Assessment of MRI worker exposures to pulsed magnetic fields produced by gradient coils has recently attracted a lot of awareness in the field of occupational health and safety. To accurately model the exposures, a full three-dimensional distribution of the magnetic field in the vicinity of the magnet end is required. Unfortunately, for many MRI installations, the coil pattern that generates this magnetic field is often not provided by the manufacturer. A method is presented in which the prediction of a current distribution that generates a nearly identical magnetic field pattern is constrained by a number of experimentally measured magnetic field sample points outside the gradient set of interest. The method takes into consideration other important descriptors such as field uniformity in the working volume, gradient coil geometry, driving current, gradient strength, active shielding etc. To demonstrate the application of the method, current density and matching magnetic field distributions of x- and z-axis gradient coils are derived. This enables robust, accurate evaluations of exposures of tissue equivalent numerical worker models without pre-knowledge of gradient coil patterns.

Synopsis: This paper presents a new method for biplanar active and passive shim design using an Equivalent Magnetizing Current (EMC) method. The EMC induced by the rotational component of the magnetization is equivalent to that of the stream function (SF) and hence the SF is proportional to the magnetization. Using this approach, the magnetic field generated by a magnetized disk of finite thickness is related directly to the SF and hence no intermediate step to transform the current density into SF is required. Optionally, instead of a current pattern, a set of iron pieces can be employed so that the magnetized shims can be placed at equally spaced contours of the magnetization-stream function (MSF). The MSF is expressed as a sum of orthogonal functions of the azimuthal angle and shim domain radius and so it is tailored in the source domain in order to generate a particular magnetic field harmonic or a combination of these inside the DSV. The method is validated using known examples and the potential to generate new solutions is demonstrated.

This study presents a biconjugate gradient method (BiCG) that can significantly improve the performance of the quasi-static finite-difference scheme, which has been widely used to model field induction phenomena in voxel phantoms. The proposed BiCG method offers remarkable computational advantages in terms of convergence performance and memory consumption over the conventional iterative, successive over-relaxation algorithm. The wide application capability and computational performance of the BiCG method is demonstrated by modelling the exposures of MRI healthcare workers to fields produced by pulsed field gradients, which is presently an important topic of research in light of the Physical Agents Directive 2004/40/EC

Synopsis: This paper presents a new alternative shimming procedure to correct the magnetic field inhomogeneities generated by horizontal and C-shape biplanar MRI magnets. A magnetization map obtained through the application of the Equivalent Magnetizing Current (EMC) method is used to define the domain where the discrete iron shim set is placed to generate a given field harmonic [1]. Optionally, instead of iron a current pattern is used, then the magnetization is related to the stream function (SF) and the current pattern is placed at equally spaced contours of the SF [1]. If a set of discrete iron pieces with no reversible magnetization direction is employed to mimic the continuous magnetization function (MF) map, then iron shims of unit strength are placed only in the positive domain (valid domain) of the MF map. The field source matrix is calculated only for the discrete elements located in the valid domain, which leads to a better conditioned matrix and superior solutions. An LP algorithm is used to calculate the optimal thickness and location of the discrete ferroshims to produce the target harmonics. Examples of simulated shimming of horizontal/C-shape magnets are presented. In the case of permanent open magnets, the magnetic coupling among the iron pieces and its influence over the magnetic field harmonics is studied for linear and nonlinear iron cases. The influence of the selection and arrangement of individual shim sizes over the field source matrix conditioning is also analysed.

To numerically evaluate the electric field/current density magnitudes and spatial distributions in healthcare workers when moving through strong, nonuniform static magnetic fields generated by the magnetic resonance imaging (MRI) system and to understand the relationship between the field characteristics and levels/distributions of induced field quantities.

To numerically evaluate the electric field/current density magnitudes and spatial distributions in healthcare workers when they are standing close to the gradient coil windings near the magnetic resonance imaging (MRI) scanner ends.

In modern magnetic resonance imaging (MRI), there are concerns for the health and safety of patients and workers repeatedly exposed to magnetic fields, and therefore accurate and efficient evaluation of in situ electromagnetic field (EMF) distributions has gained a lot of significance. This paper presents a Biconjugate Gradient Method (BiCG) to efficiently implement the quasi-static finite-difference scheme (QSFD), which has been widely utilized to model and analyze magnetically induced electric fields and currents within the human body during the operation of the MRI systems and in other settings. The proposed BiCG method shows computational advantages over the iterative, successive over-relaxation (SOR) algorithm. The scheme has been validated against other known solutions on a lossy, multilayered ellipsoid phantom excited by an ideal loop coil. Numerical results on a 3D human body model demonstrate that the convergence time and memory consumption is significantly reduced using the BiCG method.

