Open-Type Magnetically Shielded Room Using Only Canceling Coil Without a Ferromagnetic Wall for Magnetic Resonance Imaging

In order to improve amenities for patients in hospitals, a new open type of magnetically shielded room (MSR) for magnetic resonance imaging (MRI), which uses only canceling coils without a ferromagnetic wall, has been developed. The device was designed by three-dimensional magnetic field analysis and experimentally investigated using an actual MRI of 1.5 T. First, a coil and ferromagnetic material were modeled on an actual MRI, so that the distribution of the leakage magnetic flux vectors was in agreement with that measured. Second, the optimal shaped canceling coil, a saddle-shaped coil which can reduce the leakage flux from the edge plane to the side plane of the room, was designed using the modeled MRI, and realized. The effectiveness of this coil for shielding and its influence on MRI imaging were confirmed experimentally using an actual MRI. In addition, an optimal U-shaped enclosure that can sufficiently shield the leakage flux generated by the coil itself was designed; its effectiveness was confirmed experimentally by using an actual canceling coil.

I. Introduction

Magnetic resonance imaging (MRI) is now widely used in hospitals, etc. as a way to create images of the inside of a human body. MRI has some advantages over other methods; for example, it does not entail radiation exposure, as do both Computed Tomography (CT) scans and X-rays. However, as a strong flux density of more than 1.5 T is applied inside the MRI scanner, flux leakage outside the room becomes a problem because it can lead to malfunctions in heart pacemakers, quartz clocks, and so on. Therefore, the MRI scanner is usually installed in a magnetically shielded room (MSR), in order to reduce the leakage flux density to less than 0.5 mT outside the room [1]. In an ordinary MSR, the room is surrounded by ferromagnetic materials and the inner space is completely enclosed by a wall with the exception of a hall operation window. The small space enclosed by the wall causes patients a great deal of stress and discomfort because they are isolated from the operators and from the scenery outside. As the need to improve patient amenities in hospitals increases, the development of an open-type MSR becomes more pressing, in order to improve the experience for patients. An open-type MSR using magnetic bars instead of magnetic walls has already been proposed [2]. However, that MSR does not provide a sufficiently open feeling because the width and number of bars cannot be made small enough, due to the shielding performance requirements. We therefore investigated an open-type MSR composed of magnetic walls combined with canceling coils, using three-dimensional magnetic field analysis; the validity of the analysis method and practical realization of the facility were confirmed by experiments using a small MSR model [3].