Abstract:
A medical electrical lead system for neurological applications has a distal portion having a plurality of independently positionable seed electrodes, each of which may be connected via an interface to an implantable medical device. The interface allows the seed electrodes to be positioned, then excess wire trimmed, facilitating simplified connection of multiple independent electrodes to a single device. Seed electrodes according to the invention are small, have relatively low mass, and are minimally destructive of surrounding tissue.

Description:
FIELD OF THE INVENTION 
   The invention is related to implantable medical leads, and more particularly to implantable electrical leads used to sense electrographic signals from a patient&#39;s brain or to apply electrical stimulation to the brain. 
   BACKGROUND OF THE INVENTION 
   In the 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. For a review of this technology, see Ajmone-Marsan, C., Electrocorticography: Historical Comments on its Development and the Evolution of its Practical Applications, Electroencephalogr. Clin. Neurophysiol. Suppl. 1998, 48:10-16; there are numerous other applications. These electrographic signals are commonly known as electroencephalogram (EEG) signals when originating or received at the surface of the brain, such as from scalp electrodes, or electrocorticogram (ECoG) signals when originating or received below the surface of the brain, such as from intracranial electrodes. The term “EEG” will be used generically herein to refer to both types of signals. 
   It is also becoming accepted to apply electrical stimulation to various structures of the brain for both diagnostic and therapeutic purposes. For an exemplary diagnostic application, see Black, P. M. &amp; Ronner S. F., Cortical Mapping for Defining the Limits of Tumor Resection, Neurosurgery 1987, 20:914-919, which addresses the use of electrical stimulation via deep brain electrodes to identify functional portions of the brain prior to and as a planning stage in surgical resection. For an example of a therapeutic application, see Cooper, I. S. &amp; Upton, A. R. M., Effects of Cerebellar Stimulation on Epilepsy, the EEG and Cerebral Palsy in Man, Electroencephalogr. Clin. Neurophysiol. Suppl. 1978, 34: 349-354. In both of these examples, acutely implanted brain electrodes are connected to external equipment. 
   It is also contemplated that chronic electrical stimulation can be used as a direct treatment for disorders such as epilepsy. See, e.g., U.S. Pat. No. 6,016,449 to Fischell, et al., which describes an implantable neurostimulator that is coupled to relatively permanent deep brain electrodes. 
   Although it is frequently possible to employ scalp electrodes for certain types of EEG monitoring and analysis, it has been found that ambient electrical noise (such as from the 50/60 Hz power system) can adversely impact signal-to-noise ratio, and certain signal components of interest may be filtered out by the patient&#39;s intervening cranium and scalp tissue. Moreover, precise localization is less feasible with scalp electrodes. 
   Accordingly, intracranial signal analysis, that is, the consideration of signals that originate from within a patient&#39;s cranium, whether by internal or external apparatus, is best accomplished with brain surface electrodes, such as strip and grid electrodes, cortical depth leads, or some combination of surface electrodes and depth leads. 
   Typical brain surface strip and grid electrode arrays consist of flat, disk-shaped electrodes that are placed on the surface of the patient&#39;s brain. In a typical strip or grid electrode array, each electrode has an exposed diameter of approximately 3 mm (or ⅛ inch), and the electrodes are distributed along a line (for a strip electrode array) or in a rectangular grid (for a grid electrode array) at a pitch of approximately 10 mm. 
   Unfortunately, brain surface strip and grid electrode arrays have a tendency, particularly with long-term chronic use, to move away from the surface of the brain. This can be caused by atrophy or other mechanisms associated with cerebrospinal fluid (CSF) dynamics. The result is frequently unsatisfactory or intermittent electrical contact between the electrodes and the desired brain tissue. It frequently requires further surgery (with the associated risks for the patient) or electronic compensation for the change in characteristics (with a potentially harmful increase in stimulation current being delivered to the brain, or at minimum, decreased signal-to-noise ratio), and may result in long-term performance deterioration. There is no known acceptable way to anchor a traditional strip or grid electrode array to the surface of the brain. While the electrode may be anchored to the patient&#39;s cranium or dura mater, the brain tends to recede from these structures in certain cases. Moreover, the electrodes are spaced evenly along a line or grid, and while it is possible to orient a strip or grid electrode array in a desired manner, it is generally not possible to position the individual electrodes independently. 
