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PubMed2003

[Influence of pulsating magnetic field used in magnet therapy and magnet stimulation on cortisol secretion in human].

The aim of our study was to test the influence of magnetic fields during magnetotherapy and magnetostimulation over a longer period of time (like in physiotherapy) on cortisol secretion in humans. The study population was divided into two groups: magnetotherapy group (16 men) and magnetostimulation group (10 men). Magnetotherapy in the form of magnetic field induction (2.9 microT; frequency--40 Hz; square wave; bipolar; Magnetronic MF--10 apparatus) was applied for 20 min to the lumbar area in patients with chronic low back pain. Magnetostimulation (Viofor JPS system; M2P2 program; induction--25-80 microT; frequency--200 Hz, complex saw-like shape with a plateau halfway the height of the wave; bipolar) was applied every day for 12 min in patients with the same health problem. In both groups, the procedures were repeated 15 times (about 10:00 a.m.) with weekend breaks. Serum samples were collected at 6:00, 12:00, 16:00 and 24:00 and estimated by the micromethod of chemiluminescence (DPC Poland; Cat. No. LKC01). The circadian profile of cortisol was determined prior to the application, a day and a month after application. The data were analyzed statistically, using paired and unpaired Student's test. Magnetotherapy affects the cortisol secretion in the circadian profile by decreasing its level at 16:00 a day after 15 applications, whereas magnetostimulation by increasing its level at 12:00 a month after 15 applications, which may suggest its long-term effect on hypothalamic-pituitary axis. The comparison of the results indicated that a day after magnetotherapy and magnetostimulation, the circadian curves of cortisol secretion differed significantly by about 100%. All hormone oscillations did not exceed the physiological norms of the circadian cortisol level, not reaching the level so high as in an intense stress. This suggests rather their controlling effect on the cortisol level than their significant stressogenic nature.

Adult↗

PubMed2002

Value of t2-weighted magnetic resonance imaging early after myocardial infarction in dogs: comparison with bis-gadolinium-mesoporphyrin enhanced T1-weighted magnetic resonance imaging and functional data from cine magnetic resonance imaging.

RATIONALE AND OBJECTIVES: Magnetic Resonance Imaging (MRI) has proved to provide noninvasive methods to investigate the functional repercussion of myocardial infarction and to measure infarct size with specific contrast agents. In this study, we evaluate whether the combination of T2-weighted and contrast-enhanced T1-weighted MRI could detect and discern necrotic and ischemic, but salvageable, myocardium. METHODS: Reperfused myocardial infarction was surgically induced in 14 dogs. T1- and T2-weighted MRI was performed 6 hours after administration of the necrosis avid contrast agent Gadophrin-2 at 0.05 mmol/kg. Gradient-echo cine MRI series were performed at baseline and at 6 hours. Quantification of myocardial infarction was performed with triphenyltetrazolium chloride staining. RESULTS: There was a strong correlation between of postcontrast T1-weighted MRI and histomorphometry (r2 = 0.98, P < 0.01). T2-weighted MRI overestimated the infarct size by 10.5% +/- 4.3% of left ventricular area. A good correlation was found between hyperintense areas on T2-weighted images and the percentage of dysfunctional areas on cine MRI (r2 = 0.84, P < 0.01). In regions with increased signal intensity on T2-weighted MRI, a decreased maximal systolic thickening (11.8% +/- 4.9%, P = 0.043) was found. CONCLUSION: In this study, the difference between the hyperintense areas on T2-weighted and enhanced T1-weighted images after myocardial infarction likely represents viable myocardium.

Animals↗

PubMed2004

Perfusion magnetic resonance imaging and magnetic resonance spectroscopy of cerebral gliomas showing imperceptible contrast enhancement on conventional magnetic resonance imaging.

