Abstract:
An apparatus for determining a location of a sensor in a surgical navigation domain includes a first magnetic field generator having a first coil set, a second magnetic field generator having a second coil set. The first and second coil sets are disposed substantially within a common plane. The apparatus further includes a processor configured to receive a plurality of signals. The processor calculates the location of the sensor from the plurality of signals. The sensor produces the plurality of signals in response to magnetic fields generated by the first and second magnetic field generators.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The following United States patent applications, which were concurrently filed with this one on Oct. 28, 1999, are fully incorporated herein by reference: Method and System for Navigating a Catheter Probe in the Presence of Field-influencing Objects, by Michael Martinelli, Paul Kessman and Brad Jascob, Ser. No. 60/161,991; Patient-shielding and Coil System, by Michael Martinelli, Paul Kessman and Brad Jascob, Ser. No. 60/161,989; Navigation Information Overlay onto Ultrasound Imagery, by Paul Kessman, Troy Holsing and Jason Trobaugh, Ser. No. 10/047,927; Registration of Human Anatomy Integrated for Electromagnetic Localization, by Mark W. Hunter and Paul Kessman, Ser. No. 09/429,569; System for Translation of Electromagnetic and Optical Localization Systems, by Mark W. Hunter and Paul Kessman, Ser. No. 09/429,568; Surgical Communication and Power System, by Mark W. Hunter, Paul Kessman and Brad Jascob, Ser. No. 09/428,722; and Surgical Sensor, by Mark W. Hunter, Sheri McCoid and Paul Kessman, Ser. No. 09/428,721. 
     This application claims the benefit of U.S. Provisional Application No. 60/161,990, filed Oct. 28, 1999, the contents of which are incorporated herein by reference in their entirety, and from which priority is claimed. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     REFERENCE TO MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates to methods of and devices for generating magnetic fields, and more particularly to the physical characteristics of magnetic field generating coils. 
     There are various known methods for determining the position of a medical instrument during surgery. For instance, U.S. Pat. No. 5,592,939 to Martinelli, hereby incorporated by reference, discloses a method and apparatus for detecting the position of a medical instrument during surgery. This invention, however, is not limited to any specific method of determining the position of a medical instrument during surgery. For example, FIG. 1 is a diagram of an examination deck  200  with a medical instrument in a surgical environment. During surgery, for example, examination deck  200  lies below a patient. The medical device, such as a catheter  203 , is placed inside the patient. Catheter  203  has a coil  14  at its distal end. Methods and systems consistent with the &#39;939 patent determine the location and orientation of catheter  203  inside the patient relative to examination deck  200 . 
     Catheter  203  includes a conductor  16  that leads along catheter  203  to a location outside the patient. Examination deck  200  comprises magnetic field generating coils that produce magnetic fields within a navigational domain  12 . The magnetic fields induce voltage signals in sensing coil  14 . Measurements taken at conductor  16  of the induced voltage signals provide sufficient information to compute the orientation and position of sensing coil  14 . 
     FIGS. 2A,  2 B,  2 C, and  3  show magnetic field generating coils. FIG. 2A is a diagram of a coil set  202  for generating a substantially uniform magnetic field in the X direction. Driver  28  supplies current in the direction indicated by the arrows. Coil elements  20  and  22  are horizontal, while coil elements  24  and  26  are vertical. Elements  24  and  26  are “compensation” coils, i.e. “Cunard” coils, which cancel some undesirable field components generated by elements  20  and  22  in the Y and Z directions. As a result, coil set  202  generates a substantially uniform X direction field as indicated by field line  27 . 
     FIG. 2B is a diagram of a coil set  204  for generating a substantially uniform magnetic field in the Y direction. Coil set  204  includes element  30  spaced from element  32 , but parallel to element  32 . Driver  34  supplies current in the direction indicated by the arrows. Coil set  204  generates a substantially uniform Y direction field as indicated by field line  33 . 
     FIG. 2C is a diagram of a coil set  206  for generating a substantially uniform magnetic field in the Z direction. Driver  44  supplies current in the direction indicated by the arrows. Coil elements  36  and  38  are horizontal, while elements  40  and  42  are vertical. Elements  40  and  42  are compensation coils, i.e. Cunard coils, that cancel some undesirable field components in the X and Y directions. As a result, coil set  206  generates a substantially uniform Z direction magnetic field as indicated by field line  43 . 
