Transverse Homogeneous Magnetic Field Research Articles

Dose response of selected ion chambers in applied homogeneous transverse and longitudinal magnetic fields

The magnetic fields of an integrated MR-Linac system will alter the paths of electrons that produce ions in the ionization chambers. The dose response of selected ion chambers is evaluated in the presence of varying transverse and longitudinal magnetic fields. The investigation is useful in calibration of therapeutic x-ray beams associated with MR-Linac systems. The Monte Carlo code PENELOPE was used to model the irradiation of NE2571, and PR06C ionization chambers in the presence of a transverse and longitudinal (with respect to the photon beam) magnetic fields of varying magnitude. The long axis of each chamber was simulated both parallel and perpendicular to the incident photon beam for each magnetic field case. The dose deposited in each chamber for each case was compared to the case with zero magnetic field by means of a ratio. The PR06C chamber's response was measured in the presence of a transverse magnetic field with field strengths ranging from 0.0 to 0.2 T to compare to simulated results. The simulations and measured data show that in the presence of a transverse magnetic field there is a considerable dose response (maximum of 11% near 1.0 T in the ion chambers investigated, which depends on the magnitude of magnetic field, and relative orientation of the magnetic field, radiation beam, and ion chamber. Measurements made with the PR06C chamber verify these results in the region of measurement. In contrast, a longitudinal magnetic field produces only a slight increase in dose response (2% at 1.5 T) that rises slowly with increasing magnetic field and is seemingly independent of chamber orientation. Response trends were similar for the two ion chambers and relative orientations considered, but slight variations are present from chamber to chamber. Care must be taken when making ion chamber measurements in a transverse magnetic field. Ion chamber responses vary not only with transverse field strength, but with chamber orientation and type, and can be considerable. Longitudinal magnetic fields influence ion chamber responses relatively little (2% at 1.5 T), and only at field strengths in excess of 1.0 T.

SU‐E‐T‐12: Radiation Detector Responses to Applied Homogeneous Transverse and Parallel Magnetic Fields

To determine the relative dose response of a diamond detector and a ion chamber in a clinical photon beam within uniform magnetic fields, endeavoring to evaluate and refine reference dosimetry techniques for use in integrated MR-linac systems. The Monte Carlo code PENELOPE was used to model the structure and materials of the PTW60003 diamond detector and PR06 ion chamber in a 6MV beam in the presence of a homogeneous magnetic field. The magnetic field strength was varied from 0 to 1.5T, and both the parallel and transverse magnetic field orientations with respect to the beam central axis were simulated. The long axes of the detectors were oriented both perpendicular and parallel to the radiation beam direction for each magnetic field orientation. All simulations determined the detectors' signal in air. A small electromagnet was used to experimentally determine the detectors' response in transverse magnetic fields up to 0.2T to validate the simulations. The simulated response of both detectors matched to the experimental data within the estimation error. The relative response of PR06 and diamond detector varied up to ±8.5% (depending on chamber orientation) and >9% respectively with increasing transverse magnetic field strength. In contrast, both detectors were found to be relatively insensitive to the increasing magnetic fields irrespective of the detector orientation in parallel magnetic field. A maximum change of 2% in PR06 response was observed at 1.5T parallel magnetic field and in the parallel orientation of chamber. This work has significant impact on dosimetry protocols for integrated MR-linac systems, where detector response may be altered by the presence of a magnetic field. The need for a magnetic field dependent correction factor is strongly indicated for the transverse magnetic field cases, while such changes in detector response can be largely ignored in parallel magnetic fields < 1T. CIHR (Canadian Institutes of Health Research) - funding support Faculty of Medicine and Dentistry, University of Alberta - funding support.

WE‐C‐BRB‐02: A Monte Carlo Feasibility Study for Calculating Dose Perturbations in Patient Geometries Due to Homogenous Magnetic Fields from MRIgRT

Purpose: Magnetic resonance image‐guided radiotherapy (MRIgRT) has emerged as a promising new technology to reduce tumor position uncertainty during photon radiotherapy. The use of MRI during beam delivery complicates treatment planning since the magnetic field will perturb the radiation dose distribution especially in fields prescribed in heterogeneous regions of the body. The purpose of this study was to investigate the magnetic field effect on dose distributions in patient geometry using Monte Carlo methods. Methods: The Monte Carlo code Geant4 was used for all simulations performed in this study. We investigated dose distributions from a 6‐MV photon beam energy spectra in patient geometries subjected to homogeneous transverse magnetic fields with varying field strengths. For this study perturbations in the dose distribution from a single rectangular field were simulated in a prostate patient scan. Results: Preliminary results show perturbations in the overall dose profile due to the magnetic field can be seen when a transverse magnetic field was applied to the patient geometry. Regions of under‐ and over‐dosing occur throughout the beam path near the penumbra of the photon beam. Conclusions: This study demonstrated that patient‐specific dose calculations for MRIgRT treatments are feasible using Geant4. Work is currently ongoing to include treatment specific field parameters and 4D time‐resolved geometries.

