Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “MAGNETICS”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24Linked to original sources
PubMed2005

[Value of magnetic resonance angiography combined with magnetic resonance cholangiopancreatography on diagnosis for patients with carcinoma of head of pancreas].

OBJECTIVE: To study the value of magnetic resonance imaging (MRI) combined with magnetic resonance angiography (MRA) and magnetic resonance cholangiopancreatography (MRCP) in the preoperative diagnosis of patients with carcinoma of head of pancreas. METHODS: Forty-two patients were examined by MRI, MRCP and MRA. The results were compared with the intraoperative exploration findings and operative procedures. RESULTS: The MRCP and MRA results of 42 cases demonstrated that peripancreatic tissues and blood vessels were not invaded in 24 cases, while portal vein and superior mesenteric vein were invaded by the tumor in 5 cases, and peripancreatic vessels were infiltrated in 10 cases, and 3 cases were detected with organ metastasis. Thirty-five of forty-two cases were consistent with the intraoperative exploration findings, and the accurate rate was 89.7%. Pancraticoduodenectomy was performed in 28 patients, internal drainage in 11 and interventional therapy in 3 cases. The resection rate was 66.7%. CONCLUSIONS: The results of MRCP and MRA could definitely demonstrate the preoperative organ metastasis and peripancreatic organization invasion, especially blood vessel infiltration. So the imaging results played an important role in estimating the level of the process and guiding the treatment.

Adult↗
PubMed1984

Early experience with clinical magnetic resonance imaging using a 0.5 Tesla magnet.

Magnetic resonance imaging (MRI) is a new medical tool that may revolutionize the field of diagnostic radiology. It uses no ionizing radiation and may be suitable for imaging all parts of the body. MRI is undergoing rapid development, and early experience with magnets of varying strength is beginning to accumulate in the radiologic literature. We present our initial experience with MRI using a powerful 0.5 Tesla magnet.

Abdominal Neoplasms↗
PubMed1997

Detection of magnetism in the red imported fire ant (Solenopsis invicta) using magnetic resonance imaging.

Red imported fire ant (Solenopsis invicta) workers, queens, and alates were analyzed by magnetic resonance imaging (MRI) for the presence of natural magnetism. Images of ants showed distortion patterns similar to those of honey bees and monarch butterflies, both of which possess ferromagnetic material. The bipolar ring patterns of MRI indicated the presence in fire ants of small amounts of internal magnetic material, which may be used in orientation behaviors, as in the honey bees.

Animals↗
PubMed2004

A preliminary diffusion tensor and magnetization transfer magnetic resonance imaging study of early-onset multiple sclerosis.

BACKGROUND: Early-onset multiple sclerosis (MS) typically has a more favorable course than adult-onset disease. OBJECTIVE: To assess the extent of microscopic tissue damage in the brain and cervical cord of patients with early-onset MS. DESIGN: During a single magnetic resonance imaging session, images of the brain and spinal cord were obtained using diffusion tensor and magnetization transfer magnetic resonance imaging. PATIENTS: We studied 13 patients with early-onset MS and 10 healthy volunteers. RESULTS: Compared with control subjects, patients with early-onset MS showed only a slight increase of the average mean diffusivity of the normal-appearing brain tissue. CONCLUSION: The relatively modest central nervous system damage detected in these patients might explain why early-onset MS typically has a more favorable clinical course than adult-onset MS.

Adolescent↗
PubMed1999

Magnetic coupling of creatine/phosphocreatine protons in rat skeletal muscle, as studied by (1)H-magnetization transfer MRS.

Off-resonance saturation caused a reduction of the 3.04 ppm NMR signal from the methyl protons of creatine in rat hindleg skeletal muscle. (1)H-NMR spectra were recorded over a 200 kHz range of off-resonance saturation frequencies. The span of frequencies over which the creatine signal was reduced greatly exceeded that expected for direct saturation by the off-resonance RF-field. This suggests that there is a motionally restricted proton pool which exchanges magnetization with the free creatine pool. The experimental data were fitted to characterize the immobilized proton pool and the exchange kinetics, using a two-pool exchange model. The immobile pool was estimated to amount to ca. 2.5% of the mobile pool of free creatine, while the rate of exchange between the mobile and immobile configurations is ca. 2.3 sec(-1). After depletion of phosphocreatine by termination of the animal, the MT effect on the creatine methyl protons remained unchanged. This indicates that phosphocreatine and creatine both contribute to the MT phenomenon. Selective saturation of the mobile water pool also led to a reduction in the intensity of the total creatine methyl signal, suggesting that water and creatine are magnetically coupled via a macromolecular interface. The precise mechanism responsible for and the biological significance of the pronounced creatine magnetization transfer effect in rat skeletal muscle remains to be established. Magn Reson Med 42:665-672, 1999.

