Speaker
Description
The demand for magnetic resonance imaging (MRI) devices with low cost and reasonable weight continues to increase as applications keep requiring diverse fields of view (FOV). In response to this demand, research on open MRI employing permanent magnets has been actively pursued.
The use of permanent magnets eliminates the need for power consumption and cooling systems, making the setup environment for MRI devices significantly more cost-effective. Moreover, as long as the operational temperature remains below the Curie point, deviations in magnetism by external effects do not need to be considered.
To maintain a portable size and low weight while ensuring the homogeneity of the static magnetic field within the FOV, researchers develop numerous design methods of permanent magnets.
There are several methods to optimize the shape and topology of magnets or magnetic materials in various applications. These include gradient-free approaches such as genetic algorithms and gradient-based approaches that use sensitivity analysis. Previous studies proposed design optimization methods that utilize continuum sensitivity analysis to optimize the orientation and position of magnets or the shape of magnetic materials.
Optimization based on continuum sensitivity offers advantages compared to gradient-free methods, such as flexibility in defining design variables and independence from discrete models. However, gradient-based methods have limitations, including dependency on the initial shape and a tendency to converge to local optimum solutions.
In addition, earlier design approaches use finite element methods (FEM) to numerically analyze the system with high accuracy. During the optimization process, iterative finite element analyses result in long computation times and costs.
In this paper, we propose a design method for permanent magnets in the open MRI device by applying the advantages of the aforementioned approaches. We introduce a simplified algorithm based on dot sensitivity, a concept that falls within the continuum topology sensitivity.
The permanent magnet dot sensitivity is defined as the change in the objective function when a permanent magnet dot is introduced into a vacant region. This derivation uses an analytical solution for the magnetic field within the dot created under a uniform external field. The magnitude and direction of the permanent magnetization are assumed to be uniform and fixed, respectively.
The dot sensitivity distribution over the design domain is computed in a numerically modeled system using FEM. This distribution is then used to derive the permanent magnet configuration that satisfies the desired field distribution.
The process begins by discretizing the design domain into 𝑛×𝑛 grid of cells. Cells are randomly selected to integrate dot sensitivity within each selected region. This integrated value is referred to as cell sensitivity. Among all cell configurations, the permanent magnet distribution yielding a cell sensitivity most close to the integral value of the objective function in the initial model is identified. The configuration with the fewest number of selected cells is the optimized solution.
To demonstrate the applicability of the proposed method for open MRI magnet design, the C-shaped and H-shaped magnets were designed. The derived results showed that the method achieved field homogeneity using a low quantity of magnets. The designed geometries, assumptions, and specifications will be detailed in the full paper.
In this paper, permanent magnet dot sensitivity formulations are derived for a restricted application with fixed magnetization direction, and the optimization process is established. In the future work, the method may be extended to systems with variable magnetization directions, such as the Halbach cylinder, which offers advantages in weight reduction.