SU‐E‐T‐678: Monte Carlo Investigation on Dose Perturbations in the Lung Due to Constant Magnetic Fields during MRIgRT

Purpose: Magnetic resonance image‐guided radiotherapy (MR‐IGRT) has emerged as a promising new technology to reduce tumor position uncertainty during photon radiotherapy treatments of lung cancer. MRIGRT improves upon traditional IGRT techniques by providing real‐time intrafraction imaging of tumor motion. However, the use of MRI during beam delivery complicates treatment planning since the magnetic field will perturb the radiation dose distribution especially in fields prescribed in heterogeneous regions of the body such as the lung. In this study we used Monte Carlo methods to investigate the magnetic field effects on dose distributions near interfaces of high‐ and low‐density regions where the influence of magnetic fields will be significant. Methods: The Monte Carlo code EGS4nrc was used for all simulations performed in this study. We investigated dose distributions from both Co‐60 and 6‐MV photon beam energy spectra in simple heterogeneous phantoms. The phantoms consisted of alternating slabs of PMMA and air or lung material with different thicknesses. Homogeneous transverse magnetic fields of varying strengths were applied to the phantom during the simulations. Results: Large perturbations in the dose distributions near PMMA‐air or PMMA‐lung interfaces were recorded when a transverse magnetic field was applied to the phantom, which can be attributed to electron return effect. It was demonstrated that the size of the perturbation at an interface depends not only on the magnetic field strength, but also on the primary photon beam energy spectrum and the length of the air cavity within the phantom. Conclusions: Preliminary results demonstrated that magnetic fields alter the delivered dose distributions near interfaces of high‐ and low‐density regions. Therefore, accurate planning for MR‐IGRT treatments of lung cancer may need to consider magnetic field effects on the dose distribution. Future work will use Monte Carlo methods to investigate dose perturbations due to magnetic fields in 4D patient geometry.

Richtmyer–Meshkov instability of a stratified fluid in transverse magnetic field

In the present work, the Richtmyer–Meshkov instability is examined in an N-layer stratified fluid which is impulsively accelerated and immersed in a homogeneous transverse magnetic field. By solving the initial value problem and the second-order linear differential perturbation equation, the expressions of interface amplitudes are analytically obtained. Two special cases N=2 and N=3 are discussed, and it is found that the Richtmyer–Meshkov instability is suppressed by the transverse magnetic field. It is also shown for the N=3 case, when a transverse magnetic field is present, that the interfaces oscillate with a high frequency and the oscillations repeat themselves periodically with a low frequency.

Eddy currents generated in a system of two cylindrical conductors by a transverse alternating magnetic field

In the paper, using the Bubnov–Galerkin method coupled with the separation of variables method, the eddy currents in a system of two cylindrical conductors placed in a transverse homogeneous magnetic field sinusoidally varying with time are investigated. The power losses caused by these currents are also determined. By dint of computer calculations, the graphs of these losses are traced.

Cylindrical two-dimensional electron gas in a transverse magnetic field

We compute the single-particle states of a two-dimensional (2D) electron gas confined to the surface of a cylinder immersed in a magnetic field. The envelope-function equation is solved exactly for both a homogeneous and a periodically modulated magnetic field perpendicular to the cylinder axis. The nature and energy dispersion of the quantum states reflects the interplay between different length scales, namely, the cylinder diameter, the magnetic length, and, possibly, the wavelength of the field modulation. We show that a transverse homogeneous magnetic field drives carrier states from a quasi-2D (cylindrical) regime to a quasi-one-dimensional regime where carriers form channels along the cylinder surface. Furthermore, a magnetic field which is periodically modulated along the cylinder axis may confine the carriers to tunnel-coupled stripes, rings, and dots on the cylinder surface depending on the ratio between the field periodicity and the cylinder radius. Results in different regimes are traced to either incipient Landau-level formation or Aharonov-Bohm behavior.

