Mutzenhardt, A. Retournard, B. Diter, R. Raulet, J. Brondeau, and D. Canet, J. Magn. Reson. A 107, 40 (1994)) provides spatially resolved maps of longitudinal relaxation times (T1). In addition to accurate T1 values, enhanced spatial resolution is obtained.     

Spectral simulations incorporating gradient coherence selection.

Computer-aided methods can considerably simplify the use of the product operator formalism for theoretical analysis of NMR phenomena, which otherwise becomes unwieldy for anything but simple spin systems and pulse sequences. In this report, two previously available programming approaches using symbolic algebra (J. Shriver, Concepts Magn. Reson. 4, 1-33, 1992) and numerical simulation using object-oriented programming (S. A. Smith, T. O. Levante, B. H. Meier, and R. R. Ernst, J. Magn. Reson. A 106, 75-105, 1994) have been extended to include the use of gradient operators for simulation of spatially localized NMR spectroscopy and gradient coherence selection. These methods are demonstrated using an analysis of the response of an AX(3) spin system to the STEAM pulse sequence and verified with experimental measurements on lactate.