Distant Dipolar Field Research Articles

Visualization of the distant dipolar field: A numerical study

AbstractThe magnetization of liquid water in an external field generates an intrinsic magnetic field in the sample called the distant dipolar field (DDF). To visualize the spatial distribution of the DDF a numerical study was performed for the case of liquid–state 1H NMR at 7 T. 2D maps of the frequency offset caused by the DDF in pure water were calculated for homogenously magnetized spherical and cylindrical samples as well as for the case of a spatially modulated magnetization distribution occurring e.g., in CRAZED (Cosy Revamped by Asymmetric Z‐Gradient Echo Detection) experiments. The calculation yielded DDF induced frequency offsets in the range of 0.58 Hz to 10.24 Hz inside the homogeneously magnetized cylinders, while DDF‐induced frequency offsets ≤10−5 Hz were obtained inside the sphere. The calculated frequency offsets were in good agreement with analytical results available for a sphere and an infinitely long cylinder. In the case of a spatially modulated magnetization distribution, DDF‐induced frequency offsets with maximum values of +0.83 Hz were obtained inside the sphere. The presented 2D maps of the DDF‐induced frequency offset have tutorial character and may help to visualize this phenomenon in a direct manner. © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part A 34A:357–364, 2009.

Apparent diffusion behaviour of intermolecular double-quantum coherence modulated by a distant dipolar field in solution NMR

A modified correlated spectroscopy (COSY) revamped with asymmetric Z-gradient echo detection sequence was designed to investigate the influence of diffusion behaviour on intermolecular double-quantum coherence signal attenuation during the pre-acquisition period. Theoretical formulas were deduced and experimental measurements and simulations were performed. It is found that the diffusion behaviour of intermolecular double-quantum coherence in the pre-acquisition period may be different from that of conventional single-quantum coherence, depending on the relative orientation of diffusion weighting gradients to coherence selection gradients. When the orientation of the diffusion weighting gradients is parallel or anti-parallel to the orientation of the coherence selection gradients, the diffusion is modulated by the distant dipolar field. This study is helpful for understanding the signal properties in intermolecular double-quantum coherence magnetic resonance imaging.

Intermolecular double-quantum coherence NMR spectroscopy in moderate inhomogeneous fields

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. Once the field inhomogeneity reaches a certain degree, the appearance of undesirable intermolecular cross-peaks due to the distant dipolar field produced by solute spins is inevitable. The spectral quality may vary with sample even in the same moderate inhomogeneous fields, depending on the chemical shift distributions and the J coupling networks of the components under study. The conclusions drawn in this paper are generally applicable to all high-resolution iMQC methods utilizing selective RF pulses.

Nuclear magnetic resonance signal dynamics of liquids in the presence of distant dipolar fields, revisited.

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. The solution can be organized as a perturbative series, whereby access to higher order terms allows one to set better boundaries on validity regimes for analytic first-order approximations. Finally, the method legitimizes the use of simple analytic first-order approximations under less demanding experimental conditions, it predicts new pulse-sequence parameter dependencies for the range of validity, and clarifies weak points in previous calculations.

Roles of magnetic gradient field and transverse relaxation in distant dipolar field signal

AbstractDistant dipolar field (DDF) effect as an alternative signal formation mechanism has attracted a lot of attention in the magnetic resonance community in the past two decades. The signal formed under the influence of DDF is usually much smaller than the signal acquired with the conventional method, and is observable only when magnetic field gradients are utilized to modulate the magnetization distribution spatially. This article reviews the formation of the DDF signals and explains in detail the roles of the gradient fields in observing the DDF effect by examining the evolution of magnetization according to the classical Bloch equation. The magnetization evolution is calculated analytically in two common pulse sequences: i) the COrrelation SpectroscopY (COSY) sequence and ii) the COSY Revamped by Asymmetric Z‐gradient Echo Detection (CRAZED) sequence. The detailed comparison of the two sequences elucidates how the implementation of the gradient fields introduces extra dephasing to the signal and why gradient fields are indispensable in observing the DDF effect in biological samples. In addition, the DDF signal is analytically shown to have an intrinsically higher sensitivity to the change of transverse relaxation (T2) than the signal acquired with the conventional method for biological tissues. This manifests that the DDF signal has a higher T2 contrast than the conventional signal under the same signal‐to‐noise ratio and provides a potential new contrast mechanism in magnetic resonance imaging. © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part A 34A: 76–90, 2009.

