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 . turning now to the drawings , reference is initially made to fig1 , 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 . 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 above 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 . 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 . 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 . catheter electrodes ( not shown ) 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 . temperature sensors ( not shown ), typically a thermocouple or thermistor , may be mounted on ablation surfaces on the distal portion of the catheter 14 as described below . 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 , ultrasound 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 . 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 . 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 . 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 . 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 . reference is now made to fig2 , which is a perspective view of a catheter 37 according to an embodiment of the invention , which is suitable for use with the system 10 ( fig1 ). the catheter 37 comprises an elongated shaft 39 having proximal and distal ends , a control handle 41 at the proximal end of the catheter body , and a basket - shaped electrode assembly 43 mounted at the distal end of the shaft 39 . the shaft 39 comprises an elongated tubular construction having a single , axial or central lumen ( not shown ), but can optionally have multiple lumens if desired . the shaft 39 is flexible , i . e ., bendable , but substantially non - compressible along its length . the shaft 39 can be of any suitable construction and made of any suitable material . a presently preferred construction comprises an outer wall made of polyurethane or polyether block amide . the outer wall comprises an imbedded braided mesh of stainless steel or the like to increase torsional stiffness of the shaft 39 so that , when the control handle 41 is rotated , the distal end of the shaft 39 rotates in a corresponding manner . the outer diameter of the shaft 39 is not critical , but is preferably no more than about 8 french , more preferably 7 french . likewise the thickness of the outer wall is not critical , but is preferably thin enough so that the central lumen can accommodate a puller wire , lead wires , sensor cables and any other wires , cables or tubes . if desired , the inner surface of the outer wall is lined with a stiffening tube ( not shown ) to provide improved torsional stability . an example of a catheter body construction suitable for use in connection with the present invention is described and depicted in u . s . pat . no . 6 , 064 , 905 , the entire disclosure of which is incorporated herein by reference . the basket - shaped electrode assembly 43 is mounted to the distal end of the shaft 39 . as shown in fig2 , the basket - shaped electrode assembly 43 comprises five splines 45 or arms mounted , preferably generally evenly - spaced , around a contraction wire 47 , which is connected to the distal extremity of the electrode assembly 43 , and which contracts , retracts and expands the electrode assembly 43 when a tractive or a pushing force is applied longitudinally to the contraction wire 47 as the case may be . the contraction wire 47 forms a longitudinal axis of symmetry for the electrode assembly 43 . the splines 45 are all attached , directly or indirectly , to the contraction wire 47 at their distal ends , and to the shaft 39 at their proximal ends . when the contraction wire 47 is moved longitudinally to expand and contract the electrode assembly 43 , in the expanded position the splines 45 are bowed outwardly and in the contracted position the splines 45 are generally straight . as will be recognized by one skilled in the art , the number of splines 45 can vary as desired depending on the particular application , so that the electrode assembly 43 has at least two splines , preferably at least three splines , and as many as eight or more splines . as used herein , the term “ basket - shaped ” in describing the electrode assembly 43 is not limited to the depicted configuration , but can include other designs , such as spherical or egg - shaped designs , that include a plurality of expandable arms connected , directly or indirectly , at their proximal and distal ends . each of the splines 45 comprises a flexible wire with a non - conductive covering on which one or more ring spline electrodes 49 are mounted . in a preferred embodiment , the flexible wires each comprise a flat nitinol wire and the non - conductive coverings each comprise a biocompatible plastic tubing , such as polyurethane or polyimide tubing . alternatively , the splines 45 can be designed without the internal flexible wire if a sufficiently rigid nonconductive material is used for the non - conductive covering to permit expansion of the electrode assembly 43 , so long as the spine has an outer surface that is non - conductive over at least a part of its surface for mounting of the ring spline electrodes 49 . each of the ring spline electrodes 49 on the splines 45 is electrically connected to an appropriate mapping or monitoring system and / or source of ablation energy by means of an electrode lead wire ( not shown ). the electrode lead wires extend through the control handle 41 , through a lumen in the shaft 39 , into the non - conductive covering of corresponding splines 45 , and attach to their corresponding ring spline electrodes 49 by any suitable method . the contraction wire 47 is provided with a far - field electrode 51 , e . g ., a cylindrical electrode , the function of which is described below . additional details of the catheter 37 are described in the above - referenced u . s . pat . no . 