     FIG. 3 is a diagram of three pairs of delta coil sets  300  for generating three gradient magnetic fields. The configuration includes a first delta coil pair  50 - 52 , a second delta coil pair  54 - 56 , and a third delta coil pair  58 - 60 . Delta coil pairs  50 - 52 ,  54 - 56 , and  58 - 60  are arranged in a circular orientation about the Y axis such that there is an axis perpendicular to the direction of elongation of the coils at ,  120  , and  240  relative to the Z axis. The magnetic field generated by long delta coil  50  and short delta coil  52  is shown by the field lines extending from coils  50 - 52 . The field lines from delta coils  50 - 52  group form a family of substantially constant signal surfaces, i.e. the magnetic fields have a spatial gradient in two of the axis dimensions and a substantially zero field value in the remaining axial dimension. 
     Discussion of FIGS. 1,  2 A,  2 B,  2 C, and  3  are for illustrative purposes only. See U.S. Pat. No. 5,592,939 for further examples. 
     FIG. 3B is a diagram of a patient undergoing cranial surgery with a device consistent with this invention. In FIG. 3B, the medical device is a probe  302  that is placed inside a head  308  of a patient. 
     Coil sets  202 - 204 ,  300  in FIGS. 2A-2C, and  3  are contained within the examination deck  200  of FIG.  1 . Placing all these coils in examination deck  200 , however, causes examination deck  200  to be relatively thick. It is desirable, however, that examination deck  200  be relatively thin for a number of reasons. First, a thinner examination deck  200  is lighter, less cumbersome, and requires less space in a crowded surgery room. Second, if coil sets  202 - 204 ,  300  are arranged so that each is a different distance from navigational domain  12 , then the magnetic field strength in navigational domain  12  from each coil set is different. Different magnetic field strengths reduce accuracy of the positioning system. Further, it can be less expensive and easier to manufacturer a thin examination deck as compared to a thick examination deck. 
     Examination deck  200 , in turn, is placed on an examination table  306 . FIG. 3A is a diagram of examination deck  200  placed on the examination fable  306 , consistent with this invention, in a medical setting. Examination table  306  introduces other design constraints,including the width and length of the examination deck  200 , which introduces design constrains on the size and shape of coils inside examination deck  200 . Preferably, the magnetic field generating coils are such that examination deck easily fits onto standard size examination tables, such as examination table  306 . 
     Therefore, it is desirable to provide an apparatus that allows coil sets to be arranged substantially coplanar with respect to navigational domain  12 . It is also desirable to provide an apparatus that allows examination deck  200  to fit on a standard examination table. 
     It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     The foregoing and other objects are achieved by the invention which in one aspect comprises an apparatus for determining a location of a sensor in a surgical navigation domain. The apparatus includes a first magnetic field generator having a first coil set, a second magnetic field generator having a second coil set. The first and second coil sets are disposed substantially within a common plane. The apparatus further includes a processor configured to receive a plurality of signals. The processor calculates the location of the sensor from the plurality of signals. The sensor produces the plurality of signals in response to magnetic fields generated by the first and second magnetic field generators. 
     In another embodiment of the invention, the first coil set includes at least one delta coil pair for generating a gradient magnetic field-in the navigation domain. 
     In another embodiment of the invention, each delta coil pair further includes one or more end correction coils. Each delta coil pair is electrically coupled to the corresponding end correction coil, and current flows through the end correction coil in a direction opposite of the direction of the current flowing through the corresponding delta coil pair. 
     In another embodiment of the invention, the second coil set includes at least one uniform coil pair for generating a uniform magnetic field in the navigational domain. 
     In another embodiment of the invention, the first coil set includes a first delta coil pair longitudinally oriented along a first axis, a second delta coil pair longitudinally oriented along a second axis, and a third delta coil pair longitudinally oriented along a third axis. The three delta coil pairs are arranged such that the second axis is rotated within the common plane substantially sixty degrees with respect to the first axis, and the third axis is rotated within the common plane substantially one hundred and twenty degrees with respect to the first axis. 
