Presence Of Field Gradient Research Articles

Robust diffusion tensor imaging by spatiotemporal encoding: Principles and in vivo demonstrations.

Evaluate the usefulness of single-shot and of interleaved spatiotemporally encoded (SPEN) methods to perform diffusion tensor imaging (DTI) under various preclinical and clinical settings. A formalism for analyzing SPEN DTI data is presented, tailored to account for the spatially dependent b-matrix weightings introduced by the sequence's use of swept pulses acting while in the presence of field gradients. Using these b-matrix calculations, SPEN's ability to deliver DTI measurements was tested on phantoms as well as ex vivo and in vivo. In the latter case, DTI involved scans on mice brains and on human lactating breasts. For both ex vivo and in vivo investigations, SPEN data proved less sensitive to distortions arising from Bo field inhomogeneities and from eddy currents, than conventional single-shot alternatives. Further resolution enhancement could be achieved using referenceless methods for interleaved SPEN data acquisitions. The robustness of SPEN-based sequences vis-à-vis field instabilities and heterogeneities, enables the implementation of DTI experiments with good sensitivity and resolution even in challenging environments in both preclinical and clinical settings. Magn Reson Med 77:1124-1133, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Electric field gradients during electrophoretic fractionation in microfabricated post arrays

Electrokinetic separation methods have been used extensively in microfluidic devices due to their separation power and compatibility with miniaturization platform. Electrophoresis (EP) fractionation of DNA and proteins in microfluidic devices has been conducted successfully and incorporated toward developing diagnostic devices. Porous structures as well as microfabricated pillars are often used as separation media for separation of biomolecules. The presence of the solid structures can create electric field gradients resulting in dielectrophoresis (DEP) which most often was neglected during DNA and protein fractionation. In this study, we evaluated the effect of induced field gradients by calculating the magnitude of the three most occurring phenomena during electrophoretic fractionation of biomolecules, i.e., EP, Brownian diffusion, and DC DEP in uniformly patterned post arrays. A mathematical model of convection–diffusion–migration of these charged entities was developed and solved using finite element analysis. Non-dimensional parameters were defined as a measure to predict the particles transport mechanism within the microfabricated post arrays in the presence of field gradients caused by the posts. Based on the properties of particles and the medium, our numerical analysis provided criteria to predict when the DEP can significantly alter the fate of the charged entities through uniform post arrays and therefore cannot be neglected. In order to identify these criteria, a generalized approach based on a systematic parametric study of particle size and zeta potentials was conducted. The criteria obtained have the potential to shed light on some of the challenges such as irreversible trapping and large band broadening during fractionation of charged macromolecules.

Measurement of the true transverse nuclear magnetic resonance relaxation in the presence of field gradients

A measure of the nuclear spin transverse relaxation time T2, as determined using the nuclear magnetic resonance Carr-Purcell Meiboom-Gill (CPMG) experiment, provides unique information characterizing the microstructure of porous media which are themselves ubiquitous across fields of petrophysics, biophysics, and chemical engineering. However, the CPMG measurement is sensitive to diffusion in large magnetic field gradients. Under such conditions an effective relaxation time T2,eff is observed instead, described by a combination of relaxation and diffusion exponents. The relaxation exponent always varies as nte (where n is the number, and te is the temporal separation, of spin echoes). The diffusion exponent varies as nte (k), where 1 < k ≤ 3, although the exact analytic form is often unknown. Here we present a general approach to separating the influence of relaxation and diffusion by utilizing a composite diffusion exponent. Any T2,eff component with a power of k > 1 is removed to provide a measure of the true T2 relaxation time distribution from CPMG data acquired in the presence of a strong background gradient. We apply the technique to discriminate between the effects of relaxation and diffusion in porous media using catalysts and rocks as examples. The method is generally applicable to any CPMG measurements conducted in the presence of a static magnetic field gradient.

High-resolution magnetic resonance spectroscopy in unstable fields via intermolecular zero-quantum coherences

Intermolecular zero-quantum coherences (iZQCs) have been utilized to achieve high-resolution nuclear magnetic resonance (NMR) proton spectra under inhomogeneous and/or unstable fields. In this paper, we demonstrated that despite the insensitivity of iZQCs to B(0) variations, the influence of unstable fields on the observable single-quantum coherence signals causes strong t(1) noises in the high-resolution iZQC spectra. Short-time acquisition (STA) and phase spectrum schemes were proposed for noise suppression in in vivo iZQC magnetic resonance spectroscopy (MRS) under temporal B(0) variations. The feasibility of these schemes were verified by localized spectroscopic studies under B(0) variations generated by the Z0 coil current oscillations and by voxel position variations in the presence of field gradients, which simulate the field conditions of MRS in the presence of physiological motions. The phase scheme not only improves the signal-to-noise ratio but also further reduces the linewidth by half.

