Abstract:
This invention is an invasive probe apparatus including flexible elongate probe ( 20 ) having a distal portion adjacent to a distal end ( 22 ) thereof for insertion into the body of a subject, which portion assumes a predetermined curve form when a force is applied thereto. First and second sensors ( 28, 30 ) are fixed to the distal portion of the probe ( 20 ) in known positions relative to the distal end ( 22 ), which sensors generate signals responsive to bending of the probe. Signal processing circuitry ( 36 ) receives the bend responsive signals and processes them to find position and orientation coordinates of at least the first sensor ( 28 ), and to determine the locations of a plurality of points along the length of the distal portion of the probe.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Applications Nos. 60/034,703 and 60/034,704, filed Jan. 3, 1997, which are assigned to the assignee of the present patent application and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to cardiac diagnostic and therapeutic systems, and specifically to invasive medical probes that may be used to map the interior surfaces of the heart. 
     BACKGROUND OF THE INVENTION 
     Position-responsive cardiac catheters are known in the art. Such catheters are generally inserted percutaneously and fed through one or more major blood vessels into a chamber of the heart. A position-sensing device in the catheter, typically near the catheter&#39;s distal end, gives rise to signals that are used to determine the position of the device (and hence of the catheter) relative to a frame of reference that is fixed either externally to the body or to the heart itself. The position-sensing device may be active or passive and may operate by generating or receiving electrical, magnetic or ultrasonic energy fields or other suitable forms of energy known in the art. 
     U.S. Pat. No. 5,391,199, which is incorporated herein by reference, describes a position-responsive catheter comprising a miniature sensor coil contained in the catheter&#39;s distal end. The coil generates electrical signals in response to externally-applied magnetic fields, which are produced by field-generator coils placed outside the patient&#39;s body. The electrical signals are analyzed to determine three-dimensional position coordinates of the coil. 
     PCT patent publication number WO96/05768, filed Jan. 24, 1995, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference, describes a position-responsive catheter comprising a plurality of miniature, preferably non-concentric sensor coils fixed in its distal end. As in the U.S. Pat. No. 5,391,199 patent, electrical signals generated by these coils in response to an externally-applied magnetic field are analyzed so as to determine, in a preferred embodiment, six-dimensional position and orientation coordinates of the coils. 
     Multiple position-sensing devices may be placed in a known, mutually-fixed spatial relation at or adjacent to the distal end of a catheter, as described, for example, in PCT patent application no. PCT/IL97/00009, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference. This application describes a catheter having a substantially rigid structure at its distal end, to which one or more position sensors are fixed. The sensors are used to determine the position and orientation of the structure, preferably for use in mapping electrical activity in the heart. Although the structure itself is substantially rigid, the remainder of the catheter is generally flexible, and the position sensors do not provide coordinate information regarding any points on the catheter proximal to the structure. 
     PCT publication WO95/04938, which is also incorporated herein by reference, describes a miniature magnetic field sensor coil and method of remotely determining the coil&#39;s location. The sensor coil may be used to determine the spatial configuration or course of flexible endoscope within the body of a subject in one of two ways: (1) By passing the coil through an internal lumen of the endoscope, for example, the endoscope&#39;s biopsy tube, and externally tracking the coil&#39;s location while the endoscope is held stationary; or (2) By distributing a plurality of the coils, preferably about a dozen, along the length of the endoscope and determining all of the coils&#39; locations. The position coordinates determined with respect to each location of the coil (when a single coil is used) or to all the coils (when the plurality of coils are used) are taken together to interpolatively reconstruct the spatial configuration of the endoscope within the intestines of the subject, for example, and thereby estimate the corresponding spatial configuration of the intestines. 
     The accuracy of this endoscope in estimating the spatial configuration of the intestines depends on having a relatively large number of position measurements and/or of coils. Passing the coil (or other sensor element) through a lumen in the endoscope is time consuming and physically not practical for use in thin probes, such as cardiac catheters that must be passed through blood vessels. Using a large number of coils, however, undesirably increases the weight and cost of the catheter and reduces its flexibility. 