     In another embodiment of the invention, each of the first, second arid third delta coil pairs lies within a distinct plane that is parallel to the common plane, such that the delta coil pairs overlap one another. 
     In another embodiment of the invention, each of the first, second and third delta coil pairs includes two or more distinct coil elements, electrically coupled, such that the aggregate of the distinct coil elements produces the corresponding gradient magnetic field. 
     In another embodiment of the invention, intersecting delta coil pairs share one or more common coil elements. 
     In another embodiment of the invention, intersecting delta coil pairs include distinct coil elements in an intersecting region where the delta coil pairs overlap. 
     In another embodiment of the invention, each of the delta coil pairs further include one or more end correction coils. Each of the delta coil pairs is electrically coupled to the corresponding end correction coil, and electrical current flows through the end correction coils in a direction opposite of the direction of the current flowing through the corresponding delta coil pair. 
     In another embodiment of the invention, at least one of the delta coil pairs is characterized by a length that is different from the length of the other delta coil pairs. 
     In another embodiment of the invention, each of the delta coil-pairs includes a short coil and a long coil. The short coil further includes a first end correction element and a second end correction element for reducing unwanted magnetic field components. Electrical current flows through the end correction coils in a direction opposite of the direction of the current flowing through the corresponding short coil. The long coil further includes a central compensating coil for reducing unwanted magnetic field components. Electrical current flows through the central compensating coil in a direction opposite of the direction of the current flowing through the corresponding long coil. 
     In another embodiment of the invention, one or more of the delta coil pairs overlap a coplanar uniform coil pair. 
     In another embodiment of the invention, each of the one or more overlapping delta coil pairs includes two or more distinct coil elements. The distinct coil elements are electrically coupled, such that the aggregate of the distinct coil elements produces the corresponding gradient magnetic field. 
     In another aspect, the invention comprises an apparatus for determining a location of a sensor in a surgical navigation domain. The apparatus includes a first magnetic field generator including at least one delta coil pair for generating a gradient magnetic field in said navigation domain. The at least one delta coil pair disposed within a first plane. The apparatus further includes a second magnetic field generator including at least one uniform coil pair for generating a uniform magnetic field in the navigational domain. The at least one uniform coil pair disposed within a second plane. The first plane is offset from the second plane by an offset angle calculated to reduce undesirable uniform field components. The apparatus also includes a processor, configured to receive a plurality of signals, for calculating the location of the sensor from the plurality of signals. The sensor produces the plurality of signals in response to magnetic, fields generated by the first and second magnetic field generators. 
     In another aspect, the invention comprises an apparatus for determining a location of a sensor in a surgical navigation domain, including a first magnetic field generator having a common coil, a second magnetic field generator also including the common coil, and a processor for calculating the location of the sensor. The sensor produces a plurality of signals in response to a first magnetic field generated by the first magnetic field generator, and in response to a second magnetic field of a different shape, with respect to the first magnetic field generated by the second magnetic field generator. 
     In yet another aspect, the invention comprises a method of determining a location of a sensor in a surgical navigation domain. The method includes generating a first magnetic field using a first magnetic field generator having a first coil set, and generating a second magnetic field using a second magnetic field generator having a second coil set. The first and second coils are disposed substantially within a common plane. The method further includes calculating the location of the sensor from a plurality of signals. The sensor produces the plurality of signals in response to magnetic fields generated by the first and second generated magnetic fields. 
     In another embodiment, the method further includes generating a gradient magnetic field in said navigation domain using at least one delta coil pair for generating. 
     In another embodiment, the method further includes generating a gradient magnetic field in said navigation domain using two or more distinct coil elements, electrically coupled, such that the aggregate of the distinct coil elements produces the corresponding gradient magnetic field. 
     In another embodiment, the method further includes generating a gradient magnetic field in said navigation domain using delta coil pairs having one or more end correction coils. Each of the delta coil pairs is electrically coupled to the corresponding end correction coil, and electrical current flows through the end correction coils in a direction opposite of the direction of the current flowing through the corresponding delta coil pair. 