Magnetoelastic interactions in a soft ferromagnetic body with a nonlinear law of magnetization: Some applications

Linearized equations and boundary conditions of a magnetoelastic ferromagnetic body are obtained with the nonlinear law of magnetization. Magnetoelastic interactions in a multi-domain ferromagnetic materials are considered for magneto soft materials, i.e. the case when the magnetic field intensity vector and magnetization vector are parallel. As a special case, the following two problems are considered: (1) the magnetoelastic stability of a ferromagnetic plate-strip in a homogeneous transverse magnetic field; (2) the stress–strain state of a ferromagnetic plane with a moving crack in a transverse magnetic field. It is shown that the modeling of magnetoelastic equations with a nonlinear law of magnetization provides qualitative and quantitative predictions on physical quantities including critical loads and stresses. In particular, it is shown that the critical magnetic field in plate stability problems found with the nonlinear law of magnetization is in better agreement with the experimental finding than the one found with a linear law. Furthermore, it is also shown that the stress concentration factor around a crack predicted with the nonlinear law of magnetization is more accurate than the one obtained with a linear counterpart. Numerical results are presented for above mentioned two problems and for various forms of nonlinear laws of magnetization.

Optimization of saddle coils for magnetic resonance imaging

In Nuclear Magnetic Resonance (NMR) experiments, besides the apparatus designed to acquire the NMR signal, it is necessary to generate a radio frequency electromagnetic field using a device capable to transduce electromagnetic power into a transverse magnetic field. We must generate this transverse homogeneous magnetic field inside the region of interest with minimum power consumption. Many configurations have been proposed for this task, from coils to resonators. For low field intensity ( < 0.5 T) and small sample dimensions ( < 30 cm), the saddle coil configuration has been widely used. In this work we present a simplified method for calculating the magnetic field distribution in these coils considering the current density profile. We propose an optimized saddle configuration as a function of the dimensions of the region of interest, taking into account the uniformity and the sensitivity. In order to evaluate the magnetic field uniformity three quantities have been analyzed: Non-uniformity, peak-to-peak homogeneity and relative uniformity. Some experimental results are presented to validate our calculation.

Quantum phase transitions in antiferromagnetic planar cubic lattices

Motivated by its relation to an $\cal{NP}$-hard problem, we analyze the ground state properties of anti-ferromagnetic Ising-spin networks embedded on planar cubic lattices, under the action of homogeneous transverse and longitudinal magnetic fields. This model exhibits a quantum phase transition at critical values of the magnetic field, which can be identified by the entanglement behavior, as well as by a Majorization analysis. The scaling of the entanglement in the critical region is in agreement with the area law, indicating that even simple systems can support large amounts of quantum correlations. We study the scaling behavior of low-lying energy gaps for a restricted set of geometries, and find that even in this simplified case, it is impossible to predict the asymptotic behavior, with the data allowing equally good fits to exponential and power law decays. We can therefore, draw no conclusion as to the algorithmic complexity of a quantum adiabatic ground-state search for the system.

Magnetohydrodynamics of trickle bed reactors: Mechanistic model, experimental validation and simulations

An isothermal one-dimensional two-fluid magnetohydrodynamic (MHD) model based on the volume average mass, charge, and momentum balance equations and the Maxwell's equations coupled via the Lorentz force and Ohm's law was developed for the prediction of the two-phase pressure drop and the total liquid holdup in trickle bed reactors experiencing a homogeneous transverse magnetic field. The slit model approximation and the drift flux Kozeny–Carman approach were extended for the derivation of appropriate drag force closures required in the conservation equations, respectively, in the trickle flow regime and in the dispersed bubble flow regime. The expression of liquid–solid drag was adapted to take into account the influence of the magnetic field on the laminar term and the damping of turbulent/inertial term via the Hartman number and the liquid-to-bed electrical resistance ratio. Associating these drag forces with the proposed model resulted in a fully predictive MHD approach for trickle beds. Several model limiting formulations were derived for an electrically conducting fluid flowing downwards with a stagnant gas (pure trickle flow) to yield liquid holdup, as well as for single-phase upward conditions to yield the single-phase pressure drops.

A method for the design of MRI radiofrequency coils based on triangular and pulse basis functions.

This paper presents a numerical technique for the design of an RF coil for asymmetric magnetic resonance imaging (MRI) systems. The formulation is based on an inverse approach where the cylindrical surface currents are expressed in terms of a combination of sub-domain basis functions: triangular and pulse functions. With the homogeneous transverse magnetic field specified in a spherical region, a functional method is applied to obtain the unknown current coefficients. The current distribution is then transformed to a conductor pattern by use of a stream function technique. Preliminary MR images acquired using a prototype RF coil are presented and validate the design method.

