Abstract:
A floating shield current trap provides two resonance loops formed of split concentric tubular conductors joined radially at their axial ends. Adjustment of the separation of these loops provides a change in coupling between the loops effecting a simplified tuning of the resonance of the trap for different expected frequencies of interfering shield current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 10/145,229, hereby incorporated by reference. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to radio frequency traps and in particular to a floating trap suitable for use with magnetic resonance imaging equipment.  
           [0003]    Electrical conductors used for transmitting signals susceptible to external electromagnetic noise often employ a center conductor surrounded by a conductive shield. The shield is grounded to prevent external electric fields from influencing the signal on the central conductor. A common “coaxial cable” shielded conductor, used for radio-frequency signals, employs a braided shield surrounding a central multi-strand conductor separated from the braided shield by an insulator of predetermined diameter and dielectric properties. The braded shield is surrounded in turn by a second insulator that protects the shield from damage or electrical contact with other conductors.  
           [0004]    In applications where there are intense external electrical/magnetic fields, for example, in magnetic resonance imaging (MRI), significant current may be induced in the shield causing failure of the shielding effect and possibly damage to the shield and its adjacent insulation from heating. One method of reducing shield current is with ferrite “beads” which fit over the shield to resistively damp eddy currents induced by the shield currents and thus the shield currents themselves. It is also known to reduce such shield currents by creating an S-trap in which the coaxial cable is wound in a first direction and then optionally a second direction about a cylindrical form to produce a self-inductance among the coils of each winding set. A capacitance is connected in parallel with the inductance (by attaching leads of a capacitor to the shield at separated points in each winding) providing parallel resonant circuits tuned to the particular frequency of the offending external radio frequency field. The resonance provides the shield with a high impedance at the frequency of the interference, resisting current flow at this frequency, while the counter-winding reduces inductive coupling of the trap to the noise.  
           [0005]    While the S-trap may successfully reduce current flow in the shield, it requires additional cable length for the windings and thus may contribute to a loss of signal strength and may introduce an undesirable phase change in the signal. Further, manufacture of the S-trap is cumbersome, requiring modification of the coaxial cable, including a removal of portions of its external insulation for attachment of a capacitor. The fixed position of the S-trap makes it difficult to adjust the S-trap to a location on the shield having maximum current, as is desirable. Ferrite beads are unsuitable in areas of intense magnetic fields, such as are found in magnetic resonance imaging machines.  
           [0006]    Co-pending U.S. application Ser. No. 10/145,229 filed May 13, 2002 describes a shield current trap having a first and second, concentric, tubular conductor electrically connected to provide a resonance-induced high impedance to current flow in a surrounded shielded cable. The shield current trap so described does not require a direct electrical connection to the shielded cable and so may float on the cable to be easily added, removed, or adjusted in position.  
           [0007]    The effectiveness of this floating shield current trap requires that it be closely tuned to the expected frequency of the shield current. When such a trap is used with MRI equipment, the predominant shield currents will be equal to the Larmor frequency of processing hydrogen protons within the magnetic field of the MRI machine.  
           [0008]    The Larmor frequency depends on the strength of the magnet and varies among manufacturers for a given magnet size (e.g. 1.5 Tesla) and for different magnet sizes among a single manufacturer. Ideally, one such shield current trap could be used for all systems despite this variation in frequency.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The present invention provides an improved floating shield current trap that provides a simple tuning mechanism so that the trap may be used with different machines. Generally, the floating trap is divided along its axis into separate resonant loops. By adjusting the separation between these loops, the coupling between the loops is changed, adjusting the trap&#39;s resonant frequency.  
           [0010]    Specifically, the present invention provides a shield current trap having a first trap element with a first inner conductive channel joined at a respective first and second axial ends, via a first and second radial conductor, to corresponding first and second ends of a first outer conductive shell to form a first resonant loop. A second trap element having a second conductive channel joined at respective first and second axial ends, via third and fourth radial conductors, to first and second ends of a second outer conductive shell forms a second resonant loop. The first and second conductive channels may be assembled in opposition to enclose an axially extending shielded cable. A clamp assembly controls the separation of the first and second trap elements to control the coupling between the first and second resonant loops.  
