fig1 shows a patient 10 with a low - profile imd 12 deployed beneath his scalp 14 . in fig1 , imd 12 is a neurostimulator that provides deep brain stimulation via leads 16 a , 16 b deployed in the brain of patient 10 . in the example of fig1 , imd 12 is deployed in proximity to site of stimulation therapy . imd 12 may be used to treat any nervous system disorder including , but not limited to , epilepsy , pain , psychological disorders including mood and anxiety disorders , movement disorders ( mvd ) such as , but not limited to , essential tremor and parkinson &# 39 ; s disease and neurodegenerative disorders . although imd 12 is depicted as a neurostimulator , the invention is not limited to applications in which the imd is a neurostimulator . the invention may be employed with imds that perform any monitoring or therapeutic functions . the invention is not limited to imds that include leads deployed in the brain , but may also be employed with leads deployed anywhere in the head or neck including , for example , leads deployed on or near the surface of the skull , leads deployed beneath the skull such as near or on the dura mater , leads placed adjacent cranial or other nerves in the neck or head , or leads placed directly on the surface of the brain . nor is the invention limited to imds that are coupled to electrodes . the invention may be employed with low - profile imds coupled to any sensing or therapeutic elements , such as temperature sensors or motion sensors . the invention may also be employed with different types of imds including , but not limited to , imds operating in an open loop mode ( also referred to as non - responsive operation ), imds operating in a closed loop mode ( also referred to as responsive ), and imds for providing monitoring and / or warning . in the example of fig1 , imd 12 is deployed beneath scalp 14 of patient 10 , but on top of the cranium of patient 10 . the invention may be applied to other types of implantation as well , such as implantation of imd 12 in a trough cut into the cranium of patient 10 . a surgeon may implant imd 12 using any surgical technique . in a typical implantation , the surgeon makes an incision through the scalp 14 of patient 10 , and pulls back the resulting flap of skin to expose the desired area of the cranium . the incision may be a “ c - flap ” incision , for example . the surgeon drills holes , called “ burr holes ,” in the cranium and deploys leads 16 through the burr holes into the brain . the surgeon typically places caps , called “ burr hole caps ,” over the burr holes . before connecting leads 16 to imd 12 , the surgeon typically “ manages ” the leads . lead management includes arranging the excess length of leads 16 using techniques such as coiling and anchoring with anchoring plates . in a typical implantation , the surgeon arranges the leads to provide some slack to reduce the risk of lead migration . lead management also reduces the risk that the leads will be accidentally damaged during explantation , as described below . the surgeon implants imd 12 between scalp 14 and the skull . in one surgical procedure , the surgeon uses a tool to form a pocket beneath the scalp proximate to the burr holes , and positions imd 12 in the pocket . the surgeon may fix imd 12 to the cranium using an attachment mechanism such as bone screws . the surgeon closes the skin flap over imd 12 , and then staples or sutures the incision . at a later date , it may be necessary to remove imd 12 from patient 10 . explantation involves considerations that are distinct from implantation . for example , the surgeon may desire to remove imd 12 but may desire to keep leads 16 deployed as they are . in addition , the surgeon may desire to recover imd 12 in an undamaged condition . it may also be possible that the implanting surgeon and the explanting surgeon are different people , and the explanting surgeon may be unaware of what implantation and lead management techniques were used by the implanting surgeon . because of considerations such as these , the explanting surgeon plans the surgery to avoid accidentally damaging the leads or the implanted device when making an incision . fig2 illustrates a procedure for explantation of imd 12 shown in fig1 . fig2 is a diagram showing the top of the head of patient 10 . patient 10 may be under local anesthetic . the surgeon begins explantation by making an incision such as c - flap incision 18 in scalp 14 . in general , the surgeon has discretion concerning the making of an incision based upon the circumstances of each individual patient . accordingly , the incision need not be a c - flap incision as shown in fig2 , but may include a straight incision or an s - shaped incision , for example . the incision chosen by the surgeon may be a function of the location of imd 12 , the location of the leads , or other factors . as shown in fig2 , the surgeon