Abstract:
Apparatus and methods are provided for determining in near realtime the position of a probe placed within a living body. Electric currents are driven between one or more electrodes on the probe and electrodes placed on the body surface. The impedance between the probe and each of the body surface electrodes is measured, and three-dimensional position coordinates of the probe are determined based on the impedance measurements. Dynamic compensation is provided for changing impedance of the body surface and its interface with the electrodes, resulting from such causes as electrode peel-off and changes in moisture and temperature. The compensation improves the accuracy of, inter alia, medical procedures, such as mapping the heart or performing ablation to treat cardiac arrhythmias.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to sensing the position of an object placed within a living body. More particularly, this invention relates to detection and compensation for artifacts experienced during position sensing of a probe in a living body using impedance measurements.  
         [0003]     2. Description of the Related Art  
         [0004]     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.  
         [0005]     Many such position sensing systems have been developed or envisioned in the prior art. Some systems involve attaching sensors to the internal object in the form of transducers or antennas, which can sense magnetic, electric, or ultrasonic fields generated outside of the body. For example, U.S. Pat. Nos. 5,697,377 and 5,983,126 to Wittkampf, whose disclosures are incorporated herein by reference, describe a system in which three substantially orthogonal alternating signals are applied through the subject. A catheter is equipped with at least one measuring electrode, and a voltage is sensed between the catheter tip and a reference electrode. The voltage signal has components corresponding to the three orthogonal applied current signals, from which calculations are made for determination of the three-dimensional location of the catheter tip within the body.  
         [0006]     Similar methods for sensing voltage differentials between electrodes are disclosed by U.S. Pat. No. 5,899,860 to Pfeiffer; U.S. Pat. No. 6,095,150 to Panescu; U.S. Pat. No. 6,456,864 to Swanson; and U.S. Pat. Nos. 6,050,267 and 5,944,022 to Nardella, all of whose disclosures are incorporated herein by reference.  
       SUMMARY OF THE INVENTION  
       [0007]     Embodiments of the present invention provide efficient apparatus and methods for determining in near realtime the position of a probe placed within a living body. In these embodiments, electric currents are driven between one or more electrodes on the probe and electrodes placed on the body surface. In this manner, the impedance between the probe and each of the body surface electrodes is measured, and three-dimensional position coordinates of the probe are determined based on the impedance measurements. Dynamic compensation for changing local impedance of the body surface and its interface with the electrodes is performed using a novel electrode design and driving circuit. Such impedance variations can be sudden, from causes such as electrode peel-off, or can be more gradual, due to changes in skin moisture or skin temperature. The compensation improves the accuracy of, inter alia, medical procedures, such as mapping the heart or performing ablation to treat cardiac arrhythmias.  
         [0008]     The invention provides a method for position sensing in a living body, which is carried out by positioning a first electrode and a second electrode on at least one of a plurality of surface locations, wherein the second electrode is electrically isolated from the first electrode, determining a local first impedance by passing a first electrical current from the first electrode to the second electrode via the surface of the body at the one surface location, inserting a probe including at least one probe electrode into the body, passing a plurality of second electrical currents between the probe electrode and at least one of said first electrode and said second electrode via the surface of the body at respective ones of the surface locations, and determining second impedances to the second electrical currents. The method is further carried out responsively to the second impedances by determining internal body impedances of the second electrical currents by compensating for the first impedance, and determining position coordinates of the probe responsively to the internal body impedances.  
         [0009]     In an aspect of the method, positioning comprises adhering an electroconductive surface pad containing the first electrode and the second electrode to the one surface location.  
         [0010]     In another aspect of the method, compensating is performed by determining a component of one of the second impedances that is caused by the pad and the surface at the one surface location and subtracting the component from the one second impedance.  
         [0011]     According to a further aspect of the method, the first electrical current and the second electrical currents are alternating currents.  
         [0012]     According to yet another aspect of the method, determining the first impedance includes arranging the first electrode in a series circuit with the second electrode, and determining the second impedances includes arranging the first electrode in a parallel circuit with the second electrode.  
         [0013]     Still another aspect of the method includes repeating determining a local first impedance at predetermined intervals.  
         [0014]     According to an additional aspect of the method, inserting the probe includes performing a medical treatment on the body using the probe.  
