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At least 127 records · Page 7Linked to original sources

PubMed1993

A cylindrically symmetric magnetic shield for a large-bore 3.0 Tesla magnet.

A 3.0 Tesla, 0.80-m bore magnet replaced our previous 1.9 Tesla, 0.76-m magnet. The 3.0 Tesla replacement magnet had a dipole moment of 1.7 with respect to that of the 1.9 Tesla magnet. The pre-existing cylindrically symmetric passive steel shielding was modified to confine the fringe fields of the replacement magnet to match those of the previous magnet. A cylindrically symmetric inner shield insert of about 20,000 kg was designed, fabricated, and installed. Upon energization of the magnet, the combined shielding met all design criteria. Alternative designs, optimization of cylindrically symmetric designs, and costs of fabrication, are presented herein.

Carbon↗

PubMed2006

Orthodontic springs and auxiliary appliances: assessment of magnetic field interactions associated with 1.5 T and 3 T magnetic resonance systems.

The objective of this paper is to evaluate magnetic field interactions at 1.5 and 3 T for 20 orthodontic devices used for fixed orthodontic therapy. Twenty springs and auxiliary parts made from varying ferromagnetic alloys were tested for magnetic field interactions in the static magnetic field at 1.5 and 3 T. Magnetic translational force F(z) (in millinewtons) was evaluated by determining the deflection angle beta [American Society for Testing and Materials (ASTM standard test method)]. Magnetic-field-induced rotational force F(rot) was qualitatively determined using a five-point scale. beta was found to be >45 degrees in 13(15) devices at 1.5(3) T and translational force F(z) exceeded gravitational force F(g) on the particular object [F(z) 10.17-261.4 mN (10.72-566.4 mN) at 1.5(3) T]. F(z) was found to be up to 24.1(47.5)-fold higher than F(g) at 1.5(3) T. Corresponding to this, F(rot) on the objects was shown to be high at both field strengths (> or = +3). Three objects (at 1.5 T) and one object (at 3 T) showed deflection angles <45 degrees , but F(rot) was found to be > or = +3 at both field strengths. For the remaining objects, beta was below 45 degrees and torque measurements ranged from 0 to +2. Of 20 objects investigated for magnetic field interactions at 1.5(3) T, 13(15) were unsafe in magnetic resonance (MR), based on the ASTM criteria of F(z). The implications of these results for orthodontic patients undergoing MRI are discussed.

Electromagnetic Fields↗

PubMed1995

Effective targeting of magnetic radioactive 90Y-microspheres to tumor cells by an externally applied magnetic field. Preliminary in vitro and in vivo results.

Magnetic biodegradable poly(lactic acid) microspheres that incorporate both magnetite and the beta-emitter 90Y were prepared. By applying a directional external magnetic field gradient in excess of 0.02 Tesla/cm across a 96-well plate containing neuroblastoma cells incubated with the 90Y magnetite loaded microspheres, the radiation dose to the cells could be enhanced or reduced relative to the dose from a uniform loading of the well with 90Y-DTPA. Using the MTT assay, cell survival was measured for the magnetic field directed from above (cell sparing) and from below (cell targeting) the well plate, resulting in 65 +/- 8% or 18 +/- 5% survival respectively. This method was then applied to an in vivo murine tumor model. The biodistribution of intraperitoneally injected magnetic radioactive microspheres, after 24 h in mice, showed that 73 +/- 32% of the radioactivity was found on the subcutaneous tumor that had a rare earth magnet fixed above it. In contrast, the tumor radioactivity with no attached magnet was 6 +/- 4%. Magnetically targeted radiopolymers such as 90Y-microspheres show great promise for regional or intracavitary radiotherapy.

Animals↗

PubMed2005

Crystal structures and magnetic properties of complexes of M(II)Cl2 (M = Cu, Ni, and Co) coordinated with 4-(N-tert-butyloxyamino)-2-(methoxymethylenyl)pyridine: 2D magnetic anisotropy of the aminoxyl-Co(II) complex in the crystalline state.