   Typical brain depth leads are flexible small-diameter (usually 1-1.5 mm) round leads having distal electrodes. It is known for depth leads to have multiple independent distal electrodes on the same lead shaft, but such electrodes are generally located coaxially along a distal portion of the shaft. It is difficult, and usually impractical, to attempt to position the individual electrodes independently. 
   Accordingly, it would be desirable to have an implantable medical electrical lead that provides the advantages of both surface electrodes and depth leads, along with other advantages. Such an electrical lead would have multiple distal electrodes that are independently positionable near the surface of the brain or in deep brain structures, and would remain in contact with the desired neural tissue regardless of atrophy or other adverse conditions. 
   SUMMARY OF THE INVENTION 
   A medical electrical lead system in accordance with the present invention addresses the shortcomings of existing implantable lead systems by providing multiple independently positionable “seed electrodes”. A seed electrode system according to the invention has independent leads that have minimal excess slack, and thus are resistant to breakage and erosion, and take up little space. The seed electrodes are small and have minimal mass, thereby avoiding inertial movement, and resisting changes due to brain atrophy. Seed electrodes according to the invention are relatively easy to implant, stay in place, and are believed to be less traumatic than traditional depth leads (thereby enabling new surgical strategies, including implanting many seed electrodes and interfacing relatively few of them to a device). 
   The inventive medical electrical lead system has a multi-conductor proximal segment configured to connect to an implantable medical device or external equipment. At its distal end, the proximal segment connects to an interface module, which in one embodiment of the invention also serves as a burr hole cover. The interface module connects and maps one or more of the conductors in the proximal segment to one or more individually positionable insulated conductor filaments, each of which has a distal “seed electrode” adapted for implantation according to the invention. In one embodiment of the invention, the interface module includes an active multiplexer circuit, enabling more seed electrodes to be coupled to a medical device than the device would otherwise support. 
   An introducer apparatus is specially configured to implant the seed electrodes of the present invention. The introducer apparatus is adapted to interface with one or more commonly used Stereotactic head frame assemblies and to accurately position the seed electrodes of the invention within the patient&#39;s brain. In an embodiment of the invention, the introducer apparatus includes a relatively rigid insertion cannula, a push tube, and a depth calibrating spacer. The seed electrode is releasably attached to the insertion cannula such that the push tube can be manipulated to break the attachment and leave the seed electrode in place while the cannula is retracted. 
   With a lead system in accordance with the present invention, it is possible to realize several additional advantages. With plural individually positionable distal electrodes, it is possible to reach a larger number of separate brain sites while limiting the number of leads necessary to do so. The number of leads connected to an external apparatus or implanted neurostimulator (or other device) is minimized, thereby improving the ease of treating the patient, improving ease of lead management, reducing the possibility of lead breakage, and reducing the possibility of discomfort or erosion under the patient&#39;s scalp. 
   A seed electrode system according to the invention can be used for sensing applications, stimulation applications, or dual-purpose applications. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features, and advantages of the invention will become apparent from the detailed description below and the accompanying drawings, in which: 
       FIG. 1  is an illustration showing the use of a neurostimulator, lead, and interface according to the invention in an exemplary patient cranium; 
       FIG. 2  is a block diagram illustrating the use of two seed electrode interfaces and eight seed electrodes in a system according to the invention; 
       FIG. 3  is a cross-sectional illustration showing an exemplary seed electrode and lead assembly according to the invention; 
       FIG. 4  is a cross-sectional illustration showing the seed electrode and lead assembly of  FIG. 3  in conjunction with an insertion cannula and push tube according to the invention; 
       FIG. 5  is an illustration of an exemplary proximal lead end and seed electrode interface according to the invention; 
       FIG. 6  depicts the use of a set of four seed electrodes according to the invention in an exemplary section of a patient&#39;s head, including the patient&#39;s brain, dura mater, and cranium, where the seed electrode interface constitutes at least a part of a burr hole cover; 
       FIG. 7  depicts the use of a set of four seed electrodes according to the invention in an exemplary section of a patient&#39;s head, including the patient&#39;s brain, dura mater, and cranium, where the seed electrode interface constitutes a subcutaneous interface positioned between the patient&#39;s scalp and cranium; and 
       FIG. 8  shows an exemplary introducer apparatus according to the invention, used for positioning the seed electrode and lead assembly of  FIG. 3  in a patient&#39;s brain. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention is described below, with reference to detailed illustrative embodiments. It will be apparent that a system according to the invention may be embodied in a wide variety of forms. Consequently, the specific structural and functional details disclosed herein are representative and do not limit the scope of the invention. 
