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At least 253 records · Page 14Linked to original sources
PubMed2004

Algebraic reconstruction for 3D magnetic resonance-electrical impedance tomography (MREIT) using one component of magnetic flux density.

Magnetic resonance-electrical impedance tomography (MREIT) algorithms fall into two categories: those utilizing internal current density and those utilizing only one component of measured magnetic flux density. The latter group of algorithms have the advantage that the object does not have to be rotated in the magnetic resonance imaging (MRI) system. A new algorithm which uses only one component of measured magnetic flux density is developed. In this method, the imaging problem is formulated as the solution of a non-linear matrix equation which is solved iteratively to reconstruct resistivity. Numerical simulations are performed to test the algorithm both for noise-free and noisy cases. The uniqueness of the solution is monitored by looking at the singular value behavior of the matrix and it is shown that at least two current injection profiles are necessary. The method is also modified to handle region-of-interest reconstructions. In particular it is shown that, if the image of a certain xy-slice is sought for, then it suffices to measure the z-component of magnetic flux density up to a distance above and below that slice. The method is robust and has good convergence behavior for the simulation phantoms used.

Electric Impedance↗
PubMed1990

Magnetic susceptibility effects and their application in the development of new ferromagnetic catheters for magnetic resonance imaging.

Newly developed ferromagnetic catheters (Fe-Caths) are more conspicuous than conventional radiographic catheters (Rad-Caths) on magnetic resonance (MR) images because they produce recognizable ferromagnetic signal patterns (FSPs). To determine how MRI parameters influence these patterns, the imaging characteristics of nine Fe-Caths (ferromagnetic concentration 0.01 to 1.0 weight/weight %) were studied systematically and compared with three Rad-Caths. All catheters were studied in stationary and moving phantoms at mid-field (0.38 T) and high-field (1.5 T) strength using spin-echo and gradient-echo pulse sequences. Rad-Caths always produced a signal void. Fe-Caths produced FSPs, the size of which depended on the orientation of the catheter with respect to the main magnetic field, the concentration of ferromagnetic agent in the catheter, and the direction and strength of the frequency encoding gradient. When Fe-Caths were positioned perpendicular to the main magnetic field, they produced FSPs; however, when they were parallel to the main magnetic field, Fe-Caths produced no FSP, thus having a similar appearance to the Rad-Caths. Ferromagnetic catheters produce conspicuous patterns on MR images that depend on catheter orientation in the main magnetic field and vary predictably with the MRI parameters.

Animals↗
PubMed1994

Optimization of an oral magnetic particle formulation as a gastrointestinal contrast agent for magnetic resonance imaging.

RATIONALE AND OBJECTIVES: Magnetically susceptible iron oxide (MSIO) contrast agents for magnetic resonance imaging (MRI) of the gastrointestinal (GI) tract are limited because they produce magnetic susceptibility artifacts. To determine whether oral magnetic particles (WIN 39996) can be an effective MRI contrast agent without producing induced image artifacts, we optimized a liquid formulation of WIN 39996. METHODS: A range of concentrations (25-250 micrograms iron/mL) and viscosities (1-600 cP) was imaged in a phantom at 1.5 T using conventional spin-echo and gradient-recalled echo pulse sequences. Some formulations also contained titanium. RESULTS: All concentrations of WIN 39996 at 1 cP produced susceptibility artifacts. For formulations in the 150 to 600 cP range, the 125 to 150 micrograms/mL concentrations produced signal blackening and magnetic susceptibility image distortion comparable to an air control. Concentrations greater than 150 micrograms/mL were unacceptable because they produced significant susceptibility artifacts, while concentrations less than 125 micrograms/mL were undesirable because they produced insufficient signal blackening. CONCLUSIONS: These preliminary in-vitro studies suggest that an optimized liquid formulation of WIN 39996 can be produced that yields excellent negative contrast without producing image artifacts.

Artifacts↗
PubMed1992

Which disc as visualized by magnetic resonance imaging is actually a source of pain? A correlation between magnetic resonance imaging and discography.

Sixty-three discs in 25 consecutive patients with nonradicular discogenic pain were studied with magnetic resonance imaging and lumbar discography. Sagittal T2 magnetic resonance imaging sequences were blindly classified according to the nuclear signal intensity and to the status of the posterior anulus. A positive discogram had to include abnormal morphology and a provocative pain response of significant intensity and familiar quality. Specific magnetic resonance imaging patterns with a high probability for positive and negative discograms are identified. Also identified are magnetic resonance imaging patterns with intermediate probability, ie, ones that should not be presumed either asymptomatic or symptomatic. In many cases, magnetic resonance imaging could not reliably predict or replace discography.

