Abstract:
In a flexible catheterization probe a resilient member couples the tip to the distal portion of the probe and is configured to deform in response to pressure exerted on the tip when engaging tissue. A position sensor in the distal portion of the probe senses the position of the tip relative to the distal portion of the probe. The relative position changes in response to deformation of the resilient member. The position sensor generates a signal indicative of the position of the tip responsively to a magnetic field produced by a magnetic field generator located in the position sensor. The position sensor has a first coil of conductive wire having first windings, and three second coils of conductive wire having respective second windings. The second coils are symmetrically distributed about the longitudinal axis of the first coil.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention. 
         [0002]    This invention relates to medical devices. More particularly, this invention relates to a medical device for measuring force applied to parts of the body. 
         [0003]    2. Description of the Related Art. 
         [0004]    Cardiac arrhythmias, such as atrial fibrillation, occur when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. 
         [0005]    Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. 
         [0006]    Verification of physical electrode contact with the target tissue is important for controlling the delivery of ablation energy. Attempts in the art to verify electrode contact with the tissue have been extensive, and various techniques have been suggested. For example, U.S. Pat. No. 6,695,808 describes apparatus for treating a selected patient tissue or organ region. A probe has a contact surface that may be urged against the region, thereby creating contact pressure. A pressure transducer measures the contact pressure. This arrangement is said to meet the needs of procedures in which a medical instrument must be placed in firm but not excessive contact with an anatomical surface, by providing information to the user of the instrument that is indicative of the existence and magnitude of the contact force. 
         [0007]    As another example, U.S. U.S. Pat. No. 6,241,724 describes methods for creating lesions in body tissue using segmented electrode assemblies. In one embodiment, an electrode assembly on a catheter carries pressure transducters, which sense contact with tissue and convey signals to a pressure contact module. The module identifies the electrode elements that are associated with the pressure transducer signals and directs an energy generator to convey RF energy to these elements, and not to other elements that are in contact only with blood. 
         [0008]    A further example is presented in U.S. Pat. No. 6,915,149. This patent describes a method for mapping a heart using a catheter having a tip elecrode for measuring local electrical activity. In order to avoid artifacts that may arise from poor tip contact with the tissue, the contact pressure between the tip and the tissue is measured using a pressure sensor to ensure stable contact. 
         [0009]    U.S. Patent Application Publication 2007/0100332 describes systems and methods for assessing electrode-tissue contact for tissue ablation. An electromechanical sensor within the catheter shaft generates electrical signals corresponding to the amount of movement of the electrode within a distal portion of the catheter shaft. An output device receives the electrical signals for assessing a level of contact between the electrode and a tissue. 
         [0010]    Impedance-based methods for assessing catheter-tissue contact that are known in the art typically rely on measurement of the magnitude of the impedance between an electrode on the catheter and a body-surface electrode. When the magnitude is below some threshold, the electrode is considered to be in contact with the tissue. This sort of binary contact indication may be unreliable, however, and is sensitive to changes in the impedance between the body-surface electrode and the skin. 
         [0011]    U.S. Patent Application Publication Nos. 2008/0288038 and 2008/0275465, both by Sauarav et al., which are herein incorporated by reference, describe an electrode catheter system, which may comprise an electrode adapted to apply electric energy. A measurement circuit adapted to measure impedance may be implemented between the electrode and ground as the electrode approaches a target tissue. A processor or processing units may be implemented to determine a contact condition for the target tissue based at least in part on reactance of the impedance measured by the measurement circuit. In another embodiment, the contact condition may be based on the phase angle of the impedance. 
       SUMMARY OF THE INVENTION 
       [0012]    According to disclosed embodiments of the invention, a contact force sensor for a catheter has enhanced immunity to metal interference. In one embodiment a cylindrical receiving coil is centrally disposed in a nitinol housing and is operative for estimation of force value. Three elliptic coils are assembled about the cylindrical receiving coil, with 120 degrees between them. The elliptic coils are used for estimation of force value and direction. Because the receiving coil is installed centrally, deep within the center of the sensor, and remote from the nitinol housing, metal objects located less than about 1 mm to the sensor, such as the catheter shaft, do not cause a significant force measurement error. Moreover, the effects of metallic objects are unaffected by the orientation of the catheter tip due to the symmetrical design of the contact force sensor. 