   An implantable neurostimulator device  110  according to the invention, as implanted intracranially in a patient  112 , is illustrated in  FIG. 1 . The device  110  is affixed in the patient&#39;s cranium  114  by way of a ferrule  116 . The ferrule  116  is a structural member adapted to fit into a cranial opening, attach to the cranium  114 , and retain the device  110 . 
   To implant the device  110 , a craniotomy is performed in the parietal bone anterior to the lambdoidal suture to define an opening slightly larger than the device  110 . The ferrule  116  is inserted into the opening and affixed to the cranium  114 , ensuring a tight and secure fit. The device  110  is then inserted into and affixed to the ferrule  116 . 
   As shown in  FIG. 1 , the device  110  is attached to a lead  118  by way of a lead connector  120  adapted to receive one or more electrical leads, such as the illustrated lead  118 . The lead connector  120  acts to physically secure the lead  118  to the device  110 , and facilitates electrical connection between a conductor in the lead  118  coupling a seed electrode according to the invention to circuitry within the device  110 . The lead connector  120  accomplishes this in a substantially fluid-tight environment with biocompatible materials. 
   A proximal portion of the lead  118 , as illustrated, and other leads for use in a system or method according to the invention, is a flexible elongated member having one or more conductors. As shown, the lead  118  is coupled to the device  110  via the lead connector  120 , and is generally situated on the outer surface of the cranium  114  (and under the patient&#39;s scalp), extending between the device  110  and a seed electrode interface  122  taking the form of a burr hole cover, where the lead  118  enters the cranium  114  and is coupled to one or more seed electrodes implanted in desired locations in the patient&#39;s brain (such as the GPi, the thalamus, or the subthalamic nucleus). If the length of the lead  118  is substantially greater than the distance between the device  110  and the interface  122 , any excess may be urged into a coil configuration under the scalp. 
   The device  110  includes a durable outer housing fabricated from a biocompatible material. Titanium, which is light, extremely strong, and biocompatible, is used in analogous devices, such as cardiac pacemakers, and would serve advantageously in this context. As the device  110  is self-contained, the housing encloses a battery and any electronic circuitry necessary or desirable to provide the functionality described herein, as well as any other features. As will be described in further detail below, a telemetry coil may be provided outside of the housing (and potentially integrated with the lead connector  120 ) to facilitate communication between the device  110  and external devices. 
   The neurostimulator configuration described herein and illustrated in  FIG. 1  provides several advantages over alternative designs. First, the self-contained nature of the neurostimulator substantially decreases the need for access to the device  110 , allowing the patient to participate in normal life activities. Its small size and intracranial placement causes a minimum of cosmetic disfigurement. The device  110  will fit in an opening in the patient&#39;s cranium, under the patient&#39;s scalp, with little noticeable protrusion or bulge. The ferrule  116  used for implantation allows the craniotomy to be performed and fit verified without the possibility of breaking the device  110 , and also provides protection against the device  110  being pushed into the brain under external pressure or impact. A further advantage is that the ferrule  116  receives any cranial bone growth, so at explant, the device  110  can be replaced without removing any bone screws—only the fasteners retaining the device  110  in the ferrule  116  need be manipulated. 
     FIG. 2  is a block diagram illustrating how the implantable neurostimulator device  110  ( FIG. 1 ) is coupled to two interface modules in the disclosed embodiment of the invention. As illustrated in  FIG. 1 , the device  110  includes a lead connector  120  adapted to receive two multi-conductor leads, although only one lead  118  is illustrated in  FIG. 1 . Accordingly, one lead is used to couple a first seed electrode interface  212  to device  110 . In turn, the first seed electrode interface  212  is coupled to four seed electrode assemblies,  214 ,  216 ,  218 , and  220 . Each of the seed electrode assemblies  214 - 220  is electrically coupled to the device  110  through the first interface  212  as illustrated. Similarly, a second seed electrode interface  222  electrically couples four seed electrode assemblies  224 ,  226 ,  228 , and  230  to the device  110 . It should be observed that other configurations of the device  110  and one or more interfaces and seed electrode assemblies are possible. In one embodiment of the invention, seed electrode assemblies can be connected directly to the device  110 . However, and interface (such as the two interfaces  212  and  222 ) interposed between the device  110  and any seed electrodes will allow the device  110  to be explanted and replaced, or additional seed electrodes added, without the need to disturb existing seed electrode assemblies 
     FIG. 3  depicts an exemplary seed electrode assembly  310  of the type illustrated in block form in  FIG. 2 . The illustrated seed electrode assembly  310  includes a metallic seed electrode  312  at its distal end. In the disclosed embodiment, the seed electrode  312  is about 1 mm in diameter and is fabricated from a biocompatible conductive material such as platinum or a platinum-iridium alloy. The seed electrode  312  is coupled to a length of wire  314 , typically also a biocompatible material such as Pt or Pt—Ir. The wire  314  is significantly thinner than the diameter of the seed electrode  312  (and in the disclosed embodiment has a diameter of approximately 0.1 mm) and is covered with an insulating coating  316 . The disclosed wire  314  has a length of at least 30 cm, and may be cut to a desired shorter length upon use. It is possible to use shorter or longer lengths without departing from the scope of the invention. 
