Patent Publication Number: US-11642060-B2

Title: Multi-electrode impedance sensing

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 14/301,923, filed 11 Jun. 2014, now allowed, which claims the benefit of U.S. provisional application No. 61/833,739, filed 11 Jun. 2013, which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     a. Technical Field 
     The instant disclosure relates to electrical impedance-based contact sensing, including contact between tissue and electrodes disposed in or on a medical device. 
     b. Background Art 
     Catheters are used for an ever-growing number of procedures. For example, catheters are used for diagnostic, therapeutic, and ablative procedures, to name just a few examples. Typically, the catheter is manipulated through the patient&#39;s vasculature and to the intended site such as, for example, a site within the patient&#39;s heart. The catheter typically carries one or more electrodes, which may be used for ablation, diagnosis, and the like. 
     In many procedures, it may be beneficial to know the contact status of an electrode (e.g., in contact with tissue, in a blood pool) on a catheter. For example, in an electrophysiology mapping procedure, the electrical signal present on an electrode may vary depending on whether the electrode is in contact with tissue, or adjacent to the tissue in a blood pool, and that difference may be accounted for in software. In another example, in an ablation procedure, it may be desirable to only drive an ablation current when an electrode is in contact with the tissue to be ablated. 
     One existing methodology that may be used to determine whether an electrode on a catheter is in contact with tissue includes driving a current between the electrode and an electrode elsewhere within the patient (e.g., at a stable position within the patient) or on the exterior of the patient (e.g., on the patient&#39;s skin) and assessing the impedance between the electrodes. To determine an impedance between those electrodes, the electric potential of the electrode on the medical device may be referenced to a third electrode, which may also be elsewhere within the patient or on the exterior of the patient. 
     The foregoing discussion is intended only to illustrate the present field and should not be taken as a disavowal of claim scope. 
     BRIEF SUMMARY 
     An embodiment of a system for assessing contact between a medical device and tissue may comprise an electronic control unit (ECU) configured to be coupled to a medical device, the medical device comprising a first electrode and a second electrode. The ECU may be further configured to select the first electrode as an electrical source and the second electrode as an electrical sink, to cause an electrical signal to be driven between the source and sink, to detect respective electric potentials on the first electrode and the second electrode while the electrical signal is driven, and to determine an impedance respective of one of the first electrode and the second electrode according to both of the respective electric potentials. 
     An embodiment of a method for determining impedances respective of a first electrode and a second electrode, the first and second electrodes disposed on an elongate medical device, may comprise a number of steps. A first step may include driving an electrical signal between the first electrode and the second electrode and a second step may include measuring respective electric potentials on the first electrode and the second electrode while driving the electrical signal. A third step may include determining impedances respective of the first electrode and the second electrode, both impedances determined according to both of the respective electric potentials. 
     An embodiment of a system may comprise an elongate medical device comprising a shaft and a first electrode and a second electrode disposed on or in the shaft. The system may further comprise an ECU coupled to the elongate medical device and configured to select the first electrode as an electrical source and the second electrode as an electrical sink, to cause an electrical signal to be driven between the source and sink, to detect respective electric potentials on the first electrode and the second electrode while the electrical signal is driven, and to determine an impedance respective of one of the first electrode and the second electrode according to the respective electric potentials. 
     Systems and methods according to the present disclosure may improve on known systems and methods for assessing contact between a medical device and tissue by reducing the electrical current to which the patient is exposed. Further, systems and methods according to the present disclosure may provide improved reliability relative to known systems and methods by reducing the path length of one or more driven currents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an isometric view of an exemplary embodiment of an elongate medical device having multiple electrodes. 
         FIG.  2    is an isometric view of an exemplary embodiment of an elongate medical device having a distal lariat portion. 
         FIG.  3    is an end view of the distal end portion of the elongate medical device of  FIG.  2   , illustrating multiple electrodes that may be used on the device. 
         FIG.  4    is a diagrammatic view of an exemplary embodiment of a system for determining a biological impedance at two electrodes on an elongate medical device. 
         FIG.  5    is a flow chart illustrating an exemplary embodiment of a method of determining a biological impedance at two electrodes on an elongate medical device. 
         FIG.  6    is a diagrammatic depiction of an exemplary embodiment of a mapping and navigation system that may include functionality for determining a biological impedance at two electrodes on an elongate medical device. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the figures, in which like numerals indicate the same or similar elements in the various views,  FIG.  1    is an isometric view of an exemplary embodiment of the elongate medical device  10 . The elongate medical device  10  may comprise, for example, a diagnostic and/or therapy delivery catheter, an introducer or sheath, or other like devices. For purposes of illustration and clarity, the description below will be with respect to an embodiment wherein the elongate medical device  10  comprises a catheter (i.e., catheter  10 ). It will be appreciated, however, that embodiments wherein the elongate medical device  10  comprises an elongate medical device other than a catheter remain within the spirit and scope of the present disclosure. 
     The catheter  10  may comprise a shaft  12  having a distal end portion  14  and a proximal end portion  16 . The catheter  10  may be configured to be guided through and disposed in the body of a patient. Accordingly, the proximal end portion  16  may be coupled to a handle  18 , which may include features to enable a physician to guide the distal end portion  14  to perform a diagnostic or therapeutic procedure such as, for example only, an ablation or mapping procedure on the heart of the patient. Accordingly, the handle  18  may include one or more manual manipulation mechanisms such as, for example, rotational mechanisms and/or longitudinal mechanisms, coupled to pull wires for deflecting the distal end portion  14  of the shaft  12 . Exemplary embodiments of manipulation mechanisms, pull wires, and related hardware are described, for example only, in U.S. patent application publication no. 2012/0203169, hereby incorporated by reference in its entirety. The handle  18  may further include one or more electromechanical connectors for coupling to a mapping and navigation system, an ablation generator, and/or other external systems. The handle  18  may also include one or more fluid connectors for coupling to a source and/or destination of fluids such as, for example only, a gravity feed or fixed or variable-rate pump. 
     The distal end portion  14  of the shaft  12  may include a number of ring electrodes  20 A,  20 B (which may be referred to herein individually and generically as a ring electrode  20  or in the multiple as the ring electrodes  20 ) and a tip electrode  22  for applying ablation energy to tissue, acquiring electrophysiology data from tissue, determining the position and orientation (P&amp;O) of the shaft  12 , and/or other purposes. The electrodes  20 ,  22  may be coupled to electrical wiring within the shaft  12 , which may extend to the handle  18  and to electromechanical connectors for coupling to external systems. The ring electrodes  20  may be placed about six (6) millimeters away from each other (e.g., measuring from the center of one ring electrode  20  to the center of another ring electrode  20  along the longitudinal axis of the shaft  12 ), and each may have a radius of about one (1) millimeter, in an embodiment. 
