Attachment with one or more sensors for precise position determination of endoscopes

A locatable endoscope (14) attachment including an attachment (20) connectable to an insertion tube portion of an endoscope for determining the endoscope's (14) position; and one or more sensors (22), fixedly positioned with respect to the attachment (20), which are used for determining the positions of the one or more sensors (22). Preferably, when the attachment (20) is fixedly attached to the endoscope (14), the one or more sensors (22) are distanced from elements of the endoscope which interfere with determining the positions of the one or more sensors (22).

FIELD OF THE INVENTION
 The present invention relates generally to the field of endoscopy, and
 specifically to endoscope assemblies with position sensors.
 BACKGROUND OF THE INVENTION
 The use of endoscopes for diagnostic and therapeutic indications is rapidly
 expanding. There are now many types of specialized endoscopes, such as
 endoscopes for the upper esophagus, stomach, and duodenum; angioscopes for
 blood vessels; bronchoscopes for the bronchi; arthroscopes for joint
 spaces; colonscopes for the colon; and laparoscopes for the peritoneal
 cavity. The present invention applies to all types of endoscopes.
 Typically, endoscopes have a long and flexible insertion tube with a
 diameter ranging between 15-25 millimeters. The insertion tube is inserted
 into a patient's body, along a selected path, during an endoscopic
 procedure. Multiple work channels usually extend along the length of the
 endoscope within the insertion tube. The work channels may allow inserting
 biopsy tools into and taking biopsies from the patient's body. Other
 mechanisms, which may be incorporated in the endoscope, are a visual
 imaging device, an illumination device, and a deflection mechanism. The
 proximal end of the endoscope usually has a handle in which the controls
 of the endoscope residue. Ordinarily, endoscopes are made of metallic,
 electrically conducting, materials. For example, U.S. Pat. No. 4,869,238
 whose disclosure is incorporated herein by reference, describes a standard
 three-layer wall for endoscopes, containing metal coils and wire mesh.
 Cleaning and sterilizing endoscopes are expensive and tedious procedures.
 Endoscopes incorporate expensive and delicate apparatus which may be
 damaged during cleaning. Also, the long and narrow work channels in the
 insertion tube are difficult to clean.
 Disposable endoscopic sheaths have been developed, to avoid the need for
 cleaning and sterilizing endoscopes. These sheaths substantially isolate
 the endoscope from the patient, and thus prevent the endoscope from being
 contaminated. Some of these sheaths have thick walls containing work
 channels within them, leaving only part of their cross-section for a lumen
 which receives the insertion tube of the endoscope. The walls of the work
 channels and the area between the work channels usually comprise the same
 material as the outer wall.
 A sheath with thick walls is described, for example, in PCT publication WO
 94/287282, whose disclosure is incorporated herein by reference. WO
 94/28782 describes a disposable sheath which may include work channels.
 The sheath removably receives a cylindrical insertion tube which contains
 controls and other delicate apparatus of the endoscope. Another disposable
 sheath is described in U.S. Pat. No. 5,483,951, whose disclosure is
 incorporated herein by reference. This disposable sheath comprises a thin
 outer wall, inner work channels, and a lumen with a "D" shaped
 cross-section. The lumen is adapted to receive and substantially isolate a
 non-disposable insertion tube of an endoscope, which is accordingly "D"
 shaped.
 many endoscopic procedures involve irreversible actions such as taking
 tissue samples and ablation at the distal end of the insertion tube of the
 endoscope. Performing these actions at an incorrect position can damage
 important blood vessels or nerves, puncture the intestine, or otherwise
 cause severe damage to the patient. Therefore it is useful to have a
 method of determining the position and/or orientation of the distal end of
 the endoscope.
 Through a visual imaging device the user can observe images transmitted
 from the distal end of the endoscope. From these images and from knowledge
 of the path the endoscope has followed, the user can ordinarily determined
 the position of the endoscope. However, there are organs of the human body
 in which the images and knowledge of the path do not suffice to determine
 the position of the endoscope to sufficient accuracy. Some organs, such as
 the brain, have a homogeneous appearance in which it is very hard or even
 impossible to find a specific point based only on the images from the
 imaging device. In addition, determining the position of he endoscope from
 the images could be very time consuming. In many endoscopic procedures,
 such as endoscopic bypass surgery, the amount of the time a patient can
 endure the endoscopic procedure is limited.
