Abstract:
A medical probe includes a flexible insertion tube, having a distal end for insertion into a body cavity of a patient, and a distal tip, which is disposed at the distal end of the insertion tube and is configured to be brought into contact with tissue in the body cavity. A resilient member couples the distal tip to the distal end of the insertion tube and is configured to deform in response to pressure exerted on the distal tip when the distal tip engages the tissue. A position sensor within the probe senses a position of the distal tip relative to the distal end of the insertion tube, which changes in response to deformation of the resilient member.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to invasive medical devices, and specifically to methods and devices for sensing pressure exerted against a probe, such as a catheter, inside the body of a patient. 
       BACKGROUND OF THE INVENTION 
       [0002]    Intracardiac radio-frequency (RF) ablation is a well-known method for treating cardiac arrhythmias. Typically, 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. It is important to ensure proper contact between the electrode and the endocardium during ablation in order to achieve the desired therapeutic effect without excessive damage to the tissue. 
         [0003]    Various techniques have been suggested for verifying electrode contact with the tissue. For 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. 
         [0004]    As another example, U.S. Pat. No. 6,241,724, whose disclosure is incorporated herein by reference, describes methods for creating lesions in body tissue using segmented electrode assemblies. In one embodiment, an electrode assembly on a catheter carries pressure transducers, 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. 
         [0005]    A further example is presented in U.S. Pat. No. 6,915,149, whose disclosure is incorporated herein by reference. This patent describes a method for mapping a heart using a catheter having a tip electrode for measuring the 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. 
         [0006]    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. 
       SUMMARY OF THE INVENTION 
       [0007]    The embodiments of the present invention that are described hereinbelow provide a novel design of an invasive probe, such as a catheter, as well as systems and methods making use of such a probe. The design is particularly useful in achieving and verifying proper contact between the distal tip of the probe and tissue that the probe engages inside the body. 
         [0008]    In some embodiments, the probe comprises a flexible insertion tube, having a distal end for insertion into a body cavity of a patient. The distal tip of the probe is coupled to the distal end of the insertion tube by a resilient member, such as a spring, which deforms in response to pressure exerted on the distal tip when it engages the tissue. A position sensor within the probe senses the position of the distal tip relative to the distal end of the insertion tube, which is indicative of deformation of the resilient member, and is thus able to give an indication of the pressure. 
         [0009]    In a disclosed embodiment, the sensor may comprise a magnetic field sensor in the distal tip, and the probe may thus be used as part of a system that determines the coordinates of the distal tip within the body using magnetic fields. For this purpose, a first magnetic field generator, disposed outside the body of the patient, generates a magnetic field within the body. The distal end of the insertion tube contains a second (typically much smaller) magnetic field generator. The sensor in the distal tip generates signals responsively to the magnetic fields of both the first and second field generators. These signals are processed both to determine coordinates of the distal tip within the body and to detect changes in the position of the distal tip relative to the distal end of the insertion tube, which are indicative of deformation of the resilient member and hence of the pressure exerted on the distal tip. 
         [0010]    Alternatively, the distal tip may contain a magnetic field generator, and the field that it generates may be measured by sensors in the distal end of the insertion tube and outside the body for the purposes of detection of sensing pressure on and position coordinates of the distal tip. 
         [0011]    There is therefore provided, in accordance with an embodiment of the present invention, a medical probe, including: 
         [0012]    a flexible insertion tube, having a distal end for insertion into a body cavity of a patient; 
         [0013]    a distal tip, which is disposed at the distal end of the insertion tube and is configured to be brought into contact with tissue in the body cavity; 
         [0014]    a resilient member, which couples the distal tip to the distal end of the insertion tube and is configured to deform in response to pressure exerted on the distal tip when the distal tip engages the tissue; and 
         [0015]    a position sensor within the probe for sensing a position of the distal tip relative to the distal end of the insertion tube, which changes in response to deformation of the resilient member. 
         [0016]    In disclosed embodiments, the position sensor is configured to generate a signal indicative of an axial displacement and an orientation of the distal tip relative to the distal end of the insertion tube. In some embodiments, the position sensor is configured to generate the signal responsively to a magnetic field that is generated in a vicinity of the distal tip. In one embodiment, the position sensor is disposed in the distal end of the insertion tube, and the probe includes a magnetic field generator within the distal tip for generating the magnetic field. In another embodiment, the position sensor is disposed in the distal tip, and the probe includes a magnetic field generator within the distal end of the insertion tube for generating the magnetic field. Typically, the position sensor and the magnetic field generator include coils. 