         [0015]     According to one aspect of the method, the probe includes a catheter, and performing the medical treatment comprises mapping the heart of the body.  
         [0016]     The invention provides an apparatus for position sensing, including a probe having at least one probe electrode, which is adapted to be inserted into a body of a living subject. A plurality of electroconductive body surface patches are fixed to a body surface at respective surface locations. The body surface patches each comprise a first electrode and a second electrode, and the second electrode is electrically isolated from the first electrode. The apparatus includes electrical circuitry for arranging the first electrode in a series circuit with the second electrode in a calibration mode of operation and for arranging the first electrode in a parallel circuit with the second electrode in a sensing mode of operation, and a controller, which is operative to control the electrical circuitry, and which is adapted to be coupled to the probe and to the body surface patches so as to pass first electrical signals through the first electrode and the second electrode thereof in the calibration mode of operation, and to pass respective second electrical signals through the body between the probe electrode and the body surface patches. The controller is operative in the calibration mode of operation to determine a local electrical impedance of the body surface and of an interface thereof with a respective one of the body surface patches, and to determine position coordinates of the probe in the sensing mode of operation by measuring respective impedance characteristics of the second electrical signals. The impedance characteristics of the second electrical signals are adjusted by the controller according to respective instances of the local electrical impedance.  
         [0017]     According to another aspect of the apparatus, the controller is adapted to maintain a constant voltage between each of the surface locations and the probe electrode, and to measure respective impedance characteristics by measuring currents at the constant voltage.  
         [0018]     According to a further aspect of the apparatus, the controller is adapted to maintain a constant current between each of the surface locations and the probe electrode, and to measure respective impedance characteristics by measuring voltages at the constant current.  
         [0019]     According to yet another aspect of the apparatus, the probe is adapted to perform a medical treatment on the subject.  
         [0020]     According to still another aspect of the apparatus, the body surface patches comprise an adhesive layer that contacts the body surface.  
         [0021]     In an additional aspect of the apparatus, the controller is operative to alternate the calibration mode of operation and the sensing mode of operation responsively to time intervals measured by an interval timer.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein:  
         [0023]      FIG. 1  is an illustration of a position sensing system, which is constructed and operative in accordance with a disclosed embodiment of the invention;  
         [0024]      FIG. 2  is a schematic view of a catheter used in the system shown in  FIG. 1 , which is constructed and operative in accordance with a disclosed embodiment of the invention;  
         [0025]      FIG. 3  is a block diagram showing electrical circuitry and control of the system shown in  FIG. 1 , in accordance with a disclosed embodiment of the invention;  
         [0026]      FIG. 4  is a schematic diagram showing an electroconductive body surface patch and associated electrical circuitry configured in a sensing mode of operation in accordance with a disclosed embodiment of the invention;  
         [0027]      FIG. 5  is a schematic diagram showing the electroconductive body surface patch and circuitry of  FIG. 4  configured in a calibration mode of operation in accordance with a disclosed embodiment of the invention; and  
         [0028]      FIG. 6  is a flow chart illustrating a method of detecting body surface impedance of a living subject in accordance with a disclosed embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art, however, that the present invention may be practiced without these specific details.  
         [0030]     In other instances, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the present invention unnecessarily.  
         [0000]     System Overview  
         [0031]     Turning now to the drawings, reference is initially made to  FIG. 1 , which is an illustration of a position sensing system  20 , which is constructed and operative in accordance with a disclosed embodiment of the invention. The system  20  is used in determining the position of a probe, such as a catheter  22 , which is inserted into an internal body cavity, such as a chamber of a heart  24  of a subject  26 . Typically, the catheter is used for diagnostic or therapeutic medical procedures, such as mapping electrical potentials in the heart or performing ablation of heart tissue. The catheter or other intrabody device may alternatively be used for other purposes, by itself or in conjunction with other treatment devices.  
         [0032]     The distal tip of the catheter  22  comprises one or more electrodes, described below. These electrodes are connected by wires through the insertion tube of the catheter  22  to driver circuitry in a control unit  28 , as described below. The control unit is connected by wires through a cable  30  to body surface electrodes, which typically incorporated in adhesive electroconductive body surface patches  32 ,  34 ,  36 . The patches  32 ,  34 ,  36  may be placed at any convenient locations on the body surface in the vicinity of the probe. For example, for cardiac applications, the patches  32 ,  34 ,  36  are typically placed around the chest of the subject  26 .  
