Effect Of Magnetic Field Inhomogeneity Research Articles

Impact of resonator on direct-detected rapid-scan EPR at 9.8 GHz

Rapid-scan electron paramagnetic resonance spectra at 9.8 GHz were obtained on a Bruker E580 spectrometer. Spectra of lithium phthalocyanine (LiPc) needles (T1=8 μs,T2=3.4 μs) and of a 0.2 mM aqueous solution of Nycomed triarylmethyl (trityl-CD3) radical (T1=11.5 μs,T2=8 μs) were recorded at scan rates between 3.4·103 and 7.5·105 G/s at the center of sinusoidal scans. Signals for LiPc were obtained with a split-ring resonator, a rectangular resonator and a dielectric resonator. At faster scan rates the small bandwidth of the high-Q dielectric resonator filters, out high-frequency components of the rapid-scan signals. Field inhomogeneities induced by the rapidly changing magnetic field increase with scan rate and are greater with the dielectric and split-ring resonantors than with the rectangular resonator. Data for trityl-CD3 were recorded with the rectangular resonator. The extended trityl sample, about 3 mm long, shows larger effects of magnetic field inhomogeneities than the small LiPc crystals.

Conjugate phase MRI reconstruction with spatially variant sample density correction

A new image reconstruction method to correct for the effects of magnetic field inhomogeneity in non-Cartesian sampled magnetic resonance imaging (MRI) is proposed. The conjugate phase reconstruction method, which corrects for phase accumulation due to applied gradients and magnetic field inhomogeneity, has been commonly used for this case. This can lead to incomplete correction, in part, due to the presence of gradients in the field inhomogeneity function. Based on local distortions to the k-space trajectory from these gradients, a spatially variant sample density compensation function is introduced as part of the conjugate phase reconstruction. This method was applied to both simulated and experimental spiral imaging data and shown to produce more accurate image reconstructions. Two approaches for fast implementation that allow the use of fast Fourier transforms are also described. The proposed method is shown to produce fast and accurate image reconstructions for spiral sampled MRI.

Single TrAjectory Radial (STAR) imaging.

The number of MRI applications that use radial k-space data acquisition have been increasing because of their inherent robustness to motion-induced reconstruction image artifacts relative to Cartesian acquisition methods. However, images reconstructed from radial data are more prone to image degrading effects due to magnetic field inhomogeneities than images made from Cartesian data. Presented here is a method for acquiring several radial k-space data lines in one trajectory, the Single TrAjectory Radial, or STAR method, that is a variation of radial EPI. The STAR method allows for angular oversampling without the increase in imaging time that occurs with angularly oversampled single line imaging. It is shown that such oversampling potentially reduces the image degrading effect of magnetic field inhomogeneities so that the motion robust features of radial imaging may be realized in a segmented EPI approach.

Calculation of NMR line shape in rotating superfluid 3He–B

We have calculated numerically the equilibrium order-parameter texture of superfluid 3He–B in a rotating cylinder. The shape of the NMR absorption spectrum is deduced taking into account the broadening effects of magnetic field inhomogeneity and Leggett–Takagi spin relaxation. Results are compared with experimental data.

Cherenkov‐Curvature Radiation and Pulsar Radio Emission Generation

Electromagnetic processes associated with a charged particle moving in a strong circular magnetic field is considered in cylindrical coordinates. We investigate the relation between the vacuum curvature emission and Cherenkov emission and argue, that for the superluminal motion of a particle in the inhomogeneous magnetic field in a dielectric, the combined effects of magnetic field inhomogeneity and presence of a medium give rise to the synergetic Cherenkov-curvature emission process. We find the conditions when the operator relations between electric field and electric displacement in cylindrical coordinates may be approximated by algebraic relations. For nonresonant electromagnetic wave the interaction with particles streaming along the curved magnetic field may be described in the WKB approximation. For the resonant waves interacting with superluminal particles we find a particular case of plane-wave-like approximation. This allows a fairly simple calculation of the dielectric tensor of a plasma in a weakly inhomogenous magnetic field. We find in this approximation the polarization of normal modes in the plasma, Cherenkov-curvature and Cherenkov-drift emissivities and growth rates.