         [0017]    In one embodiment, the resilient member includes a spring, and the position sensor is configured to generate a signal, responsively to the deformation, which is indicative of the pressure exerted on the distal tip. 
         [0018]    In a disclosed embodiment, the distal tip includes an electrode, which is configured to make electrical contact with the tissue, wherein the electrode is coupled to apply electrical energy to the tissue so as to ablate a region of the tissue. 
         [0019]    There is also provided, in accordance with an embodiment of the present invention, apparatus for performing a medical procedure inside a body of a patient, the apparatus including: 
         [0020]    a first magnetic field generator, for disposition outside the body of the patient, for generating a first magnetic field within the body; 
         [0021]    a probe, which includes:
       an insertion tube having a distal end for insertion into a body cavity of a patient;   a second magnetic field generator within the distal end of the insertion tube for generating a second magnetic field;   a distal tip, which is flexibly coupled to the distal end of the insertion tube; and   a sensor, which is disposed within the distal tip and is configured to generate first and second signals responsively to the first and second magnetic fields, respectively; and       
 
         [0026]    a processor, which is coupled to receive and process the first signal so as to determine coordinates of the distal tip within the body and to receive and process the second signal so as to detect changes in a position of the distal tip relative to the distal end of the insertion tube. 
         [0027]    In some embodiments, the distal tip is rigid, and the probe includes a resilient member, which couples the distal tip to the distal end of the insertion tube. Typically, the resilient member is configured to deform in response to pressure exerted on the distal tip when the distal tip engages tissue inside the body, and the changes in the position of the distal tip are indicative of deformation of the resilient member, while the processor is configured to generate, responsively to the deformation, an output that is indicative of the pressure exerted on the distal tip. Optionally, the processor may be configured to generate a control input for automatically controlling motion of the probe within the body cavity responsively to the first and second signals. 
         [0028]    There is additionally provided, in accordance with an embodiment of the present invention, a method for contacting tissue in a body cavity of a patient, the method including: 
         [0029]    inserting a probe into the body cavity, the probe including a flexible insertion tube and a distal tip, which is coupled to a distal end of the insertion tube by a resilient member, and including a position sensor, which generates a signal indicative of a position of the distal tip relative to the distal end of the insertion tube, which changes in response to deformation of the resilient member; 
         [0030]    advancing the probe within the body cavity so that the distal tip engages and applies a pressure against the tissue, thereby causing the resilient member to deform; and 
         [0031]    processing the signal while the distal tip engages the tissue so as to provide an indication of the pressure. 
         [0032]    In a disclosed embodiment, advancing the probe includes bringing an electrode on the distal tip into electrical contact with the tissue, and the method includes applying electrical energy to the electrode so as to ablate a region of the tissue that is engaged by the distal tip. Applying the electrical energy may include controlling application of the energy responsively to the indication of the pressure, so that the electrical energy is applied to the electrode when the pressure is within a desired range. 
         [0033]    There is further provided, in accordance with an embodiment of the present invention, apparatus for performing a medical procedure inside a body of a patient, the apparatus including: 
         [0034]    a probe, which includes: 
         [0035]    an insertion tube having a distal end for insertion into a body cavity of a patient; 
         [0036]    a distal tip, which is flexibly coupled to the distal end of the insertion tube; 
         [0037]    a magnetic field generator, which is disposed within the distal tip and is configured to generate a magnetic field; and 
         [0038]    a first sensor within the distal end of the insertion tube for generating a first signal in response to the magnetic field; and 
         [0039]    a second sensor, for disposition outside the body of the patient, for generating a second signal in response to the magnetic field; 
         [0040]    a processor, which is coupled to receive and process the second signal so as to determine coordinates of the distal tip within the body and to receive and process the first signal so as to detect changes in a position of the distal tip relative to the distal end of the insertion tube. 
         [0041]    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 
         [0042]      FIG. 1  is a schematic, pictorial illustration of a catheter-based medical system, in accordance with an embodiment of the present invention; 
           [0043]      FIG. 2  is a schematic, cutaway view showing details of the distal end of a catheter, in accordance with an embodiment of the present invention; and 
           [0044]      FIG. 3  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. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0045]      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. 
