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Multiphysics Coil Design: Applications in Novel Magnetic Resonance Imaging Systems

Final Report Summary - MCODE (Multiphysics Coil Design: Applications in Novel Magnetic Resonance Imaging Systems)

Introduction

Magnetic resonance imaging (MRI) systems are essential tools for the diagnosis of a wide range of medical conditions. Recent developments in MRI technology have seen a move towards novel systems dedicated to particular medical niches, such as ultra-high field systems, combined MRI-positron emission tomography (PET) scanners and systems for musculoskeletal or brain imaging. These novel systems require radical redesign of the MRI hardware, including one essential sub-system known as the gradient coils. The gradient coils produce images from signals coming from the subject through frequency encoding of the MR signal.

Current design methods for gradient coils optimise electromagnetic properties and then empirical testing is needed to ensure that the designs are practical and safe. This is fine for standard MRI systems but the complexity of novel MRI systems means that this is likely to result in suboptimal gradient coils. In this project, we will incorporate a number of properties other than electromagnetics into the design of gradient coils, including temperature, vibration and practical manufacturing constraints. This will ensure optimal performance of gradient coils in novel MRI systems. Preliminary experiments have seen more than 100% improvement in performance in cases where the size of the wire in the coil limits performance. Improved performance of the gradient sub-system will allow increased image resolution, shorter scan times and importantly, will make more novel systems feasible.

The project requires development of appropriate models of the physics, software development, empirical validation and the production of gradient coils for a 9.4 T MR-PET system. The outcomes will be a greater understanding of the thermodynamic and vibration processes in gradient coils, new knowledge about the possibility for reduced temperature, vibration and acoustic noise and the production of gradient coils with improved performance to enable new methods in metabolic imaging.

Completed parts of the project

The project was terminated early due to unforeseen circumstances. Figure 1 shows, schematically, the plan for the project with respect to the prior state of the art. Prior to the project, the design of gradient coils included only electromagnetic considerations, and the aim was to include the physics of thermodynamics and vibration. It has been modified to illustrate the completed parts of the project.

The mathematics was developed from first principles and software was written to dovetail with the IBEM coil design code, which simulates the temperature of electromagnetic coils. The forward simulation was verified against previously measured temperatures of coils. The agreement between the temperatures measured using a thermal imaging camera and those simulated with developed thermodynamic model was excellent.

The aim of this part of the project was then to solve the inverse problem. That is, to invert the thermodynamic model in order to design coils with lower maximum temperature. Coils with lower temperature can consequently be operated at higher duty cycle as demanded by advanced MR imaging techniques developed recently. Software was written to solve the minimum maximum temperature coil design problem using convex optimisation techniques. In words, this optimisation minimises the maximum temperature that occurs in a coil subject to the creation of a specified magnetic field (for gradient coils in MRI that means a field that varies linearly in x, y and z), and other electromagnetic constraints.

The key to realistic results in manufactured coils is to construct a standard coil, measure the temperature distribution in it and fit the thermal simulation parameters (thermal conductivity, cooling coefficients, etc.) to the results. Then, using these parameters to design new coils, with the described optimisation, that have lower peak temperatures. Simulations with different thermodynamic parameters were performed, but the measurement of the temperature of coils was not performed. Nevertheless, this demonstrates the method and coils were redesigned using the minimum maximum temperature technique. Figure 2 shows examples of the temperature simulations of a standard coil, a), and a coil that has been optimised to have lower peak temperature, b), for the same magnet field. A significant reduction in peak temperature was predicted. The simulation results shown in c) illustrate the ability of the developed method to design coils of a different geometry – in this case, a gradient coil for a portable MRI system designed on two flat discs rather than the cylindrical shapes in a) and b).

It is hoped that similar techniques can be taken up by the MRI manufacturers, which will result in MRI systems of higher performance and therefore improved medical imaging diagnostics.
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