Static Background Magnetic Field Research Articles

Postprocessing technique to correct for background gradients in image-based R*(2) measurements.

Background static magnetic field gradients are a source of signal loss in gradient-echo imaging, as they typically result from discontinuity in the magnetic susceptibility at air-tissue boundaries. Moreover, these induced gradients severely compromise the measurement of R*(2), the effective transverse relaxation rate, which is of interest in many biomedical applications of MRI. Since the slice thickness is usually larger than the in-plane pixel dimensions, gradients parallel to the slice-select direction are of particular concern. In this work, a post-processing technique is introduced which attempts to correct the signal on the assumption that the background gradients are approximately linear across the voxel and the signal decay in the absence of these gradients is exponential. In this case, the time-domain signal is weighted by a sinc function characterized by the amplitude G(b) of the background gradient, which is typically not known a priori. The algorithm searches for the estimate of G(b) which yields the optimum fit of the corrected experimental data to an exponential. It is shown to be effective as long as this gradient is below a critical threshold. Evaluation in a phantom and in the human brain at 1.5 and 4 T demonstrates that this method can restore R(2)* in spite of the apparent rate constant exceeding the true value by up to 100%. Contrary to prospective correction techniques, the approach presented in this study does not prolong scan time.

Low frequency wave propagation in a cold magnetized dusty plasma

In this paper several characteristics of low frequency waves in a cold magnetized dusty plasma propagating parallel and perpendicular to the static background magnetic field have been investigated. In the case of parallel propagation the negatively charged dust particles resonate with the right circularly polarized (RCP) component of em waves when the wave frequency equals the dust cyclotron frequency. It has been shown that an RCP wave in dusty plasma consists of two branches and there exists a region where an RCP wave propagation is not possible. Dispersion relation, phase velocity and group velocity of RCP waves have been obtained and propagation characteristics have been shown graphically. Poynting flux and Faraday rotation angles have been calculated for both lower and upper branches of the RCP wave. It has been observed that sense of rotation of the plane of polarization of the RCP wave corresponding to two distinct branches are opposite. Finally, the effect of dust particles on the induced magnetization from the inverse Faraday effect (IFE) due to the interaction of low frequency propagating and standing em waves with dusty plasmas has been evaluated.

Diffusion Measurements Using Radiofrequency Field Gradient: Artifacts, Remedies, Practical Hints

The two major advantages of experiments carried out with radiofrequency (RF) field-gradient NMR are the instrumental simplicity and the insensitivity to background static magnetic field gradients. These features combined with large RF gradients, which became available only recently, should make this technique especially attractive for molecular translational diffusion studies. However, a critical evaluation of the method shows that under some circumstances (small and/or heterogeneous samples, weak diffusion coefficients, very short relaxation times) the quality of measurements may be affected by a number of artifacts. Their origin has been investigated and several remedies have been considered; in particular, a new improved sequence is presented. The success of various experimental tests demonstrates the efficiency of the proposed solutions which thus open the way to much wider application fields.

Occupational Exposure to Low Frequency Magnetic Fields in Health Care Facilities

A characterization of health care workers' exposures to magnetic fields in the 40–1000 Hz range was performed in a multistory research hospital and single-story outpatient surgery unit. Area survey, source, and personal measurements of broadband and frequency-specific magnetic flux densities were performed with frequency analyzer dosimeters; the background static field was measured with a portable magnetometer. Dosimetry measurements were cross-referenced to workers' time/activity patterns. Spot measurements of magnetic flux density in the research hospital varied between 0.8 mG (.08 µT) and 65 mG (6.5 µT). Workers' time-weighted average exposures ranged from 1.2 mG (0.12 µT)to 10.4 mG (1.04 µT), with peak exposures ranging from 8.9 mG (0.89 µT) to 103.7 mG (10.37 µT). Frequency spectra of the exposures were variable and included harmonic bands as well as 60 Hz. The background static magnetic field in the hospital varied between 250 mG (25 µT) and 480 mG (48µT) suggesting shielding of the geomagnetic field from building materials. These results indicate that health care workers are exposed to 40–1000 Hz magnetic fields during routine patient care. The fields are characterized by a switching (on/off, in/out)pattern of occurrence, variable frequencies that include 60 Hz and harmonic bands, and peak excursions that may occur at regular intervals. These characteristics vary both within and among job categories, suggesting that job title may be a poor surrogate for exposure classification. Although limited in scope, the results indicate that spot measurement of broadband or 60-Hz magnetic fields, or summary indices such as time-weighted average, mean or median, are insufficient to characterize occupational exposures.