         [0033]     Typically, the patches  32 ,  34 ,  36  are placed on external surfaces of the body, but in some applications some or all of them could be placed on internal surfaces. There is no special requirement regarding the orientation of patches relative to each other or to the coordinates of the body, although greater accuracy may be achieved if the patches are spaced apart, rather than clustered in one location. There is no requirement that the placement of the patches be along fixed axes. Consequently, patch placement can be determined so as to interfere as little as possible with the medical procedure being performed. The control unit  28  determines position coordinates of the catheter  22  inside the heart  24  based on an adjusted impedance measured between the catheter  22  and the patches  32 ,  34 ,  36 . Details of the adjustment are given below. The control unit drives a display  40 , which shows the catheter position inside the body. The catheter  22  may be used in generating a map  42  of the heart, for example, an electrical map, wherein the electrodes on the catheter are used alternately for position sensing and for measuring electrical potentials generated in the heart tissue. The catheter position may be superimposed on this map or on another image of the heart.  
         [0034]     Reference is now made to  FIG. 2 , which is a detailed schematic view of the catheter  22  ( FIG. 1 ), which is constructed and operative in accordance with a disclosed embodiment of the invention. Interaction is shown between electrodes  44 ,  46 ,  48  disposed on the catheter  22  and the patches  32 ,  34 ,  36 . The electrodes  44 ,  46 ,  48  may be of any suitable shape and size, and may be used for other purposes, as well, such as for electrophysiological sensing or ablation. In the pictured embodiment, each of the electrodes  44 ,  46 ,  48  communicates with all of the patches  32 ,  34 ,  36  ( FIG. 1 ). The control unit  28  ( FIG. 1 ) drives a current between each catheter electrode and all body surface electrodes, and uses the current to measure the impedances between the catheter electrode and the patches  32 ,  34 ,  36 . Based on the measured impedances, the control unit  28  determines the catheter position relative to the body surface electrodes. Alternatively, greater or smaller numbers of electrodes may be used. For example, the control unit  28  may be set to multiplex the currents between one catheter electrode and multiple body surface electrodes. As another example, more than three body surface electrodes may be used for enhanced accuracy.  
         [0035]     Reference is now made to  FIG. 3 , which is a block diagram showing elements of the system  20  ( FIG. 1 ) in accordance with a disclosed embodiment of the invention. The control unit  28 , described above, comprises circuitry for driving currents and for measuring impedance. Each of three circuits  50 ,  52 ,  54  drives a current through a closed loop consisting of a catheter electrode and the body surface of the patches  32 ,  34 ,  36 . Specifically, the circuit  50  drives a current through body tissue  58 , which lies between the electrode  44  and the patches  32 ,  34 ,  36 ; the circuit  52  drives a current through body tissue  60 , which lies between the electrode  46  and the patches  32 ,  34 ,  36 ; and the circuit  54  drives a current through body tissue  62 , which lies between the electrode  48  and the patches  32 ,  34 ,  36 . Each of the currents generated by the driver circuits may be distinguished by setting the circuits  50 ,  52 ,  54  to operate at different frequencies.  
         [0036]     Each of circuits  50 ,  52 ,  54  measures the electrical impedance in its respective loops through the body tissues  58 ,  60 ,  62 . These impedance readings are passed to a controller or processing unit  56 , which uses the readings to calculate the position coordinates of the catheter relative to the body surface electrodes. Based on these position coordinates, the processing unit  56  then generates the real-time information appearing on the display  40 , as described above.  
         [0037]     In one embodiment of the invention, the circuits  50 ,  52 ,  54  generate constant voltage signals. The circuits  50 ,  52 ,  54  measure the currents flowing through the respective loops to determine impedances, which are then used to calculate the position coordinates.  
         [0038]     In a second embodiment of the invention, the circuits  50 ,  52 ,  54  generate constant current signals. Measurement of the voltage across the current drivers can therefore be measured by the processing unit  56  to determine impedances, which are used to calculate position coordinates.  
