Abstract:
A method, using multiple patches fixed to a surface of a body, the patches including respective electrodes in contact with the surface, and at least one of the patches configured to output a signal in response to a magnetic field applied to the body. Initially, the signal is processed to compute first magnetic and first electrical locations of the at least one of the patches. Subsequently, the signal is processed to compute second magnetic and second electrical locations of the at least one of the patches. A first relation is computed between the first magnetic and electrical locations, and a second relation is computed between the second magnetic and electrical locations. When there is a difference between the first and the second relations, a magnetic location correction is computed responsively to the difference, and the correction is applied in tracking a position of a magnetic tracking sensor inside the body.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 62/214,273, filed Sep. 4, 2015, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to medical imaging, and specifically to a method for correcting measurements indicating an inconsistent field-based location coordinates of a skin patch affixed to a patient. 
       BACKGROUND OF THE INVENTION 
       [0003]    A wide range of medical procedures involve placing objects, such as sensors, tubes, catheters, dispensing devices, and implants, within the body. Real-time imaging methods are often used to assist doctors in visualizing the object and its surroundings during these procedures. In most situations, however, real-time three-dimensional imaging is not possible or desirable. Instead, systems for obtaining real-time spatial coordinates of the internal object are often utilized. 
         [0004]    U.S. Patent Application 2007/0016007, to Govari et al., whose disclosure is incorporated herein by reference, describes a hybrid magnetic-based and impedance-based position sensing system. The system includes a probe adapted to be introduced into a body cavity of a subject. 
         [0005]    U.S. Pat. No. 6,574,498, to Gilboa, whose disclosure is incorporated herein by reference, describes a system for determining the position of a work piece within a cavity of an opaque body. The system claims to use a transducer that interacts with a primary field, and several transducers that interact with a secondary field. 
         [0006]    U.S. Pat. No. 5,899,860, to Pfeiffer, et al., whose disclosure is incorporated herein by reference, describes a system for determining the position of a catheter inside the body of a patient. A correction function is determined from the difference between calibration positions derived from received location signals and known, true calibration positions, whereupon catheter positions, derived from received position signals, are corrected in subsequent measurement stages according to the correction function. 
         [0007]    Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
         [0008]    The description above is presented as a general overview of related art in this field and should not be construed as an admission that any of the information it contains constitutes prior art against the present patent application. 
       SUMMARY OF THE INVENTION 
       [0009]    There is provided, in accordance with an embodiment of the present invention a method for sensing, using an array of patches fixed to a surface of a body of a subject, the patches including respective electrodes in contact with the surface, and at least one of the patches including a patch sensor configured to output a signal in response to a magnetic field applied to the body, the method including at a first time, processing the signal so as to compute first field-based location coordinates of the at least one of the patches, and computing first impedance-based location coordinates of the at least one of the patches by measuring an impedance to an electrical current applied to the body, at a second time, subsequent to the first time, processing the signal so as to compute second field-based location coordinates of the at least one of the patches, and computing second impedance-based location coordinates of the at least one of the patches by measuring the an impedance to the electrical current, computing a first relation between the first field-based location coordinates and the first impedance-based location coordinates, and a second relation between the second field-based location coordinates and the second impedance-based location coordinates, when there is a difference between the second relation and the first relation, computing a field-based location coordinate correction responsively to the difference, and applying the field-based location coordinate correction in tracking a position of a magnetic tracking sensor inside the body, based on signals received from the magnetic tracking sensor in response to the applied magnetic field. 
         [0010]    In embodiments of the present invention, the first relation for a given patch may include a first distance and a first orientation from the first impedance-based location coordinates of the given patch to the first field-based location coordinates of the given patch, and wherein the second relation for the given patch includes a second distance and a second orientation from the second impedance-based location coordinates of the given patch to the second field-based location coordinates of the given patch. 
         [0011]    In some embodiments, the field-based location coordinate correction for the second field-based location coordinates of the given patch includes the first distance and the first orientation. In additional embodiments, the method may include at a third time, subsequent to the second time, processing the signal so as to compute third field-based location coordinates of the at least one of the patches, computing third impedance-based location coordinates of the at least one of the patches by measuring the an impedance to the electrical current, and applying the field-based location coordinate correction to the third field-based location coordinates. 