Energy losses in a thin-wall tube multifilament composite superconductor carrying a transport current in a time-varying transverse magnetic field

An approach and formulae are given to calculate the hysteresis, coupling, eddy-current and Ohmic losses in a thin-wall tube multifilament composite superconductor carrying a transport current under low excitation by a homogeneous transverse time-varying magnetic field, taking into account possible filament saturation and current sharing effects. The technique can be applied to a single layer superconducting cable twisted around a thin-wall normal conducting tube.

Transverse electron focusing as a way of studying surface crystallography

Abstract Conduction electrons represent a unique probe for studying surfaces (and interfaces) due to their extremely low excitation energies and to the fact that they impinge upon the surface from inside the sample. Focusing of conduction electrons by means of a transverse homogeneous magnetic field—transverse electron focusing (TEF)—provides a means for probing the atomic structure and composition of surfaces and interfaces, including both regular and irregular roughness. This article explains what TEF is, reviews what has been learned from it about surface structure, and describes what can be learned in the future.

Review of theoretical models of electromagnetic NDT and their applications : NDT-86. Proceedings of the 21st Annual British Conference on nondestructive testing, Newcastle-upon-Tyne (United Kingdom), 16–18 Sep. 1986. pp. 541–548. Edited by J.M. Farley and P.D. Hanstead. Engineering materials advisory services ltd. (1987)

An approach and formulae are given to calculate the hysteresis, coupling, eddy-current and Ohmic losses in a thin-wall tube multifilament composite superconductor carrying a transport current under low excitation by a homogeneous transverse time-varying magnetic field, taking into account possible filament saturation and current sharing effects. The technique can be applied to a single layer superconducting cable twisted around a thin-wall normal conducting tube.

Ac free-electron laser.

A possible new source of tunable coherent radiation is analyzed. By the passing of a relativistic electron beam (relativistic factor ${\ensuremath{\gamma}}_{0}$) through a homogeneous transverse ac electric or magnetic field of frequency ${f}_{0}$, electromagnetic radiation of frequency $f\ensuremath{\sim}2{\ensuremath{\gamma}}_{0}^{2}{f}_{0}$ is generated by its coupling the negative-energy electrostatic beam modes with the high-frequency electromagnetic modes. Existing superconductor cavities which can supply ac fields of frequency ${f}_{0}\ensuremath{\gtrsim}10$ GHz with amplitude ${E}_{max}&gt;20$ MV/m may be used to lead to a new class of submillimeter or even visible generating devices.

Laser acceleration of particles using resonance inversions of the Cherenkov effect

The possibility of Cherenkov accelerator development for physics experiments in the 10 2–10 6 TeV range is discussed in connection with the realization of some practical schemes of Cherenkov effect inversion using both the transverse homogeneous magnetic field and several Cherenkov waves. The required fields of 10 −2–10 5 TV/m can be obtained by double inversion of the Cherenkov effect in conventional and artificial ionic media. Calculations of particle dynamics were made taking into account the Coulomb scattering and radiation losses. The possibility of ∼ 10 5 T magnetic fields generation by relativistic particle beams is also discussed.

Skin effect in cylindrical segments placed in a harmonical transverse magnetic field

In the paper, eddy current losses and electrodynamic forces in cylindrical segments (Joffe's conductor) placed in a transverse homogeneous magnetic field sinusoidally changing with time were determined by means of the Bubnov-Galerkin method coupled with the separation of variables.

Cylindrical Plasma Column in a Transverse Magnetic Field

The effect of a transverse homogeneous magnetic field B on a stationary cylindrical plasma column is considered. The problem is solved for the collision-dominated and the collision-free case using first-order perturbation theory in B. The potential and density distributions, the temperature and the Hall reduction factor α = UHall/2Rvd-B are calculated. The limitations of the theory are considered in detail.

The Influence of a Transverse Magnetic Field on a Cylindrical Plasma

The conditions in a cylindrical plasma under the influence of a transverse homogeneous magnetic field are treated by known methods. It is shown that the magnetic field deflects the column towards the wall with the result that the total loss of electrons and ions is increased. This causes an increase in electron temperature and in the axial field strength. Formulae relating the axial electric field, the magnetic field and the electron temperature are derived as well as equations describing the electron density distribution. Some measurements have been made in order to test the theory. The agreement is fair, taking account of the limits of experimental accuracy and of uncertainty in the constants used in the calculations.