MRI Magnet Design Research Articles

Mechanical analysis of an MgB2 1.5 T MRI main magnet protected using Coupling Loss Induced Quench

Mechanical analysis of the stress and strains developed in the coils were calculated for a ten coil 1.5 T MRI magnet design with magnesium diboride (MgB2) wire protected with Coupling Loss Induced Quench (CLIQ). The temperature distribution inside the coils was first simulated in MATLAB to solve the governing heat and circuit equations. Simulations were performed on the magnet, in which each coil was divided into two subsections, with two CLIQ units while the capacitor ranged from 5 to 20 mF and the initial charging voltage ranged from 2.6 kV to 1.3 kV in order to keep the total stored energy in the CLIQ system constant. The wire’s filamentary twist pitch remained constant at 5 cm for all simulations. The exported temperature distribution was expanded to form a representative unit cell (RUC) representing the wire composite and then imported into ANSYS to calculate the 1st principle strain in the MgB2 filament and shear stress across the epoxy for the coils. A peak temperature of 191 K occurred inside the coil with the initial quench when the CLIQ unit had a 20 mF capacitor charged to 1.3 kV. According to the mechanical simulations, the largest resulting peak strain in the wire was 0.034%, and peak shear stress was 44 MPa.

Parametric Relations in Multicoil Cylindrical MRI Magnet Design

In MRI magnet design, optimized configurations are generated by different manufacturers, which are driven by the pursuit of competitive cost and performance. It is the magnet envelope and the field constraints such as homogeneity and stray field that largely define the coil topology. Understanding the scaling laws and relations that govern sensitivities in the design space gives a designer a tool for scoping and tradeoff analyses, guiding his selection of design options. A study is presented that produces scaling relations and characterizes the amount of conductor, the number and location of coils as a function of the magnet envelope, and field requirements. Described is the birth of coils and their movement with an expanding field of view, as well as impacts of shielding and of shorter envelopes. Derived relations are given in parametric form, which is described by dimensionless complexes.

Numerical study on the quench propagation in a 1.5 T MgB2 MRI magnet design with varied wire compositions

To reduce the usage of liquid helium in MRI magnets, magnesium diboride (MgB2), a high temperature superconductor, has been considered for use in a design of conduction cooled MRI magnets. Compared to NbTi wires the normal zone propagation velocity (NZPV) in MgB2 is much slower leading to a higher temperature rise and the necessity of active quench protection. The temperature rise, resistive voltage, and NZPV during a quench in a 1.5 T main magnet design with MgB2 superconducting wire was calculated for a variety of wire compositions. The quench development was modeled using the Douglas–Gunn method to solve the 3D heat equation. It was determined that wires with higher bulk thermal conductivity and lower electrical resistivity reduced the hot-spot temperature rise near the beginning of a quench. These improvements can be accomplished by increasing the copper fraction inside the wire, using a sheath material (such as Glidcop) with a higher thermal conductivity and lower electrical resistivity, and by increasing the thermal conductivity of the wire’s insulation. The focus of this paper is on the initial stages of quench development, and does not consider the later stages of the quench or magnet protection.

Design of Superconducting Magnet for 1.5 T Dedicated Extremity MRI System

In this paper, an effective method to design the superconducting magnet for 1.5 T dedicated extremity MRI is proposed. The feasible current carrying zones of superconducting magnet are subdivided by an array of grid elements. The size of each grid element is determined by the actual superconducting wire. The initial current distribution of superconducting magnet is optimized using 0-1 integer programming by a comprehensive consideration of superconductivity wire consumption, central magnetic field intensity, imaging region homogeneity, and the range of leakage file. The final rectangular section of magnet is obtained using the genetic algorithm optimization with artificial limitation of the coil position and section size considering the initial current distribution of superconducting magnet. The method based on the actual superconducting wire makes the MRI magnet design more feasible. A superconducting magnet for 1.5 T dedicated extremity MRI system is designed using this method. This magnet can offer 1.5 T central field with high homogeneity in diameter sphere volume (DSV), with total length of 430 mm, inner diameter of 350 mm. This method can also be used for short whole-body MRI superconducting magnet design.