         [0012]    In further embodiments, the magnetic field is applied to the body by positioning the body over multiple coils configured to generate the magnetic field. In supplemental embodiments, the object includes a medical probe having a probe electrode, and wherein the electrical current is applied to the body by conveying the electrical current to the probe electrode. In additional embodiments, the signal includes a first signal, and wherein measuring the impedance includes receiving, from the at least one patches, a second signal in response to the impedance of the electrical current conveyed by the probe electrode. 
         [0013]    There is also provided, in accordance with an embodiment of the present invention an apparatus for method for sensing, including an array of patches fixed to a surface of a body of a subject, the patches including respective electrodes in contact with the surface, and at least one of the patches including a patch sensor configured to output a signal in response to a magnetic field applied to the body, and a control console configured at a first time, to process the signal so as to compute first field-based location coordinates of the at least one of the patches, and to compute first impedance-based location coordinates of the at least one of the patches by measuring an impedance to an electrical current applied to the body, at a second time, subsequent to the first time, to process the signal so as to compute second field-based location coordinates of the at least one of the patches, and to compute second impedance-based location coordinates of the at least one of the patches by measuring the an impedance to the electrical current, to compute a first relation between the first field-based location coordinates and the first impedance-based location coordinates, and a second relation between the second field-based location coordinates and the second impedance-based location coordinates, when there is a difference between the second relation and the first relation, to compute a field-based location coordinate correction responsively to the difference, and to apply the field-based location coordinate correction in tracking a position of a magnetic tracking sensor inside the body, based on signals received from the magnetic tracking sensor in response to the applied magnetic field. 
         [0014]    There is further provided, in accordance with an embodiment of the present invention, a computer software product for sensing, using an array of patches fixed to a surface of a body of a subject, the patches including respective electrodes in contact with the surface, and at least one of the patches including a patch sensor configured to output a signal in response to a magnetic field applied to the body, the product including a non-transitory computer-readable medium, in which program instructions are stored, which instructions, when read by a computer, cause the computer to process, at a first time, the signal so as to compute first field-based location coordinates of the at least one of the patches, and to compute first impedance-based location coordinates of the at least one of the patches by measuring an impedance to an electrical current applied to the body, to process at a second time, subsequent to the first time, the signal so as to compute second field-based location coordinates of the at least one of the patches, and to compute second impedance-based location coordinates of the at least one of the patches by measuring the an impedance to the electrical current, to compute a first relation between the first field-based location coordinates and the first impedance-based location coordinates, and a second relation between the second field-based location coordinates and the second impedance-based location coordinates, when there is a difference between the second relation and the first relation, to compute a field-based location coordinate correction responsively to the difference, and to apply the field-based location coordinate correction in tracking a position of a magnetic tracking sensor inside the body, based on signals received from the magnetic tracking sensor in response to the applied magnetic field. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The disclosure is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a schematic pictorial illustration of a medical system configured to correct an inconsistent location of one or more adhesive skin patches while performing a procedure on a heart, in accordance with an embodiment of the present invention; 
           [0017]      FIG. 2  is a schematic pictorial of a catheter in the heart, in accordance with an embodiment of the present invention; 
           [0018]      FIG. 3  is a flow diagram that illustrates a method of correcting an inconsistent physical location of a given adhesive skin patch by using location measurements from additional skin patches, in accordance with an embodiment of the present invention; 
           [0019]      FIGS. 4A-4E  are schematic diagrams illustrating rigid bodies that are constructed from locations of the adhesive skin patches in order to correct the inconsistent physical location of the given skin patch, in accordance with an embodiment of the present invention; 
           [0020]      FIG. 5  is a flow diagram that illustrates a method of correcting an inconsistent apparent location of multiple adhesive skin patches caused by magnetic interference, in accordance with an embodiment of the present invention; and 
           [0021]      FIGS. 6A-6C  are schematic diagrams illustrating first, second and corrected second location coordinates for the multiple adhesive skin patches, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
       [0022]    Various diagnostic and therapeutic procedures involve mapping of the electrical potential on the inner surface of a cardiac chamber. Electrical mapping can be performed, for example, by inserting a medical probe (e.g., a catheter), whose distal end is fitted with a position sensor and a mapping electrode, into the cardiac chamber. The cardiac chamber is mapped by positioning the probe at multiple points on the inner chamber surface. At each point, the electrical potential is measured using the electrode, and the distal end position is measured using the position sensor. The measurements are typically presented as a map of the electrical potential distribution over the cardiac chamber surface. 
