Abstract:
The present invention generally provides an implantable medical lead having markings. The lead is easier to implant and improves the accurate localization of lead electrodes at a specific point of the brain for neurostimulation. Also provided is an implantable medical lead having a removable extension that advantageously provides a minimal length of excess lead protruding from the lead insertion site. The improved lead and method of implantation facilitates use of a neurostimulator device that is implanted directly in a patient&#39;s cranium.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to implantable medical leads, and more particularly, to leads designed for improved lead positioning and placement.  
         BACKGROUND  
         [0002]    In medical diagnosis and treatment of various brain disorders, including epilepsy, Parkinson&#39;s disease, sleep disorders, and psychiatric ailments, it is customary and frequently useful to analyze electrical signals originating in the brain. It is also becoming accepted to apply electrical stimulation to various structures of the brain for both diagnostic and therapeutic purposes. Furthermore, chronic electrical stimulation is contemplated as a direct treatment for various disorders. For example, an implantable neurostimulator that is coupled to an implantable medical lead having one or more deep brain electrodes or depth electrodes is described in U.S. Pat. No. 6,016,449 to Fischell, et al.  
           [0003]    Generally, external equipment, such as a cranially implantable neurostimulator device, is connected to acutely implanted brain electrodes located on an implantable medical lead. Typical implantable medical leads are flexible small-diameter (usually 1-1.5 mm) round leads having one or more distal electrodes located coaxially along the distal end of the lead. It is also possible to employ an implantable medical lead having scalp electrodes instead of depth electrodes for certain types of electroencephalogram monitoring and analysis along the scalp; however, precise localization of the electrode is less feasible with scalp electrodes.  
           [0004]    Generally, implantation of an implantable medical lead involves surgically exposing an insertion site in the cranium that is an operative distance from an electrode site. An electrode site may be chosen at any appropriate stage of the surgical procedure, including pre-surgically in an operative planning stage; intraoperatively after a craniotomy has been performed or a burr hole has been made; or intraoperatively after one or more other procedures, such as functional mapping, have been performed. After an insertion site is created, the lead is then inserted to the desired electrode site along a predetermined direction to a predetermined depth.  
           [0005]    Common requirements for implantable medical leads include flexibility, strength, and durability. The extent of such qualities, of course, is dependent upon the nature of the use, for example, temporary or permanent implantation. While material selection and certain construction techniques can be tailored to assist in meeting these prescribed characteristics, one consideration in the design of implantable leads is increasing the ease with which the delicate lead is implanted.  
           [0006]    A conventional lead placement technique is illustrated in FIGS. 1 a - 1   d . Referring first to FIG. 1 a , a traditional deep brain lead  10  is shown positioned in a desired deep brain structure  12  with the assistance of a cannula  14 , a rigid tunneling and positioning tool capable of sliding over the brain lead  10 . Positioning the lead  10  is complicated by the fact that existing leads do not provide a visual indicator showing how deep the lead  10  has been inserted and/or whether the lead  10  has been accidentally moved. Even one millimeter of travel of a properly positioned lead may cause unsatisfactory results. Accordingly, reliable methods and apparatus for locating the lead in the brain are necessary.  
           [0007]    Referring now to FIG. 1 b , after the cannula  14  and lead  10  is appropriately placed, the cannula  14  is retracted. While the cannula  14  is retracted, a stylet  16  inserted at a proximal end  18  of the lead  10  may be employed to hold the lead  10  in place. The lead  10  and its position within the brain must be continually monitored to ensure proper placement.  
           [0008]    After the cannula  14  is retracted such that its distal end  20  is exposed as shown in FIGS. 1 b  and  1   c , the physician grasps the exposed portion of the lead  10  adjacent to the distal end  20  of the cannula  14 , holds it in position, and releases the stylet  16  so that the cannula  14  can be removed by sliding it longitudinally over the lead  10 . If a split cannula is employed, it may be removed in a lateral direction without sliding the cannula longitudinally over the lead  10 . However, removal of a split cannula in a lateral direction may risk damage to the delicate lead  10  as well as risk dislocation of the lead  10 .  
