High Field Magnet Construction in NMR Devices Explained

Introduction

Nuclear magnetic resonance (NMR) devices are playing an increasingly important role in healthcare and research today. As the term implies, magnets — specifically high field magnets — are an essential part of these sophisticated devices, with important implications for a wide range of valuable healthcare and research applications. To gain additional insights into how these devices operate, this paper provides a discussion concerning the standard construction of modern high field magnets used in NMR devices, including a detailed graphic illustrating the different components of a representative magnet. An examination of the effects of transitions to higher magnet strengths on cooling systems is followed by an analysis of the superconducting materials used and a brief description of magnet construction. A discussion concerning the differences between shielded and non-shielded magnets and innovations in technology that may allow room-temperature magnet applications that avoid evaporation in the future is followed by a summary of the research, with important findings and trends presented in the conclusion.

Standard Construction of High Field Magnets

The history of the construction of modern high field magnets can be traced to the late 20th century, when magnetic resonance applications were being routinely used for medical diagnosis (Carlisle 2004; Leroy 2003). According to Jacoby and Youngson (2005), "When high field magnets were introduced in the 1980s, scans became quicker to do and improved computer technology made the images much clearer" (p. 190). Initially termed "nuclear magnetic resonance" when applied to human patients, the medical community determined that the use of the word "nuclear" was counterproductive, and the process was renamed magnetic resonance imaging for all intents and purposes (Goldberg 2007).

The construction of the high field magnets used in these diagnostic devices improved over time as materials and manufacturing processes continued to introduce innovations in high-field magnet performance (High field magnets 2011). At present, typical high-field magnets use multifilamentary Nb3Sn windings to generate magnetic fields ranging between 10 Tesla and more than 15 Tesla at 4.2K (High field magnets 2011). All such high-field magnets are comprised of an Nb3Sn inner coil (high field region) surrounded by an 8 to 9 Tesla NbTi outer coil (background field) (High field magnets 2011). According to this vendor, "Coil windings are permeated with epoxy to ensure the absence of voids and prevent wire movement and its subsequent 'training' effects" (High field magnets 2011, p. 3). These recent innovations have meant that nuclear magnetic resonance devices have become increasingly valuable research tools in the laboratory (Wanjek 2003). In fact, a high field magnet research facility at Oxford University has been in the vanguard of developing world-class superconducting materials (High field magnet facility 2011).

Effects of Higher Magnet Strengths on Cooling Systems

Phase transitions in higher magnet strengths represent a particularly challenging phenomenon because of the "dome-shape" they describe. Furthermore, the operation of the various types of cooling systems used in different superconductors is affected by a wide range of performance metrics, including design, materials composition, and the type of superconductor involved (Johnston 2009). Likewise, an effective energy gap in superconductors has been demonstrated in microwave absorption experiments that clearly illustrate the effects of such transitions to higher magnet strengths on the level of cooling required for optimal operation (Wang, Ono, Onose, Gu, Ando, Tokura, Uchida, and Ong 2002).

According to a recent patent application from Bruker Biospin GmbH (2011), "High temperature superconductors (HTS) of oxidic ceramic material have been known since 1986. They are particularly characterized by very high transition temperatures of up to 120K as well as very high critical magnetic field upper limits (BC2)" (Superconducting magnet coil for very high field 2011, p. 2). The functionality of these ceramic materials, however, is constrained by their fragility and the complex nature of the process steps involved in maintaining temperatures appropriate for optimal superconduction (Superconducting magnet coil for very high field 2011).

For instance, the patent application for a superconducting magnet coil for a very high field points out that "In a processing step, thermal treatment is carried out in an oxidizing atmosphere at temperatures in the range of 800°C. To maintain the optimum superconducting properties, the oxygen content of the atmosphere must be controlled with high precision and must be continuously provided to the superconductor in the required concentrations in accordance with a desired processing procedure" (2011, p. 2). The patent application adds that optimal operating temperatures are typically within a narrow range of tolerances, but that improvements in material composition and design are reducing this constraint (Superconducting magnet coil for very high field 2011, p. 2).

Conclusion

This paper delivered a review of the relevant peer-reviewed, scholarly, and industry literature regarding the standard construction of modern high field magnets used in nuclear magnetic resonance devices, along with detailed graphics illustrating the components of a high field magnet, and a discussion concerning the effects of transitions to higher magnet strengths on superconductor cooling systems. An examination of the types of superconducting materials used and a description of magnet construction was followed by a discussion of the respective differences between non-shielded and shielded magnets. Finally, an analysis of the effect of recent innovations in technology that may allow room-temperature magnet applications that avoid evaporation in the future concluded the paper.

References

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