相关论文
2001 - The Astrophysical Journal
Three-dimensional Magnetohydrodynamic Simulations of Radiatively Cooling, Pulsed Jets

We here investigate, by means of fully three-dimensional smoothed particle magnetohydrodynamic numerical simulations, the effects of different initial magnetic field configurations on the evolution of overdense, radiatively cooling, pulsed jets using the following different initial magnetic field topologies: (1) longitudinal, (2) helical geometry permeating both the jet and the ambient medium, and (3) purely toroidal geometry permeating the jet only. We explore the effects of different pulsational periods, as well as different values of the magnetic field strength (? 0.1-? or B 260-0 ?G). The presence of a helical or toroidal field tends to affect the global characteristics of the fluid more than a longitudinal field. However, the relative differences that have been previously detected in two-dimensional simulations involving distinct magnetic field configurations are diminished in the three-dimensional flows. While the presence of toroidal magnetic components can modify the morphology close to the jet head, inhibiting its fragmentation in the early evolution of the jet, as previously reported in the literature, the impact of the pulse-induced internal knots causes the appearance of a clumpy, complex morphology at the jet head (as required by the observations of Herbig-Haro [HH] jets) even in the MHD jet models with helical or toroidal configurations. The detailed structure and emission properties of the internal working surfaces can also be significantly altered by the presence of magnetic fields. The increase of the magnetic field strength (decrease of ?) improves the jet collimation and amplifies the density (by factors up to 1.4 and 4) and the H? intensity (by factors up to 4 and 5) behind the knots of jets with a helical field and ? 1-0.1 relative to a nonmagnetic jet. As a consequence, the corresponding I[S II]/IH? ratio (which is frequently used to determine the excitation level of HH objects) can be decreased in the MHD models with toroidal components relative to nonmagnetic calculations by about the same amounts, although the intensity estimates above are very approximate. We also find that the helical mode of the Kelvin-Helmholtz instability can be triggered in MHD models with helical magnetic fields, causing some wiggling of the jet axis. No evidence for the formation of the nose cones that are commonly detected in two-dimensional jet simulations with initial toroidal magnetic fields is found in the three-dimensional flows or even in the ? 0.1 case. The implications of our results for HH jets are briefly discussed.

2015 - ACS applied materials & interfaces
Numerical characterization of magnetically aligned multiwalled carbon nanotube-Fe3O4 nanoparticle complex.

Alignment states of one-dimensional multiwalled carbon nanotubes containing various contents of zero-dimensional ferriferrous oxide nanoparticles (MWCNT-Fe3O4) were numerically characterized. MWCNT-Fe3O4 complexes were successfully prepared via in situ surface-initiated atom transfer radical polymerization, followed by a coprecipitation process. The complexes showed strong magnetism, which endowed them with the ability to be aligned under the action of an external magnetic field. The intensity of the magnetic field, loading content of Fe3O4 nanoparticles, and viscosity of dispersing medium, however, all had substantial effects on the alignment degree. To evaluate the alignment effectively and quantitatively, an orientation tensor description based on marking the direction of a single MWCNT in a selected region of optical images was employed. The results showed that MWCNT-Fe3O4 complex containing 26 wt % of Fe3O4 nanoparticles achieved a desirable alignment in deionized water under a magnetic field intensity of 0.10 T. Accordingly, epoxy composites reinforced with such aligned MWCNT-Fe3O4 complexes displayed 12.3 and 10.9% enhancement in tensile strength and modulus, as well as 8.9 and 6.1% enhancement in flexural strength and modulus, respectively.

2002 - cond-mat/0205429
Field-induced magnetic ordering in the quantum spin system KCuCl 3

KCuCl 3 is a three-dimensional coupled spin-dimer system and has a singlet ground state with an excitation gap Δ/k B =31 K. High-field magnetization measurements for KCuCl 3 have been performed in static magnetic fields ofup to 30 T and in pulsed magnetic fields of up to 60 T. The entire magnetization curve including the saturation region was obtained at T= 1.3 K. From the analysis of the magnetization curve, it was found that the exchange parameters determined from the dispersion relations of the magnetic excitations should be reduced, which suggests the importance of the renormalization effect in the magnetic excitations. The field-induced magnetic ordering accompanied by the cusplike minimum of the magnetization was observed, as in the isomorphous compound TlCuCl 3 . The phase boundary was almost independent of the field direction, and is represented by the power law. These results are consistent with the magnon Bose-Einstein condensation picture for field-induced magnetic ordering.