draws scalp flap 20 away to expose the portion of the patient &# 39 ; s skull 22 beneath scalp flap 20 , and to expose at least a portion of imd 12 . in the example shown in fig2 , patient 10 has leads 16 a and 16 b deployed in the brain through burr holes 24 a and 24 b . a portion of the bodies of leads 16 a and 16 b , identified with reference numerals 26 a and 26 b , is deployed outside of the brain on the surface of skull 22 . the burr holes may be sealed with burr hole caps , with leads 26 a and 26 b passing therethrough . leads 26 a and 26 b are depicted as coiled and are anchored by anchoring plates 28 a and 28 b . leads 26 a and 26 b are coupled to imd 12 . in fig2 , imd 12 is a low - profile device , allowing it to be implanted between scalp 14 and skull 22 , with little discomfort or adverse cosmetic consequences to patient 10 . in addition , low - profile imd 12 can have the advantages of reducing skin erosion and infection . imd 12 comprises one or more modules that carry out the various functions of imd 12 . as shown in fig2 , imd 12 includes at least three modules : a control module 30 , a power supply module 32 and a recharge module 34 . one or more of modules 30 , 32 , 34 includes a housing that can carry out a variety of functions , including encasing the components of the modules , sealing the modules against contamination , electrically isolating electrical components , and the like . in some embodiments of the invention , at least one of the modules comprises a radiopaque material . the modules are coupled to member 36 , which may be made of a soft , biocompatible material . member 36 at least partially encapsulates one or more housings of modules 30 , 32 , 34 , and generally serves as a smooth interface between the modules and the body tissue . leads 26 a and 26 b are coupled to imd 12 at lead connectors 38 a and 38 b . imd 12 may be anchored with an anchoring mechanism such as a metallic tab 40 that includes an opening for receiving a bone screw . in general , member 36 integrates modules 30 , 32 and 34 into a desired form factor , but , where flexible , allows relative intermodule motion . in some embodiments , member 36 incorporates mechanical features to restrict intermodule motion to certain directions or within certain ranges . member 36 may be made from silicone , and is some embodiments may be made from two or more materials of differing flexibility , such as silicone and a polyurethane . an exemplary polyurethane for this purpose is tecothane ®, which is commercially available from hermedics polymer products , wilmington , mass . member 36 may also be referred to as an “ overmold ,” but use of the term “ overmold ” herein is not intended to limit the invention to embodiments in which member 36 is a molded structure . member 36 may be a molded structure , or may be a structure formed by any process . the invention is not limited to the particular imd depicted in fig2 , but includes a number of embodiments , some of which are described in more detail below . in fig2 , it is assumed that the surgeon has successfully made incision 18 , avoiding leads 26 and imd 12 . the surgeon may also have successfully removed bone screws that anchored imd 12 to skull 22 . the surgeon can decouple leads 26 a and 26 b from lead connectors 38 a and 38 b by hand or with a tool . in many cases , however , imd 12 does not easily separate itself from the site of implantation , and the surgeon applies force to remove imd 12 . fibrous tissue growth proximate to the implantation site , for example , may resist the efforts of the surgeon to remove imd 12 . imd 12 includes a grippable access structure 42 that aids in explantation . in fig2 , grippable access structure 42 is a small handle or handle - like formation in or otherwise coupled to member 36 that can be gripped with a hand or an instrument , so that the surgeon may apply force to remove imd 12 . a surgeon presented with imd 12 as shown in fig2 , for example , can grip imd 12 at handle 42 with a forceps , and apply force to pull or twist imd 12 . the invention is not limited to the grippable access structure shown in fig2 . other exemplary embodiments of grippable access structures will be described below . some embodiments of grippable access structures have the advantage that they give more implantation and explantation options to the surgeon . in particular , the surgeon can plan an explantation procedure in which the incision is close to the grippable access structure , but safely away from the imd and the leads . fig3 is a conceptual imaging diagram of the top of a head of a patient , illustrating an exemplary technique for identifying the location of an implanted low - profile imd . before explanting the implanted device , the surgeon should know where the device is . accordingly , the surgeon may direct that patient 10 be imaged using one or more medical imaging