     In another aspect, the invention comprises a method of determining a location of a sensor in a surgical navigation domain, including generating a gradient magnetic field in said navigation domain using a first magnetic field generator including at least one delta coil pair disposed within a first plane. The method further includes generating a uniform magnetic field in the navigational domain using a second magnetic field generator including at least one uniform coil pair. The at least one uniform coil pair is disposed within a second plane. The first plane is offset from the second plane by an offset angle calculated to reduce undesirable uniform field components. The method also includes calculating the location of the sensor from a plurality of signals. The sensor produces the plurality of signals in response to magnetic fields generated by the first and second generated magnetic field. 
     In another aspect, the invention comprises a method of determining a location of a sensor in a surgical navigation domain, including generating a first magnetic field using a magnetic field generator that includes a common coil. The method further includes generating a second magnetic field of a different shape than the first magnetic field, using a second magnetic field generator that includes the common coil. The method also includes calculating the location of the sensor from a plurality of signals. The sensor produces the plurality of signals in response to magnetic fields generated by the first and second magnetic field generators. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which: 
     FIG. 1 is a diagram of an examination deck  200  with a medical instrument in a medical environment; 
     FIGS. 2A,  2 B,  2 C, and  3  show magnetic field generating coils; 
     FIG. 3A is a diagram of the examination deck in FIG. 1 placed on an examination table, consistent with this invention, in a medical setting; 
     FIG. 3B is a diagram of a patient undergoing cranial surgery with a device consistent with this invention; 
     FIG. 4 is a top view of coil sets arranged to be disposed within in examination deck  200 ; 
     FIG. 5 is a top view of coil sets, consistent with this invention, arranged substantially coplanar for generating magnetic fields; 
     FIG. 6 is an exploded top view of a portion of two delta coils in FIG. 5, consistent with this invention, arranged substantially coplanar; 
     FIG. 6A is an electrical diagram of the coils shown in FIG. 6 configured with switches; 
     FIG. 7 is an exploded top view of a portion of two delta coils in FIG. 5, consistent with this invention, arranged substantially coplanar for generating magnetic fields; 
     FIG. 7A is an electrical diagram of the coils shown in FIG. 7; 
     FIG. 8 is a top view of coil sets, consistent with this invention, arranged substantially coplanar for generating magnetic fields; 
     FIG. 8A is an electrical diagram of the coils shown in FIG. 9; 
     FIG. 9 is an exploded top view of a delta coil and a uniform coil in FIG. 8, consistent with this invention, arranged substantially coplanar; 
     FIG. 9A is an exploded side view of a uniform coil set, consistent with this invention; 
     FIG. 10 is a top view of coil sets, consistent with this invention, arranged substantially coplanar for generating magnetic fields; 
     FIG. 11 is a diagram of a top view of a delta coil arrangement relative to an inner circular space; 
     FIG. 12 schematically depicts an examination deck; 
     FIGS. 13A,  13 B,  13 C, and  13 D show magnetic field generating coils; 
     FIG. 14 is an engineering drawing of the coil sets, consistent with this invention, shown in FIG. 5; 
     FIG. 15 is an engineering drawing of the coil sets, consistent with this invention, shown in FIG. 10; 
     FIGS. 16 and 17 are engineering drawings, consistent with this invention, of the coil sets shown in FIG. 8; 
     FIG. 18 shows a constant signal pattern for a catheter in a magnetic field generated by the delta coil set of FIG. 10; 
     FIG. 19 is a diagram of a surgical table with an integrated examination deck; and 
     FIG. 20 is a top view, side view, and an end view of an examination deck with an integrated examination deck. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of embodiments of this invention refers to the accompanying drawings. Where appropriate, the same reference numbers in different drawings refer to the same or similar elements. 
     FIG. 4 is a top view of coil sets, consistent with this invention, arranged to be placed in examination deck  200 . An arrangement  400  comprises a first delta coil pair  402 - 406 , a second delta coil pair  408 - 410 , and a third delta coil pair  412 - 414 . First through third delta coil pairs  402 - 414  create gradient fields similar to those described with respect to FIG. 3 above. Arrangement  400  also comprises a first uniform coil pair  420 ,  424 , and a second uniform coil pair  426 ,  422 . First and second uniform coil pairs generate substantially uniform magnetic fields similar to the fields described with respect to FIGS. 2A and 2C above. Arrangement  400  also comprises a girth coil  428  that creates a substantially uniform magnetic field similar to the magnetic field described with respect to FIG. 2B above. Arrows indicate a possible direction of current flowing in the coils. Arrangement  400  is advantageous because it may be configured to in examination deck  200  so that it can fit on and be integrated into a standard examination or surgical table. 
