Attenuation of morphine-induced analgesia in mice by exposure to magnetic resonance imaging: separate effects of the static, radiofrequency and time-varying magnetic fields

Exposure of adult male mice to a magnetic resonance imaging (MRI) procedure has been shown to abolish the nocturnal analgesic responses observed following treatment with morphine. The field component(s) responsible for this inhibitory effect were examined by exposing mice to either the static, time-varying or rf magnetic field components associated with an MRI procedure. In the middle of the night portion of their day-night cycle, mice were exposed for 23.2 min to one of the above field components, intraperitoneally injected with morphine sulphate (10 mg/kg) and then exposed to the field conditions for another 23.2 min, after which analgesic responses were determined. Analgesia was quantitated by determining the length of time mice were content to be on a hot surface (50 degrees C) before they showed discomfort by licking their paws. It was observed that the time-varying magnetic field completely abolished, the rf field significantly reduced, while the static field component (0.15 T) had no evident effect on morphine-induced analgesia. These results indicate that the time-varying, and to a lesser extent the rf, fields associated with the MRI procedure inhibit morphine-induced analgesia in mice. These data also raise the possibility that exposure in humans to some of the magnetic field components associated with MRI may have clinically relevant effects on the actions of narcotic drugs such as morphine.     

Radiofrequency magnetic field gradient echoes have reduced sensitivity to susceptibility gradients

The amplitudes of gradient-echoes produced using static field gradients are sensitive to diffusion of tissue water during the echo evolution time. Gradient-echoes have been used to produce MR images in which image intensity is proportional to the self-diffusion coefficient of water. However, such measurements are subject to error due to the presence of background magnetic field gradients caused by variations in local magnetic susceptibility. These local gradients add to the applied gradients. The use of radiofrequency (RF) gradients to produce gradient-echoes may avoid this problem. The RF magnetic field is orthogonal to the offset field produced by local magnetic susceptibility gradients. Thus, the effect of the local gradients on RF gradient-echo amplitude is small if the RF field is strong enough to minimize resonance offset effects. The effects of susceptibility gradients can be further reduced by storing magnetization longitudinally during the echo evolution period. A water phantom was used to evaluate the effects of background gradients on the amplitudes of RF gradient-echoes. A surface coil was used to produce an RF gradient of between 1.3 and 1.6 gauss/cm. Gradient-echoes were detected with and without a 0.16 gauss/cm static magnetic field gradient applied along the same direction as the RF gradient. The background static field gradient had no significant effect on the decay of RF gradient-echo amplitude as a function of echo evolution time. In contrast, the effect of the background gradient on echoes produced using a 1.6 gauss/cm static field gradient is calculated to be significant. This analysis suggests that RF gradient-echoes can produce MR images in which signal intensity is a function of the self-diffusion coefficient of water, but is not significantly affected by background gradients.     

Distortion-free magnetic resonance imaging in the zero-field limit

MRI is a powerful technique for clinical diagnosis and materials characterization. Images are acquired in a homogeneous static magnetic field much higher than the fields generated across the field of view by the spatially encoding field gradients. Without such a high field, the concomitant components of the field gradient dictated by Maxwell’s equations lead to severe distortions that make imaging impossible with conventional MRI encoding. In this paper, we present a distortion-free image of a phantom acquired with a fundamentally different methodology in which the applied static field approaches zero. Our technique involves encoding with pulses of uniform and gradient field, and acquiring the magnetic field signals with a SQUID. The method can be extended to weak ambient fields, potentially enabling imaging in the Earth’s field without cancellation coils or shielding. Other potential applications include quantum information processing and fundamental studies of long-range ferromagnetic interactions.     

Scattering polarization in the presence of magnetic and electric fields

The polarization of radiation by scattering on an atom embedded in combined external quadrupole electric and uniform magnetic fields is studied theoretically. Limiting cases of scattering under Zeeman effect, and Hanle effect in weak magnetic fields are discussed. The theory is general enough to handle scattering in intermediate magnetic fields (Hanle-Zeeman effect) and for arbitrary orientation of magnetic field. The quadrupolar electric field produces asymmetric line shifts, and causes interesting level-crossing phenomena either in the absence of an ambient magnetic field, or in its presence. It is shown that the quadrupolar electric field produces an additional depolarization in the Q/I profiles and rotation of the plane of polarization in the U/I profile over and above that arising from magnetic field itself. This characteristic may have a diagnostic potential to detect steady-state and time-varying electric fields that surround radiating atoms in solar atmospheric layers.     