         [0039]     In either of the two embodiments described above, the impedance measured is proportional to the distance between the electrode and the patch. These distances may then be used to triangulate the position at the tip of the catheter  22  using well known methods, described, e.g., in U.S. Pat. No. 5,443,489, and PCT Patent Publication WO 96/05768 to Ben-Haim et al., whose disclosures are incorporated herein by reference. The measurement accuracy may be further improved by making initial reference measurements with the catheter at known anatomical locations (i.e., landmarks within the heart), or by using a separate, reference catheter at a known location to calibrate the impedance scale.  
         [0040]     The system  20  ( FIG. 1 ) represents an embodiment of the invention as it may be used in a catheter-based procedure for diagnosis or treatment of conditions of the heart, such as arrhythmias. The system  20  can be used, as well, in the diagnosis or treatment of intravascular ailments, which may involve angioplasty or atherectomy. The principles of system  20  may also be applied, mutatis mutandis, in position-sensing systems for the diagnosis or treatment of other body structures, such as the brain, spine, skeletal joints, urinary bladder, gastrointestinal tract, prostrate, and uterus.  
         [0000]     Body Surface Patches  
         [0041]     Reference is now made to  FIG. 4 , which is a schematic diagram showing an electroconductive body surface patch  70  and its associated electrical circuitry configured for a sensing mode of operation. The body surface patch  70  is used as one or more of the patches  32 ,  34 ,  36  ( FIG. 1 ). Typically, all of the patches  32 ,  34 ,  36  are realized as instances of the body surface patch  70 . In order to perform the impedance measurements described above accurately, a stable baseline of body surface impedance, e.g., skin impedance, is required. Using the body surface patch  70  in accordance with the present invention reduces the impact that body surface impedance variations have on the position measurements of the catheter  22  ( FIG. 1 ).  
         [0042]     The body surface patch  70  is an adhesive electrode pad, comprising two electrically isolated body surface electrodes  72 ,  74 , which are adhered to a body surface  76  by an adhesive layer  78 . The electrodes  72 ,  74  are separated by an insulator  80 . As was described above, the catheter  22  ( FIG. 2 ) passes a current through the subject&#39;s body to the electrodes  72 ,  74 . Dual wires  82 ,  84  convey current from the respective electrodes  72 ,  74  to the processing unit  56  and to a driver  86 . The driver  86  can be the AC voltage or current source in a respective one -of the circuits  50 ,  52 ,  54  ( FIG. 3 ). Alternatively, the driver  86  can be a different source. When the position of the catheter  22  is being sensed, the wires  82 ,  84  are electrically connected by a switch  88 , so that the electrodes  72 ,  74  are arranged in parallel with the driver  86 , as shown in  FIG. 4 . REM PolyHesive™ II Patient Return Electrodes, available from Valleylab, 5920 Longbow Drive, Boulder, Colo. 80301-3299, are suitable for use as the body surface patch  70 .  
         [0043]     The total impedance Z seen by the driver  86  is the sum of the local electrical impedance Z local  of the body surface and its interface with the body surface patch  70 , and the intrabody impedance Z b  between the catheter  22  and the body surface  76 . The components of the local electrical impedance Z local  are the impedance Z s1  of the electrode  72  and the body surface  76 , and impedance Z s2  of the electrode  74  and the body surface  76  in parallel:  
         Z   local     =           Z     s   ⁢           ⁢   1       ⁢     Z     s   ⁢           ⁢   2             Z     s   ⁢           ⁢   1       +     Z     s   ⁢           ⁢   2           .         
 
         [0044]     The total impedance Z computed by the processing unit  56  can thus be approximated by the equation  
       Z   =       Z   b     +           Z     s   ⁢           ⁢   1       ⁢     Z     s   ⁢           ⁢   2             Z     s   ⁢           ⁢   1       +     Z     s   ⁢           ⁢   2           .           
 
 In this mode of operation, the body surface patch  70  functions as a conventional, single-piece body surface electrode. 
 
         [0045]     Reference is now made to  FIG. 5 , which is a schematic diagram of the body surface patch  70  and its associated circuitry configured for a calibration mode of operation.  FIG. 5  is similar to  FIG. 4 . However, the catheter  22  is now disconnected from the driver  86  by a switch  90 . The switch  88  now connects the electrodes  72 ,  74  in a series circuit with the driver  86 . Consequently, the driver  86  sees a calibration impedance Z cal  in which the impedances Z s1  and Z s2  are in series. The processing unit  56  computes Z cal  as follows: 
 
 Z   cal   =Z   s1   +Z   s2 . 
