Abstract:
A shield for controlling a magnetic field emanating from a magnetic in a magnetic medical treatment instrument has the form of a circular shaped band with opposite open ends and a center axis extending perpendicularly between the open ends. The magnet in the magnetic medical treatment instrument moves about an operating table having a surface upon which a patient being operated on by a physician reposes. The circular shaped shield surrounds the magnet and is spaced away from the magnet in a manner to allow the physician to enter the interior of the band and operate on the patient. The band has opposite first and second parallel planar portions. The top surface of the operating table and the first and second planar portions of the band are parallel to each other. The operating table has a length and a width, and the length of the operating table is parallel to the center axis of the band.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to magnetic shielding and in particular to passive magnetic shielding used to contain the fringe magnetic field emanating from a source magnet contained in a magnetic medical treatment instrument. 
     2. Description of the Related Art 
     Magnetic medical treatment instruments have long been used in the medical field as a tool in performing non-invasive medical procedures, stereotaxic mapping, and magnetic resonance imaging (MRI). The versatility for providing care with these types of instruments has increased with the advent of more powerful computers to assist the physician in controlling the magnet and processing the data developed by the treatment instrument. However, the fields and gradients created by the source magnets used by these treatment instruments are strong and usually require extensive shielding at many health care facilities. 
     In the environment of a health care facility, a magnetic field may cause interference with the proper operation of health care monitoring equipment and other electronic health aids. In areas where magnetic medical treatment instruments are used, advisory signs are generally posted to warn about the potential dangers of entering the area. Rooms housing magnetic medical treatment instruments or treatment rooms, have restricted access when the instrument is being used so that patients and other personnel using magnetically sensitive electronic equipment do not inadvertently enter the treatment room are become adversely effected from the operation of the instrument. Since space is at a premium in health care facilities, magnetic medical treatment instruments may be placed in rooms that are adjacent other areas where health care monitoring equipment and other electronic health aids are being used. To limit the areas in the health care facility having restricted access, the treatment room must be properly shielded. Some institutions and agencies impose limits as low as 5 Gauss on the field strength of magnetic fields outside the treatment room. 
     Conventional shielding systems in treatment rooms at health care facilities are rather elaborate and generally fixed in the structural portions and foundations of the treatment rooms. For example, the walls, the ceiling, and the floor of the treatment room may be lined with iron-based or other magnetic permeable materials in order to deflect and control the magnetic field generated by the source magnet. This shielding is extremely heavy, and may require additional structural support. Thus installation can be difficult, time consuming and expensive. 
     Often, conventional room shielding must be customized on the site once the magnetic medical equipment is installed to ensure proper attenuation of the magnetic field from the source magnet. These unexpected problems sometimes pose logistical problems for technicians during the initial set-up of the treatment instrument. Highly permeably magnetic materials are generally not easily machined and assembled on the job site. Congruent and matching shapes must be created between adjoining sections of shield to reduce the possibility of fringing the magnetic field. 
     A primary reason that room magnetic shielding is so heavy and complete is that it is used for MRI machines. These are generally hug cylindrical coils which are kept at a strong current continuously for months and years. Because of the size, the field wall (unless the new, compensating coils are used) is relatively strong. In addition, the size and direction of the field is constant, so that a strongly magnetized shielding, tending in part toward saturation at the wall, will take on a somewhat permanent magnetization of its own, with thermal variations and mechanical vibrations present to realign the magnetic domains more completely. It is observed when MRI machines are turned off that the wall shielding is quite highly magnetized. Unless such shielding is thick and of more expensive very low carbon steel, such a magnetization can be sever, even while maintaining safe levels outside the room, and will be insufficient to maintain safe levels outside after a period of use. 
     Therefore, in the Magnetic Stereotaxis System, or other strong magnetic field sources which take random directions in a procedure room, it is possible to judiciously design a system with much lighter, thinner, and less encompassing shielding. 
