Abstract:
A method for position tracking includes receiving signals from a main position transducer at a distal end of a medical probe via wiring traversing the probe to a connector at a proximal end of the probe, for connection to a processor, which processes the signals to find a first position of the distal end. Calibration data with respect to an interference introduced into the signals at the connector is collected as a function of a position of the proximal end. A second position of an auxiliary position transducer at the proximal end of the probe is measured. The interference in the signals is canceled responsively to the measured second position and the calibration data. The first position is calculated based on the signals, after canceling the interference.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to invasive probes, and specifically to determining the position of a medical probe inside a body cavity. 
     BACKGROUND 
     A wide range of medical procedures involve placing objects, such as sensors, tubes, catheters, dispensing devices and implants, within the body. Position sensing systems have been developed for tracking such objects. Magnetic position sensing is one of the methods known in the art. In magnetic position sensing, magnetic field generators are typically placed at known positions external to the patient. One or more magnetic field sensors within the distal end of a probe generate electrical signals in response to these magnetic fields, which are processed in order to determine the position coordinates of the distal end of the probe. These methods and systems are described in U.S. Pat. Nos. 5,391,199, 6,690,963, 6,484,118, 6,239,724, 6,618,612 and 6,332,089, in PCT International Publication WO 1996/005768, and in U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are all incorporated herein by reference. 
     U.S. Pat. No. 6,370,411, whose disclosure is incorporated herein by reference, describes a probe having two parts: a catheter of minimal complexity which is inserted into a patient&#39;s body, and a connection cable that connects between the proximal end of the catheter and the console. The catheter comprises a microcircuit that carries substantially only information specific to the catheter, which is not in common with other catheters of the same model. The cable comprises an access circuit which receives the information from the catheter and passes it in a suitable form to the console. In some embodiments, the cable operates with all catheters of a specific model or type, and therefore when a catheter is replaced, there is no need to replace the cable. Catheters that are planned for one-time use do not require replacement of the cable, which does not come in contact with patients. 
     U.S. Patent Application Publication 2006/0074289 A1, whose disclosure is incorporated herein by reference, discusses an endoscopic probe, whose handle has an orientation sensor that generates signals indicative of the orientation of the handle in an external frame of reference. The output of the orientation sensor may be used to sense movement of the handle relative to its initial position and orientation at the beginning of the endoscopic procedure. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention that is described herein provides a method for position tracking, including: 
     receiving signals from a main position transducer at a distal end of a medical probe via wiring traversing the probe to a connector at a proximal end of the probe, for connection to a processor, which processes the signals to find a first position of the distal end; 
     collecting calibration data with respect to an interference introduced into the signals at the connector as a function of a position of the proximal end; 
     measuring a second position of an auxiliary position transducer at the proximal end of the probe; 
     canceling the interference in the signals responsively to the measured second position and the calibration data; and 
     calculating the first position based on the signals, after canceling the interference. 
     In some embodiments, the medical probe includes a catheter. In an embodiment, the signals are generated by the main position transducer in response to one or more magnetic fields that are applied in a vicinity of the probe and sensed by the main position transducer. In another embodiment, the auxiliary position transducer is fitted adjacent to the connector. The auxiliary position transducer and the connector may be coupled to a handle of the probe. In another embodiment, collecting the calibration data includes placing the proximal end at a plurality of positions relative to a source of the interference, collecting auxiliary position signals from the auxiliary position transducer indicative of the respective positions of the proximal end, and measuring the interference as a function of the auxiliary position signals. 
     In yet another embodiment, measuring the second position includes applying one or more magnetic fields in a vicinity of the proximal end, receiving from the auxiliary position transducer signals that are generated by the auxiliary position transducer responsively to the magnetic fields, and calculating the second position based on the received signals. In still another embodiment, the method includes presenting the calculated first position to an operator. 
     There is additionally provided, in accordance with an embodiment of the present invention, apparatus, including: 
     a medical probe, which includes a distal end including a main position transducer, a proximal end including an auxiliary position transducer, a connector connecting the distal end to the proximal end, and wiring traversing the probe and coupling the main position transducer to the connector; and 
     a processor, which is configured to receive from the main position transducer over the wiring signals, which are indicative of a first position of the distal end, to collect calibration data with respect to an interference introduced into the signals at the connector as a function of a position of the proximal end, to measure a second position of the auxiliary position transducer, to cancel the interference in the signals responsively to the measured second position and the calibration data, and to calculate the first position based on the signals, after canceling the interference. 