 In some procedures, the endoscope is used to map a section of an organ. The
 map is produced by systematically bringing the distal end of the endoscope
 in contact with a plurality of points within the organ and registering the
 positions of the points. To confirm that the entire section of the organ
 has been mapped, a sufficient density of points must be registered within
 the section. To insure use of a sufficient density of points it is
 necessary to have unique position identification for every point.
 Another problem which arises, for example, in colonscope procedures, is
 formation of loops in the long and narrow tube of the colonscope. Such
 loops may arise when the insertion tube encounters an obstacle, or gets
 stuck in a narrow passage. Instead of progressing, the tube forms loops
 within the patient. In an attempt to proceed in insertion of the
 colonscope, excess force may be exerted, damaging delicate tissue in the
 patient's body. The user may proceed with the attempted insertion of the
 endoscope without realizing there is a problem. The ability to see the
 configuration of the endoscopic insertion tube within the patient's body,
 allows early discovery of the existence of loops and makes straightening
 them simpler.
 One method used in the art of determine the configuration of the insertion
 tube is x-ray imaging. Another method used is magnetic field positioning,
 which avoids the x-ray exposure to the patient and the operator. PCT
 application PCT/GB93/01736, whose disclosure is incorporated herein by
 reference, describes a method of magnetic filed position determination
 using low frequency magnetic fields to determine the position of a
 miniature sensor embedded within a colonscope tube. Based on the position
 of the sensor at sequential time periods, an image of the configuration of
 the colonscope tube is produced.
 In tests mentioned in PCT/GB93/01736 it was found that there were some
 distortions in the image due to the metallic construction of the
 colonscope. The metallic construction of the colonscope reacts with the
 sensing magnetic filed in that currents are induced in the colonscope by
 the magnetic filed. These currents, called eddy currents, generate a
 disturbing magnetic field which is overlaid on the sensing magnetic field.
 Thus, the amplitude and/or phase of the magnetic field used by the
 position determining system are changed in proximity of metallic
 substances. The magnitude and effect of the eddy currents depend on the
 size and geometry of the metallic substance. For example, large metal
 rings change the magnetic field substantially in their proximity.
 Conversely, small metal objects and objects with a relatively high
 resistance, within which substantially no eddy currents are formed, does
 not substantially affect the magnetic field.
 Magnetic filed position determining systems typically determine positions
 according to the magnetic filed's amplitude and/or its phase. Changes in
 the amplitude and/or phase due to eddy currents cause inaccuracies in
 determined positions and interfere with precise determination of
 positions. Interference can also arise from ferro-magnetic materials in
 the endoscope, which concentrate the magnetic filed in their proximity.
 Thus, ferro-magnetic materials cause distortions in the magnetic field,
 changing the amplitude and phase of the field at measured points.
 The interference is dependent on the frequency of a drive signal which
 generates the magnetic field. A high drive signal frequency is preferred
 in order to enhance sensor sensitivity, but must be limited so as not to
 intensify the interference to the position determining system. Therefore,
 the PCT/GB93/10736 system makes a compromise in its choice of the
 frequency used. If a different method to minimize the interference is
 used, it would be possible to enjoy the advantages of a higher drive
 signal frequency.
 Existing catheters have a metal coil (for structural purposes) within them.
 The coil extends along the length of the catheter except for a small part
 of the distal end of the catheter. A sensor coupled with a magnetic filed
 position determining system is embedded within the distal end of the
 catheter.
 SUMMARY OF THE INVENTION
 It is an object of some aspects of the present invention to provide
 accurate positioning of an endoscope using a magnetic field position
 determining system.
 It is further object of some aspects of the present invention to provide an
 endoscopic sheath which, when it receives an insertion tube of an
 endoscope, allows accurate position determination of points within the
 insertion tube.
 Another object of some aspects of the present invention is to distance one
 or more position sensors, embedded at any point along an insertion tube of
 an endoscope, from materials, structures and signal sources within the
 endoscope which may interfere with position determination. Such materials,
 structures and signals sources are referred to herein as "interference
 causing structures".