2004
Longitudinal magnetic field effect on the electrical breakdown in low pressure gases

The electrical breakdown has been investigated for low-pressure argon and nitrogen discharges under the influence of an external longitudinal magnetic field. Plane-parallel aluminum electrodes (5 cm diameter) separated by a variable distance d (4.0 cm < d < 11.0 cm) were sustained with a dc voltage (0 < V < 1 kV). A Helmholtz coil was used to produce an uniform magnetic field(B) parallel to the discharge axis. Paschen curves were obtained and the secondary electron emission coefficient (g), the first Townsend ionization coefficient (a) and the ionization efficiency(h), were plotted with respect to the variation of the reduced field (E/P). To observe the effect of the magnetic field these curves were plotted for fixed values of B=0 and B=350 Gauss. As consequence of the longitudinal magnetic field, the free paths of the electrons in the Townsend discharge are lengthened and their lateral diffusion is reduced, thus reducing electron losses to the walls. The data presented in this paper give a quantitative description of the B-field effect on the Townsend's coefficients and overall it is concluded that the DC electrical breakdown of the gases is facilitated if a longitudinal magnetic field is applied along the discharge axis.

2004 - Monthly Notices of the Royal Astronomical Society
Axisymmetric p‐mode pulsations of stars with dipole magnetic fields

The effect of a dipole magnetic field on adiabatic axisymmetric non-radial p-mode pulsations is studied numerically. The angular dependence of pulsation, which cannot be represented by a single spherical harmonic in the presence of a magnetic field, is expanded into a series of spherical harmonics with different degrees l. The presence of a magnetic field not only shifts the pulsation frequency, the pulsations are also damped due to the generation of magnetic slow waves. In agreement with the results of Cunha & Gough, who used a different approach from ours, we find that the effect of a magnetic field on the intermediate-to-high-order p-modes is not monotonic but cyclic with respect to the pulsation frequency and the magnetic field strength. The damping rate of a high-order p-mode becomes very small at about 3 kG and 8 kG; the corresponding field strengths are higher for lower overtones. The diminished magnetic damping is favourable for the corresponding modes, if they are excited by the classical K-mechanism, to survive even in the presence of a strong magnetic field. This picture could explain the mode selection as observed in the rapidly oscillating Ap stars. For a low-order p-mode, the damping rate increases as the strength of the magnetic field increases. We find that in the presence of a magnetic field of a few kG, magnetic damping seems to exceed the driving owing to the K-mechanism of oscillations representative of δ Scuti variability. This may explain why δ Scuti-type oscillations are unlikely to be seen in magnetic Ap stars. The amplitude of a mainly dipole (or quadrupole) mode is strongly confined to the magnetic axis in the outer layers. Furthermore, horizontal motion can be comparable to radial motion even for high-order p-modes. We discuss the influence of the magnetic distortion of the eigenfunction on the pulsation amplitude modulation with respect to the rotation phase.

2003 - Magnetic Resonance in Medicine
Neurobehavioral effects among subjects exposed to high static and gradient magnetic fields from a 1.5 Tesla magnetic resonance imaging system—A case‐crossover pilot study

The interactive use of magnetic resonance imaging (MRI) techniques is increasing in operating theaters. A study was performed on 17 male company volunteers to assess the neurobehavioral effects of exposure to magnetic fields from a 1.5 Tesla MRI system. The subjects' neurobehavioral performances on a neurobehavioral test battery were compared in four 1‐hr sessions with and without exposure to magnetic fields, and with and without additional movements. Adverse effects were found for hand coordination (–4%, P < 0.05; Pursuit Aiming II) and near visual contrast sensitivity (–16% and –15%, P < 0.10; Vistech 6000™). The results from the remaining tests were inconclusive due to a strong learning effect. No additional effect from gradient fields was detected. The results indicate that working near a 1.5 Tesla MRI system may lead to neurobehavioral effects. Further research is recommended, especially in members of operating teams using interactive MRI systems. Magn Reson Med 50:670–674, 2003. © 2003 Wiley‐Liss, Inc.