           [0011]    Thus, it is one object of the invention to provide a simple method of adjusting the frequency at which the trap is resonant and thereby accommodating both for manufacturing tolerances and variations between shield current frequencies in different applications.  
           [0012]    At least one of the inner conductive channels, the outer conductive shell, and the first through fourth radial conductors include a series capacitor.  
           [0013]    Thus, it is another object of the invention to allow capacitive tuning of the loops such as may provide more compact trap size.  
           [0014]    The clamping means may be a spring clip fitting around the first and second trap elements to draw them together.  
           [0015]    Another object of the invention is to provide a simple mechanical means for holding the trap elements together while allowing adjustment.  
           [0016]    The clamping means may include adjustable standoffs extending from the first trap element to space the second trap element therefrom. In one embodiment, the standoffs may be set screws partially extending from threaded holes in the first trap element to extend outward therefrom to abut a portion of the second trap element.  
           [0017]    Thus, it is another object of the invention to provide for easy adjustability of the shield trap without the need for a variety of shims or the like.  
           [0018]    The clamping means may alternatively be a machine screw having a head engaging the first trap element and threads engaging a threaded hole in the second trap element to draw the first and second trap elements together with a tightening of the machine screw. The separation between the first and second trap elements may include a spring urging the first and second trap elements apart. That spring may be an elastomeric polymer.  
           [0019]    Thus, it is another object of the invention to provide a simplified alternative adjustment mechanism.  
           [0020]    The invention may include alignment guides holding the first and second trap elements in alignment for a range of separation of the first and second trap elements, for example, dowels and interfitting bores.  
           [0021]    Thus, it is another object of the invention to hold the shells in alignment during the adjustment process simplifying the adjustment process and further preventing shifting after the adjustment is complete.  
           [0022]    Another object of the invention is to provide an adjustment mechanism that allows separation of the trap for insertion of the shielded cable into the trap after the cable is connected to equipment or connectors such as would prevent the cable from being threaded through the bore of the trap.  
           [0023]    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  
       [0024]    [0024]FIG. 1 is a perspective view of a shield current trap of the present invention as fit over two coaxial cables, showing the invention&#39;s concentric outer and inner conductive structures;  
         [0025]    [0025]FIG. 2 is a cross-sectional view of the shield current trap of FIG. 1 taken along lines  2 - 2  showing the successive layers of conductors of the outer conductive structure, the inner conductive structure and the shield of the coaxial cables;  
         [0026]    [0026]FIG. 3 is a schematic representation of the shield current trap of FIG. 1 taken in cross section along lines  3 - 3  of FIG. 1 as positioned around a single coaxial cable showing the suppression of shield currents through tuning of the inner and outer conductive structures;  
         [0027]    [0027]FIG. 4 is a fragmentary, schematic, cross-section similar to FIG. 3 showing an alternative embodiment in which high voltages on the outer conductive structure are displaced toward the center of the current trap to be covered by an insulating outer housing;  
         [0028]    [0028]FIG. 5 is a figure similar to that of FIG. 4 showing yet an alternative embodiment eliminating the tuning capacitor;  
         [0029]    [0029]FIG. 6 is a cross sectional view similar to FIG. 2 showing an embodiment in which the shield current trap is assembled from two halves so that it may be placed about a shielded cable without a threading of the cable through the shield current trap;  
         [0030]    [0030]FIG. 7 is a fragmentary cross-sectional view of an interface between conductive structures of the embodiment of FIG. 6 in which the separation of the halves is along longitudinal eddy current reducing slots;  