           [0009]    Finally, the stylet  16  must be removed without grossly affecting the position of the lead  10 . A positioned lead  10  with the stylet  16  removed is shown in FIG. 1 d . As can be seen in FIGS. 1 a - 1   d , the length  22  of the lead  10  remaining outside the insertion site is approximately at least as long as the cannula  14  itself so that the lead  10  may be held in position during installation. However, the length  22  of the lead  10  that remains outside the brain may be too long for aesthetic reasons as well as for functional attachment to a cranially implanted neurostimulator device. Also, a long lead segment  22  outside the cranium may create a site for traction on the lead  10  potentially resulting in its dislocation and/or a site for potential damage to the lead  10  itself. In some circumstances, leads with an excess of coiled-up length may have an increased susceptibility to reliability problems.  
           [0010]    In general, implanting a lead within the brain is difficult and, therefore, a lead that is easy to implant is desirable. A lead that is easier to implant improves the accurate localization of the electrode at a specific point of the brain. As a result, subsequent treatment can be carried out more precisely. Also desirable is a lead that advantageously has a minimal length of excess lead protruding from the insertion site.  
         SUMMARY OF THE INVENTION  
         [0011]    In accordance with one aspect of the present invention, there is provided an implantable lead including a body having a proximal end and a distal end. At least one electrode is located at the distal end and connected to at least one terminal that is located at the proximal end. The lead includes a plurality of markings on the body.  
           [0012]    In accordance with another aspect of the present invention, there is provided a method of implanting a lead. The method includes providing a lead that has a proximal end and a distal end. The lead has at least one electrode at the distal end that is connected to at least one terminal at the proximal end. The lead includes a plurality of markings located between the distal end and the proximal end. The method includes the step of determining an electrode site for stimulation. An insertion site is provided in the patient&#39;s cranium and the lead is inserted into the insertion site. The lead is positioned and a visual indication of lead position is provided by the markings on the lead.  
           [0013]    In accordance with yet another aspect of the present invention, there is provided an implantable lead comprising a lead portion having a proximal end and a distal end. The lead portion further includes a body between the proximal end and the distal end. The lead portion also includes at least one electrode at the distal end and at least one terminal at the proximal end. The at least one electrode is connected to the at least one terminal. The implantable lead further includes an extension removably connected to the lead portion. The extension has a proximal end and a distal end. The extension includes a body between the proximal end and the distal end.  
           [0014]    In accordance with another aspect of the present invention, there is provided an implantable lead comprising a lead portion having a proximal end and a distal end. The lead portion further includes at least one electrode at the distal end electrically connected to at least one terminal at the proximal end. The implantable lead further includes an extension removably connected to the lead portion at a breakaway zone. The extension has a proximal end and a distal end.  
           [0015]    In accordance with another aspect of the invention there is provided an implantable lead. The implantable lead includes a lead portion having a proximal end and a distal end. The lead portion includes at least one electrode at the distal end and at least one terminal at the proximal end. The at least one electrode is electrically connected to the at least one terminal. The implantable lead further includes an extension removably coupled to the lead portion. The extension has a proximal end and a distal end. The extension includes a sleeve portion that has a fixed end and a lead portion receiving end. The fixed end is fixed to the distal end of the extension. The lead portion is removably coupled within the lead portion receiving end of the sleeve.  
           [0016]    In accordance with yet another aspect of the present invention, there is provided a method of implanting a lead in a patient&#39;s brain comprising the step of providing a lead that has a lead portion and an extension. The lead portion is removably coupled to the extension. The lead portion has at least one electrode at a distal end that is electrically coupled to at least one terminal at the proximal end. The method further includes determining an electrode site for stimulation. An insertion site is provided in the patient&#39;s cranium and a cannula is inserted into the patient&#39;s brain. The lead is inserted into the cannula such that the distal end of the lead portion is inserted first. The lead is positioned at the electrode site and the cannula removed. The extension is removed from the lead portion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0018]    [0018]FIGS. 1 a - 1   d  illustrate a conventional lead placement technique;  
         [0019]    [0019]FIG. 2 is a lead of the present invention;  
         [0020]    [0020]FIG. 3 is a cross-sectional view of a lead of the present invention;  
         [0021]    [0021]FIGS. 4 a - 4   d  illustrate a lead placement technique of the present invention;  
         [0022]    [0022]FIG. 5 is a lead of the present invention;  
         [0023]    [0023]FIG. 6 is a cross-sectional view of a lead of the present invention; and  
         [0024]    [0024]FIGS. 7 a - 7   d  illustrate a lead placement technique of the present invention. 