2010 - Journal of Materials Chemistry
Non-contact actuation of triple-shape effect in multiphase polymer network nanocomposites in alternating magnetic field

Triple-shape polymers (TSP) can memorize two independent shapes, which are recovered when the temperature is subsequently increased. Certain applications do not allow triggering of the triple-shape effect (TSE) by environmental heating (e.g. potential damaging of surrounding tissue) and therefore require a non-contact activation. Here we explored whether polymer nanocomposites can be designed, which enable non-contact activation of TSE in an alternating magnetic field. A series of nanocomposites were synthesized by incorporation of silica coated iron(III)oxide nanoparticles into a polymer network matrix containing poly(e-caprolactone) (PCL) and poly(cyclohexyl methacrylate) (PCHMA) segments. Triple-shape functionalization of the materials was realized by application of different thermomechanical procedures (single or two dual-step), in which samples were deformed by bending to minimize changes in S/V ratio during shape recovery. For quantification of triple-shape properties inductive heating experiments were conducted in an alternating magnetic field at frequency of f = 258 kHz. By increasing the magnetic field strength H the triple-shape effect was triggered, while the maximum achievable temperature Tmax and the shape change was monitored using an infrared video camera. Excellent triple-shape properties were achieved for nanocomposites containing 40 wt-% of PCL exhibiting a two-step recovery of shapes B and C, when stimulated by step-wise increasing the magnetic field strength. In this way the TSE could be characterized by two distinct switching magnetic strengths Hsw,1(A → B) and Hsw,2(B → C) corresponding to the switching temperatures determined in cyclic, thermomechanical tensile tests for thermally-induced TSP.

2001 - Analytical chemistry
Magnetohydrodynamic electrochemistry in the field of Nd-Fe-B magnets. Theory, experiment, and application in self-powered flow delivery systems.

Nd-Fe-B permanent magnets are easily available, powerful, and inexpensive and generate strong quantifiable convective effects during electrolysis, similar to those obtained with rotating electrodes or large electromagnets. The magnetic field of Nd-Fe-B magnets has been simulated numerically and mapped. Its most characteristic difference from the field of most commercial electromagnets is the presence of magnetic field gradients, which introduce additional body forces in the electrolytic solution and create new modes of mass transfer due to the attraction of electrogenerated radicals into areas of stronger field. The effect of those new forces on the radial distribution of the flow profile in the vicinity of the electrode has been monitored with generation-collection experiments and optical photography. The emerging utility of Nd-Fe-B magnets in systems of chemical interest is demonstrated with flow control and delivery devices, based on galvanic cells configured as self-powered magnetohydrodynamic pumps.

2017 - IEEE Photonics Journal
A Tunable Polarization Beam Splitter Based on Magnetic Fluids-Filled Dual-Core Photonic Crystal Fiber

A tunable polarization beam splitter (PBS) is proposed based on a dual-core photonic crystal fiber (DCPCF). The DCPCF is filled with magnetic fluids in the air holes, whose refractive index varies with the magnetic field strength. The two polarized modes are separated into the two cores at the output ports in a magnetic-free condition. When magnetic field is applied, the output intensities of the two polarized modes would fluctuate. The polarized modes can be ported out with arbitrary proportion by adjusting the magnetic field intensity. Moreover, the polarized mode would be completely converted at some magnetic field strength. The properties of the DCPCF are investigated by the finite element method (FEM), including mode-conversion magnetic field strength, extinction ratio, fabrication tolerance of the PBS, and the performance of PBS at various temperatures. The numerical results show that at 1550-nm wavelength, the length of the tunable PBS is 8.13 mm. The polarized mode converts at a magnetic intensity of 25 mT with a high extinction ratio greater than −100 dB. In addition, the tunable PBS sustains a fabrication tolerance of at least 0.5% and works well at temperatures in the range of 0–55 °C by adjusting the magnetic field strength.

2006 - Physical review letters
Anisotropic turbulent spectra in the terrestrial magnetosheath as seen by the cluster spacecraft.

Here we report the first three-dimensional spatial spectrum of the low frequency magnetic turbulence obtained from the four Cluster spacecraft in the terrestrial magnetosheath close to the magnetopause. We show that the turbulence is compressible and dominated by mirror structures, its energy is injected at a large scale kp approximately 0.3 (l approximately 2000 km) via a mirror instability well predicted by linear theory, and cascades nonlinearly and unexpectedly up to kp approximately 3.5 (l approximately 150 km), revealing a new power law in the inertial range not predicted by any turbulence theory, and its strong anisotropy is controlled by the static magnetic field and the magnetopause normal.

2016 - Physical review letters
Enhanced Multi-MeV Photon Emission by a Laser-Driven Electron Beam in a Self-Generated Magnetic Field.