 In some preferred embodiments of the invention, the position determining
 system uses magnetic fields to determine positions. In these embodiments
 the interference causing structures usually consist of electrically
 conducting and/or ferro=magnetic materials. The interference causing
 structures could be in any of various components of the endoscope, such as
 control wires, metal coils, reinforcements of the endoscope's walls,
 electric wires, etc. The interference is dependent on the size and
 geometry of the interference causing structures, and on the distance
 between the sensors and the interference causing structures substance.
 More specifically, the interference is approximately inversely
 proportional to the cube of the distance between the sensors and the
 interference causing structures. When using an amplitude-based position
 determining system, a distance between 1.varies.3 mm is typically
 sufficient to reduce the interference to less than 1% of the field used by
 the position determining system.
 In accordance with preferred embodiments of the invention, an endoscopic
 sheath preferably comprises only non-interference causing structures.
 Preferably, the walls of the endoscopic sheath contain, within them, work
 channels and therefore are thick. Preferably, the sheath comprises a lumen
 which receives the insertion tube of an endoscope. One or more sensors are
 embedded within the sheath, preferably in regions farthest from the lumen.
 Thus, when the insertion tube is placed in the lumen, the sensors are
 distanced from the interference causing structures. The insertion tube is,
 preferably, tightly and precisely positioned within the sheath so that
 positions of points within the insertion tube are easily determined
 relative to the sensors.
 In further embodiments of the present invention, the distal end of the
 sheath extends beyond the insertion tube and isolates the tip of the
 insertion tube from the patient's body. In some sheaths of the invention
 the distal end comprises a transparent window allowing the distal end of
 the endoscope a clear view. Preferably, the distal end of the sheath
 comprises substantially no interference causing structures. A sensor is
 embedded within the distal end of the sheath away from interference
 causing structures which are within the insertion tube. Preferably, the
 sensor does not obscure the view through the transparent window. Thus,
 when inserting the insertion tube into the sheath a sensor is situated at
 the tip of the endoscope with substantially not interference causing
 structures in it proximity. This is true even if the tip of the insertion
 tube is comprised of interference causing structures.
 It is noted that, in some prior art sheaths, there are some metal
 components within the sheath. In accordance with preferred embodiments of
 the present invention, in these sheaths, the sensors are preferably
 embedded at points at which the interference is minimal when the insertion
 tube is within the sheath. Preferably, tests are performed during a design
 stage to find these points. In these tests, performed when the insertion
 tube is within the sheath, the interference induced by the interference
 causing structures is measured, and the positions of the sensors are
 chosen accordingly. In this embodiment it is important to identify the
 preferred orientation of the insertion tube within the sheath, in order
 that the interference to the sensors will be the same as at the time of
 the tests.
 Israeli Patent Application No. 119,262, filed by Biosense Ltd. on Sep. 17,
 1996, which is assigned to the assignee of the present invention, and is
 incorporated herein by reference, describes a sheath having an embedded
 position sensor suitable for a disposable biopsy needle. In accordance
 with a preferred embodiment of the present invention the biopsy needle is
 made of a non interference causing substance, such as carbon, so that the
 structure of the biopsy needle does not interfere with the operation of
 the position determining system. Alternatively, the sheath is thick enough
 to distance the sensor from the biopsy needle. Israeli Patent Application
 No. 117,148, titled "Catheter with a Lumne", filed by Biosense LTD. on
 Feb. 15, 1996, the disclosure of which is incorporated herein by
 reference, describes catheters having a large lumen and a position sensor
 which does not block the lumen during its operation.