Animals↗
PubMed2003

Directly mapping magnetic field effects of neuronal activity by magnetic resonance imaging.

Magnetic resonance imaging (MRI) of brain functional activity relies principally on changes in cerebral hemodynamics, which are more spatially and temporally distributed than the underlying neuronal activity changes. We present a novel MRI technique for mapping brain functional activity by directly detecting magnetic fields induced by neuronal firing. Using a well-established visuomotor paradigm, the locations and latencies of activations in visual, motor, and premotor cortices were imaged at a temporal resolution of 100 msec and a spatial resolution of 3 mm, and were found to be in consistent with the electrophysiological and functional MRI (fMRI) literature. Signal strength was comparable to traditional event-related fMRI methods: about 1% of the baseline signal. The magnetic-source MRI technique greatly increases the temporal accuracy in detecting neuronal activity, providing a powerful new tool for mapping brain functional organization in human and animals.

Action Potentials↗
PubMed2002

Application of magnetization transfer at 3.0 T in three-dimensional time-of-flight magnetic resonance angiography of the intracranial arteries.

PURPOSE: To apply magnetization transfer (MT) at 3.0 T in three-dimensional time-of-flight magnetic resonance angiography of the intracranial arteries. MATERIALS AND METHODS: This study was performed on phantoms and seven volunteers to determine the effects of MT at 3.0 T. By using a modulated MT approach and an altered phase encode order, the specific absorption rate (SAR) was kept below 3 W/kg over any 8-second time period. RESULTS: For a 20-degree flip angle and 36 msec repetition time, the background suppression at 3.0 T was improved with MT by 52 +/- 5% for white matter and 40 +/- 8% for grey matter, making the distal intracranial vasculature significantly more discernible. CONCLUSIONS: MT at 3.0 T can significantly improve background suppression in 3D time-of-flight magnetic resonance angiography (MRA) of the intracranial arteries without exceeding SAR guidelines.

Adult↗
PubMed2002

Phase coherent averaging in magnetic resonance spectroscopy using interleaved navigator scans: compensation of motion artifacts and magnetic field instabilities.

The quality of spectra in (1)H magnetic resonance spectroscopy (MRS) is strongly affected by temporal signal instabilities during the acquisition. One reason for these instabilities are hardware imperfections, e.g., drifts of the main magnetic field in superconducting magnets. This is of special concern in high-field systems where the specification of the field stability is close to the spectral linewidth. A second major potential source of artifacts, particularly in clinical MRS, is patient motion. Using standard acquisition schemes of phase-cycled averaging of the individual acquisitions, long-term effects (field drifts) as well as changes on a shorter time scale (motion) can severely reduce spectral quality. The new technique for volume-selective MRS presented here is based on the additional interleaved acquisition of a navigator signal during the recovery time of the metabolite acquisition. It corrects for temporal signal instabilities by means of a deconvolution of the metabolite and the navigator signal. This leads to phase-corrected individual metabolite scans and upon summation to a phase-coherent averaging scheme. The interleaved navigator acquisition does not require any user interaction or supervision, while sequence efficiency is maintained.

Animals↗
PubMed1989

Magnetic shielding of magnetic resonance systems.

Basic theoretical concepts of static magnetic shielding are summarized. Expressions describing the magnetic field produced by a solenoidal coil confined coaxially inside a long thin ferromagnetic cylinder of constant permeability are derived. Conditions for the optimum arrangement of a magnetic screen for whole-body NMR systems are discussed.

Magnetic Resonance Imaging↗
PubMed1996

Quantitative magnetic resonance imaging of perfusion using magnetic labeling of water proton spins within the detection slice.

A technique for noninvasive quantitative magnetic resonance imaging of perfusion is presented. It relies on using endogenous water as a freely diffusible tracer. Tissue water proton spins are magnetically labeled by slice-selective inversion, and longitudinal relaxation within the slice is detected using a fast gradient echo magnetic resonance imaging technique. Due to blood flow, nonexcited spins are washed into the slice resulting in an acceleration of the longitudinal relaxation process. Incorporating this phenomenon into the Bloch equation yields an expression that allows quantification of perfusion on the basis of a slice-selective and a nonselective inversion recovery experiment. Based on this technique, quantitative parameter maps of the regional cerebral blood flow (rCBF) were obtained from eight rats. Evaluation of regions of interest within the cerebral hemispheres yielded an average rCBF value of 104 +/- 21 ml/min/100 g, which increased to 219 +/- 30 ml/min/100 g during hypercapnia. The measured rCBF values are in good agreement with previously reported literature values.

Animals↗
PubMed1997

Magnetic susceptibility measurement of insoluble solids by NMR: magnetic susceptibility of bone.

