Superconducting MRI Magnet Research Articles

Design of Open High Magnetic Field MRI Superconducting Magnet With Continuous Current and Genetic Algorithm Method

An optimization design method of short-length actively shielded and open structure superconducting MRI magnets is suggested in the paper. Firstly, the section of the solenoid coil is simplified as a current loop with zero section to solve a linear programming problem. The position coordinates in the radius and axial, and current for the loop can be calculated by the linear programming method. Then, the cross-section of the coil is optimized with a genetic algorithm to get appropriate section size. The method of linear programming, especially combining with genetic algorithm, reduces optimizing variables, which makes the design of a magnet feasible. Based on the method, a full open MRI superconducting magnet is designed with maximum radii of 0.8 m and 1.2 m. In the paper, the detailed optimization technologies are presented.

A Novel Design Method of Shapes of Ferromagnetic Materials for the Superconducting MRI Magnets

We present a novel non-stochastic method for optimizing the shapes of ferromagnetic materials to generate a target static magnetic field. A magnetizing current corresponding to an error field is calculated as an inverse problem and then the current is replaced with an equivalent ferromagnetic material. Iteration of this process leads to an error field of almost zero. It should be noted that the initial condition of the material shape is critical for convergence. Consequently, we propose a combined method: the initial shape of the material is generated with a stochastic optimization algorithm, and then the shape is updated using an explicit method. We show that this method works well with a test 2D axisymmetric magnet.

Design and construction of a prototype high‐power B0 insert coil for field‐cycled imaging in superconducting MRI systems

AbstractField‐cycled MRI provides a noninvasive quantification of the concentration of bound (activated) targeted contrast agents. For field‐cycled imaging to be performed within a clinical full‐body superconducting MRI magnet, an auxiliary insertable magnet is required that can dynamically change the main field strength during the MR pulse sequence. Herein, the design and construction of a solenoidal insert coil is discussed. Design aspects including active shielding, thermal performance, forces, and ramping times are examined. Proper materials, construction methods, and interfacing for the insert coil with a clinical MRI are also covered. Finally, a field‐cycled experiment in a 1.5 T clinical MRI system is described, demonstrating the feasibility of this method and then future novel contrasts available with field‐cycled MRI. © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 35B: 1–10, 2009

Iterative EM Design of an MRI Magnet Using HTS Materials

Conventional superconducting MRI magnet electromagnetic (EM) design involves the critical parameters of the magnet field and dimensional requirements, plus the low temperature superconductor (LTS) wire properties. When using HTS material for an MRI magnet, in addition to the conventional characteristics of a superconductor, the wire and the joint resistance need also to be considered. This is mainly due to the relatively low n-values of the BSCCO tape when compared to that of a conventional LTS wire, and it is also due to the more resistive joint nature for HTS joints. This paper discusses an iterative EM design procedure that calculates the resistance of the HTS wire/tape for the entire magnet, which is a strong function of the magnetic fields at different locations in the coils. The resistance is fed back to the regular MRI EM design for optimization, in order to meet resistance target and the ordinary MRI design goals. This iterative design process results in an optimized HTS MRI design with overall low resistance for magnet drifting and power supply requirements.

A 0.6 T/650 mm RT Bore Solid Nitrogen Cooled MgB2 Demonstration Coil for MRI-a Status Report.

Aiming to demonstrate feasibility and practicality of a low cost superconducting MRI magnet system targeted for use in small hospitals, rural communities and underdeveloped countries, MIT-Francis Bitter Magnet Laboratory has developed a 0.6 T/650 mm room temperature bore demonstration coil wound with multifilament MgB2 conductor and cooled via an innovative cryogenic design/operation. The coil is to be maintained cold by solid nitrogen kept in the solid state by a cryocooler. In the event of a power failure the cryocooler is automatically thermally decoupled from the system. In this paper we present details of the MgB2 conductor, winding process, and preliminary theoretical analysis of the current-carrying performance of the conductively cooled coils in zero background field and over the 10-30 K temperature range.

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.

Problems of measurement of the helium boil off rate of tomographic magnets

The problems connected with the evaluation of the helium boil off of a superconducting MRI magnet are discussed. The influence of changes in atmospheric pressure on the evaporation rate of a 1.5 T whole body tomograph magnet was investigated in experiments. The necessity of application of a pressure stabilizer for the boil off measurement was demonstrated. At the continuously stabilized helium bath pressure, the basic evaporation rate and also the boil off increase due to the eddy currents produced during the scanning procedure were measured.

A study on optimal coil configurations in a split-type superconducting MRI magnet

Optimal coil configurations in a split-type superconducting MRI magnet have been investigated, and the feasibility of the compact and high performance magnet is presented. A highly homogeneous magnetic field of about 10 ppm/50 cm diameter spherical volume (DSV) is attainable by eliminating error fields of higher orders and, moreover, the radius of the outermost coil can be reduced to about 70 percent of that of the magnet with positive current simply by alternately arranging coils with positive and negative currents.

A compact 0.8T superconducting MRI magnet

A compact, cryogen-free 0.8T superconducting MRI magnet has been woundusing Nb 3Sn tape conductor and is maintained at 8K using a two-stage Gifford-McMahon refrigerator. The magnet is housed in a low eddy-current cryostat which incorporates a fiberglass-reinforced epoxy (FRE) vacuum enclosure with an integral stainless-steel permeation barrier, a composite copper/fiberglass thermal shield heat stationed to the first stage of the cryocooler, and a FRE magnet support structure heat stationed to the second stage of the cryocooler. The low eddy-current components eliminate the need to shield the gradient coils. The magnet components and the measured performance characteristics of the system are described in detail.

A 4.7 tesla magnet for magnetic resonance imaging and spectroscopy

This paper describes the design of a superconducting MRI magnet for imaging experimental animals. The magnet is designed to have an 0.5 m warm bore with a central induction of 4.7 tesla. The field uniformity will be good enough to do NMR spectroscopy on living animals as well as magnetic resonance imaging. The magnet is designed so that the field will be less than 0.1 tesla at the cryostat outside boundary and the 0.0001 tesla (1 gauss) line will be about 5 meters from the center of the magnet when the central magnetic induction is 4.7 tesla.