     U.S. Pat. No. 5,042,486, whose disclosure is further incorporated herein by reference, describes a method of locating a catheter within the body of a subject, generally within a blood vessel, by tracking the position of an electromagnetic or acoustic transmitter or receiver in the tip of the catheter. The position readings are registered with a previously acquired X-ray image of the blood vessel. This method is practical, however, only when the catheter is moving within a vessel or other physiological structure that defines a narrow channel within which the catheter&#39;s movement is constrained. 
     PCT publication WO 92/03090, whose disclosure is also incorporated herein by reference, describes a probe system, such as an endoscope, including sensing coils mounted at spaced positions along the probe. An array of antennas in a vicinity of the probe are driven by AC electrical signals, so as to induce corresponding voltage signals in the sensing coils. These signals are analyzed to determine three-dimensional coordinates of the coils. The locations of points along the probe, intermediate a pair of the sensing coils, may be determined by interpolation between the respective coordinates of the coils. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a generally flexible catheter, for insertion into the body of a subject, wherein the course and/or position of the catheter within the body are determined using a minimal number of sensors fixed to the catheter. 
     It is a further object of the present invention to provide a catheter having a distal portion that assumes a predetermined shape or curvature, dependent on a force is applied thereto, and a method of determining the course of the distal portion within the body. 
     In one aspect of the present invention, the entire course of the distal portion is determined by measuring position coordinates of two points on the portion and using the coordinates to find the shape or curvature of the portion. 
     In another aspect of the present invention, the entire course of the distal portion is determined by measuring position coordinates of a point on the portion and measuring the curvature of the portion. 
     It is yet another object of the current invention that the course of the catheter may be determined within body cavities in which the catheter is free to move in three dimensions, and not only within constraining lumens as in the prior art. 
     In preferred embodiments of the present invention, a flexible catheter, having a distal end for insertion into the body of a subject, comprises first and second sensors, fixed at known, respective positions along a generally distal portion of the length of the catheter, in a known relation to one another and to the distal end. The distal portion of the catheter is sufficiently resilient so as to assume a predetermined, curved form when a force is applied thereto. At least one of the sensors is a position sensor, which generates signals responsive to the position coordinates thereof. The outputs of the first and second sensors are processed jointly to determine the curvature of the portion of the catheter, so as to find the positions of a plurality of points along the length of the distal portion, inside the subject&#39;s body. 
     Preferably, the at least one position sensor comprises a magnetic-field-responsive coil, as described in the above-mentioned U.S. Pat. No. 5,391,199 patent, or more preferably, a plurality of such coils, as described in the above-mentioned PCT publication WO96/05768. The plurality of coils enables six-dimensional position and orientation coordinates to be determined. Alternatively, any suitable position sensor known in the art may be used, such as electrical, magnetic or acoustic sensors. 
     In some preferred embodiments of the present invention, both the first and second sensors comprise position sensors, preferably of the type described above with reference to the PCT publication, which allows their six-dimensional coordinates to be determined. The coordinates of the second sensor, relative to those of the first sensor, are determined and taken together with other, known information pertaining to curvature of the catheter. As will be described below, this information is used to find the positions of a plurality of points along the length of the catheter in a vicinity of the first and second sensors. 
     In some of these preferred embodiments, the catheter has an elasticity that is generally constant over at least a portion of its length, for example, due to internal reinforcement of the catheter with a resilient longitudinal member, as is known in the art. In this case, absent significant deformation of the catheter due to external forces, the known position and orientation coordinates of the first and second position-sensing elements, determined as described above, are sufficient to establish the curvature of catheter intermediate the elements. 
