Distant dipolar field

Intermolecular multiple-quantum coherences (iMQCs) can be utilized to retrieve high-resolution NMR spectra in inhomogeneous magnetic fields. The application of selective pulses in pulse sequences can greatly simplify 2D iMQC spectra. However, so far high-resolution iMQC methods are mainly used in relatively small field inhomogeneities. In this paper, we took the IDEAL-II sequence as an example to study their applicability in moderate inhomogeneous magnetic fields. The experimental and simulation results show that high-resolution NMR spectra can be obtained in moderate inhomogeneous fields if the excitation range of selective pulse is properly set...

Establishing correlations among distant (>3A) spins remains an outstanding problem for both spectral assignment and elucidation of interhelical contacts in solid-state NMR of oriented membrane proteins. Here we present a pulse sequence which incorporates the previously established mismatched Hartmann-Hahn mixing of dilute spins via the proton bath together with high-resolution local field spectroscopy. In addition to providing structural information, the use of dipolar couplings in the indirect dimension helps eliminate the spectral crowdedness compared to the standard homonuclear correlation techniques...

The description of the nuclear magnetic resonance magnetization dynamics in the presence of long-range dipolar interactions, which is based upon approximate solutions of Bloch-Torrey equations including the effect of a distant dipolar field, has been revisited. New experiments show that approximate analytic solutions have a broader regime of validity as well as dependencies on pulse-sequence parameters that seem to have been overlooked. In order to explain these experimental results, we developed a new method consisting of calculating the magnetization via an iterative formalism where both diffusion and distant dipolar field contributions are treated as integral operators incorporated into the Bloch-Torrey equations...

A mixed-valence Co(II)/Co(III) heptanuclear wheel [Co(II)3Co(III)4(L)6(MeO)6] (LH2 = 1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one) has been synthesized and its crystal structure determined using single-crystal X-ray diffraction. The valence state of each cobalt ion was established by bond valence sum calculations. Studies of the temperature dependence of the magnetic susceptibility and the field dependence of the magnetization evidence ferromagnetic interactions within the compound. In order to understand the magnetic properties of this Co7 wheel, we performed ab initio calculations for each cobalt fragment at the CASSCF/CASPT2 level, including spin-orbit coupling effects within the SO-RASSI approach...

A number of contrast enhancement effects based on the use of intermolecular multiple-quantum coherences, or distant dipolar field effects are known. This phenomenon is characterized by the dependence on the mth power of the initial magnetization (where m is the coherence order used). In this paper, we describe the contrast enhancement based on chemical exchange saturation transfer and NOE, which is achieved by the use of intermolecular double-quantum coherences (iDQC). The method was validated using clinically relevant systems based on glycosaminoglycans and a sample of cartilage tissue, showing that the CEST contrast, as well as, NOE are enhanced by iDQC...

Distant dipolar fields among nuclear spins on macroscopic scales in the gas phase are reported for the first time. Their observation via interatomic multiple quantum coherences requires high nuclear spin polarization corresponding to spin temperatures of a few mK, which is generated in laser-polarized 3He, and proper control of the gas diffusion through a heavier buffer gas. This combination of physics at low and ambient temperatures opens up new ways of studying the relative translational diffusion of atoms and of gas diffusion in structures with a large range of length scales...

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...

Diffusion weighting in MRI is commonly achieved with the pulsed-gradient spin-echo (PGSE) method. When combined with spin-warping image formation, this method often results in ghosts due to the sample's macroscopic motion. It has been shown experimentally (Kennedy and Zhong, MRM 2004;52:1-6) that these motion artifacts can be effectively eliminated by the distant dipolar field (DDF) method, which relies on the refocusing of spatially modulated transverse magnetization by the DDF within the sample itself. In this report, diffusion-weighted images (DWIs) using both DDF and PGSE methods in the presence of macroscopic sample motion were simulated...

The presence of highly concentrated dissolved laser-polarized xenon (approximately 1mol/L, polarization up to 0.2) induces numerous effects on proton and xenon NMR spectra. We show that the proton signal enhancements due to (129)Xe-(1)H cross-relaxation (SPINOE) and overall shifts of the proton resonances due to the average dipolar shift created by the intense xenon magnetization are correlated. Protons behave as very useful sensors of the xenon magnetization. Indeed the xenon resonances exhibit many features such as superimposition of narrow lines on the main resonance due to clustering effects, or such as a polarization-dependent line broadening that is tentatively assigned to the effects of temperature fluctuations that decorrelate some distant dipolar field effects from local interactions, transforming xenon spins from "like" to "unlike" spins...

It has been observed recently that the finite duration of refocusing rf pulses in a multiecho acquisition of the signal formed under the influence of the dipolar field leads to significant signal attenuation [S. Kennedy, Z. Chen, C.K. Wong, E.W.-C. Kwok, J. Zhong, Investigation of multiple-echo spin-echo signal acquisition under distant dipole-dipole interactions, Proc. Int. Soc. Magn. Reson. Med. 13 (2005) 2288]. Hereto, we quantify the phenomenon by evaluating analytically the influences of both the distant dipolar field (DDF) and transverse relaxation T2 on the magnetization in a multiecho pulse sequence based on correlation spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED)...