We use numerical simulations to demonstrate that a source of collimated multi-MeV photons with high conversion efficiency can be achieved using an all-optical single beam setup at an intensity of 5×10^{22}  W/cm^{2} that is already within reach of existing laser facilities. In the studied setup, an unprecedented quasistatic magnetic field (0.4 MT) is driven in a significantly overdense plasma, coupling three key aspects of laser-plasma interactions at high intensities: relativistic transparency, direct laser acceleration, and synchrotron photon emission. The quasistatic magnetic field enhances the photon emission process, which has a profound impact on electron dynamics via radiation reaction and yields tens of TW of directed MeV photons for a PW-class laser.

2019 - Physica A: Statistical Mechanics and its Applications
An experimental investigation for study the rheological behavior of water–carbon nanotube/magnetite nanofluid subjected to a magnetic field

Abstract This study aims at experimentally investigating the influence of magnetic field on the rheological behavior of water–carbon nanotube (CNT)/magnetite (Fe3O4) nanofluid. Tetramethylammonium hydroxide (TMAH) and Gum arabic (GA) are respectively used to stabilize the nanofluid. Scanning Electron Microscope (SEM), Dynamic Light Scattering (DLS) and X-ray Diffraction (XRD) methods were used to characterize the prepared nanofluid samples. Experiments were performed to evaluate the viscosity of water–CNT/magnetite nanofluid in the shear rate range of 1–100 s−1, volume fraction range of 0.5-1.5%, and magnetic field strength range of 0–480 mT. It was found that the presence of an external magnetic field causes an increase in the viscosity of the water–CNT/magnetite nanofluid. In addition, the results depicted that the viscosity of the nanofluid augments by boosting the concentration of the nano-materials. Moreover, it was reported that increasing the magnetic field strength to more than 360 mT has a negligible influence on the augmentation of nanofluid viscosity. Furthermore, the non-Newtonian shear thinning behavior was depicted because of the decrease in viscosity with intensifying the shear rates.

2008 - Magnetic resonance in medicine
DWI of the spinal cord with reduced FOV single‐shot EPI

Single‐shot echo‐planar imaging (ss‐EPI) has not been used widely for diffusion‐weighted imaging (DWI) of the spinal cord, because of the magnetic field inhomogeneities around the spine, the small cross‐sectional size of the spinal cord, and the increased motion in that area due to breathing, swallowing, and cerebrospinal fluid (CSF) pulsation. These result in artifacts with the usually long readout duration of the ss‐EPI method. Reduced field‐of‐view (FOV) methods decrease the required readout duration for ss‐EPI, thereby enabling its practical application to imaging of the spine. In this work, a reduced FOV single‐shot diffusion‐weighted echo‐planar imaging (ss‐DWEPI) method is proposed, in which a 2D spatially selective echo‐planar RF excitation pulse and a 180° refocusing pulse reduce the FOV in the phase‐encode (PE) direction, while suppressing the signal from fat simultaneously. With this method, multi slice images with higher in‐plane resolutions (0.94 × 0.94 mm2 for sagittal and 0.62 × 0.62 mm2 for axial images) are achieved at 1.5 T, without the need for a longer readout. Magn Reson Med 60:468–473, 2008. © 2008 Wiley‐Liss, Inc.

1999 - Mutation research
Mutagenicity and co-mutagenicity of static magnetic fields detected by bacterial mutation assay.

Possible mutagenic and co-mutagenic effects of strong static magnetic fields were estimated using bacterial mutagenicity test. Mutagenic potential of static magnetic fields up to 5T (T:1T=10,000 G) was not detected by the bacterial mutagenicity test using four strains of Salmonella typhimurium (TA98, TA100, TA1535 and TA1537) and Escherichia coli WP2 uvrA either in the pre-incubation method or in the plate incorporation method. In the co-mutagenicity test, E. coli WP2 uvrA cells were treated with various chemical mutagens and were simultaneously exposed to a 2T or a 5T static magnetic field. Mutation rate in the exposed group was significantly higher than that in the non-exposed group when cells were treated with N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), ethylmethanesulfonate (EMS), 4-nitroquinoline-N-oxide (4-NQO), 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline (IQ) or 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide (AF-2). The mutagenicity of 2-aminoanthracene (2-AA), 9-aminoacridine (9-AA), N4-aminocytidine and 2-acetoamidofluorene (2-AAF) was not affected by the magnetic field exposure. Possible mechanisms of the co-mutagenicity of magnetic fields are discussed.

