Patent Publication Number: US-11382531-B2

Title: Systems and methods for positioning implanted devices in a patient

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/148,987, filed May 6, 2016, which is a continuation of U.S. patent application Ser. No. 14/598,114, now issued as U.S. Pat. No. 9,345,891, filed Jan. 15, 2015, which is a continuation of U.S. patent application Ser. No. 12/895,403, now issued as U.S. Pat. No. 8,965,482, filed Sep. 30, 2010, which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL HELD 
     The present technology is directed generally to systems and methods for positioning implanted devices in a patient. 
     BACKGROUND 
     Neurological stimulators have been developed to treat pain, movement disorders, functional disorders, spasticity, cancer, cardiac disorders, and various other medical conditions. Implantable neurological stimulation systems generally have an implantable pulse generator and one or more leads that deliver electrical pulses to neurological tissue or muscle tissue. For example, several neurological stimulation systems for spinal cord stimulation (SCS) have cylindrical leads that include a lead body with a circular cross-sectional shape and multiple conductive rings spaced apart from each other at the distal end of the lead body. The conductive rings operate as individual electrodes and the SCS leads are typically implanted either surgically or percutaneously through a large needle inserted into the epidural space, with or without the assistance of a stylet. 
     Once implanted, the pulse generator applies electrical pulses to the electrodes, which in turn modify the function of the patient&#39;s nervous system, such as by altering the patient&#39;s responsiveness to sensory stimuli and/or altering the patient&#39;s motor-circuit output. During pain treatment, the pulse generator applies electrical pulses to the electrodes, which in turn can generate sensations that mask or otherwise alter the patient&#39;s sensation of pain. For example, in many cases, patients report a tingling or paresthesia that is perceived as more pleasant and/or less uncomfortable than the underlying pain sensation. In other cases, the patients can report pain relief without paresthesia or other sensations. 
     In any of the foregoing systems, it is important for the practitioner to accurately position the stimulator in order to provide effective therapy. One approach to accurately positioning the stimulator is to implant the stimulator in a surgical procedure so that the practitioner has a clear visual access to the implantation site. However, many patients and practitioners wish to avoid the invasiveness and associated likelihood for complications typical of a surgical procedure. Accordingly, many patients and practitioners prefer a less invasive (e.g., percutaneous) implantation technique. With a percutaneous approach, the practitioner typically is unable to see exactly where the device is positioned because the device is beneath the patient&#39;s skin and in most SCS cases, within the patient&#39;s spinal column. In addition, the process typically requires the patient to provide feedback to the practitioner based on that patient&#39;s sensations. Accordingly, the industry has developed a variety of techniques for visualizing medical devices and anatomical features below the patient&#39;s skin as the device is implanted. Such techniques include fluoroscopy, which is commonly used to aid the practitioner when implanting SCS leads. However, a drawback with fluoroscopy is that it results in added expense to the SCS implantation procedure, it may be cumbersome to implement, it limits the implantation procedure to sites with fluoroscopy equipment, and it exposes the patient to unwanted x-ray radiation. Accordingly, there remains a need in the art for improved visualization techniques that can be used to implant patient devices with greater ease, accuracy, and lower cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a partially schematic illustration of an implantable spinal cord modulation system positioned at a patient&#39;s spine to deliver therapeutic signals in accordance with several embodiments of the present disclosure. 
         FIG. 1B  is a partially schematic, cross-sectional illustration of a patient&#39;s spine, illustrating representative locations for an implanted lead in accordance with an embodiment of the disclosure. 
         FIG. 2  is a partially schematic illustration of a representative signal delivery device, signal transmission device, and signal detection device, configured in accordance with an embodiment of the disclosure. 
         FIG. 3  is a partially schematic illustration of a signal detector system positioned over the patient&#39;s spine in accordance with an embodiment of the disclosure. 
         FIG. 4  is an enlarged illustration of a portion of the patient&#39;s spinal cord and a representative signal detector device. 
         FIG. 5  is a schematic illustration of a signal detector system and display for presenting results obtained during procedures in accordance with embodiments of the disclosure. 
         FIG. 6  is a schematic illustration of an intrathecal penetration detector configured in accordance with an embodiment of the disclosure. 
         FIGS. 7A-7D  illustrate processes for implanting patient devices in accordance with several embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present technology is directed generally to systems and methods for positioning implanted devices in a patient. In at least some contexts, the systems and methods are used to implant leads proximate to the patient&#39;s spinal cord to deliver high frequency signals that modulate neural activity at the patient&#39;s spine, in particular embodiments, to address chronic pain. In other embodiments, however, the systems and associated methods can have different configurations, components, and/or procedures. Still other embodiments may eliminate particular components or procedures. A person of ordinary skill in the relevant art, therefore, will understand that the present technology may include other embodiments with additional elements, and/or may include other embodiments without several of the features shown and described below with reference to  FIGS. 1A-7D . 