     
    
       [0025]    While the invention is susceptible to various modifications and alternative forms, specific variations have been shown by way of example in the drawings and will be described herein. However, it should be understood that the invention is not limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0026]    Referring initially to FIG. 2, an implantable medical lead  100  is illustrated. For the purposes of this specification and claims, the term “lead” is used herein in its broadest sense and includes a stimulation lead, a sensing lead, a combination thereof or any other elongated member, such as a catheter, which may usefully be passed through an insertion site such as a cranial burr hole.  
         [0027]    The lead  100  includes a proximal end  102  and a distal end  104  and a body  101  extending therebetween. The proximal end  102  includes a plurality of electrically conductive terminals  106  for operatively coupling the proximal end  102  to a neurostimulator device or some kind of equipment. The distal end  104  includes at least one electrically conductive electrode  108  to electrically stimulate or for sensing the patient&#39;s brain. While typically each terminal  106  is electrically connected to a single electrode  108  by a conductor (shown in FIG. 3), a terminal  106  can be connected to two or more electrodes  108 . Terminals  106 , electrodes  108  and conductors are preferably formed of a non-corrosive, highly conductive material such as stainless steel, platinum, or platinum alloys such as a platinum-iridium alloy.  
         [0028]    It should further be noted that sensors other than electrodes  108  may be employed. For example, a temperature sensor (e.g., a thermocouple), a chemical concentration sensor (such as a dissolved oxygen sensor), or a pressure sensor may be advantageously employed in lieu of one or more of the distal electrodes  108  in some variations.  
         [0029]    The body  101  is made primarily from a medical grade, substantially inert material such as silicone, urethane or any of a number of suitable biocompatible materials. The body  101  provides a flexible and durable exterior structure for the components of the lead  100  and insulate adjacent terminals  106  and/or electrodes  108 . Serving as a sheath, the body  101  contains the internalized elements of the lead  100 . For example, the body  101  provides an enclosure for each conductor that connects a terminal  106  with one or more electrodes  108 .  
         [0030]    The body  101  further includes a plurality of markings  112 , preferably graduated markings  112  located along the outside of the body  101 , preferably between the electrodes  108  and terminals  106 . The markings  112  provide the physician with a visual indication of the depth to which the lead  100  has been inserted during the implant procedure and/or whether the lead  100  has been accidentally moved. Various markings  112  are within the scope of the invention. For example, any markings such as markings  112  indicating length in metric or English units, with or without numerals or letters are within the scope of the invention. Also, the markings  112  need not be graduated and nor be equally spaced, as long as the markings  112  are useful to the physician in locating the lead  100  during implantation. Even if the lead  100  stretches while being positioned, the markings  112  provide a gross estimate of positioning indication that is useful to the physician.  
         [0031]    The markings  112  are generally formed by using a silicone-based adhesive with carbon black pigment that is transferred to the body  101 . This technique is employed for radio-opaque or non-radio-opaque embodiments. In one variation, the markings  112  are printed on the body  101  of the lead  100  using ion beam assisted deposition. Various materials including metallic or ceramic materials are used for the markings  112  to provide biocompatibility or radiopacity. Alternative methods known in the art may be used for creating the markings  112 . In one variation, the markings  112  are made of radio-opaque material to facilitate implantation when X-ray or other known techniques are used during the installation. Non-radio-opaque markings  112  are also within the scope of the invention.  
         [0032]    The body  101  of the lead  100  has a diameter between approximately 0.5 mm and approximately 2.0 mm, preferably has a diameter between 1.0 and 1.3 mm. In general, a smaller diameter is better, consistent with sufficient ruggedness and ease of handling for surgical applications. A suitable overall length for the lead  100  is between approximately 250 and 500 mm.  
         [0033]    [0033]FIG. 3 illustrates an exemplary longitudinal cross section of a lead  100  representative of a distal end  104 . The lead  100  of FIG. 3 includes four ring electrodes  108  on a single distal end  104 . Varying the number of electrodes  108  may vary the illustrated structure in ways that should be apparent. It should be noted that the cross-section of FIG. 3 may also be representative of a proximal end  102 ; the construction is preferably similar or identical, though dimensions and some materials may vary.  
         [0034]    The distal end  104  of the lead  100  illustrated in FIG. 3 includes a helically arranged conductor set  114  that includes four independently insulated conductors  116 , one for each of the electrodes  108 . Each of the conductors  116  in the conductor set  114  is affixed to and in conductive communication with a respective one of the electrodes  108 .  