1995 - Bioelectromagnetics
Measurement and analysis of static magnetic fields that block action potentials in cultured neurons.

To characterize the properties of static magnetic fields on firing of action potentials (AP) by sensory neurons in cell culture, we developed a mathematical formalism based on the expression for the magnetic field of a single circular current loop. The calculated fields fit closely the field measurements taken with a Hall effect gaussmeter. The biological effect induced by different arrays of permanent magnets depended principally on the spatial variation of the fields, quantified by the value of the gradient of the field magnitude. Magnetic arrays of different sizes (macroarray: four center-charged neodymium magnets of approximately 14 mm diameter; microarray; four micromagnets of the same material but of approximately 0.4 mm diameter) allowed comparison of fields with similar gradients but different intensities at the cell position. These two arrays had a common gradient value of approximately 1 mT/mm and blocked > 70% of AP. Alternatively, cells placed in a field strength of approximately 0.2 mT and a gradient of approximately 0.02 mT/mm produced by the macroarray resulted in no significant reduction of firing; a microarray field of the same strength but with a higher gradient of approximately 1.5 mT/mm caused approximately 80% AP blockade. The experimental threshold gradient and the calculated threshold field intensity for blockade of action potentials by these arrays were estimated to be approximately 0.02 mT/mm and approximately 0.02 mT, respectively, In conclusion, these findings suggest that spatial variation of the magnetic field is the principal cause of AP blockade in dorsal root ganglia in vitro.

2006 - Bioelectromagnetics
Biological effects of 6 mT static magnetic fields: A comparative study in different cell types

The present work was a comparative study of the bio-effects induced by exposure to 6 mT static magnetic field (MF) on several primary cultures and cell lines. Particular attention was dedicated to apoptosis. Cell viability, proliferation, intracellular Ca(2+) concentration and morphology were also examined. Primary cultures of human lymphocytes, mice thymocytes and cultures of 3DO, U937, HeLa, HepG2 and FRTL-5 cells were grown in the presence of 6 mT static MF and different apoptosis-inducing agents (cycloheximide, H(2)O(2), puromycin, heat shock, etoposide). Biological effects of static MF exposure were found in all the different cells examined. They were cell type-dependent but apoptotic inducer-independent. A common effect of the exposure to static MF was the promotion of apoptosis and mitosis, but not of necrosis or modifications of the cell shape. Increase of the intracellular levels of Ca(2+) ions were also observed. When pro-apoptotic drugs were combined with static MF, the majority of cell types rescued from apoptosis. To the contrary, apoptosis of 3DO cells was significantly increased under simultaneous exposure to static MF and incubation with pro-apoptotic drugs. From these data we conclude that 6 mT static MF exposure interfered with apoptosis in a cell type- and exposure time-dependent manner, while the effects of static MF exposure on the apoptotic program were independent of the drugs used.

2004 - Mutation research
Effects of a static magnetic field on cell growth and gene expression in Escherichia coli.

Escherichia coli cultures exposed to a 300mT static magnetic field (SMF) were studied in order to analyse possible induced changes in cellular growth and gene expression. Biomass was evaluated by visible-light spectrometry and gene expression analyses were carried out by use of RNA arbitrarily primed PCR. The bacterial strain XL-1Blue, cultivated in traditional and modified Luria-Bertani medium, was exposed to SMF generated by permanent neodymium magnetic disks. The results show alterations induced by SMF in terms of increased cell proliferation and changes in gene expression compared with control groups. Three cDNAs were found to be expressed only in the exposed cells, whereas one cDNA was more expressed in the controls. One clone, expressed only in the exposed cells, corresponds to a putative transposase. This is of particular interest in that it suggests that exposure to a magnetic field may stimulate transposition activity.