 In another preferred embodiments of the invention, one or more position
 sensors are embedded within a sheath which covers an invasive tool. In an
 exemplary embodiment of the invention the invasive tool is a pacemaker
 electrode, which is usually inserted by guiding the electrode through a
 vein into the right atrium, and then, the right ventricle, where the
 electrode is fixed at the apex using a screw, clip or other type of
 suture. Knowledge of the exact location of attachment of the electrode is
 desirable for various reasons, in particular those described in a PCT
 application filed in the Israeli Receiving office on Jan. 8, 1997 and
 title "Cardiac Electro-Mechanics" to inventors Shlomo Ben-Haim and Maier
 Fenster, the disclosure of which is incorporated herein by reference. This
 PCT application describes various methods of determining the activation
 profile of the heart and, based on that profile and/or an electrical
 activation map and/or a mechanical activation map of the heart,
 determining an optimal location for such a pacemaker electrode. One aim is
 to avoid attaching the electrode to diseased tissue (which will increase
 the drain on the pacemaker power supply and/or hurt the patient). Another
 aim is to achieve a particular activation profile of the heart using the
 pacemaker electrode. The position sensor assists in guiding the insertion
 of the pacemaker electrode to a required position in the patient's heart.
 After the pacemaker electrode is attached at it required position the
 sheath is peeled off of the pacemaker electrode and taken out of the
 patient's body along with the sensors. In a preferred embodiment of the
 invention, the sheath includes a steering mechanism, as known in the art,
 to deflect the electrode while navigating through blood vessels and in the
 heart, towards a desirable attachment location. It should be appreciated
 that since pacemaker electrodes are usually inside the patient's heart and
 flex with each heart beat, the probability of some foreign material
 breaking off the electrodes is great, as is the possible damage to the
 patient. Pacemaker electrodes are typically specially engineered to resist
 corrosion and breakage under this type of constant strain, while it is not
 desirable to apply the same engineering principles to position sensors,
 due to considerations of cost and to technological limitations. This
 embodiment of the invention is also especially useful with other invasive
 tools, such as infusion tubes, which remain in the patient's body for long
 periods.
 In some preferred embodiments of the present invention, the sensors are not
 embedded within a sheath. Rather, the sensors are embedded within
 attachments which are affixed to the endoscope. These attachments are of
 substances which are substantially not interference causing to the
 operation of the position determining system, and a preferably thick
 enough so as to form an effective separation between the sensors and
 interference causing structures within the endoscope. The attachments may
 be attached to the endoscope using an adhesive, or by any other connection
 method. In some embodiments of the invention a sheath is used to cover
 both the endoscope and the attachments thereon. The sheath isolates the
 endoscope from the surroundings and also keeps the attachments, and
 sensors, fixed to the endoscope. In a preferred embodiment one long
 attachment is laid along the length of the endoscope, and within it are
 one or more sensors.
 In further preferred embodiments of the present invention, separation of
 the sensors from the interference causing structures is achieved without
 using a separate disposable part. Rather, the insertion tube itself is
 partitioned, forming regions free of substances interference causing to
 the operation of position determining systems. Within these regions one or
 more sensors are embedded, distanced for the interference causing
 structures. Thus, the sensors and the interference causing structures are
 separated, substantially eliminating interference to the operation of the
 position determining system.
 In some preferred embodiments of the present invention, a widthwise
 separation is employed. In widthwise separations the regions free of
 interference causing structures are located at predetermined regions of
 the cross-section of the insertion tube. In one preferred embodiment of
 the invention, the separation is such that interference causing structures
 are axially centered, and regions adjacent the circumference are free of
 interference causing structures. Thus, most parts of these regions are
 substantially free of interference, if the diameter of the sheath is large
 enough. In another preferred embodiment of the invention, the interference
 causing structures are confined to a "D" shaped area of the ordinarily
 circular cross-section of the endoscope. Thus, the rest of the
 cross-section is free of interference causing structures, and therefore a
 major part of the cross-section is substantially free of interference.
 In one preferred embodiment of the invention based on a lengthwise
 separation, one or more sections along the length of the insertion tube
 are substantially free of interference causing structures. In another
 embodiment of the invention, interference causing structures run along the
 entire length of the insertion tube. However, there are one or more
 segments along the length of the insertion tube in which the interference
 causing structures are confined to a part of the cross-section. Thus, some
 regions of the cross-section of these segments are free of interference
 causing structures. Accordingly, the sensors of the position determining
 system are embedded within these regions at points which are substantially
 free of interference.
 Preferably, these points are the points in the insertion tube with the
 least interference. Preferably, these points are chosen according to tests
 as described above regarding previous embodiment of the invention.
 Alternatively, the sensors are embedded at points substantially farthest
 from all interference causing structures within the insertion tube.