Electron spin transition solution applicable to an ensemble of isolated electrons.

An electron spin aligned with a static magnetic field changes its orientation when subjected to a time-varying magnetic field which is directed perpendicular to the static magnetic field. This well-known phenomenon is readily calculated when the time-varying magnetic field is circularly polarized; however, the evolution of the spin-state wavefunctions becomes much more difficult to calculate when the time-varying magnetic field is linearly polarized. For linear polarization and isolated spins, an analytic solution has been derived for the dynamical spin-state wavefunctions. Part of the solution procedure relies on an expansion using a small parameter, which is the ratio of the amplitude of the time-varying magnetic field to the static magnetic field. To verify the validity of the expansion technique, a numerical solution of the basic equations is compared to the analytic solution. Results are found to agree to better than 10% for exact resonance and better than 5% in general.     

Potential hazards of NMR imaging. No evidence of the possible effects of static and changing magnetic fields on cardiac function of the rat and guinea pig

Clinical proton NMR imaging uses magnetic field strengths in the range 0.1 to 0.5 T. In addition to the large static magnetic field, patients are exposed to magnetic field gradients during imaging and under extreme conditions, such as power failure or quenching, the field may collapse precipitously. A potential source of hazard to patients under these conditions is the induction of thoracic currents which may trigger ventricular fibrillation. In the present experiments, a 0.16 T resistive magnet with a time constant of 60 ms, powered by a programmable power supply, was used to examine any possible effects of static and changing magnetic field on the ECG and arterial blood pressure of anesthetized rats and guinea pigs. Animals were exposed to the following field conditions: static fields of 0.16 T; sine, triangular, and square wave modulated fields from 0.1 to 2 Hz; rapid field switches in excess of 2.0 T/s for 25 ms timed to occur at different stages of the cardiac cycle, including the vulnerable period during ventricular repolarization; and AC fields of 50 Hz. No change was observed in the blood pressure, heart rate, or ECG under any of the field conditions examined.     

Harmonic Fields in a Non Uniform Plasma

The magnitudes of the harmonic fields which are generated when an alternating electric field is applied to a plasma are calculated. The plasma has a density gradient and is immersed in a uniform static magnetic field. The harmonics are very strongly excited near the upper hybrid frequency. A fluid theory is employed to describe the highly non linear behavior near resonance and a kinetic theory is used to find the effect of a finite temperature. It is found that kinetic effects are important if the radius of gyration is comparable in size to the scale length.     

Non-inertial electromagnetic effects in matter. Gyromagnetic effect

Non-inertial electromagnetic effects in matter, i.e. electromagnetic fields created by a non-inertial motion of material bodies, are discussed within the Drude–Lorentz (plasma) model of matter polarization. It is shown that an oscillatory motion of a point-like body, or wavelike motion in an extended body gives rise to electromagnetic fields with the same frequency as the frequency of the original motion, while shock-like movements of a point-like body generate electromagnetic fields with the characteristic (atomic scale) frequency of the bodies. The polarization of a rigid body induced by rotations is discussed in various circumstances. A uniform rotation produces a static electric field in a dielectric and a stationary current (and a static magnetic field) in a conductor. The latter corresponds to the gyromagnetic effect (while the former may be called the gyroelectric effect). Both fields are computed for a sphere and the gyromagnetic coefficient is derived. A non-uniform rotation induces emission of electromagnetic fields. The equations of motion for the polarization are linearized for slight non-uniformities of the angular velocity and solved both for a dielectric and a conducting sphere. The electromagnetic field emitted by a dielectric spherically shaped body in (a slightly) non-uniform rotation has the characteristic (atomic scale) frequency of the body (slightly shifted by the uniform part of the angular frequency). In the same conditions, a conducting sphere emits an electromagnetic field whose frequency is double the uniform part of the angular frequency.     

Velocity imaging by ex situ NMR

A pulsed field gradient stimulated spin-echo NMR sequence is combined with imaging methods to spatially resolve velocity distributions and to measure 2D velocity maps ex situ. The implementation of these techniques in open sensors provides a powerful non-invasive tool to measure molecular displacement in a large number of applications inaccessible to conventional closed magnets. The method is implemented on an open tomograph that provides 3D spatial localization by combining slice selection in the presence of a uniform static magnetic field gradient along the depth direction with pulsed field gradients along the two lateral directions. Different pipe geometries are used to demonstrate that the sequence performs well even in the extremely inhomogeneous B0 and B1 fields of these sensors.     