Three metal complexes, [M(II)Cl2(4NOPy-OMe)2] (M = Cu (1), Ni (2), and Co (3)), were prepared by mixing the corresponding metal chloride and 4-(N-tert-butyloxyamino)-2-(methoxymethylenyl)pyridine, 4NOPy-OMe, in 1:2 ratio. Complex 1 has two structures (complexes A and B) with similar coordination geometries, compressed octahedrons. In the crystal structure, complexes A and B locate alternately in short distances (C(radical)...C(beta) = 3.17 and 3.23 A) to form a 1-D chain structure. Complexes 2 and 3 are isomorphous and have a slightly distorted octahedral structure. In the crystal structure, both complexes have intermolecular short contacts (C(radical)...C(alpha) = 3.46 and 3.52 A for 2 and 3, respectively) to form the 2-D structures. The temperature dependence of the chi(mol)T values for the three complexes indicated that the magnetic interactions between the radicals and the metal ions within the complexes were ferromagnetic. By fitting a modified Fisher 1-D model to the data of the chi(mol)T vs T plot for 1, we estimated the intra- and intermolecular (intrachain) exchange coupling constants to be J1/kB = 60.2 and J2/kB = -7.02 K, respectively. On the other hand, complexes 2 and 3 showed steep increases of the chi(mol)T value below ca. 3 K, indicating that the long-range magnetic ordering is operating. The 1/chi(mol) vs T plot for 2 was analyzed by a Curie-Weiss model to give theta = 6.25 K and C = 2.02 cm3 K mol(-1) with g(Ni) = 2.25. Complex 3 was investigated in more detail using an orientated sample. Magnetic behavior strongly depends on the direction of the applied field, in which the c axis perpendicular to the ab plane is an easy axis for magnetization. Direct current (dc) and alternating current (ac) magnetic susceptibility measurements revealed that complex 3 had a magnetic phase transition of T(c) = 2.14 K and exhibited a glasslike magnetic behavior below T(c).

Cobalt↗

PubMed1980

Magnetization curves of haemoproteins measured by low-temperature magnetic-circular-dichroism spectroscopy.

The magnetic-circular-dichroism (m.c.d.) spectra of methymyoglobin cyanide and oxidized horse heart cytochrome c were measured in the region of the Soret band over a range of temperatures from 1.5 to 50 K and in fields from 0 to 5T. A similar study has been made with reduced bovine heart cytochrome c oxidase, which contains one high-spin ferrous haem, namely a3. M.c.d. magnetization curves characteristic of an isolated Kramer's ground state with spin S = 1/2. These curves contrast with the magnetization curve of the high-spin ferrous haem with spin S = 2. The electronic ground state of the latter compound contains zero-field components that are thermally accessible over the temperature range of the experiment. Hence the magnetization curves are a complex nested set. The magnetization curves of the S = 1/2 proteins were analysed and it is shown that it is possible to make estimates of the ground-state g-factors even in the presence of rhombic anisotropy, provided that some knowledge of the polarizations of the electronic transitions is available. The striking difference between the m.c.d. magnetization curves of a simple S = 1/2 paramagnet and magnetically complex ground state should prove extremely useful when m.c.d. spectroscopy is sued to probe the magentic properties of metal centres in proteins, and should have wide application beyond the field of haemoproteins.

Circular Dichroism↗

PubMed1999

Patient positioning using artificial intelligence neural networks, trained magnetic field sensors and magnetic implants.

