Abstract:
A cylindrical magnet assembly for use in magnetic resonance imaging apparatus has a radially compact construction which eliminates prior manufacturing steps. Recesses are formed in insulation layers for receiving complimentary shaped bus bars. Because the bus bars are dimensioned to fit flush within the recesses, they do not add to the radial growth of the magnet assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to laminated cylindrical coils for producing magnetic fields and relates in particular to such apparatus wherein insulated bus bars are inset within grooves formed in the coils. 
     2. Description of Prior Developments 
     The manufacture and construction of a gradient coil assembly for use in magnetic resonance imaging (MRI) apparatus can be complicated and costly. A typical gradient coil assembly includes the main coil subassembly which includes three cylindrical main coils known as the x, y and z main coils and the shield coil subassembly which includes corresponding cylindrical shield coils known as the x, y and z shield coils. The entire gradient coil assembly is mounted within the bore of an MRI magnet for producing the accurately controlled intense magnetic fields required to produce MRI images. 
     The x and y gradient coils can be formed from flat panels which are bent into 90° or 180° arcs and assembled into cylindrical fabrications. Such panel constructions used for the x and y coils are known as “thumb-print” 90° or 180° section panel coils. Each panel includes an electrical conducting layer laminated to a backing layer of insulation which may be of a resin base formulation such as G-10 fiberglass. 
     In order to properly distribute electrical current to the x and y gradient coils, bus bars are inserted radially between the x and y coils and between y and z coils. This placement of the bus bars takes up space between the x and y gradient coils as well as between the y and z gradient coils. The result is an overall gradient coil assembly having a relatively large radial thickness due to the placement of the bus bars. 
     When the bus bars are initially mounted to a panel coil, a layer of resinous composite material is generally applied over the entire outer surface of the bus bars and panel coils to form a cylindrical sleeve around the bus bars and their underlying gradient coils. This layer of composite material must be allowed to harden and cure. The time required for curing slows down the magnetic coil manufacturing process. 
     After the layer of composite material cures, it must be carefully machined to provide a smooth accurate cylindrical surface beneath and within which the bus bars are embedded. The carefully machined surface on the composite resin material is required for maintaining an accurate coaxial alignment between the x, y and z coils. This accurate alignment is needed to produce accurate magnetic field gradients. 
     The next layer of gradient coil material is then mounted over the machined resinous composite layer. It can be appreciated that the machining of the composite material adds significant time and cost to the fabrication of the gradient coil assembly. 
     The conventional assembly of the bus bars between the x and y gradient coils and between the y and z gradient coils not only requires significant time and effort, it results in a large diameter coil assembly. This is a drawback, since the end result is an MRI magnet having a smaller central opening. Small openings are considered unfavorably by patients who may experience claustrophobia when positioned in such small openings. 
     Moreover, if the main gradient coil subassembly and the shield gradient coil subassembly can be made more radially compact, the gradient coil assembly becomes more magnetically efficient insofar as it requires less electrical current to produce the same magnetic field as a more radially enlarged design. 
     Accordingly, a need exists for a more radially compact gradient coil assembly for an MRI magnet. A further need exists for a gradient coil assembly which eliminates one or more of the time consuming manufacturing steps associated with the fabrication of conventional gradient coils. Yet another need exists for a gradient coil assembly which facilitates the mounting of bus bars between concentric cylindrical gradient coils in an MRI magnet assembly. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed to fulfill the needs noted above and therefore has an object the provision of a radially compact gradient coil assembly for use in MRI apparatus. 
     A further object of the invention is the provision of a bus bar mounting arrangement for mounting bus bars between adjacent concentric coils, such as gradient coils used in MRI apparatus. 
     Yet another object of the invention is the provision of an improved method of assembling gradient coils such that one or more conventional manufacturing steps are eliminated. 
     Another object of the invention is the provision of locating pins for facilitating the accurate alignment of consecutive layers of magnet coils, both axially and circumferentially. 
     These and other objects are met in accordance with the present invention which is directed to a compact gradient coil assembly wherein grooves are machined within the coil backing insulation material to receive electrical bus bars. The bus bars are used to distribute electrical power to the magnetic field generating coils concentrically mounted around one another. 