         [0046]    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. 
         [0047]    Console  34  uses magnetic position sensing to determine position coordinates of distal end  30  inside heart  22 . For this purpose, 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  (shown in  FIG. 2 ) 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. 
         [0048]    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 the catheter and other components of system  20 , processor  36  drives a display  42  to give operator  26  visual feedback regarding the position of distal end  30  in the patient&#39;s body, as well as status information and guidance regarding the procedure that is in progress. 
         [0049]    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. These signals are indicative of both the position of the distal end of the catheter and of force exerted on the distal end, as explained further hereinbelow. 
         [0050]      FIG. 2  is a schematic, cutaway view of distal end  30  of catheter  28 , showing details of the structure of the catheter in accordance with an embodiment of the present invention. Catheter  28  comprises a flexible insertion tube  54 , with a distal tip  52  connected to the distal end of tube  54  at a joint  56 . The insertion tube is covered by a flexible, insulating material  60 , such as Celcon® or Teflon®. The area of joint  56  is covered, as well, by a flexible, insulating material, which may be the same as material  60  or may be specially adapted to permit unimpeded bending and compression of the joint, (This material is cut away in  FIG. 2  in order to expose the internal structure of the catheter.) Distal tip  52  may be covered, at least in part, by an electrode  50 , which is typically made of a metallic material, such as a platinum/iridium alloy. Alternatively, other suitable materials may be used, as will be apparent to those skilled in the art. Further alternatively, the distal tip may be made without a covering electrode. The distal tip is typically relatively rigid, by comparison with the flexible insertion tube. 
         [0051]    Distal tip  52  is connected to the distal end of insertion tube  54  by a resilient member  58 . In  FIG. 2 , the resilient member has the form of a coil spring, but other types of resilient components may alternatively be used for this purpose. For example, resilient member  58  may comprise a polymer, such as silicone, polyurethane, or other plastics, with the desired flexibility and strength characteristics. Resilient member  58  permits a limited range of relative movement between tip  52  and insertion tube  54  in response to forces exerted on the distal tip. Such forces are encountered when the distal tip is pressed against the endocardium during an ablation procedure. The desired pressure for good electrical contact between the distal tip and the endocardium during ablation is on the order of 20-30 grams. The spring serving as the resilient member in this embodiment may be configured, for example, to permit axial displacement (i.e., lateral movement along the axis of catheter  28 ) of the distal tip by about 1-2 mm and angular deflection of the distal tip by up to about 30° relative to the distal end of the insertion tube, in response to the desired pressure. 
         [0052]    As noted above, distal tip  52  contains a magnetic position sensor  62 . Sensor  62  may comprise one or more miniature coils, and typically comprises multiple coils oriented along different axes. Alternatively, sensor  62  may comprise another type of magnetic sensor, such as a Hall effect or magnetoresistive sensor, for example. The magnetic fields created by field generators  32  cause these coils to generate electrical signals, with amplitudes that are indicative of the position and orientation of sensor  62  relative to the fixed frame of reference of field generators  32 . Processor  36  receives these signals via wires (not shown in the figures) running through catheter  28 , and processes the signals in order to derive the location and orientation coordinates of distal tip  52  in this fixed frame of reference, as described in the patents and patent applications cited above. 
         [0053]    In addition, insertion tube  54  contains a miniature magnetic field generator  64  near the distal end of the insertion tube. Typically, field generator  64  comprises a coil, which is driven by a current conveyed through catheter  28  from console  34 . The current is generated so as to create a magnetic field that is distinguishable in time and/or frequency from the fields of field generators  32 . For example, the current to field generator  64  may be generated at a selected frequency in the range between about 16 kHz and 25 kHz, while field generators  32  are driven at different frequencies. Additionally or alternatively, the operation of generators  32  and  64  may be time-multiplexed. 
         [0054]    The magnetic field created by field generator  64  causes the coils in sensor  62  to generate electrical signals at the drive frequency of field generator  64 . The amplitudes of these signals will vary depending upon the location and orientation of distal tip  52  relative to insertion tube  54 . Processor  36  processes these signals in order to determine the axial displacement and the magnitude of the angular deflection of the distal tip relative to the insertion tube. (Because of the axial symmetry of the field generated by a coil, only the magnitude of the deflection can be detected using a single coil in field generator  64 , and not the direction of the deflection. Optionally, field generator  64  may comprise two or more coils, in which case the direction of deflection may be determined, as well.) The readings of displacement and deflection are typically accurate to within a few tenths of a millimeter and about one degree, respectively. The magnitudes of the displacement and deflection may be combined by vector addition to give a total magnitude of the movement of distal tip  52  relative to the distal end of insertion tube  54 . 