Formulation of the spherical harmonic coefficients of the entire magnetic field components generated by magnetic moment and current for shimming

For MRI and NMR magnet design, a highly homogeneous and high magnetic field has to be achieved by active and/or passive shimming. The active and passive shimming commonly homogenize the magnetic field around the magnet center by cancellation of the higher terms of a spherical harmonic series than the 0th term. So far, the spherical harmonic expression in the cylindrical coordinate system is well known for a circular coil, a solenoid coil, and magnetization of ferromagnetic material with/without its volume. In a shimming design, only the z-component of an inhomogeneous magnetic field is compensated because it is dominant to the performance of MRI/NMR. However, various kinds of MRI and NMR systems have recently been developed, and these magnets sometimes have an axially asymmetric configuration or generate a tilted magnetic field. Therefore, the entire x-, y-, and z-components have to be homogenized for such magnets. I derive the spherical harmonic expression of the entire components of magnetic field generated by a circular coil, a dipole coil, and a straight line current. In addition, since the entire magnetization components of the ferromagnetic material having the volume contribute the entire components of the magnetic field around the magnet center, I also derive the equations of the spherical harmonic coefficients of the magnetic field generated by ferromagnetic material. Since these equations need a numerical integration, such as the Gauss quadrature integration, the computation accuracy of the spherical harmonic coefficients is investigated against the number of the evaluation points. We can calculate the highly accurate spherical harmonic coefficients with the small number of the evaluation points.

A Target Field Approach to Open MRI Magnet Design

The design of an fMRI magnet for the study of the human motor cortex poses a number of challenges due to the necessity of maintaining the subject in a natural, erect position, with free access to the environment. This paper presents the design of an asymmetric superconducting magnet composed of racetrack coils. The coil configuration is derived from the one presented in [1] from which the non-planar coils have been eliminated, with significant simplifications of the manufacturing process. The position of the coils is defined along the principles described in [1] and the current in the coils is determined by means of an approach similar to the Inverse Boundary Method [2-4] in which the goal is established as a set of spherical harmonic coefficients. The major challenge in the design lies in limiting the coil currents to sustainable values without compromising the field homogeneity. The current limit requires an increase in the number of winding layers that, in turn, induces a change in the physical dimensions, making the system nonlinear. It is shown how this can be counteracted by means of an iterative procedure that converges to acceptable results. In the final configuration the magnet accommodates a sitting individual and produces a field intensity of 1.4 T. It consists of planar coils only with current densities and maximum field intensities compatible with currently available superconductors.

Field Stabilization of the Iseult/Inumac Magnet Operating in Driven Mode

A neuroscience research center with very high field MRI equipments was opened in November 2006 by the CEA life sciences division. Three MRI systems operating at 3, 7 and 17 T have been already installed. One of the imaging systems will require a 11.75 T magnet with a 900 mm warm bore. The large aperture and high field strength of this magnet provide a substantial engineering challenge compared to the largest MRI systems ever built. This magnet is being developed within an ambitious R&D program, Iseult, whose focus is high field MRI. Traditional MRI magnet design principles are not readily applicable and thus concepts taken from high energy physics or fusion experiments, namely the Tore Supra tokamak magnet system, will be used. The coil will be made of a niobium-titanium conductor cooled by a He II bath at 1.8 K, permanently connected to a cryoplant. Due to its design the magnet will be operated in a non-persistent mode. As the field stability needed for MRI imaging requires a field drift of less than 0.05 ppm/h, it is hardly feasible to directly transpose these requirements in the power supply specification. Two existing solutions developed for other applications have been selected: one using a semi-persistent mode, and the other using a short-circuited superconducting coil in the inner bore. In order to make a decision on experimental basis, an ambitious R&D field stability program has been set-up based on magnet prototypes, high field test facility (Seht, a 44 H and 8 T magnet with a warm bore to 600 mm). We will present development and experimental results of the two stabilization solutions. In conclusion, the stability solution selected for the Iseult magnet is given.

The Iseult/Inumac Whole Body 11.7 T MRI Magnet R&D Program

A neuroscience research center with very high field MRI equipments has been opened in November 2006 by the CEA life science division. One of the imaging systems will require a 11.75 T magnet with a 900 mm warm bore. Regarding the large aperture and field strength, this magnet is a real challenge when compared to the largest MRI systems ever built, it is being developed within an ambitious R&D program, Iseult, focused on high field MRI. The conservative MRI magnet design principles are not readily applicable, other concepts taken from high energy physics or fusion experiments, namely the Tore Supra tokamak magnet system, will be used. The coil will thus be made of a niobium-titanium conductor cooled by a He II bath at 1.8 K, permanently connected to a cryoplant. Due to the high level of stored energy, about 340 MJ, and a relatively high nominal current, about 1500 A, the magnet will be operated in a non-persistent mode with a conveniently stabilized power supply. In order to take advantage of superfluid helium properties and regarding the high electromagnetic stresses on the conductors, the winding will be made of wetted double pancakes meeting the Stekly criterion for cryostability. The magnet will be actively shielded to fulfill the specifications regarding the stray field. In order to develop the magnet design on an experimental basis, an ambitious R&D program has been set-up based on magnet prototypes, high field test facility (Seht) and stability experiments. The main results from these experiments and their impact on the Iseult magnet design will be discussed.