         [0035]    Although the conductor set  114  is depicted in FIG. 3 as helical, or coiled about the longitudinal axis of the distal end to provide some longitudinal resilience, the conductors  116  may alternatively extend longitudinally through the distal end  104 , particularly in the case of a single uninsulated conductor. Each conductor  116  is formed of a conductive material such as stainless steel, platinum, or platinum-iridium alloy. The conductor set  114  exhibits the desired mechanical properties of low resistance, corrosion, resistance, flexibility, and strength.  
         [0036]    The electrodes  108  are fabricated as rings arranged around and somewhat embedded into the flexible body  101 . A portion of the distal end  104  includes a reinforcing tube  118 , preferably fabricated from a more rigid polymer material such as polyimide, polyamide, or polyetheretherketone (PEEK). Together the body  101  and the tube  118  define a longitudinal lumen  120 , which maybe employed to receive a stylet to facilitate implantation. A stylet may not always be necessary, especially if the distal end  104  is sufficiently rigid from the use of a helical conductor set  114  and/or reinforcing tube  118 .  
         [0037]    The tip of the distal end  104  is closed by a relatively rigid plug  122  adapted to fit within the reinforcing tube  118 , or if no such tube  118  is present, within the body  101 . The plug  122  is preferably fabricated from a relatively rigid biocompatible polymer, and is crimped, glued, molded, or fused in place. It should be observed that numerous materials are possible for the plug  122 , body  101 , and reinforcing tube  118 . Various categories of polymers and plastics such as polyester, polyimide, polyamide, polyetheretherketone (PEEK), and specific materials falling into those categories such as nylon and aramid (e.g., Kevlar®), are particularly well suited.  
         [0038]    Placement of a graduated depth lead  100  is now discussed in reference to FIGS. 4 a - 4   d . An electrode site  124  is chosen at any appropriate stage of the surgical procedure, including pre-surgically in an operative planning stage; intraoperatively after a craniotomy has been performed or a burr hole  126  has been made; or intraoperatively after one or more other procedures, such as functional mapping, have been performed. Referring first to FIG. 4 a , after an incision in the patient&#39;s scalp  128  is formed, an insertion site or bore hole  126  is formed in the patient&#39;s cranium  130 . An incision is made in the dura mater  132  and a cannula  134  is inserted.  
         [0039]    The cannula  134  may be of any commercially available type. Generally, it has an open proximal end  136 , an open distal end  138 , and a central bore  140 . Prior to insertion, the cannula  134  may be provided with an obturator (not shown) to assist in penetrating the tissue structure. The obturator is inserted into the central bore  140  of the cannula  134  to close the central bore  140  of the cannula  134  and prevent tissue from entering the cannula  134  during insertion. The cannula  134  is generally constructed of surgical stainless steel or other material.  
         [0040]    Still referencing FIG. 4 a , with the cannula  134  in position and the obturator removed, a brain lead  100  is passed into the bore  140  of the cannula  134  until positioned in a desired deep brain structure. The positioning of the brain lead  100  is assisted with a stylet  142 . The stylet  142 , or guide wire is designed to be inserted into the lumen  120  of the brain lead  100  to assist in positioning the brain lead  100 . The brain lead  100  is gently pushed into the cannula  134  to the electrode site  124 . The positioned brain lead  100  is shown in FIG. 4 a . Alternate methods and devices may be used to achieve similar results.  
         [0041]    As seen in FIG. 4 a , the markings  112  on the body  101  of the brain lead  100  provide the physician with a visual indicator of how much of the brain lead  100  has been inserted. The markings  112  appearing outside of the cannula  134  are viewed. Also, markings  112  that are not outside of the cannula  134  are viewed using X-ray, fluoroscopy, and other known techniques during the installation. Viewing the markings  112  at any location along the brain lead  100  and at any stage of the implantation procedure facilitates accurate placement and positioning of the brain lead  100 . Also, with the brain lead  100  in position, the markings  112  serve as indicator in the event the brain lead  100  is accidentally moved during the remainder of the procedure and after the procedure is completed.  
         [0042]    Referring now to FIG. 4 b , after the cannula  134  and lead  100  is appropriately placed, the cannula  134  is retracted. As the cannula  134  is retracted, a stylet  142  may be employed to hold the lead  100  in place. The lead  100  and its position within the brain must be continually monitored to ensure proper placement.  