The purpose of the present paper was to evaluate the utility of perfusion MRI in cerebral gliomas showing imperceptible contrast enhancement on conventional MRI, and to evaluate the relationships of perfusion MRI and magnetic resonance (MR) spectroscopic results in these tumours. Twenty-two patients with histopathologically proven cerebral gliomas and showing insignificant contrast enhancement on conventional MR were included in the present study. All patients underwent perfusion MRI and MR spectroscopy on a 1.5-T MR system. Significant differences of the relative cerebral blood volume (rCBV) values and the choline : creatine ratios were noted between low-grade and anaplastic gliomas (P < 0.01). Good correlation was found between the rCBV values and the choline : creatine values (y = 0. 532x + 1.5643; r = 0.67). Perfusion MRI can be a useful tool in assessing the histopathological grade of non-contrast-enhancing cerebral gliomas. Along with MR spectroscopic imaging it can serve as an important technique for preoperative characterization of such gliomas, so that accurate targeting by stereotactic biopsies is possible.

Adult↗

PubMed1995

Magnetic resonance imaging: biophysical basis for the proton magnetic resonance appearance of normal cartilage; other magnetic resonance techniques.

Various attempts to explain the proton magnetic resonance imaging (MRI) appearance of cartilage at the histochemical level are presented, with a discussion of their supporting experimental evidence. Other MR techniques that have been applied to cartilage are presented and their potential for the evaluation of cartilage structure is discussed.

Animals↗

PubMed2004

[Measurements of the flux densities of static magnetic fields generated by two types of dental magnetic attachments and their retentive forces].

OBJECTIVE: To measure and analyze the flux densities of static magnetic fields generated by two types of commonly used dental magnetic attachments and their retentive forces, and to provide guidance for the clinical application of magnetic attachments. METHODS: A digital Gaussmeter was used to measure the flux densities of static magnetic fields generated by two types of magnetic attachments, under four circumstances: open-field circuit; closed-field circuit; keeper and magnet slid laterally for a certain distance; and existence of air gap between keeper and magnet. The retentive forces of the magnetic attachments in standard closed-field circuit, with the keeper and magnet sliding laterally for a certain distance or with a certain air gap between keeper and magnet were measured by a tensile testing machine. RESULTS: There were flux leakages under both the open-field circuit and closed-field circuit of the two types of magnetic attachments. The flux densities on the surfaces of MAGNEDISC 800 (MD800) and MAGFIT EX600W (EX600) magnetic attachments under open-field circuit were 275.0 mT and 147.0 mT respectively. The flux leakages under closed-field circuit were smaller than those under open-field circuit. The respective flux densities on the surfaces of MD800 and EX600 magnetic attachments decreased to 11.4 mT and 4.5 mT under closed-field circuit. The flux density around the magnetic attachment decreased as the distance from the surface of the attachment increased. When keeper and magnet slid laterally for a certain distance or when air gap existed between keeper and magnet, the flux leakage increased in comparison with that under closed-field circuit. Under the standard closed-field circuit, the two types of magnetic attachments achieved the largest retentive forces. The retentive forces of MD800 and EX600 magnetic attachments under the standard closed-field circuit were 6.20 N and 4.80 N respectively. The retentive forces decreased with the sliding distance or with the increase of air gap between keeper and magnet. CONCLUSION: The magnetic attachments have flux leakages. When they are used in patients' oral cavities, if keeper and magnet are not attached accurately, the flux leakage will increase, and at the same time the retentive force will decrease. Therefore the keeper and magnet should be attached accurately in clinical application.

Denture Precision Attachment↗

PubMed2003

Prefrontal cortex transcranial magnetic stimulation does not change local diffusion: a magnetic resonance imaging study in patients with depression.