Pairwise entanglement of a three-qubit Heisenberg XY chain in a nonuniform magnetic field with intrinsic decoherence

Taking into account the intrinsic decoherence, the concurrence of the nearest and the next-to-nearest neighbor qubits in a three-qubit Heisenberg XY chain are investigated when a nonuniform magnetic field is included. We show that the effects of the external magnetic field, including the uniform and inhomogeneous magnetic fields, on the time evolution of entanglement between the nearest and the next-to-nearest neighbor qubits rely deeply on the initial states. We can moderate the destructive effect of intrinsic decoherence by controlling the uniform and inhomogeneous magnetic fields, so that a proper value of uniform and inhomogeneous magnetic fields can, to a great extent enhance the stationary entanglement.     

Biological effects of weak electromagnetic fields from 0 Hz to 200 MHz: a survey of the literature with special emphasis on possible magnetic resonance effects

The literature on biological effects of weak electromagnetic fields of a frequency of 200 MHz or less is surveyed. The topic has been extraordinarily controversial, in part because of disputed assertions about a role for electromagnetic fields in carcinogenesis or production of abnormalities in growth and development. There is fairly widespread acceptance of certain beneficial effects, particularly the stimulation of healing. An increasing number of reports point to interactions between static magnetic fields and time-varying fields in the production of some effects. Safety implications are noted along with the hypothetical possibility of production of experimental artifacts by electromagnetic fields in MRS research.     

Theory of MRI in the presence of zero to low magnetic fields and tensor imaging field gradients

Today, all commonly practiced magnetic resonance imaging (MRI) reconstruction methods assume that the magnetic field created by the gradient coils is everywhere truncated by a dominant static uniform magnetic field. However, with the advent of SQUID detected MRI at microtesla fields, the opposite limit attracts attention, i.e., image formation in the unperturbed tensor field of the gradient coils. Here, we show by numerical simulations that, in principle, it is possible to reconstruct the image of an object in the absence of a uniform static field, working with the same gradient field setup as used in conventional MRI. Our calculations show that this approach could increase the image resolution limit attainable at low fields with a minimal incorporation of additional hardware and pulse sequences.     

Correction of concomitant gradient artifacts in experimental microtesla MRI

Magnetic resonance imaging (MRI) suffers from artifacts caused by concomitant gradients when the product of the magnetic field gradient and the dimension of the sample becomes comparable to the static magnetic field. To investigate and correct for these artifacts at very low magnetic fields, we have acquired MR images of a 165-mm phantom in a 66-microT field using gradients up to 350 microT/m. We prepolarize the protons in a field of about 100 mT, apply a spin-echo pulse sequence, and detect the precessing spins using a superconducting gradiometer coupled to a superconducting quantum interference device (SQUID). Distortion and blurring are readily apparent at the edges of the images; by comparing the experimental images to computer simulations, we show that concomitant gradients cause these artifacts. We develop a non-perturbative, post-acquisition phase correction algorithm that eliminates the effects of concomitant gradients in both the simulated and the experimental images. This algorithm assumes that the switching time of the phase-encoding gradient is long compared to the spin precession period. In a second technique, we demonstrate that raising the precession field during phase encoding can also eliminate blurring caused by concomitant phase-encoding gradients; this technique enables one to correct concomitant gradient artifacts even when the detector has a restricted bandwidth that sets an upper limit on the precession frequency. In particular, the combination of phase correction and precession field cycling should allow one to add MRI capabilities to existing 300-channel SQUID systems used to detect neuronal currents in the brain because frequency encoding could be performed within the 1-2 kHz bandwidth of the readout system.     

Frame Dragging Effect on Properties of Rotating Neutron Stars with Strong Magnetic Field

The general relativistic frame dragging effect on the properties, such as the moments of inertia and the radii of gyration of fast rotating neutron stars with a uniform strong magnetic field, is calculated accurate to the first order in the uniform angular velocity. The results show that compared with the corresponding non-rotating static spherical symmetric neutron star with a weaker magnetic field, a fast rotating neutron star (millisecond pulsar) with a stronger magnetic field has a relative smaller moment of inertia and radius of gyration.     

Effect of Weak Magnetic Fields on the Electric Properties of CdTe Crystals

The effect of static and pulsed magnetic fields (~1 T) on the electrical conductivity of CdTe crystals has been revealed. With a delay after the magnetic exposure of crystals, the effect is observed in the form of two peaks of their conductivity with the subsequent relaxation return. The first peak at both types of magnetic treatment is observed ~1 h after exposure and its amplitude exceeds the background value by ~23–36% (the larger value corresponds to the static field). The second peak appears in both cases also at commensurate but much larger delays of ~50–60 h, and its amplitudes are much different for the two types of exposure, exceeding the background by ~60% for the static field and only by ~11% for the pulsed field. Possible mechanisms of the observed effects have been discussed.     