     There is also provided, in accordance with an embodiment of the present invention, a computer software product, operated in conjunction with a medical probe that includes a distal end including a main position transducer, a proximal end including an auxiliary position transducer, a connector connecting the distal end to the proximal end, and wiring traversing the probe and coupling the main position transducer to the connector, the product including a non-transitory computer-readable medium, in which program instructions are stored, which instructions, when read by a computer, cause the computer to receive from the main position transducer over the wiring signals, which are indicative of a first position of the distal end, to collect calibration data with respect to an interference introduced into the signals at the connector as a function of a position of the proximal end, to measure a second position of the auxiliary position transducer, to cancel the interference in the signals responsively to the measured second position and the calibration data, and to calculate the first position based on the signals, after canceling the interference. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are schematic, pictorial illustrations of a medical position tracking system that uses interference cancellation, in accordance with an embodiment of the present invention; and 
         FIG. 3  is a flow diagram that schematically illustrates a method of measuring the position of a catheter using interference cancellation, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Various diagnostic and therapeutic procedures, such as intracardiac electrical mapping and cardiac ablation, use an invasive probe that is inserted into a patient&#39;s body. In these procedures, it is sometimes important to ascertain the location of the probe within a body cavity. The location can be determined by a console which processes signals from a position transducer fitted in the distal tip. 
     Probe assemblies are sometimes implemented with a disposable distal part (e.g., the part of the catheter to be inserted in the body cavity) and a reusable proximal part (e.g., a cable carrying signals from the distal part to a processing console). The distal and proximal parts of the probe are typically connected to one another using a connector. The connector may be fitted, for example, in a handle of the probe. In this “split handle” configuration, wires conveying the signals from the position transducer in the distal tip to the console may be shielded against interference pickup, e.g., using shielded and/or twisted pair wiring. In the vicinity of the connector, however, continuous shielding may be difficult to achieve, because the wiring may need to be unwound in order to connect to the connector pins. 
     In some position tracking systems, the position transducer in the distal tip generates signals in response to a magnetic field that is generated by external field generators. In many practical implementations, the signals sent over the wiring in the probe are weak in comparison with the external magnetic field. As a result, the wiring may pick up interference from the external magnetic field, and this interference may distort the position measurements of the system. Since, as noted above, shielding may be degraded in the vicinity of the connector, interference pickup in that area may be particularly severe. 
     Embodiments of the present invention provide methods and systems for canceling interference that is picked-up in the vicinity of the connector. In some embodiments, an additional auxiliary position transducer is fitted in the handle, in close proximity to the connector. The signals produced by the auxiliary position transducer are indicative of the location and orientation of the handle (and thus of the connector). In a preparatory calibration procedure, the interference is measured as a function of the handle position, according to the signals produced by the auxiliary position transducer. 
     During an actual medical procedure, the console receives position measurements from the position transducer the distal tip (referred to as a main position transducer), as well as from the auxiliary position transducer in the handle. The console determines the position of the distal tip by canceling out the interference in the signals received from the main position transducer using the calibration data, based on the signals received from the auxiliary position transducer in the handle. Thus, the position of the distal tip can be measured with high accuracy, even in the presence of strong interference. 
     System Description 
       FIG. 1  is an illustration of a medical position tracking system  20  that uses interference cancellation, in accordance with an embodiment of the invention. System  20  may be based, for example, in the CARTO™ system, produced by Biosense Webster Inc. (Diamond Bar, Calif.). System  20  comprises a medical probe  22 , such as a catheter, and a control console  24 . In the embodiment described hereinbelow, it is assumed that probe  22  is used for diagnostic or therapeutic treatment, such as mapping electrical potentials in a heart  26  or performing ablation of heart tissue. Alternatively, probe  22  may be used, mutatis mutandis, for other therapeutic and/or diagnostic purposes in the heart or in other body organs. 
     An operator  28 , such as a cardiologist, inserts probe  22  through the vascular system of a patient  30  so that a distal end  32  of probe  22  enters a chamber of the patient&#39;s heart  26 . Holding probe  22  at a handle  34 , operator  28  advances the probe, positioning a distal tip  36  at a desired location. Handle  34  couples probe  22  to a cable  38 , which connects to console  24  via a suitable connector. The configuration of probe  22 , and particularly handle  34 , is shown in greater detail in  FIG. 2  below. 