 There is therefore provided in accordance with a preferred embodiment of
 the invention, a locatable endoscope attachment connectable to an
 endoscope for determining the endoscope's position; and one or more
 sensors, fixedly positioned with respect to the attachment, which are used
 for determining the positions of the one or more sensors.
 Preferably when the attachment is fixedly attached to the endoscope, the
 one or more sensors are distanced from elements of the endoscope which
 interferes with determining the positions of the one or more sensors.
 Preferably the one ore more sensors are embedded within the attachment.
 Preferably the attachment does not contain elements which substantially
 interfere with the determining of positions of the one or more sensors.
 In a preferred embodiment of the invention determining the positions of the
 one or more sensors is performed by transmitting and receiving magnetic
 fields.
 Preferably there are markings on the outside of the attachment which
 indicate the positions of the one or more sensors.
 In a preferred embodiment of the invention the attachment includes a tube.
 There is further provided, the in accordance with a preferred embodiment of
 the invention, an endoscopic positioning and sensing kit including one or
 more attachments as described above; and a thin sheath which covers the
 endoscope and the one or more attachments.
 In a preferred embodiment of the invention the attachment includes a sheath
 which has a lumen and a circumference, preferably the lumen has a "D"
 shaped cross-section and the one or more sensors are spaced from materials
 which interfere with the determining of positions, by an amount sufficient
 to substantially avoid the interfering effect of the materials.
 Alternatively, or additionally, the lumen is axially centered within the
 sheath, and the one or more sensors are positioned adjacent the
 circumference of the sheath.
 Preferably, the sheath has a distal end which extends beyond the lumen and
 at least one of the one or more sensors is embedded within the distal end.
 There is further provided, in accordance with a preferred embodiment of the
 invention, a locatable endoscope including an insertion tube having an
 elongate body with a cross-section, a circumference, and a tip; and one or
 more sensors positioned at fixed points of the body, preferably the one or
 more sensors are used for determining the positions of the one or more
 sensors, and portions of the cross-section of the elongate body, distal
 from the tip and adjacent the one or more sensors, do not contain
 substances which substantially interfere with determining the positions of
 the one or more sensors.
 Preferably, the one or more sensors are positioned at points of the
 cross-section of the elongate body where elements of the insertion tube
 which interfere with determining the positions of the one or more sensors
 have the last effect.
 Preferably, the one or more sensors are embedded within the insertion tube.
 Preferably, determining the positions of the one or more sensors is
 performed by transmitting and receiving magnetic fields.
 Preferably, the one or more sensors are positioned at points within the
 insertion tube substantially farthest from electrically conducting and
 ferro-magnetic materials.
 Preferably, the substances which interfere with determining the positions
 of the one or more sensors are confined to a "D" shaped cross-section of
 the elongate body, and the one or more sensors are distanced from
 materials which interfere with the determining of positions, by an amount
 sufficient to substantially avoid the interfering effect of the materials.
 Alternatively the substances which interfere with determining the positions
 of the one or more sensors are confined to a round, axially centered,
 cross-section of the elongate body; and the one or more sensors are
 positioned adjacent the circumference of the insertion tube.
 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:

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Reference is made to FIG. 1 which shows a flexible endoscopic sheath 20,
 installed over an endoscopic insertion tube 14 of a colonscope. Colonscope
 10, comprising a control unit 12 and insertion tub 14 which as a distal
 end 15. Colonscope 10 is placed in a flexible endoscopic sheath 20, which
 is adapted to tightly receive tube 14. Insertion tube 14 and sheath 20 are
 inserted together into a patient's body, such that tube 14 is essentially
 isolated from the patient's body. One or more sensors 22 are embedded
 along sheath 20 as described below. A position determining system (not
 shown) determined the position in space of sensors 22, preferably,
 according to magnetic fields transmitted to and/or from sensors 22.
 Insertion tube 14 is a long and narrow flexible tube with durable walls,
 and preferably has a "D" shaped cross-section. A deflection mechanism, a
 visual imaging device, and possible other apparatus are located within the
 tube 14. Wires serving the apparatus within the colonscope, run along
 insertion tube 14 from its distal end 15 to control unit 12. Ordinarily,
 tube 14 is a non-disposable elongate tube, which comprises electrical
 conducting materials.