         [0055]    The relative movement of the distal tip relative to the distal end of the insertion tube gives a measure of the deformation of resilient member  58 . Generally speaking, this deformation is proportional to the force that is exerted on the resilient member, which is roughly equal to the force that is exerted on the distal tip by the heart tissue with which the distal tip is in contact. Thus, the combination of field generator  64  with sensor  62  serves as a pressure sensing system, for determining the approximate pressure exerted by the endocardial tissue on the distal tip of the catheter (or equivalently, the pressure exerted by electrode  50  against the endocardial tissue). By virtue of the combined sensing of displacement and deflection, this pressure sensing system reads the pressure correctly regardless of whether the electrode engages the endocardium head-on or at an angle. The pressure reading is insensitive to temperature variations and free of drift, unlike piezoelectric sensors, for example. 
         [0056]      FIG. 3  is a schematic detail view showing distal end  30  of catheter  28  in contact with endocardium  70  of heart  22 , in accordance with an embodiment of the present invention. Pressure exerted by the distal tip against the endocardium deforms the endocardial tissue slightly, so that electrode  50  contacts the tissue over a relatively large area. Since the electrode engages the endocardium at an angle, rather than head-on, distal tip  52  bends at joint  56  relative to the insertion tube of the catheter. The bend facilitates optimal contact between the electrode and the endocardial tissue. 
         [0057]    Processor  36  receives and processes the signals generated by sensor  62  in response to the magnetic field of generator  64 , in order to derive an indication of the pressure exerted by distal tip  52  on endocardium  70 . As noted earlier, for good ablation, pressure of about 20-30 grams is desirable. Lower pressure means that there may be inadequate contact between electrode  50  and the endocardial tissue. As a result, much or all of the RF energy may be carried away by the blood inside the heart, and the tissue will be ablated inadequately or not at all. Higher pressure means that the electrode is pressing too hard against the endocardial tissue. Excessive pressure of this sort may cause severe cavitation in the tissue, leading to extensive tissue damage and possibly even perforation of the heart wall. 
         [0058]    To avoid these eventualities, console  34  outputs an indication of the pressure measured using sensor  62  to operator  26 , and may issue an alarm if the pressure is too low or too high. Optionally, RF generator  40  may be interlocked, so as to supply RF power to electrode  50  only when the pressure against the tissue is in the desired range. Alternatively or additionally, the pressure indication may be used in closed-loop control of an automated mechanism for maneuvering and operating catheter  28 , as described hereinabove, to ensure that the mechanism causes the distal end of the catheter to engage the endocardium in the proper location and with the appropriate pressure against the tissue. 
         [0059]    In an alternative embodiment, the roles of sensor  62  and magnetic field generators  32  and  64  may be reversed. In other words, driver circuit  38  may drive a magnetic field generator in distal tip  52  to generate one or more magnetic fields. The coils in generators  32  and  64  may be configured to sense and generate signals indicative of the amplitudes of the components of these magnetic fields. Processor  36  receives and processes these signals in order to determine the pressure of the distal tip against the tissue and the position coordinates of the distal tip within the heart. 
         [0060]    Although the operation of sensor  62  and field generator  64  in sensing pressure is described above in the context of catheter-based ablation, the principles of the present invention may similarly be applied in other therapeutic and diagnostic applications that use invasive probes, both in the heart and in other organs of the body. As one example, the devices and techniques for position and pressure sensing that are implemented in system  20  may be applied, mutatis mutandis, in guiding and controlling the use of a catheter insertion sheath. If the position of the sheath is not properly controlled and excessive force is used in its insertion, the sheath may perforate the heart wall or vascular tissue. This eventuality can be avoided by sensing the position of and pressure on the distal tip of the sheath. In this regard, the term “distal tip” as used herein should be understood to include any sort of structure at the distal end of a probe that may be bent and/or displaced relative to the main body of the probe. 
         [0061]    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.