Abstract:
A local coil for magnetic resonance imaging equipment employs mirror conductors on opposite sides of an insulating substrate to produce lower resistance, higher Q and improved signal-to-noise ratio for a given foil thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to magnetic resonance imaging (MRI) and in particular to a method of manufacturing local coils used in MRI imaging.  
           [0002]    Magnetic resonance imaging (MRI) reconstructs images, for example of the human body, by detecting a nuclear magnetic resonance (NMR) signal emitted from processing hydrogen protons, principally in watery tissue of the body. The NMR signal is faint and thus it is known in the art to place electrical coils on the patient in close proximity to the imaged tissue to improve the signal-to-noise ratio of the detected NMR signal.  
           [0003]    A wide variety of different local coil designs are known in the art including: single loop coils, paired loop coils (Helmholtz), quadrature coils, “bird cage” coils and phased array coils. Coils of these designs and others are shown generally in U.S. Pat. Nos. 4,734,647; 5,136,244; 5,166,618; 5,277,183; 5,370,118; 5,256,791; 5,370,118; 5,619,996; and 6,438,402, all assigned to the assignee of the present invention and hereby incorporated by reference.  
           [0004]    The signal-to-noise ratio of the detected NMR signal may also be improved by reducing noise in the detected signal. To this end, local coils are usually tuned to a resonant frequency close to the expected Larmor frequency of the hydrogen protons whose resonance is being detected. For a 1.5 Tesla magnetic field in an MRI device, this frequency will be approximately 64 megahertz.  
           [0005]    The tuning creates a sensitivity in the coil at the Larmor frequency of the expected NMR signal and a desensitivity to external noise sources at other frequencies. The sharpness of this frequency discrimination is a function of the electrical Q of the local coil which in turn is a function of the resistance of the coil conductors at the relevant NMR frequency.  
           [0006]    Low resistance in the conductors of a local coil may be obtained by using conductors having large cross sections such as copper tubing. These conductors may be bent into the desired shape of the coil and tuned with series capacitances to the desired resonant frequency.  
           [0007]    It can be difficult to manufacture coils using such conductors and the local coil that is produced is relatively rigid and may not readily conform to the patient. Accordingly, it is known in the art to construct local coils using printed circuit board material where the coils are precisely etched out of metal foil supported on an insulating substrate or formed using well-known additive techniques. The substrate may be flexible allowing the formed coils to better conform to the patient.  
           [0008]    One disadvantage to the use of printed circuit board techniques is that the resistance of metal foil conductors is generally greater than the resistance of wire or tubular conductors. This higher resistance decreases the Q of the resultant local coil.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The present inventors have found a way to significantly increase the Q of local coils manufactured of printed circuit board materials by forming duplicate patterns in metal foil on both sides of the insulating substrate. The duplicate patterns may, but need not be, joined with conductors. At high frequencies, the duplicate patterns share current flow decreasing the intrinsic resistance of the local coil. The proximity of the two patterns is also believed to promote more uniform current distribution in the metal foil decreasing the effective resistance of the foil. Alternatively, for a given desired Q, this technique allows thinner and more flexible metal foil to be used. Importantly, the inventors have determined that two separated layers of metal foil, of a given thickness, provide benefits beyond those obtained with a single layer of foil of twice the thickness. Further, the invention allows coil designs normally requiring foil layers thicker than is available in standard printed circuit boards, for example four ounce copper, to be realized using standard circuit board materials in which the copper foil is normally limited to two ounces or less.  
           [0010]    Specifically, the present invention provides an MRI local coil in which a first conductive pattern is formed on a front surface of an insulating sheet to provide a resonant antenna sensitive to NMR signals. A second conductive pattern is formed on a rear surface of the insulating sheet in alignment with the first conductive pattern and, signal terminals are attached to the first conductive pattern to conduct an NMR signal to an MRI machine for processing.  
           [0011]    Thus, it is one object of the invention to provide a method of significantly increasing the electrical Q of an MRI coil constructed using standard printed circuit materials and techniques.  
           [0012]    The first and second conductive patterns may be substantially identical.  
           [0013]    Another object of the invention is to provide a technique that may make use of existing patterns used for printed circuit board coils simply by duplicating the patterns on the rear surface of the printed circuit board.  
           [0014]    The insulating sheet may be a flexible material.  
           [0015]    Thus, it is another object of the invention to provide an improved tradeoff between improved flexibility of thin metal foil used in flexible local coils and high electrical Q.  
           [0016]    The first and second conductive patterns may be electrically connected by direct current.  
           [0017]    Thus, it is another object of the invention to allow improved control of current flow in the first and second patterns by defining points of common voltage.  
           [0018]    The electrical connections between the first and second conductors may be via plate-through holes.  
           [0019]    Thus, it is another object of the invention to permit an electrical interconnection between the patterns using standard printed circuit board techniques.  
           [0020]    The first conductive pattern may include series capacitances to tune it into resonance and, the second conductive pattern may have corresponding series capacitances.  
           [0021]    Thus, it is another object of the invention to allow distribution of the necessary tuning capacitances on both the first and second conductive patterns to employ lower value capacitances with higher electrical Q values.  
           [0022]    The first and second conductive patterns may be loops.  
           [0023]    Thus, it is an object of the invention to allow the manufacturer of a building block of many local coils using this technique.  
           [0024]    The insulating sheet may be in the form of a cylindrical tube.  