Measurements of the volume magnetic susceptibility of solids using the classical Gouy balance approach are hampered by variations in apparent density of the packed powder. In this paper a quantitative NMR measurement of the volume magnetic susceptibility of powdered solids is described and the volume susceptibility of bone is reported. The technique is based on the measurement of changes in incremental linewidth (1/pi T2') induced in a marker fluid whose susceptibility can be predictably modified by changing the composition, such as by addition of a soluble diamagnetic compound. The spectroscopic linewidth of the marker fluid is determined by the susceptibility difference between the fluid and the suspended solid. Changes in the linewidth are accompanied by bulk magnetic susceptibility induced frequency shifts in the fluid resonance. Correlating the two dependencies allows measurement of the absolute volume susceptibility of the solid. The susceptibility of bovine rib bone was found to be -0.9 +/- 0.02 x -10(-6) (CGS) confirming previous estimates which suggested bone to be more diamagnetic than the marrow constituents. Knowledge of the susceptibility of bone is relevant in view of the growing interest in MRI osteodensitometric techniques.

Animals↗
PubMed2006

Dual-echo spiral in/in acquisition method for reducing magnetic susceptibility artifacts in blood-oxygen-level-dependent functional magnetic resonance imaging.

MRI signal dropout in gradient recalled echo acquisitions limits the capability of blood-oxygen-level-dependent functional magnetic resonance imaging (fMRI) to study activation tasks that involve the orbitofrontal, temporal, and basal areas of the brain where significant macroscopic magnetic susceptibility differences exist. Among the various approaches aimed to address this issue, the acquisition method based on spiral in/out trajectories is one of the most time-efficient and effective techniques. In this study, we extended further the spiral in/out approach into 3D acquisition and compared the effectiveness of the different spiral in/out trajectory combinations in reducing signal dropout. The activation results from whole brain fMRI studies using complex finger tapping and breath-holding tasks demonstrate that the acquisition method based on dual-echo spiral in/in (DSPIN) trajectories is the most favorable. The DSPIN acquisition method has the following advantages: (1) It reduces most effectively signal dropout in the brain where magnetic susceptibility inhomogeneity is problematic and significantly improves the sensitivity to detect functional activations in those regions. (2) It significantly improves SNR in the whole brain by dual echo averaging without compromising functional contrast. (3) There is no reduction in time-efficiency and spatial resolution.

Adult↗
PubMed1994

Variation of the magnetic relaxation rate 1/T1 of water protons with magnetic field strength (NMRD profile) of untreated, non-calcified, human astrocytomas: correlation with histology and solids content.

The magnetic relaxation rate 1/T1 of tissue water protons was measured over a wide range of magnetic field strengths (NMRD profile) for 92 fresh surgical specimens of astrocytomas to search for correlations of 1/T1 with tumor histology, as determined by light microscopy, and to assess the diagnostic potential of NMRD profiles for grading astrocytomas. A third goal was to elucidate the molecular determinants of 1/T1. Each specimen was histologically graded and inspected for evidence of mineral deposits (Ca, Fe); its dry weight was determined and expressed in % of original wet weight. To minimize variability not directly related to tumor grade, this initial report is limited to NMRD profiles of 47 non-calcified, non-hemorrhagic, untreated astrocytomas. For these, the mean value of 1/T1 at very low magnetic field strengths was found to increase with increasing grade of malignancy; no clear correlation could be demonstrated at high fields where most imaging is done. The spread of 1/T1 for different grades of malignancy is large, however, and the overlap significant, even at the lowest field, so that astrocytomas can not be graded by NMRD profiles alone. Average 1/T1 and average dry weight increase with grade of malignancy; but the variability of 1/T1 among specimens of the same dry weight is large, indicating that at least one other cellular parameter, not variable in normal tissue, influences 1/T1 strongly. We hypothesize that this parameter reflects changes at the molecular level in size distribution, mobility, or intermolecular interaction of cytoplasmic proteins. Which specific changes are induced by malignant transformation in astrocytomas remains to be investigated.

Adolescent↗
PubMed1998

[Stimulation of peripheral nerves by time-varying magnetic field gradients in magnetic resonance tomography].

PURPOSE: The objective of this study is to explain how peripheral nerves can be stimulated by magnetic resonance imaging (MRI) and to describe the associated potential risk. Present knowledge on the bioeffects of time-varying magnetic fields used in MRI are summarized and discussed. MATERIAL AND METHODS: This review summarizes current reports on peripheral nerve stimulation during MRI and studies which determine threshold values for stimulation effects. RESULTS/DISCUSSION: The comparison of the different studies indicates that it is reasonable to express the stimulation threshold in terms of the gradient amplitude rather than in terms of the rate of change of the magnetic field gradient. Further studies are necessary to obtain reliable threshold values for peripheral nerve stimulation during MRI and, even more important, to find out how these results can be used to define threshold values in order to avoid excitation of the myocardium during MRI.