Sources of distortion in functional MRI data.

Functional magnetic resonance image (fMRI) experiments rely on the ability to detect subtle signal changes in magnetic resonance image time series. Any areas of signal change that correlate with the neurological stimulus can then be identified and compared with a corresponding high-resolution anatomical scan. This report reviews some of the several artefacts that are frequently present in fMRI data, degrading their quality and hence their interpretation. In particular, the effects of magnetic field inhomogeneities are described, both on echo planar imaging (EPI) data and on spiral imaging data. The modulation of these distortions as the subject moves in the magnet is described. The effects of gradient coil nonlinearities and EPI ghost correction schemes are also discussed.

The Effect of Magnetic Field Inhomogeneity on the Transverse Relaxation of Quadrupolar Nuclei Measured by Multiple Quantum Filtered NMR

The effects of magnetic fieldsB0andB1inhomogeneities on techniques which are commonly used for the measurements of triple-quantum-filtered (TQF) NMR spectroscopy of23Na in biological tissues are analyzed. The results of measurements by pulse sequences with and without refocusing ofB0inhomogeneities are compared. It is shown that without refocusing the errors in the measurement of the transverse relaxation times by TQF NMR spectroscopy may be as large as 100%, and thus, refocusing of magnetic field inhomogeneity is mandatory. Theoretical calculations demonstrate that without refocusingB0inhomogeneities the spectral width and phase depend on the interpulse time intervals, thus, leading to errors in the measured relaxation times. It is shown that pulse sequences that were used for the refocusing of the magnetic field (B0) inhomogeneity also reduce the sensitivity of the experimental results to radiofrequency (B1) magnetic field inhomogeneity.

Nuclear magnetic resonance study of lung water compartments in the rat.

Nuclear magnetic resonance transverse relaxation time (T2) was previously measured in studies of lung water. The T2 decay curves for peripheral lung tissue were found to be multiexponential with two T2 components: T2 fast (T2f) and T2 slow (T2s). This behavior was explained by the compartmentalization of water, in which the protons of water are restricted and do not undergo rapid exchange between the compartments. We investigated the origin of the water for these T2 components using excised rat lungs. The effect of magnetic field inhomogeneity due to air-tissue interfaces was examined by degassing some lungs. The contribution of intravascular water was examined by perfusing the lungs with oil or NaCl solutions. Degassing produced a greater increase in the T2f than the T2s component, indicating that the water in the alveolar walls exposed to air spaces contributed to the T2f. Perfusion with oil decreased the T2s, indicating that intravascular water contributed to the T2s component. The effects of intravascular osmotic pressure on the T2f and T2s components suggest that intracellular water is related to the T2f component.

Phase unwrapping in the three-point Dixon method for fat suppression MR imaging.

TO present an algorithm for performing phase unwrapping for fat and water signal separation in the three-point Dixon (3PD) method of magnetic resonance (MR) imaging. The algorithm is based on a two-dimensional region-growing approach, which tracks phase evolution in 3PD images and unwraps the phase when a 2 pi jump is detected. The unwrapped phase data are consecutively used to calculate fat and water signal components on a per-pixel basis. The method was tested in eight volunteers. The algorithm creates four output images: standard spin-echo, fat suppression, water suppression, and phase map images. This method corrects for the effects of magnetic field inhomogeneities and provides spatially uniform fat signal suppression superior to that achievable with the standard method. This method of 3PD data reconstruction appears to be promising for routine application of fat suppression MR imaging in a clinical setting.

Brillouin limit for non-neutral plasma in inhomogeneous magnetic fields

The effect of magnetic field inhomogeneity upon the maximum density of non-neutral plasma that can be confined in hydrostatic equilibrium under the combined influence of electrostatic and magnetostatic fields is explored in two cases of azimuthally symmetric geometries: first, a plasma in an arbitrary vacuum poloidal magnetic field, B=Br(r,z)r̂+Bz(r,z)ẑ, and second, a toroidal plasma in a toroidal vacuum magnetic field. For the poloidal field case, it is demonstrated that the Brillouin limit on number density can be achieved only with a homogeneous axial magnetic field. For the toroidal field case, it is demonstrated that although the Brillouin limit does not provide a limitation on the local number density confined, it nevertheless does provide an upper limit on the total amount of non-neutral plasma confined in equilibrium. It is also demonstrated that when viscosity is present in the toroidal field case, its effect to first order in the inverse aspect ratio can be opposed by a readjustment of only the external electrostatic potential.