 
         [0046]     Assuming that Z s1 ≈Z s2 , then Z cal ≈2Z s1 , and Z s1 ≈Z cal /2. When the switches  90 ,  88  are returned to the sensing configuration shown in  FIG. 4 , the processing unit  56 , using the above relations, can compute the body impedance Z B , compensating for the surface impedances as follows  
                 Z   =       Z   b     +       Z     s   ⁢           ⁢   1       2         ,           ⁢   and                 Z   b     =     Z   -         Z   cal     4     .               ⁢     
         
 
         [0047]     The processing unit  56  includes suitable interval timing circuitry or software for alternating the switches  90 ,  88  between the configurations of  FIG. 4  and  FIG. 5 .  
         [0048]     Optionally, if a change in the calibration impedance Z cal  as determined in successive intervals exceeds a threshold, an alarm may be activated. Such an excursion may indicate electrode peel-off or disconnection, and in any case could impair the reliability of the respective body impedance measurement, even after compensation.  
         [0000]     Operation  
         [0049]     Reference is now made to  FIG. 6 , which is a flow chart illustrating a method of detecting body surface impedance while using impedance measurements to locate a probe inside a living subject in accordance with a disclosed embodiment of the invention. The process steps are shown in a particular sequence in  FIG. 6  for clarity of presentation. However, it will be evident that many of them can be performed in parallel, asynchronously, or in different orders.  
         [0050]     At initial step  92 , dual-electrode body surface pads, as described above with reference to  FIG. 4  and  FIG. 5 , are emplaced at convenient locations on the body surface of a subject, generally near the location of a probe to be inserted and mapped.  
         [0051]     Next, at step  94  one of the body surface pads is chosen.  
         [0052]     Next, at step  96  the electrodes of the current body surface pad are connected in a series circuit as shown in  FIG. 5 . A test current is passed from one of the electrodes to the other through the subject&#39;s skin or other body surface. The baseline impedance is measured and memorized.  
         [0053]     Control now proceeds to decision step  98 , where it is determined if more pads remain to be evaluated. If the determination at decision step  98  is affirmative, then control returns to step  94 .  
         [0054]     If the determination at decision step  98  is negative, then control proceeds to step  100 . A probe is configured and inserted into an operational area of the body, for instance the left ventricle of the subject&#39;s heart.  
         [0055]     Next, at step  102  an interval timer is set. The purpose of the timer is to establish a time interval, which, upon its passage, triggers a repeat of the impedance measurements that were performed in step  94 , step  96 , and decision step  98 . The length of the interval is not critical. However, if the interval is too long, intervening changes in one or more of the body surface impedances may introduce errors in locating the probe. If the interval is too short, system overhead incurred in testing the body surface pads may limit the frequency at which measurements of the probe&#39;s position can be taken. Intervals of about one second have been found to be practical.  
         [0056]     Next, at step  104  the position of the probe in the body is mapped by configuring the body surface pads as shown in  FIG. 4 , measuring impedances between the probe and the body surface pads, and performing triangulation as described above. In determining the impedances to be used for the triangulation calculations, the memorized values of the local impedances of the respective body surface pads are subtracted from the total impedance in order to determine the true intrabody impedances between the probe and the body surface. Thus, the effects of local variations in body surface impedance are eliminated. This increases the accuracy of the determination of the probe&#39;s position.  
         [0057]     Control now proceeds to decision step  106 , where it is determined if the timer that was set in step  102  has expired. If the determination at decision step  106  is negative, then control returns to step  104  to update the determination of the probe&#39;s location.  
         [0058]     If the determination at decision step  106  is affirmative, then new measurements of the body surface impedances are to be taken. Control proceeds to a sequence of steps consisting of step  108 , step  110 , and decision step  112 . These are performed identically to step  94 , step  96  and decision step  98 , respectively, and their details are not repeated in the interest of brevity. When the determination at decision step  112  indicates completion of the sequence, control returns to step  102 , where the interval timer is reset. Position coordinates of the probe may now be determined by compensating for the updated body surface impedances, as described above.  
         [0059]     It will be appreciated by persons skilled in the art 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 sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.