OBJECTIVE: To determine whether transcranial magnetic stimulation over the left dorsolateral prefrontal cortex produces pathologic changes or leakage of the blood-brain barrier in patients with depression by using apparent diffusion coefficient magnetic resonance imaging. BACKGROUND: Transcranial magnetic stimulation is a new technology for noninvasively stimulating the brain. It appears to be a relatively safe technique, with some important exceptions. Its neurobiologic mechanisms of action are poorly understood. One theory to explain its apparent antidepressant effects involves a potential change in local blood-brain barrier settings, allowing passage of peripheral substances directly into brain parenchyma. Knowing whether transcranial magnetic stimulation changes local brain diffusion is important as well from a safety perspective. To test whether transcranial magnetic stimulation changes local brain diffusion, we used apparent diffusion coefficient magnetic resonance imaging in depressed patients undergoing interleaved transcranial magnetic stimulation/functional magnetic resonance imaging over the left prefrontal cortex. METHODS: Within a 1.5 Tesla magnetic resonance imaging scanner, 14 depressed patients were stimulated with a figure-eight transcranial magnetic stimulation coil over the left prefrontal cortex. Apparent diffusion coefficient magnetic resonance imaging was acquired before, and immediately after, 1 Hertz transcranial magnetic stimulation (147 stimuli) intermittently delivered at a motor threshold of more than 7.35 minutes. Phase maps of the transcranial magnetic stimulation magnetic fields were used to guide region-of-interest placement. RESULTS: No significant qualitative apparent diffusion coefficient differences were observed before and after 1 Hertz transcranial magnetic stimulation underneath the coil. CONCLUSIONS: One Hertz transcranial magnetic stimulation over the left dorsolateral prefrontal cortex as applied in this study did not result in pathologic changes or leakage of the blood-brain barrier in patients with depression. If prefrontal transcranial magnetic stimulation at these usage parameters changes local diffusion, it is not an obvious or large effect.

Adolescent↗

PubMed1989

[Sandwich type dental magnetic devices of Nd-Fe-B magnet and permendur].

Nd-Fe-B magnets have a very high maximum energy product (BH max), which is defined as the attractive strength between a magnet and opposing magnetic materials. Permendur (Fe-49 Co-2 V) has the greatest magnetic saturation (Bs), which makes it strongly magnetized. If magnetic retainers were made with Nd-Fe-B magnets and a permendur yoke, they would be small and have strong retention. The purpose of this investigation was to develop small dental magnetic devices with Nd-Fe-B magnets and permendur yokes. The magnetic devices form sandwich-type magnetic circuits with magnetic stainless steel keepers. A 4 x 3 x 2 mm rectangular prism Nd-Fe-B magnet was used. The magnet was sandwitched between the semi-columnar yokes. To protect the magnet from corrosion, the devices were encapsulated with 304 stainless steel by silver brazing and adhesion bonding of a stainless steel tube and foil. The optimum cross-sectional area of the yoke was determined experimentally. The dimensions of the devices were phi 5 x 3.5 mm for 4 x 3 x 2 magnet. The breakaway retention for a keeper of magnetic stainless steel (Type XM 27) was 852 g on average. This breakaway retention is sufficient for dental prosthetic applications.

Beryllium↗

PubMed2004

Systematic correlation of transcranial magnetic stimulation and magnetic resonance imaging in cervical spondylotic myelopathy.

STUDY DESIGN: A prospective study over a 3.5-year period involving transcranial magnetic stimulation and magnetic resonance imaging. OBJECTIVES: To assess the correlation of transcranial magnetic stimulation and magnetic resonance imaging in cervical spondylotic myelopathy qualitatively and statistically. SUMMARY OF BACKGROUND DATA: Cervical spondylotic myelopathy presents with different degrees of cord compression, which can be assessed by magnetic resonance imaging. There are no large studies correlating transcranial magnetic stimulation and magnetic resonance imaging findings in this condition. METHODS: A total of 141 patients with a clinical diagnosis of cervical spondylotic myelopathy were prospectively studied over a 3.5-year period. They were classified into Groups 1 to 4 based on severity of cervical cord changes on magnetic resonance imaging. All had transcranial magnetic stimulation and central motor conduction time measurements within 2 months of the magnetic resonance imaging study. RESULTS: Twenty-eight, 49, 28, and 36 patients were classified into Groups 1 to 4, respectively. Mean upper limb and lower limb central motor conduction times correlated with the severity of magnetic resonance cord compression. The absence of central motor conduction time abnormalities correlated reliably with the absence of cervical cord impingement as in Group 1. Statistically significant right left difference in central motor conduction time in the lower limbs was seen between Groups 1 (no cord changes) and Group 2 (mild cord impingement). Eight other patients with diagnoses other than cervical spondylotic myelopathy all showed central motor conduction time abnormalities. The sensitivity and specificity for transcranial magnetic stimulation for differentiating the presence from absence of magnetic resonance imaging cord abnormality were 100% and 84.8%, respectively. CONCLUSIONS: Transcranial magnetic stimulation showed excellent correlation with magnetic resonance imaging findings and can be considered as an effective technique for screening patients for cervical cord abnormalities before magnetic resonance imaging in the clinical setting. The findings in this study have relevant implications in the pathophysiology, management, and health costs of cervical spondylotic myelopathy.