Elimination of the effects of magnetic field inhomogeneities to the measurement of RF excitation profiles by using double gradient reversals

Elimination of the effects of magnetic field inhomogeneities to the measurement of RF excitation profiles by using double gradient reversals

Extraction of 1H1H and 1H13C dipolar couplings from spectra acquired in inhomogeneous magnetic fields

AbstractExperimental procedures are presented for the extraction of 1H1H and 1H13C dipolar couplings from molecules oriented in high magnetic fields without a requirement for high field homogeneity. These couplings impose restrictions on structure in a manner similar to NOE distances and can serve as useful additional constraints for defining structure. The experimental procedures involve pulse sequences which refocus the effects of magnetic field inhomogeneities but which retain dipolar couplings, scalar couplings and, in cases of chemically distinct groups, chemical shift differences. The methods are important for the extension of experiments to higher fields and more complex media where field homogeneity is hard to maintain. The experiments are illustrated using samples containing sodium acetate‐99% 13C2 or alanine‐99% 13C3 dissolved in a nematic liquid crystal and oriented in an inhomogeneous magnetic field. Coupling constants obtained from the studies are in good agreement with couplings extracted from one‐dimensional spectra obtained in homogeneous fields.

Measurement of the magnetic susceptibility effect in high-field NMR imaging

The magnetic susceptibility effect often obtained in clinical NMR imaging is analysed and a measurement method of the magnetic susceptibility effect is proposed. The method of extraction of the susceptibility effect alone in high-field NMR imaging where usually the chemical shift and main magnetic field inhomogeneity effects are intermingled with the susceptibility effect is discussed. The proposed susceptibility measurement uses the susceptibility weighted echo-time encoding technique (SWEET) to modulate the NMR signal as a function of echo time with which the image intensity can be weighted. Both phantom-oriented physical experiments and human volunteer experiments were performed to demonstrate the usefulness of the technique for human imaging.

Total inhomogeneity correction including chemical shifts and susceptibility by view angle tilting.

A correction technique of the total magnetic field inhomogeneity effects including the localized object induced inhomogeneities such as chemical shift and susceptibility is developed and its usefulness is experimentally demonstrated. With this new and simple technique all the inhomogeneity induced artifacts can be corrected simultaneously. The basic idea of this method is to add a compensation gradient of the same amplitude as the selection gradient in simultaneity with the reading gradient in such a way that the view angle is tilted. Thereby all the inhomogeneity induced geometrical shifts and hence the intensity changes are corrected, since the addition of the compensation which is independent from the field inhomogeneities including both chemical shifts and susceptibility. This technique has been theoretically examined and its usefulness is demonstrated by experiments.

4656425 Method to eliminate the effects of magnetic field inhomogeneities in NMR imaging and apparatus therefor

NMR imaging apparatus comprising apparatus for applying a linear magnetic field gradient to a sample, apparatus for varying the magnetic field gradient in a predetermined manner, apparatus for obtaining an NMR signal based on the linear magnetic field gradient, apparatus for storing the NMR signal, apparatus for reconstructing an NMR image from the stored NMR signal, and display apparatus for displaying the reconstructed NMR image, and wherein the apparatus for obtaining an NMR signal includes apparatus for obtaining the NMR signal at times when cancellation of the signal component dependent on magnetic field inhomogeneities occurs. There is also provided a technique for NMR imaging using the above apparatus.

Field inhomogeneity correction and data processing for spectroscopic imaging.

To obtain separate NMR images of spatially resolved high-resolution chemical-shift information the effects of magnetic field inhomogeneity must be accounted for. A suitable correction method is described which relies on first generating a plot of the magnetic field distribution. Using these data the spectroscopic imaging data can be processed to display the spatial distributions of separate resonances without distortion from field inhomogeneity. The field plot is obtained by using the same data acquisition sequence while imaging a phantom object or in particular cases the plot may be derived from the spectroscopic imaging data set itself. The correction procedure is illustrated using proton in vivo imaging of a cat. Some additional data processing techniques are presented which offer alternative methods of displaying spectroscopic imaging data.

