WO2001012093A1

WO2001012093A1 - Method and system for navigating a catheter toward a target in a body cavity

Info

Publication number: WO2001012093A1
Application number: PCT/US2000/021669
Authority: WO
Inventors: Pinhas Gilboa
Original assignee: Super Dimension Ltd.; Friedman, Mark, M.
Priority date: 1999-08-16
Filing date: 2000-08-09
Publication date: 2001-02-22
Also published as: AU6531500A; WO2001012093A9

Abstract

A system and method for navigating a catheter (10) toward a target in a body cavity (22). A local motion reference frame is established for the catheter (10) by effecting incremental elementary displacements of the tip (24) while measuring the disposition (position, and optionally orientation) of the tip (24).

Description

METHOD AND SYSTEM FOR NAVIGATING A CATHETER TOWARD A

TARGET IN A BODY CAVITY

FIELD AND BACKGROUND OF THE INVENTION The present invention relates to biomedical devices and, more particularly, to a method of and system for navigation of a catheter toward a target within a body cavity.

Catheters are employed for executing numerous procedures within a patient's body or a body cavity thereof. For example, catheters are employed in various procedures to record heart electrophysiological records from within the heart and to ablate selected portions of the heart tissue. Such ablation must be performed with maximal precision, so as to minimize unnecessary damage and risk to the patient.

Mechanisms for steering catheters towards targets in a patient's body are taught, for example, in U. S. Patents Nos. 5,395,327 and 5,531,686, to Lundquist et al., in U. S. Patent No. 5,456,664, to Heinzelman et al., in U. S. Patent No. 5,728,144, to Edwards et al., and in PCT application WO 97/42996.

PCT application WO 00/16684, which is incorporated by reference as if fully set forth herein, teaches a system and method which enable to simultaneously obtain location data of a treated body, of a catheter inserted into the body and of an imaging instrument used to image the catheter and the body, to thereby record and display in context of the image the location of at least one point-of-interest in the body even when the relative location between any of the above locatable items is changed.

Thus, the above system and method can be used to manually navigate a catheter to a target or point-of-interest within the body. However, for higher precision automatically navigating and moving the catheter is preferred.

In U. S. Patent No. 5,492,131, a closed-loop servo-mechanism is disclosed, including a position sensor, for guiding a catheter along a prescribed course of travel through branched bodily passages. The described servo-mechanism operates on the assumption that there is a direct relationship (a well-determined transfer function) between the driving mechanism and the resulting movements of the catheter tip. The disclosed mechanism is significantly limited with respect to navigating and moving a catheter through an actual cavity, having content and boundary constraints, in search of a target. The disclosure fails to include an enabling description of how the closed-loop operation of this type of mechanism provides flexibility and deflection of the catheter at any given instant along the path to the target.

U.S. Patent No. 5,711,299 to Manwaring et al. discloses a surgical guidance method and system for approaching a target within a body. The invention is based on magnetically detecting and tracking positions of a surgical instrument relative to a trajectory as the instrument approaches a surgical target within the body. The detection system is based on an overall two-point, instrument-target, global environment, to be used for free-hand guiding a rigid surgical instrument to the selected target. The invention is limited to providing means of automatically detecting and monitoring the instrument, while leaving movements of the instrument inside the body dependent upon the skill and accuracy of manual means.

In U.S. Patent No. 5,876,325, Mizuno et al. describe a surgical manipulator system including a surgical manipulator featuring rigid arms and multi -joints, a surgical device, and a guide. A detector detects the geometric relationship between the surgical device and the guide. Driven by the surgical manipulator, the guide guides the surgical device to a bodily location where an operation is to take place using the surgical device.