     In other preferred embodiments of the present invention, the first sensor comprises a position sensor, as described above, while the second sensor comprises a bend sensor, which generates signals responsive to a bend radius of the catheter in a vicinity thereof. Preferably, the bend sensor comprises one or more piezoelectric sensors, as are known in the art, which generate electrical signals proportional to a force or torque exerted thereon when the catheter bends. Alternatively, the bend sensor may comprise one or more strain sensors, as are known in the art. Further alternatively, the bend sensor may comprise a fiberoptic sensor fixed in the catheter, wherein the bend radius is determined by measuring the loss and/or back-reflection of light in an optical fiber, as is known in the art. 
     Further alternatively, the catheter may include a user-controlled bending mechanism, such as a pull-wire or other mechanism known in the art, or bending mechanisms of other types as described in PCT patent application no. PCT/IL97/00159, which is assigned to the assignee of the present invention, and whose disclosure is incorporated by reference. Preferably, the bending mechanism is calibrated, so that the bend radius of the catheter in a vicinity thereof is known, and is used in determining the positions of the plurality of points along the catheter. 
     In some preferred embodiments of the present invention, the catheter includes physiological sensors, such as electrophysiological sensing electrodes, or, additionally or alternatively, therapeutic devices, such as ablation electrodes, at some or all of the plurality of points along its length. Such embodiments are particularly useful, for example, in diagnosis and treatment of abnormal electrical conduction paths in the heart. Devices and methods for use in accordance with these preferred embodiments are further described in U.S. provisional patent application No. 60/034,704 which is assigned to the assignee of the present patent application, and whose disclosure is incorporated herein by reference. 
     Although preferred embodiments are described herein with reference to certain types of position and orientation sensors, the principles of the present invention may be implemented in catheters including other types and combinations of such sensors, as are known in the art. It is generally unnecessary to determine six-dimensional position and orientation coordinates of the sensors. It is sufficient, for example, that the first position sensor provide five-dimensional position and orientation data (to determine its three-dimensional translational coordinates and two-dimensional rotational azimuth and elevation), and the second position sensor provide three-dimensional position information. Under these conditions, the positions of the plurality of points along the catheter can be determined, as described above. 
     While the preferred embodiments of the present invention are generally described herein with reference to one or two position sensor and/or a single bend sensor, it will be appreciated that the inventive principles that they embody may be similarly applied to catheters, or other probes, having a plurality of position sensors and/or a plurality of bend sensors. Preferably, however, the number of such sensors is held to the minimum needed to achieve the desired accuracy of determination of the plurality of points along the length of the catheter, generally along the portion of the catheter adjacent the distal end thereof. 
     Furthermore, although the preferred embodiments described herein make reference to catheters, and particularly to intracardiac catheters, it will be appreciated that the principles of the present invention may similarly be applied to other types of flexible medical probes, such as endoscopes. 
     There is therefore provided, in accordance with a preferred embodiment of the present invention, invasive probe apparatus including: 
     a flexible, elongate probe, having a distal portion adjacent to a distal end thereof, for insertion into the body of a subject, which portion assumes a predetermined, curved form when a force is applied thereto; 
     first and second sensors, fixed to the distal portion of the probe in known positions relative to the distal end, which sensors generate signals responsive to bending of the probe; and 
     signal processing circuitry, which receives the bend-responsive signals and processes them to find position and orientation coordinates of at least the first sensor and to determine the locations of a plurality of points along the length of the distal portion of the probe. 
     Preferably, the first sensor comprises three coils, which generate signals responsive to an externally-applied magnetic field. 
     Preferably, the probe has a generally constant elasticity over the length of the distal portion thereof and includes a resilient longitudinal member. 
     In some preferred embodiments of the present invention, the second sensor includes a position-sensing element, and the signal processing circuitry processes the signals generated by the second sensor to find position and orientation coordinates thereof. 
     Preferably, the position and orientation coordinates found by the signal processing circuitry include six-dimensional position and orientation coordinates. 
     In other preferred embodiments of the present invention, the second sensor includes a bend-sensing element, which generates signals responsive to a direction of bending of the probe. 