   The seed electrode  312  and the wire  314  are electrically coupled to each other; typically the two pieces may be welded or crimped together. The wire  314  may be a single strand of conductive material, or if some compliance is desired, may comprise plural strands of thinner material wound together. The seed electrode assembly  310  should be strong enough to resist fracture and failure during the implant process and chronic use by the patient thereafter; this principle would guide selection of materials, dimensions, and tolerances by a practitioner of ordinary skill. 
   In  FIG. 4 , the seed electrode assembly  310  ( FIG. 3 ) is shown in relation to other parts of a system according to the invention; these parts are used to implant the seed electrode assembly  310 . Specifically, the seed electrode  312  is disposed at a distal end of a tube-shaped insertion cannula  418  of substantially the same outer diameter as the seed electrode  312 . The seed electrode  312  is mechanically coupled to the distal end of the insertion cannula  418  via a quantity of releasable biocompatible adhesive  420 . The wire  314  (with its insulating coating  316 ) is threaded longitudinally through a bore of the insertion cannula  418 . The insertion cannula is fabricated from a relatively rigid biocompatible material that is capable of tunneling through the patient&#39;s brain. 
   A relatively rigid push tube  422  is positioned between (and coaxial with) the wire  314  and the insertion cannula  418 . The push tube does not appreciably adhere to the seed electrode  312  or the cannula  418 ; rather, it is preferably free to move longitudinally when manipulated by a clinician. As will be discussed in further detail below (in connection with  FIG. 8 ), the push tube  422  is used to disengage the seed electrode  312  from the cannula  418  when a desired position within the patient&#39;s brain is reached. 
   The cannula  418  and push tube  422  are fabricated from a material suitable for use in acute surgical settings, such as stainless steel. As described above, the outer diameter of the insertion cannula  418  is generally defined by the diameter of the seed electrode  312 ; other inner and outer diameters (for the cannula  418  and push tube, for example) may be determined on a relative basis by considering the desired mechanical characteristics of a system according to the invention as would be understood by a practitioner of ordinary skill in the art. 
   An embodiment of a seed electrode interface  510  is illustrated in  FIG. 5 . In the disclosed embodiment, the interface  510  is a relatively thin disc-shaped unit coupled to a proximal terminal end  512  adapted to be coupled to a neurostimulator (such as the device  110  of  FIG. 1 ). The proximal terminal end  512  includes four ring terminals  514 ,  516 ,  518 , and  520 , each of which is electrically coupled to a conductor embedded within a substantially cylindrical lead body  522 . The lead body  522  connects each of the ring terminals  514 - 520  to a housing  524  of the interface  510 . In the disclosed embodiment of the invention, the lead body  522  is fabricated from biocompatible silicone, and the ring terminals  514 - 520  are Pt—Ir alloy. The lead body  522  has a diameter between 1.0 and 1.5 mm and a length between 5 and 20 cm, although other dimensions are possible. 
   The interface  510  further includes a cover  526  adapted to fit the housing  524 . In the disclosed embodiment, the cover defines an aperture  528  configured to receive a fastener to attach the cover  526  to the housing  524 , such as a captive screw (not shown). The cover  526  is intended to substantially exclude body fluids and growth from entering an interior portion of the housing, although it is to be expected that some leakage is probable. 
   The interface  510 , within the housing  524 , includes four punchdown terminals  530 ,  532 ,  534 , and  536 . Each of the punchdown terminals  530 - 536  includes a conductive blade electrically coupled to a corresponding ring terminal  514 - 520  at the proximal terminal end  512  of the interface  510 . 