         [0023]    While positioning the medical probe within the cardiac chamber, impedance-based and/or magnetic-based position sensing systems can be used to determine a location of the probe within the cardiac chamber. Location sensing systems, such as those described in U.S. Pat. No. 8,456,182, whose disclosure is incorporated herein by reference, can determine a location of the probe by using locations of a set of three adhesive skin patches (also referred to herein as patches) that are affixed to a back of a patient. Location measurements received from the patches can be used to define a rigid body in a body coordinate system, and to determine a location of the probe within the rigid body. The body coordinate system can be updated as the adhesive skin patches move due to normal patient activities such as breathing. 
         [0024]    Typically, the adhesive skin patches move and have respective locations that are consistent with one another so that the rigid body referred to above does not deform, but there may be instances when movement of one or more of the patches results in each of the one or more patches having a location that is not consistent with locations of the remaining patches. Embodiments of the present invention provide methods and systems for detecting and correcting an inconsistent location of one or more of the adhesive skin patches. 
         [0025]    In a disclosed embodiment, the inconsistent location comprises a physical location of one of the adhesive skin patches. For example, if the patient is lying on a table, the one adhesive skin patch may “stick” to the table as the patient moves. In an alternative embodiment, the inconsistent location comprises apparent locations of a plurality of the patches. For example, the positioning system may be based on magnetic sensors, and magnetic interference may cause an “apparent” movement (i.e., not a physical movement) of the plurality of the patches to their respective apparent inconsistent locations. 
       System Description 
       [0026]      FIG. 1  is a schematic pictorial illustration of a medical system  20 , and  FIG. 2  is a schematic illustration of a probe used in the system, in accordance with an embodiment of the present invention. System  20  may be based, for example, on the CARTO® system, produced by Biosense Webster Inc. (Diamond Bar, Calif.). System  20  comprises a medical probe  22 , such as a catheter, and a control console  24 . In embodiments described hereinbelow, it is assumed that probe  22  is used for diagnostic or therapeutic treatment, such as performing ablation of heart tissue in a heart  26 . Alternatively, probe  22  may be used, mutatis mutandis, for other therapeutic and/or diagnostic purposes in the heart or in other body organs. 
         [0027]    An operator  28  inserts probe  22  through the vascular system of a patient  30  so that distal end  32  ( FIG. 2 ) of probe  22  enters a chamber of heart  26 . In the configuration shown in  FIG. 1 , operator  28  uses a fluoroscopy unit  34  to visualize distal end  32  inside heart  26 . Fluoroscopy unit  34  comprises an X-ray source  36 , positioned above patient  30 , which transmits X-rays through the patient. A flat panel detector  38 , positioned below patient  30 , comprises a scintillator layer  40  which converts the X-rays which pass through patient  30  into light, and a sensor layer  42  which converts the light into electrical signals. Sensor layer  42  typically comprises a two dimensional array of photodiodes, where each photodiode generates an electrical signal in proportion to the light detected by the photodiode. 
         [0028]    Control console  24  comprises a processor  44  that converts the electrical signals from fluoroscopy unit  34  into an image  46 , which the processor presents as information regarding the procedure on a display  48 . Display  48  is assumed, by way of example, to comprise a cathode ray tube (CRT) display or a flat panel display such as a liquid crystal display (LCD), light emitting diode (LED) display or a plasma display. However other display devices can also be employed to implement embodiments of the present invention. In some embodiments, display  48  may comprise a touchscreen configured to accept inputs from operator  28 , in addition to presenting image  46 . 
         [0029]    System  20  can use magnetic position sensing to determine position coordinates of distal end  32  inside heart  26 . In configurations where system  20  uses magnetic based position sensing, console  24  comprises a driver circuit  50  which drives field generators  52  to generate magnetic fields within the body of patient  30 . Typically, field generators  52  comprise coils, which are placed below the patient at known positions external to patient  30 . These coils generate magnetic fields in a predefined working volume that contains heart  26 . A magnetic field sensor  54  (also referred to herein as position sensor  54 ) within distal end  32  of probe  22  generates electrical signals in response to the magnetic fields from the coils, thereby enabling processor  44  to determine the position of distal end  32  within the cardiac chamber. Magnetic position tracking techniques are described, for example, in U.S. Pat. Nos. 5,391,199, 6,690,963, 5,443,489, 6,788,967, 5,558,091, 6,172,499 and 6,177,792, whose disclosures are incorporated herein by reference. 