         [0013]    There is provided according to embodiments of the invention a flexible probe whose distal portion is adapted for insertion into a body cavity of a patient. The distal tip of the probe is configured to be brought into contact with tissue in the body cavity. A resilient member couples the distal tip to the distal portion of the probe and is configured to deform in response to pressure exerted on the distal tip when the distal tip engages the tissue. A position sensor is disposed in the distal portion of the probe for sensing a position of the distal tip relative to the distal portion of the probe, which changes in response to deformation of the resilient member. The position sensor is configured to generate a signal indicative of the position of the distal tip responsively to a magnetic field. A magnetic field generator is provided within the distal tip for generating the magnetic field, wherein the position sensor includes a first coil of conductive wire having first windings, and three second coils of conductive wire having respective second windings. The second coils are symmetrically distributed about the longitudinal axis of the first coil. 
         [0014]    According to still another aspect of the apparatus there are exactly three second coils. 
         [0015]    According to yet another aspect of the apparatus, the first windings are directed about the longitudinal axis of the first coil. 
         [0016]    According to still another aspect of the apparatus, the second coils are elliptical coils. 
         [0017]    According to an additional aspect of the apparatus, the second coils are in contact with the first coil. 
         [0018]    According to one aspect of the apparatus, the major axes of the elliptical coils are parallel to the longitudinal axis of the first coil. 
         [0019]    According to another aspect of the apparatus, the second windings are directed from the first vertex to the second vertex of the elliptical coils, respectively. 
         [0020]    According to a further aspect of the apparatus, the first coil is wound about a hollow tube. 
         [0021]    According to yet another aspect of the apparatus, the second coils are air core inductors. 
         [0022]    There is further provided according to embodiments of the invention a method, which is carried out by inserting the distal portion of a flexible probe into a body cavity of a patient, bringing the distal tip of the probe into contact with tissue in the body cavity, coupling the distal tip to the distal portion of the probe with a resilient member that is configured to deform in response to pressure exerted on the distal tip when the distal tip engages the tissue, and sensing a position of the distal tip relative to the distal portion of the probe with a position sensor disposed in the distal portion of the probe. The position of the distal tip changes in response to deformation of the resilient member. The method is further carried out by generating a signal indicative of the position of the distal tip responsively to a magnetic field that is generated in a vicinity of the distal tip, providing a magnetic field generator within the distal tip for generating the magnetic field. The position sensor includes a first coil of conductive wire and three second coils of conductive wire. The second coils are symmetrically distributed about the longitudinal axis of the first coil. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0023]    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: 
           [0024]      FIG. 1  is a pictorial illustration of a system for performing catheterization procedures on a heart, in accordance with a disclosed embodiment of the invention; 
           [0025]      FIG. 2  is a schematic oblique elevation of a contact force sensor  37  in accordance with an embodiment of the invention; 
           [0026]      FIG. 3  is a sectional view through a sensor in accordance with an embodiment of the invention; 
           [0027]      FIG. 4  is a schematic partial sectional view through an air core elliptical coil in accordance with an embodiment of the invention; 
           [0028]      FIG. 5  is an elevation of a distal portion of a cardiac catheter in accordance with an embodiment of the invention; 
           [0029]      FIG. 6  is a schematic sectional view through the distal portion of a cardiac catheter including a contact force sensor that is constructed and operative in accordance with an embodiment of the invention; and 
           [0030]      FIG. 7  is a flow chart of a method of determining contact between a probe and a tissue in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily needed for practicing the present invention. In this instance, 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 general concepts unnecessarily. 
       System Overview. 
       [0032]    Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for evaluating electrical activity and performing ablative procedures on a heart  12  of a living subject, which is constructed and operative in accordance with a disclosed embodiment of the invention. The system comprises a catheter  14 , which is percutaneously inserted by an operator  16  through the patient&#39;s vascular system into a chamber or vascular structure of the heart  12 . The operator  16 , who is typically a physician, brings the catheter&#39;s distal tip  18  into contact with the heart wall, for example, at an ablation target site. Electrical activation maps may be prepared, according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are herein incorporated by reference. One commercial product embodying elements of the system  10  is available as the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765. This system may be modified by those skilled in the art to embody the principles of the invention described herein. 