     Therefore, what is needed is a magnetic shield that guides and shapes the magnetic field emanating from the source magnet of a magnetic medical treatment instrument in such a manner as to attenuate the field in a relatively short distance away from the magnet. The shield would be arranged in close proximity to the magnet to contain the magnetic field around the magnet but still allow proper operation of the magnetic medical treatment instrument. By attenuating the field in a short distance away from the source magnet, rooms adjacent to the treatment room may have unrestricted access. The shield would allow other magnetic sensitive equipment found in rooms adjacent to the treatment room to be operated without interference. The magnetic shield would be standard equipment for a particular medical treatment instrument and obviate the requirement for custom placement and configuration of special shielding in the treatment room. The magnetic shield would be smaller and be of less weight so that the structural requirements of the treatment room may be reduced. The magnetic shield would have a lower cost than conventional methods of shielding the entire ceiling, floor, and walls of the treatment room. 
     SUMMARY OF THE INVENTION 
     According to the principals of the present invention, a magnetic shield is arranged in close proximity to the source magnet of a magnetic medical treatment instrument and forms an operating space within the volume defined by the shield. The magnetic shield shapes and channels the fringe magnetic field from the source magnetic to allow the magnetic medical treatment instrument to be operated in an environment where other magnetically sensitive electronic equipment may be used. 
     In one embodiment of the current invention, a magnetic shield is provided around a magnet medical device that generates magnetic fields to shape and channel the field so as to contain the magnetic field in the immediate vicinity of the magnet. Thus, the shield prevents the magnetic field from radiating from the treatment room into surrounding rooms which may have other sensitive electronic equipment that may be disturbed or disrupted by the magnetic field generated by the magnetic medical treatment instrument. The magnetic shield has a ceiling and opposite floor section, and left and right side sections extending between the ceiling and floor sections. The ceiling and floor sections and left and right sections of the shield define the operating space. The left and right side sections are curved members having concave surfaces facing toward each other that give the operating space a generally cylindrical shape. The magnetic reduced device is positional within the operating space. 
     According to another embodiment of the invention, the shield has the form of a tube with cylindrically shaped sides spaced apart by parallel, planar ceiling and floor sections. The tube has opposite open ends separated by the sides, and a center axis extending between the open ends. The tube has an interior defining an operating space in which the magnetic medical treatment instrument is placed. The operating space has sufficient area to allow a physician to enter and exit the operating space to operate on a patient with the magnetic medical treatment instrument. 
     According to another embodiment of the invention, a shield is used for controlling the magnetic field emanating from a magnetic medical device. The shield has the form of a circular shaped band having opposite open ends and a center axis extending perpendicularly between the open ends. The band is positioned to surround the magnet at distance from the magnet to allow the physician to enter the interior of the band and operate on the patient. The band has first and opposite, second parallel planar portions. The top surface of the operating table and first and second planar portions are parallel to each other. 
     In this arrangement the magnetic field emanating from the magnet used in the magnetic medical device may be shaped and contained with the immediate area of the magnet. By guiding the magnetic field in this way, the field may be directed out the open ends of the band into the surrounding room in selected directions. Because magnetic field falls off quickly with distance from the source, the magnetic field channeled out through the open ends of the shield may be dissipated in selected directions without the use of additional shielding in the walls of the treatment room. By providing the shield in close proximity to the source magnet, the magnetic field emanating from a magnetic medical device may be contained within the boundaries of the treatment room so that the operation of the magnetic medical device does not disrupt other electronic and other sensitive medical monitoring and treatment equipment in adjacent rooms. This also allows the construction of smaller treatment rooms. Additionally, by constructing a shield in close proximity to the source magnet, a smaller shield may be provided with a reduced weight and lower cost than room shielding. 
     In some cases a room is large enough, or a magnet source has small enough projected field, that shields on the ceiling and floor will suffice. In those cases, it is desirable to minimize their sizes and weights. This can optimally be done with appropriate consideration of the relative sizes and distances of these flat shields to that of the source magnet. Usually the floor plate is closer to the source magnet than is the ceiling plate. Therefore its design will need to incorporate greater thickness to reduce saturation, while it may be somewhat smaller in extent because it subtends a larger solid angle at the closer distance to the magnet. In essence, these plate provide regions which do not surround the source magnet, but which instead provide effective return paths for the flux at room boundaries that are closer than the side walls. 