     Console  24  uses magnetic position sensing to determine position coordinates of distal tip  36  inside heart  26 . To determine the position coordinates, a driver circuit  40  in console  24  drives field generators  42  to generate magnetic fields within the body of patient  30 . Typically, field generators  42  comprise coils, which are placed below the patient&#39;s torso at known positions external to patient  30 . These coils generate magnetic fields in a predefined working volume that contains heart  26 . Magnetic field transducers that are coupled to distal tip  36  and handle  34  generate electrical signals in response to these magnetic fields. A signal processor  44  in console  24  processes the electrical signals in order to determine the position coordinates of distal tip  36  and handle  34 , typically including both location and orientation coordinates. As discussed supra, processor  44  can cancel out the interference in the signals received from a main position transducer in distal tip  36 , based on the signals received from an auxiliary position transducer in handle  34 . Both position transducers are shown in  FIG. 2  below. 
     Processor  44  typically comprises a general-purpose computer, with suitable front end and interface circuits for receiving signals from probe  22  and controlling the other components of console  24 . Processor  44  may be programmed in software to carry out the functions that are described herein. The software may be downloaded to console  24  in electronic form, over a network, for example, or it may be provided on non-transitory tangible media, such as optical, magnetic or electronic memory media. Alternatively, some or all of the functions of processor  44  may be carried out by dedicated or programmable digital hardware components, or using a combination of hardware and software elements. 
     An input/output (I/O) interface  46  enables console  24  to interact with probe  22 . Based on the signals received from probe  22  (via interface  46  and other components of system  20 ), processor  44  drives a display  48  to present operator  28  with an image  50  showing the position of distal tip  36  in the patient&#39;s body, as well as status information and guidance regarding the procedure that is in progress. 
     Alternatively or additionally, system  20  may comprise an automated mechanism (not shown) for maneuvering and operating probe  22  within the body of patient  30 . Such mechanisms are typically capable of controlling both the longitudinal motion (advance/retract) of probe  22  and transverse motion (deflection/steering) of distal end  32 . In such embodiments, processor  44  generates a control input for controlling the motion of probe  22  based on the signals provided by the magnetic field transducers in the probe and the handle, as explained further hereinbelow. 
       FIG. 2  is another schematic, pictorial illustration of system  20 , in accordance with an embodiment of the present invention.  FIG. 2  shows the configuration of probe  22 , and in particular handle  34 , in greater detail. As can be seen in the figure, handle  34  connects probe  22  to cable  38 , and comprises a distal part  52  and a proximal part  54  that mate via a suitable connector  56 . Proximal part  54  of the handle and cable  38  are sometimes referred to as the proximal part of the probe. Distal part  56  of the handle, and catheter  22 , are sometimes referred to as the distal part of the probe. 
     Distal tip  36  comprises a main position transducer  58 , which generates a signal to console  24  that is indicative of the position coordinates of the distal tip relative to field generators  42 . An auxiliary position transducer  60  is fitted in proximal part  54  of handle  34 , and generates a signal to console  24  that is indicative of the position coordinates of the handle relative to field generators  42 . Each of position transducers  58  and  60  may comprise one or more miniature coils, and typically comprise multiple coils oriented along different axes. Alternatively, position transducers  58  and  60  may comprise either another type of magnetic transducer, an electrode which serves as a position transducer, or position transducers of other types, such as impedance-based or ultrasonic position transducers. Although  FIG. 2  shows a probe with a single position transducer in distal tip  36 , embodiments of the present invention may utilize probes with more than one position transducer in the distal tip and/or distal end  32 . When distal tip  36  is positioned in heart  26  during a medical procedure, processor  44  uses the signals received from position transducers  58  and  60  to calculate the position of the distal tip. 
     As discussed supra, position transducers  58  and  60  may generate weak signals due to their configuration. An amplifier  62  coupled to proximal part  54  amplifies the signals received from position transducers  58  and  60 . The “split handle” configuration shown in  FIG. 2  permits components such as amplifier  62  and auxiliary position transducer  60  to be contained in proximal part  54 , which is reusable, while probe  22  is disposed of after use. Further aspects of split-handle configurations are addressed in U.S. Pat. No. 6,370,411, cited above. 
     In an alternative embodiment, the roles of position transducers  58 ,  60  and magnetic field generators  42  may be reversed. In other words, driver circuit  40  may drive magnetic field generators in position transducers  58  and  60 , so as to generate magnetic fields. Coils  42  may be configured to sense the fields and generate signals indicative of the amplitudes of the components of these magnetic fields. In this embodiment, processor  44  receives and processes the signals from coils  42  in order to determine the position coordinates of distal tip  36  within heart  26 . 