         [0043]    After the cannula  134  is retracted such that its distal end  138  is exposed at the insertion site as shown in FIGS. 4 b  and  4   c , the physician releases the stylet  142  and grasps the exposed portion of the brain lead  100  adjacent to the distal end  138  of the cannula  134 . The physician holds the brain lead  100  in position so that the cannula  134  can be removed by sliding it longitudinally over the brain lead  100 . If a split cannula is employed, it may be removed in a lateral direction without sliding the cannula longitudinally over the lead. However, removal of a split cannula in a lateral direction may risk damage to the delicate lead  100  as well as risk dislocation of the lead  100 . In either case, the markings  112  on the brain lead  100  assist in keeping the brain lead  100  stationary by providing a visual indication of whether the brain lead  100  has been moved while the cannula  134  is being extracted.  
         [0044]    Finally, the stylet  142  must be removed without grossly affecting the positioning of the lead  100 . A positioned lead  100  with the stylet  142  removed is shown in FIG. 4 d . Again, the markings  112  on the brain lead  100  assist in maintaining the lead  100  in position as the stylet  142  is removed from the lumen  120  of the brain lead  100 . Also, fine positioning of lead  100  either before or after removal of the stylet  142  is aided by the markings  112  as they are viewed outside the cranium  130  or within the cranium  130 .  
         [0045]    Referring now to FIG. 5, an implantable medical lead  200  according to another embodiment is illustrated. The lead  200  includes a lead portion  201  and an extension  203  removably coupled to the lead portion  201 . The lead portion  201  is similar to the lead  100  described with respect to FIGS.  2 - 4  and like numerals are used to refer to like elements. The lead portion  201  includes a proximal end  202 , a distal end  204 , and a body  201  therebetween. The proximal end  202  includes a plurality of electrically conductive terminals  206  for operatively coupling the proximal end  202  to a neurostimulator device that is implanted in the patient&#39;s cranium or some kind of equipment. The distal end  204  includes at least one electrically conductive electrode  208  to electrically stimulate or for sensing the patient&#39;s brain. Typically, each terminal  206  is electrically connected to a single electrode  208  by a conductor (not shown), however, a terminal  206  can be connected to two or more electrodes  208 . The conductor exhibits the desired mechanical properties of low resistance, corrosion, resistance, flexibility, and strength. Terminals  206 , electrodes  208  and conductors are preferably formed of a non-corrosive, highly conductive material such as stainless steel, platinum, or platinum alloys such as a platinum-iridium alloy. It should further be noted that sensors other than electrodes  208  may be employed on some or all of the distal end  204 . For example, a temperature sensor (e.g., a thermocouple), a chemical concentration sensor (such as a dissolved oxygen sensor), or a pressure sensor may be advantageously employed in lieu of one or more of the distal electrodes  208  in some variations.  
         [0046]    The body  201  is made primarily from a medical grade, substantially inert material such as silicone, urethane or any of a number of suitable biocompatible materials. The body  201  provides a flexible and durable exterior structure for the components of the lead  200  and insulate adjacent terminals  206  and/or electrodes. Serving as a sheath, the body  201  contains the internalized elements of the lead portion  201 . For example, the body  201  provides an enclosure for each conductor that connects a terminal  206  with one or more electrodes  208 .  
         [0047]    The body  201  of the lead portion  201  has a diameter between approximately 0.5 mm and approximately 2.0 mm, preferably has a diameter between 1.0 and 1.3 mm. In general, a smaller diameter is better, consistent with sufficient ruggedness and ease of handling for surgical applications. A suitable overall length for the lead portion  201  is between approximately 250 and 500 mm. In one variation, the body  201  further includes a plurality of markings  212  of the type described with respect to FIGS.  2 - 5  above. The markings  212  are preferably graduated markings  212  located along the outside of the body  201 , preferably between the electrodes  208  and terminals  206 . The longitudinal cross section of the lead  201  representative of the distal end  204  and proximal end  206  is similar to the cross-section depicted in FIG. 3.  