     Methods and systems consistent with this invention arrange the coil sets so that they are substantially coplanar. FIG. 5 is a top view of coil sets, consistent with this invention, arranged substantially coplanar for generating magnetic fields. An arrangement  500  comprises a first delta coil pair  502 - 506 , a second delta coil pair  508 - 510 , and a third delta coil pair  512 - 514 . First through third delta coil pairs  508 - 514  create gradient fields similar to those described with respect to FIG. 3 above. Arrangement,  500  also comprises a first uniform coil pair  520 ,  524 , and a second uniform coil pair  526 ,  522 . First and second uniform coil pairs generate uniform magnetic fields similar to the fields described with respect to FIGS. 2A and 2C above. Arrangement  500  also comprises a girth coil  528  that creates a substantially uniform magnetic field similar to the magnetic field described with respect to FIG. 2B above. Arrows indicate a possible direction of current flowing in the coils. 
     In FIG. 5, coil  506  has nine elements  506   a-i . When coil  506  generates a magnetic field, nine elements  506   a -I are electrically connected in series so such that they produce magnetic fields that are nearly identical to coil  406  in FIG.  4 . Coil  508  also has nine separate elements  508   a-i . When coil  508  generates a magnetic field, nine elements  508   a -I are electrically connected in series such that they produce magnetic fields that are nearly identical to coil  408  in FIG.  4 . Coils  510 ,  512 ,  514 , and  502  also comprise nine elements and are configured in a similar fashion. 
     FIG. 6 is an exploded top view of a portion of two delta coils, consistent with this invention, arranged substantially coplanar. In the embodiment shown in FIG. 6, element  506   b  and element  508   b  are the same element. When coil  508  generates a magnetic field, switches  602 ,  604  create one electrical path through elements  508   a ,  508   b  ( 506   b ), and  508   c . When coil  506  generates a magnetic field, switches  602 ,  604  create one electrical path through elements  506   a ,  506   b  ( 508   b ), and  508   c . Arrows indicate a possible direction of current flowing in coils  506   a-c  and  508   a-c . In this fashion, coil  508  and coil  506  share an element so that the coils can be arranged substantially coplanar. 
     FIG. 6A is an electrical diagram of coils  506   a-c  and  508   a-c  shown in FIG. 6 configured with switches. When coil  508  generates a magnetic field, switches  602 ,  604  are in position A, which creates one electrical path through elements  508   a ,  508   b  ( 506   b ), and  508   c . When coil  506  generates a magnetic field, switches  602 ,  604  are in position B, which creates one electrical path through elements  506   a ,  506   b  ( 508   b ), and  506   c . Arrows indicate a possible direction of current flowing in coils  506   a-c  and  508   a-c.    
     FIG. 7 is an exploded top view of a portion of two delta coils, consistent with this invention, arranged substantially coplanar for generating magnetic fields. In the embodiment shown in FIG. 7, element  506   b  and element  508   b  are separate elements comprising two separate coils wrapped around substantially the same area. In the embodiment shown in FIG. 7, no switches are necessary, which eliminates complexity and hardware. Instead, elements  508   a ,  508   b , and  508   c , are always connected in series to form one electrical path. Likewise, elements  506   a ,  506   b , and  506   c  are always connected in series to form one electrical path. FIG. 7 also provides a cross sectional view of elements  506   b  and  508   b . As shown in this cross sectional view, coil  506   b  is wound outside coil  508   b  such that coil  506   b  encloses a greater area than coil  506   b . In order to compensate for the smaller area enclosed by  508   b , it is possible to include one or more extra windings of element  508   b  compared to element  506   b.    
     FIG. 7A is an electrical diagram of coils  506   a-c  and  508   a-c  of FIG.  7 . In this embodiment, there are no switches and there is one-electrical path through elements  508   a ,  508   b , and  508   c . Likewise, there is one electrical path through elements  506   a ,  506   b , and  506   c . Arrows indicate a possible direction of current flowing in coils  506   a-c  and  508   a-c.    