         [0030]    Additionally, system  20  can use impedance-based position sensing to determine position coordinates of distal end  32  inside heart  26 . In configurations where system  20  uses impedance-based position sensing, position sensor  54  is configured as a probe electrode, typically formed on an insulating exterior surface  76  of the distal end, and console  24  is connected by a cable  56  to body surface electrodes, which comprise three primary adhesive skin patches  58  and one or more ancillary adhesive skin patches  60 . In some embodiments, primary adhesive skin patches  58  are affixed to a back  62  of patient  30 , and the one or more ancillary adhesive skin patches are affixed to a front  64  of the patient. In operation, processor  44  can determine position coordinates of probe  22  inside heart  26  based on the impedance measured between the probe electrode and patches  58  and  60 . Impedance-based position tracking techniques are described, for example, in U.S. Pat. Nos. 5,983,126, 6,456,864 and 5,944,022, whose disclosures are incorporated herein by reference. 
         [0031]    In some embodiments, each patch  58  and  60  may also comprise magnetic field sensors (e.g., coils) that can measure the magnetic fields produced by field generators  52 , and convey the magnetic field measurements to console  24 . Based on the measurements received from patches  58  and  60 , processor  44  can determine current positions for each of the primary and the ancillary adhesive skin patches. Both magnetic-based and impedance-based systems described hereinabove generate signals which vary according to the position of distal end  32 . 
         [0032]    Processor  44  receives and processes the signals generated by position sensor  54  in order to determine position coordinates of distal end  32 , typically including both location and orientation coordinates. The method of position sensing described hereinabove is implemented in the above-mentioned CARTO™ system and is described in detail in the patents and patent applications cited above. 
         [0033]    Processor  44  typically comprises a general-purpose computer, with suitable front end and interface circuits for receiving signals from probe  22  and controlling the other components of console  24 . Processor  44  may be programmed in software to carry out the functions that are described herein. The software may be downloaded to console  24  in electronic form, over a network, for example, or it may be provided on non-transitory tangible media, such as optical, magnetic or electronic memory media. Alternatively, some or all of the functions of processor  44  may be carried out by dedicated or programmable digital hardware components. 
         [0034]    Based on the signals received from probe  22  and other components of system  20 , processor  44  drives display  48  to update image  46  to present a current position of distal end  32  in the patient&#39;s body, as well as status information and guidance regarding the procedure that is in progress. Processor stores data representing image  46  in a memory  66 . In some embodiments, operator  28  can manipulate image  46  using one or more input devices  68 . In embodiments, where display  48  comprises a touchscreen display, operator  28  can manipulate image  46  via the touchscreen display. 
         [0035]    In the configuration shown in  FIG. 2 , probe  22  also comprises a force sensor  70  contained within distal end  32  and an ablation electrode  72  mounted on a distal tip  74  of probe  22 . Force sensor  70  measures a force applied by distal tip  74  on the endocardial tissue of heart  26  by generating a signal to the console that is indicative of the force exerted by the distal tip on the endocardial tissue. In one embodiment, the force sensor may comprise a magnetic field transmitter and receiver connected by a spring in distal tip  74 , and may generate an indication of the force based on measuring the deflection of the spring. Further details of this sort of probe and force sensor are described in U.S. Patent Application Publications 2009/0093806 and 2009/0138007, whose disclosures are incorporated herein by reference. Alternatively, distal end  32  may comprise another type of force sensor. 
         [0036]    Electrode  72  typically comprises one or more thin metal layers formed over exterior surface  76  of distal end  32 . Console  24  also comprises a radio frequency (RF) ablation module  78 . Processor  44  uses ablation module  78  to monitor and control ablation parameters such as the level of ablation power applied via electrode  72 . Ablation module  78  may also monitor and control the duration of the ablation that is provided. 
       Single Patch Location Correction 
       [0037]      FIG. 3  is a flow diagram that illustrates a method of correcting an inconsistent physical location of a single primary adhesive skin patch  58  by using location measurements from ancillary patches  60 , and  FIGS. 4A-4E , referred to collectively as  FIG. 4 , are schematic diagrams illustrating rigid bodies  100 - 108  that are constructed from locations  110 - 134  of the primary and the ancillary skin patches, in accordance with an embodiment of the present invention. In the example shown in  FIG. 4 , locations  110 - 132  comprise three-dimensional coordinates in a coordinate system  136  comprising an X-axis  138 , a Y-axis  140 , and a Z-axis  142 . 