techniques such as x - ray , magnetic resonance imaging ( mri ), ct - scan or fluoroscopy . some of the imaging techniques employ electromagnetic radiation . fig3 represents an image 50 obtained with radiation , such as an x - ray . the image may include images of features or landmarks 52 , 54 of the skull , which assist in locating the implanted device . in addition , fig3 shows images of modules 56 of the implanted device . images of modules 56 appear in contrast to the most of the balance of image 50 . modules 56 appear because the housings include a radiopaque material that causes the modules to stand out in image 50 . in the exemplary illustration of fig3 , the member , being made of a non - radiopaque material such as silicone , does not appear in image 50 . in some embodiments of the invention , however , the member includes one or more radiopaque markers , so that the location of the member can be identified as well . the invention supports any of several techniques for including one or more radiopaque markers in the member , such as outlining the member with radiopaque wire and loading the member with radiopaque powders or fibers . in fig3 , the image of leads 58 is visible as well , as the leads may include radiopaque markers . in addition , image 50 includes a radiopaque incision mark 60 , which may have been created by the surgeon who implanted the device . the surgeon can use a radiopaque marker to make radiopaque incision mark 60 on the skull of the patient during the implantation procedure . in some cases , radiopaque incision mark 60 can assist the surgeon in locating the imd and leads by providing a reference on the skull itself . in addition to imaging as shown in fig3 , the surgeon could palpate for the imd and could use the implantation incision scar as a reference . radiopaque incision mark 60 may show the surgeon whether the implantation incision scar is proximate to its original site , or whether the implantation incision scar has migrated anteriorly or posteriorly . the surgeon can correct for scar migration , thereby reducing the risk of making an incision that cuts across the imd . in addition , the surgeon can reduce the risk of making an incision that inadvertently cuts across the leads , which may be difficult to locate by palpation . in general , the explanting surgeon takes one or more images of the patient , and uses the images to determine the location of the implanted device and the leads . in particular , the surgeon uses the image to learn about the size and configuration of the implanted device , and the lead management techniques that have been employed . the surgeon may also take into consideration the site of an incision used during the implantation procedure . using this information , the surgeon plans an incision strategy . the incision strategy takes into account the safety and effectiveness of a given incision , based upon the information obtained from the images . the surgeon implements the incision strategy in the operating room and removes the implanted device . fig4 shows an alternate exemplary embodiment of the invention . imd 70 is a low - profile imd that includes one or more modules 72 with housings that are at least partially encapsulated by a member 74 . in addition , radiopaque markers 76 , 78 are coupled to member 74 . markers 76 , 78 , which appear more plainly on an image than member 74 , can assist the surgeon in locating the position of the member . markers 76 , 78 may include additional information about member 74 , such as a model number , that would assist the surgeon in identifying the shape and dimensions of member 74 . markers 76 , 78 may be affixed to exterior of member 74 or may be embedded in member 74 . in addition , imd 70 includes a grippable access structure 80 coupled to member 74 , in the form of a loop . loop 80 , like handle 42 in fig2 , can be formed integral with the member or may be mechanically coupled to the member . loop 80 can be dimensioned such that a surgeon may grip loop 80 with an instrument such as a forceps , or the surgeon has the option to grip loop 80 with her fingers . loop 80 may include a wire or other radiopaque element ( not shown ) that would make loop 80 visible during imaging . fig5 illustrates another exemplary embodiment of the invention . imd 90 is a low - profile imd that includes one or more modules 92 with housings that are at least partially encapsulated by a member 94 . in addition , radiopaque markers 96 , 98 , 100 are coupled to member 94 , and can assist the surgeon in locating the position of member 94 in an image . in particular , radiopaque markers 96 , 98 , 100 assist the surgeon in identifying the edges of imd 90 . radiopaque markers 96 , 98 , 100 may be affixed to exterior of member 94 or may be embedded in member 94 , such as by loading radiopaque powders or fibers in member 94 . in addition , imd 90 includes a