 Flexible endoscopic sheath 20 is an elongate disposable tube which
 generally comprises materials which do not interfere with the operation of
 the position determining system. Interfering materials include
 electrically conducting materials, and ferro-magnetic materials.
 Preferably, sheath 20 comprises polyethylene or polyvinylchloride ("PVC"),
 but can comprise any durable lubricious material. It is noted that in some
 prior art descriptions sheath 20 also comprises a metal spring or other
 electrically conducting part. In the present invention, where such sheath
 are used, measurements are preferably made, in the design stage, to
 determine the interference induced by the conducting part. The influence
 of the conducting part of the sheath may be ignored if the interference it
 induces is small relative to the interference induced by materials within
 the colonscope. Alternatively, in accordance with the present invention, a
 similar sheath may be used in which the conducting part is replaced by a
 functionally equivalent part, comprising non-conducting material.
 As can best be seen in FIG. 2, which is a cross-sectional view of
 endoscopic sheath 20 of FIG. 1, without the colonscope's insertion tube,
 sheath 20 preferably has a circular external cross-section. Within sheath
 20 there is a lumen 17, shaped and sized to tightly receive insertion tube
 14. The rest of the cross-section of sheath 20 contains several work
 channels 25, 26 and 27 running substantially parallel to the longitudinal
 axis of sheath 20. Work channels 25, 26 and 27 are used to pass materials
 and apparatus in and out of the patient's body, such materials as air,
 water and also biopsy taking apparatus. In an exemplary embodiment channel
 25 is an air channel, channel 26 is a water channel, and channel 27 is
 used for passing biopsy taking apparatus into the patient and is called a
 biopsy channel. It is noted that occasionally, electrically conducting
 apparatus, which induces interference, is passed through biopsy channel
 27. Ordinarily, substantially no interference to the operation of the
 position determining system is caused by such apparatus, because of the
 small size of the apparatus, and the distance between the apparatus and
 sensors 22. However, preferably, work channels near sensors 22 are
 allocated tasks which do not include passing interfering materials through
 them. Accordingly, sensors 22 are embedded farthest from biopsy channel
 27. In other words, the channel farthest from sensors 22 is used for
 biopsy apparatus.
 As mentioned above, along sheath 20 there are one or more miniature sensors
 22, which are used in conjunction with the position determining system. In
 a preferred embodiment of the invention the sensors sense the amplitude
 and/or phase of the magnetic field in their proximity. The position
 determining system uses the amplitude and/or phase to determine positions
 within the endoscope. Each of sensors 22 measures at least three
 coordinates. Preferably, each sensor 22 allows determination of the six
 coordinates of position and orientation. Alternatively, a few sensors
 which measure only two coordinates, may be used, if the sensors are
 fixedly positioned relative to each other. The sensors are, preferably,
 miniature coils such as described, for example, in PCT/GB93/01736, U.S.
 Pat. No. 5,391,199, PCT publication WO95/04938, PCT publication
 WO96/05768, or U.S. Provisional patent application No. 60/011,724, filed
 Feb. 15, 1996, which is assigned to the assignee of the present
 application, all of which patents, publications and applications are
 incorporated herein by reference. Sensors 22 are located on the inner side
 of sheath 20 near its circumference, spaced from interfering materials by
 an amount sufficient to avoid interference to the operation of the
 position determining system. In a preferred embodiment of the invention,
 the sensors are diametrically opposite lumen 17. As can best be seen in
 FIG. 1, wires 24, running along sheath 20, connect sensors 22 to the
 position determining system (not shown). Wires 24 are then enough so as to
 take up minimal space of the interior of sheath 20, and also so as not to
 interfere with the operation of the position determining system.
 Alternatively, sensors 22 are wireless. In a preferred embodiment of the
 invention, at least one sensor 22 is coupled with a transmitter on an
 integrated circuit.
 Before insertion of tube 14 into a patient, it is tightly placed and
 precisely oriented within sheath 20. In addition, distal end 15 of tube 14
 is preferably brought to a re-locatable depth within sheath 20. Thus,
 precise positional co-ordination between sensors 22 and insertion tube 14,
 is achieved.
