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Gradient Fields (gradient + field)
Selected AbstractsBiological dosimetry of magnetic resonance imaging,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 5 2002Concepción Guisasola MD Abstract Purpose To check the bioeffects of the components of magnetic resonance imaging (MRI). MRI is based on an assumedly harmless interaction between certain nuclei in the body when placed in a strong magnetic field and radio wave fields. There are three key factors actuating on the examining body: a powerful static magnetic field (SMF), magnetic gradient fields (MGFs), and pulsed radiofrequency (RF) radiation. Materials and Methods In vitro cells (L-132 cells) were used as biosensors, and different cellular compounds were used as biomarkers (heat shock proteins [HSPs] and their messenger ribonucleic acids [mRNAs], calcium, and adenosine-3,,5,-cyclic monophosphate [cAMP]). The biosensors were placed in the bore of a 1.5-T MRI machine and the different electromagnetic fields were operated. Results HSPs and their mRNAs and cAMP did not respond to SMF, MGFs, or RF radiation. RF radiation increased cytosolic calcium concentration (18%, P < 0.05). Conclusion Although MRI procedures do not induce any cellular stress response, it may cause an unfathomable calcium increase in vitro. Although the in vitro experimental conditions are not totally comparable to clinical situations, the usefulness of the in vivo biological dosimetry, circulating leukocytes as biosensors, and HSPs and/or calcium as biomarkers is suggested. J. Magn. Reson. Imaging 2002;15:584,590. © 2002 Wiley-Liss, Inc. [source] Automatic slice positioning (ASP) for passive real-time tracking of interventional devices using projection-reconstruction imaging with echo-dephasing (PRIDE)MAGNETIC RESONANCE IN MEDICINE, Issue 4 2009S. Patil Abstract A novel and fast approach for passive real-time tracking of interventional devices using paramagnetic markers, termed "projection-reconstruction imaging with echo-dephasing" (PRIDE) is presented. PRIDE is based on the acquisition of echo-dephased projections along all three physical axes. Dephasing is preferably set to 4, within each projection ensuring that background tissues do not contribute to signal formation and thus appear heavily suppressed. However, within the close vicinity of the paramagnetic marker, local gradient fields compensate for the intrinsic dephasing to form an echo. Successful localization of the paramagnetic marker with PRIDE is demonstrated in vitro and in vivo in the presence of different types of off-resonance (air/tissue interfaces, main magnetic field inhomogeneities, etc). In order to utilize the PRIDE sequence for vascular interventional applications, it was interleaved with balanced steady-state free precession (bSSFP) to provide positional updates to the imaged slice using a dedicated real-time feedback link. Active slice positioning (ASP) with PRIDE is demonstrated in vitro, requiring approximately 20 ms for the positional update to the imaging sequence, comparable to existing active tracking methods. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc. [source] Experimental determination of human peripheral nerve stimulation thresholds in a 3-axis planar gradient systemMAGNETIC RESONANCE IN MEDICINE, Issue 3 2009Rebecca E. Feldman Abstract In MRI, strong, rapidly switched gradient fields are desirable because they can be used to reduce imaging time, obtain images with better resolution, or improve image signal-to-noise ratios. Improvements in gradient strength can be made by either increasing the gradient amplifier strength or by enhancing gradient efficiency. Unfortunately, many MRI pulse sequences, in combination with high-performance amplifiers and existing gradient hardware, can cause peripheral nerve stimulation (PNS). This makes improvements in gradient amplifiers ineffective at increasing safely usable gradient strength. Customized gradient coils are one way to achieve significant improvements in gradient performance. One specific gradient configuration, a planar gradient system, promises improved gradient strength and switching time for cardiac imaging. The PNS thresholds for planar gradients were characterized through human stimulation experiments on all three gradient axes. The specialized gradient was shown to have significantly higher stimulation thresholds than traditional cylindrical designs (y-axis SRmin = 210 ± 18 mT/m/ms and ,Gmin = 133 ± 13 mT/m; x-axis SRmin = 222 ± 24 mT/m/ms and ,Gmin = 147 ± 17 mT/m; z-axis SRmin = 252 ± 26 mT/m/ms and ,Gmin = 218 ± 26 mT/m). This system could be operated at gradient strengths 2 to 3 times higher than cylindrical configurations without causing stimulation. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc. [source] Basic aspects of geopotential field approximation from satellite-to-satellite tracking dataMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 11 2001W. Freeden Abstract The satellite-to-satellite tracking (SST) problems are characterized from mathematical point of view. Uniqueness results are formulated. Moreover, the basic relations are developed between (scalar) approximation of the earth's gravitational potential by ,scalar basis systems' and (vectorial) approximation of the gravitational field by ,vectorial basis systems'. Finally, the mathematical justification is given for approximating the external geopotential field by finite linear combinations of certain gradient fields (for example, gradient fields of multi-poles) consistent to a given set of SST data. Copyright © 2001 John Wiley & Sons, Ltd. [source] No influence of magnetic fields on cell cycle progression using conditions relevant for patients during MRIBIOELECTROMAGNETICS, Issue 4 2003Ilka B. Schiffer Abstract The purpose of this study was to examine whether exposure to magnetic fields (MFs) relevant for magnetic resonance imaging (MRI) in clinical routine influences cell cycle progression in two tumor cell lines in vitro. HL60 and EA2 cells were exposed to four types of MFs: (i) static MF of 1.5 and 7.05 T, (ii) extremely low frequency magnetic gradient fields (ELFMGFs) with ±,10 mT/m and 100 Hz, as well as ±,100 mT/m and 100 Hz, (iii) pulsed high frequency MF in the radiofrequency (RF) range (63.6 MHz, 5.8 ,T), and (iv) a combination of (i,iii). Exposure periods ranged from 1 to 24 h. Cell cycle distribution (G0/G1, S, and G2/M phases) was analyzed by flow cytometry. Cell cycle analysis did not reveal differences between the exposed and the control cells. As expected, positive controls with irradiated (8 Gy) HL60 and EA2 cells showed a strong G2/M arrest. Using conditions that are relevant for patients during MRI, no influence of MFs on cell cycle progression was observed in these cell lines. Care was taken to control secondary parameters of influence, such as vibration by the MR scanner or temperature to avoid false positive results. Bioelectromagnetics 24:241-250, 2003. © 2003 Wiley-Liss, Inc. [source] On the numerical treatment of initial strains in biological soft tissuesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 8 2006E. Peña Abstract In this paper, different methodologies to enforce initial stresses or strains in finite strain problems are compared. Since our main interest relies on the simulation of living tissues, an orthotropic hyperelastic constitutive model has been used to describe their passive material behaviour. Different methods are presented and discussed. Firstly, the initial strain distribution is obtained after deformation from a previously assumed to be known stress-free state using an appropriate finite element approach. This approach usually involves important mesh distortions. The second method consists on imposing the initial strain field from the definition of an initial incompatible ,deformation gradient' field obtained from experimental data. This incompatible tensor field can be imposed in two ways, depending on the origin of the experimental tests. In some cases as ligaments, the experiment is carried out from the stress-free configuration, while in blood vessels the starting point is usually the load-free configuration with residual stresses. So the strain energy function would remain the same for the whole simulation or redefined from the new origin of the experiment. Some validation and realistic examples are presented to show the performance of the strategies and to quantify the errors appearing in each of them. Copyright © 2006 John Wiley & Sons, Ltd. [source] | ||||||||||