Effect of magnetic field inhomogeneity on exact mass determination in ICR spectrometry

At present, in ICR spectrometry increasing interest exists in the development of ICR spectrometers utilizing superconducting solenoid magnets with magnetic field strength up to about 80 kG, which extend the measurable mass range into the region of higher masses. But, due to such high magnetic field strengths, inhomogeneities, which grow in proportion to field strength increasingly affect the accuracy of measurements. Therefore, magnets with the highest possible homogeneous fields are the aim of technical developments. The extent to which it is necessary to improve the homogeneity of solenoid magnets used in ICR spectrometers by the aid of expensive technology in order to achieve a still measurable effect with respect to exact mass determination is investigated in this work. For this purpose, the change of cyclotron frequency due to magnetic field inhomogeneity will be calculated and the resulting difference between measured and actual masses determined. As the mathematical form of this problem is complicated in the frame of classical mechanics, it is advantageous to use a different method. Regarding the quantum mechanical Schrödinger equation of this problem, we calculate the contributions due to inhomogeneity perturbation theoretically with the aid of a wavepacket formalism. The relation between shift of cyclotron frequency and experimental parameters is given and the order of magnitude of the perturbation is estimated for limiting conditions. It is shown that, even in the most unfavorable case, the influence of magnetic field inhomogeneity is negligibly small.

Pulsed nuclear magnetic resonance study of39K within Halobacteria

The39K contents of isolated pellets and supernatant solutions from suspensions ofHalobacterium halobium were studied at 21–22°C by pulsed NMR spectroscopy. The rates of transverse relaxation were measured directly from the free induction decay (FID). The rate of longitudinal relaxation was measured by studying the FID after pairs of pulses of approximately 90°. Care was exercised to minimize the effect of magnetic field inhomogeneity; its contribution to the FID was approximately 25–30 sec−1. The transverse relaxation process was found to consist of at least two components, whose rates were 321–449 sec−1 and 1,122–2,067 sec−1. In one preparation where the longitudinal relaxation process was studied, the data could be well fit to a single exponential relaxing at 253±33 (mean ±95% confidence limits) sec−1. Comparison of the relative intensities of the NMR signals with the results of atomic absorption photometric analyses indicated that the great bulk of the intracellular39K was detected by the NMR techniques used. The data obtained from the current NMR ofH. halobium are consistent with: (1) fractional binding of <3% of the total intracellular K+, (2) a small ordering factor characterizing all of the intracellular K+, or (3) some combination of the two.

Motional narrowing of Zeeman resonance lineshapes. IV. Analytical results

For pt.III see ibid., vol.10, p.959 (1977). The effects of magnetic field inhomogeneities and the motion of the spins on Zeeman resonance lines are examined for a simple system of non-interacting spins contained in a sphere. The values of the field strength parameter Delta and the relaxation time tau , which in the combination eta = Delta tau characterize the lineshapes, are calculated exactly. In the absence of motion (1<< eta ) the lineshapes are easy to evaluate and depend on the full field inhomogeneity. In the opposite limit of motional narrowing ( eta <<1) the lineshapes are narrowed and may be shifted are obtained and are in excellent agreement with Monte-Carlo calculations. Finally, a simple way of estimating possible frequency shifts in the limit of motional narrowing is confirmed.

Computer simulation of multipulse and Fourier transform NMR experiments. I. Simulations using the Bloch equations

A variety of multipulse and Fourier transform NMR experiments have been simulated using a computer program based on the Bloch equations. The program can calculate the effect of any sequence of pulses and delays on the magnetization vector. Examples considered are the conventional Fourier experiment and its alternating phase modifications, the partially relaxed and progressive saturation approaches to T 1 measurements, Carr-Purcell and modified Carr-Purcell experiments for T 1 and T 1 ϱ measurements and for signal-to-noise enhancement, and finally the conventional time-shared NMR experiment. The effects of magnetic field inhomogeneity and finite pulse power are included in the calculations where necessary.