Adult↗

PubMed2005

Magnetically labeled human natural killer cells, accumulated in vitro by an external magnetic force, are effective against HOS osteosarcoma cells.

We evaluated the efficacy of a novel natural killer (NK) cell delivery system in vitro, and also investigated the antitumor effect of the accumulated cells on HOS osteosarcoma cells. Human peripheral blood mononuclear cells were isolated and co-cultured with inactivated K562 erythroleukaemic cells in the presence of IL-2 for 5 days. CD3- CD56+ NK cells were labeled with immunomagnetic beads and separated using a magnetic cell sorting system. Purity and cytotoxicity against K562 cells and HOS cells of the magnetically labeled NK cells were measured. To evaluate whether magnetically labeled NK cells could be accumulated in a specific area by magnetic force, the NK cells were placed in chamber slides in the presence, or not, of an external magnetic force of a neodymium magnet (diameter: 1.5 mm, height: 3 mm, total magnetic flux density: 0.282 T). Moreover, to investigate the antitumor effect on HOS cells, the magnetically labeled NK cells were added to HOS cells in chamber slides in the presence, or not, of an external magnetic force for various times. HOS cells were subsequently stained with Papanicolaou for histological examination. It was found that the magnetically labeled NK cells were highly purified and had cytotoxicity against target cells. The NK cells were accumulated effectively by the magnetic field and, when the NK cells were added to HOS cells in a chamber slide with a magnet placed beneath, a significantly larger number of HOS cells detached from the magnet zone than from other zones. Apoptosis was detected in most detached HOS cells. In conclusion, these findings indicate that magnetically accumulated NK cells efficiently induced apoptosis in HOS cells, suggesting that magnetic targeting therapy using magnetically labeled NK cells holds promise as an immunotherapy for osteosarcoma.

Antineoplastic Agents↗

PubMed2002

Nonequilibrium Brownian dynamics analysis of negative viscosity induced in a magnetic fluid subjected to both ac magnetic and shear flow fields.

We study the rheological and magnetic characteristics of a magnetic fluid. The system, which we investigate, is as follows. Ferromagnetic particles are dispersed in a solvent, which is subjected to both ac magnetic and shear flow fields. The translational and rotational motions of particles are calculated by the Brownian dynamics method based on Langevin equations and the rheological and magnetic characteristics of the magnetic fluid system are estimated. First, we investigate the rheological and magnetic characteristics of the system in a dc magnetic field and then we analyze the effect of an ac magnetic field on those characteristics. We find that the negative viscosity effect is induced at a certain frequency range of the ac magnetic field. We also find that there are two main mechanisms responsible for the occurrence of negative viscosity. (1) Resonance between the rotational motions of the dipoles of particles and the fluctuation of ac magnetic fields occurs when applied magnetic fields are weak compared to the shear rate, in which case particles can still rotate in magnetic fields. Beyond this resonance frequency, negative viscosity appears. (2) The magnetic dipole moments of particles are forced to stay in the direction of the magnetic field when strong magnetic fields are applied in relatively low shear flow fields. However, negative viscosity occurs when the frequency of external magnetic fields exceeds a critical value, in which case the dipoles rotate continuously in a shear flow without stopping. In both cases, the mean angular velocity of the particles becomes higher than that of the solvent.