     The distal end portion  16  of the shaft  12  may also include one or more fluid ports or manifolds for distributing or collecting fluids such as, for example only, irrigation fluid during an ablation procedure. The fluid ports may be fluidly coupled with one or more fluid lumens extending through the shaft  12  to the handle  18  and a fluid connector for coupling to external fluid sources and/or destinations. One or more lumens may also be provided through the shaft  12  for passing a second elongate medical device. In some embodiments, for example, the elongate medical device  10  may comprise an introducer that includes at least one lumen configured to receive another device such as a catheter or probe. 
       FIG.  2    is an isometric view of another exemplary embodiment of an elongate medical device  24 , the elongate medical device  24  having a distal lariat portion.  FIG.  3    is a distal end view of a distal end portion  26  of the shaft  28  of the device  24 . For purposes of illustration and clarity, the description below will be with respect to an embodiment wherein the elongate medical device  24  comprises a catheter (i.e., catheter  24 ). It will be appreciated, however, that embodiments wherein the elongate medical device comprises an elongate medical device other than a catheter remain within the spirit and scope of the present disclosure. 
     Referring to  FIGS.  2  and  3   , the catheter  24  may comprise a shaft  28  having a distal end portion  26  and a proximal end portion  30 . The catheter  24  may be configured to be guided through and disposed in the body of a patient. Accordingly, the proximal end portion  30  of the shaft  28  may be coupled to a handle  32 , which may include features to enable a physician to guide the distal end portion to perform a diagnostic or therapeutic procedure such as, for example only, an ablation or mapping procedure on the heart of the patient. Accordingly, the handle  32  may include one or more manual manipulation mechanisms  34  such as, for example, rotational mechanisms and/or longitudinal mechanisms, coupled to pull wires for deflecting the distal end portion of the shaft. Exemplary embodiments of manipulation mechanisms, pull wires, and related hardware are described, for example only, in U.S. patent application publication no. 2012/0203169, referenced above. The handle  32  may further include one or more electromechanical connectors for coupling to a mapping and navigation system, an ablation generator, and/or other external systems. The handle  32  may also include one or more fluid connectors  36  for coupling to a source and/or destination of fluids such as, for example only, a gravity feed or fixed or variable-rate pump. 
     The distal end portion  26  of the shaft  28  may have a portion having a lariat shape. In an embodiment, the lariat shape may be formed by, for example, a shape memory wire disposed within the shaft. A tip electrode  22  and a number of ring electrodes  20 A,  20 B,  20 C,  20 D,  20 E,  20 F,  20 G,  20 H,  20 I (which, again, may be referred to herein individually and generically as a ring electrode  20  or in the multiple as the ring electrodes  20 ) may be disposed on the distal end portion  26  of the shaft  28 , in an embodiment. For example, the tip electrode  22  and ring electrodes  20  may be disposed on the lariat portion of the shaft  28 . In an embodiment, the distal end portion  26  may include nine (9) ring electrodes  20  (i.e., a “decapolar” catheter having ten total electrodes, as illustrated in  FIG.  3   ) or nineteen (19) ring electrodes  20  (i.e., a “duo-decapolar” catheter having twenty total electrodes). Like the catheter  10  illustrated in  FIG.  1   , the electrodes  20 ,  22  on the catheter  24  illustrated in  FIGS.  2  and  3    may be used for applying ablation energy to tissue, acquiring electrophysiology data from tissue, determining the position and orientation (P&amp;O) of the shaft, and/or other purposes. The electrodes  20 ,  22  may be coupled to electrical wiring within the shaft  28 , which may extend to the handle  32  and to electromechanical connectors for coupling to external systems. The ring electrodes  20  may be placed in pairs, in an embodiment, with the two electrodes  20  in a pair disposed a first distance away from each other along the length of the shaft  28 , and non-pair electrodes  20  separated by a second distance along the length of the shaft  28 . For example, electrodes  20 B and  20 C may be considered a first pair, electrodes  20 D and  20 E may be considered a second pair, and so on. The first distance may be less than the second distance, in an embodiment. For example, the first distance may be about one (1) mm, and the second distance may be about 4.5 mm. 
     The catheters  10 ,  24  illustrated in  FIGS.  1 - 3    are exemplary in nature only. The teachings of the present disclosure may find use with numerous other medical devices, such as circular mapping catheters, other known mapping and diagnostic catheters, and other known medical devices, in embodiments. Furthermore, two or more ring electrodes may be provided on various embodiments of the medical device, and the methods, steps, techniques, and algorithms discussed below may find use with potentially many electrodes on the medical device. 
     An elongate medical device having multiple electrodes, such as the catheter  10  or the catheter  24 , may find use in a system for assessing a state of contact between the elongate medical device and the tissue of a patient. As mentioned in the Background, in some known systems, an electrical current may be driven between an electrode on an elongate medical device disposed within the body and a cutaneous electrode to assess such contact. The electric potential on the in-body electrode may be measured with reference to a third electrode (e.g., another cutaneous electrode), and an impedance may be calculated, which impedance may indicate a contact state. Such a system and methodology may be improved upon by a system for assessing a contact state according to an electrical current driven between two electrodes on the same device (e.g., on the same elongate medical device within the patient&#39;s body). 
       FIG.  4    is a diagrammatic view of a system  40  for assessing a contact state according to an electrical current driven between two electrodes on the same device. The system  40  may include a medical device  42  comprising at least two electrodes A, B having respective impedances Z A , Z B , a detection amplifier  44 , and a signal generator  46 . The detection amplifier may include, in an embodiment two operational amplifiers (op amps)  52   A ,  52   B , a reference electrode R, and an impedance sensor in the form of an electronic control unit (ECU)  50 . In an embodiment, the signal generator may be incorporated in or may be considered a part of the detection amplifier  44 . 
     The medical device  42  may be or may include an elongate medical device, in an embodiment. For example, the medical device  42  may be the catheter  10  (see  FIG.  1   ) or the catheter  24  (see  FIGS.  2  and  3   ). The medical device  42  may have exactly two electrodes A, B or more than two electrodes. For example, the medical device  42  may have three (3) electrodes, like the catheter  10  illustrated in  FIG.  1   , may have ten (10) electrodes, like the catheter  24  illustrated in  FIGS.  2  and  3   , or may have some other number of electrodes. 
     The electrodes A, B may be any two electrodes on the device, in an embodiment. For example, referring to  FIGS.  1  and  4   , the electrodes A, B may be the tip electrode  22  and the first ring electrode  22 A. Alternatively, referring to  FIGS.  3  and  4   , the electrodes A, B may be two ring electrodes  20 D and  20 E, or  20 F and  20 G, etc. 
     The signal generator  46  may be configured to generate (e.g., among other signals), an excitation signal across the electrodes A, B (i.e., using one electrode as a source and the other as a sink). In an embodiment, the excitation signal may have a frequency within a range from about 1 kHz to over 500 kHz, more preferably within a range of about 2 kHz to 200 kHz, and even more preferably about 20 kHz. In an embodiment, the excitation signal may be a constant current signal, preferably in the range of between 20-200 μA, and more preferably about 100 μA. 