The prior art clearly fails to teach automatic navigation and moving methods for flexible catheters, especially those catheters having a bendable distal portion. The reason for that is the fact that such catheters are not amenable to close-loop navigation and movement as is effected by a servo mechanism because their response to a movement signal is not a function solely depending on the signal's magnitude. Thus, one cannot determine the signal required for achieving a certain movements response. This is especially the case in flexible catheters having a bendable distal portion, because such a portion may acquire any number of configurations. Thus, for example, if the catheter is extended, reaching a target in front of its tip is effected by pushing, whereas, if the catheter distal portion acquires a U shape, reaching a target in front of its tip is effected by pulling. The flexibility itself adds complexity, because the response of flexible elements to a movement signal depends to a great extent on non-predictable interaction with the surrounding environment. Essentially, the surrounding environment introduces sufficient uncertainty to the transfer function of the servomechanism to render the servomechanism useless for navigation. Thus, as already mentioned, close-loop servo mechanisms are at all not applicable for navigating and moving flexible catheters, especially those catheters having a bendable distal portion. There is thus a need for, and it would be highly advantageous to have, a method of and a system for automatically navigating a catheter to a target within a body cavity, which enable highly accurately incremental navigating and moving of the catheter to the target according to an open-loop format, including a navigating system and a moving system which are mechanized and automatically controllable by a central controller such as a computer.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method of navigating a catheter toward a target within a body cavity, the catheter including a distal end having a tip, the method including the steps of: (a) displacing the tip of the catheter while measuring a plurality of dispositions of the tip of the catheter; (b) from the plurality of dispositions, inferring a local motion reference frame for the catheter; and (c) moving the tip of the catheter toward the target in accordance with the reference frame. According to the present invention there is provided a system for navigating a catheter toward a target within a body cavity, the catheter including a distal end having a tip, the system including: (a) a navigation subsystem for measuring a disposition of the tip of the catheter; (b) an actuating mechanism for moving the tip of the catheter; and (c) a control subsystem for inferring, from a plurality of the dispositions that are measured using the navigation subsystem in coordination with a corresponding at least one displacement of the tip of the catheter that is effected using the actuating mechanism, a local motion reference frame for the catheter.

According to the present invention there is provided a catheter, including: (a) a proximal portion; (b) a distal portion including a tip; and (c) at least two wires running from the tip towards the proximal portion.

The term "disposition", as used herein, refers to the position and orientation, separately or jointly, of an object in space. Thus, depending on the context, the "disposition" of the tip of a catheter preferably may be either only the position of the tip or the combination of both the position of the tip and the orientation of the tip. The scope of the present invention also includes less preferred contexts in which the "disposition" of the tip of a catheter refers to only the orientation of the tip. The term "displacing", as used herein, refers to an incremental movement of an object.

The term "sliding", as used herein, refers to both pushing an object, such as a catheter, away from an operator and pulling an object towards an operator.

Figure 1 shows a catheter 10 of the present invention, inserted, via an introducer shaft 28, to a body 20 of a medical or veterinary patient for the purpose of treating or diagnosing a target 26 in a body cavity 22 of the patient. Distal portion 12 of catheter 10 is inside body cavity 22. At the distal end of distal portion 12 is a tip 24. For example, body cavity 22 may be a chamber of the heart of the patient, target 26 may be tissue which is to be ablated, and tip 24 may include electrodes that are used to effect the ablation. The present invention is premised on the ability to measure the position, and preferably also the orientation, of tip 24, at the distal end of distal portion 12, and also the position of target 26 toward which tip 24 must be moved, as taught, for example, in the above-referenced WO 00/16684. To this end, a disposition sensor 30 is installed in tip 24. Disposition sensor 30, along with auxiliary equipment 32, is used to measure the disposition of tip 24. For example, auxiliary equipment 32 may include a set of antennas for transmitting low frequency electromagnetic fields, and sensor 30 may include a set of receiver coils for receiving the transmitted fields, as taught for example in PCT application WO 00/10456, which is incorporated by reference as if fully set forth herein. Note that the nomenclature used in WO 00/10456 differs from the nomenclature used herein: the equivalent of sensor 30 in WO 00/10456 is called a "receiver", and the individual receiver coils in WO 00/10456 are called "sensors".

Proximal portion 14 of catheter 10 terminates in a handle 16 that includes a lever 18. Tip 24 is moved within body 20 by sliding (pushing or pulling) handle 16, rotating handle 16 and/or using lever 18 to bend distal portion 12.

