High-Field Superconducting Magnets

Abstract

Over the past 30 years, magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) have developed into the key technologies assisting medical diagnosis and many kinds of scientific research. MRI and MRS, however, remain fields characterized by a great deal of innovation. The main goal of this innovation is the improvement in the basic signal to noise ratio. One way whereby the signal to noise ratio can be improved is by increasing the strength of the main magnetic field, and this has initiated a lot of development in the field of magnet technology. The exploitation of superconductivity is the only practical and economic way of reaching higher magnetic field strengths. This paper will describe the basic principles of superconducting magnet design starting with the generation of magnetic fields and the handling of the associated forces. MRI and MRS applications pose considerable demands on the homogeneity and stability of such magnets. These demands are reflected in the design of the magnet coil windings, and the superconducting materials that are used in magnet construction. Furthermore, methods for the fine adjustment of the magnet homogeneity (often referred to as shimming) are described. For operational and economic reasons superconducting coil structures need to be well thermally insulated and therefore the cryostat technology is also described with particular reference to active and passive cooling technologies. Finally, different specifications and parameters for existing High-Field Superconducting MRI magnets are presented beginning with 7 T magnets for human applications and ending with 11.7 T human systems that are currently in the design phase. For each magnetic field strength typical specifications and parameters for small animal MRI superconducting magnets used in pre-clinical research are also described.