     The ECU  50  may include conventional filters (e.g., bandpass filters) to block frequencies that are not of interest, but permit appropriate frequencies, such as the excitation frequency, to pass, as well as conventional signal processing software used to obtain the component parts of the measured complex impedance. Accordingly, the impedance sensor  50  may include a memory storing such signal processing software and a processor configured to execute the signal processing software, in an embodiment. 
     The ECU  50  may include any processing apparatus such as, as noted above, a memory and a processor. Additionally or alternatively, the impedance sensor may include an application-specific integrated circuit (ASIC), programmable logic device (PLD), field-programmable gate array (FPGA), and/or other processing device. 
     The detection amplifier  44  may have a positive polarity connector which may be electrically connected to a first electrode A and a negative polarity connector which may be electrically connected to a second electrode B. The positive and negative polarity connectors may be disposed relative to the other components of the detection amplifier  44  so as to form the circuit diagrammatically shown in  FIG.  4    when connected with the electrodes A, B. It should be understood that the term connectors as used herein does not imply a particular type of physical interface mechanism, but is rather broadly contemplated to represent one or more electrical nodes. 
     The detection amplifier may drive a current between electrodes A, B on the same device to assess a contact state between the electrodes A, B and tissue. Impedances may be calculated based on those driven currents to determine a contact state. The system may be configured to execute the method  60  of  FIG.  5   , as described below, to determine impedances respective of the first and second electrodes A, B to determine a contact state. 
       FIG.  5    is a flow chart illustrating a method  60  of determining impedances respective of a pair of electrodes. Referring to  FIGS.  4  and  5   , the electrodes referred to in the method  60  may be electrodes A and B (shown in  FIG.  4   ). The method  60  will be described with reference to electrodes A and B. It should be understood, though, that the method  60  may find use with numerous different electrode arrangements—both adjacent and non-adjacent electrodes, ring and other types of electrodes, etc. Additionally, in embodiments, more than two electrodes may be used in variations of the method  60 . 
     The method  60  may begin with a step  62  of driving an electrical signal between electrodes A and B, with one of electrodes A and B selected as a source, and the other as a sink. The source and sink selection may be made by the ECU  50 , and the current driven by the signal generator  46 . The signal may have predetermined characteristics (e.g., frequency and amplitude). In an embodiment, the signal may have a frequency within a range from about 1 kHz to over 500 kHz, more preferably within a range of about 2 kHz to 200 kHz, and even more preferably about 20 kHz. In an embodiment, the signal may be a constant current signal, preferably in the range of between 20-200 μA, and more preferably about 100 μA. 
     The method  60  may further include a step  64  of measuring respective electric potentials on electrodes A and B while driving the current between electrodes A and B. The potentials may be measured by a detection amplifier, in an embodiment. The detection amplifier may present a very high impedance (for example, about 100 kΩ or more, in an embodiment, and/or 50 times or more greater than the nominal impedance of one of the electrodes A, B, in an embodiment, and/or 100 times or more greater than the nominal impedance of one of the electrodes A, B, in an embodiment) relative to the path between electrodes A and B, so the effect of measurements with the detection amplifier on the potential on the electrodes A, B may be negligible. 
     The measuring step  64  may further include referencing the measured electric potentials to a reference electrode, such as electrode R (shown in  FIG.  4   ). Reference electrode R may be a cutaneous electrode, such as a body patch electrode, in an embodiment. Alternatively, the reference electrode R may be another in-patient electrode. Such referencing may be performed, in an embodiment, by inputting the potential on electrode A into a first input of the first op amp  48   A , the potential on electrode B into a first input of the second op amp  48   B , and the potential on the reference electrode into respective second inputs on both the first op amp  48   A  and the second op amp  48   B . The output of the op amps  48   A ,  48   B  may be input to the ECU  50 , in an embodiment, for impedance determinations, contact assessment, and/or other calculations. In another embodiment, hardware separate from the ECU  50  may be provided to perform some or all of the impedance and/or contact determinations and other functions of the detection amplifier described above and/or below. 
     For driving the current between electrodes A and B and determining electric potentials on electrodes A and B, well known methods of driving a current at a particular carrier frequency and synchronously demodulating the respective potentials on electrodes A and B may be used. The detection amplifier may amplify the signals manifest on each electrode A, B, and after demodulation a voltage related to the impedance of each electrode is available. 
     In the case of electrode B, the recovered voltage will be negative (i.e., assuming electrode A is selected as the source and B as the sink), so a trivial conversion to a positive quantity may be applied by the ECU  50  or other device. Since the current source-sink electrode pair may comprise a closely spaced dipole, the potential at the reference electrode R with respect to the dipole will be low, and thus the physical location of R may vary with little effect on the voltages between A and R and B and R. 
     The method  60  may further include a step  66  of determining impedances at each of the electrodes according to the measured electric potentials on the electrodes. The determining step may include mathematically accounting for the effects of each electrode A, B, on the potential measured on the other electrode A, B. Such accounting may include, among other things, considerations of the radius of each electrode A, B and the distance between the electrodes A, B. 
     The electric potential measured on each electrode A, B will carry a component of influence from the potential at the other electrode A, B. Generally, for purposes of assessing the impedance of a given electrode, influence from the neighboring electrode is not desired, so this component of potential may be accounted for and corrected. As a result, measurements based on a current driven between two electrodes A, B on the same medical device may be more predictable than measurements based on a current driven between an electrode (e.g., electrode A) and a cutaneous electrode, in which undesired effects on the potential measured on the electrode (e.g., electrode A) (due to, for example, unpredictable variations in the impedance of the patient&#39;s body) are more difficult to account for and correct. As a result, contact assessments based on a current driven between electrodes A, B may be more accurate than contact assessments based on a current driven between an electrode on a catheter and an exterior electrode. 
     For a given electrode geometry for which impedance is measured at a sufficiently high frequency, such as twenty (20) kHz or higher for platinum iridium electrodes, for example only, the potential measured for a current driven between electrodes A, B may be essentially resistive in a pure saline or blood medium, and may reflect the electrode&#39;s geometry and the solution conductivity. For example, a spherical electrode in a homogeneous medium will have an electric potential for a current driven through the electrode according to equation (2) below: 
                   V   =       ρ   ⁢           ⁢   I       4   ⁢   π   ⁢           ⁢   r               (   2   )               
where V is electric potential, I is the applied current, ρ is the media resistivity, and r is the distance from the center of the electrode to the point on the electrode at which the potential measurement is made. The measured impedance may be taken as the measured potential on the electrode divided by the applied current, as set forth in equation (3) below:
 