           [0025]    Thus, it is another object of the invention to allow the construction of coils having non-planar topologies.  
           [0026]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is an exploded perspective view of a single loop MRI coil joining first and second aligned patterns on opposite sides of an insulating sheet;  
         [0028]    [0028]FIG. 2 is a cross-sectional view of the coil of FIG. 1 showing what is believed to be an improved distribution of charge during current flow caused by the opposed plates of the first and second conductors of FIG. 1;  
         [0029]    [0029]FIG. 3 is a cross-sectional view along lines  3 - 3  of FIG. 1 showing the introduction of series capacitances into the loops of the first and second conductive patterns and showing the connection of the first and second patterns through the insulating substrate through the use of plate through holes; and  
         [0030]    [0030]FIG. 4 is a perspective view of a simplified tubular local coil showing positioning of multiple loops on the inside and outside of the coil as may implement the present invention in a non-planar form. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    Referring now to FIG. 1, a local coil  10  constructed according to the present invention includes a planar insulating substrate  12  such as may be a conventional, flexible, or rigid printed circuit board material having a first and second surface  16  and  18 , respectively.  
         [0032]    A first conductive pattern  20  of metal foil is attached to the first surface  16  and provides a loop current path terminating at first and second terminals  22  and  24 . During use of the local coil  10 , terminals  22  and  24  are connected via leads  28  to the input circuitry of a standard magnetic resonance imaging (MRI) system.  
         [0033]    The first conductive pattern  20  is broken periodically by gaps  30  bridged by tuning capacitors  32  to provide a series resonant circuit tuned to the desired Larmor frequency as is generally understood in the art.  
         [0034]    A similar second conductive pattern  20 ′ is attached to the reverse surface  18  of the insulating substrate  12  and aligned with first conductive pattern  20 . In this respect, the second conductive pattern  20  may have corresponding gaps  30 ′ bridged by corresponding capacitors  32 ′ (visible in FIG. 3).  
         [0035]    Alternatively, in a simpler embodiment, the capacitors  32 ′ and gaps  30 ′ may be omitted.  
         [0036]    In the preferred embodiment, the first and second conductive patterns  20  and  20 ′ are formed using printed circuit techniques operating on standard printed circuit board material. Such standard material may include a substrate  12  of epoxy-fiberglass, for example, clad on the front and rear surface with a continuous layer of copper foil. Standard thicknesses of copper foil useful for the present invention are one or two ounces of copper per square foot. In one subtractive technique, the copper foil is coated with a photo resist that is selectively treated by light passing through a mask defining the first and second conductive patterns  20  and  20 ′. Photo resist outside of the first and second conductive patterns  20  and  20 ′ is removed and an etchant applied to remove the copper in that area. Precise first and second conductive patterns  20  and  20 ′ may thus be easily reproduced.  
         [0037]    Alternatively, the copper can be removed by a milling cutter operating under computer control, or copper can be selectively applied by additive printed circuit board processing techniques, both well known in the art.  
         [0038]    Referring now to FIG. 2, the proximity and orientation of first and second conductive patterns  20  and  20 ′ will cause high frequency current flow  21  on the local coil  10  to be shared between the first and second conductive patterns  20  and  20 ′. This will be true even without a direct current passing conductor connecting the first and second conductive patterns  20  and  20 ′. The effect of this division of current flow is to increase the effective cross-section of conductor, lowering resistance to current flow and increasing the Q of the local coil  10 .  
         [0039]    As is understood in the art, charge  34  forming the high frequency current flow  21  tends to move toward edges of the conductive patterns  20  and  20 ′. This concentration of charge  31  increases the effective resistance of the first and second conductive patterns  20  and  20 ′. Although the inventors do not wish to be bound by a particular theory, it is believed that the electric field  36  from mirror image charges  34 , near the edge of the aligned first and second conductive patterns  20  and  20 ′, create a mutual repulsion that resists this edge charge build up. The resulting more uniform current flow further lowers the effective resistance of the first and second conductive patterns  20  and  20 ′ beyond the effect obtained simply by current sharing.  
         [0040]    Referring now to FIG. 3, in an alternative embodiment, gaps  30  and  30 ′ bridged by capacitors  32  and  32 ′ may be flanked by plate-through holes  40  on opposite sides of the capacitors  32  and  32 ′. The plate through holes provide a direct current passing path between the first and second conductive patterns  20  and  20 ′ ensuring that the voltage at these points on both of the first and second conductive patterns  20  and  20 ′ are equal and improve current sharing.  
         [0041]    The plate through holes  40  may be manufactured using normal printed circuit techniques as are well known in the art and eliminated the need for auxiliary wiring to make this connection. Alternatively, other methods of joining the first and second conductive patterns  20  and  20 ′ may be used including wires, rivets and the like.  
         [0042]    Referring to FIG. 4, the insulating substrate  12  may be a rigid or a flexible material and may be formed into non-planar sheets, for example, into a cylindrical tube  50 . In this case, the first and second conductive patterns  20  and  20 ′ will be matching loops opposed on the outside and inside of the tube  50 , respectively. The tubular shape is useful for the production of certain phased array, quadrature, and birdcage coils as is understood in the art.  
         [0043]    It will be understood that the principle of the present invention may be extended to matching patterns separated by insulating layers in multi-layer circuit boards and, that the first and second conductive patterns  20  and  20 ′ need not be precisely identical so long as they function in unison to share currents induced by the NMR signal from a single volume of interest. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.