     With the principle just stated, a finite element analysis (FEA) program can be used to provide specific design by trial and error. In this method, a first trial set of floor and ceiling plates is located in the FEA volume and a worst case magnetic source field applied. The resulting fields in regions beyond the plates and within the plates are calculated. From these the degree of saturation or lack of it in the plates is noted, and sizes and thickness of the plates adjusted accordingly, depending on the material used. A difficulty in this procedure occurs because the source field is so much stronger than the tolerable field outside the shields that sufficient resolution in space and field strength cannot be obtained in practical times with readily available computers. At this juncture in the procedure it is important to assess the field leakage around the plates along with the fields in the plates. If the leakage bulges sharply and significantly at the plate edges, then the plate is not overly saturated, but is too small, at least in one dimension. If the field drops across the plate, but is still too large beyond it, and there are not sharp bulges at the edge, then the plate is too thin, or of a material which saturates too easily, or is of too low permeability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and features of the invention are revealed in the following detailed description of the preferred embodiment of the invention and in the drawings wherein: 
     FIG. 1 is a perspective view of a magnetic medical treatment instrument of the present invention with a close-in magnetic shield system in place around the instrument; 
     FIG. 2 is a perspective view of the magnetic shield of FIG. 1 with a cut-away view of a connection between a left section of the shield and a floor section of the shield; 
     FIG. 3 is a cross-sectional view of a joint in a section of the magnetic shield of FIG. 1; 
     FIG. 4 is a partial, side cross-sectional view of a typical portion of the magnetic shield used to form a floor section of the magnetic shield of FIG. 1; 
     FIG. 5 is a partial, side cross-sectional view of a typical portion of the magnetic shield used to form ceiling, left, and right sections of the magnetic shield of FIG.  1 . 
     FIG. 6 is an exploded, perspective view of the magnetic shield of FIG. 1; 
     FIG. 7 is a partial, side view of the connection between the ceiling section of the shield and the left side section of the magnetic shield of FIG. 1; 
     FIG. 8 is a partial, side cross-sectional view of the connection between the floor section of the shield and left side section of the magnetic shield of FIG. 1; 
     FIG. 9 is a side view of the magnetic shield of FIG. 1 showing the magnetic field distribution from the side of the shield; and 
     FIG. 10 is a front view of the magnetic shield of FIG. 1 showing the magnetic field distribution from the front of the shield. 
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 show a general construction of a magnetic shield constructed according to the principles of the present invention. The magnetic shield, generally indicated at reference number  10 , includes a ceiling section  12  and an opposite, floor section  14  spaced apart by a left and right side sections  16 ,  18 . The ceiling section  12  and floor section  14  are preferably rectangular in shape and form parallel planes spaced apart from one another by the side sections  16 ,  18 . The left and right side sections  16 ,  18  are curved members with concave surfaces facing each other that give the shield  10  a generally cylindrical appearance with opposite open ends  20 ,  22 . Together, the ceiling and floor sections  12 ,  14 , and the left and right side sections  16 ,  18  define the boundaries of an operating space  24 . The operating space  24  has a center axis extending between the open ends  20 ,  22  that is aligned parallel with the axial direction of the cylindrically shaped shield  10  and the left and right side sections  16 ,  18  of the shield. Each of the ceiling and floor sections  12 ,  14  extend outward beyond the open ends  20 ,  22  of the operating space beyond the side edges of the left and right side sections  16 ,  18  in the direction of the center axis. Preferably, the ceiling section  12  extends outward beyond the left and right side sections  16 ,  18  in a direction perpendicular to the center axis to form an overhang  26  with each of the left and right side sections  16 ,  18 . 
     A magnetic medical device  28  is positioned within the operating space  24 . The magnetic medical device  28  includes a patient table  30  with a longitudinal axis that is aligned with the center axis of the operating space  24 , and a source magnet  32 . As shown in FIGS. 1 and 2, the floor  34  of the treatment room is equipped with a track  36  to allow the source magnet  32  to be positioned with respect to the patient table  30 . In an alternative construction, the treatment instrument may have a patient table capable of moving into the operating space. The track  36  for the treatment instrument  28  is aligned parallel with the center axis of the operating space  24 . The track  36  stops in a position where the source magnet  32  is fully encompassed by the left and right side sections  16 ,  18  when the source magnet  32  is properly positioned relative to the patient table  30 . 
     Inside the operating space  24 , the longitudinal axis of the patient table  30  is preferably positioned parallel with the center axis of the shield  10  as shown in FIGS. 1 and 2. A top surface  31  of the patient table  30  upon which the patient reposes during the procedure preferably forms a plane that is parallel with the ceiling and floor sections  12 ,  14  of the shield. Preferably, the patient table  30  is positioned such that the portion of the patient&#39;s body to be treated using the source magnet  32  is positioned within the travel range of the source magnet  32 . A portion of the patient table  30  may extend through one of the open ends of the shield  10  so as to provide a more compact arrangement of the shield  10  around the patient table and the properly positioned source magnet  32 . 