Journal Article↗

PubMed1995

Extent and flux density of static magnetic fields generated by orthodontic samarium-cobalt magnets.

The aim of this study was to measure and to analyze the extent and flux density of static magnetic fields generated by commercially available samarium-cobalt magnets used in orthodontics. The flux density was measured with a gaussmeter and a Hall probe with the magnets mounted in clinically relevant positions, i.e., in attractive and in repelling positions and also in the single position. Furthermore, the flux density between new and clinically used and recycled magnets was compared. It was found that the maximum flux density was generated at the pole faces and that magnets in attractive positions produced the highest flux density (2.2 kG), followed by the single magnet (2.0 kG) and the repelling magnets (1.7 kG). The flux density decreased rapidly (exponentially) with increased distance from the magnets. The flux density was approximately the same or less than the flux density of the earth magnetism (0.3 to 0.7 G) 60 mm from the attractive magnets, 50 mm from the single magnets, and 35 mm from the repelling magnets. The difference in flux density between new and clinically used and recycled samarium-cobalt magnets was negligible. Thus, the static magnetic field exposure of surrounding tissues can be assumed to be low, and the conceivable risk of harmful biologic effects must be regarded as small and limited when the tested orthodontic magnets are used clinically.

Cobalt↗

PubMed2005

Establishment and implications of a characterization method for magnetic nanoparticle using cell tracking velocimetry and magnetic susceptibility modified solutions.

Magnetic micro and nanoparticles conjugated to affinity labels have become a significant, commercial reagent. It has been demonstrated that the performance of cell separation systems using magnetic labels is a function of the magnitude of the magnetic force that can be generated through labeling. This magnetic force is proportional to the number of magnetic particles bound to the cell, the magnetic energy gradient, and the particle-field interaction parameter. This particle-field interaction parameter, which is the product of the relative volumetric, magnetic susceptibility and the volume of the micro or nanoparticle, is a fundamental parameter which can be used to characterize the magnetic particles. An experimental technique is presented which measures the volumetric magnetic susceptibility of particles through the use of susceptibility modified solutions and an experimental instrument, Cell Tracking Velocimetry, CTV. Experimental studies were conducted on polystyrene microspheres alone and those bound to four different magnetic nanoparticles. The experimentally determined values of the magnetic susceptibility of the polystyrene microspheres are consistent with values found from literature. Consequently, magnetic susceptibility measurements of these polystyrene microspheres bound with the magnetic nanoparticles combined with particle size measurements using commercial dynamic light scattering instrument allowed estimates of the particle-field interaction parameter to be made for four commercial, magnetic nanoparticles. The value found for MACS beads is close to what is reported from an independent study. The values for MACS beads and Imag beads are found to agree with what is observed from experiments. Finally, an experimental demonstration of the impact that differences in this field interaction parameter has on the labeling of human lymphocytes is presented.

Cell Movement↗

PubMed2005

Localization and distribution of magnetic chemotherapeutic drugs with magnetic targeting in rat brain.