         [0038]    In embodiments described hereinbelow, locations  110 - 132  are indicative of spatial relationships that correspond to rigid bodies  100 - 106 . Thus, in the example shown in  FIG. 4 , locations  110 ,  112 ,  114  are indicative of first spatial relationships which define rigid body  100 , locations  122 ,  124 ,  126  are indicative of second spatial relationships which define rigid body  102 , locations  110 ,  112 ,  114 ,  116 ,  118 ,  120  are indicative of third spatial relationships which define rigid body  104 , and locations  122 ,  126 ,  128 ,  130  and  132  are indicative of fourth spatial relationships which define rigid body  106 . In embodiments described herein, rigid body  100  may also be referred to as a first rigid body, rigid body  102  may also be referred to as a second rigid body, rigid body  104  may also be referred to as a third rigid body, and rigid body  106  may also be referred to as a fourth rigid body. 
         [0039]    In an initial step  80 , operator  28  affixes primary adhesive skin patches  58  to back  62  of patient  30 , and affixes ancillary skin patches  60  to front  64  of the patient. In a first receive step  81 , processor  44  receives, at a first time, first position-dependent signals from patches  58  and  60 . In the flow diagram shown in  FIG. 3 , primary patches  58  may be referred to as back patches, and ancillary patches  60  may be referred to as front patches. 
         [0040]    In a first compute step  82 , processor  44  computes respective first location coordinates  110 ,  112 ,  114  for patches  58 , and respective first location coordinates  116 ,  118 ,  120  for patches  60 . In a first identification step  83 , processor  44  identifies the first spatial relationships between patches  58 , using, as shown in  FIG. 4A , the respective first location coordinates of locations  110 ,  112  and  114  of the primary adhesive skin patches, i.e., as rigid body  100 . 
         [0041]    In a second receive step  84 , processor  44  receives, at a second time subsequent to the first time, second position-dependent signals from patches  58  and  60 . In a second compute step  85 , processor  44  computes respective second location coordinates  122 ,  124 ,  126  for patches  58  and respective second location coordinates  128 ,  130 ,  132  for patches  60 . In a second identification step  86 , processor  44  identifies the second spatial relationships between patches  58 , using, as shown in  FIG. 4B , the respective second location coordinates of locations  122 ,  124  and  126  of primary adhesive skin patches  58 , i.e., as rigid body  102 . 
         [0042]    In a detection step  87 , processor  44  detects a discrepancy between the first and the second spatial relationships. The discrepancy is caused by a change of location of only one primary patch  58  relative to the other primary patches. The detected discrepancy indicates that the second location of the only one primary patch is inconsistent with the second locations of the remaining primary patches  58 . 
         [0043]    In the present example, the inconsistent location is a result of a physical movement of the only one primary patch  58  from location  112  ( FIG. 4A ) to location  124  ( FIG. 4B ) not being consistent with movements of the remaining primary patches from locations  110  and  114  to locations  122  and  126  (i.e., both locations  112  and  124  comprise physical locations of the only one primary patch). For example, processor  44  may detect the discrepancy between the first and the second spatial relationships by detecting that rigid body  100  and rigid body  102  are no longer congruent, and that the non-congruency is effectively caused by the movement of only one of the patch locations defining the bodies. In other words, by detecting the incongruence between rigid bodies  100  and  102 , processor  44  detects a discrepancy between the first and the second spatial relationships caused by a given patch  58  that has first location  112  and the other patches  58  that have respective first locations  110  and  114 . 
         [0044]    In a third identification step  88 , processor  44  identifies the third spatial relationships between patches  58  and  60 , using, as shown in  FIG. 4C , the respective first location coordinates indicated by locations  110 ,  112 ,  114 ,  116 ,  118 , and  120  of the primary and the ancillary skin patches, i.e., as rigid body  104 . 
         [0045]    During a medical procedure, processor  44  receives signals from all of the primary and the ancillary adhesive skin patches. Typically, as shown in  FIGS. 4A and 4B , the processor defines rigid bodies  100  and  102  based on respective locations of primary patches  58 . In embodiments of the present invention, upon detecting an inconsistent movement/location of a given patch  58 , processor  44  can calculate a correction for location  124  of the given patch by using locations of ancillary patches  60  and the remaining primary patches to create rigid bodies  104  ( FIG. 4C ),  106  ( FIG. 4D ) and  108  ( FIG. 4E ), as explained hereinbelow. 