         [0033]    Areas determined to be abnormal, for example by evaluation of the electrical activation maps, can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires in the catheter to one or more electrodes at the distal tip 18, which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating it to a point (typically about 60° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. The principles of the invention can be applied to different heart chambers to diagnose and treat many different cardiac arrhythmias. 
         [0034]    The catheter  14  typically comprises a handle  20 , having suitable controls on the handle to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator  16 , the distal portion of the catheter  14  contains position sensors (not shown) that provide signals to a processor  22 , located in a console  24 . The processor  22  may fulfill several processing functions as described below. 
         [0035]    Ablation energy and electrical signals can be conveyed to and from the heart  12  through one or more ablation electrodes  32  located at or near the distal tip  18  via cable  34  to the console  24 . Pacing signals and other control signals may be conveyed from the console  24  through the cable  34  and the electrodes  32  to the heart  12 . Sensing electrodes  33 , also connected to the console  24  are disposed between the ablation electrodes  32  and have connections to the cable  34 . 
         [0036]    Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning sub-system for measuring location and orientation coordinates of the catheter  14 . The processor  22  or another processor (not shown) may be an element of the positioning subsystem. The electrodes  32  and the body surface electrodes  30  may be used to measure tissue impedance at the ablation site as taught in U.S. Pat. No. 7,536,218, issued to Govari et al., which is herein incorporated by reference. A temperature sensor (not shown), typically a thermocouple or thermistor, may be mounted on or near each of the electrodes  32 . 
         [0037]    The console  24  typically contains one or more ablation power generators  25 . The catheter  14  may be adapted to conduct ablative energy to the heart using any known ablation technique, e.g., radiofrequency energy, ultra-sound energy, and laser-produced light energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are herein incorporated by reference. 
         [0038]    In one embodiment, the positioning subsystem comprises a magnetic position tracking arrangement that determines the position and orientation of the catheter  14  by generating magnetic fields in a predefined working volume and sensing these fields at the catheter, using field generating coils  28 . The positioning subsystem is described in U.S. Pat. No. 7,756,576, which is hereby incorporated by reference, and in the above-noted U.S. Pat. No. 7,536,218. 
         [0039]    As noted above, the catheter  14  is coupled to the console  24 , which enables the operator  16  to observe and regulate the functions of the catheter  14 . Console  24  includes a processor, preferably a computer with appropriate signal processing circuits. The processor is coupled to drive a monitor  29 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14 , including signals generated by sensors such as electrical, temperature and contact force sensors, and a plurality of location sensing electrodes (not shown) located distally in the catheter  14 . The digitized signals are received and used by the console  24  and the positioning system to compute the position and orientation of the catheter  14 , and to analyze the electrical signals from the electrodes. 
         [0040]    In order to generate electroanatomic maps, the processor  22  typically comprises an electroanatomic map generator, an image registration program, an image or data analysis program and a graphical user interface configured to present graphical information on the monitor  29 . 
         [0041]    Typically, the system  10  includes other elements, which are not shown in the figures for the sake of simplicity. For example, the system  10  may include an electrocardiogram (ECG) monitor, coupled to receive signals from one or more body surface electrodes, in order to provide an ECG synchronization signal to the console  24 . As mentioned above, the system  10  typically also includes a reference position sensor, either on an externally-applied reference patch attached to the exterior of the subject&#39;s body, or on an internally-placed catheter, which is inserted into the heart  12  maintained in a fixed position relative to the heart  12 . Conventional pumps and lines for circulating liquids through the catheter  14  for cooling the ablation site are provided. The system  10  may receive image data from an external imaging modality, such as an MRI unit or the like and includes image processors that can be incorporated in or invoked by the processor  22  for generating and displaying images. 
       Contact Force Sensor. 