     Axial, circumferential, helical or other shaped grooves are machined in the insulating backing material typically provided on the sheet-like panels used to form the coils., These grooves allow the bus bars to be inserted within or inlaid into these recesses cut into the backing material. In this manner, the bus bars do not radially project above or beyond the surface of the backing material but are instead enshrouded within the backing material as opposed to being completely embedded in an overlay of cured adhesive resin. This recessed mounting of the bus bars substantially flush with the surface of the panel backing layer of the overlying panel allows the next adjacent overlying gradient coil to be mounted substantially directly to the surface of the underlying coil without the need for accommodating a radially projecting bus bar of the type necessitated by prior coil assembly constructions. 
     A particular advantage of this type of bus bar and coil mounting assembly is the reduction in the overall radial thickness of the main coil and shield coil subassemblies. That is, prior designs simply positioned a bus bar on the conductive outer surface of the panel coil and then applied a cylindrical layer of adhesive resin over the bus bar and the underlying panel coil. This layer of resin required time to cure and added to the radial growth of the main and shield coil subassemblies. 
     Once the resin dried or cured, it had to be carefully machined, as noted above. It can be appreciated that by inserting a bus bar into a groove in the coil backing material, so that the bus bar does not radially project to any significant degree above or beyond the surface of the backing material, a supplemental layer of resin is not needed to form a continuous cylindrical support layer around the radially protruding bus bar and its underlying coil. Such a supplemental resin layer was required in prior panel constructions to provide a continuous cylindrical mounting surface around which the next cylindrical coil was mounted. 
     By eliminating the supplemental resin layer between adjacent coils, the assembly time of the coil assembly can be reduced. Moreover, the time required for curing the supplemental resin layer can also be eliminated. A significant additional advantage of eliminating the supplemental resin layer is the elimination of the machining step previously required to form an accurate cylindrical support surface on the cured supplemental resin material. 
     The bus bars may be specially adapted for use with the MRI coil assembly of the present invention by having a layer of insulation provided along the radially inner surface of each bus bar. In this manner, the bus bars are prevented from electrically shorting the current flowing in the coils upon which the bus bars are mounted. Bus bars insulated in accordance with the invention are particularly well suited to the simplified x and y coil construction noted above. 
     Another feature of the invention is the use of radily-extending positioning pins for axially and circumferentially positioning a y coil over an x coil during their assembly. Such pins facilitate accurate fabrication and alignment of flat panels as they are flexed into accurate segments to form a cylindrical coil. 
     These and other objects of the invention will become more readily apparent from the following detailed description, taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In The Drawings: 
     FIG. 1 is a schematic radial sectional view taken through the location of the bus bars in a magnetic coil sub-assembly constructed in accordance with the invention. 
     FIG. 2 is a perspective view of a bus bar constructed in accordance with the invention. 
     FIG. 3 is a schematic top plan view of a main x-coil constructed from four panels interconnected with bus bars in accordance with the invention. The panels are shown laid out flat prior to being curled into a cylinder. 
     In the various views of the drawings, like reference characters designate like or similar parts. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described in conjunction with the drawings, beginning with FIG. 1, which shows a radial section taken through the full axial length of a hollow cylindrical laminated magnet coil assembly  10  constructed in accordance with the invention. The radial dimension or height of FIG. 1 is greatly increased or exaggerated for clarity while the axial dimension or length of FIG. 1 is greatly reduced or compressed. In practice, the length of the magnet assembly of FIG. 1 is, for example, 2 meters while the height of FIG. 1 is a small fraction of that length, for example, less than 100 mm. 
     The magnet coil assembly  10  is constructed on a cylindrical support tube  12  having a central axis  14 . The lower, mirror image of the section through FIG. 1 is not shown. Tube  12  is typically constructed of a wound glass fiber material such as “G10” fiberglass. Resin adhesive is used to fix the fiber material in a permanent cylindrical shape. The open cylindrical area  16  defined by the inner surface  18  of tube  12  provides for an open imaging region within which a patient is typically positioned during MRI procedures. 
     Once the resin material in tube  12  begins to harden and cure, radial bores or holes  20  are drilled through the fiberglass. Locating and alignment pins  22  are then inserted into the holes  20 . Although only two pins  22  are shown in FIG. 1, preferably more are used, such as eight pins circumferentially spaced 90° apart and arranged in four separate pairs as the pair shown in FIG.  1 . 
