Effects of static and radiofrequency magnetic field inhomogeneity in ultra-high field magnetic resonance imaging

To characterize the severe static (B(0)) and radiofrequency (B(1)) magnetic field inhomogeneity in ultra-high field (> or =7 T) magnetic resonance imaging, gradient echo (GE) and spin echo (SE) images of in vivo and postmortem human brains were acquired. The B(0) and B(1) inhomogeneity were experimentally mapped and/or numerically simulated, and correlated with the image artifacts. Whereas B(0) inhomogeneity affects predominantly GE images near air/tissue interfaces, B(1) inhomogeneity affects SE images more severely and shows non-intuitive patterns. Mapping of the B(0) and B(1) inhomogeneity is important in characterizing image artifacts. This will help develop better B(0) and B(1) inhomogeneity correction methods.     

Determining pore sizes using an internal magnetic field

A concept is proposed to measure the pore size length scale by the internal magnetic field (B_i) in porous materials. The spatial distribution of the magnetic field inhomogeneity, a result of the magnetic susceptibility contrast between the porous material and the fluid, reflects the underlying pore geometry. Diffusion in B_i causes the initial decay of magnetization. At long times, the effect of B_i saturates when the diffusion length reaches a characteristic pore size. This method is independent of surface spin relaxation in determining pore sizes. Nuclear magnetic resonance experiments on packed glass beads and sedimentary rock samples will be presented.     

Anisotropic diffusion of metabolites in peripheral nerve using diffusion weighted magnetic resonance spectroscopy at ultra-high field

Although the diffusivity and anisotropy of water has been investigated thoroughly in ordered axonal systems (i.e., nervous tissue), there have been very few studies on the directional dependence of diffusion of metabolites. In this study, the mean apparent diffusion coefficient (Trace/3 ADC) and fractional anisotropy (FA) values of the intracellular metabolites N-acetyl aspartate (NAA), creatine and phosphocreatine (tCr), choline (Cho), taurine (Tau), and glutamate and glutamine (Glx) were measured parallel and perpendicular to the length of excised frog sciatic nerve using a water suppressed, diffusion-weighted, spin-echo pulse sequence at 18.8T. The degree of anisotropy (FA) of NAA (0.41+/-0.09) was determined to be less than tCr (0.59+/-0.07) and Cho (0.61+/-0.11), which is consistent with previously reported human studies of white matter. In contrast, Glx diffusion was found to be almost isotropic with an FA value of 0.20+/-0.06. The differences of FA between the metabolites is most likely due to their differing micro-environments and could be beneficial as an indicator of compartment specific changes with disease, information not readily available with water diffusion.     

Plasma diffusion across a magnetic field

Using a simple model of a slowly diffusing plasma across a strong magnetic field, it is demonstrated that plasma mass and energy evolves from an initially given density and temperature distribution into isothermal state with a self-similar diffusion profile that depends only on its initial mass and energy.     

A rotating probe for measuring the inhomogeneity of a magnetic field

The theory is given of a rotating probe for measuring the inhomogeneity of a magnetic field. The design of the instrument and its calibration are described, giving its sensitivity and some methods of use such as the measurement of saturation magnetization, the Curie point and hysteresis loops.     

Hole-burning diffusion measurements in high magnetic field gradients

We describe methods for the measurement of translational diffusion in very large static magnetic field gradients by NMR. The techniques use a "hole-burning" sequence that, with the use of fringe field gradients of 42 T/m, can image diffusion along one dimension on a submicron scale. Two varieties of this method are demonstrated, including a particularly efficient mode called the "hole-comb," in which multiple diffusion times comprising an entire diffusive evolution can be measured within the span of a single detected slice. The advantages and disadvantages of these methods are discussed, as well as their potential for addressing non-Fickian diffusion, diffusion in restricted media, and spatially inhomogeneous diffusion.     

磁性液体磁悬浮原理演示实验研究

基于磁性液体的Bernoulli方程推导了磁性液体中非磁性物体所受的磁悬浮力,通过演示实验研究了2种磁性液体对不同密度非磁性物体的磁悬浮情况．磁性液体中非磁性物体所受的磁悬浮力受磁性液体的磁化率、磁场强度、磁场梯度的影响．相同条件下,随磁场强度和磁场梯度增强,M FP-1磁性液体分别在不同电流时将6种非磁性双锥体浮起,当电流达到3．00 A时,还出现了Rosensweig尖峰;MFP-2磁性液体的磁化率小于MFP-1磁性液体,仅浮起了聚氟乙烯、玻璃、铝双锥体．随着电流增大,中心磁场梯度较强,边缘磁场梯度较弱,出现非磁性双锥体向侧壁移动现象．     

Polarization and transmission of microwaves in the magnetic suspension in the applied magnetic field

The production method of magnetic suspension consisting of ferromagnetic particles dispersed in cedarwood oil is presented at the beginning of this article. Next, the set-up for microwaves generation using a klystron is described. The main part of this paper concerning microwave transmission and polarization during its passage in samples of the produced magnetic suspension placed in a magnetic field is based on the following parameters: induction of this field, filling factor of magnetic suspension by ferromagnetic particles, dimensions of particles, viscosity of liquid carrier, and ratio of the magnetic field changes. Conducted investigations show that microwaves are damped and polarized in these magnetic suspensions. Obtained results are discussed and observed effects are explained by ordering of ferromagnetic particles in magnetic suspension by applied magnetic field.     