     Although  FIGS. 1 and 2  show a particular system configuration, other system configurations can also be employed to implement embodiments of the present invention, and are thus considered to be within the spirit and scope of this invention. For example, the methods described hereinbelow may be applied using position transducers of other types, such as impedance-based or ultrasonic position transducers. The term “position transducer” as used herein refers to an element mounted on probe  22  or handle  34  which causes console  24  to receive signals indicative of the coordinates of the respective element. The position transducer may thus comprise a receiver on the probe or the handle, which generates a position signal to the control unit based on energy received by the transducer; or it may comprise a transmitter, emitting energy that is sensed by a receiver external to the probe or the handle. Furthermore, the methods described hereinbelow may similarly be applied in mapping and measurement applications using not only catheters, but also probes of other types, both in the heart and in other body organs and regions. 
     Position Measurement Using Interference Cancellation 
     Cable  38  conveys signals from main position transducer  58  to console  24  via handle  34 . As discussed hereinabove, cable  38  may pick up interference that may distort the signals of the main position transducer. As a result, console  24  may err is calculating the position of distal tip  36 . The interference picked-up by cable  38  may be caused by the relatively strong magnetic fields generated by generators  42 , by various electrical signals in the vicinity of the probe, or by any other source. 
     Cable  38  typically comprises shielded, twisted-pair wires in order to avoid such undesired interference pickup. In the vicinity of connector  56 , however, the shielding performance may be degraded because of the interconnection to the connector pins. Thus, some residual interference is sometimes picked-up in the vicinity of the connector. 
     System  20  reduces the effect of interference pickup in connector  56  by pre-calibrating and canceling this interference using auxiliary position transducer  60 . In some embodiments, processor  44  first measures the interference pickup as a function of the position (location and orientation) of handle  34  relative to the source of the interference. Processor  44  then uses this calibration data for canceling the interference in the signals received from main position transducer  58  during an actual medical procedure. The position of distal tip  36  can thus be calculated with high accuracy, even in the presence of strong interference. Moreover, the disclosed techniques may permit relaxing of the shielding requirements of cable  38 . 
       FIG. 3  is a flow diagram that schematically illustrates a method of measuring the position of distal tip  36  of probe using interference cancellation, in accordance with an embodiment of the present invention. At a preliminary calibration step  70 , operator  28  positions handle  34  in multiple positions (locations and orientations) relative to field generators  42  (or other interference source). At each handle position, processor  44  measures the interference pickup at connector  56  as a function of the position of handle  34  (as measured by auxiliary position transducer  60 ). Processor  44  thus calibrates the interference amplitude as a function of the output of the auxiliary position transducer in the handle. The measured interference as a function of handle position is referred to as calibration data. Main position transducer  58  is typically disabled during the calibration procedure. 
     During a medical procedure, operator  28  manipulates handle  34  to position probe  22  in heart  26 , at a probe positioning step  72 . Processor  44  receives position signals from main position transducer  58  indicating the position of distal tip  36 , at a main measurement step  74 . Additionally, processor  44  receives position signals from auxiliary position transducer  60  indicating the position of handle  34 , at an auxiliary measurement step  76 . 
     Processor  44  cancels the interference in the signal received from main position transducer  58  based on the measured position of handle  34 , at an interference cancellation step  78 . Typically, processor  44  queries the calibration data with the current position of the handle, as measured at step  76 , so as to determine the expected interference level at this handle position. Processor  44  then subtracts the expected interference level from the signal of main position transducer  58 , measured at step  74  above. 
     After canceling the interference, processor  44  computes the position of distal tip  36 , at a tip positioning step  80 . The calculation is performed using the position signal received from the main position transducer, after the interference has been canceled out from the signal. Finally, processor  44  presents image  50  on display  48 , so as to display the location of distal tip  36  to operator  28 , at an output step  82 . The method returns to step  72  above. 
     Alternatively or additionally, the position measurements and interference cancellation scheme may be used in closed-loop control of an automated mechanism for maneuvering and operating probe  22 , as described hereinabove, to ensure that the automated mechanism positions distal tip  36  in the proper location. 
     Although the embodiments described herein refer mainly to interference cancellation in medical position tracking systems, the disclosed techniques can be used for canceling position-dependent interference in various other applications. 
     The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limiting to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated. 
     It is intended that the appended claims cover all such features and advantages of the disclosure that fall within the spirit and scope of the present disclosure. As numerous modifications and changes will readily occur to those skilled in the art, it is intended that the disclosure not be limited to the limited number of embodiments described herein. Accordingly, it will be appreciated that all suitable variations, modifications and equivalents may be resorted to, falling within the spirit and scope of the present disclosure.