     Locating pins  22  can have threaded end portions  24  which can be screwed into bores  20  for added retention and rigidity. Pins  22  serve to axially and circumferentially locate, align and accurately position the overlying laminations as discussed below. 
     Once the cylinder  12  fully cures, its outer cylindrical surface  26  is carefully machined to a predetermined outer diameter as calculated for proper coil performance. After machining, a thin layer of resin, epoxy or other adhesive  28  is evenly applied over the machined surface  26 . 
     A main x-coil  30  in the form of a thin, flat rectangular sheet is then assembled around the tube  12 . Although the invention will be described using main coils as an example, x and y shield coils are also constructed using the same procedure and materials. 
     The main x-coil  30  is, in this example, actually formed from four separate flat o rectangular panels  32 ,  34 ,  36 ,  38  (FIG.  3 ). Each panel includes one or more positioning bores  40  for receiving the respective guide pins  22 . The individual panels are thin and flexible and are easily bent, curved and curled into 180° arcs. The panels are fitted over the guide pins  22  and deformed around the tube  12 . The adhesive resin  28 , which is typically applied to tube  12  in the manner of paint, securely holds the individual panels in their semi-cylindrical flexed shape. 
     As further seen in FIG. 1, two panels  32 ,  34  are abutted end to end along an interface or butt joint  42 . Each panel is provided in the form of a laminated sheet having an electrical insulation layer or backing layer  44  and an electrical conductive layer  46 . The layer  44  may be formed of a glass or resin material and the layer  46  is typically formed of copper. Spiral grooves  48  are cut into and through the conductive layer  44  in a known manner to form an electrically conductive circuit pattern similar to a thumb print. 
     For this reason, panels  32 ,  34 ,  36  and  38  are commonly referred to as thumb-print panel coils, and in this case 180° thumb-print panel coils. Of course, one 360° panel, four 90° panels or any other suitable number and arrangement of panel coils can be used to construct a main coil or shield coil in accordance with the invention. 
     At this point, one or more bus bars  50  are assembled across the butt joint  42  so as to electrically connect each pair of axially abutting panels  32 ,  34  and  36 ,  38 . As seen in FIG. 2, each bus bar has an electrically conductive portion  52  and an electrically insulating portion  54 . The bus bars  50  can be rigidly attached to the panels of the main x coil  30  with recessed fasteners such as brass screws or bolts  56 . 
     The bus bar conductive portions  52  can be formed of copper and the insulating portions  54  can be formed of an epoxy composite or glass based material such as fiberglass. As further seen in FIG. 2, each bus bar has a pair of upstanding end portions or legs  58  separated by a central insulated portion  60 . The insulated portion prevents the bus bars from electrically shorting the current flowing through the spiral paths cut into the respective panels. Countersunk bores  60  may be formed through the legs  58  to receive the fasteners  56 . 
     A thin layer of adhesive  62  is next applied over the exposed surface of the main x coil  30 . The main y coil  64  is then assembled over the main x coil  30 . In the same manner as described above with respect to the main x coil  30 , the main y coil  64  is fabricated in this example from four flat flexible panels arranged similarly to those panels shown in FIG. 3, i.e., four 180° thumb print y panel coils, two of which  66 ,  68  are shown in FIG.  1 . 
     Each y panel  66 ,  68  is formed with one or more positioning bores  70  for receiving the positioning pins  22  and accurately locating the panels axially and circumferentially with respect to each other and with respect to the main x coil  30 . The main y coil is constructed and assembled similar to the main x coil except for the provision of preformed recesses or pockets  72  formed in the panel coils  66 ,  68 . 
     The recesses  72  are formed, or cut, such as by machining grooves into the insulation or backing layers  74  when the panel coils are laid out flat, prior to being bent or curled. This greatly facilitates the formation of the recesses. The recesses  72  are cut deep enough into the backing layers  74  to fully receive the complimentary shaped bus bars  50 . In this manner, the presence of the bus bars does not increase the radial separation between the x and y main coils  30 ,  64 . 
     Because the bus bars  50  do not cause any radial separation between the main x and y coils, a magnet designer can design the spacing between the electrically conductive portions  46 ,  76  of the respective x and y main coils substantially equal to the radial thickness of the backing layer  74  of the main coil. The thickness of the adhesive layer  62  is negligible. 