Multiple Layer Superconducting Magnet Design for Magnetic Resonance Imaging

A design method of multiple layers superconducting magnet for MRI is introduced. Considering the actual coils layout, the candidate domain is divided into several layers. The current density curves of each layer are solved by Quadratic Programming optimization with Minimum Stored Energy. The seed coils are arranged at the peak positions of current curves. Based on the regularization solution, the current of the seed coils can be solved. According to the coil's current, selecting the current density of the superconducting wires, the original sections for the coils can be obtained. Then a further optimization about the homogeneity based on constrained nonlinear multivariable optimization method is employed to determine the final coils' geometries. The method is especially suitable for short superconducting MRI magnet design.

Racetrack Coils for Dedicated MRI Magnets

The need to optimize the magnet shape, size and region of homogeneity with respect to the anatomy of the patient is particularly strong in dedicated MRI magnet design: cost and weight are often determining factors for success in the market. An elongated coil geometry could be useful in cases where the scanner is designed for imaging non axisymmetric parts of the human body (e.g. extremities): in this paper we present a fast design method based on Linear Programming (LP), in which racetrack coils are used to optimize the magnetic field homogeneity over an ellipsoidal region, together with some results.

The Iseult/Inumac Whole Body 11.7 T MRI Magnet Design

A neuroscience research center with very high field MRI equipments has been opened in November 2006 by the CEA life science division. One of the imaging systems will require a 11.75 T magnet with a 900 mm warm bore. Regarding the large aperture and field strength, this magnet is a real challenge as compared to the largest MRI systems ever built, and is then developed within an ambitious R&D program, Iseult, focus on high field MRI. The conservative MRI magnet design principles are not readily applicable and other concepts taken from high energy physics or fusion experiments, namely the Tore Supra tokamak magnet system, will be used. The coil will thus be made of a niobium-titanium conductor cooled by a He II bath at 1.8 K, permanently connected to a cryoplant. Due to the high level of stored energy, about 340 MJ, and a relatively high nominal current, about 1500 A, the magnet will be operated in a non-persistent mode with a conveniently stabilized power supply. In order to take advantage of superfluid helium properties and regarding the high electromagnetic stresses on the conductors, the winding will be made of wetted double pancakes meeting the Stekly criterion for cryostability. The magnet will be actively shielded to fulfill the specifications regarding the stray field.

A design method for superconducting MRI magnets with ferromagneticmaterial

The emphasis of this work is on the optimal design of MRI magnets with bothsuperconducting coils and ferromagnetic rings. The work is directed to theautomated design of MRI magnet systems containing superconducting wire andboth ‘cold’ and ‘warm’ iron. Details of the optimization procedure are given andthe results show combined superconducting and iron material MRI magnets withexcellent field characteristics. Strong, homogeneous central magnetic fields areproduced with little stray or external field leakage. The field calculations areperformed using a semi-analytical method for both current coil and iron materialsources. Design examples for symmetric, open and asymmetric clinical MRImagnets containing both superconducting coils and ferromagnetic material arepresented.

Genetic algorithms for MRI magnet design

Continuing advances in the field of parallel computing have allowed nonlinear optimization techniques to be applied to many problems previously considered too computationally demanding. We describe a general magnet design software package, CamGASP, which uses genetic algorithms (GAs) for the design of large whole-body MRI systems. The method of GAS allows a population of many designs to evolve with a bias toward the fittest designs continuing to later generations. Central to all nonlinear optimization techniques is the cost function, which decreases for designs that match the required specifications and are hence deemed to be fitter. Multiple evaluations of the cost function are necessary to complete a single generation and this task can readily be shared across a network of processors, working in parallel. Thus GAs are especially suited to running on parallel computer systems. We present results of the performance of the GA software and also discuss methods for rapid calculation of magnetic fields from circular coils. We also present specific superconducting MRI magnet designs including a split coil optimized for simultaneous PET and MRI.