 One method of tightly attaching a sheath to an endoscope is to shrink the
 sheath around the endoscope using chemical or heat methods. Preferably,
 such a sheath includes a "rip cord", such as a Kevlar.RTM. cable running
 along the inside of the sheath, to facilitate removal of the sheath after
 usage. Pulling such a rip cord, perpendicular to the endoscope, rips the
 sheath so that is may be easily removed from the endoscope. Such a cord is
 especially important if the cross-section of the endoscope is not
 constant, such as due to attachments.
 Sensors 22 are precisely fixed, relative to sheath 20 and hence to
 insertion tube 14, so that the position determining system will be able to
 determine the position of any point along sheath 20 and insertion tube 14.
 In addition, sensors 22 are preferably embedded within sheath 20 to
 protect sensors 22 from the surroundings.
 In some preferred embodiments of the invention the position determining
 system uses DC currents. In these embodiments, conducting materials do not
 interfere with the operation of the position determining system. Therefore
 the sensors are distanced only from ferro-magnetic materials. In these
 systems, the decision of where to place the sensors is performed according
 to the locations of ferro-magnetic materials within the insertion tube.
 Reference is now made to FIG. 3 which shows an endoscopic sheath in
 accordance with a preferred embodiment of the present invention. In some
 embodiments of the present invention, sheath 20 has a distal end 28 which
 extends beyond distal end 15 of tube 14. Distal end 28 preferably isolates
 distal end 15 from the patient's body. Sheath 20 preferably has a
 transparent window 30 on its distal end 28, allowing an imaging device
 within tube 14 an unobstructed field of view. Window 30 preferably
 comprises a clear transparent optical grade plastic, as described, for
 example, in U.S. Pat. No. 5,402,768, which is incorporated herein by
 reference. In accordance with the present invention, distal end 28 is
 substantially free of substances interference causing to the operation of
 the position determining system. Therefore, substantially no interference
 is induced on a sensor 22 embedded within distal end 28. Preferably,
 distal end 28 is thick enough to contain at least one sensor 22 in such a
 way that does not obscure the view thorough window 30. In a preferred
 embodiment of the present invention, the interference to the position
 determining system at different points within distal end 28 is measured,
 and sensor 22 is embedded at a point which has the least interference.
 Reference is made to FIG. 4 which shows the cross-section of a sheath 120
 in accordance with another embodiment of the invention. Sheath 120
 comprises a cylindrical lumen 117 which is preferably, axially centered
 within sheath 120. Lumen 117 is shaped to tightly receive an endoscopic
 insertion tube. Several work channels run parallel to lumen 117, within
 sheath 120, radially surrounding lumen 117. Preferably, there are three
 channels, for example, for air 125, for water 126 and for biopsy apparatus
 127. One or more sensors 22 are embedded along sheath 120, preferably, on
 the outer circumference of sheath 120, as far as possible from lumen 117.
 Preferably, sensors 22 are embedded near air channel 125, and water
 channel 126, so as to keep them away from any metal apparatus passing
 through biopsy channel 127.
 On the insertion tube, there is preferably a marking indicating the correct
 orientation of the insertion tube within the lumen. In a preferred
 embodiment of the invention, the insertion tube has a key, and the lumen
 has corresponding slot. Thus, the insertion tube can be inserted into the
 lumen only in the correct orientation. In addition, the distal end of the
 insertion tube contacts the inner surface of the distal end of the lumen.
 Thus, the position determining system can precisely register the location
 and orientation of the insertion tube according to the position of sensors
 22.
 In some preferred embodiments of the present invention, the sensors are
 embedded within attachments to the endoscope. Reference is now made to
 FIG. 5 which shows an endoscope with sensors in accordance with a
 preferred embodiment of the invention. As shown in FIG. 5, and endoscope
 90 has one or more attachments 94 which incorporate sensors 22.
 Preferably, endoscope 90 has a groove 96 along at least part of its
 length. Attachment 94 are, preferably, situated within groove 96 and are
 preferably shaped to smoothly fit in groove 96 without protruding from it.