grippable access structure 102 coupled to member 94 , in the form of a tab . like loop 80 in fig4 , tab 102 can be formed integral with the member or may be mechanically coupled to the member , and can be dimensioned to give the surgeon flexibility to grip the structure by hand or with an instrument . in the embodiment shown in fig5 , tab 102 includes a radiopaque marker 104 that would make tab 102 visible during imaging . fig6 shows a further exemplary embodiment of the invention in an explantation procedure . in particular , fig6 demonstrates a technique for lead management that may be advantageous during explantation . fig6 shows the top of the head of the patient , with the scalp being invisible for clarity . as in fig2 , leads 26 a and 26 b are coiled proximate to burr holes 24 a and 24 b , and imd 12 is implanted nearby . in fig6 , leads 26 a and 26 b are coupled to imd 12 via tethered interconnect module 110 . in the embodiment shown in fig6 , tethered interconnect module 110 couples to the lead connectors 38 a and 38 b of imd 12 and leads 26 a and 26 b , and is interposed between the lead connectors and the leads . with tethered interconnect module 110 , the surgeon has more options for coupling leads 26 a and 26 b to imd 12 . the surgeon may elect , for example , to deploy leads 26 a and 26 b so as to create a substantial space between the leads and imd 12 . during explantation , an incision 112 can cause damage to the interconnecting leads 114 of tethered interconnect module 110 . even so , the integrity of leads 26 a and 26 b is preserved . in other words , tethered interconnect module 110 can be sacrificed during explantation to avoid damage to imd 12 and leads 26 a and 26 b by the incision . once tethered interconnect module 110 is decoupled from imd 12 and from leads 26 a and 26 b , the surgeon can remove imd 12 without disturbing from leads 26 a and 26 b . tethered interconnect module 110 may include a radiopaque material that enhances its visibility during imaging . in addition , tethered interconnect module 110 may include one or more anchoring structures ( not shown ) that hold tethered interconnect module 110 in position . the configuration of tethered interconnect module 110 shown in fig6 is exemplary , and the invention is not limited to the particular configuration shown . fig7 is a perspective view of an embodiment of a low - profile imd 120 that includes a lead management structure . imd 120 includes one or more modules 122 within housings and a member that at least partially encapsulates the housings . imd 120 is configured to be implanted between a scalp and a skull of a patient . leads 126 a and 126 b are coupled to lead connectors 128 a and 128 b . leads 126 a and 126 b are deployed around imd 120 in a lead management structure . a lead management structure is a structure in imd 120 that is configured to receive and protect the bodies of leads that are coupled to the imd . in particular , a lead management structure is a structure that is configured to receive and protect the bodies of the leads as opposed to the terminals of the leads . lead management structures include , but are not limited to , structures that route , fixate or anchor the lead bodies . examples of a lead management structure include a groove or a cavity that receives a lead body . one of the practical problems associated with the leads is that the leads can be difficult to manage . the leads can twist , bend , slide and otherwise move . the propensity of leads to move can be inconvenience during implantation , and can also be a problem during explantation . if the leads move after implantation , there is an increased risk of damage to leads during explantation . in fig7 , the lead management structure is a groove 130 formed in member 124 , and leads 126 a and 126 b are wrapped around imd 120 in groove 130 . the dimensions of the groove may a function of the length of the leads and the dimensions of imd . the placement of groove 130 around the periphery of imd 120 is for illustrative purposes , and the invention is not limited to the particular lead management structure shown in fig7 . the lead management structure need not be formed in member 124 . in some embodiments , the lead management structure can be constructed of a separate material , such as a protective material that would resist damage in the event the incision should cut across imd 120 . cut - resistant materials include , but are not limited to , metals and materials including embedded wire or polymer meshes . furthermore , the lead management structure need not be located around the periphery as shown in fig7 , but in some embodiments can be located underneath member 124 and modules 122 . lead management structures can not only direct lead bodies around imd 120 , but can direct the lead bodies over or under imd 120 . the lead management structure offers several possible