Spectral and temporal features of multiple spontaneous NMR-maser emissions

When a system composed of dissolved laser-polarized xenon with negative spin temperature is put inside a high field NMR magnet, a series of spontaneous maser emissions can be observed. We report here their spectral and temporal features using a processing model derived from the solution of the Bloch equations in the presence of radiation damping. We show, in particular, that by combining Fourier transformation and squared modulus, a parameter allowing the characterization of the burst of transverse magnetization can be determined. This parameter is shown to be correlated with the radiated energy. Moreover, this processing clearly reveals features which can probably be assigned to effects resulting from distant dipolar fields. Finally, the analysis of the experimental data reveals an unexpected behavior of the 129 Xe transverse self-relaxation.

Intermolecular multiple-quantum coherence NMR signals modulated by double distant dipolar fields

A modified CRAZED pulse sequence was applied to obtain the intermolecular multiple-quantum coherence NMR signals from double distant dipolar fields in highly polarized spin systems. Complete theoretical analyses were explicitly derived from the dipolar field treatment combined with product operator formalism. Two typical samples containing several different components were chosen for the experimental verifications. The computer simulations and experimental observations are consistent with the theoretical predictions. The results presented herein provide a convenient way to understand the combined effects of multiple distant dipolar fields from the different components in complicated chemical or biological solutions. When experimental conditions such as selective radio-frequency pulses are not optimal, it may be helpful to identify possible unexpected signals or artefacts of high-resolution NMR spectroscopy in inhomogeneous fields.

Chemical exchange saturation transfer by intermolecular double-quantum coherence

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 in Laser-Polarized Gases on Macroscopic Scales

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.

Localized intermolecular zero‐quantum coherence spectroscopy in vivo

AbstractResolution enhancement in NMR spectra, acquired in spatially or temporally varying magnetic fields, can be achieved with 2D pulse sequences detecting intermolecular zero‐quantum coherences (iZQC). The insensitivity towards long range field distortions renders these methods particularly appealing for in vivo NMR spectroscopy, where ample sources of field inhomogeneities are encountered. This article provides a comprehensive description of iZQC spectroscopy, following a classical treatment. A pictorial explanation is given of how iZQC signal is formed under the action of the distant dipolar field in the sample and how this local refocusing process leads to line narrowing in the indirect dimension of 2D spectra. Signal evolution and peak positions in the spectra are analyzed by solving the modified Bloch equations. Finally, it is explained how water suppression and localization can be combined with the iZQC preparation sequence, and recent in vivo applications are discussed. The given examples illustrate that iZQC spectroscopy can provide either resolution or sensitivity enhancement in vivo. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A: 117–133, 2008.

NMR Time Reversal Experiments in Highly Polarised Liquid 3He-4He Mixtures

Long-range magnetic interactions in highly magnetised liquids (laser-polarised 3He-4He dilute mixtures at 1 K in our experiment) introduce a significant non-linear and non-local contribution to the evolution of nuclear magnetisation that leads to instabilities during free precession. We recently demonstrated that a multi-echo NMR sequence, based on the magic sandwich pulse scheme developed for solid-state NMR, can be used to stabilise the magnetisation against the effect of distant dipolar fields. Here, we report investigations of echo attenuation in an applied field gradient that show the potential of this NMR sequence for spin diffusion measurements at high magnetisation densities.

Visualizing feedback‐enhanced contrast in magnetic resonance imaging

AbstractA new approach to magnetic resonance imaging (MRI) contrast enhancement has recently been developed that exploits nonlinear feedback interactions to amplify contrast arising from small variations in the underlying MRI parameters. A unified framework for understanding feedback‐enhanced contrast is presented here based on the concepts of instability and positive feedback. The specific mechanisms governing contrast enhancement under the feedback interactions of radiation damping, the distant dipolar field, and their joint effect are elucidated through numerical simulations illustrating the involved spin dynamics. Experimental demonstrations of feedback‐enhanced contrast are shown on samples of in vitro human brain tissue, and applications to improving lesion detection in disease states such as epilepsy and cancer are discussed. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 378–393, 2007.

Numerical simulations of motion-insensitive diffusion imaging based on the distant dipolar field effects

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. Numerical simulation results quantify the dependence of signals in DWI on several key motion parameters and demonstrate that the DDF DWIs are much less sensitive to macroscopic sample motion than the traditional PGSE DWIs. The results also show that the dipolar correlation distance (d(c)) can alter contrast in DDF DWIs. The simulated results are in good agreement with the experimental results reported previously.

1H and 129Xe NMR absorption line shapes in the presence of highly polarized and concentrated xenon solutions in high magnetic field

The presence of highly concentrated dissolved laser-polarized xenon (∼1 mol/L, polarization up to 0.2) induces numerous effects on proton and xenon NMR spectra. We show that the proton signal enhancements due to 129Xe– 1H 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. These spectral features make difficult the determination of the average dipolar field by means of the xenon resonance but have interesting consequences on the heteronuclear polarization transfer experiment in Hartmann–Hahn conditions (SPIDER).