     This spacing between the conductive portions provides a design advantage in that additional insulation material is not required between the main x and y coils for the purpose of obtaining a desired radial spacing with respect to the production and interaction of magnetic fields. This simplifies the design and reduces manufacturing time, effort and expense. That is, the magnet designer can design the spacing between the conductive portions of the x and y main coils to be equal to the standard thickness of commercially available backing layers provided on the panel coils. 
     Once the many coil  64  is assembled over the main x coil  30 , bus bars  80  are connected to the panel coils  66 ,  68  across butt joint  78 . Fasteners such as brass screws  56  may be screwed through the legs  58  of the bus bars  80 , as described above, to anchor the bus bars  80  on the conductive portions  76  of the main y coil  64 . The insulation  54  on the underside or radially inner surface of bus bar  80  prevents electrical shorting of the respective 180° thumb print panel coils  66 ,  68 . 
     At this stage in the construction of the magnet assembly  10 , an outer layer of composite insulation material  82 , such as “pre-preg” material is applied over the conductive portion  76  of the main y coil and over the bus bar  80 . Pre-preg material is an uncured composite material, typically in the form of glass, carbon or other fiber which is wetted with a resin such as epoxy. Pre-preg is available in flexible sheets and is kept refrigerated below its curing temperature until it is needed. 
     The pre-preg sheets are warmed to room temperature, and are manually placed over the main y coil and its bus bars and allowed to cure. After the pre-preg sheets cure, they form a solid composite structure. 
     An important feature of the invention is the forming or cutting of recesses  84  into the pre-preg or other electrical insulating material  82  in its pre-cured condition when the material is laid out in a planar configuration prior to its application over the main y coil  64 . Recesses or pockets  84  are dimensioned to fully compliment and fully receive the bus bars  80  such that the insulation layer  82  directly contacts the outer surface of the electrically conductive portion  76  of the main y coil  64  without any obstruction or interference with or from the bus bars  80 . Alignment bores  86  are drilled or punched through the pre-preg material to receive the alignment pins  22 . 
     By having the recesses  84  receive and radially envelope bus bars  80 , the radial separation between the insulation material  82  and the main y coil is minimized. This provides the significant design and operating benefits noted above. 
     The next step in the manufacture of the magnet assembly  10  is the winding of a layer of glass material  88 , such as “G10” fiberglass, along with an adhesive, over the insulation layer  82 . The fiberglass material is wound under maximum tension in order to apply radially compression to the insulation layer  82 . 
     Once the wound glass material  88  cures, its outer diameter  90  is machined as required. Spiral grooves  92  are then machined into the glass material  88  for receiving the windings of a main z coil. A thin layer of adhesive resin is applied to the grooves  98  and a z coil wire is then wound into grooves  92  in a known fashion. At this point, the same set of steps can be used to assemble the x, y and z shield coils, as the invention is equally applicable to constructing shield coils. 
     Details of the four 180° thumb print panel coils  32 ,  34 ,  36  and  38  which together form a main x coil such as the main x coil  30  are shown schematically in FIG.  3 . The directional arrows represent the direction of current flow through the bus bars  50  and through the panel coils. Electric current flows through the panels via input leads  94  and output leads  96 . 
     It can be appreciated that the four panel coils  32 ,  34 ,  36  and  38  are shown in FIG. 3 as being laid out on a flat surface. In practice, the panel coils are rolled into a tube from the top of FIG. 3 to the bottom around the cylindrical tube  12  of FIG.  1 . 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 
     For example, the concept of inserting bus bars into performed grooves or recesses in the backing material of one or more layers of a gradient coil can also be readily applied to flat gradient magnet assemblies as compared to the cylindrical panels. In this case, the sectional view in FIG. 1 is taken through a flat planar stack of magnet coil layers forming a parallepipedic assembly. Curved, arcuate or other shaped gradient magnet assemblies are also within the scope of the invention. 
     Another variation of the invention includes varying the sequence and locations of the individual magnet coils. In the example discussed above, the main coil subassembly was ordered in the sequence of x, y and z coils and the shield coil subassembly was ordered in the coil sequence of z, x, y. However, any sequence for either the main or shield coil subassemblies may be used, such as a y, x, z sequence for the main coil subassembly. 
     Moreover, the invention covers not only the combination of main and shield coil subassemblies, but also the insertion of bus bars into preformed grooves in an unshielded gradient coil assembly which includes a main coil subassembly, but doe not include a shield coil subassembly.