         [0031]    [0031]FIG. 8 is a figure similar to FIG. 7 in which the conductive structures of FIG. 6 are connected by electrically conductive fingers forming a releasable electrical connector;  
         [0032]    [0032]FIG. 9 is a simplified cross section along lines  2 - 2  showing an alternative method of creating eddy current suppressing slots in the conductors of the inner and outer conductive structures;  
         [0033]    [0033]FIG. 10 is a fragmentary perspective view of an alternative embodiment of the inner and outer conductive structures showing a simplified construction technique and elimination of eddy current suppressing slots;  
         [0034]    [0034]FIG. 11 is a figure similar to that of FIG. 10 showing an alternative embodiment of the shield current trap that provides simplified tuning;  
         [0035]    [0035]FIG. 12 is a cross-sectional fragmentary view taken along line  12 - 12  of FIG. 11 showing the use of set screws and a spring clamp to adjust the separation of two halves of the shield current trap of FIG. 11; and  
         [0036]    [0036]FIG. 13 is a figure similar to that of FIG. 12 showing an alternative embodiment of the adjustment mechanism in which a machine screw pulls the two halves of the shield current trap together against the resisting force of a trapped elastomer. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     General Shield Current Trap  
       [0037]    Referring now to FIGS. 1 and 2, a shield current trap  10  of the present invention may include a tubular inner portion  12  having a central lumen  14  receiving one or more coaxial cables  16  of conventional design. The size of the central lumen  14  may be varied depending on how many cables are to be accepted. Each coaxial cable, known in the prior art, includes an outer insulating sheath  18  fitting around a braided, rigid, or similar shield  20  covering an insulator  22  having a central signal-carrying conductor  24 .  
         [0038]    The tubular inner portion  12  of the shield current trap  10  may be insulating and have a conductive surface, typically of copper foil, to produce an inner conductor  26  extending along its entire surface, broken only by optional longitudinal slots  28  intended to prevent circumferential eddy currents at low frequencies produced, for example, by gradient magnetic fields used in the MRI machine. The foil may be aligned by shallow longitudinal grooves cut in the outer surface of the tubular inner portion  12 . The slots  28  are optional and are unnecessary if the problem of eddy currents is not significant in the particular application. The tubular inner portion  12  may have an arbitrary cross section not limited to circular but including rectangular and other shapes. Bridging capacitors  30  are attached across the slots  28  to provide a conductive path for radio frequencies and thereby a substantially unbroken conductor at the frequency of the expected interference. For a typical MRI machine with a 1.5 Tesla magnet, the expected radio-frequency interference will be at approximately 64 megahertz, but the invention is not limited to a particular frequency range and may find use in frequencies ranging from 40 to 500 megahertz.  
         [0039]    Fitting around the tubular inner portion  12  of the shield current trap  10  is a large diameter tubular outer portion  32  whose outer surface is conductive to provide an outer conductor  34 . Again, the tubular outer portion need not have a circular cross section. The outer conductor  34  is broken by longitudinal slots  36  (like the inner conductor  26  of the tubular inner portion  12 ) preventing low-frequency eddy currents and bridged by optional radio frequency conducting capacitors  38  to provide a substantially unbroken radio frequency conductor. Again, the outer conductor  34  may be applied as foil aligned by means of shallow longitudinal grooves cut in the outer surface of the tubular outer portion  23 . The slots  36  are particularly useful when low-frequency magnetic fields will be present as is the case with magnetic resonance imaging.  
         [0040]    In one embodiment, the tubular outer portion  32  tapers inward while maintaining its cylindrical aspect at either end of the tubular outer portion  32  to approach the tubular inner portion  12 . At a first end  40  of the shield current trap  10 , the outer conductor  34  of the tubular outer portion  32  is electrically connected to the inner conductor  26  of the tubular inner portion  12  by capacitors  42 . These capacitors  42  are selected to be large enough to provide essentially no impedance at the expected radio frequency of the interference.  