Occupational Exposure to Low Frequency Magnetic Fields in Health Care Facilities

A characterization of health care workers' exposures to magnetic fields in the 40-1000 Hz range was performed in a multistory research hospital and single-story outpatient surgery unit. Area survey, source, and personal measurements of broadband and frequency-specific magnetic flux densities were performed with frequency analyzer dosimeters; the background static field was measured with a portable magnetometer. Dosimetry measurements were cross-referenced to workers' time/activity patterns. Spot measurements of magnetic flux density in the research hospital varied between 0.8 mG (.08 microT) and 65 mG (6.5 microT). Workers' time-weighted average exposures ranged from 1.2 mG (0.12 microT) to 10.4 mG (1.04 microT), with peak exposures ranging from 8.9 mG (0.89 microT) to 103.7 mG (10.37 microT). Frequency spectra of the exposures were variable and included harmonic bands as well as 60 Hz. The background static magnetic field in the hospital varied between 250 mG (25 microT) and 480 mG (48 microT) suggesting shielding of the geomagnetic field from building materials. These results indicate that health care workers are exposed to 40-1000 Hz magnetic fields during routine patient care. The fields are characterized by a switching (on/off, in/out) pattern of occurrence, variable frequencies that include 60 Hz and harmonic bands, and peak excursions that may occur at regular intervals. These characteristics vary both within and among job categories, suggesting that job title may be a poor surrogate for exposure classification. Although limited in scope, the results indicate that spot measurement of broadband or 60-Hz magnetic fields, or summary indices such as time-weighted average, mean or median, are insufficient to characterize occupational exposures.

Characteristic modes of anisotropic two-component plasma

A convenient formulation is presented for the numerical calculation of the complete set of characteristic modes in a two-component, fluid-model, warm, unbounded, fully ionized, and anisotropic plasma. The anisotropy in this analysis is due to a static background magnetic field. Quantitative plots of the dispersion relations and field polarizations are presented for various modes of the system using an assumed set of typical plasma parameters. An interesting property of the hybrid low-frequency ion mode is shown to display a critical dependence on the angle between the wave propagation vector and the background magnetic field as this angle is varied near 90°.

External production and control of electrojet irregularities

Analytical investigation of parametric coupling of a large-amplitude 'pump' wave to a natural plasma mode of the ionospheric E layer. It is shown that the E layer plasma mode can be either stabilized or destabilized by the parametric process. Two necessary conditions for parametric interaction are (1) near frequency matching (i.e., the pump wave frequency should be near the local upper hybrid frequency) and (2) a finite electric field component of the pump wave perpendicular to the background static magnetic field. An investigation of the propagation characteristics of the pump wave in the magnetic equatorial region indicates that an obliquely propagating O mode in the magnetic meridian plane meets the two necessary conditions in an optimal manner. From typical values of ionospheric parameters it is estimated that the required incident energy flux of the pump wave for parametric stabilization or destabilization of the E layer plasma mode (near its natural marginal stability) is much less than 1 mW/sq m. Thus parametric coupling experiments for the ionospheric E layer are technically feasible.

Dispersion Relations and Mode Polarizations for Plane-Wave Oscillations in Two-Component Anisotropic Plasma (Computer Program Descriptions)

The dispersion relations /spl omega/=/spl omega/(k) and field polarizations are computed and corresponding graphical outputs are given for the six nontrivial modes that can propagate in unbounded collisionless fluid-model plasma with a static background magnetic field.