<title>Magnetic Resonance Imaging Using Permanent Magnets</title>

Abstract

Magnetic Resonance Imaging Using Permanent MagnetsC. G. MasiMagnicon Corp., 59 Birch St., Derry, NH 03038AbstractUntil recently the idea of using permanent magnets for magnetic resonance imaging hasbeen ignored in the literature. This paper discusses the technical aspects of the design ofPermanent magnets for whole body imaging and describes the differences between permanentmagnet systems and those using resistive or superconducting electromagnets. Material proper-ties, magnetic circuit characteristics, and the problems of scaling to whole body size arediscussed. Also discussed are the problems of site preparation, maintenance and repair.IntroductionThe development of magnetic resonance imaging was initially carried out using electro-magnets for several reasons. The technique is an outgrowth of NMR spectroscopy whereelectromagnets have a decided advantage over permanent magnets because of their high fieldstrengths and the ability to vary the field strength over a wide range. Manufacturers havehad considerable experience in building electromagnets to produce high quality fields inlarge sizes.Early studies showed that permanent magnets would be relatively heavy and limited infield strength. At that time these factors were felt to make permanent magnets unsuitablefor magnetic resonance imaging. Since then it has been shown that some of the problems ofelectromagnets may be as serious as those of permanents magnets and the latter are againbeing seriously considered. Two companies have built permanent magnets for whole bodymagnetic resonance imaging: Fonar Corporation and Oldendorg Magnetic Resonance (OMR).Fonar has had several years experience in building these magnets for their model QED 80imager. In the Fall of 1982 they introduced their model QED 3000 which produced images ofacceptable quality with a 3000 gauss permanent magnet. At the same time OMR presented aprototype of a 1500 gauss permanent magnet. In the Spring of 1983 they completed a fullsize version. As of this writing they have not trimmed the magnet to the uniformity general-ly thought necessary for whole body imaging.ElectromagnetsSuperconducting and resistive electromagnets designed for whole body magnetic reson-ance imaging are all constructed according to the same general plan. Usually circular coilsof wire are arranged on a common axis. In the resistive version shown in Figure 1 the endcoils have a smaller diameter than those in the center in order to keep the lines of forceconcentrated until they emerge from the magnet. Outside of the magnet bore the lines spreadout over a large volume as they curve around to form closed loops. If these large loops aredistorted by the presence of large ferromagnetic objects.such as elevator counterweights,the uniformity of the magnetic field at the center will be degraded. Also, the magneticfield represented by these loops is strong enough to disrupt many of the devices necessaryin a modern hospital, such as magnetic recording media and diagnostic instruments.As shown in Figure 2, the external field of a moderate size electromagnet cannot becontained in a 40 foot by 50 foot room. From above, the field can be seen to spill intoadjacent corridors and rooms (a). All computer equipment, large ferromagnetic objects andmagnetic recording media must be located outside the 10 gauss ellipse. More sensitiveequipment such as nuclear medicine instruments must be kept well outside the 3 gaussellipse. Viewed from the front (b), these effects can be seen to extend two floors up fromthe magnetic resonance imaging suite (assuming 10 -feet per storey). Permanent magnets do nothave this problem. The author s calculations indicate that the external field will notextend significantly beyond the magnet itself (c and d).Permanent Magnet DesignThe magnetic field of a piece of permanent magnet material is generated by the valenceelectrons of its constituent atoms, which are in stable quantum states that have a magneticmoment. The moments of adjacent atoms spontaneously line up parallel to each other, forminglarge regions of uniform magnetization called domains shown in Figure 3. The domains canpoint in any direction independently of one another. When a permanent magnet is unmagnetizedthe domains point in random directions (a). About half of them have their north poles facingup and about half have their north poles facing down. The net field is zero. The material is