Abstract:
A method for displaying information includes receiving a measurement with respect to an invasive probe inside a body of a subject of at least one probe parameter, selected from a group of parameters consisting of a bend angle of the probe and a pressure on the probe. Responsively to the measurement, an icon is displayed on a display screen representing the at least one probe parameter for viewing by an operator of the probe.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to invasive medical devices, and specifically to methods and devices for displaying characteristics of a probe, such as a catheter, inside the body of a patient. 
     BACKGROUND OF THE INVENTION 
     In some diagnostic and therapeutic techniques, a catheter is inserted into a chamber of the heart and brought into contact with the inner heart wall. In such procedures, it is generally important that the distal tip of the catheter engages the endocardium with sufficient pressure to ensure good contact. Excessive pressure, however, may cause undesired damage to the heart tissue and even perforation of the heart wall. 
     For example, in intracardiac radio-frequency (RF) ablation, a catheter having an electrode at its distal tip is inserted through the patient&#39;s vascular system into a chamber of the heart. The electrode is brought into contact with a site (or sites) on the endocardium, and RF energy is applied through the catheter to the electrode in order to ablate the heart tissue at the site. Proper contact between the electrode and the endocardium during ablation is necessary in order to achieve the desired therapeutic effect without excessive damage to the tissue. 
     A number of patent publications describe catheters with integrated pressure sensors for sensing tissue contact. As one example, U.S. Patent Application Publication 2007/0100332, whose disclosure is incorporated herein by reference, describes systems and methods for assessing electrode-tissue contact for tissue ablation. An electro-mechanical 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. 
     As another example, U.S. Pat. No. 6,695,808, whose disclosure is incorporated herein by reference, 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. 
     Other catheters with pressure sensors are described in U.S. Pat. No. 6,241,724 and U.S. Pat. No. 6,915,149, whose disclosures are incorporated herein by reference. 
     PCT International Publication WO 2007/067938, whose disclosure is incorporated herein by reference, describes a method for displaying catheter electrode-tissue contact in an electro-anatomic mapping and navigation system. The system provides an indication to the physician concerning the electrical coupling of an electrode, such as an ablative or mapping electrode, with a patient. The indication may be provided by changing the color or other display characteristics of the electrode on the navigation system display or by way of providing a waveform indicating the electrode coupling. This manner of providing electrode coupling information is said to minimize physician distraction. 
     SUMMARY OF THE INVENTION 
     The embodiments of the present invention that are described hereinbelow provide novel means and methods for displaying parameters associated with the quality of engagement between an invasive probe and tissue within the body of a subject. This sort of display can assist the operator of the probe in visualizing the situation of the probe and thus in ensuring the effectiveness and safety of diagnostic and/or therapeutic procedures that are performed using the probe. 
     An embodiment of the present invention provides a method for displaying information. The method includes receiving a measurement with respect to an invasive probe inside a body of a subject of at least one probe parameter, selected from a group of parameters consisting of a bend angle of the probe and a pressure on the probe. Responsively to the measurement, an icon is displayed on a display screen representing the at least one probe parameter for viewing by an operator of the probe. 
     In some embodiments, the probe includes a distal tip that bends at a resilient joint, and receiving the measurement includes measuring a deformation of the joint due to engagement of tissue in the body by the distal tip. In a disclosed embodiment, the probe includes a catheter, which is inserted into a chamber of a heart of the subject and engages myocardial tissue. The method may include applying energy via the distal tip so as to ablate the myocardial tissue, wherein the operator controls application of the energy responsively to the icon. Additionally or alternatively, measuring the deformation includes sensing both the deformation of the joint and a position of the probe within the body by transmitting and receiving one or more magnetic fields. 
     Typically, displaying the icon includes positioning the icon on the display screen so as to represent a location of the probe within the body. Positioning the icon may include locating the icon on the display screen relative to a map of a surface of an organ of the body, and wherein receiving the measurement includes measuring the pressure between the probe and the surface. 
     In a disclosed embodiment, displaying the icon includes presenting the icon so as to show both the bend angle and the pressure, wherein the icon is articulated to show the bend angle and is colored to represent the pressure. 
     There is also provided, in accordance with an embodiment of the present invention, apparatus for performing a medical procedure, including a display screen arranged for viewing by an operator of an invasive probe inside a body of a subject. A processor is coupled to receive a measurement with respect to the invasive probe of at least one probe parameter, selected from a group of parameters consisting of a bend angle of the probe and a pressure on the probe, and to display an icon on the display screen, responsively to the measurement, representing the at least one probe parameter. 
     There is additionally provided, in accordance with an embodiment of the present invention, a computer software product, including a computer-readable medium in which program instructions are stored, which instructions, when read by a computer, cause the computer to receive a measurement with respect to an invasive probe inside a body of a subject of at least one probe parameter, selected from a group of parameters consisting of a bend angle of the probe and a pressure on the probe, and to display, responsively to the measurement, an icon on a display screen representing the at least one probe parameter for viewing by an operator of the probe. 