6 , 748 , 255 . the catheter 37 typically has multiple electrodes arranged on multiple flexible splines of the “ basket .” the catheter 37 is introduced into the heart 12 ( fig1 ) in a collapsed form , where the splines 45 are relatively close together . once in the heart 12 , the splines 45 may be formed into their expanded basket shape by the contraction wire 47 , which holds distal ends of the splines 45 , the splines 45 being pulled in a proximal direction . reference is now made to fig3 , which is a detailed schematic view of the electrode assembly 43 ( fig2 ), in accordance with an embodiment of the invention . in expanded form of the electrode assembly 43 at least a portion of the spline electrodes 49 of the splines 45 contact endocardial surface 53 of the heart 12 and acquire signals corresponding to electropotentials generated at their points of contact with the surface . however , since the spline electrodes 49 are in a conductive medium ( the blood ), in addition to the electropotentials from the points of contact the acquired signals also include far - field components from other regions of the heart 12 . the far - field components constitute an interfering signal on the endocardial surface electropotentials . to counteract the interference , embodiments of the present invention position the far - field electrode 51 on the contraction wire 47 . in the expanded configuration of the electrode assembly 43 , the far - field electrode 51 is located on the contraction wire 47 so as to be approximately equidistant from all corresponding spline electrodes 49 , i . e ., spline electrodes 49 that are equidistant from a fixed reference point on the long axis of the catheter , such as reference point 55 at the proximal end of the electrode assembly 43 , and is prevented from contacting the surface of the heart by the splines . for example , electrodes 57 , 59 are equidistant from reference point 55 , and are also equidistant from the far - field electrode 51 , as indicated by broken lines 61 , 63 , respectively . when the far - field electrode 51 is at least 0 . 5 cm removed from the spline electrodes 49 in the expanded configuration of the electrode assembly 43 it acquires a far - field signal , but not a near - field signal from the endocardial surface 53 . however the signals acquired by the spline electrodes 49 have both a far - field and a surface ( near - field ) component . the far - field component signal x ( t ) acquired by the far - field electrode 51 is removed from signals e ( t ) acquired by the spline electrodes 49 so as to counteract the interference suffered by these electrodes , i . e ., by subtraction of the signals : e ( t )- x ( t ). additionally or alternatively , removal of the far - field component may be accomplished using the teachings of copending , commonly assigned application ser . nos . 14 / 574 , 578 , 14 / 585 , 828 and 62 / 093 , 773 , the disclosures of which are herein incorporated by reference . in some embodiments , the catheter 37 is provided with a distal electrode location sensor 65 mounted at or near the position where the distal ends of the spines are connected , and a proximal electrode location sensor 67 mounted at or near the proximal end of the electrode assembly 43 , whereby , in use , the coordinates of the electrode location sensor 65 relative to those of the electrode location sensor 67 can be determined and taken together with known information pertaining to the curvature of the splines 45 to find the positions of each of the spline electrodes 49 . reference is now made to fig4 , which is a prospective graphical example of signals that may be obtained from a subject using the arrangement shown in fig3 , in accordance with an embodiment of the invention . graph 69 shows an electrogram e ( t ) obtained from a unipolar or bipolar configuration of the spline electrodes 49 . graph 71 is a signal tracing x ( t ) of the far - field electrode 51 , which may be a concurrent tracing . graph 73 is a tracing of the signal obtained when the far - field component in the electrogram e ( t ) is removed by subtraction of the signal of graph 71 from the graph 69 or by application , mutatis mutandis , of the algorithms described in the above - noted application ser . nos . 14 / 574 , 578 , 14 / 585 , 828 and 62 / 093 , 773 . 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 .
cardiac catheterization is performed with a catheter having a basket - shaped assembly at its distal end . a plurality of spline electrodes are disposed on the splines of the assembly . the assembly is configurable in an expanded arrangement wherein the splines bow radially outwardly and in a collapsed arrangement , wherein the splines are arranged generally along the longitudinal axis of the catheter body . a far - field electrode is disposed in the interior of the assembly . an intracardiac electrogram and a far - field electrogram are obtained with at least one of the spline electrodes and the far - field electrode , respectively . the far - field component is removed from the intracardiac electrogram using the far - field electrogram .