According to the present invention, tip 24 is moved incrementally towards target 26. Using handle 16 or lever 18, tip 24 is displaced within body cavity 22, not necessarily with reference to target 26. This displacement is an elementary displacement corresponding to one of the three modes of motion, sliding rotating and bending, that are effected using handle 16 and lever 18. Two dispositions of tip 24 are measured using sensor 30, before and after the displacement. Optionally, the elementary displacement is repeated, after which at third disposition of tip 24 is measured using sensor 30. From these dispositions, a component of the local motion reference frame of tip 24 is inferred. Typically, the component corresponding to sliding handle 16 is a line along which tip 24 is moved by sliding, the component corresponding to rotating handle 16 is a circle along which the rotation moves tip 24, and the component corresponding to operating lever 18 is a circle along which tip 24 moves as distal portion 12 is bent by lever 18. Once one or more, preferably all three, of the components of the local motion reference frame are established, handle 16 and/or lever 18 are again operated to move tip 24 towards target 26 in accordance with the local motion reference frame. Sensor 30 and auxiliary equipment 32 are components of a navigation subsystem of the present invention. Handle 16 and lever 18 are components of an actuating mechanism of the present invention for moving tip 24. In addition to a navigation subsystem and a mechanism for moving tip 24, a system of the present invention includes a control subsystem for inferring the local motion reference frame and for determining the extent to which each mode of motion should be effected using the actuating mechanism to move tip 24 towards target 26. In an embodiment of the present invention in which the actuating mechanism is operated manually, the control subsystem indicates to the operator how to effect the incremental displacements that are needed to determine the local motion reference frame, and then how much corresponding motion of tip 24 to effect in order to move tip 24 toward target 26. In a preferred embodiment of the present invention, however, the control subsystem itself operates both the navigation subsystem and the actuating mechanism to obtain the data needed to infer the local motion frame of reference, and then operates the actuating mechanism to move tip 24 toward target 26, thereby achieving the above-described object of automatic navigation of tip 24 toward target 26. According to another aspect of the present invention, at least two wires are provided in catheter 10, running from tip 24 towards proximal portion 14. Preferably, the wires are rigidly attached to tip 24 at respective attachment points that are equispaced circumferentially around tip 24. Preferably, distal portion 12 includes an interior region, proximal to tip 24, that is surrounded by a shielding region, with the each wire departing tip 24 between the interior region and the shielding region, and then entering the interior region. Most preferably, there are at least three such wires, with two of the wires entering the interior region at a common distance from tip 24, and a third wire entering the interior region twice as far as the common distance from tip 24.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is a general illustration of a catheter of the present invention inserted in a body cavity of a medical or veterinary patient;

FIG. 2 illustrates the establishing of the first component of a local motion reference frame;

FIG. 3 illustrates the establishing of the second component of a local motion reference frame;

FIG. 4 illustrates the establishing of the third component of a local motion reference frame;

FIG. 5 illustrates how the catheter is slid to move the catheter tip toward the target in accordance with the local motion reference frame; FIG. 6 illustrates how the catheter is rotated to move the catheter tip toward the target in accordance with the local motion reference frame;

FIG. 7 illustrates how the distal portion of the catheter is bent to move the catheter tip toward the target in accordance with the local motion reference frame;

FIG. 8 is a schematic depiction of a manually operated system of the present invention;

FIG. 9 is a schematic depiction of an automatic system of the present invention; FIG. 10 shows a catheter of the present invention of improved interior construction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a method and system for navigating a catheter within a body cavity of a medical or veterinary patient. Specifically, the present invention can be used to manually or automatically navigate the tip of the catheter towards a target in the body cavity.

The principles and operation of catheter navigation according to the present invention may be better understood with reference to the drawings and the accompanying description.

Sensor 30 of Figure 1 may be a three-degree-of- freedom (3DOF) sensor that measures only the position of tip 24, expressed, for example, as Cartesian coordinates (x, y, z). Alternatively, sensor 30 may be a six-degree-of-freedom (6DOF) sensor that measures both the position of tip 24, expressed, for example, as Cartesian coordinates, and the orientation of tip 24, expressed, for example, as Euler angles (a, β, γ). For simplicity of exposition, the description below is primarily in terms of a 3DOF sensor 30.

Referring again to the drawings, Figures 2, 3 and 4 illustrate the establishing of a local motion reference frame for distal portion 12 of catheter 10. The preferred local motion reference frame has three components. Figure 2 illustrates how the first component of the reference frame is established. Distal portion 12 of catheter 10 starts with tip 24 in an initial position a, as measured using 3DOF sensor 30. By sliding (pushing or pulling) proximal portion 14 of catheter 10, distal portion 12 of catheter 10 is moved incrementally to place tip 24 in a final position b, also as measured using 3DOF sensor 30. The line 34 that intersects points a and b is the first component of the local motion reference frame.