   Accordingly, then, one or more seed electrode assemblies (as in the seed electrode assembly  310  of  FIG. 3 ) may be coupled to the interface  510 ; the disclosed embodiment of the interface  510  receives up to four seed electrode assemblies. A single seed electrode assembly  310  ( FIG. 3 ) is connected to the interface  510  by routing its wire  314  through a hole (such as one of the two visible holes  540  and  542 ) in the housing, positioning the wire  314  over a desired and selected punchdown terminal one of the punchdown terminals  530 - 536 ), and pressing the wire  314  down into the selected punchdown terminal at substantially any location along the length of the wire  314 . A tool may be provided for the purpose; punchdown terminals and tools are generally understood by practitioners of ordinary skill in the art to which this invention pertains. 
   As is well known, the act of pushing the wire  314  down into a punchdown terminal will cause the insulating cover  316  over the wire  314  to be penetrated by the blade of the punchdown terminal, enabling electrical conduction between the blade and the wire  314 , and hence a closed electrical circuit between one of the ring terminals  514 - 520  and the seed electrode  312 . If there is any unused length of wire  314  after it is connected to the interface  510 , it may be trimmed. It should be apparent that after trimming, different custom lengths of wire  314  may be present, thereby reducing any undesired slack in wires leading up to the seed electrode  312 . 
   The process set forth above is repeated for up to four seed electrode assemblies in a single interface, or eight seed electrode assemblies with two interfaces (provided the disclosed embodiment of the device  110  is used; as stated above, other configurations are possible). It will be recognized that if two interfaces are used, they may be positioned in different locations. In an embodiment of the invention, the interface  510  includes active multiplexing circuitry to allow a relatively large number of seed electrode assemblies to be driven by or responsive to a relatively small number of input/output channels on the device. 
   In an embodiment of the invention, the cover  526  defines a lower surface adapted to mechanically clamp a short portion of seed electrode assemblies within the housing  524  when the cover  526  is engaged with the housing  524 ; this provides additional strain relief on the seed electrode assemblies and serves to exclude fluids and tissue growth to some extent. 
   The interface  510  illustrated in  FIG. 5  is adapted to be positioned between the patient&#39;s scalp and cranium; suture holes or other fastening means may be provided as desired to anchor the interface  510  in a preferred position. 
     FIG. 6  illustrates an embodiment of the interface  510  incorporated into a burr hole cover assembly  610 . As illustrated, In general, an interface  510  according to the invention is advantageously situated below a patient&#39;s scalp  612 , and in the illustrated embodiment the burr hole cover  610  (incorporating the functions of the interface  510 ) extends through the patient&#39;s cranium  614 , and seed electrode assemblies penetrate the dura mater  616  to access the patient&#39;s cortex  618 . 
   As illustrated, and consistent with the illustration of  FIG. 6 , the disclosed burr hole cover  610  has a lead body  620  connected to a neurostimulator (such as the device  110 , not shown) and accommodates four seed electrode assemblies  622 ,  624 ,  626 , and  628 , which are shown inserted into desired electrode sites. Contrary to the illustration of  FIG. 5 , this embodiment receives the lead body  620  at a top location and the seed electrode assemblies  622 - 628  at a bottom location. Choosing desired electrode sites may be performed at any appropriate stage of the surgical procedure, including presurgically in an operative planning stage; intraoperatively after a craniotomy have been performed or a burr hole has been made; or intraoperatively after one or more other procedures, such as functional mapping, have been performed. 
   Each of the seed electrode assemblies  622 - 628  is inserted a short distance into the cortex  618 , enough to ensure their distal electrodes are fully embedded in neural tissue. This configuration can be adapted to serve as a replacement for a strip electrode, with four electrodes inserted shallowly in the cortex in an essentially collinear configuration. Other configurations (including different implantation depths and non collinear configurations) are, of course, possible, and are described elsewhere herein. 
   The seed electrode assemblies  622 - 628  are preferably inserted into the cortex relatively perpendicular to the surface of the brain (although different trajectories are possible and at times clinically desirable, as determined in surgical planning); this arrangement minimizes tissue damage and orients the electrodes consistently with respect to each other. In the absence of external forces, the distal end segments will ordinarily remain implanted in the desired electrode sites without any affixation means. 
   By way of explanation, it is anticipated that implantation of the seed electrode assemblies  622 - 628  will be performed before the burr hole cover  610  is in place. After the seed electrode assemblies  622 - 628  are positioned, their corresponding wires are routed into the burr hole cover  610 , the burr hole cover  610  is placed and affixed, and the wires for the seed electrode assemblies  622 - 628  are connected to punchdown terminals within the burr hole cover  610  and trimmed as desired. The cover  526  and housing  524  are then attached to each other, and the lead body  620  is then routed to the device  610 , over the cranium  614 , as desired. 