         [0046]    In a fourth identification step  89 , processor  44  identifies the fourth spatial relationships between patches  60  and the other patches  58  (i.e., the fourth spatial relationships do not include the given patch  58  that moved inconsistently), using, as shown in  FIG. 4D , the respective second location coordinates of locations  122 ,  126 ,  128 ,  130  and  132  of the primary and the ancillary adhesive skin patches, i.e., as rigid body  106 . 
         [0047]    In a calculation step  90 , processor  44  calculates, based on the spatial relationships, a location correction for the only one primary patch. In some embodiments, the spatial relationships comprise the third and the fourth spatial relationships. Finally, in an application step  91 , processor  44  applies the location correction to the second location of the only one primary patch, thereby determining a corrected second location for the only one primary patch, and the method ends. In some embodiments, processor  44  applies the location correction while using the second location coordinates of patches  58  in order to track an object such as probe  22  in the patient&#39;s body. 
         [0048]    To calculate the location correction using the third and the fourth spatial relationships (i.e., rigid bodies  104  and  106 ), processor  44  can determine corrected second location  134  for the only one primary patch by determining, based on rigid body  104 , an expected second location (i.e., the corrected second location) for the only one primary patch in rigid body  106  (as indicated by an arrow  144 ), thereby defining rigid body  108 . Location  134  comprises a three-dimensional coordinates in coordinate system  136 . 
         [0049]    Once processor  44  has calculated the location correction for the only one primary patch, processor  44  can apply the location correction to subsequent signals indicating subsequent locations of the only one primary patch. Therefore, upon processor  44  receiving, at a third time subsequent to the second time, third position-dependent signals from the only one primary patch, the processor can compute, based on the third position-dependent signals, third location coordinates for the only one primary patch, and apply the location correction to the third location of the only one primary patch, thereby determining a corrected third location for the only one primary patch. 
         [0050]    While embodiments described herein use three ancillary patches  60  to correct an inconsistent movement of only one primary patch  58 , configurations comprising any number of ancillary patches  60  whose respective location measurements can be used to define rigid bodies  104 ,  106  and  108  are considered to be within the spirit and scope of the present invention. Therefore, in embodiments of the present invention, at least four adhesive patches (i.e., three primary patches  58  and at least one ancillary patch  60 ) may be affixed to patient  30 . 
       Multiple Patch Location Correction 
       [0051]      FIG. 5  is a flow diagram that illustrates a method of correcting inconsistent apparent locations of a plurality of primary adhesive skin patch  58 , and  FIGS. 6A-6C , referred to collectively as  FIG. 6 , are schematic diagrams illustrating first patch location coordinates  170 - 174 , second patch location coordinates  176 - 180  and corrected second patch location coordinates  182 - 186 , in accordance with an embodiment of the present invention. 
         [0052]    In the example shown in  FIG. 6 , locations  170 - 186  comprise three-dimensional coordinates in a coordinate system  188  comprising an X-axis  190 , a Y-axis  192 , and a Z-axis  194 . In embodiments described herein, locations  170 - 174  are indicative of first spatial relationships represented by a rigid body  196 , and locations  176 - 180  are indicative of second spatial relationships indicated by a rigid body  198 . 
         [0053]    In an initial step  150 , operator  28  affixes primary adhesive skin patches  58  to back  62  of patient  30 , and in a first receive step  152 , processor  44  receives, at a first time, first position-dependent signals from patches  58 . The first position-dependent signals are generated using the magnetic position sensing referred to above. In embodiments of the present invention, the first position-dependent signals may also indicate a first magnetic interference level for each primary patch  58 . In the example shown in  FIG. 1 , the magnetic interference level(s) typically provide a measure of a proximity of X-ray source  36  to field generators  52 . In the flow diagram shown in  FIG. 5 , primary patches  58  may also be referred to as back patches. 
         [0054]    In a first compute step  154 , processor  44  computes respective first location coordinates and computes a first magnetic interference index (i.e., a value) based on the first magnetic interference levels. In a first identification step  156 , processor  44  identifies the first spatial relationships between primary patches  58 , using, as shown in  FIG. 6A , the respective first location coordinates of locations  170 ,  172  and  174  of the primary adhesive patches, i.e., as rigid body  196 . 