Figure 3 illustrates how the second component of the local motion reference frame is established. Distal portion 12 of catheter 10 starts with tip 24 in an initial position c, as measured using 3DOF sensor 30. By rotating handle 16, distal portion 12 of catheter 10 is rotated about an axis of rotation 36, and so tip 24 is moved incrementally, first to an intermediate position d, as measured using 3DOF sensor 30, and then to a final position e, as measured using 3DOF sensor 30. The circle 38 that intersects points c d and e is the second component of the local motion reference frame. Note that circle 38 is centered on axis 36, and lies in a plane perpendicular to axis 36. Figure 4 illustrates how the third component of the local motion reference frame is established. Distal portion 12 of catheter 10 starts with tip 24 in an initial position f, as measured by 3DOF sensor 30. By bending distal portion 12 of catheter 10, tip 24 is rotated about another axis of rotation 40, and so is moved incrementally, first to an intermediate position g, as measured using 3DOF sensor 30, and then to a final position h, as measured using 3DOF sensor 30. The circle 42 that intersects points f, g and h is the third component of the local motion reference frame. Note that circle 42 is centered on axis 40, and lies in a plane perpendicular to axis 40.

Usually, axis 36 is approximately parallel to line 34, and axis 40 is approximately perpendicular to axis 36, so that the local motion reference frame is approximately orthogonal.

The magnitudes of the incremental elementary displacements depend on the resolution and the noise level of the navigation subsystem. Typically, the magnitudes of the incremental elementary displacements of tip 24 of catheter 10 are between three millimeters and six millimeters. Note that, for clarity of illustration, the sizes of the elementary displacements in Figures 2, 3 and 4 are exaggerated.

With the local motion reference frame for catheter 10 established, tip 24 is moved toward target 26 in accordance with the local motion reference frame, as illustrated in Figures 5, 6 and 7. As illustrated in Figure 5, there is a closest point pi, to target 26, on line 34, at a distance qi from point b. Similarly, as illustrated in Figure 6, there is a closest point p₂, to target 26, on circle 38, at an arc length q₂ from point e; and, as illustrated in Figure 7, there is a closest point p3, to target 26, on circle 42, at an arc length q₃ from point h. Catheter 10 is slid a fraction of qi, typically between O.lqi and 0.4qι, towards pi. Then catheter 10 is rotated a fraction of q₂, typically between 0.1 q₂ and 0.4q₂, towards p₂, and finally, distal end 12 of catheter 10 is bent a fraction of q₃, typically between 0.1 q₃ and 0.4q₃, towards p₃. After tip 24 has been thus moved toward target 26 in accordance with the local motion reference frame, if tip 24 is not sufficiently close to target 26, this procedure is repeated. Tip 24 is again moved incrementally to establish a new local motion reference frame, and then tip 24 is moved toward target 26 in accordance with the new local motion reference frame.

The establishment of the second component of the local motion reference frame starts with tip 24 at the position where the establishment of the first component of the local motion reference frame ends, and the establishment of the third component of the local motion reference frame starts with tip 24 at the position where the establishment of the second component of the local motion reference frame ends. In other words, point c is coincident with point b, and point f is coincident with point e. Alternatively, after the first component of the local motion reference frame is established, tip 24 is withdrawn approximately to position a, and after the second component of the local motion reference frame is established, tip 24 is rotated back approximately to position c. The alternative procedure for establishing the local motion reference frame has the advantage of producing a local motion reference frame that is more localized in space, at the expense of requiring more time.

In an alternative sequence of motions, the establishment of partial (single-component) local motion reference frames is interleaved with movement of tip 24 according to those components. After line 34 is established, tip 24 is slid the full distance qi to point i, which then serves as point c for establishing circle 38. After circle 38 is established, tip 24 is rotated the full arc length q₂ to point p₂, which then serves as point f for establishing circle 42. After circle 42 is established, distal portion 12 is bent to move tip 24 the full arc length q₃ to point p₃, which, if necessary, then serves as point a for establishing a new line 34.