     
       
         
           
             
               
                 
                   Z 
                   = 
                   
                     V 
                     I 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The spherical electrode model of equation (2) may be utilized even for certain non-spherical electrodes such as, for example, ring electrodes. It has been shown in computer models and testing that, for typical ring electrodes used in heart mapping catheters, of six (6) or seven (7) French diameter (i.e., a radius of about one (1) millimeter (mm)), and ring thicknesses of one (1) or two (2) mm, the spherical electrode model applies with acceptable accuracy. The spherical model may apply to ring electrodes of different diameters and thicknesses, as well. In both spherical and ring electrodes, the electrode driven with a current creates a potential that varies inversely with distance in a predictable manner. 
     In the spherical model (which, as noted above, may be applied to ring electrodes), a current may be driven between a pair of electrodes A, B, with respective electrode radii r A  and r B  and local resistivity values ρ A  and ρ B . Local resistivity may vary between different locations within a patient, leading to a change in computed impedance respective of the electrode A, B, which may be used to determine the position of the electrode A, B. For example, an electrode in blood will encounter a lower local resistivity than an electrode embedded in tissue, and this difference may be manifest in the impedance measurement. 
     If a current is driven between electrodes A and B, a voltage measurement made on one of them (with respect to a distant or infinite reference, such as reference electrode R) will, by virtue of superposition, contain both the potential owing to the current emitted from the measured electrode (the value of which is shown in equation (2)), as well as potential due to the current through the second electrode. The distance between electrodes A, B (from the center of electrode A to the center of electrode B), denoted d AB  (see  FIG.  3   ), may be relevant to the amount that the current through one electrode (e.g., the sink) affects the potential measured on the other electrode (e.g., the source). Both electrode radii r A  and r B  and the distance between the electrodes d AB  may be stored (e.g., in the memory) or otherwise known (e.g., by the ECU  12 ). It may be assumed that the electrodes&#39; radii are small enough that the electrodes do not overlap (again, modeling the electrodes A, B as spheres), satisfying equation (4) below:
 
 r   A   +r   B   &lt;d   AB   (4)
 
     Electrode A may have a measured potential that is dependent on the current flowing through both electrodes A and B, as well as the physical configuration of electrode A (i.e., radius and local resistivity), the local resistivity electrode B, and the distance between electrodes A and B. The measured potential on electrode A, V Am , may be characterized according to equation (5) below: 
                     V   Am     =           ρ   A     ⁢   I       4   ⁢   π   ⁢           ⁢     r   A         +         ρ   B     ⁢   I       4   ⁢   π   ⁢           ⁢     d   AB                   (   5   )               
where the first term on the right hand side of equation (5) follows directly from equation (2), and the second term on the right hand side of equation (5) results from the potential on electrode B. As shown in  FIG.  3   , V Am  is the voltage that results from referencing the potential on electrode A to the potential on reference electrode R. Analogously, the measured potential V Bm  on electrode B may be characterized according to equation (6) below:
 
     
       
         
           
             
               
                 
                   
                     V 
                     Bm 
                   
                   = 
                   
                     
                       
                         
                           ρ 
                           B 
                         
                         ⁢ 
                         I 
                       
                       
                         4 
                         ⁢ 
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           r 
                           B 
                         
                       
                     
                     + 
                     
                       
                         
                           ρ 
                           A 
                         
                         ⁢ 
                         I 
                       
                       
                         4 
                         ⁢ 
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           d 
                           AB 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     In both equations (5) and (6), it is desirable to account for and remove the influence of the other electrode (i.e., the influence of electrode B on the potential measured for electrode A, and vice-versa) by solving for the first term in the right hand side of equations (5) and (6). Because all quantities in equations (5) and (6) are known (i.e., stored or measured) except for the resistivity values ρ A  and ρ B , the resistivity values may be solved for. Doing so, and collecting certain terms on the left hand side, results in equation (7) below: 
     
       
         
           
             
               
                 
                   
                     
                       
                         ρ 
                         A 
                       
                       ⁢ 
                       I 
                     
                     
                       4 
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         r 
                         A 
                       
                     
                   
                   = 
                   
                     
                       
                         
                           
                             d 
                             AB 
                           
                           ⁢ 
                           
                             V 
                             Am 
                           
                         
                         - 
                         
                           
                             r 
                             B 
                           
                           ⁢ 
                           
                             V 
                             Bm 
                           
                         
                       
                       
                         
                           d 
                           AB 
                         
                         - 
                         
                           
                             