The purpose of this study was to evaluate the precision of a sensor and to ascertain the maximum distance between the sensor and the magnet, in a magnetic positioning system for external beam radiotherapy using a trained artificial intelligence neural network for position determination. Magnetic positioning for radiotherapy, previously described by Lennernäs and Nilsson, is a functional technique, but it is time consuming. The sensors are large and the distance between the sensor and the magnetic implant is limited to short distances. This paper presents a new technique for positioning, using an artificial intelligence neural network, which was trained to position the magnetic implant with at least 0.5 mm resolution in X and Y dimensions. The possibility of using the system for determination in the Z dimension, that is the distance between the magnet and the sensor, was also investigated. After training, this system positioned the magnet with a mean error of maximum 0.15 mm in all dimensions and up to 13 mm from the sensor. Of 400 test positions, 8 determinations had an error larger than 0.5 mm, maximum 0.55 mm. A position was determined in approximately 0.01 s.

Humans↗

PubMed2004

Analytical electron microscopy and electron holography on microstructures and magnetic domain structures of Sm-Co 2:17 magnets.

Microstructures and magnetic domain structures of precipitation-hardened Sm-Co permanent magnets were systematically investigated by analytical electron microscopy and electron holography. By an elemental mapping method with energy-dispersive X-ray spectroscopy, the change in the local distribution of additive elements, i.e. Cu, Fe and Zr, in Sm-Co magnets with various heat treatments was visualized and the enrichment of Zr in the Z-phase with a width of approximately 1 nm was clarified directly. Detailed analysis with electron holography revealed that considerable fluctuation in the distribution of lines of magnetic flux in the step-aged magnet was due to the chemical partitioning of additives and resulted in magnetic hardening during the magnetization process.

Holography↗

PubMed1997

Measurement of AC magnetic field distribution using magnetic resonance imaging.

Electric currents are applied to body in numerous applications in medicine such as electrical impedance tomography, cardiac defibrillation, electrocautery, and physiotherapy. If the magnetic field within a region is measured, the currents generating these fields can be calculated using the curl operator. In this study, magnetic fields generated within a phantom by currents passing through an external wire is measured using a magnetic resonance imaging (MRI) system. A pulse sequence that is originally designed for mapping static magnetic field inhomogeneity is adapted. AC current in the form of a burst sine wave is applied synchronously with the pulse sequence. The frequency of the applied current is in the audio range with an amplitude of 175-mA rms. It is shown that each voxel value of sequential images obtained by the proposed pulse sequence is modulated similar to a single tone broadband frequency modulated (FM) waveform with the ac magnetic field strength determining the modulation index. An algorithm is developed to calculate the ac magnetic field intensity at each voxel using the frequency spectrum of the voxel signal. Experimental results show that the proposed algorithm can be used to calculate ac magnetic field distribution within a conducting sample that is placed in an MRI system.

Algorithms↗

PubMedDate not supplied

Enhancement of electron beam dose distributions by longitudinal magnetic fields: Monte Carlo simulations and magnet system optimization.

A Monte Carlo electron-photon transport code was developed in order to determine the effects of static, longitudinal, magnetic fields on dose distributions produced by high-energy electron beams, and to optimize the design of a superconducting magnet system. As a result of these simulations, a 20-cm-i.d., 30-cm-o.d., 15-cm-tall, single-coil, magnet system was designed that could be incorporated into a mobile treatment table for use with a standard radiation therapy accelerator. Operating at a current density of 18 kA/cm2, the magnet would produce field strengths of 1-4 T in the phantom and 0.01 T at the accelerator exit window. Magnetically enhanced dose distributions, calculated for 20- and 30-MeV electron beams, show a pronounced Bragg peak, steeper gradients to the sides and rear, and a roughly fourfold increase in the peak dose to entrance dose ratios relative to those similarly calculated without a magnetic field. These magnetically enhanced dose distributions have the potential for sparing intervening tissue when high-energy electrons are used for the treatment of deep-seated tumors.

Electrons↗

PubMed2004

Initial experience with remote catheter ablation using a novel magnetic navigation system: magnetic remote catheter ablation.