BACKGROUND: Magnetic targeting therapy may be a new method for the treatment of malignent tumors. The purpose of this study was to investigate the localization and distribution of ferrofluid microsphere of human serum albumin methotrexate (FM-HSA-MTX) carriers in the brain and to explore the magnetic targeting chemotherapy for malignant brain tumor. METHODS: Ninety SD rats were divided into three groups: targeting group, non-magnetic targeting group, and control group. Synthesized FM-HSA-MTX carriers (MTX 25 mg/kg) were injected into the systemic circulation via the caudal vein (magnetic targeting group, n = 30). A 0.6 T magnetic field was placed around the right hemisphere. The non-magnetic targeting group (n = 30) was administered with FM-HSA-MTX without external magnetic field, meanwhile the control group (n = 30) was treated with MTX and a magnetic field. Random serial sacrifices (n = 10) were conducted at 15, 30 and 45 minutes after drug administration. Bilateral hemispheres were collected respectively, and analyzed for total MTX content. RESULTS: MTX content in the right hemisphere of the magnetic targeting group was significantly higher than that in the other two groups at 15, 30 and 45 minutes after drug administration (P < 0.05) No difference was seen between the non-targeting group and control group. In the magnetic targeting group, MTX returned to the peak level [(0.564 +/- 0.018) mg/g, q15-45 = 32.252, P < 0.05] 45 minutes after the injection but it deceased in the other two groups [non-magnetic targeting group: (0.060 +/- 0.015) mg/g, q15-45 = 9.245, P < 0.05, control group: (0.074 +/- 0.045) mg/g, q15-45 = 6.299, P < 0.05]. In the magnetic targeting group, the concentration of MTX in the right hemisphere was significantly higher than that in the left hemisphere (t45min = 21.135, P = 0.000) but no difference was observed between bilateral hemispheres in the other two groups (non-magnetic targeting group: t45min = 0.434, P = 0.670; control group: t45min = 0.533, P = 0.600). CONCLUSION: In the presence of the external magnetic field, FM-HSA-MTX can distribute successfully in the targeting areas of the brain.

Animals↗

PubMed2004

Bending of magnetic filaments under a magnetic field.

Magnetic beads and superparamagnetic (SP) colloid particles have successfully been employed for micromechanical manipulation of soft material, in situ probing of elastic properties, and design of smart materials (ferrogels). Here we derive analytical expressions for the equilibrium shape of magnetic fibers, considering two end-member cases, (a) SP or single-domain particles concentrated at the free end of cantilevered rods or tubes, and (b) filaments consisting of SP particles, with this case being mathematically equivalent to tubes containing SP particles. Our analysis yields also metastable equilibrium states (MES's), which only exist above a critical filament length, but become more stable with increasing magnetic field. The MES's for case (a) are, like the ground state, circular arcs, but more strongly bent. The multiform MES's in case (b), which comprise hairpin, sinuous, or even closed shapes, have recently been observed in experiments, too. We also study the effect of gravity on the balance between bending and magnetic energy, which leads to curves with inflection point if the influence of gravity is stronger than that of the magnetic field. Because of their simple experimental realization, case (a) magnetic filaments are deemed highly suitable for micromechanical experiments on long chains of polymer molecules. Another potential application of cantilevered magnetic filaments with magnetic material attached to the free end is in scanning probe microscopes. Because the magnetic field due to the magnetic tip is comparatively weak, the magnetization structure of the sample to be investigated would not be affected by the probe. Thus, for the examination of magnetically soft materials, probes in the form of magnetic filaments may hold advantages over tips usually employed in magnetic force microscopy.

Journal Article↗

PubMed1995

[Clinical investigation of transcranial magnetic stimulation of the facial nerve--an early prognostic diagnosis of patients with peripheral facial palsy and the facial nerve magnetic stimulation site].