       [0042]    Reference is now made to  FIG. 2 , which is a schematic oblique elevation of a contact force sensor  37  in accordance with an embodiment of the invention. A central coil  39  comprises one or two layers of 10 μ enameled copper wire are wound about a cylindrical air-filled polyimide tube  41  to form a central air core inductor  43 . The diameter of the tube  41  is typically about 0.8-0.9 mm. Typical dimensions for one layer of 10 μm wire wound about the polyimide tubing are: outer diameter 0.947 mm, length 2.15 mm, and 350 turns. The central coil  39  is wound generally transverse to the longitudinal axis of the tube  41 . 
         [0043]    Surrounding the inductor  43  are a plurality of elliptical coils. Three elliptical coils  45 ,  47 ,  49  are shown in  FIG. 2 . Each comprises more than 10 layers of 10 μ enameled copper wire to create an air core elliptic coil having major and minor axes, which are typically 2.15-2.35 and 0.6-0.8 cm, respectively. In this embodiment elliptical coils  45 ,  47 ,  49  are disposed about the central coil  39  with the major axis of each ellipse being parallel to the longitudinal axis of the central coil  39 . The windings of the elliptical coils  45 ,  47 ,  49  are each generally directed from one vertex to the other vertex of the respective ellipses. The elliptical coils  45 ,  47 ,  49  are symmetrically distributed about the longitudinal axis of the tube  41 . Leads  51  conduct signals from the elliptical coils  45 ,  47 ,  49  to a processor (not shown). A lead  53  conducts signals from the central coil  39  to the processor. 
         [0044]    Reference is now made to  FIG. 3 , which is a sectional view through a sensor in accordance with an embodiment of the invention. Distribution of the elliptical coils  45 ,  47 ,  49  at 120° intervals is demonstrated on this view. There is a contacting relationship between each of the forms of the elliptical coils  45 ,  47 ,  49  and the central coil  39 . Wire leads  55  are provided for conducting signals from the coils. 
         [0045]    Reference is now made to  FIG. 4 , which is a schematic partial view through an air core elliptic coil  57 , in accordance with an embodiment of the invention. As shown in a cross section  59  taken through line A-A of coil  57 , more than ten layers of wire  61  are wound in an elliptical pattern to form the elliptical air coil. 
         [0046]    Reference is now made to  FIG. 5 , which is an elevation of the distal portion of a cardiac catheter  63 , in accordance with an embodiment of the invention. A contact force sensor constructed in accordance with an embodiment of the invention is disposed in a segment  65  of the catheter. Except for the contact force sensor, the catheter  63  may be the catheter described in commonly assigned U.S. Patent Application Publication No. 2009/0093806 by Govari et al., which is herein incorporated by reference. The catheter  63  is a flexible insertion tube, having a distal end  67  for insertion into a body cavity of a patient, and a distal tip  69 , which is configured to be brought into contact with tissue in a body cavity. A resilient member  71  couples the distal tip  69  to the distal end  67  and deforms in response to pressure exerted on the distal tip  69 . When the distal tip  69  engages the tissue. The contact force sensor within the probe senses a position of the distal tip  69  relative to the distal end  67  of the catheter  63 , The position and the sensor readings change in response to deformation of the resilient member  71 . 
         [0047]    Reference is now made to  FIG. 6 , which is a schematic longitudinal sectional view through the distal portion of a cardiac catheter  73 , which has been modified by replacement of a conventional contact force sensor by a contact force sensor  75  that is constructed and operative in accordance with an embodiment of the invention. From the perspective of the operator, the operation of the catheter  63  does not differ from an unmodified version. However, there is one less coil and one less electrical channel than in the unmodified version. A transmitting coil  77  is provided as a signal source for the central coil and the elliptical coils in the contact force sensor  75 . Four receiving coils  79 , (best seen in  FIG. 2  as the elliptical coils  45 ,  47 ,  49  and central coil  39 ) are present. The contact force sensor  75  receives signals from external field generating coils  28  ( FIG. 1 ) and the transmitting coil  77 , so that the four receiving coils  79  are exposed to four electromagnetic fields at respective frequencies. Other components of the contact force sensor  75  include a spring  81  disposed between the transmitting coil  77  and the receiving coils  79 . Various typically asymmetric metallic structures  83  having functions that are beyond the scope of this disclosure may be present in the cardiac catheter  73 . As noted above, the metallic structures  83  can adversely affect readings of the contact force sensor  75 . 