     The ceiling and floor sections  14 ,  16  may be formed from smaller modules  38  to reduce the overall weight and cost of manufacturing the sections of shield  10 . Preferably, the ceiling and floor sections  14 ,  16  are made from three modules  38  and each of the left and right side sections  16 ,  18  are made from a single module  38 . Each module  38  is preferably an industry standard size of 4′×8′ flat stock material. As shown in FIG. 3, each module to be joined to form a section has a joint edge  40  with steps  42 . The steps  42  allow the modules  38  to be joined with an overlap that forms a smooth interlocking surface between adjacent modules  38  of the sections. The outer most corners of the modules may have fastener holes to secure adjacent modules. The left and right side sections  16 ,  18  of the shield are preferably rolled to form the desired radius of curvature to generate the needed volume in the operating space  24 . 
     As shown in FIGS. 3,  4  and  5 , the modules  38  are preferably made from layers of carbon steel  44  and aluminum  46 . Experimentally, it has been found that a laminate of AISI 1008 steel and aluminum (any grade) provide adequate materials for the shield. The carbon steel  44  is a magnetically permeable material and the aluminum  46  decreases the overall weight of the module  38 . Preferably, each layer of the carbon steel  44  is {fraction (1/32)}″ thick and each layer of the aluminum  46  is {fraction (1/16)}″ thick. The floor section  14  of the magnetic shield  10  is comprised of four layers of carbon steel  44  interposed among three layers of aluminum  46 , as shown in FIG.  4 . The ceiling section  12 , and left and right side sections  16 ,  18  of the magnetic shield have a similar construction except that the laminate is comprised of three alternating layers of carbon steel interposed among two layers of aluminum, as shown in FIG.  5 . To bond the carbon steel  44  to the aluminum  46 , an adhesive  48  is used. Preferably, the adhesive  48  is sprayed on and has a thickness of no more than 0.010″ to promote adequate field conduction. 
     The ceiling section  12  of the magnetic shield is preferably held in position above the operating space  24  by attaching it to a structure of the room in which the magnetic medical treatment instrument  28  is to be used. The ceiling section  12  is suspended from the structural members that comprise the ceiling structure between adjacent floors in the building. The ceiling section  12  is rectangular in shape and extends beyond the operating space  24  to overhang the patient table  30  at one open end  20  of the operating space  24  and to overhang the magnetic medical treatment instrument  28  as it travels along its tracks  36  adjacent the opposite open end  22  of the operating space  24 . 
     Spaced away and parallel to the ceiling section  12  is the floor section  14 . In a similar arrangement with the ceiling section  12 , the floor section  14  preferably extends beyond the operating space  24  in the directions of the open ends  20 ,  22  of the operating space  24  along the center axis of the operating space  24 . The floor section  14  may project the same distance as the ceiling section  12  beyond the open ends  20 ,  22  of the operating space  24  in the same direction as the center axis of the operating space  24 . As shown in FIG. 2, the floor  34  of the treatment room is preferably formed with a rectangular recess  50  having a length and width equal to the floor section  14  of the magnetic shield  10  so that the recess  50  may receive the floor section  14  therein. The depth of the recess  50  is sized for the thickness of the floor section  14  and to accommodate the height of the tracks  36  upon which the source magnet or patient table slides. The side perimeter edges of the floor shield  14  operably connect to the left and right side sections  16 ,  18  of the shield  10  and do not extend beyond the left and right sections  16 ,  18 . 