Dose-Response Relationship, Radiation↗
PubMed1993

Magnetic resonance imaging of female pelvic masses and local recurrent tumors at an ultra low (0.02 T) magnetic field: correlation with computed tomography.

Pelvic MR (41 patients) and CT (36 patients) examinations were performed on 14 females with a primary pelvic complaint, and on 28 females with a suspicion of local recurrent disease of gynecologic malignant tumor. Benign cystic tumors were found in eight patients, five patients had endometriomas, one had a lymphoma, and one had a small androblastoma. Ten local recurrent tumors were confirmed histopathologically or cytologically. All cysts, one endometrioma, the lymphoma, and six recurrent tumors were detected on images obtained by our ultra low field magnetic unit. The smallest cyst detected was 16 mm in diameter. Small scattered implants of endometriosis were not discerned. The appearance of the tumors did not differ essentially from those described at high magnetic fields. Physical examination detected all 10 recurrent tumors, CT detected 8 of them, and MRI 6 out of 9 cases. The sensitivities of physical examination, CT and MRI to find recurrent diseases were 100%, 80%, and 67%, respectively. Corresponding specificities were 93%, 67%, and 80%. The results indicate that physical examination is most important in recurrent diseases. CT is the basic method for imaging malignant pelvic tumors. MR imaging at 0.02 T magnetic field provides a convenient and inexpensive method for more specific information, if CT findings are equivocal. MRI at 0.02 T is also accurate in detecting benign pelvic masses, but the findings are not very specific.

Evaluation Studies as Topic↗
PubMed2005

NMR detection and one-dimensional imaging using the inhomogeneous magnetic field of a portable single-sided magnet.

A portable, nuclear magnetic resonance (NMR) probe is described which utilises the intrinsic inhomogeneity of the field produced by a single-sided magnet to provide spatial encoding of the NMR signal. The probe uses a longitudinally magnetized hollow cylinder, and a figure-8 radiofrequency (RF) surface coil. The system has been used to measure NMR relaxation times and one-dimensional NMR profiles of rubber phantoms.

Equipment Design↗
PubMed1997

Source localization of the N400 response in a sentence-reading paradigm using evoked magnetic fields and magnetic resonance imaging.

The aim of the present investigation was to localize the sources of the N400 response elicited in a sentence-reading paradigm. Eight neurologically healthy adults viewed sentences that were presented one word at a time in the center of a computer screen. Half of the sentences ended with a semantically inappropriate word, while the other half had appropriate endings. Event-related potentials recorded at Fz and Pz showed a negative-going deflection, the amplitude of which was strongly affected by semantic congruity (N400). Evoked magnetic fields that were simultaneously recorded over the left hemisphere showed clear magnetic field extrema in seven subjects during the time course of the N400. Underlying sources were successfully modeled as single equivalent current dipoles. Anatomical regions that contained the dipoles were localized by superimposing dipole coordinates onto magnetic resonance scans. Dipole regions were found in temporal lobe structures, in the vicinity of the hippocampus and the parahippocampal gyrus (in two subjects) and in posterior temporal neocortical regions (in the vicinity of the middle temporal gyrus; in five subjects). These findings are consistent with the view that posterior association cortices in the left hemisphere are involved in word recognition and semantic comprehension during reading.

Adolescent↗
PubMedDate not supplied

Safety aspects of chronic low-frequency transcranial magnetic stimulation based on localized proton magnetic resonance spectroscopy and histology of the rat brain.

Because repetitive transcranial magnetic stimulation (rTMS) is capable of inducing lasting alterations of cortical excitability, it represents a promising therapeutic tool in several neuropsychiatric disorders. However, rTMS, especially when applied chronically, may cause harmful effects in the stimulated tissue. To study the safety of chronic rTMS we used a novel small stimulation coil, which was specially designed to treat rats, and investigated brain tissue using in vivo localized proton magnetic resonance spectroscopy (MRS) and post mortem histological analysis. Histology was based on a modified stereology method in combination with immunohistochemistry applying antibodies against OX-6, OX-42, ED, and GFAP to detect any microglial and/or astrocytic activation 48 h after the last TMS session. Conscious rats were treated with a daily suprathreshold rTMS regimen of 1000 stimuli applied on 5 consecutive days at a frequency of 1 Hz. In comparison with control animals receiving magnetic stimulation over the lumbar spine, quantitative evaluations of cerebral metabolite concentrations by proton MRS revealed no significant alterations of N-acetyl-aspartate, creatine and phosphocreatine, choline-containing compounds, myo-inositol, glucose and lactate after chronic rTMS. Similarly to the in vivo results, post mortem histology revealed no changes in microglial and astrocytic activation after rTMS. In conclusion, these data provide support for the safety of chronic rTMS. However, they do not exclude acute changes on neurotransmitters systems or other physiologic responses during or directly after the rTMS treatment.

Animals↗