Back Pain↗
PubMed2004

Hysteresis effect in FePd magnetic stripes studied by coherent soft X-ray resonant magnetic scattering.

An FePd thin film sample, showing magnetic stripe domains as imaged by magnetic force microscopy, has been measured by soft X-ray resonant magnetic scattering in reflection geometry. Illumination with coherent radiation, produced by inserting a 20 microm pinhole in front of the sample, leads to a magnetic speckle pattern in the scattered intensity that gives access to the domain morphology. Application of an in-plane magnetic field for a few seconds gives a strong change in the observed intensity fluctuations, which indicates a large degree of variation between the two patterns taken before and after field exposure. From the speckle pattern we calculate a degree of coherence of beta = 0.5 for the incident beam.

Anisotropy↗
PubMed2001

Improving the homogeneity of the magnetic field in the magnetic search coil technique.

The magnetic search coil technique is used to obtain accurate eye-movement measurements. For the data analysis it must be assumed that the magnetic field does not change over time (apart from the required modulation). To comply with this assumption either no translational movements of the eye coil(s) are allowed or the magnetic field has to be perfectly homogeneous. Both are normally not the case, i.e., measurement errors occur. These errors can be minimized by keeping the magnetic field as homogeneous as possible. Larger field coils are typically chosen to achieve this aim. However, sometimes the experimental setup imposes limitations on the size of the configuration. We present a method for improving the homogeneity of the magnetic field by adding supplementary field coils without changing the size of the configuration.

Equipment Design↗
PubMed2005

Compensation of magnetic disturbances improves inertial and magnetic sensing of human body segment orientation.

This paper describes a complementary Kalman filter design to estimate orientation of human body segments by fusing gyroscope, accelerometer, and magnetometer signals from miniature sensors. Ferromagnetic materials or other magnetic fields near the sensor module disturb the local earth magnetic field and, therefore, the orientation estimation, which impedes many (ambulatory) applications. In the filter, the gyroscope bias error, orientation error, and magnetic disturbance error are estimated. The filter was tested under quasi-static and dynamic conditions with ferromagnetic materials close to the sensor module. The quasi-static experiments implied static positions and rotations around the three axes. In the dynamic experiments, three-dimensional rotations were performed near a metal tool case. The orientation estimated by the filter was compared with the orientation obtained with an optical reference system Vicon. Results show accurate and drift-free orientation estimates. The compensation results in a significant difference (p < 0.01) between the orientation estimates with compensation of magnetic disturbances in comparison to no compensation or only gyroscopes. The average static error was 1.4 degrees (standard deviation 0.4) in the magnetically disturbed experiments. The dynamic error was 2.6 degrees root means square.

Acceleration↗
PubMed2000

Magnetic fields with photon beams: Monte Carlo calculations for a model magnetic field.

Strong transverse magnetic fields can produce very large dose enhancements and reductions in localized regions of a patient under irradiation by a photon beam. We have suggested a model magnetic field which can be expected to produce such large dose enhancements and reductions, and we have carried out EGS4 Monte Carlo calculations to examine this effect for a 6x6 cm2 photon beam of energy 15, 30, or 45 MV penetrating a water phantom. Our model magnetic field has a nominal (center) strength B0 ranging between 1 and 5 T, and a maximum strength within the geometric beam which is 2.2xB0. For all three beam energies, there is significant dose enhancement for B0 = 2 T which increases greatly for B0 = 3 T, but stronger magnetic fields increase the enhancement further only for the 45-MV beam. Correspondingly, there is major reduction in the dose just distal to this region of large dose enhancement, resulting from secondary electrons and positrons originating upstream which are depositing energy in the dose-enhancement region rather than continuing further into the patient. The dose peak region is fairly narrow (in depth), but the magnetic field can be shifted along the longitudinal axis to produce a flat peak region of medium width (approximately 2 cm) or of large width (approximately 4 cm), with rapid dose dropoffs on either side. For the 30-MV beam with B0 = 3 T, we found a dose enhancement of 55% for the narrow-width configuration, 32% for the medium-width configuration, and 23% for the large-width configuration; for the 45-MV beam with B0 = 3 T, the enhancements were quite similar, but for the 15-MV beam they were considerably less. For all of these 30-MV configurations, the dose reductions were approximately 30%, and they were approximately 40% for the 45-MV configurations.

Computer Simulation↗
PubMed2003

Magnetic fields and the melatonin hypothesis: a study of workers chronically exposed to 50-Hz magnetic fields.