Chaos suppression by feedback control in nuclear magnetic resonance

The strength of static magnetic field and the sensitivity of probe increase with the technique development of modern nuclear magnetic resonance (NMR) spectrometer. Radiation damping effect thus can no longer be neglected in solution NMR experiments with abundant spins. The cooperative effects of radiation damping and distant dipolar field (DDF) generate spin turbulence, which makes the evolution of magnetization unpredictable by traditional NMR theory and causes the uncertainty of acquired signals. The chaos phenomena can be described by introducing a radiation damping term into the Bloch equations. In this paper, we used two kinds of feedback control method to suppress the chaotic process. Numerical simulation indicates their feasibility and efficiency.

Theoretical studies of the effect of the dipolar field in multiple spin-echo sequences with refocusing pulses of finite duration

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 T 2 on the magnetization in a multiecho pulse sequence based on correlation spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED). Analytic expressions for the magnetization were obtained, which demonstrate explicitly the origin of rephased signal in the presence of the finite π pulses in the multiecho train. The expressions also explain the effects of the DDF and T 2 during the refocusing pulses on the signal strength, and show the substantial signal dependence on the phase of the rf pulses. We show that when the DDF effect during the pulse is canceled, the signal rises primarily during the free evolution time in the acquisition period. This elucidates the signal attenuation when the rf pulses cover a significant proportion of time in the sequence. In addition, we performed an optimization on the number of refocusing pulses that maximizes the total acquired signal using parameters for water, brain white matter, and muscle. We found that maximal signal-to-noise ratio is obtained when the pulse duration approximately equals the free evolution time in the samples with a wide range of T 2.

Designing feedback-based contrast enhancement for in vivo imaging

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. 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. These methods were demonstrated experimentally on live guppy fish, developing frog embryos, and blood in in vitro tissue samples by microimaging at 14.1 T. The developed feedback-based methods yielded images that identified distinct morphological features with superior contrast compared with conventional MR images and those acquired under radiation damping only. Positive contrast due to evolution under radiation damping and local dipole fields was also observed in SPIOs and blood. Approaches to enhancing feedback-based contrast were successfully designed and demonstrated in vitro and in vivo. The newly devised methods were less sensitive to field inhomogeneity and prolonged evolution under the feedback fields, allowing for better visualization of contrast in vivo.

Diffraction-like phenomena in a periodic magnetization distribution at 1.5 T using the distant dipolar field (DDF)

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 τ) 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 = π/( γGτ) between pairs of dipolar-coupled spins. Diffraction-like phenomena may result for periodically structured samples. In this study, we report the observation of diffraction owing to the DDF at 1.5 T using a clinical whole-body tomograph. Based on the semi-classical treatment of the problem by Robyr and Bowtell, diffraction conditions were obtained for a CRAZED-type pulse sequence that selects iMQC of order N. The predicted distinct difference in N = 2 and N ≠ 2 coherences, i.e., a dominant continuous course as a function of τ ( N = 2) and prominent diffraction peaks otherwise, could be verified in CRAZED experiments in a periodically structured sample selecting coherence orders N = 2 and N = 3. The diffractive signal component contains information on the geometric structure of the sample. Applications of this technique may permit the detection of changes in composition and geometry of periodic structures.

Understanding spin turbulence in solution magnetic resonance through phase space dynamics and instability

AbstractThe joint action of two feedback fields in solution magnetic resonance (MR), radiation damping and the distant dipolar field, are known to give rise to nonlinear evolution of the magnetization at high fields. The different stages of the resulting complex dynamics are explored here, and standard tools and terminology adopted from the theory of nonlinear dynamics are introduced to analyze the underlying dynamics. The nonlinear spin dynamics are shown to generate unexpected artifacts in routine MR experiments involving bulk solvents. Approaches to curtail the effects of the joint feedback fields are discussed, as well as applications exploiting these fields as new contrast enhancement mechanisms for MR imaging. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part A 28A: 410–421, 2006

Direct enhancement of any solution NMR signal using the distant dipolar fields created by highly polarized and concentrated nuclear spin systems

Peculiar nuclear spin systems can be polarized at a level of thousands times the value obtained at thermal equilibrium, for instance by optical pumping. When concentrated, these systems create a sizeable average dipolar field which is experienced by any nuclear spin. We propose to use these distant dipolar fields for performing a polarization transfer in the Hartmann-Hahn conditions. We report the maximum enhancement value calculated using the spin temperature approach and first theoretical insights on the polarization transfer rate. Using, as an example, dissolved laser-polarized xenon, we show that by spin-locking both xenon spins and a proton spin of a solute, the polarization of the latter is enhanced. This is obtained without the existence of chemical interaction between the two entities and with characteristic rising time not directly correlated to the proton self-relaxation time. By its generality and its non-local feature, this approach could make possible nuclear magnetic resonance spectroscopy on very dilute systems.