To obtain an early prognostic diagnosis of patients with peripheral facial palsy, a magnetic stimulator (Dantec Mag 2) was used to directly stimulate the intracranial portion of the facial nerve in 15 normal subjects and 108 patients with peripheral facial palsy. In normal subjects and patients with facial palsy, compound muscle action potentials (CMAPs) of the orbicularis oris muscle elicited by transcranial magnetic stimulation were compared with CMAPs elicited by electrical stimulation at a peripheral site of the stylomastoid foramen. This technique is similar to electroneurography (ENoG) and is regularly used in our department. In normal subjects, the latency of magnetically evoked CMAPs was longer (1.0ms, SD 0.39ms) than that of CMAPs evoked by electrical stimulation. There were two categories of patients; the first group consisted of patients who visited our hospital within 2 weeks after palsy onset with a record of electrically evoked CMAPs (ENoG) and magnetically evoked CMAPs, the second group consisted of all others. The first group was then divided into four subgroups based on minimal ENoG values obtained within 2 weeks after the onset of palsy. In patients, ENoG values declined until the seventh day after palsy onset, and then plateaued. However, the amplitude ratio of magnetically evoked CMAPs between the affected side and normal side showed no tendency to deline until the seventh day after palsy onset. Thus, whether magnetically evoked CMAPs could be recorded must be discussed in relation to the prognosis of facial palsy. The patients in whom magnetically evoked CMAPs could be recorded within the seven days after the onset of palsy were classified into a group in which the minimal ENoG value was greater than 20%. These patients recovered almost 2 months after the onset of palsy, and were significantly better than the recovery rates of those patients in whom magnetically evoked CMAPs could not be recorded. The site at which the facial nerve is magnetically stimulated remains controversial. In patients with peripheral facial palsy, recovery of the stapedial reflex, blink reflex and magnetically evoked CMAPs were examined to investigate the site of magnetic stimulation. From the clinical perspective, the facial nerve is thought to be magnetically stimulated near the meatal foramen that Fisch reported the site of damage in Bell's palsy. This stimulation site was almost the same point as that calculated from the mean latency difference between magnetically evoked CMAPs and ENoG in normal controls.

Adolescent↗

PubMed2001

Effects of paramagnetic ferrocenium cations on the magnetic properties of the anionic single-molecule magnet [Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)]-.

The preparation and physical characterization are reported for the single-molecule magnet salts [M(Cp')(2)](n)()[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (M = Fe, n = 1, Cp' = C(5)Me(5) (2a), C(5)H(5) (2b); M = Co, n = 1, Cp' = C(5)Me(5) (2c), C(5)H(5) (2d); M = Fe, n = 2, Cp' = C(5)Me(5) (2e), C(5)H(5) (2f)) to investigate the effects of paramagnetic cations on the magnetization relaxation behavior of [Mn(12)]- anionic single-molecule magnets. Complex 2a.2H(2)O crystallizes in the orthorhombic space group Aba2, with cell dimensions at 173 K of a = 25.6292(2) A, b = 25.4201(3) A, c = 29.1915(2) A, and Z = 4. Complex 2c.2CH(2)Cl(2).C(6)H(14) crystallizes in the monoclinic space group P2(1)/c, with cell dimensions at 173 K of a = 17.8332(6) A, b = 26.2661(9) A, c = 36.0781(11) A, beta = 92.8907(3) degrees, and Z = 4. These two salts consist of either paramagnetic [Fe(C(5)Me(5))(2)]+ cations or diamagnetic [Co(C(5)Me(5))(2)]+ cations, and [Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)]- anions. The structures of the anions in the two salts are similar, consisting of a central Mn(4)O(4) cubane moiety, surrounded by a nonplanar ring of eight Mn atoms that are bridged by and connected to the cube via mu(3)-O(2)- ions. The oxidation states of four Mn sites out of eight outer Mn ions in complex 2a were assigned to be +2.75 from the valence bond sum analysis although the disordering of bridging carboxylates prevents more precise determination. On the other hand in complex 2c, one Mn site out of eight outer Mn ions was identified as a Mn(II) ion, accommodating the "extra" electron; this was deduced by a valence bond sum analysis. Thus, the anion in complex 2c has a Mn(II)(1)Mn(III)(7)Mn(IV)(4) oxidation state description. The Jahn-Teller axes of the Mn(III) ions in both anions are roughly aligned in one direction. All complexes studied exhibit a single out-of-phase ac magnetic susceptibility (chi"(M)) signal in the 4.6-4.8 K range for complexes 2a-2d and in the 2.8-2.9 K range for complexes 2e and 2f at 1 kHz ac frequency. The temperature of the chi"(M) peaks is frequency dependent, as expected for single-molecule magnets. From Arrhenius plots of the frequency dependence of the temperature of the chi"(M) maxima, the effective energy barriers U(eff) for changing spin from "up" to spin "down" were estimated to be 50-54 K for complexes 2a-2d and 27-28 K for complexes 2e and 2f. The least-squares fits of the reduced magnetization data indicate that both complexes 2a and 2d have ground states of S = (21)/(2). High-frequency EPR spectra were recorded for complex 2a at frequencies of 217, 327, and 434 GHz in the 4.5-30 K range. The observed transition fields were least-squares fit to give g = 1.91, D = -0.35 cm(-1), and B(4)(0) = -3.6 x 10(-7) cm(-1) for the S = (21)/(2) ground state. The effective energy barrier U(eff) is slightly lower than U estimated from D, which is consistent with the thermally assisted tunneling model. Magnetization hysteresis loops were observed for complexes 2a and 2c. Although 2a was oriented in a different manner as expected by strong magnetic field, both complexes show clear hysteresis loops with some steps on them, indicating that the effect of the magnetic cation on the magnetization relaxation of the anionic [Mn(12)]- complex is rather small. An 11% (57)Fe enriched complex 2b was studied by means of Mössbauer spectroscopy down to as low as 1.7 K. Slow paramagnetic relaxation broadening and magnetic hyperfine splitting were evident in the low-temperature spectra, indicating that the iron atoms feel a growing magnetic field owing to slow magnetization reversal in the [Mn(12)]- anions.