     Several aspects of the technology are embodied in computing devices, e.g., programmed pulse generators, controllers and/or other devices. The computing devices on which the described technology can be implemented may include one or more central processing units, memory, input devices (e.g., input ports), output devices (e.g., display devices), storage devices, and network devices (e.g., network interfaces). The memory and storage devices are computer-readable media that may store instructions that implement the technology. In many embodiments, the computer readable media are tangible media. In other embodiments, the data structures and message structures may be stored or transmitted via an intangible data transmission medium, such as a signal on a communications link. Various suitable communications links may be used, including but not limited to a local area network and/or a wide-area network. 
       FIG. 1A  schematically illustrates a representative patient system  100  for providing relief from chronic pain and/or other conditions, arranged relative to the general anatomy of a patient&#39;s spinal cord  191 . The overall patient system  100  can include a signal delivery system  110 , which may be implanted within a patient  190 , typically at or near the patient&#39;s midline  189 , and coupled to a pulse generator  121 . The signal delivery system  110  can provide therapeutic electrical signals to the patient during operation. The overall patient system  100  can further include a signal transmission system  130  and a signal detector system  140 . The signals handled by the signal transmission system  130  and the signal detector system  140  can function primarily to identify the location of the signal delivery system  110 , rather than to provide therapy to the patient. Accordingly, the signal transmission system  130  and signal detector system  140  can operate independently of the signal delivery system  110  to guide the practitioner as he/she positions elements of the signal delivery system  110  within the patient. Nevertheless, in particular embodiments, certain elements of the signal transmission system  130  can be shared with the signal delivery system  110 . Aspects of the signal delivery system  110  are described immediately below, followed by a description of the signal transmission system  130  and the signal detector system  140 . 
     In a representative example, the signal delivery system  110  includes a signal delivery device  111  that carries features for delivering therapy to the patient  190  after implantation. The pulse generator  121  can be connected directly to the signal delivery device  111 , or it can be coupled to the signal delivery device  111  via a signal link  113  (e.g., an extension). In a further representative embodiment, the signal delivery device  111  can include an elongated lead or lead body  112 . As used herein, the terms “lead” and “lead body” include any of a number of suitable substrates and/or support members that carry devices for providing therapy signals to the patient  190 . For example, the lead  112  can include one or more electrodes or electrical contacts that direct electrical signals into the patient&#39;s tissue, such as to provide for patient relief. In other embodiments, the signal delivery device  111  can include structures other than a lead body (e.g., a paddle) that also direct electrical signals and/or other types of signals to the patient  190 . 
     The pulse generator  121  can transmit signals (e.g., electrical signals) to the signal delivery device  111  that up-regulate (e.g., stimulate or excite) and/or down-regulate (e.g., block or suppress) target nerves. As used herein, and unless otherwise noted, the terms “modulate” and “modulation” refer generally to signals that have either type of the foregoing effects on the target nerves. The pulse generator  121  can include a machine-readable (e.g., computer-readable) medium containing instructions for generating and transmitting suitable therapy signals. The pulse generator  121  and/or other elements of the system  100  can include one or more processors  122 , memories  123  and/or input/output devices. Accordingly, the process of providing modulation signals, providing guidance information for locating the signal delivery device  111 , and/or executing other associated functions can be performed by computer-executable instructions contained by computer-readable media located at the pulse generator  121  and/or other system components. The pulse generator  121  can include multiple portions, elements, and/or subsystems (e.g., for directing signals in accordance with multiple signal delivery parameters), carried in a single housing, as shown in  FIG. 1A , or in multiple housings. 
     In some embodiments, the pulse generator  121  can obtain power to generate the therapy signals from an external power source  118 . The external power source  118  can transmit power to the implanted pulse generator  121  using electromagnetic induction (e.g., RF signals). For example, the external power source  118  can include an external coil  119  that communicates with a corresponding internal coil (not shown) within the implantable pulse generator  121 . The external power source  118  can be portable for ease of use. 
     During at least some procedures, an external programmer  120  (e.g., a trial modulator) can be coupled to the signal delivery device  111  during an initial procedure, prior to implanting the pulse generator  121 . For example, a practitioner (e.g., a physician and/or a company representative) can use the external programmer  120  to vary the modulation parameters provided to the signal delivery device  111  in real time, and select optimal or particularly efficacious parameters. These parameters can include the location from which the electrical signals are emitted, as well as the characteristics of the electrical signals provided to the signal delivery device  111 . In a typical process, the practitioner uses a cable assembly  114  to temporarily connect the external programmer  120  to the signal delivery device  111 . The practitioner can test the efficacy of the signal delivery device  111  in an initial position. The practitioner can then disconnect the cable assembly  114  (e.g., at a connector  117 ), reposition the signal delivery device  111 , and reapply the electrical modulation. This process can be performed iteratively until the practitioner obtains the desired position for the signal delivery device  111 . Optionally, the practitioner may move the partially implanted signal delivery element  111  without disconnecting the cable assembly  114 . 