 Attachments 94 are comprised substantially of non-interference causing
 materials and are preferably thick enough so as to substantially separate
 between sensors 22 and interference causing structures within the
 endoscope. Preferably, an adhesive connects attachments 94 to endoscope
 90. Alternatively, or additionally, attachments 94 are connected to
 endoscope 90 using any connection method known in the art. Sensor 22 are
 preferably embedded within attachments 94 in the portion of attachment 94
 farthest from endoscope 90. Thus, most of each attachment 94 separates its
 associated sensor 22 from interference causing structures within endoscope
 90.
 In a preferred embodiment of the invention, a thin sheath 92 covers
 endoscope 90 and attachments 94, and thus isolates endoscope 90 from the
 patient's body and also keeps attachment 94 in fixed positions on
 endoscope 94. Before insertion of endoscope 90 into the patient, endoscope
 90 is preferably brought to an external reference calibration point and
 the position determining system registers the positions of the sensors
 relative to endoscope 90.
 In a preferred embodiment of the invention, one or more sensors are
 situated within one long attachment which is placed along the endoscope.
 Reference is now made to FIG. 6, which shows a cross-section of an
 endoscopic assembly, in accordance with another embodiment of the present
 invention. As shown in FIG. 6, a disposable sheath 150 isolates an
 endoscope 147 from the patient's body. A groove 142 runs along the length
 of endoscope 147. A disposable tube 144 is laid along endoscope 147 within
 groove 142, such that sheath 150 covers endoscope 147 and tube 144.
 Endoscope 147 and tube 144 can be, for example, as described in U.S. Pat.
 No. 4,646,722, whose disclosure is incorporated herein by reference. In
 accordance with the present invention, one or more sensors 22 are fixed
 along disposable tube 144, and preferably are embedded within its wall.
 Sensors 22 are embedded substantially along a straight line parallel to
 the longitudinal axis of tube 144. On the outer side of tube 144, this
 straight line is preferably marked to identify where sensors 22 are
 embedded. The marking helps the user lay tube 144 in groove 142 such that
 sensors 22 are adjacent to sheath 150, and therefore are distanced from
 interference causing structures within endoscope 147. Also, the position
 of the sensors in relation to endoscope 147 is thus accurately
 established.
 It is noted, that although in the above embodiments the sheath is separate
 from the insertion tube of the endoscope, the sheath can also be a
 non-separable part of the insertion tube. In such embodiments, there is no
 separable sheath, but rather there is one endoscopic insertion tube with
 two parts. One part contains the circumference of the insertion tube and
 the work channels and is substantially free of interference causing
 structures, and the other part is the core of the insertion tube which
 contains interference causing structures. In accordance with preferred
 embodiments of the invention, the sensors are embedded in the part which
 is free of interference causing structures.
 Reference is now made to FIG. 7 which shows an endoscopic insertion tube
 160 with a combined lengthwise and widthwise division in accordance with a
 preferred embodiment of the invention. As shown in FIG. 7, insertion tube
 160 has an axial metal core 162, and sections 164 which contain various
 apparatus of insertion tube 160. The apparatus in sections 164 are parts
 of a deflections system or other systems such as a visual imaging system,
 and contain interference causing structures. Other sections 168, which do
 not contain interference causing structures, have embedded within them one
 or more sensors 22. Axial core 162 is preferably thin and contains wires
 which connect the device within insertion tube 160 with the proximal end
 of insertion tube 160. Preferably, also wires 170, which connect sensors
 22 to a position determining system outside of the endoscope, run within
 axial core 162.
 It is noted, that although the invention has been described in conjunction
 with a magnetic field position determining system, it can also be used
 with other position determining systems, such as, acoustic position
 determining systems. When using acoustic position determining, hard
 substances within the insertion tube reflect acoustic waves and therefore
 interfere with the operation determining positions. Therefore, in
 accordance with the present invention, the sensors, which are ordinarily
 transducers, are distanced from hard substances within the insertion tube.
 The distancing is performed in a manner similar to the described above
 distancing of magnetic field sensors from conducting and ferro-magnetic
 materials.
 It is also noted, that although the invention has been described in
 conjunction with an endoscope, it can also be used with any other medical
 instrument for which position determination is desired. In particular, the
 invention can be used with invasive tools such as catheters and feeding
 tubes.
 It will 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.