Journal Article↗

PubMed1997

Magnetic orientation and the magnetic sense in arthropods.

The physical properties of the earth's magnetic field are summarized with the aim of emphasizing their significance as cues that can be exploited in orientational tasks. Past work has revealed magnetic orientation in vertebrates as well as invertebrates, including arthropods. The key finding to date has been that, as opposed to many vertebrates, the magnetic compass of arthropods responds to the polarity, rather than to the inclination of the earth's magnetic field. As in the case of vertebrates, the debate over how arthropods detect magnetic fields has yet to be resolved. Currently, evidence has been reported in support of a detection system based on magnetite crystals together with a variety of detection systems based on events occurring at the molecular level. Interactions between the magnetic and other compasses in orientation experiments suggest the existence of an area in the brain where spatial orientation information from magnetic and other stimuli converges. The slow advance of our knowledge on magnetic orientation in arthropods, as opposed to the much better understanding of magnetic orientation in vertebrates, arises from difficulties in identifying the appropriate behavioural contexts in which arthropods respond to magnetic fields in both laboratory and field situations. Arthropods thus present challenges not only in demonstrating magnetic orientation, but also in elucidating the sensory mechanisms involved in the perception of magnetic fields.

Animals↗

PubMed2006

Safety study of the Cochlear Nucleus 24 device with internal magnet in the 1.5 Tesla magnetic resonance imaging scanner.

OBJECTIVES: To evaluate the effect of the 1.5 Tesla magnetic resonance imager (MRI) on the Cochlear Nucleus 24 Device without removing the internal magnet. To determine whether device fixation using a compression dressing could prevent internal magnet displacement in the MRI scanner and potentially obviate the need for surgical removal of the internal magnet. STUDY DESIGN: Prospective cadaveric study. METHODS: Four cadaver heads were implanted bilaterally with the Nucleus device with the internal magnet in place and placed into the 1.5 Tesla MRI scanner. The devices were then explanted after interaction with the MRI and evaluated for displacement of the internal magnet. Conditions tested include device fixation with a commercially available compression dressing and no fixation (worst-case scenario). Magnet strength was measured before and after each of the test conditions. RESULTS: Moderate to severe displacement of the magnet from the internal device occurred in 14 of 16 (87%) implants when no compression dressing was placed. Displacement occurred in 0 of 16 (0%) implants when the compression dressing was applied. No decrease in the strength of the implant magnet was found with the initial or subsequent MRI/implant interactions. CONCLUSIONS: Use of the 1.5 Tesla MRI on subjects with Cochlear Nucleus 24 implants did not result in any significant demagnetization of the internal magnet and did not cause displacement of the magnet when an external compression dressing was applied. Surgical removal of the internal magnet before scanning with the 1.5 Tesla MRI may not be necessary if a compression dressing is applied.

Cadaver↗