                               r 
                               A 
                             
                             ⁢ 
                             
                               r 
                               B 
                             
                           
                           
                             d 
                             AB 
                           
                         
                       
                     
                     + 
                     
                       
                         
                           ρ 
                           B 
                         
                         ⁢ 
                         I 
                       
                       
                         4 
                         ⁢ 
                         π 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     In comparing equation (7) to equation (2), it can be seen that the left hand side of equation (7) is the desired potential on electrode A, free of the influence of electrode B. Accordingly, equation (7) may be re-written as equation (8) below for the potential on electrode A: 
     
       
         
           
             
               
                 
                   
                     V 
                     A 
                   
                   = 
                   
                     
                       
                         
                           d 
                           AB 
                         
                         ⁢ 
                         
                           V 
                           Am 
                         
                       
                       - 
                       
                         
                           r 
                           B 
                         
                         ⁢ 
                         
                           V 
                           Bm 
                         
                       
                     
                     
                       
                         d 
                         AB 
                       
                       - 
                       
                         
                           
                             r 
                             A 
                           
                           ⁢ 
                           
                             r 
                             B 
                           
                         
                         
                           d 
                           AB 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     A counterpart equation for the potential on electrode B, free of the influence of electrode A, may be written as equation (9) below: 
     
       
         
           
             
               
                 
                   
                     V 
                     B 
                   
                   = 
                   
                     
                       
                         
                           d 
                           AB 
                         
                         ⁢ 
                         
                           V 
                           Bm 
                         
                       
                       - 
                       
                         
                           r 
                           A 
                         
                         ⁢ 
                         
                           V 
                           Am 
                         
                       
                     
                     
                       
                         d 
                         AB 
                       
                       - 
                       
                         
                           
                             r 
                             A 
                           
                           ⁢ 
                           
                             r 
                             B 
                           
                         
                         