Effect of a magnetic field in photodetachment microscopy

The effect of an external static magnetic field of arbitrary orientation with respect to the electric field, on the electron interference ring patterns observed by the photodetachment microscope is studied both experimentally and theoretically. The design of the interaction chamber has been modified to superimpose a controlled uniform magnetic field on the whole volume accessible to the interfering electron. Contrary to a previous study in weaker fields, where the overall dimension of the interferogram was not modified, the effect of the magnetic field here encompasses a regime of magnetic refocusing. A quantitative analysis is carried out using a closed-orbit perturbative calculation of the interference phase at the centre of the ring pattern. The essential result of this work is still the invariance of the extreme interference phase whatever the direction and magnitude of the applied magnetic field, up to values 100 times larger than in the previous experimental study. This property can be applied to revise former electron affinity measurements. Partly due to the previously unsuspected robustness of the electron interferograms vs. magnetic fields, partly thanks to the 2006 CODATA revision of the energy conversion factors, one can update the values of the electron affinities of ¹⁶O, ²⁸Si and ³²S to 1.4611134(9), 1.3895210(7) and 2.0771040(6) eV respectively.     

On the role of combined static hyperfine interactions for the interpretation of in-beam nuclear Larmor-precession measurements

Differential perturbed angular-distribution measurements have been performed for¹⁰⁷Cd and¹⁰⁹Cd in silver hosts. The data have been obtained at various target temperatures with and wthout applied magnetic fields. The theory of static, combined electric and magnetic hyperfine interaction has been applied for comparison of the zero-field quadrupole data and the external field Larmor-precession data.It is concluded that this combined interaction of the external magnetic field and the damage-induced, electric field gradients quantitatively accounts for the observed dependence of Larmor-precession amplitudes on time, temperature and field strength. The significance of the experimental time range for the appearance and thus for the interpretation of this dependence is demonstrated.     

Nonlinear oscillation characteristics of magnetic microbubbles under acoustic and magnetic fields

Microbubbles loaded with magnetic nanoparticles (MMBs) have attracted increasing interests in multimode imaging and drug/gene delivery and targeted therapy. However, the dynamic behaviors generated in diagnostic and therapeutic applications are not clear. In the present work, a novel theoretical model of a single MMB was developed, and the dynamic responses in an infinite viscous fluid were investigated under simultaneous exposure to magnetic and acoustic fields. The results showed that the amplitude reduces and the resonant frequency increases with the strength of the applied steady magnetic field and the susceptibility of the magnetic shell. However, the magnetic field has a limited influence on the oscillating. It is also noticed that the responses of MMB to a time-varying magnetic field is different from a steady magnetic field. The subharmonic components increase firstly and then decrease with the frequency of the magnetic field and the enhanced effect is related to the acoustic driving frequency. It is indicated that there may be a coupling interaction effect between the acoustic and magnetic fields.     

行波管中静态轨迹的研究

 研究了行波管中均匀聚焦磁场和周期永磁聚焦磁场对轨迹波动的影响。推导了这两种磁场下轨迹波动周期和幅值,讨论了磁场强度对轨迹的影响。解释了聚焦磁场存在小的波动的原因,并且通过计算得出小波动的周期为磁场周期的1/2,揭示了在周期永磁聚焦磁场下,电子轨迹近似等效于周期为周期永磁聚焦磁场1/2的小波动和均匀磁场形成的波动轨迹的叠加。利用电子科技大学编写的微波管模拟套装中的3维注-波互作用模块进行了静态轨迹计算,验证了理论推导。     

Reduction of Effect of Concomitant Gradients in Low Magnetic Field MRI via Optimization of Gradient Magnetic System

The effect of concomitant magnetic fields emerging in conjunction with encoding gradients, which is important in the process of the magnetic resonance imaging in low fields, has been considered. The manifestations of concomitant magnetic fields in a concrete gradient system, namely in the system of two coaxial gradient coils, have been thoroughly analyzed. It has been suggested to improve the gradient system via optimization of the interspace between coils on the basis of the standard criterion of the minimum of root-mean-square deviation of the encoding field dependence from a linear one. It has been shown that the optimal interspace is not the Maxwell condition.