In an alternative, less preferred protocol for establishing a local motion reference frame using a 3DOF sensor 30, instead of using a circle as the rotational component of the local motion reference frame, the line connecting points c and d is used. Similarly, instead of using a circle as the bending component of the local motion reference frame, the line connecting points f and g is used. This protocol is less accurate than the preferred 3DOF protocol, but has the advantage of requiring fewer position measurements. The advantages of the two 3DOF protocols are achieved without their respective disadvantages by using a 6DOF sensor 30. Line 34 is established as before. Circle 38 is established from position and orientation measurements at points c and d. Specifically, axis 36 is determined as the axis of rigid rotation that transforms a body that has the measured orientation at point c into a similar body that has the measured orientation at point d. Points c and d define a plane perpendicular to axis 36, and the circle in this plane that is centered on axis 36 and that passes through points c and d is circle 38. Note that circle 38 is overdetermined by this procedure, which uses twelve inputs (for example, six Cartesian coordinates for the two points c and d and six Euler angles for the respective orientations) to determine nine unknowns. (In the corresponding 3DOF procedure, nine inputs (for example, nine Cartesian coordinates of the three points c, d and e) are used to determine the nine unknowns.) Circle 42 is established similarly, based on position and orientation measurements at points f and

Figure 8 shows, schematically, a system 50 of the present invention in which the incremental elementary displacements for establishing local motion reference frames, and the subsequent movements of catheter 10, are effected manually by an operator. The necessary measurements and calculations are effected by a computer-based processor 44 that issues instructions to the operator using a display device 46. Processor 44 is connected to sensor 30 by a communication line 48 and to auxiliary equipment 32 by a control line 52. For example, in a low frequency electromagnetic embodiment of system 50, in which sensor 30 is the receiver of WO 00/10456 and auxiliary equipment 32 includes the transmitting antennas of WO 00/10456, control line 52 is used to control the transmission of low frequency electromagnetic fields by the antennas of auxiliary equipment 32, and communication line 48 typically is a set of twisted wire pairs.

To establish a local motion frame of reference, processor 44 displays, at display device 46, recommended incremental elementary displacements. The operator effects these elementary displacements using handle 16 and lever 18. Before and after each elementary displacement, processor 44 receives position measurements from sensor 30. If sensor 30 is a 6DOF sensor, processor 44 also receives orientation measurements from sensor 30 before and after each elementary displacement. From these measurements, processor 44 computes the local motion reference frame. Processor 44 then instructs the operator, via display device 46, how to manipulate catheter 10, using handle 16 and lever 18, to move tip 24 toward target 26. Insofar as processor 44 operates auxiliary equipment 32 and receives and processes signals from sensor 30, processor 44 functions as a portion of a navigation subsystem of system 50, with the remainder of the navigation subsystem including sensor 30 and auxiliary equipment 32. Insofar as processor 44 infers, from the measured positions of tip 24, a local motion frame of reference for catheter 10, and then calculates the extent to which catheter 10 must be slid and rotated, and distal portion 12 must be bent, to move tip 24 toward target 26, processor 44 functions as a portion of control subsystem of system 50, with the remainder of the ^" control subsystem including display device 46.

Figure 9 shows, schematically, a fully automatic system 60 of the present invention. As in system 50, the measurements and calculations needed to establish local reference frames are effected by a computer-based processor 44' that is connected to sensor 30 by communication line 48 and to auxiliary equipment 32 by a control line 52. Processor 44' also moves catheter 10 directly, using a slide actuator 62 and a rotation actuator 64 to slide and rotate catheter 10 via handle 16, and using a bend actuator 66 to bend distal portion 12 of catheter 10 via lever 18. Actuators 62, 64 and 66 are connected to processor 44' by respective control lines 54, 56 and 58. To establish a local motion frame of reference, processor 44' uses activators 62, 64 and 66 to move tip 24 incrementally, while receiving position measurements, and optionally orientation measurements, from sensor 30. From these measurements, processor 44' computes the three components of the local motion reference frame. Then, using activators 62, 64 and 66, processor 44' moves tip 24 toward target 26 in accordance with the thus-computed local motion reference frame. System 60 thus overcomes the deficiencies, as discussed above, of the system taught in the above-referenced US 5,492,131. In effect, system 60 measures an effective local transfer function for tip 24 and activators 62 every step of the way to target 26, instead of assuming an a priori transfer function. Insofar as processor 44' operates auxiliary equipment 32 and receives and processes signals from sensor 30, processor 44' functions as a portion of a navigation subsystem of system 60, with the remainder of the navigation subsystem including sensor 30 and auxiliary equipment 32. Insofar as processor 44' infers, from the measured positions of tip 24, a local motion frame of reference for catheter 10, and then uses actuators 62, 64 and 66 to slide and rotate catheter 10 and to bend distal portion 12 to move tip 24 toward target 26, processor 44' functions as a portion of control subsystem of system 60, with the remainder of the control subsystem including, inter alia, actuators 62, 64 and 66. Another aspect of the present invention is a flexible, bendable catheter 110 of improved interior construction, as illustrated in Figure 10. Like catheter 10, catheter 110 includes a proximal portion 114 and a distal portion 112, with distal portion 112 terminating in a tip 124. Catheter 100 includes an interior region 80, whose configuration is preferably, but not necessarily, tubular, and an exterior shielding region 82, whose configuration also is preferably, but not necessarily, tubular, with the configuration of exterior region 82 generally coinciding with the configuration of interior region 80. Catheter tip 124, catheter interior region 80 and catheter exterior shielding region 82 are shown in phantom in Figure 10. Catheter interior region 80 and catheter exterior shielding region 82 are made of flexible materials, enabling flexible movement of catheter 110.