         [0048]    A portion of the distal end  204  includes a reinforcing tube (not shown) preferably fabricated from a more rigid polymer material such as polyamide, polyimide, or polyetheretherketone (PEEK). Together the body  201  and the tube define a longitudinal lumen  220 , which may be employed to receive a stylet to facilitate implantation. A stylet may not always be necessary, especially if the distal end  204  is sufficiently rigid from the use of a helical conductor set (not shown) and/or reinforcing tube.  
         [0049]    The tip of the distal end  204  is closed by a relatively rigid plug  222  adapted to fit within the reinforcing tube, or if no such tube is present, the plug  222  is adapted to fit within the body  201 . The plug  222  is preferably fabricated from a relatively rigid biocompatible polymer, and is crimped, glued, molded, or fused in place. It should be observed that numerous materials are possible for the plug  222 , body  201 , and reinforcing tube. Various categories of polymers and plastics such as polyester, polyimide, polyamide, polyetheretherketone (PEEK), and specific materials falling into those categories such as nylon are particularly well suited.  
         [0050]    The lead portion  201  is removably coupled to an extension  203 . The removable extension  203  includes a proximal end  244  and a distal end  246 . The distal end  246  of the extension  203  is removably coupled to the proximal end  202  of the lead portion  201  at a separation zone or break-away zone  248 . The extension  203  includes a lumen  250  extending longitudinally along the length of the extension  203  from the proximal end  244  of the extension  203  to the distal end  246  of the extension  203 . The proximal end  244  of the extension  203  includes a lumen opening  252  for receiving a stylet. At the distal end  246  of the removable extension  203 , the lumen  250  of the extension  203  connects with the lumen  220  of the lead portion  201  allowing a stylet to pass through the extension  203  and into the lead portion  201 .  
         [0051]    The removable extension  203  has a diameter that is substantially the same as the diameter of the lead portion  201 . The diameter of the removable extension  203  is between approximately 0.5 mm and approximately 2.0 mm, preferably has a diameter between 1.0 and 1.3 mm. In one variation, the break-away zone  248  has a reduced diameter as discussed below. A suitable overall length for the removable extension  203  is at least as long as the length of a standard cannula or approximately 250 and 500 mm. In one variation, the removable extension  203  further includes a plurality of markings  254  of the type described with respect to FIGS.  2 - 5  above. The markings  254  are preferably graduated  254  markings located along the outside of the extension  203 . In one variation, the lead portion  201  includes markings  212  and the extension  203  does not. In another variation, the extension  203  includes markings  254  and the lead portion  201  does not. In yet another variation, the lead portion  201  includes markings  212  and the extension  203  includes markings  254 . Markings  212  on the lead portion  201  can be used independently from the markings  254  on the extension  203  and markings  254  on the extension  203  can be used independently of markings  212  on the lead portion  201 . Also, the markings  212  on the lead portion  201  can be used in combination with the markings  254  on the extension  203 . It should be noted that the markings on the lead portion can be used with or without an extension and vice versa.  
         [0052]    The extension  203  is made from medical grade, substantially inert material such as silicone, urethane or any of a number of suitable biocompatible materials and provides a flexible and durable structure. The extension  203  is made from the same material having the same characteristics as the body  201  of the lead portion  201 . Alternatively, the same material as the lead portion  201  or material different from the lead portion  201  having different characteristics is employed. In essence, the material of the removable extension  203  is customized to provide the desired flexibility, durability and ease of installation. In one variation, for example, the removable extension  203  is less flexible than the lead portion  201 . A stiffer removable extension  203  relative to the lead portion  201  helps guide the lead portion  201  into position.  
         [0053]    The removable extension  203  is removably coupled to the lead portion  201  in a connecting yet separable fashion to the proximal end  202  of the lead portion  201 . This can be accomplished in a variety of ways. For example, the removable extension  203  may be coupled to the lead portion  201  with adhesive. Alternatively, the removable extension  203  is coupled to the lead portion  201  in a snap-fit engagement. In another example, the removable extension  203  is coupled to the lead portion  201  in a compression-fit engagement. In yet another example, the removable extension  203  is crimped to the lead portion  201 . Also, the removable extension  203  can be coupled to the lead portion  201  in a friction-fit engagement. Any suitable connecting yet separable fashion is within the scope of the invention for connecting the removable extension  203  to the lead portion  201 . The removable extension  203  is removed by application of manual force or with a removal tool such as scissors. The amount of force required to remove the extension  203  is generally greater than the force endured by the lead  200  during normal implantation such that the extension  203  will not be accidentally separated from the lead portion  201 . The removable extension  203  is either integrally formed with the lead portion  201  or formed separately from the lead portion  201  and then removably coupled to the lead portion  201 .  