                           d 
                           AB 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Thus, for each potential measured as a current is driven between electrodes A and B, equations (8) and (9) may be solved (e.g., by the ECU  12 ) to determine the voltages on each of electrodes A and B (relative to reference electrode R). Accordingly, equations (8) and (9) may be stored in the memory of the ECU  50  for execution by the processor of the ECU  50 . Those voltages may be applied to equation (2) to determine impedances respective of each of electrode A and B (again, by the ECU  50 , for example). Based on those impedances, a contact state between electrodes A and B and the tissue of a patient may be assessed. Such measurements may be carried out numerous times, multiplexed with the patch excitation and measurement process described above. Furthermore, such measurements may be carried out for numerous sets of electrodes A and B. 
     For electrodes other than ring and spherical electrodes, the calculations according to the equations above may be modified to account for the particular geometry of the electrode. Alternatively, instead of mathematically accounting for the effect of one electrode on the electric potential on the other electrode, a compensation factor may be determined experimentally. 
     In an embodiment, the steps of the method  60  may be carried out repeatedly for numerous different pairs of electrodes to determine a contact state for each of those electrodes. For example, referring to  FIGS.  3  and  5   , the steps of the method  60  may first be carried out on electrodes  22  and  20 A, then on  20 B and  20 C, then on  20 D and  20 E, and so on. Any two electrodes on the catheter  24  may be used in the method  60 , in an embodiment (i.e., the method  60  may be applied to adjacent electrodes, but is not limited to use with adjacent electrodes). 
     Referring to  FIG.  4   , the system  40  may be a standalone system, in an embodiment. Alternatively, the system  40 , or the functionality of the system  40 , may be incorporated into a system having additional functionality. For example, the functionality of the system  40  may be incorporated into a mapping and navigation system. 
       FIG.  6    is a diagrammatic depiction of an embodiment of an exemplary mapping and navigation system  70  that may incorporate the contact sensing functionality of the system  40  (see  FIG.  4   ) and the method  60  (see  FIG.  5   ), in addition to various other functionality including, but not limited to, determining the location (i.e., position and orientation) of an elongate medical device within the body of a patient, mapping the anatomy of the patient, etc. The system  70  may include various visualization, mapping and navigation components as known in the art, including, for example, an EnSite™ Velocity™ system commercially available from St. Jude Medical, Inc., or as seen generally, for example, by reference to U.S. Pat. No. 7,263,397, or U.S. patent application publication no. 2007/0060833, both of which are hereby incorporated by reference in their entireties as though fully set forth herein. 
     The system  70  may include an electronic control unit (ECU)  72 , an analog-to-digital converter (A-to-D)  74 , a lowpass filter (L.P.)  76 , a switch  78 , a signal generator  80 , and a plurality of body surface patch electrodes  82 . The system  70  may be electronically and/or mechanically coupled with an elongate medical device  84 . The system  70  may be configured for a number of functions for guiding the elongate medical device  84  to a target site within the body of a patient  86 , such as the heart  88 , and for assessing contact between the elongate medical device  84  and the tissue of the patient  86 . The system  70  may further include a conventional set of ECG leads  90  for the capture and measurement of patient ECG data. 
     The elongate medical device  84  may be one of the catheters  10 ,  24  described herein (see  FIGS.  1 - 3   ), or some other elongate medical device. The elongate medical device  84  may have one or more electrodes  92 . The one or more electrodes may include a tip electrode  22  (see  FIGS.  1 - 3   ), one or more ring electrodes  20  (see  FIGS.  1 - 3   ), and/or another type of electrode. Two or more electrodes  92  may be used as the electrodes A, B illustrated in  FIG.  4    and used in the method of  FIG.  5   . 
     Referring again to  FIG.  6   , the signal generator  80  may output an excitation signal for assessing a contact state of an electrode  92 . The excitation signal may have a frequency within a range from about 1 kHz to over 500 kHz, more preferably within a range of about 2 kHz to 200 kHz, and even more preferably about 20 kHz, in an embodiment. The excitation signal may be a constant current signal, preferably in the range of between 20-200 μA, and more preferably about 100 μA, in an embodiment. The signal generator  80  may also generate signals involved in, for example, determining a location of the electrodes  92  within the body of the patient. 
     The ECU  72  may include a memory  94  and a processor  96 . The memory  94  may be configured to store data respective of the elongate medical device  84 , of the patient  86 , and/or other data. Such data may be known before a medical procedure, or may be determined and stored during a procedure. The memory  94  may also be configured to store instructions that, when executed by the processor  96 , cause the ECU  72  to perform one or more methods, steps, functions, or algorithms described herein. For example, but without limitation, the memory  94  may include data and instructions for determining impedances respective of one or more electrodes  92  on the elongate medical device  84 , as described in conjunction with  FIGS.  3  and  4   . Such impedances may be used, for example, to assess contact between one or more electrodes  92  and patient tissue. 
     In addition to impedance calculations and contact state determinations, the system  70  may be configured to determine the position and orientation (P&amp;O) of the elongate medical device  84  (e.g., of a distal end portion of the elongate medical device  84 ) within the body of the patient  86 . Accordingly, the ECU  72  may be configured to control generation of one or more electrical fields and determine the position of one or more electrodes  92  within those fields. The ECU  72  may thus be configured to control signal generator  80  in accordance with predetermined strategies to selectively energize various pairs (dipoles) of body surface patch electrodes  82  and catheter electrodes  92 . 