     The left and right side sections  16 ,  18  of the shield are concave members that are shaped to increase the volume of the operating space  24 . The radius of curvature generally is proportional to the shape of the magnet field emanating from the source magnet  32  so as to contain the flux emanating from the magnet  32 . The curvature is also arranged so that a smooth transition may be provided between the left and right side sections  16 ,  18  and each of the ceiling and floor sections  12 ,  14  of the shield. The left and right side sections  16 ,  18  are positioned so that when the magnet  32  is properly positioned with respect to the patient table  30 , the left and right side sections  16 ,  18  fully encompass the source magnet  32 . The left and right side sections  16 ,  18  are also spaced away from the source magnet  32  to allow the physician to enter the operating space  24  and operate on a patient on the patient table  30 . The left and right side sections  16 ,  18  are spaced sufficiently away from the patient table  30  to allow the physician to enter and exit the operating space  24  and access the control panels and other instrumentation that are used to control the treatment instrument  28 . Since the ceiling and floor sections  12 ,  14  are permanently affixed to the structures of the treatment room, the left and right side sections  16 ,  18  do not bear the weight of the ceiling section  12 . The left and right side sections  16 ,  18  of the magnetic shield  10  are constructed to support each section&#39;s  16 ,  18  respective own weight while the curvature provides ample room in the operating space  24 . 
     The left and right side sections  16 ,  18  of the shield act as a flux connector between the ceiling and floor sections  12 ,  14  of the shield. In order to guide the field between each of the side sections  16 ,  18  and the floor and ceiling sections  12 ,  14  without causing the field to fringe, a smooth transition between the sections is needed. To provide the smooth transition between the left and right side shields  16 ,  18  and the ceiling and floor sections  12 ,  14  of the magnetic shield, ceiling and floor angle plates  52 ,  54  are provided. 
     As shown in FIG. 6, two ceiling angle plates  52  are provided to join the left and right side sections to the ceiling section  12 , and two floor angle plates  54  are provided to join the left and right side sections  16 ,  18  to the floor section  14  of the shield. Preferably, the angle plates  52 ,  54  have the same width as the left and right side sections  16 ,  18 . The ceiling angle plates  52  have a side engagement portion  56  that attaches a top edge  58  of the each of the left and right side sections  16 ,  18  of the shield, and a ceiling engagement portion  60  that is obliquely angled to the side engagement portion  56 . The top edge  58  of each of the left and right side sections  16 ,  18  and the side engagement portion  56  of the ceiling angle plates  52  have a series of matching fastening holes  62  that permit attachment of each of the side sections  16 ,  18  to the respective ceiling angle plate  52  to the ceiling section  12  of the shield. As shown in FIG. 7, the oblique angle at which the ceiling engagement portion  60  is formed with the side engagement portion  56  provides a smooth transition between the left and right side sections  16 ,  18  and the ceiling section  12 . The angle plate  52  may have a thickness that allows mechanical fasteners  66  to be countersunk into the ceiling angle plate  52  so as to prevent a fringing field to be developed from sharp protrusions that may extend beyond the interior surfaces of the shield. 
     To join the left and right side sections  16 ,  18  of the shield to the floor section  14 , the floor angle plates  54  are provided. The floor angle plates  54  have a similar construction to the ceiling angle plates  52  in that the floor angle plates  54  have a side engagement portion  68  that attaches to the left and right side sections  16 ,  18  of the magnetic shield and a floor shield engagement portion  70  which engages and attaches to the floor section  14 . The side engagement portion  68  of the floor angle plate  54  and the bottom portion of each of the left and right side sections  16 ,  18  have a series of matching holes  72 . In this way, each left and right side section  16 ,  18  may be attached to the respective floor angle plate  54 . Preferably, the mechanical fasteners  66  are used to join the left and right side sections  16 ,  18  to the floor angle plate  54 . The side engagement portion  68  is obliquely angled to the floor engagement portion  70 . The floor engagement portion  70  also has a series of holes through it along its width  74  for attaching the floor angle plate  54  to the floor section  14 . 
     On the side perimeter edges of the floor section  14  in the area where the left and right side sections  16 ,  18  are joined to the floor section  14 , a floor box  76  is provided. As shown in FIG. 6, each floor box  76  is rectangular in shape with four walls extending outward from a bottom wall  78  to form a rectangular box with an open top. The floor box  76  has side walls  80  that are slightly wider than the width of the left and right side sections  16 ,  18  of the shield, and end walls  82  that are wider than the combined size of the side engagement portion  68  and the floor engagement portion  70  of the floor angle plate  54 . As shown in FIG. 8, the interior volume of the floor box  76  is sized to receive the floor angle plate  54  and the bottom portion of the respective left and right side section  16 ,  18 . The bottom wall  78  of the floor box  76  is positioned on top of the floor section  14  to expose the open top. The side perimeter edge of the floor section  14  is substantially even with the outboard side wall  80  of the floor box  76 . The floor box  76  is anchored to the floor  34  of the treatment room through a floor box anchor  84 . The floor box anchor  84  also partially secures the floor section  14  to the floor  34  of the room. 