         [0041]    At a second, opposing end  44  of the shield current trap  10 , the outer conductor  34  of the tubular outer portion  32  is also attached to the inner conductor  26 , but through capacitors  46 , selected to tune the shield current trap  10  to resonance at the frequency of the expected external interference. The resonance is “parallel resonance” creating a high impedance to longitudinal current flow traveling in a path in a first direction in the outer conductor  34  of the tubular outer portion  32  and in the opposite direction in inner conductor  26  of the tubular inner portion  12 . Capacitors  46  are selected to adjust the path length in this path to be substantially equal to an odd multiple of one-quarter of the wavelength of the expected external interfering signal. This condition creates a high impedance resisting current flow at the frequency of the expected interfering signal. In the event that the expected interfering signal is comprised of many frequencies, as will often be the case, the adjustment of the electrical length of the path may be made with respect to a dominant frequency component or multiple shield current traps  10  may be used. The suppression of shield current flow is accomplished by proper selection of the dimensions of the inner conductor  26  and the outer conductor  34 , or those dimensions and the value of capacitor  46  so that a high impedance is created in those conductors at the resonant frequency of the external interfering field, preventing current  52 .  
         [0042]    Significantly, this high impedance and suppression of current  52  requires no direct electrical connection between elements of the shield current trap  10  and the coaxial cable  16  such as would require cutting the outer shield away from the coaxial cable  16 .  
         [0043]    Referring now to FIG. 4, the path formed by outer conductor  34  and inner conductor  26  may be broken into multiple (in this example: two) sub-paths by joining outer conductor  34  and inner conductor  26  through low impedance connections at the two opposite ends of the shield current trap  10  and joining outer conductor  34  and inner conductor  26  at midpoints with capacitor  46 . Again, the value of the capacitor  46  is selected so that each sub-path has an electrical length being an odd multiple of one quarter of the wavelength of the expected interfering external signal. In this case, high voltages on the outer conductor  34  are displaced toward the center of the shield current trap  10 . An insulating covering  56  is placed around the outer conductor  34  to protect the user from these voltages. Clearly, an arbitrary number of sub-paths may be created in this manner. The insulating covering  56  may conform generally to the outer conductor  34  and the inner conductor  26 , the extent of the inner conductor  26  is exposed at either end of the shield current trap  10 , so as to fully insulate all exposed conductive surfaces of the shield current trap  10 .  
         [0044]    Referring to FIG. 5, it will be understood that the path formed by outer conductor  34  and inner conductor  26  may be sized to be of the desired electrical length (an odd multiple of one quarter of the wavelength of the interfering signal) without the need for the capacitor  46  simply by adjusting the actual length of the inner and outer conductors  26  and  34 . This generally will increase the length of the shield current trap  10  but may be appropriate for certain applications.  
         [0045]    It will be understood to those of ordinary skill in the art that the paths of FIGS.  3 - 5  may be repeated an arbitrary number of times and that further, each separate path may be tuned independently to address a different frequency of interference.  
         [0046]    The ability of the shield current trap  10  to operate without direct electrical connection to the contained coaxial cable  16  allows the shield current trap  10  to be installed or removed freely at any time by simply threading the coaxial cable  16  through the lumen  14  of the tubular inner portion  12 . In certain situations, however, threading the coaxial cable  16  through the tubular inner portion  12  will be obstructed by electrical connectors or the like attached to the coaxial cable  16 . Accordingly, the present invention contemplates that the shield current trap  10  may be constructed in two halves split longitudinally along an axis of symmetry.  
         [0047]    Referring now to FIG. 6, the tubular inner portion  12  may be split into two hemi-cylindrical portions  12   a  and  12   b . Likewise, the tubular outer portion  32  (including the tapered ends) may be split into two corresponding hemi-cylindrical portions  32   a  and  32   b  and, the insulating cover may be split into two hemi-cylindrical covers  56   a  and  56   b . Each of hemi-cylindrical covers  56   a  and  56   b  may include transversely extending the tabs  60   a  and  60   b , respectively, abutting along a separation plane when the two halves of the shield current trap  10  are assembled together. Clearly, this principle can be extended to a splitting of the tubular inner and outer portions  12  and  32  into more than two pieces, as well.  