     Thus, referring back to FIG. 5, delta coil pairs  502 - 506 ,  508 - 510 , and  512 - 514  lie substantially coplanar because of the configurations in FIGS. 6 and 7. In FIG. 5, there are twelve elements that “intersect,” i.e., they may implement the configuration in FIGS. 6 and 7. Further, first uniform coil pair  520 ,  524  and second uniform coil pair  522 ,  526  are coplanar with first through third delta coil pairs  502 - 514 . Lastly, girth coil  526  is coplanar with first through third delta coil pairs  502 - 514  and first unidirectional coil pair  520 ,  524  and second unidirectional coil pair  522 ,  528 . Thus, all coil sets in FIG. 5 are coplanar. 
     FIG. 5 also shows “end correction” coils  530 - 540 . Coils  530 - 540  are electrically in series with the nearest delta coil pairs, but carry current in the reverse direction. End correction coils  530 - 540  help to reduce unwanted magnetic field components in the non-gradient direction. 
     FIG. 8 is a top view of coil sets, consistent with this invention, arranged substantially coplanar for generating magnetic fields. The arrangement  800  comprises a first delta coil pair  802 - 804 , a second delta coil pair  806 - 808 , and a third delta coil pair  810 - 812 . First through third delta coil pairs  802 - 812  create gradient fields similar to those described above with respect to FIG.  3 . Arrangement  800  also comprises a first uniform coil pair  814 ,  818  and a second uniform coil pair  816 ,  820 . First and second uniform coil pairs  816 ,  820  generate uniform magnetic fields similar to the magnetic fields discussed with respect to FIGS. 2A and 2C above. Arrangement  800  also comprises a girth coil  824  that generates a substantially uniform magnetic field similar to the magnetic fields discussed with respect to FIG. 2B above. 
     Also in FIG. 8, first delta coil set  802 - 804  intersects first uniform coil pair  814 ,  818  at element  802   b . First delta coil set  802 - 804  also intersects second uniform coil pair  820 ,  816  at element  802   c . Likewise, third delta coil set  810 - 812  intersects first uniform coil pair  814  at element  812   b . Third coil set  810 - 812  also intersects second uniform coil pair  820 ,  816  at element  812   c . Despite these intersections, a however, first and third delta coil set  802 - 804 ,  810 - 812  are arranged coplanar with first and second uniform coil set  814 ,  818 , and  816 ,  820 . 
     Also in FIG. 8, first delta coil set  802 - 804  intersects first uniform coil pair  814 ,  818  at element  802   b . First delta coil set  802 - 804  also intersects second uniform coil pair  820 ,  816  at element  802   c . Likewise, second delta coil set  810 - 812  intersects first uniform coil pair  814  at element  812   b . Second delta coil set  810 - 812  also intersects second uniform coil pair  820 ,  816  at element  812   c . Despite these intersections, however, first and second delta coil set  802 - 804 ,  810 - 812  are arranged coplanar with first and second uniform coil set  814 ,  818 , and  816 ,  820 . 
     FIG. 9A is an exploded side view of a uniform coil set, consistent with this invention. In FIG. 9A, first, second, and third delta coil sets (not shown) are substantially in a plane  902 . First uniform coil set  816 ,  820 , in this embodiment is displaced by an angle α from plane  902  as shown. Offset angle α eliminates the need for compensation coils  24  and  26  in FIG. 2A while achieving the same result. Thus, displacing first uniform coil set  816 ,  820  by angle α cancels undesirable magnetic field components in the Y and Z directions. As a result, uniform coil set  816 ,  820  generates a substantially uniform X direction field. Second uniform coil set  814 ,  818  may also be displaced by angle α. It is evident to one of ordinary skill in the art how to calculate angle α necessary to create equivalent correction to eliminate elements  24 ,  26 . 
     FIG. 9A is an exploded side view of a uniform coil set, consistent with this invention. In FIG. 9A, first, second, and third delta coil sets (not shown) are substantially in a plane  902 . First uniform coil set  816 ,  812 , in this embodiment is displaced by an angle α from plane  902  as shown. Offset angle α eliminates the need for compensation coils  24  and  26  in FIG. 2A while achieving the same result. Thus, displacing first uniform coil set  816 ,  820  by angle α cancels undesirable magnetic field components in the Y and Z directions. As a result, uniform coil set  816 ,  820  generates a substantially uniform X direction field. Second uniform coil set  814 ,  818  may also be displaced by angle α. It is evident to one of ordinary skill in the art how to calculate angle α necessary to create equivalent correction to eliminate elements  24 ,  26 . 