BACKGROUND: Catheters are typically stiff and incorporate a pull-wire mechanism to allow tip deflection. While standing at the patient's side, the operator manually navigates the catheter in the heart using fluoroscopic guidance. METHODS AND RESULTS: A total of 42 patients (32 female; mean age, 55+/-15 years) underwent ablation of common-type (slow/fast) or uncommon-type (slow/slow) atrioventricular nodal reentrant tachycardia (AVNRT) with the use of the magnetic navigation system Niobe (Stereotaxis, Inc). It consists of 2 computer-controlled permanent magnets located on opposite sides of the patient, which create a steerable external magnetic field (0.08 T). A small magnet embedded in the catheter tip causes the catheter to align and to be steered by the external magnetic field. A motor drive advances or retracts the catheter, enabling complete remote navigation. Radiofrequency current was applied with the use of a remote-controlled 4-mm, solid-tip, magnetic navigation-enabled catheter (55 degrees C, maximum 40 W, 60 seconds) in all patients. The investigators, who were situated in the control room, performed the ablation using a mean of 7.2+/-4.7 radiofrequency current applications (mean fluoroscopy time, 8.9+/-6.2 minutes; procedure duration, 145+/-43 minutes). Slow pathway ablation was achieved in 15 patients, whereas slow pathway modulation was the end point in the remaining patients. There were no complications. CONCLUSIONS: The Niobe magnetic navigation system is a new platform technology allowing remote-controlled navigation of an ablation catheter. In conjunction with a motor drive unit, this system was used successfully to perform completely remote-controlled mapping and ablation in patients with AVNRT.

Adult↗

PubMedDate not supplied

Magnetic nanostructures and materials in magnetic random access memory.

The advances in magnetic random access memory provide a remarkable showcase for the rapid development and application of nanodevices. Several aspects of state-of-the-art magnetic nanoscience and nanotechnology are developed and utilized in this single device. Current magnetic random access memory design is built upon the discovery and understanding of physics issues such as giant magnetoresistance, spin-dependent tunneling, exchange bias, and magnetic anisotropy in small elements. Successful magnetic random access memory development requires future research in some of the key areas involving nanotechnology. For example, the uniformity of the barrier thickness across the entire device and magnetic switching stability of a nanosized element are challenging issues that lie ahead. The spin degree of freedom of electrons is an added dimension that is both unique and useful in electronic transport and information technology. Nonvolatile, high-density, high-speed, and low-power magnetic random access memory is one of the first examples of the application of spintronics.

Computer Systems↗

PubMed2003

Targeted gene delivery to human osteosarcoma cells with magnetic cationic liposomes under a magnetic field.

Gene delivery using cationic liposomes results in relatively low transfection, especially under in vivo conditions. This system, however, can overcome some of the problems associated with viral delivery systems. The present study was carried out in order to improve the transfection efficiency of cationic liposomes by preparing magnetic cationic liposomes (MCL). Small MCL approximately 40 nm in diameter and incorporating one or two magnetite particles were prepared with phosphatidylethanolamine and 3beta-[N-(N', N'-dimethylaminoethane)-carbamoyl] cholesterol. The efficiency of MCL in gene delivery was evaluated by using plasmid DNA containing a luciferase reporter gene and human osteosarcoma Saos-2 cells. Without a magnetic field, maximum luciferase activity was observed when DNA was mixed with MCL at a 1:5 ratio and incubated with cells for 6 h. Under a magnetic field, maximum luciferase activity was achieved by 30-min magnetic induction. This improvement in transfection efficiency by magnetic induction was approximately 3.5-fold. The feasibility of this active transgenic system was further shown by measuring apoptosis rates after transfection of the p53 gene to Saos-2 cells. Apoptosis rates increased to 18.9% from 2.4% by magnetic induction. In conclusion, a gene delivery system including MCL and magnetic induction was found to achieve rapid and enhanced gene delivery in vitro. Such a gene delivery system may be applicable under in vivo conditions, and is expected to offer numerous clinical advantages.

Bone Neoplasms↗

PubMed1994

[Limits of exposure to magnetic fields and national standards for the safety of the patient during magnetic resonance examination].