     Referring to  FIGS.  4  and  6   , the ECU  72  may include one or more portions of and/or may be configured to perform the functionality of one or more portions of, the detection amplifier  44 , such as the op amps  48  and impedance sensor for determining the detected impedance, in an embodiment. Additionally or alternatively, one or more portions of the detection amplifier  44  may be provided in the system  70  in hardware separate from the ECU  72 . 
     Referring again to  FIG.  6   , the mapping and navigation functionality of the system  70  will be briefly described. The body surface patch electrodes  82  may be used to generate axes-specific electric fields within the body of the patient  86 , and more specifically within the heart  88 . Three sets of patch electrodes may be provided: (1) electrodes  82   X1 ,  82   X2 , (X-axis); (2) electrodes  82   Y1 ,  82   Y2 , (Y-axis); and (3) electrodes  82   Z1 ,  82   Z2 , (Z-axis). Additionally, a body surface electrode (“belly patch”)  82   B , may be provided as an electrical reference. The body patch electrodes  82   X1 ,  82   X2 ,  82   Y1 ,  82   Y2 ,  82   Z1 ,  82   Z2 ,  82   B  may be referred to herein generically as a body patch electrode  82  or as the body patch electrodes  82 . Other surface electrode configurations and combinations are suitable for use with the present disclosure, including fewer body patch electrodes  82 , more body patch electrodes  82 , or different physical arrangements, e.g. a linear arrangement instead of an orthogonal arrangement. 
     Each patch electrode  82  may be independently coupled to the switch  78 , and pairs of patch electrodes  82  may be selected by software running on the ECU  72  to couple the patch electrodes  82  to the signal generator  80 . A pair of electrodes, for example the Z-axis electrodes  82   Z1 ,  82   Z2 , may be excited by the signal generator  80  to generate an electrical field in the body of the patient  86  and, more particularly, within the heart  88 . In an embodiment, this electrode excitation process occurs rapidly and sequentially as different sets of patch electrodes  82  are selected and one or more of the unexcited surface electrodes  82  are used to measure voltages. During the delivery of the excitation signal (e.g., current pulse), the remaining (unexcited) patch electrodes  82  may be referenced to the belly patch  82   B  and the voltages impressed on these remaining electrodes  82  may be measured. In this fashion, the patch electrodes  82  may be divided into driven and non-driven electrode sets. The low pass filter  76  may process the voltage measurements. The filtered voltage measurements may be transformed to digital data by the analog to digital converter  74  and transmitted to the ECU  72  for storage (e.g. in the memory  94 ) under the direction of software. This collection of voltage measurements may be referred to herein as the “patch data.” The software may store and have access to each individual voltage measurement made at each surface electrode  82  during each excitation of each pair of surface electrodes  82 . 
     Generally, in an embodiment, three nominally orthogonal electric fields may be generated by the series of driven and sensed electric dipoles in order to determine the location of the elongate medical device  84  (i.e., of one or more electrodes  92 ). Alternately, these orthogonal fields can be decomposed and any pair of surface electrodes (e.g., non-orthogonal) may be driven as dipoles to provide effective electrode triangulation. 
     The patch data may be used, along with measurements made at one or more electrodes  92  and measurements made at other electrodes and devices, to determine a relative location of the one or more electrodes  92 . In some embodiments, electric potentials across each of the six orthogonal patch electrodes  82  may be acquired for all samples except when a particular surface electrode pair is driven. In an embodiment, sampling electric potentials may occur at all patch electrodes  82 , even those being driven. 
     As a part of determining locations of various electrodes, the ECU  72  may be configured to perform one or more compensation and adjustment functions, such as motion compensation. Motion compensation may include, for example, compensation for respiration-induced patient body movement, as described in U.S. patent application publication no. 2012/0172702, which is hereby incorporated by reference in its entirety. 
     Data sets from each of the patch electrodes  82  and the electrodes  92  are all used to determine the location of the electrodes  92  within the patient  86 . After the voltage measurements are made for a particular set of driven patch electrodes  82 , a different pair of patch electrodes  82  may be excited by the signal generator  80  and the voltage measurement process of the remaining patch electrodes  82  and electrodes  92  takes place. The sequence may occur rapidly, e.g., on the order of 100 times per second in an embodiment. The voltage on the electrodes  92  within the patient  86  may bear a linear relationship with the position of the electrodes between the patch electrodes  82  that establish the electrical fields, as more fully described in U.S. Pat. No. 7,263,397 referred to above. 
     In summary,  FIG.  6    shows an exemplary system  70  that employs seven body patch electrodes  82 , which may be used for injecting current and sensing resultant voltages. Current may be driven between two patches  82  at any time. Positioning measurements may be performed between a non-driven patch  82  and, for example, belly patch  82   B  as a ground reference. An electrode bio-impedance may be computed according to the following equation (1): 
                       BioZ   ⁡     [     n   -&gt;   m     ]       ⁡     [   k   ]       =       V   k       I     n   -&gt;   m                 (   1   )               
where V k  is the voltage measured on electrode k and I n→m  is a known constant current driven between electrodes n and m. The position of an electrode  92  may be determined by driving current between different sets of patches and measuring one or more impedances. Some impedances that may be measured may be according to currents driven between two electrodes  92  on the elongate medical device  84 . In one embodiment, time division multiplexing may be used to drive and measure all quantities of interest. Position determining procedures are described in more detail in, for example, U.S. Pat. No. 7,263,397 and publication no. 2007/0060833 referred to above.
 