         [0054]    As shown in FIG. 5, the body  201  of the lead portion  201  is integrally formed with the removable extension  203  wherein the break-away zone  248  includes a score  256  for facilitating removal of the extension  203 . As a result of the score  256 , the break-away zone  248  has a reduced diameter or cross-sectional area of material at the break-away zone  248  that effectively weakens the break-away zone  248 . The score  256  also provides a visual indicator of break-away zone  248 . Alternatively, the break-away zone  248  is not substantially weaker relative to the remainder of the extension  203  and/or the lead portion  201 . In such a variation, the extension  203  is preferably removed from the lead portion  201  with a removal tool and the score  256  or other marking serves as a visual indicator for an ideal separation point. Methods other than scoring are also possible. For example, forming a groove by molding or machining, cutting a notch or forming a crimp are within the scope of the invention for creating a visual indicator and/or a break-away zone  248  that is weaker or visually defined relative to the lead portion  201  and/or remaining portion of the extension  203 .  
         [0055]    Referring now to FIG. 6, there is shown another variation of a separation zone or break-away zone  258  in which the removable extension  203  is removably coupled to the lead portion  201 . In this variation the removable extension  203  includes a sleeve portion  260  having a fixed end  262  and a lead portion receiving end  264 . The fixed end  262  of the sleeve  260  is fixed to the distal end  246  of the extension  203 . The lead portion receiving end  264  receives the lead portion  201  such that the lead portion  201  is retained in the lead portion receiving end  264 . The lead portion  201  is removably retained in the lead portion receiving end  264  such that the extension  203  is coupled to the lead portion  201  in a compression-fit engagement. Alternatively, the removable extension  203  can be coupled in a separable fashion to the lead portion  201  in a variety of ways. For example, the removable extension  203  may be coupled to the lead portion  201  with adhesive. Alternatively, the removable extension  203  is coupled to the lead portion  201  in a snap-fit engagement. In yet another example, the removable extension  203  is crimped to the lead portion  201 . Also, the removable extension  203  can be coupled to the lead portion  201  in a friction-fit engagement. Any suitable connecting yet separable fashion is within the scope of the invention for removably coupling the removable extension  203  to the lead portion  201 . In one variation, the break-away zone includes a split-sleeve. The split sleeve, for example, can be employed in a compression-fit engagement or adhered using adhesive.  
         [0056]    The removable extension  203  is removed by application of manual force such as with a pulling force or with a removal tool. The amount of force required to remove the extension  203  is generally greater than the force endured by the lead  200  during normal implantation such that the extension  203  will not be accidentally separated from the lead portion  201 .  
         [0057]    The general procedure for implanting the lead in the brain is now discussed in reference to FIGS. 7 a - 7   d . An electrode site  224  within the brain tissue is chosen. The site is chosen at any appropriate stage of the surgical procedure, including pre-surgically in an operative planning stage; intraoperatively after a craniotomy has been performed or a burr hole  226  has been made; or intraoperatively after one or more other procedures, such as functional mapping, have been performed. Referring first to FIG. 7 a , after an incision in the patient&#39;s scalp  228  is formed, an insertion site or bore hole  226  is formed in a patient&#39;s cranium  230 . An incision is made in the dura mater  232  and a cannula  234  is inserted.  
         [0058]    The cannula  234  may be of any commercially available type. Generally, it has an open proximal end  236 , an open distal end  238 , and a central bore  240 . Prior to insertion, the cannula  234  may be provided with an obturator (not shown) to assist in penetrating the tissue structure. The obturator is inserted into the central bore  240  of the cannula  234  to close the central bore  240  of the cannula  234  and prevent tissue from entering the cannula  234  during insertion.  
         [0059]    Still referencing FIG. 7 a , with the cannula  234  in position and the obturator removed, a brain lead  200  is passed into the bore  240  of the cannula  234  until positioned in a desired deep brain structure. The positioning of the brain lead  200  is assisted with a stylet  242 . The stylet  242 , or guide wire is inserted into the lumen  250  of the extension  203  and into the lumen  220  of the lead portion  201  to assist in positioning the lead portion  201 . The lead  200  is gently pushed and guided into the cannula to the electrode site  224  with the stylet  242 . The positioned brain lead  200  is shown in FIG. 7 a . Alternate methods and devices may be used to achieve similar results.  