Three metal wires 86, 88 and 90 are attached to catheter tip 124. Wires 86, 88 and 90 are attached to catheter tip 124 as follows: wire 86 at point 92, wire 88 at point 94, and wire 90 at point 96. Attachment points 92, 94, and 96, are separated by 120 degrees along the circumference of catheter tip 124. Wire 86 is positioned between catheter interior region 80 and catheter exterior shielding region 82, extending along the portion between catheter tip 124 and junction 98, where wire 86 then extends towards catheter proximal portion 114. Wire 88 is positioned between catheter interior region 80 and catheter exterior shielding region 82 (in the plane of the page), extending along the portion between catheter tip 124 and junction 100, where wire 88 then extends towards catheter proximal portion 114. Wire 90 is positioned between catheter interior region 80 and catheter exterior shielding region 82, extending along the portion between catheter tip 124 and junction 102, where wire 90 then extends towards catheter proximal portion 114.

The distance between catheter tip 22 and junction 98 is preferably twice the distance between catheter tip 22 and each of junctions 100 and 102. Pulling a single wire 86, 88, or 90, produces unbalanced tension along the corresponding catheter portion extending from junction 98, 100, or 102, respectively, to catheter tip 124, resulting in bending of the corresponding catheter portion in the direction of the pulled wire. Pulling wire 86 results in the corresponding catheter portion, extending from junction 98 to catheter tip 124, to bend a length twice that resulting from pulling either wire 88 or 90.

According to the geometrical configuration of attachment points 92, 94, and 96, featuring separation by 120 degrees along the circumference of catheter tip 22, operating the three wires 86, 88, and 90, enables corresponding bending of catheter 110 in multiple directions varying by 120 degrees, translating to complete directional navigation and movement of catheter tip 124. Sliding and rotating the handle (not shown) located beyond catheter proximal portion 114, in addition to bending or flexing catheter 110, enable highly accurate, multi-directional, and complete incremental navigation and movement of catheter 110, in general, and of catheter tip 124, in particular, to a target inside a cavity within a body. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

WHAT IS CLAIMED IS:

1. A method of navigating a catheter toward a target within a body cavity, the catheter including a distal end having a tip, the method comprising the steps of:

(a) displacing the tip of the catheter while measuring a plurality of dispositions of the tip of the catheter;

(b) from said plurality of dispositions, inferring a local motion reference frame for the catheter; and

(c) moving the tip of the catheter toward the target in accordance with said reference frame.

2. The method of claim 1, wherein each said disposition includes a position of the tip of the catheter.

3. The method of claim 1, wherein each said disposition includes an orientation of the catheter.

4. The method of claim 1, wherein each said disposition includes at least three degrees of freedom selected from among a position of the tip of the catheter and an orientation of the catheter.

5. The method of claim 1. further comprising the step of:

(d) providing a disposition sensor at the tip of the catheter; said dispositions being measured using said disposition sensor.

6. The method of claim 5, wherein said disposition sensor is operative to measure a position of the tip of the catheter.

7. The method of claim 5, wherein said disposition sensor is operative to measure an orientation of the tip of the catheter.

8. The method of claim 1, wherein said displacing effects at least one elementary displacement of the tip of the catheter.