Because epidemiological studies report clinical disorders (mainly neurobehavioral alterations and/or cancer) that may be related to diminished melatonin secretion or to changes in its circadian rhythm in subjects living or working in environments exposed to magnetic fields, research on the effects of these fields in humans is particularly important. In this study, we examine the circadian rhythm of melatonin in 15 men exposed chronically and daily for a period of 1-20 yr, in the workplace and at home, to a 50-Hz magnetic field in search of any cumulative effect from those chronic conditions of exposure. The weekly geometric mean of individual exposures ranged from 0.1 to 2.6 microT. The results are compared with those for 15 unexposed men who served as controls (individual exposures ranged from 0.004 to 0.092 microT). Blood samples were taken hourly from 2000 to 0800. Nighttime urine was also collected and analyzed. This work shows that subjects exposed over a long period (up to 20 yr) and on a daily basis to magnetic fields experienced no changes in their plasma melatonin level, their urinary 6-sulfatoxymelatonin level, or the circadian rhythm of melatonin. Our data strongly suggest that magnetic fields do not have cumulative effects on melatonin secretion in humans and thus clearly rebut the "melatonin hypothesis" that a decrease in plasma melatonin concentration (or a disruption in its secretion) explains the occurrence of clinical disorders or cancers possibly related to magnetic fields.

Circadian Rhythm↗
PubMed1997

Patient positioning using magnetic implants and magnetic field sensors.

A phantom study was conducted in order to evaluate the precision and reproducibility of a magnetic field positioning system using magnetic implants and magnetic field sensors. Three implants and three magnetic field sensors were used to position a phantom on a Simulix Y simulator; x-ray films were used as reference. The reproducibility of the position determination of the three magnetic field sensors was checked three times in five different positions in both the X and Y dimensions of the sensors. The phantom was then positioned by the system five times and the displacement errors of four markers in the periphery of the phantom were noted. A reference x-ray film was exposed before the positioning procedure. An error of one millimetre was permitted in each dimension in one sensor. Only the maximum displacement errors of one of the four markers were noted. In the first reproducibility test the system was found to miss 1 mm in one reading out of 90 position readings. In the second part, the mean for the maximum displacement error was 0.46 and 0.6 mm for the maximum frontal and lateral displacements respectively, with a maximal error of 1.3 mm.

Equipment Design↗
PubMed1997

[Study of magnetic microspheres for purging cancer cells from bone marrow. II. Preparation of polystyrene magnetic microspheres. ].

Magnetic microspheres (MS) used to purge tumor cells from human bone marrow were prepared with a two step method. Main factors that can influence the magnetization result were discussed. The MS were coated with polyacrolein which has active aldehyde group. The magnetic material content and the magnetic response of MS were determined and showed that the magnetic material y-Fe(2)O(3) by X-ray diffraction.

Acrolein↗
PubMed2005

Experimental evaluation of the magnetic properties of commercially available magnetic microspheres.

The magnetic properties of 5 commercially available magnetic microsphere samples are tested and compared with those stated by their manufacturers. A suspension of magnetic, iron oxide nanoparticles is studied for comparison. Two of the microsphere samples have magnetic properties which do not support the manufacturer's claims of superparamagnetism. The remaining 3 microsphere samples as well as the nanoparticle suspension are superparamagnetic or ferromagnetic as claimed by the manufacturers. Field cooled and zero field cooled magnetisations indicate that the non-superparamagnetic microsphere samples contain blocked magnetic particles at room temperature. This observation is supported by the open hysteresis loops of the room temperature, field dependent magnetisation measurement. There is a significant paramagnetic component in the superparamagnetic microspheres. This is also present to a lesser extent in a nanoparticle suspension.

Biocompatible Materials↗
PubMed1985

[Diagnosis of kidney diseases with magnetic resonance tomography including the use of nuclear magnetic resonance contrast media].

Magnetic resonance imaging of normal and pathological kidneys and the method of examination are demonstrated. The anatomy of the kidneys is excellently displayed by means of magnetic resonance imaging which can also distinguish between the renal cortex and renal medulla. Renal tumors and cysts can easily be recognized and differentiated by means of magnetic resonance imaging and the extension of the tumor (staging) can be defined. The principles of magnetic resonance imaging contrast media are described as well as the primary results obtained from administration of contrast media in volunteers. At the present time, ultrasound and computed tomography are the first imaging methods in recognition and differentiation of space occupying lesions of the kidneys. Magnetic resonance imaging should only be utilised in complicated cases, e.g. haemorrhagic cysts or when computed tomographic results show a suspected tumor invasion of the renal vein or tumor thrombus in the vena cava.