Simulation of the influence of magnetic field inhomogeneity and distortion correction in MR imaging

We describe a technique for simulation and correction of the effects of an arbitrary distribution of undesired components of the static and gradient magnetic fields. This technique is applicable to direct Fourier NMR imaging. The mathematical basis and details of this technique are fully described. Computer simulation demonstrates the effectiveness of this method.     

Influence of a magnetic field on diffusion and thermal diffusion in gaseous mixtures

The influence of a magnetic field on the transport properties of binary mixtures of a polyatomic and a noble gas is studied theoretically. Using an inverse operator technique, first thermal conductivity and viscosity are treated. Then diffusion and thermal diffusion are discussed in detail, with special emphasis on the composition dependence. A relation connecting the magnitudes of the field effects on thermal conductivity, diffusion, and thermal diffusion is derived. This relation is used to estimate the field effect on diffusion.     

Study of myocardial fiber pathway using magnetic resonance diffusion tensor imaging

The purpose of this study was to investigate myocardial fiber pathway distribution in order to provide supplemental information on myocardial fiber architecture and cardiac mechanics. Diffusion tensor imaging (DTI) with medium diffusion resolution (15 directions) was performed on normal canine heart samples (N=6) fixed in formalin. With the use of diffusion tensor fiber tracking, left ventricle (LV) myocardial fiber pathways and helix angles were computed pixel by pixel at short-axis slices from base to apex. Distribution of DTI-tracked fiber pathway length and number was analyzed quantitatively as a function of fiber helix angle in step of 9 degrees . The long fiber pathways were found to have small helix angles. They are mostly distributed in the middle myocardium and run circumferentially. Fiber pathways tracked at the middle and upper LV are generally longer than those near the apex. Majority of fiber pathways have small helix angles between -20 degrees and 20 degrees , dominating the fiber architecture in myocardium. Likely, such myocardial fiber pathway measurement by DTI may reflect the spatial connectiveness or connectivity of elastic myofiber bundles along their preferential pathway of electromechanical activation. The dominance of the long and circumferentially running fiber pathways found in the study may explain the circumferential predominance in left ventricular contraction.     

Interpretation of restricted diffusion in sandstones with internal field gradients.

We report on experiments to characterize internal magnetic field gradients that are caused by magnetic susceptibility differences between the solid phase and the fluids filling the pore space. Our measurements focus on low-field relaxometry of brine and oil in sandstones from various reservoirs around the world. Our results show the need to understand the dependence of internal field gradients on diffusion length, pore size- and fluid distribution in order to predict the impact of internal gradients on the interpretation of NMR experiments.     

Energy absorption by a magnetic nanoparticle suspension in a rotating field

Heat generation by viscous dissipation in a dilute suspension of single-domain ferromagnetic particles in a rotating magnetic field is analyzed by assuming that the suspended particles have a high magnetic rigidity. The problem is solved by using a kinetic approach based on a rotational diffusion equation. Behavior of specific loss power (SLP) as a function of field strength H and frequency ω is examined at constant temperature. SLP increases as either of these parameters squared when the other is constant, eventually approaching a saturation value. The function SLP(H, ω) can be used to determine optimal and admissible ranges of magnetically induced heating.     

Study of pediatric brain development using magnetic resonance imaging of anisotropic diffusion

The properties of water diffusion in human brain tissue can be characterized by diffusion tensors computed from diffusion weighted magnetic resonance images. Since these properties are strongly determined by the structural and geometrical characteristics of the tissue, the maturation process of white matter and gray matter tissue can be expected to be reflected in these images and derived tensor quantities. The purpose of this work was therefore to study the development of pediatric brain in terms of changes occurring in the observed diffusion behavior. Echo planar diffusion tensor imaging was performed on 22 (10 females and 12 males) full term newborn and infant patients, diagnosed in retrospect as neurologically healthy. The subjects were subdivided in three age categories. A number of quantities based on the diffusion images were calculated for each tissue type and age category, and the ability of these quantities to provide sensitive and consistent information about the tissue differences and evolution was evaluated. The results clearly illustrate that the rotationally invariant quantities (e.g., the highest diffusivity, anisotropy ratio and volume ratio) are superior to the rotationally variant ones (e.g., ADCs measured along the three axes of the magnet) often used in the clinic. On the basis of the anisotropy ratio and the volume ratio indices, a correlation between the white matter maturation and the evolution of the diffusion anisotropy could be established. The same quantities did not exhibit any age dependence for the gray matter tissues.     

Forced diffusion in magnetic fluids under the influence of a strong magnetic field gradient

We have investigated the forced diffusion of magnetic nanoparticles suspended in a carrier liquid under the influence of a magnetic field gradient. A cylindrical layer of the suspension was exposed to an azimuthal magnetic field with radial gradient. The radial distribution of the concentration of magnetic particles was determined for different times. The obtained experimental data are compared with a numerical solution of the diffusion equation and good agreement has been observed.