         [0048]    Machine screws  58  fabricated from nylon or other non-ferromagnetic materials, or the like (for example, interfitting snaps molded as integral parts of the housing) may be used to attach the tabs  60   a  and  60   b  together about the coaxial cable  16  without the need to thread the cable  16  through the lumen  14 . Alternatively, but not shown, the machine screws  58  may be received within bores in the cylindrical body of the shield current trap  10  eliminating the need for the tabs  60   a  and  60   b.    
         [0049]    Dielectric spacers  62   a  and  62   b  may be used to support the inner tubular portions  12   a  and  12   b  with respect to the outer tubular portions  32   a  and  32   b , the latter of which are held by the insulating housings  56   a  and  56   b . Alternatively, dielectric spacers  62   a  and  62   b  are eliminated by direct mechanical connection in the tapered portion of the housing  56   a  and  56   b  to the inner tubular portions  12   a  and  12   b  as may be better understood by also viewing FIGS. 4 and 5.  
         [0050]    Referring now to FIG. 7, the shield current trap  10  may be separated along a longitudinal slot  36  in the outer conductor  34  (and aligned slots  28  in the inner conductor  26 ) to prevent the need for electrical interconnection of the halves. Alternatively, as shown in FIG. 8, outer conductor  34  (and corresponding inner conductor  26 ) may include conductive fingers  64  pressed together at the interface between the halves of the shield current trap  10  when they are assembled providing electrical interconnection. In this latter case, eddy current reducing slots  36  may be displaced away from the parting line of the shield current trap  10 . The embodiment of FIG. 8 allows use of bridging capacitors  30  across the gaps provided to reduce any currents.  
         [0051]    In an alternative embodiment, not shown, the halves of the shield current trap  10  may be hinged to open at only one edge.  
         [0052]    Referring now to FIG. 9, in an alternative embodiment, the slots  36  or  28  may be formed not by circumferential gaps in the outer conductor  34  and inner conductor  26 , respectively, but by radial gaps  70  formed by overlap of the outer conductors  34  and inner conductors  26 . Overlap in the outer conductors  34  or inner conductors  26  may provide for the optional radio frequency conducting capacitors  38  as well, or discrete capacitors may be placed across these gaps.  
         [0053]    Referring to FIG. 10, the slots  28  and  36  may be eliminated altogether when eddy currents are not a problem. The tapering of the outer conductor  34  may be avoided by using the tuning capacitors  46  to connect the inner and outer conductors  26  and  34 .  
         [0054]    In use, the shield current trap  10  may be slid along the cable  16  so as to be located near a point of maximum shield current and thereby to have greatest effect. Such adjustment is not possible with prior art S-traps.  
       Shield Current Trap with Adjustable Tuning  
       [0055]    Referring now to FIG. 11, a shield current trap  10  providing adjustable tuning may be constructed using a split, solid wall, cylindrical tube  68 . The tube  68  is split along its axial-diametric plane to produce mirror image, arcuate, tube half  73   a  and tube half  73   b . The tube  68  provides opposed ends  71  being generally bases of the cylinder lying in a radial plane.  
         [0056]    The tubular inner portions  12   a  and  12   b  referred to above are provided by an inner arch wall of each tube half  73   a  and  73   b . The tubular outer portions  32   a  and  32   b  referred to above are provided by an outer arch wall of each tube half  73   a  and  73   b . The tube  68  may be reconstituted by joining opposed walls  72   a  and  72   b  of each tube half  73   a  and  73   b , respectively.  
         [0057]    All surfaces of the tube halves  73   a  and  73   b  are metallized except for the opposed walls  72   b  and  72   a , and arcuate slots  74   b  and  74   a  formed in tube half  73   a  and  73   b , respectively, at one end  71  of the tube  68 . The remaining end  71  (not shown) is fully metallized.  
         [0058]    This metallization may be easily accomplished by plating the tube  68 , milling a slot at a constant radius at one end  71  to remove the plating along the arcuate slots  74   a  and  74   b , then slicing the tube  68  to separate the opposed walls  72   a  and  72   b.    