         [0055]    In a second receive step  158 , processor  44  receives, at a second time subsequent to the first time, second position-dependent signals from primary patches  58 . In embodiments of the present invention, the second position-dependent signals may also indicate a second magnetic interference level for each primary patch  58 . 
         [0056]    In a second compute step  160 , processor  44  computes respective second location coordinates and respective second magnetic interference levels for each primary patch  58 , and computes a second magnetic interference index based on the second magnetic interference levels. In a second identification step  162 , the processor identifies the second spatial relationships between primary patches  58 , using, as shown in  FIG. 6B , the respective second location coordinates of locations  176 ,  178  and  180  of the primary adhesive skin patches, i.e., as rigid body  198 . 
         [0057]    In a detection step  164 , processor  44  detects a discrepancy between the first and the second magnetic indices and a discrepancy between the first and the second spatial relationships of a plurality of primary patches  58  relative to the other primary patches. The detected discrepancy indicates that the second locations of a plurality of primary patches  58  are inconsistent with the second locations of the remaining primary patches  58 . 
         [0058]    In the present example, location  176  comprises a physical first location of a first given primary patch  58 , location  178  comprises a physical first location of a second given primary patch  58 , location  182  comprises an apparent second location of the first given primary patch, and location  186  comprises an apparent second location of the second given primary patch. In embodiments of the present invention, the inconsistent (i.e., apparent) locations are a result of a difference between the first and the second magnetic field measurements, the difference causing an apparent movement of the first and the second given primary patches from locations  170 ,  172  and  174  ( FIG. 6A ) to locations  176 ,  178  and  180  ( FIG. 6B ). In some embodiments, processor  44  can detect the discrepancy between the first and the second spatial relationships by detecting a difference between rigid body  196  and rigid body  198 . 
         [0059]    In a calculation step  166 , processor  44  calculates, based on the first location coordinates, location corrections for the plurality of primary patches. In some embodiments, the location correction for a given patch  58  comprises a distance and orientation from the second location of the given patch to the first location of the given patch (or vice versa). Finally, in an application step  168 , processor  44  applies the location corrections to the second locations of the plurality of the primary patches, thereby determining corrected second locations for the plurality of the primary patches, and the method ends. 
         [0060]    In the example shown in  FIG. 6 , based on the distances and the orientations are indicated by arrows  206 ,  208  and  210 , processor  44  determines corrected second locations  200 ,  202  and  204  for the plurality of the primary patches. Locations  200 ,  202  and  204  comprise three-dimensional coordinates in coordinate system  188 . In embodiments where the detected movement of patches  58  is caused by magnetic interference (i.e., the detected movement is apparent), then the corrected location coordinates are in accordance with the first location coordinates. Therefore, in the example shown in  FIG. 6 , location  200  is in accordance with location  170 , location  202  is in accordance with location  202 , and location  174  is in accordance with location  204 . 
         [0061]    Once processor  44  has calculated the location correction for patches  58 , processor  44  can apply the location correction to subsequent signals indicating subsequent locations of the back patches. Therefore, upon processor  44  receiving, at a third time subsequent to the second time, third position-dependent signals from patches  58 , the processor can compute, based on the third position-dependent signals, third location coordinates for the back patches, and apply the location correction to the third locations of the back patches, thereby determining a corrected third location for patches  58 . 
         [0062]    In embodiments of the present invention, processor  44  can track an object (e.g., probe  22 ) in the patient&#39;s body relative to the respective location coordinates of patches  58  while applying the respective location corrections to the respective location coordinates of the patches. Additionally, while embodiments described herein use three primary patches  58  whose respective location measurements can be used to define rigid bodies  100 - 108  and  196 - 198 , configurations comprising more than three patches  58  are considered to be within the spirit and scope of the present invention. 
         [0063]    It will be understood that the description above provides two embodiments for locating and correcting inconsistent second locations of one or more patches  58 . In a first embodiment, as described supra in the description referencing  FIGS. 3 and 4 , processor  44  detects an inconsistent second location for only one patch  58 , but does not detect a discrepancy in the magnetic interference index between the first and the second times. In a second embodiment, as described supra in the description referencing  FIGS. 5 and 6 , processor  44  detects respective inconsistent second locations for a plurality of patches  58  while detecting a discrepancy in the in the magnetic interference index between the first and the second times. 
         [0064]    It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.