     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 
         FIG. 1  is a schematic, pictorial illustration of a catheter-based medical system, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic detail view showing the distal tip of a catheter in contact with endocardial tissue, in accordance with an embodiment of the present invention; and 
         FIG. 3  is a schematic representation of a display screen including an icon corresponding to a catheter tip, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic, pictorial illustration of a system  20  for cardiac catheterization, in accordance with an embodiment of the present invention. System  20  may be based, for example, on the CARTO® system, produced by Biosense Webster Inc. (Diamond Bar, Calif.). This system comprises an invasive probe in the form of a catheter  28  and a control console  34 . In the embodiment described hereinbelow, it is assumed that catheter  28  is used in ablating endocardial tissue, as is known in the art. Alternatively, the catheter may be used mutatis mutandis, for other therapeutic and/or diagnostic purposes in the heart or in other body organs. 
     An operator  26 , such as a cardiologist, inserts catheter  28  through the vascular system of a patient  24  so that a distal end  30  of the catheter enters a chamber of the patient&#39;s heart  22 . The operator advances the catheter so that the distal tip of the catheter engages endocardial tissue at a desired location or locations. Catheter  28  is typically connected by a suitable connector at its proximal end to console  34 . The console comprises a radio frequency (RF) generator  40 , which supplies high-frequency electrical energy via the catheter for ablating tissue in the heart at the locations engaged by the distal tip, as described further hereinbelow. Alternatively, the catheter and system may be configured to perform ablation by other techniques that are known in the art, such as cryo-ablation. Further alternatively or additionally, the catheter and system may be used to perform other sorts of therapeutic and/or diagnostic procedures, such as electro-anatomical mapping. 
     Console  34  uses a position sensing technique to determine position coordinates of distal end  30  of catheter  28  inside heart  22 . In the present embodiment, it is assumed that the console uses magnetic position sensing, which is also used in deriving angle and pressure information with respect to the distal end, as described further hereinbelow. Alternatively or additionally, the principles of the present invention may be applied using other position sensing and pressure sensing techniques, as are known in the art. 
     For the purpose of magnetic position sensing, a driver circuit  38  in console  34  drives field generators  32  to generate magnetic fields within the body of patient  24 . Typically, the field generators comprise coils, which are placed below the patient&#39;s torso at known positions external to the patient. These coils generate magnetic fields in a predefined working volume that contains heart  22 . A magnetic field sensor within distal end  30  of catheter  28  (not shown in the figures) generates electrical signals in response to these magnetic fields. A signal processor  36  processes these signals in order to determine the position coordinates of the distal end, typically including both location and orientation coordinates. This method of position sensing is implemented in the above-mentioned CARTO system and is described in detail 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 Patent Publication WO 96/05768, 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. 
     Processor  36  typically comprises a general-purpose computer, with suitable front end and interface circuits for receiving signals from catheter  28  and controlling the other components of console  34 . The processor may be programmed in software to carry out the functions that are described herein. The software may be downloaded to console  34  in electronic form, over a network, for example, or it may be provided on tangible media, such as optical, magnetic or electronic memory media. Alternatively, some or all of the functions of processor  36  may be carried out by dedicated or programmable digital hardware components. 
     Based on the signals received from catheter  28  and other components of system  20 , processor  36  drives a display  42  to give operator  26  visual feedback regarding distal end  30  of catheter  28  in the patient&#39;s body, as well as status information and guidance regarding the procedure that is in progress. The visual feedback shows the pressure on the distal end, as well as the bend angle of the distal tip of the catheter, as is described further hereinbelow with reference to  FIG. 3 . 
     Alternatively or additionally, system  20  may comprise an automated mechanism for maneuvering and operating catheter  28  within the body of patient  24 . Such mechanisms are typically capable of controlling both the longitudinal motion (advance/retract) of the catheter and transverse motion (deflection/steering) of the distal end of the catheter. Some mechanisms of this sort use DC magnetic fields for this purpose, for example. In such embodiments, processor  36  generates a control input for controlling the motion of the catheter based on the signals provided by the magnetic field sensor in the catheter. As noted earlier, these signals are indicative of both the position of the distal end of the catheter and force exerted on the distal end. In this case, the pressure and bend angle shown on display  42  may be used by a human operator in monitoring the status and progress of the automated procedure. 
       FIG. 2  is a schematic sectional view of a chamber of a heart  22 , showing distal end  30  of catheter  28  inside the heart, in accordance with an embodiment of the present invention. The catheter comprises an insertion tube  60 , which is typically inserted into the heart percutaneously through a blood vessel, such as the vena cava or the aorta. An electrode  50  on a distal tip  52  of the catheter engages endocardial tissue  70 . Pressure exerted by the distal tip against the endocardium deforms the endocardial tissue locally, so that electrode  50  contacts the tissue over a relatively large area. In the pictured example, the electrode engages the endocardium at an angle, rather than head-on. Distal tip  52  therefore bends at a resilient joint  56  relative to the distal end of insertion tube  60  of the catheter. The bend may facilitate optimal contact between the electrode and the endocardial tissue. 