     FIG. 10 is a top view of coil sets, consistent with this invention, arranged substantially coplanar for generating magnetic fields. An arrangement  1000  comprises a first delta coil pair  1002  a second delta coil pair  1004  and a third delta coil pair  1006 . First through third delta coil pairs  1002 - 1004  create gradient fields similar those discussed above with respect to FIG.  3 . Arrangement  1000  also comprises a first uniform coil pair  1012 - 14  and a second uniform coil pair  1008 - 1010 . First and second uniform coil pair  1012 - 1014 ,  1008 - 1010  generate uniform magnetic fields similar to those discussed above with relation to FIGS. 2A and 2C. Arrangement  1000  also comprises a girth coil  1024  that generates a substantially uniform magnetic field similar to the magnetic fields discussed above with respect to FIG.  2 B. First delta coil pair  1002 , which in this embodiment is shorter than second delta coil pair  1004  and third delta coil pair  1006 , contains end correction elements  1018 ,  1016 ,  1022 , and  1020 . End correction elements carry current in the reverse direction to reduce unwanted magnetic field components. 
     FIG. 11 is a diagram of a top view of a delta coil arrangement  1100  relative to an inner circular space  104 . In arrangement  1100 , a short coil  52  is provided with end correction elements  94 ,  96 . Long coil  50  comprises a central compensating or “sucker” coil  88 , which carries current in the opposite direction than coil  50  and eliminates some unwanted magnetic field components. Long coil  50  and short coil  52  are modified by compensation coils  80 - 82 ,  84 - 86 ,  88 ,  90 - 94 , and  92 - 96 . Long coil  50  and short coil  52  are shown schematically for sets  50 - 52 , but similar configurations likewise exist for coil sets  54 - 56  and  58 - 60 . 
     FIG. 12 schematically depicts another examination deck in accordance with another embodiment of the present invention. FIGS. 13A-C are diagrams of unidirectional coils. The assembly for the X direction unidirectional coil set is shown in FIG.  13 A and includes two coil elements  110  and  112 . Elements  110 ,  112  project a substantially uniform field in the X direction throughout the navigational domain. FIG. 13B schematically depicts the Y direction unidirectional coils including coil elements  114 ,  116 ,  118 , and  120 . FIG. 13C schematically depicts the Z direction unidirectional coils including coil elements  122 - 124 , and  126 - 128 . FIG. 13D shows the delta coil arrangement used in the railed configuration. The arrangement in FIG. 13D is the same as used in FIG. 3 described above. 
     Discussion of FIG. 11,  12 ,  13 A,  13 B,  13 C, and  13 D are for illustrative purposes only. See U.S. Pat. No. 5,592,939 for further examples. 
     FIG. 14 is an engineering drawing of the coil sets, consistent with this invention, shown in FIG.  5 . FIG. 15 is an engineering drawing of the coil sets, consistent with this invention, shown in FIG.  10 . As shown in FIG. 15, the coils can easily fit onto an operating table that is twenty inches wide and twenty-two inches long. FIGS. 16 and 17 are an engineering drawings, consistent with this invention, of the coil sets shown in FIG.  8 . FIG. 18 shows the constant signal pattern for a catheter at φ=90 and θ=90 degrees of a magnetic field generated by a delta coil set in FIG.  10 . 
     FIG. 19 is a diagram of a surgical table  1900  with an integrated examination deck. The examination deck comprising field generating coils may be integrated at any or all locations  1902 ,  1904 , and  1906 . Alternatively, field generating coils may be integrated directly into surgical table  1900  at any or all locations  1902 ,  1904 , and  1906 . FIG. 20 is a top view, side view, and an end view of surgical table  2000  with an integrated examination deck. The examination deck comprising field generating coils may be integrated at any or all locations  2002 ,  2004 , and  2006 . Alternatively, field generating coils may be integrated directly into surgical at any or all locations  2002 ,  2004 , and  2006 . 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.