Study of some magnetic properties of ferrofluids filtered in magnetic field gradient

The magnetic properties of some ferrofluids obtained by a new technique, in which the stabilization of particles was done by hydrofobization in the absence of the dispersion medium, and the filtration was done in the presence of a magnetic field gradient, are studied. Measurements were performed on three samples originated from the same ferrofluid with magnetite particles dispersed in kerosene. They are distinguished by the type of filtration: in the absence of the field gradient (sample A), in the presence of the field gradient (sample B), and in the presence of the field gradient with a filamentary matrix introduced in the filtration vessel (sample C). The measurements performed in static magnetic field show an increase of approximately 12% and 40% in the saturation magnetization of samples B and C, respectively, in comparison to sample A, due to the increase of the concentration of the magnetic phase, as a result of the filtration in the presence of the field gradient. A decrease of the mean magnetic diameter of the particles in the ferrofluid can be noticed, from the dimensional distribution of the particles, determined by the Chantrell method (from 9.15 nm (sample A) to 7.97 nm (sample C)).

Multiple Nonlinear Stimulated Echoes

Three-pulse sequences in the presence of magnetic field gradients at high magnetic fields produce multiple nonlinear stimulated echoes (NOSE) at timesnτ1after the third pulse, wherenis an integer and τ1the interval between the first two pulses. These phenomena are due to the demagnetizing field produced by the spatial modulation of the nuclear magnetization arising in the sample after the first two pulses. The theory is presented and compared with experiments. The dependence of the NOSE amplitudes on the flip angles and on the pulse intervals is described. Implications for multidimensional NMR experiments based on sequences of three or more pulses in the presence of field gradients are discussed.

Slice-selective polarization transfer (or cross-polarization) for localized solid-state NMR spectroscopy

A new principle for spatially localized solid-state NMR is reported. Polarization transfer (or cross-polarization or more generally spin-order transfer) is strongly susceptible to deviations from the resonance conditions. Hartmann—Hahn matching and matching to the local field in dipolar order states while magnetic field gradients are present have been treated theoretically and studied experimentally. The transfer process was found to be slice selective. The spatial resolution exceeds that of the localization effect by spin-locking in the presence of field gradients. Applications to imaging localized heteronuclear spectroscopy are discussed. First test experiments to this end are reported.

Spatially resolved homonuclear solid-state NMR. I. Slice selection by spin locking and localized spin temperature

Slice selection by spin-locking pulses in the presence of field gradients is a technique particularly suitable for spatially resolved NMR experiments (imaging or localized spectroscopy) with solid-like samples. The theory of the excitation profiles in the limits of liquids and solids is presented. Under idealized conditions, the shapes are Lorentzian, but severe deviations are predicted if the preparation pulse is not a hard 90° pulse and/or if the spin-locking amplitude is comparable to the local field in the rotating frame. From the physical point of view, it is also of interest that slice-selection techniques produce localized spin temperatures in solids. Corresponding gradients thus can be prepared.

Flow state of 3He in adiabatic flow of HeII

In adiabatic flow of HeII, the normal fluid components are forced to flow by the action of the superfluid component through vortices. The flow state of 3He , added into the system, in the flow path was studied with the pulsed NMR method in the presence of field gradient. The results show that the flow is almost Poiseuille type in contrast with the highly turbulent state of the superfluid component. The method also directly gives the He flow velocity in the flow path. The results show the existence of the characteristic velocity v c2 predicted by the new-type mutual friction force.

Flow state of 3He in adiabatic flow of HeII

In adiabatic flow of HeII, the normal fluid components are forced to flow by the action of the superfluid component through vortices. The flow state of 3He , added into the system, in the flow path was studied with the pulsed NMR method in the presence of field gradient. The results show that the flow is almost Poiseuille type in contrast with the highly turbulent state of the superfluid component. The method also directly gives the He flow velocity in the flow path. The results show the existence of the characteristic velocity v c2 predicted by the new-type mutual friction force.

High-field magnetoconductance of electrons on liquid helium

For the first time electrons on liquid helium have been subjected to magnetic fields as high as 22 T. The magnetoconductance has been measured in the temperature range 1.5–2 K, where the mobility of the electrons is dominated by the scattering with helium vapor atoms. Since helium is a diamagnetic liquid, the presence of field gradients has to be taken into account in high magnetic fields. The conductance data inherently contain the effect of the field-induced deformation of the helium surface. An estimation of this effect is presented. The corrected data are discussed within quantum transport theory.

Electric field gradients and band sharpening in DNA gel electrophoresis.

A mathematical study of the effect of non-uniform electric fields on the width of DNA electrophoretic bands is presented. Using a simple model, we show that field gradients sharpen these bands during an experiment if the corresponding gradient of electrophoretic velocity is large enough. This is in agreement with experimental results indicating that narrower bands form when pulsed field electrophoresis is carried out in the presence of field gradients. Moreover, it is shown that there is in fact an optimal experimental duration that maximizes separation. Finally, gradients are also predicted to reduce the relative mobilities of the DNA fragments, which is a serious drawback of this technique.

Spin-echoes for diffusion in bounded, heterogeneous media: A numerical study

The diffusive behavior of spin-bearing species in a bounded heterogeneous medium is analyzed in a manner appropriate for spin echo experiments in the presence of field gradients. A numerical method based upon the stochastic Liouville equation (SLE) is discussed that includes the discontinuities in transport and solubility properties due to the different spatial regions. The double step computational algorithm, which takes advantage of the different time scales of diffusive and spin-quantum phenomena, is then introduced as a general approximate solution of the time dependent SLE. This method is applied to the calculation of the decay of spin echo amplitudes, and it suggests a new approach for analyzing such experiments in terms of the microscopic details and chemical properties of heterogeneous systems.