benefits . first , it can protect the leads from damage in circumstances in which the incision cuts across the imd . second , it can in some circumstances offer a more efficient lead management option than coiling as illustrated in fig2 and 6 . third , if the leads include radiopaque materials , an image of the leads can show not only the position of the leads , but also the position of the imd . fig8 is a perspective view of an embodiment of a burr hole cap 140 that includes a lead management structure . burr hole cap 140 comprises a ring member 142 and a cover member 144 that couples to ring member 142 by a coupling mechanism ( not shown ). burr hole cap 140 is configured to close a burr hole in a bony structure such as a skull , while allowing a lead to pass through . ring member 142 includes a lead management structure . the lead management structure is groove 146 , which receives lead 148 . the implanting surgeon can coil lead 148 inside groove 146 , and draw lead through exit 150 , before coupling cover member 144 to ring member 142 . ring member 142 , cover member 144 or both can be constructed from a protective material that would resist damage in the event the incision should cut across burr hole cover 140 . the lead management technique illustrated in fig8 can protect the lead from damage in circumstances in which the incision is close to the burr holes . burr hole cap 140 can , in some circumstances , offer a more efficient lead management option than coiling outside of the burr hole cap . the configuration of the burr hole cap and the lead management structure are for illustrative purposes , and the invention is not limited to the burr hole cap or lead management structure shown in fig8 . for example , the invention includes burr hole caps that include a lead management structure that supports winding of a lead around the exterior of the burr hole cap . fig9 is a perspective view of a protective pouch 160 that can be used in lead management . pouch 160 is sized to slip over coils of lead 162 and protect the coils from accidental damage . pouch 160 can be constructed of a cut - resistant protective material and may also include a radiopaque material that enhances visibility of pouch 160 during imaging . pouch 160 may be constructed of any of a number of biocompatible materials , such as silicone , and may further incorporate cut - resistant materials . cut - resistant materials include , but are not limited to , metals and materials including an embedded metallic wire mesh , embedded threads , or a polymer mesh such as a dacron mesh . the invention is not limited to the particular embodiment of the pouch shown in fig9 . the invention encompasses , for example , pouches that are configured to hold more than one lead , pouches that have anchoring structures , and pouches that include closing structures that reduce the risk that the pouch will disengage from the coiled lead . although the invention has been described in connection with explantation of a device implanted on the head , the invention is not limited to the area of the head . a low - profile imd such as the devices described herein may be implanted anywhere in the body . implantation and explantation techniques may be similar to techniques for explantation and implantation under the scalp . in particular , the surgeon may make an incision in the skin of a patient . the surgeon may retract the incision to expose a bone , muscle or other anatomical structure . the surgeon may wish to avoid damage to the imd or the leads , and may wish to remove the imd without disturbing the leads . the invention supports implantation of an imd that performs any of several functions . the invention supports explantation of imds that provide monitoring , imds that administer therapy , and imds that do both . the invention is not limited to any particular number of modules or to any particular functionality . various embodiments of the invention have been described . as mentioned above , the invention is not limited to the particular embodiments described or shown in the figures . these and other embodiments are within the scope of the following claims .
in general , the invention is directed to apparatus and techniques that aid in the removal or explantation of an implantable medical device under the scalp of a patient . the various embodiments of the invention address risks associated with the explantation , such as the risk of damage to leads , the risk of damage to the imd , the risk that the incision may hinder the explantation , and the risk that the imd may be difficult to remove . in some embodiments , the invention is directed to apparatus that help the surgeon identify the location of the implanted elements , and that protect the implanted elements from inadvertent damage . in other embodiments , the invention is directed to techniques that facilitate the removal of the imd .