     FIG. 7  illustrates a subcutaneous module  710  form of the interface  510 . It does not function as a burr hole cover, but otherwise functions similarly to the embodiment of  FIG. 6 . The subcutaneous module  710  is positioned between the patient&#39;s scalp  612  and cranium  614 , and if desired, is affixed in position or embedded into a space defined by the cranium  614 . A lead body  720  connects the subcutaneous module  710  to a neurostimulator (as in the device  110 , not shown). The subcutaneous module  710  receives four seed electrode assemblies  722 ,  724 ,  726 , and  728 . Because the subcutaneous module  710  does not function as a burr hole cover, the seed electrode assemblies  722 - 728  are connected to the module  710  above the cranium  614 . Accordingly, each of the seed electrode assemblies  722 - 728  (such as a first seed electrode assembly  722 ) will penetrate the cranium  614  through a relatively small opening (such as a first opening  730 ) defined in the cranium and formed via twist drill, for example. Each relatively small opening may be sealed with a small quantity of biocompatible adhesive, such as certain cyanoacrylate and epoxy materials. 
   In the disclosed embodiment, the seed electrode assemblies  722 - 728  are inserted at different locations, with different trajectories and different depths. Each of the seed electrode assemblies  722 - 728  is independently positionable. 
     FIG. 8  illustrates an introducer apparatus  810  according to the invention, used to position seed electrodes in a patient&#39;s brain (or other) tissue. The introducer apparatus  810  includes a handle  812  and a spacer  814 , and in an embodiment of the invention the spacer  814  is removable and replaceable with spacers of different lengths, and also can be coupled to a Stereotactic head frame or other equipment adapted to position the introducer apparatus  810  accurately and precisely. The spacer may be made of any relatively rigid material suitable for acute surgical use, such as stainless steel or any of various polymers. 
   A portion of a seed electrode assembly  816  protrudes from the spacer  814  at a proximal end of the introducer apparatus  810 , to a length defined by the length of the spacer  814 . A seed electrode  818  and a portion of an insertion cannula  820  are visible; the insertion cannula  820  fits snugly (but slidably) within the spacer  814  and is coupled to the handle  812 . At a distal end of the apparatus  810 , another portion of a seed electrode assembly  816  protrudes; a distal portion of a push tube  822  is visible. The push tube  822  is preferably held in place and prevented from unintended longitudinal motion by a clip  824 , held in place by a flexible (and moveable) stanchion  826 . When longitudinal motion of the push tube  822  (and hence the seed electrode  818 ) is desired, the stanchion  826  may be deflected and the clip  824  disengaged from the push tube  822 . 
   In operation, a desired target site is identified either in surgical planning or intraoperatively. A Stereotactic head frame or other apparatus is fixed to the patient and adjusted to place the spacer  814  in line with a desired trajectory to the target site, such that the introducer apparatus  810 , when the cannula  820  is inserted into the spacer  814 , causes the seed electrode  818  to penetrate the tissue to a specified depth which may be slightly short of the target site. 
   When the target site is nearly reached, the push tube  822  and introducer apparatus  810  are manipulated (i.e., moved longitudinally with respect to one another) to apply force to the seed electrode  818  and separate it from the adhesive  420  ( FIG. 4 ) coupling it to the cannula  820 . The adhesive  420  effectively prevents the seed electrode  818  from inadvertent motion with respect to the cannula  820 , but when the seed electrode  818  is appropriately placed, the push tube  822  enables the cannula  820  to be removed while the seed electrode  818  remains in place. 
   The foregoing surgical approach is deemed representative only; other methods of implanting a seed electrode system according to the invention, either with or without an introducer apparatus  810 , are possible. 
   It should be observed that while the foregoing detailed description of various embodiments of the present invention is set forth in some detail, the invention is not limited to those details and an implantable medical electrical lead system made or used according to the invention can differ from the disclosed embodiments in numerous ways. In particular, it will be appreciated that embodiments of the present invention may be employed in many different applications for sensing or stimulation, not just in the brain. Leads according to the invention may have utility in connection with peripheral nerves, muscles, other portions of the body, and other applications. Hence, the appropriate scope hereof is deemed to be in accordance with the claims as set forth below.