OBJECTIVE: Nonlinear feedback interactions induced by the spins themselves have recently been introduced as novel MRI contrast enhancement mechanisms sensitive to small differences in MR parameters. Developing feedback-based contrast enhancement into a useful tool for in vivo imaging requires improved techniques that are robust to inhomogeneity and sensitive to subtle anatomical/physiological variations. MATERIALS AND METHODS: Three different imaging methods combining the radiation damping feedback field with the distant dipolar field, applied radio-frequency (RF) fields, and local dipole fields, respectively, were designed and tested through numerical simulations on simple phantoms...

In the CRAZED experiment (COSY revamped by asymmetric Z-gradient echo detection, Warren et al.), a spatially anisotropic magnetization distribution is created by application of a magnetic field gradient (strength G, duration tau) which in turn generates a response called the distant dipolar field (DDF). The DDF is a source of intermolecular multiple-quantum coherences (iMQC) which contain information on the distance d=pi/(gammaGtau) between pairs of dipolar-coupled spins. Diffraction-like phenomena may result for periodically structured samples...

A conceptually new approach giving rise to contrast enhancement by feedback fields in magnetic resonance imaging is proposed, and the detailed mechanism is described. Nonlinear spin dynamics under the feedback fields of the distant dipolar field and/or radiation damping are examined and shown to amplify contrast due to small variations in spin density and precession frequency. Feedback-based contrast enhancement depends on the instability of the initial magnetization configuration and is propagated by positive feedback, as shown through numerical simulations and experimental results on simple phantom samples...

Magnetic resonance spectroscopy (MRS) techniques that use the distant dipolar field (DDF) to locally refocus inhomogeneous line-broadening promise improved spectral resolution in spatially varying fields. We investigated three possible implementations of localized DDF spectroscopy. Theoretical analysis and phantom experiments at 17.6 T showed that only localization immediately prior to acquisition provides sufficient spatial selectivity and sensitivity for in vivo applications. Spectra from an (8 mm)(3) voxel of the rat brain were acquired in 25 min, and three major metabolites were resolved...

The dynamics under the joint action of radiation damping and the distant dipolar field in high-field solution magnetic resonance are investigated. Different dynamical regimes during the evolution are identified and their individual features are discussed. In the steady state, the dynamics can be associated with a strange attractor in phase space on which the motion is chaotic. The possibility of the observed chaotic motion being spatiotemporal is examined.

Nuclear spin relaxation provides useful information related to the dynamics of molecular systems. When relaxation is driven by intermolecular dipolar interactions, the relevant spectral density functions (SDFs) also have significant contributions, in principle, from distant spins all over the dynamic range typically probed by NMR experiments such as NOESY. In this work, we investigate the intermolecular dipolar spin relaxation as driven by the relative diffusion of solvent and solute molecules taking place under a central force field, and we examine the relevant implications for (preferential) solvation studies...

Experiments have been carried at magnetic-field strengths of 9.4, 14.1, and 17.6 T to explore the evolution of intermolecular multiple quantum coherences in the nonlinear regime where the system evolves for times that are much greater than the characteristic time of action of the long-range dipolar field, tau(d). The results show the expected Bessel function form of the recorded signal as a function of time of evolution, with evident zeros and sign changes. As expected, the rate of signal evolution increases at higher-field strengths as a result of the increased equilibrium magnetization...

The presence of long-range dipolar fields in liquids is known to introduce a non-linear term in the Bloch-Torrey equations which is responsible for many interesting effects in nuclear magnetic resonance as well as in magnetic resonance imaging. We show here, for the first time, that the diffusion coefficient D and the spin-spin relaxation time T2 can be obtained simultaneously from the time evolution profile of the long-range dipolar field refocused signal. In a COSY Revamped by Z-asymmetric Echo Detection sequence, the analytical first-order approximation solution of the Bloch-Torrey equations modified to include the effect of the distant dipolar field is used to demonstrate the technique in an experiment using doped water...

Solvent-localized NMR (SOLO) is a new method which allows the separation of NMR spectra of substances dissolved in different solvents. It uses the selective HOMOGENIZED pulse sequence to produce a two-dimensional NMR spectrum resulting from intermolecular zero-quantum coherences in one distinct solvent. The detected signal is locally refocused by the action of the distant dipolar field, which is created by a frequency selective pulse only in regions containing the selected solvent. The prerequisites are that the different solvents have sufficiently different chemical shifts to be excited separately and that compartments with different solvents are spatially separated by more than the typical diffusion distance...

A perturbative approach is employed to solve the Bloch-Torrey equations in the presence of distant-dipole fields in nuclear magnetic resonance. The procedure, although only carried out to first order in the perturbation parameter a=1/k2Dtaud, could, in principle, be generalized to higher orders. Here D is the diffusivity, taud the dipolar demagnetization time, and k is the wave vector of the spatial modulation of magnetization produced by the magnetic field gradient. The results are especially interesting for dilute binary mixtures consisting of molecular species with different diffusivities...