     Preferably, the bend-sensing, element includes at least one piezoelectric crystal, and more preferably, three such crystals, each crystal having an axis, wherein the axes are mutually orthogonal. 
     Alternatively, the bend-sensing element includes a fiberoptic sensor or a strain sensor. 
     Preferably, the signal processing circuitry determines a radius of curvature of the probe or, alternatively or additionally, a radius and a pitch of a helical form described by the probe. 
     Preferably, the probe comprises a deflection device within the distal portion thereof. 
     There is further provided, in accordance with a preferred embodiment of the present invention, a method for determining the course of an elongate, flexible probe inside the body of a subject, including: 
     finding position and orientation coordinates of a point on the probe; 
     measuring a bending angle of a portion of the probe adjacent to the point; and 
     processing the position and orientation coordinates and the bending angle to determine the locations of a plurality of points along the length of a portion of the probe inside the body. 
     Preferably, finding position and orientation coordinates includes finding six-dimensional position and orientation coordinates. 
     Further preferably, measuring a bending angle includes finding position coordinates of an additional point on the probe. 
     Alternatively, measuring a bending angle comprises measuring a force associated with bending the probe. 
     Preferably, processing the position coordinates and the bending angle includes calculating a radius of curvature of the probe or, alternatively or additionally, calculating a radius of a helical path described by the probe. 
     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a bend-responsive catheter system, in accordance with a preferred embodiment of the present invention, 
     FIG. 2A is a schematic illustration of a portion of the catheter shown in FIG. 1, in a first, curved configuration; 
     FIG. 2B is a schematic illustration of a portion of the catheter shown in FIG. 1, in a second, twisted configuration, 
     FIG. 3 is a schematic illustration showing a bend-responsive catheter, in accordance with another preferred embodiment of the present invention; and 
     FIG. 4 is a schematic, partial, sectional illustration showing a bend-responsive catheter, in accordance with another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1, which illustrates a bend-responsive catheter  20 , in accordance with a preferred embodiment of the present invention. Catheter  20  includes a distal end  22 , which is preferably inserted in the heart of a subject, and a proximal end  24 , which is coupled to a control console  26 . 
     Adjacent to distal end  22 , catheter  20  includes a first position-sensing element  28  and, proximal thereto, a second position-sensing element  30 , which serves to enable determination of a bending angle of catheter  20 , as will be described below. Preferably, each of elements  28  and  30  comprises three substantially orthogonal, non-concentric coils, as described in the above-mentioned PCT publication WO96/05768, which generate signals responsive to magnetic fields applied by field generators  32 . These signals are conveyed via wires  34  to signal processing and computing circuitry  36  in console  26 , which preferably also provides driver and control signals to generators  32 . Circuitry  36  analyzes the signals, as further described in the PCT publication, in order to determine the six-dimensional translational and orientational coordinates of elements  28  and  30  in relation to a frame of reference established by generators  32 . 
     Alternatively, it is sufficient that one of elements  28  and  30  comprise three such coils, and that the other of the elements comprise a single coil, as described in the above-mentioned U.S. Pat. No. 5,391,199 patent. As described in the patent, three-dimensional translational coordinates of the single-coil element are determined. 
     Further alternatively, sensors  28  and  30  may comprise other types and combinations of position sensors, known in the art. It is sufficient, for example, that element  28  be such as to enable determination of three-dimensional translational coordinates and two-dimensional angular elevation and azimuth coordinates with respect thereto, while three-dimensional coordinates are determined with respect to element  30 . If bending of catheter  20  is constrained to a plane, as shown in FIG.  2 A and described below, it is sufficient to determine two-dimensional coordinates of element  30 . 