This review summarizes experimental bases, limitations and rationale of guide-lines and recommendations adopted by national health authorities for the safety of patients exposed to the magnetic fields utilized during magnetic resonance (MR) examinations: static magnetic field (B0); time-varying magnetic field gradients (dB/dt) and radiofrequency magnetic fields. Although the mechanisms of interaction of these fields with the living matter are today sufficiently understood to identify the main categories of potentially associated health hazards, epidemiological studies and dosimetric concepts are still far from the levels of definition of protection standards adopted in the field of ionizing radiations. The definition of safety thresholds and levels of exposure to magnetic fields during MR examinations has evolved in the last decade and the matter is still under study today. This article summarizes the main steps in this continuous updating process and discusses limitations and criteria, which recently suggested the adoption in some Countries (including Italy) of a two-tier system for the levels of exposure of patients to the magnetic fields generated by clinical MR equipment.

Biophysical Phenomena↗

PubMed2005

Towards a better understanding of magnetic interactions within m-phenylene alpha-nitronyl nitroxide and imino nitroxide based radicals, part III: Magnetic exchange in a series of triradicals and tetraradicals based on the phenyl acetylene and biphenyl coupling units.

The present work completes and extends our previous reports on the determination of the magnetic ground state and on the strength of the through bond exchange coupling within series of biradicals. This knowledge was subsequently exploited for the analysis of the magnetic interactions in their crystals. We report here the studies of series of triradicals incorporating alpha-nitronyl nitroxides (NN) or alpha-imino nitroxides (IN) as terminal radical fragments connected through a m-phenylene coupling unit in one case and a phenyl acetylene unit in other case. Tetraradical derivatives have also been studied. The studies of isolated molecules (EPR in solution and DFT calculations) allow the assessment of the magnetic interactions through the magnetic coupling unit. All triradical derivatives are found to exhibit a quartet ground state, whereas a singlet ground state is determined for the tetraradical. This last result reinforces previous findings that the singlet ground state is favoured in related biradicals involving similar m-phenylene couplers. Moreover, the through bond magnetic exchange coupling for the ortho-meta connectivity could be demonstrated as being ferromagnetic, thus ascertaining our previous hypotheses. The magnetic properties of the triradicals and tetraradicals in their solid state have been rationalized by using a previously proposed methodology, allowing to identify the most relevant magnetic pathways.

Journal Article↗

PubMed2005

Disruption of magnetic orientation in hatchling loggerhead sea turtles by pulsed magnetic fields.

Loggerhead sea turtles (Caretta caretta) derive both directional and positional information from the Earth's magnetic field, but the mechanism underlying magnetic field detection in turtles has not been determined. One hypothesis is that crystals of biogenic, single-domain magnetite provide the physical basis of the magnetic sense. As a first step toward determining if magnetite is involved in sea turtle magnetoreception, hatchling loggerheads were exposed to pulsed magnetic fields (40 mT, 4 ms rise time) capable of altering the magnetic dipole moment of biogenic magnetite crystals. A control group of turtles was treated identically but not exposed to the pulsed fields. Both groups of turtles subsequently oriented toward a light source, implying that the pulsed fields did not disrupt the motivation to swim or the ability to maintain a consistent heading. However, when swimming in darkness under conditions in which turtles normally orient magnetically, control turtles oriented significantly toward the offshore migratory direction while those that were exposed to the magnetic pulses did not. These results are consistent with the hypothesis that at least part of the sea turtle magnetoreception system is based on magnetite. In principle, a magnetite-based magnetoreception system might be involved in detecting directional information, positional information, or both.

Animals↗

PubMed1999

The excitation site of the accessory nerve to the magnetic stimulation--the relationship between the orientation of the magnetic field and the excitation site.