Diagnosis, Differential↗
PubMedDate not supplied

[Magnetic susceptibility and magnetic capture of cells].

A phenomenon of magnetic capture of human erythrocytes and lymphocytes in paramagnetic and diamagnetic modes of motion near a transversely magnetized wire has been investigated. The values of magnetic susceptibility of numerous individual erythrocytes containing hemoglobin in various forms and lymphocytes were determined using a trajectory analysis. The distribution histograms of erythrocytes in various states and lymphocytes of magnetic susceptibility have been constructed. This histogram for lymphocytes cannot exclude magnetic identification of these cells. The method of determination of physiological concentration of methemoglobin in erythrocytes based on the distribution histogram has been suggested.

Erythrocytes↗
PubMed2005

[From loadstone to magnetic resonance tomography. Historical remarks on magnetism].

This contribution offers a scientific historical view of the discovery, research, and medical use of magnetic phenomena. The milestones of the scientific findings beginning with the very first account of a classic magnetic force and progressing through to the new age discoveries of the magnetic resonance phenomenon with its subsequent use in medicine in the form of magnetic resonance tomography are presented.

History, 18th Century↗
PubMedDate not supplied

From signal to image: magnetic resonance imaging physics for cardiac magnetic resonance.

Magnetic resonance imaging (MRI) is a powerful tool which enables the visualization of anatomy and the assessment of many physiological aspects of organ function. MRI and magnetic resonance angiography and magnetic resonance spectroscopy will play critical roles in cardiac applications during the next millennium. Thus, it is important to have a basic understanding of the most important physical processes in MR--the generation of nuclear magnetic resonance signals and their transformation into images. A conceptual description of these processes is the primary focus of this article. Also discussed are some additional signal manipulations specific to the needs of cardiac MRI, a field readily identified as the most significant in modern MRI technology development.

Heart Diseases↗
PubMed1990

Importance of alignment between local DC magnetic field and an oscillating magnetic field in responses of brain tissue in vitro and in vivo.

The frequency dependence of the electric and magnetic (EM)-field-induced release of calcium ions from an in vitro brain tissue preparation has been shown to be a function of the density of the local DC magnetic field (Bdc). In this study, we demonstrate that the relative orientation of the Bdc and the magnetic component (Bac) of a 315-Hz EM signal (15 Vrms/m and 61 nTrms) are crucial for the induced release to be observed. The induced release occurs only when the Bdc and the Bac are perpendicular, and not when they are parallel. This finding is consistent with a magnetic resonance-like transduction mechanism for the conversion of EM energy into a physicochemical change, and contrasts with the requirement for parallel Bdc and Bac components in the diatom-mobility experiments of Smith et al. A review of the exposure conditions in the rat behavioral experiments conducted by Thomas et al. identifies unhydrated calcium and zinc ions as alternatives to lithium ions as candidates for interaction under parallel magnetic-field orientations but fails to reject perpendicular orientations as an alternative basis for the phenomenon. Investigators that attempt to confirm the rat behavioral experiments should be aware of the conflicting exposure conditions that can be assumed to be operative, and they should design their experiments to test all conditions accordingly.

Animals↗
PubMed2004

Synthesis, nuclear, and magnetic structures and magnetic properties of [Mn3(OH)2(SO4)2(H2O)2].

[Mn(3)(OH)(2)(SO(4))(2)(H(2)O)(2)] and its deuterated analogue were synthesized by a hydrothermal technique and characterized by differential thermal analysis, thermogravimetric analysis, and IR spectroscopy. Its nuclear structure, determined by single-crystal X-ray analysis and Rietveld analysis of neutron powder-diffraction data, consists of a 3D network of chains of edge-sharing Mn(1)O(6), running along the c axis, connected by the apices of Mn(2)O(6) and SO(4) units. It is isostructural to the nickel analogue. Determination of the magnetic structure and measurements of magnetization and heat capacity indicate the coexistence of both magnetic long-range ordering (LRO) and short-range ordering (SRO) below a Néel temperature of 26 K, while the SRO is retained at higher temperatures. The moments of the two independent Mn atoms lie in the bc plane, and that of Mn(1) rotates continuously by 54 degrees towards the c axis on decreasing the temperature from 25 to 1.4 K. While the SRO may be associated with frustration of the moments within a Mn(3) trimer, the LRO is achieved by antiparallel alignment of the four symmetry-related trimers within the magnetic unit cell. A spin-flop field, measured by dc and ac magnetization on a SQUID, is observed at 15 kOe.

Journal Article↗