       Operation. 
       [0048]    As noted above, the elliptical coils  45 ,  47 ,  49  provide information on force value and direction. The central coil  39  provides information on the force value. Reverting to  FIG. 2 , the signals received in the elliptical coils  45 ,  47 ,  49  and the central coil  39  are measured and the ratio between the transmitted signal produced by transmitting coil  77  ( FIG. 6 ) and the received signal from the elliptical coils is calculated for each of the elliptical coils  45 ,  47 ,  49  and the central coil  39  using signals from the field generating coils  28  ( FIG. 1 ) at respective frequencies: 
         [0000]    
       
         
           
             
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         [0049]    The ratio between the transmitted and received signal is normalized with a measurement taken when no force is applied to the tip of the catheter. 
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         [0050]    After calibration, the force applied to the tip of the catheter is estimated as follows: 
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         [0000]    where M ij  are calibration elements calculated for a given matrix of N force measurements, each comprising components F x , F y , F z . Four force measurements can be obtained from the three elliptical coils  45 ,  47 ,  49  and the central coil  39 . 
         [0051]    Signals from all four coils provide a solution for magnitude and direction. 
         [0052]    For all four coils the force vector is: 
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         [0053]    When only a single coil is being used, the equation reduces to. 
         [0000]      {right arrow over (F)}=[M1]×[Sz1]=[Fx]   Eq. (3)
 
         [0054]    Signals from three elliptical coils provide a less precise solution for magnitude and direction than from all four coils. 
         [0055]    Signals taken only from the central coil provide a solution for magnitude, but not direction. 
         [0056]    The magnitude of readings from the contact force sensor are dependent on the hardware configuration of the catheter and the electronics. Typically, the maximum axial force detected is 150 gm. A lateral force can be accurately measured up to 30 gm, above which accuracy suffers. The resolution of the force measurement is less than 1 gm. 
         [0057]    Reference is now made to  FIG. 7 , which is a flow chart of a method of determining contact between a probe and a tissue in accordance with an embodinvent of the invention. The process steps are shown in a particular linear sequence for clarity of presentation. However, it will be evident that many of them can be performed in parallel, asynchronously, or in different orders. Those skilled in the art will also appreciate that a process could alternatively be represented as a number of interrelated states or events, e.g., in a state diagram. Moreover, not all illustrated process steps may be required to implement the method. 
         [0058]    At initial step  85  a probe is introduced conventionally into the body of a subject and brought into contact with a tissue. Metallic objects are assumed to be present in sufficient proximity to affect the readings of the contact force sensor. 
         [0059]    Next, at step  87  a force vector (A) is determined using all four coils of the sensor, e.g., elliptical coils  45 ,  47 ,  49  and central coil  39  ( FIG. 2 ) according to Equation 2, 
         [0060]    Next, at step  89  a force vector (B) is determined using the three elliptical coils  45 ,  47 ,  49  according to Equation 1. 
         [0061]    Next, at step  91  a force vector (C) is determined using only the central coil  39  according to Equation 3 
         [0062]    Next, at decision step  93 , it is determined if the force magnitude (C) obtained from the central coil  39  in step  91  is in agreement with the force magnitude (B) obtained from the elliptical coils  45 ,  47 ,  49  in step  89  according to a predetermined criterion, e.g., the two force magnitudes differ by less than 5%. This criterion may be varied in different applications. 
         [0063]    If the determination at decision step  93  is affirmative, then control proceeds to final step  95 . The force magnitude and directional readings from all four coils (A) that was obtained in step  87  are used to evaluate contact between the probe and the tissue. 
         [0064]    If the determination at decision step  93  is negative, then at final step  97  the force magnitude information obtained from the central coil  39  (C) is used to evaluate contact between the probe and the tissue. Directional information is not available. 
         [0065]    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.