OBJECTIVE: The relationship between the accessory nerve excitation site and the magnetic field direction was investigated to prove whether the cranial nerve excitation site to the transcranial magnetic stimulation is constant or not. METHODS: Compound muscle action potentials (CMAPs) elicited by the transcranial magnetic stimulation were recorded from the trapezius muscles of 7 adult cats. The waveforms of CMAPs were detected before craniectomy, after craniectomy, and after cutting the accessory nerve at the C1, at the jugular tubercle, and at the jugular foramen. The optimal orientation was determined by rotating the coil clockwise in increments of 22.5 degrees from the rostral direction. RESULTS: The accessory nerve was stimulated by the magnetic stimulation at the C1, at the jugular tubercle or at the jubular foramen, and these excitation sites varied with coil orientation. The average angles of the optimal orientation of the magnetic coil were 77.1 degrees for C1, 122.1-263.6 degrees for the jugular tubercle, and 308.6-32.1 degrees for the jugular foramen. CONCLUSIONS: The accessory nerve excitation site varied with the orientation of the magnetic coil. This study suggested the possibility of a variety of the cranial nerve excitation sites to the transcranial magnetic stimulation.

Accessory Nerve↗

PubMed2001

Influence of Jahn-Teller coupling on the magnetic properties of transition metal complexes with orbital triplet ground terms: magnetization and electronic Raman studies of the titanium(III) hexa-aqua cation.

Magnetization and electronic Raman data are presented for salts of the type Cs[Ga:Ti](SO(4))(2) x 12H(2)O, which enable a very precise definition of the electronic structure of the [Ti(OH(2))(6)](3+) cation. The magnetization data exhibit a spectacular deviation from Brillouin behavior, with the magnetic moment highly dependent on the strength of the applied field at a given ratio of B/T. This arises from unprecedented higher-order contributions to the magnetization, and these measurements afford the determination of the ground-state Zeeman coefficients to third-order. The anomalous magnetic behavior is a manifestation of Jahn-Teller coupling, giving rise to low-lying vibronic states, which mix into the ground state through the magnetic field. Electronic Raman measurements of the 1%-titanium(III)-doped sample identify the first vibronic excitation at approximately 18 cm(-1), which betokens a substantial quenching of spin-orbit coupling by the vibronic interaction. The ground-state Zeeman coefficients are strongly dependent on the concentration of titanium(III) in the crystals, and this can be modeled as a function of one parameter, representing the degree of strain induced by the cooperative Jahn-Teller effect. This study clearly demonstrates the importance that the Jahn-Teller effect can have in governing the magnetic properties of transition metal complexes with orbital triplet ground terms.

Journal Article↗

PubMed2001

Magnetic-field-dependent heat capacity of the single-molecule magnet [Mn(12)O(12)(O(2)CEt)(16)(H(2)O)(3)].

Accurate heat capacities of the single-molecule magnet [Mn(12)O(12)(O(2)CEt)(16)(H(2)O)(3)] were measured from 0.3 to 311 K by adiabatic calorimetry without an external magnetic field. Heat-capacity anomalies were separated by assuming several contributions including lattice vibration, magnetic anisotropy, and hyperfine splitting. Among them, a tiny thermal anomaly between 1 and 2 K is attributable to the presence of Jahn-Teller isomers. The heat capacities of the polycrystalline sample were also measured with applied magnetic fields from 0 to 9 T in the 2-20 K temperature region by the relaxation method. With an applied magnetic field of up to 2 T, a steplike heat-capacity anomaly was observed around the blocking temperature T(B) approximately 3.5 K. The magnitude of the anomaly reached a maximum at 0.7 T. With a further increase in the magnetic field, the step was decreasing, and finally it disappeared above 3 T. The step at T(B) under 0.7 T can be roughly accounted for by assuming that a conversion between the up-spin and down-spin states is allowed above T(B) by phonon-assisted quantum tunneling, while it is less effective below T(B). Excess heat capacity under a magnetic field revealed a large heat-capacity hump around 14 K and 2 T, which would be attributed to a thermal excitation from the S = 9 ground state to the spin manifold with different S values, where S is the total spin quantum number.

Journal Article↗