     FIGS. 2 and 8 show the typical arrangement of the floor box  76  and floor section  14 . The walls  78 ,  82  of the floor box  76  extend upward from the bottom wall  78  of the floor box  76  to enclose the floor angle plate  54  and provide a bounded volume in which the left and right side sections  16 ,  18  of the shield may be joined to the floor section  14 . The walls  78 ,  82  of the floor block  76  are spaced away from the surfaces of the floor angle plate  54  to provide an installation technician access to the mechanical fasteners  66  that attach the left and right side sections  16 ,  18  of the magnetic shield to the side engagement portion  68  of the floor angle plate  54  and access to the mechanical fasteners  66  that attach the floor engagement portion  70  of the angle plate to the floor section  14 . 
     Generally, a portion of the magnetic medical treatment instrument  28  slides on a track  36  that is positioned in the floor  34  of the treatment room. To ensure that the magnetic flux emanating from the source magnet  32  of the magnetic medical instrument  28  is properly controlled and shaped, the floor section  14  of the shield is positioned under the tracks  36 , as shown in FIG.  8 . However, to support the weight of the sliding portions of the treatment instrument  28 , the tracks  36  are positioned on a layer of concrete  86  poured on top of the floor section  14  of shield. When the concrete is poured over the floor section  14  of the shield and trenches  88  for the tracks  36  are formed, the floor boxes  76  provide a mold around the fasteners  66  and floor angle plate  54  to provide access to the floor angle plate  54  and the fasteners  66 . The floor box  76  acts a flux connector to direct the magnetic field from the left and right side sections  16 ,  18  to the floor  14  where the concrete layer  86  in the floor  34  might otherwise impede the travel of the field. Preferably, the floor blocks  76  and floor angle plates  54  are made from carbon steel or other highly magnetic permeable material to allow the magnetic field to be conducted from the ceiling section  12  to the floor section  14  of the shield. 
     FIGS. 9 and 10 provide a visual representation of the attenuation of the magnetic field using the shield  10  of the present invention. In operation, the shield  10  attenuates a magnetic field having a field strength of 0.1 Tesla to less than 5 Gauss at a distance of approximately ten feet from the top surface of the operating table in each vertical direction. The 5 Gauss line is indicated at reference numeral  90 . Similarly, the shield  10  attenuates the field to less than 5 Gauss at a distance of ten feet from the centerline of the patient table in each horizontal direction. The shield directs the magnetic field out through the open ends  20 ,  22  of the operating space  24  where air acts to attenuate the magnetic field. As shown in FIGS. 9 and 10, the field is also attenuated to less than 5 Gauss at a distance of approximately ten feet in both directions through the open ends  20 ,  22  of the shield  10 . 
     Thus, with the magnetic shield  10  of the present invention, the magnetic medical treatment instrument  28  may be positioned in any room in the hospital and an adjacent room may have unrestricted use. Although the patient table  30  extends upward from the floor  36  of the treatment room a height of roughly four feet, the distance between the rooms on floors below and above the treatment room is such that the 5 Gauss line  90  does not generally extend into these areas. Thus, rooms on floors below and above the treatment room may also be released for unrestricted use given the shield and source magnet described previously. 
     The ceiling angle plate  52  and the floor angle plate  54  provide smooth transitions to guide and shape the flux between the ceiling and floor sections  12 ,  14 . Since the shield  10  is arranged in close proximity to source magnet  32 , the size and resultant weight of the shield  10  may be reduced. This reduces the material and cost of the shield  10 . Moreover, because the shield is relatively thin and has a reduced weight, the shield may be easy installed and retrofitted into existing treatment rooms. Although the magnetic shield  10  of the present invention is formed using alternating layers of carbon steel  44  and aluminum  46 , other highly magnetic permeable materials such as 80 Ni metal may be used to form the shield  10 . Carbon steel  44  has been chosen because of its relative cost effectiveness, its each of machining and manufacturing, its bonding capabilities with other metals, and its ease of forming into required shapes. 
     As various changes could be made in the above construction with departing from the scope of the invention, it is intended that all matter contain the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in any limiting sense. The invention therefore shall be solely limited by the scope of the claims set forth below.