     Because of the elastic quality of joint  56 , the angle of bending and the axial displacement of the joint are proportional to the pressure exerted by tissue  70  on distal tip  52  (or equivalently, the pressure exerted by the distal tip on the tissue). Measurement of the deformation of the joint, in terms of bend angle and axial displacement, thus gives an indication of this pressure. The pressure indication may be used by operator  26  of system  20  in ensuring that the distal tip is pressing against the endocardium firmly enough to give the desired therapeutic or diagnostic result, but not so hard as to cause undesired tissue damage. 
     Various techniques may be used in measuring the bend angle and pressure exerted on distal tip  52 . Components and methods that may be used for this purpose are described, for example, in U.S. patent application Ser. No. 11/868,733, filed Oct. 8, 2007, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference. This patent application describes a catheter whose distal tip is coupled to the distal end of the catheter insertion tube by a spring-loaded joint (such as joint  56 ), which deforms in response to pressure exerted on the distal tip when it engages tissue. A magnetic position sensing assembly within the probe, comprising coils on opposite sides of the joint, senses the position of the distal tip relative to the distal end of the insertion tube. Changes in this relative position are indicative of deformation of the spring and thus give an indication of the pressure. 
     Joint  56  may comprise a superelastic coupling member, as described in U.S. patent application Ser. No. 12/134,592, filed Jun. 6, 2008. Alternatively, the coupling member may comprise a coil spring or any other suitable sort of resilient component with the desired flexibility and strength characteristics. U.S. patent application Ser. No. 12/327,226, filed Dec. 3, 2008, describes an arrangement of magnetic coils within the distal end of the catheter that can be used in sensing the tip angle and pressure with enhanced accuracy. Both of these two patent applications are assigned to the assignee of the present patent application, and their disclosures are incorporated herein by reference. 
       FIG. 3  is a schematic representation of a map  80  of a heart chamber, which includes an icon  84  corresponding to distal end  30  of catheter  28 , in accordance with an embodiment of the present invention. A map of this sort is typically presented on display  42 , as an aid to operator  26  in visualizing the distal end of the catheter within heart  22 . The map includes a graphical representation of an inner surface  82  of the heart chamber in which the distal end of the catheter is located. (Surface  82  may be fully reconstructed, as shown in  FIG. 3 , or only partially reconstructed.) The position of icon  84  relative to surface  82  gives the operator an indication of the location of the actual distal end of the catheter in the heart chamber. 
     Icon  84  shows not only the location of distal end  30 , but also angular and pressure characteristics. In the example shown in  FIG. 3 , the icon is articulated to show the measured bend angle of distal tip  52  relative to insertion tube  60 . If the operator sees that the distal tip of the catheter is sharply bent, for example, he or she may readjust the position of the catheter before continuing with a diagnostic or therapeutic procedure, such as ablating the heart tissue. 
     Furthermore, at least a portion  86  of the icon may be colored (represented in the figure by hatching) to indicate the pressure. For example, green coloring may indicate that the pressure is within the correct pressure range for RF ablation, while red indicates too much pressure, and blue indicates too little. The pressure ranges may be preset, or they may be adjusted by the operator. In either case, the operator will then apply the RF energy only when the pressure is within the range that will give the desired therapeutic result. 
     The graphical display of pressure and bend angle by icon  84  gives the operator additional visual information that is not provided by display techniques that are known in the art. This additional information can be useful as a complement to or in place of measurement of electrode/tissue electrical contact resistance. The pressure and/or angle display itself is important, for example, under the following circumstances:
         When touching scarred myocardium, the electrical contact resistance will not accurately reflect pressure, and therefore direct pressure measurement is needed.   When the catheter touches the heart wall sideways (along the length of the catheter), the electrical contact resistance may be low, because the contact area is large, even though the pressure exerted by the catheter on the heart wall is low. The pressure and/or angle display of  FIG. 3  allows the operator to detect and rectify this sort of situation.   Similarly, when a catheter touches a trabeculated wall, the electrical contact resistance may be low even if little or no pressure is applied. Direct pressure measurement enables the operator to detect and rectify this sort of situation, as well.       

     Although icon  84  in  FIG. 3  represents both tip angle and pressure parameters, in addition to location of the catheter tip, the examples above show that it can be useful to display either the angle or the pressure by itself. Alternatively or additionally, one or both of the angle and pressure measurements may be displayed together with a measurement of electrical contact resistance or other parameters. Furthermore, although  FIG. 3  shows a particular mode of graphical representation, other techniques for displaying angle and pressure data will be apparent to those skilled in the art and are considered to be within the scope of the present invention. The display techniques that are described or suggested hereinabove may be used not only in cardiac catheterization procedures, but also in other types of invasive diagnostic and therapeutic applications. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and 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 subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.