     In addition to a simple impedance, the systems and methods described herein may be used to determine a complex impedance respective of one or more electrodes on a catheter. Such complex impedances may be used to assess a contact state between an electrode and tissue. For example, as described in U.S. Pat. No. 8,403,925, which is hereby incorporated by reference in its entirety, a complex impedance may be used to determine an electrical coupling index (ECI), which in turn may be used to assess contact between an electrode and tissue. 
     Various embodiments are described herein to various apparatuses, systems, and/or methods. Numerous specific details are set forth to provide a thorough understanding of the overall structure, function, manufacture, and use of the embodiments as described in the specification and illustrated in the accompanying drawings. It will be understood by those skilled in the art, however, that the embodiments may be practiced without such specific details. In other instances, well-known operations, components, and elements have not been described in detail so as not to obscure the embodiments described in the specification. Those of ordinary skill in the art will understand that the embodiments described and illustrated herein are non-limiting examples, and thus it can be appreciated that the specific structural and functional details disclosed herein may be representative and do not necessarily limit the scope of the embodiments, the scope of which is defined solely by the appended claims. 
     Reference throughout the specification to “various embodiments,” “some embodiments,” “one embodiment,” or “an embodiment”, or the like, means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in various embodiments,” “in some embodiments,” “in one embodiment,” or “in an embodiment”, or the like, in places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Thus, the particular features, structures, or characteristics illustrated or described in connection with one embodiment may be combined, in whole or in part, with the features structures, or characteristics of one or more other embodiments without limitation given that such combination is not illogical or non-functional. 
     Although numerous embodiments of this invention have been described above with a certain degree of particularity, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this disclosure. All directional references (e.g., plus, minus, upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader&#39;s understanding of the present disclosure, and do not create limitations, particularly as to the position, orientation, or use of the any aspect of the disclosure. As used herein, the phrased “configured to,” “configured for,” and similar phrases indicate that the subject device, apparatus, or system is designed and/or constructed (e.g., through appropriate hardware, software, and/or components) to fulfill one or more specific object purposes, not that the subject device, apparatus, or system is merely capable of performing the object purpose. Joinder references (e.g., attached, coupled, connected, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims. 
     Any patent, publication, or other disclosure material, in whole or in part, that is said to be incorporated by reference herein is incorporated herein only to the extent that the incorporated materials does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as explicitly set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein will only be incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material.