The design of high-performance gradient coils is essential for modern magnetic resonance imaging (MRI) applications. This work presents a new and alternative design methodology that explores three-dimensional (3D) solution space by combining fuzzy membership functions to model and re-shape the coil structure, given the magnetic field, electrical and mechanical constraints. The method was applied to design a short, unshielded asymmetric gradient coil for breast imaging. The resulting dome-shape coil has superior gradient performance compared to a standard fingerprint coil. New quadrupolar gradient coil designs for breast imaging are also obtained with the proposed method.

The design of high-performance gradient coils is essential for modern magnetic resonance imaging (MRI) applications. This work presents a new and alternative design methodology that explores three-dimensional (3-D) solution space by combining fuzzy membership functions to model and reshape the coil structure, given the magnetic field and electrical and mechanical constraints. The approach includes a stream function generator, a 3-D coil structure generator, and the evaluation of an objective function. An unconventional stream function for asymmetric transverse gradient coils is defined in terms of fuzzy sets. The method was applied to design a short, unshielded asymmetric gradient coil for breast imaging. The resulting dome-shape coil has superior gradient performance compared to a standard fingerprint coil. New quadrupolar gradient coil designs for breast imaging can also be obtained with the proposed method. The paper concludes with a study of the influence of a number of design parameters on the coil performance.

This study extends our recent works on object-oriented Finite-Difference Time-Domain (FDTD) simulations into a parallel-computing framework, producing substantially improved performance with respect to computing time efficiency. The simulator is intended to be a complete, high-performance FDTD model of an MRI system including all temporal radio frequency (RF) and low-frequency field generating units and electrical models of the patient. The parallel computational structure has been designed using a Message-Passing Interface (MPI) Library. The implementation of the MRI-dedicated FDTD algorithms within the parallel computing framework is detailed. To improve the simulator performance in a realistic network-computing environment, several software design aspects have been investigated. These include: data transmission with consideration of buffered and synchronous communication, load balancing on the computing network using a weighting mechanism. The power of the optimized FDTD parallel architecture is illustrated by two distinct, large-scale field calculation problems involving the study of the interaction of RF-fields with human tissue (whole-body, male and female) in high held MRI and characterization of the temporal eddy currents induced on the cryostat vessel during gradient switching. The examples demonstrate the improved capabilities of the simulator. in addition, through the parallelization of the FDTD algorithm we are in a position to study various MRI phenomena at much higher spatial resolutions than was previously feasible. (c) 2007 Wiley Periodicals, inc.

The switching of magnetic field gradient coils in magnetic resonance imaging (MRI) inevitably induces transient eddy currents in conducting system components, such as the cryostat vessel. These secondary currents degrade the spatial and temporal performance of the gradient coils, and compensation methods are commonly employed to correct for these distortions. This theoretical study shows that by incorporating the eddy currents into the coil optimization process, it is possible to modify a gradient coil design so that the fields created by the coil and the eddy currents combine together to generate a spatially homogeneous gradient that follows the input pulse. Shielded and unshielded longitudinal gradient coils are used to exemplify this novel approach. To assist in the evaluation of transient eddy currents induced within a realistic cryostat vessel, a low‐frequency finite‐difference time‐domain (FDTD) method using the total‐field scattered‐field (TFSF) scheme was performed. The simulations demonstrate the effectiveness of the proposed method for optimizing longitudinal gradient fields while taking into account the spatial and temporal behavior of the eddy currents. Magn Reson Med 57:1119–1130, 2007. © 2007 Wiley‐Liss, Inc.

High-quality gradient coils are pivotal to advances in magnetic resonance imaging (MRI). We have studied the influence of coil dimensions and target requirements in multilayer, asymmetric, transverse gradient coils. We developed a simple linear function that defines the optimal coil length to produce a maximum figure of merit given an imaging region size and location, coil radius, and gradient nonuniformity. Our method, based on the linear function, yields high-quality solutions. The method introduces two torque/force minimization strategies in order to obtain asymmetric transverse gradient coils that balance minimum torque with a maximum figure of merit. High-performance head, asymmetric gradient coils with simple current patterns and minimum torque can be tailored to a specific magnet design, as we illustrate

In modern MRI, occupational workers are exposed to strong, non-uniform static magnetic fields generated by the main superconducting magnet. Previous studies have indicated that movement of the body through these fields can stimulate in situ electric fields/ current densities approaching physiological significance. The relationship between the magnetic field pattern/strength and the current distribution/level induced in the body is not well understood. This paper presents numerical evaluations of electric fields/currents in tissue-equivalent, whole-body male and female human models (occupational workers) at various positions and a variety of normalized body motions around three superconducting magnets with central field strengths of 1.5T, 4T and 7T. Possible correlations between the magnetic field characteristics and the induced current density distribution are described and simulations show that it is possible to induce electric fields/current densities above the ICNIRP and IEEE safety standards when the worker is moving very close to the magnets.