2003 - Journal of Magnetic Resonance Imaging
High field human imaging

This review article examines the state of knowledge regarding human imaging using MRI at high main magnetic field strengths. The article starts with a summary of the technical issues associated with magnetic field strengths in the range of 3–8 T, including magnet characteristics and the properties of radiofrequency magnetic fields, with special reference to sensitivity, power deposition, and homogeneity. The published data on tissue‐water relaxation times in the brain is tabulated and the implications for contrast and pulse sequence implementation is elucidated. The behavior of the major fast imaging sequences, fast low angle shot (FLASH), rapid acquisition with relaxation enhancement (RARE), and echo planar imaging (EPI), is examined in this context. A number of anatomical images from 3 T systems are presented as examples. Particular attention is given to various forms of vascular imaging, namely, time of flight angiography, venography, and arterial spin labeling. The most complex changes in contrast with main magnetic field strength are in activation studies utilizing the blood oxygen level dependent mechanism, which are examined in detail. Improvements in spatial specificity are emphasized, particularly in conjunction with spin‐echo imaging. The article concludes with a discussion of the current status and the potential impact of technical developments such as parallel imaging. J. Magn. Reson. Imaging 2003;18:519–529. © 2003 Wiley‐Liss, Inc.

2004 - Evidence-based complementary and alternative medicine : eCAM
Study on Application of Static Magnetic Field for Adjuvant Arthritis Rats

In order to examine the effectiveness of the application of static magnetic field (SMF) on pain relief, we performed a study on rats with adjuvant arthritis (AA). Sixty female Sprague–Dawley (SD) rats (age: 6 weeks, body weight: approximately 160 g) were divided into three groups [SMF-treated AA rats (Group I), non-SMF-treated AA rats (Group II) and control rats (Group III)]. The SD rats were injected in the left hind leg with 0.6 mg/0.05 ml Mycobacterium butyrium to induce AA. The rats were bred for 6 months as chronic pain model. Thereafter, the AA rats were or were not exposed to SMF for 12 weeks. We assessed the changes in the tail surface temperature, locomotor activity, serum inflammatory marker and bone mineral density (BMD) using thermography, a metabolism measuring system and the dual-energy X-ray absorptiometry (DEXA) method, respectively. The tail surface temperature, locomotor activity and femoral BMD of the SMF-exposed AA rats were significantly higher than those of the non-SMF-exposed AA rats, and the serum inflammatory marker was significantly lower. These findings suggest that the pain relief effects are primarily due to the increased blood circulation caused by the rise in the tail surface temperature. Moreover, the pain relief effects increased with activity and BMD of the AA rats.

2003
Effect of Magnetic Field on Solidification in Cu-Pb Monotectic Alloys

Two phases produced through the liquid-liquid separation gravitationally segregate during solidification of the monotectic alloys. This paper examined reduction of the gravity segregation for the Cu-Pb monotectic alloys by imposing a static magnetic field up to 10 T. The Cu-Pb alloys with compositions ranging from 15.5 to 84.5 at% Pb solidified at a cooling rate of 10 K/s under a magnetic field. The effect of the magnetic field on the macrosegregation was clearly recognized for the Cu-Pb alloys with compositions ranging from 65 to 70 at% Pb, while the magnetic field did not become a dominant factor of the segregation behavior in the other compositions. Furthermore, diameter of the Cu-rich liquid drops under 10 T was smaller than that under 0 T. The static magnetic field reduced not only the rising velocity of the Cu-rich drops but also the coalescence rate of the liquid drops, resulting in the reduction of the macrosegregation. The magnetohydrodynamic estimation suggested that the terminal velocity of the Cu-rich particles with typical diameters is significantly reduced by the imposed static magnetic field. The difficulty of the particle movement due to the electromagnetic force resulted in the homogenous solidified structure for the Cu-Pb alloys.

Physical interactions of static magnetic fields with living tissues.

John F Schenck
|
J. Schenck

Abstract:Clinical magnetic resonance imaging (MRI) was introduced in the early 1980s and has become a widely accepted and heavily utilized medical technology. This technique requires that the patients being studied be exposed to an intense magnetic field of a strength not previously encountered on a wide scale by humans. Nonetheless, the technique has proved to be very safe and the vast majority of the scans have been performed without any evidence of injury to the patient. In this article the history of proposed interactions of magnetic fields with human tissues is briefly reviewed and the predictions of electromagnetic theory on the nature and strength of these interactions are described. The physical basis of the relative weakness of these interactions is attributed to the very low magnetic susceptibility of human tissues and the lack of any substantial amount of ferromagnetic material normally occurring in these tissues. The presence of ferromagnetic foreign bodies within patients, or in the vicinity of the scanner, represents a very great hazard that must be scrupulously avoided. As technology and experience advance, ever stronger magnetic field strengths are being brought into service to improve the capabilities of this imaging technology and the benefits to patients. It is imperative that vigilance be maintained as these higher field strengths are introduced into clinical practice to assure that the high degree of patient safety that has been associated with MRI is maintained.

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