A High-Field Superferric NMR Magnet zyx F. Russell Huson, R. Nick Bryan, William W. MacKay, Richard C. Herrick, John Colvin, Joseph J. Ford, Sergio Pissanetzky, Gordon A. Plishker, Richard Rocha, William Schmidt, Michael Teodorescu, Gary Shotzman, John Zeigler zyx Strong, extensive magnetic fringe fields are a significant prob- lem with magnetic resonance imaging magnets. This is par- ticularly acute with 4-T, whole-body research magnets. To date this problem has been addressed by restricting an extensive zone around the unshielded magnet or by placing external unsaturated iron shielding around the magnet. This paper de- scribes a solution to this problem which uses superconduct- ing coils closely integrated with fully saturated iron elements. A 4-T, 30-cm-bore prototype, based on this design principle, was built and tested. The 5 G fringe field is cOotained within 1 meter of the magnet bore along the zaxis. Homogeneityof the raw magnetic field is 10 ppm over 30% of the magnet's diam- eter after passive shimming. Compared with an unshielded magnet, 20% less superconductor is required to generate the magnetic field. Images and spectra are presented to demon- strate the magnet's viability for magnetic resonance imaging and spectroscopy. Key words: magnets; homogeneity;shielded. INTRODUCTION A critical area for further development of magnetic reso- nance imaging (MRI)and spectroscopy (MRS) is the mag- net system. The current maximum field strength of com- mercial magnets is 2 T for 1-meter-bore, whole- body scanners. Prototypes of a 4-T 1-meter-bore magnet have been developed but pose formidable problems arising from the very strong and extensive fringe field that will have to be solved if these magnets are to be placed in a clinical setting. All NMR systems require a static magnetic field of sufficient strength to produce a detectable magnetization within a sample. Magnetic field strength and homogene- ity are critical factors in NMR measurements. Three key factors influenced by this field are Signal to noise ratio -which increases with magnetic field strength zyxwvuts (1). .Chemical shift dispersion zyxwvutsr - which is also propor- tional to magnetic field strength. Spectral resolution-which is related to the uniformity of the magnetic field (2). Human NMR investigations require large bore (1 meter) magnets to properly accommodate patients. Fast From the Texas Accelerator Center, The Woodlands, Texas (F.R.H., W.W.M., J.C., S.P., R.R., W.S., M.T., G.S., J.Z.) and Baylor College of Medicine Magnetic Resonance Center, The Woodlands, Texas (R.N.B.. R.C.H., J.J.F.. G.A.P.). Address correspondence to: Richard C. Herrick, Ph.D., Baylor College of Medicine, Department of Radiology, Mail Stop BCM-1658, 1 Baylor Plaza, Houston, TX 77030. Received July 2, 1991; revised March 20, 1992; accepted March 23, 1992. Copyright zyxwvutsrqpo 0 1993 by Williams & Wilkins All rights of reproduction in any form reserved. 0740-31 94/93 $3.00 imaging techniques and in vivo spectroscopy require high magnetic fields to obtain adequate signal-to-noise ratios in a reasonable amount of time. Maintaining the field homogeneity is critical for preserving spectral res- olution. These factors are pushing research interest and development toward higher magnetic field intensities and greater homogeneity. RF field penetration problems and RF probe design considerations may put a practical upper limit on the field intensity for proton imaging. Nonetheless, higher field intensities would greatly re- duce acquisition times for low sensitivity nuclei such as sodium and phosphorus, and make imaging of these nu- clei more practical. To advance the state of the art, the MRI magnet sys- tem must be capable of operating above zy 2 T with adequate shielding to block the magnetic fringe field. In addition, the ideal shielding should aid in maintaining a high degree of homogeneity. Inadequate shielding of these magnets allow strong fringe fields which can accelerate sizeable magnetic objects to lethal veloci- ties from distances of a few meters from the magnet. At larger distances, up to tens of meters, the fringe field can interfere with the performance of electronic instru- mentation in rooms adjacent to the magnet, even above or below on adjacent floors. Magnetic fields produced ex- ternally or external permeable iron can severely distort the homogeneity of the field within the magnet. The sys- tem itself can produce additional distortion of field if unwanted eddy currents are induced in nearby conduc- tors. Commercial MRI superconducting magnets presently used in clinical settings have not solved all of these prob- lems. They typically have a maximum field range of only 1.5 to 2 tesla with 1-meter bores. The field homogeneity of these unshimmed magnets is only about 20 ppm over a 50-cm sphere. In general, they are unshielded air-core solenoids, with 5 gauss fringe fields at distances up to 10 meters (3). This poses not only environmental safety haz- ards, but siting limitations as well. In an effort to improve MRI magnets for medical ap- plications, we have developed a high-field superferric magnet prototype with a 30-cm horizontal bore. The term zyxwvu superferric refers to a superconducting electro- magnet containing iron in which the magnetic induction is driven beyond the saturation value of 2.1 T. The su- perferric magnet design proposed by Huson et al. (4-6) is a spin-off from accelerator magnet research. Integrating the superconducting coils with saturated iron allows the iron to restrict the fringe field while improving field strength and homogeneity. This design is particularly adapted for use with magnetic field intensities greater than the magnetic saturation of iron, but could operate at or below this level. The basic magnet design and actual 25