Improvement of MRI magnet design through sensitivity analysis

Superconducting magnets for MRI are designed to provide high level of magnetic flux density in a wide testing volume with the greatest level of field homogeneity. The design of such magnets is usually performed using optimization techniques able to tune the geometrical parameters of the magnets, taking also into account constructional issues and dimensions and packaging factors of the wires for each coil and, in addition, technical and physical constraints, such as the critical current of the superconductor to prevent the quench phenomenon. Unfortunately due to manufacturing tolerances, the actual geometrical parameters of the magnet differ from the design ones, affecting the field homogeneity. In this paper the effects of the manufacturing tolerances on the field homogeneity are investigated for an MRI magnet by means of a statistical Monte Carlo analysis.

An inverse design method for elliptical MRI magnets

An inverse, current density mapping (CDM) method has been developedfor the design of elliptical cross-section MRI magnets. The method provides arapid prototyping system for unusual magnet designs, as it generates a 3Dcurrent density in response to a set of target field and geometricconstraints. The emphasis of this work is on the investigation of newelliptical coil structures for clinical MRI magnets. The effect of theelliptical aspect ratio on magnet performance is investigated. Viable designsare generated for symmetric, asymmetric and open architecture ellipticalmagnets using the new method. Clinically relevant attributes such as reducedstray field and large homogeneous regions relative to total magnet length areincluded in the design process and investigated in detail. The preliminarymagnet designs have several novel features.

2D visual design of permanent MRI magnets by a hybrid BEM-IEM computation

A hybrid boundary element method (BEM)-integral equation method (IEM) computation is proposed for the two-dimensional (2D) design of permanent MRI magnets. Over the finite element method (FEM), the hybrid BEM-IEM computation has the advantages of much less discrete elements and thus less computer memory and CPU time. As the curved shape of the ferromagnetic pole planes of permanent MRI magnets plays an important role in improving the uniformity of the magnetic field, a computer program using the hybrid computation is developed to optimize the pole plane shape visually and automatically. With a 0.2T Nd-Fe-B magnet, the proposed design method is carried out and the results are given in this paper.

A hybrid, inverse approach to the design of magnetic resonance imaging magnets.

This paper describes a hybrid numerical method of an inverse approach to the design of compact magnetic resonance imaging magnets. The problem is formulated as a field synthesis and the desired current density on the surface of a cylinder is first calculated by solving a Fredholm equation of the first kind. Nonlinear optimization methods are then invoked to fit practical magnet coils to the desired current density. The field calculations are performed using a semi-analytical method. The emphasis of this work is on the optimal design of short MRI magnets. Details of the hybrid numerical model are presented, and the model is used to investigate compact, symmetric MRI magnets as well as asymmetric magnets. The results highlight that the method can be used to obtain a compact MRI magnet structure and a very homogeneous magnetic field over the central imaging volume in clinical systems of approximately 1 m in length, significantly shorter than current designs. Viable asymmetric magnet designs, in which the edge of the homogeneous region is very close to one end of the magnet system are also presented. Unshielded designs are the focus of this work. This method is flexible and may be applied to magnets of other geometries.

A comparison of MRI magnet design using a Hopfield network and the optimized material distribution method

Two approaches to device design, the Hopfield network and optimized material distribution (OMD), are compared. The comparison is based on a previously published MRI dipole magnet design problem. Since this problem is already formulated as a material distribution problem, the results focus on the optimization procedures used in the two methods, i.e. the learning algorithm of the Hopfield network versus the conjugate gradient algorithm used in OMD. Results of the two methods are presented, and are also compared to a previously published simulated annealing solution.

Considerations in the design of MRI magnets with reduced stray fields

One of the major considerations in siting choice for an MRI system within a hospital is its interactions with its environment. This interaction places restrictions on the proximity of equipment sensitive to magnetic field and limits areas of general public access. In addition special account must be taken of the possible impact of environmental iron on magnet homogeneity. To date, the approach adopted by the MRI system scanner manufacturers to these problems has been to employ either YOKE or MIRROR iron (PASSIVE) shielding frequently requiring significant structural modifications with associated costs. An alternative approach to the shielding problem has been investigated at OXFORD using a geometry utilising superconducting counter running coils alone and prototypes with Central fields of 0.5T, 1.0T and 1.5T have been tested.