     Catheter  20  preferably includes a resilient longitudinal member  38 , for example, a coil spring element, which is fixed within the catheter along a longitudinal axis thereof. Preferably, there is a sufficient distance between metal parts of member  38  and sensors  28  and  30  so that the metal parts do not significantly distort the magnetic fields at the sensors. Such distortion may be caused, for example, by eddy currents induced in the metal parts or by bending of the magnetic field lines by ferromagnetic materials. On account of member  38 , catheter  20  has a generally constant elasticity over at least a portion  40  of its length, preferably extending at least from element  30 , or from another point proximal thereto, out to distal end  22 , or at least to element  28 . Portion  40  of catheter  20  is preferably short enough, generally less that about 9 cm long, so that it is inserted entirely into a chamber of the heart with no more than a single bend in the portion. As a result, when portion  40  is bent, whereby element  30  is translationally displaced and orientationally rotated by a known angle relative to element  28 , portion  40  will assume an arcuate or helical shape having a known radius of curvature, determined by the known angle. 
     FIG. 2A illustrates, for example, a case in which portion  40  of catheter  20  is bent in a plane, which we take to be the plane of the page without loss of generality. The length of portion  40  is taken to be L, as shown. Respective first and second local coordinate axes  50  (x o ,y o ,z o ) and  52  (x 1 ,y 1 ,z 1 ) are defined at the positions of first and second elements  28  and  30 , wherein the local z-axis is taken in every case taken to be aligned with the longitudinal axis of catheter  20 , generally parallel to member  38 . 
     The six-dimensional position coordinates of first element  28  are determined and used to define the element&#39;s translational position and first local coordinate axes  50 . The orientation coordinates of second element  30  define second local axes  52 , which together with axes  50  determine a bend angle θ, as shown. An arc is thus defined having a radius of curvature given by R=L/θ, and a center of curvature  54  at a position y=R defined with respect to coordinate axes  50  or  52 . The elasticity of member  38  ensures that portion  40  will generally follow this arc, so that the position of any point within portion  40  of catheter  20  may be conveniently determined. 
     FIG. 2B schematically illustrates the more general case, in which catheter  20  is free to twist in three dimensions. In the case shown here, portion  40  of catheter  20  has been twisted about its longitudinal axis by approximately 180°, so that axes x 1  and y 1  of second local axes  52  are oriented in generally opposite respective directions to axes x 0  and y 0  of local axes  50 . The elasticity of member  38  causes portion  40  to assume a generally right-helical form, within the bounds of a cylinder  54  having a diameter R c  and length d, as shown in the figure. The length d is defined by the translational displacement of element  30  relative to element  28 , but determining R c  generally requires solving an integral equation. Preferably, solutions to the equation are stored in the form of a look-up table, preferably within signal processing circuitry  36 , as is known in the art. R c  and d then determine the pitch of the helical form, so that the position of any point within portion  40  of catheter  20  may again be conveniently determined. 
     Preferably, portion  40  of catheter  20  will not be allowed to twist by more than 180° in either the clockwise or counterclockwise direction, so that the relative rotational coordinates of elements  28  and  30  will be unambiguous. If necessary, however, the twist of portion  40  may be continuously monitored, by analyzing the signals received from the elements, as catheter  20  is being inserted into and manipulated inside the body, so that rotations of greater than 180° will be detected. These greater twist angles are then used in appropriately determining R c , as described above. 
     In the preferred embodiments described above, it is assumed that portion  40  of catheter  20  is free to move within a body cavity, and that the shape and configuration of portion  40  are determined substantially by its own elasticity. Portion  40  is caused to bend by a combination of a compressive axial force, generally exerted from proximal end  24  of catheter  20  by a user, such as a physician, and a lateral deflecting force exerted on distal end  22  by body tissue with which the distal end is in contact. 