         [0060]    As seen in FIG. 7 a , the markings  212 ,  254  on the body  201  of the lead portion  201  and/or on the extension  203  provide the physician with a visual indicator of how much of the brain lead  200  has been inserted. The markings  212 ,  254  appearing outside of the cannula  234  are viewed. Also, in one variation, markings  212 ,  254  that are not outside of the cannula  234  are viewed using X-ray, fluoroscopy, and other known techniques during the installation. If the extension  203  includes markings  254  and the lead portion  201  does not, then the markings  254  are used. Also, if the lead portion  201  includes markings  212  and the extension does not, then the markings  212  are used by the physician. If both the lead portion  201  and the extension  203  include markings  212  and  254 , respectively, then either the markings  212  or the markings  254  may be employed by the physician in positioning the lead. By viewing the markings  212 ,  254  at any location along the lead portion  201  and/or extension  203  at any stage of the implantation procedure facilitates accurate placement and positioning of the lead  200 . Also, with the brain lead  200  in position, the markings  212 ,  254  serve as indicator in the event the brain lead  200  is accidentally moved during the remainder of the procedure and after the procedure is completed.  
         [0061]    Referring now to FIG. 7 b , after the cannula  234  and lead portion  201  is appropriately placed, the cannula  234  is retracted. As the cannula  234  is retracted, a stylet  242  is employed to hold the lead  200  in place at a proximal end  244  of the extension  203 . The lead  200  and its position within the brain must be continually monitored to ensure proper placement.  
         [0062]    After the cannula  234  is retracted such that its distal end  238  of the cannula  234  is exposed at the insertion site as shown in FIGS. 7 b  and  7   c , the physician releases the stylet  242  and grasps the exposed portion of the lead portion  201  adjacent to the distal end  238  of the cannula  234 . The physician holds the lead portion  201  in position so that the cannula  234  can be removed by sliding it longitudinally over the lead portion  201  and the extension  203 . The markings  212 ,  254  on the brain lead  200  assist in keeping the brain lead  200  stationary by providing a visual indication of whether the brain lead  200  has been moved while the cannula  234  is being extracted.  
         [0063]    Next, the stylet  242  must be removed without grossly affecting the positioning of the lead  200 . A positioned lead  200  with the stylet  242  removed is shown in FIG. 7 d . Again, the markings  212 ,  254  on the lead portion  201  and/or extension  203  assist in maintaining the lead portion  201  in position as the stylet  242  is removed from the lumen  220  of the lead portion  201  and the lumen  250  of the extension  203 . Also, fine positioning of lead portion  201  either before or after removal of the stylet  242  is aided by the markings  212 ,  254  as they are viewed outside the brain or within the brain with the aid of viewing instrumentation.  
         [0064]    Finally, the extension  203  is removed by applying a force sufficiently large to separate the extension  203  from the lead portion  201  at the break-away zone  248 . Alternatively, a removal tool is employed to sever the extension  203  at the break-away zone  248 . With the extension  203  removed, the terminals  206  at the proximal end  202  of the lead portion  201  are ready to be connected to an implantable neurostimulator device that is or is not implanted in the patient&#39;s cranium or to other equipment.  
         [0065]    The extension  203  serves as a sacrificial segment that is discarded after the implantation of the lead portion  201  is accomplished. Traditionally, the lead portion  201  is longer than minimally needed by approximately the length of the cannula  234  so that the cannula  234  can be removed while the lead portion  201  is held in position. With the extension  203  of the present invention, however, a shorter lead portion  201  is possible. Therefore, the extension  203  advantageously avoids a lead portion  201  that is too long for aesthetic reasons as well as for functional attachment to a cranially implanted neurostimulator device while increasing the ease of installation. Furthermore, post-installation, a shorter lead portion  201  reduces possible traction on the lead portion  201  that would potentially result if a longer lead portion  201  was connected to a neurostimulator device implanted in the same location of the cranium. Also, susceptibility to reliability problems may be reduced in the instant invention as compared to lead portions with an excess of coiled-up length. Furthermore, the extension  203  increases the ease of cannula  234  removal as well as lead portion  201  positioning.  
         [0066]    While the present invention has been described with reference to one or more particular variations, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof are contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.