         [0059]    The slots  74   a  and  74   b  may be bridged by capacitors  76  serving the same function as capacitors  46  described above. Thus, two electrical loops are formed by the cutting of tube  68  into tube halves  73   a  and  73   b . The first loop is along an axial path following the outer tubular portion  32   a , as metallized, passing radially along a first end  71  (not shown in FIG. 11) to the inner tubular portion  12   a  and axially along the inner tubular portion, and then radially along the second end  71  to cross the slot  74   a  through bridging capacitor  76 . A second loop is formed following the similar path but with the outer tubular portion  32   b  to the first end  71 , back along inner tubular portion  12   a  to second end  71  and, across slot  74  through capacitors  76  back to the outer tubular portion  32   b.    
         [0060]    Referring still to FIG. 11, generally the tube halves  73   a  and  73   b  may be fit together in alignment with wall  72   a  parallel to and proximate to wall  72   b  as guided by several dowel pins  80 . The dowel pins  80  extend upward perpendicularly from wall  72   b  to be received by corresponding bore  82  in wall  72   a . Multiple dowel pins  80  and bores  82  assure that the tube halves  73   a  and  73   b  are generally in alignment with walls  72   a  and  72   b  parallel to each other for a range of separations through which the dowel pins  80  retain engagement with bores  82 .  
         [0061]    Referring now to FIG. 12, the precise separation between tube halves  73   a  and  73   b  may be controlled by a set screw  84  accessible through a bore  86  cut through the outer portion  32   a  of tube half  73   a  so that a screwdriver  88  may be inserted through the bore  86  to engage the head of the set screw  84 . The set screw  84  passes through a threaded hole  90  through the tube half  73   a  to extend from wall  72   a  by a standoff distance  92 . The tip of the set screw thus may abut wall  72   b  defining a standoff distance  92  between walls  72   a  and  72   b . At least three set screws will be used contacting different points of wall  72   b  to establish a planar relationship between walls  72   a  and  72   b.    
         [0062]    A split ring, spring clamp  94  may be applied to the outer circumference of the assembly of tube half  73   a  and  73   b  to hold tube half  73   a  and  73   b  together at the standoff distance  92 . The spring clamp  94  may be a polycarbonate C-clip or the like engaging surfaces  32   a  and  32   b.    
         [0063]    As will be understood, the tube halves  73   a  and  73   b  may be fit together about an existing cable (not shown) to form a lumen  14  through which the cable may run and the spring clamp  94  opened and placed about them to hold them together on the set screws  84 .  
         [0064]    Tuning the shield current trap  10  is accomplished by coupling the trap to a conventional resonance testing apparatus, for example, a loop formed from the shield and central conductor of a shielded cable driven at a desired resonant frequency and coupled to a voltmeter. With the shield current trap  10  so connected, a screwdriver blade  88  is inserted into the set screws  84  to adjust the standoff distance  92  until the desired resonance is achieved. Generally, the capacitor  76  will be adjusted so that the tuning of the individual loops of the tube halves  73   a  and  73   b  is approximately correct (within manufacturing tolerances) for a given separation of the tube halves  73   a  and  73   b  and, the set screws  84  are adjusted to increase or decrease the separation distance as required to achieve the desired Larmor frequency for the particular equipment.  
         [0065]    Preferably, the assembly shown in FIG. 2 may be placed with a separate protective housing (not shown) and in this case, the spring clamp  94  may apply force to the housing.  
         [0066]    Referring now to FIG. 13, in an alternative embodiment, the bore  86  may receive a machine screw  100  whose head  101  rests against a bottom of the bore  86  and whose threaded portion extends out of wall  72   a  to be received in a corresponding threaded hole  102  cut into wall  72   b . Walls  72   a  and  72   b  may be separated by elastomeric washer  104  which holds the walls  72   a  and  72   b  in separation by its relaxed thickness. Tightening of the screw  100  serves to pull tube halves  73   a  and  73   b  together squeezing the washer  104  as necessary.  
         [0067]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims. For example, the trap need not be split into halves but may have a single slot making it a “C” or may be split into thirds or the like.