     FIG. 3 schematically illustrates an alternative preferred embodiment of the present invention, in which catheter  20  bends controllably, not necessarily in an arcuate or helical form, by means of a steering mechanism  56 . Preferably, mechanism  56  comprises an electronically- or mechanically-controlled deflection element, operating under the control of console  26 , as described in the above-mentioned PCT patent application no. PCT/IL97/00159. Alternatively, mechanism  56  may comprise any suitable catheter steering or deflection device known in the art. Catheter  20  is sufficiently rigid, except in an immediate vicinity of mechanism  56 , so as to bend only in the immediate vicinity of the mechanism. The position coordinates of elements  28  and  30  are used to measure the deflection angle θ, whereby the location of any point along portion  40  of catheter  20  may be determined. Preferably, the measured deflection angle is also used to provide feedback for closed-loop control of mechanism  56 . 
     FIG. 4 schematically illustrates another preferred embodiment of the present invention, which is similar to the embodiments described above except that in place of second position-sensing element  30 , catheter  20  as shown here includes a bend sensor  80 , responsive to the angle of bending of the catheter. Bend sensor  80  preferably comprises at least one piezoelectric element, or more preferably, three such elements  82 ,  84  and  86  as shown in the figure. The piezoelectric elements are mechanically coupled to resilient member  38 , so that when member  38  is bent, as described above, the bending force is conveyed to and acts upon the elements. As is known in the art, the piezoelectric crystals generate voltage signals that are generally proportional to this bending force, which signals are conveyed by wires  34  to signal processing circuitry  36  in console  26 . 
     Each of elements  82 ,  84  and  86  includes a piezoelectric crystal having a crystal axis aligned orthogonally to the axes of the other two elements, so that each crystal generates signals responsive to bending of catheter  20  about a different axis. Thus, as shown in FIG. 4, element  82  generates signals responsive to twisting of catheter  20  about its longitudinal axis, and elements  84  and  86  generate signals responsive to left-right and up-down bending, respectively. 
     Due to the generally constant elasticity of member  38 , the signals generated by elements  82 ,  84  and  86  can be used to derive the bend and twist angles of portion  40  of catheter  20 . These angles are taken together with the translational and orientational coordinates determined with respect to position-sensing element  28 , in order to determine the positions of the plurality of points of interest along the length of catheter  20 . 
     Other types of bend sensors may be used in place of sensor  80  shown in FIG.  4 . For example, strain gauges may be substituted-for piezoelectric elements  82 ,  84  and  86 . Such strain gauges have an electrical resistance that varies as a function of mechanical strain applied thereto, as is known in the art. Alternatively, fiberoptic sensors, as are known in the art, may be used to determine the bend angle of catheter  20 , by measuring the loss and back-reflection of light conveyed through an optical fiber embedded in the catheter. 
     Furthermore, additional bend sensors of other types may be positioned at different locations along the length of catheter  20 , so that multiple bends or bends of non-constant radius of curvature can be detected. 
     More generally speaking, while the preferred embodiments of the present invention have been described above with reference to one or two position-sensing elements  28  and  30  and a single bend sensor  80 , it will be appreciated that for some applications, catheter  20  may preferably comprise a greater number of position sensors and/or of bend sensors. Such additional sensors may be particularly useful when a portion of the length of the catheter must be tracked within a convoluted passage, or when the catheter is brought to bear against and is desired to conform to a convoluted surface within a body cavity. Preferably, however, the number of such sensors is held to the minimum needed to achieve the desired accuracy of determination of the plurality of points along the length of the catheter. 
     Although for simplicity of illustration, catheter  20  has been shown and described above as comprising only the sensors and other elements necessary for the operation of the present invention, in preferred embodiments of the present invention, the catheter preferably includes other sensing and/or therapeutic devices, as are known in the art. The principles of the present invention may then be applied, for example, to map physiological activity or apply local therapeutic treatment within a body cavity, such as a chamber of the heart, with greater ease and accuracy than methods and devices known in the art. 
     It will be appreciated that the principles of the present invention may be applied, as well, to other flexible medical probes, such as endoscopes. 
     It will further be appreciated that the preferred embodiments described above are cited by way of example, and the full scope of the invention is limited only by the claims.