9. The method of claim 8, wherein one of said at least one elementary displacements is effected by sliding the catheter.

10. The method of claim 8, wherein one of said at least one elementary displacements is effected by rotating the catheter.

11. The method of claim 8, wherein one of said at least one elementary displacements is effected by bending the distal end of the catheter.

12. The method of claim 8, wherein, for each of said at least one elementary displacement, a first said disposition is measured before said each elementary displacement and a second said disposition is measured after said each elementary displacement.

13. The method of claim 12, wherein said local motion reference frame includes a line defined with reference to said first and second dispositions.

14. The method of claim 13, wherein said first and second dispositions include respective positions of the tip of the catheter, and wherein said line intersects both said position of said first disposition and said position of said second disposition.

15. The method of claim 12, wherein said local motion reference frame includes a circle defined with reference to said first and second dispositions.

16. The method of claim 8, wherein said displacing effects at least two elementary displacements of the tip of the catheter, and wherein, for each of at least one pair of said elementary displacements, a first said disposition is measured before a first elementary displacement of said each pair, a second said disposition is measured between said first elementary displacement of said each pair and a second elementary displacement of said each pair, and a third said disposition is measured after said second elementary displacement of said each pair.

17. The method of claim 16, wherein said local reference frame includes a circle defined with reference to said first, second and third dispositions.

18. The method of claim 1, wherein said steps are repeated until the tip of the catheter reaches the target.

19. A system for navigating a catheter toward a target within a body cavity, the catheter including a distal end having a tip, the system comprising:

(a) a navigation subsystem for measuring a disposition of the tip of the catheter;

(b) an actuating mechanism for moving the tip of the catheter; and

(c) a control subsystem for inferring, from a plurality of said dispositions that are measured using said navigation subsystem in coordination with a corresponding at least one displacement of the tip of the catheter that is effected using said actuating mechanism, a local motion reference frame for the catheter.

20. The system of claim 19, wherein said actuating mechanism includes a plurality of submechanisms, each said submechanism operative to move the tip of the catheter according to a respective mode of motion.

21. The system of claim 20, wherein said local motion reference frame includes, for each said submechanism, a corresponding reference frame component.

22. The system of claim 20, wherein one of said modes of motion is sliding the catheter.

23. The system of claim 20, wherein one of said modes of motion is rotating the catheter.

24. The system of claim 20, wherein one of said modes of motion is bending the distal end of the catheter.

25. The system of claim 20, wherein said control subsystem is further operative to determine an extent of at least one said mode of motion, in accordance with said reference frame, that moves the tip of the catheter towards the target.

26. The system of claim 19, wherein said navigation subsystem includes a disposition sensor at the tip of the catheter.

27. The system of claim 19, wherein said disposition includes a position of the tip of the catheter.

28. The system of claim 19, wherein said disposition includes an orientation of the tip of the catheter.

29. The system of claim 19, wherein said control system is further operative to operate said actuating mechanism to effect said at least one displacement while operating said navigation subsystem to measure said plurality of subsystems.

30. The system of claim 19, wherein said control system is further operative to operate said actuating mechanism to move the tip of the catheter toward the target in accordance with said local motion reference frame.

31. A catheter, comprising:

(a) a proximal portion;

(b) a distal portion including a tip; and

(c) at least two wires running from said tip towards said proximal portion.

32. The catheter of claim 31, wherein said at least two wires are rigidly attached to said tip.

33. The catheter of claim 31, wherein said at least two wires are rigidly attached to said tip at respective attachment points that are equispaced circumferentially around said tip.

34. The catheter of claim 31 , wherein said distal portion further includes an interior region proximal to said tip and a shielding region surrounding said interior region, and wherein at least a portion of each said wire, adjacent to said tip, runs towards said proximal portion between said interior region and said shielding region.

35. The catheter of claim 34, wherein only a portion of each said wire, adjacent to said tip, runs towards said proximal portion between said interior region and said shielding region, said each wire then entering said interior region.

36. The catheter of claim 35, wherein one of said wires enters said interior region at different distances from said tip than at least one other of said wires.

37. The catheter of claim 35, including three said wires, two of said wires entering said interior region at a common distance from said tip and a third said wire entering said interior region farther from said tip than said common distance.

38. The catheter of claim 37, wherein said third wire enters said interior region twice as far from said tip as said common distance.