     After a trial period with the external programmer  120 , the practitioner can implant the implantable pulse generator  121  within the patient  190  for longer term treatment. The signal delivery parameters provided by the pulse generator  121  can still be updated after the pulse generator  121  is implanted, via a wireless physician&#39;s programmer  125  (e.g., a physician&#39;s remote) and/or a wireless patient programmer  124  (e.g., a patient remote). Generally, the patient  190  has control over fewer parameters than does the practitioner. 
       FIG. 1B  is a cross-sectional illustration of the spinal cord  191  and an adjacent vertebra  195  (based generally on information from Crossman and Neary, “Neuroanatomy,”  1995  (published by Churchill Livingstone)), along with multiple signal delivery devices  111  (shown as signal delivery devices  111   a - d ) implanted at representative locations. For purposes of illustration, multiple signal delivery devices  111  are shown in  FIG. 1B  implanted in a single patient. In actual use, any given patient will likely receive fewer than all the signal delivery devices  111  shown in  FIG. 1B . 
     The spinal cord  191  is situated within a vertebral foramen  188 , between a ventrally located ventral body  196  and a dorsally located transverse process  198  and spinous process  197 . Arrows V and D identify the ventral and dorsal directions, respectively. The spinal cord  191  itself is located within the dura mater  199 , which also surrounds portions of the nerves exiting the spinal cord  191 , including the ventral roots  192 , dorsal roots  193  and dorsal root ganglia  194 . In one embodiment, a single first signal delivery device  111   a  is positioned within the vertebral foramen  188 , at or approximately at the spinal cord midline  189 . In another embodiment, two second signal delivery devices  111   b  are positioned just off the spinal cord midline  189  (e.g., about 1 mm. offset) in opposing lateral directions so that the two signal delivery devices  111   b  are spaced apart from each other by about 2 mm. In still further embodiments, a single signal delivery device or pairs of signal delivery devices can be positioned at other locations, e.g., at the dorsal root entry zone as shown by a third signal delivery device  111   c , or at the dorsal root ganglia  194 , as shown by a fourth signal delivery device  111   d.    
     In any of the foregoing embodiments, it is important that the signal delivery device  111  be placed at a target location that is expected (e.g., by a practitioner) to produce efficacious results in the patient when activated. The following disclosure describes techniques and systems for improving the level of accuracy with which the devices are positioned. 
       FIG. 2  is a partially schematic illustration of a representative signal delivery device  111  that includes a lead  112  carrying a plurality of ring-shaped therapy contacts  126  positioned toward a distal end to deliver a therapy signal to the patient when the lead  112  is implanted. The lead  112  includes internal wires (not visible in  FIG. 2 ) that extend between the therapy contacts  126  at the distal end of the lead  112 , and corresponding connection contacts  127  positioned at the lead proximal end. After implantation, the connection contacts  127  are connected to the external programmer  120  or the implanted pulse generator  121  discussed above with reference to  FIG. 1A . During implantation, an implanting tool  160  (e.g., a stylet  161 ) is temporarily coupled to the lead  112  to support the lead  112  as it is inserted into the patient. For example, the implanting tool  160  can include a shaft  162  that is slideably and releasably inserted (via, e.g., a handle  163 ) into an axially-extending opening in the lead  112 . The shaft  162  is generally flexible, but more rigid than the lead  112  to allow the practitioner to insert the lead  112  and control its position during implantation. A stylet stop  128  at the distal end of the lead opening prevents the practitioner from over-inserting the stylet shaft  162 . 
     The lead  112  and/or other portions of the overall system  100  can include features that guide the practitioner when positioning the lead  112  at a target location. For example, the signal transmission system  130  can be carried by the lead  112  and/or the implanting tool  160 , and can communicate with the signal detector system  140  located outside the patient&#39;s body. In a particular embodiment, the signal transmission system  130  includes one or more signal transmission devices  131 . For purposes of illustration, several different signal transmission devices  131  are shown together in  FIG. 2  as first, second and third signal transmission devices  131   a ,  131   b ,  131   c , though in most embodiments, a single or single type of signal transmission device  131  will be implemented. The signal transmission devices  131  communicate with the signal detector system  140  via corresponding locator signals  132  (shown schematically as first, second and third locator signals  132   a - 132   c ). The signal transmission devices  131  can generate, emit, and/or reflect the locator signals  132  in a manner that is detected by a signal detector device  141  of the signal detector system  140 . The first signal transmission device  131   a  can be carried by the lead  112 , and can be independent of (e.g., electrically isolated from) the therapy contacts  126 . The second signal transmission device  131   b  can also be carried by the lead  112 , but can double as one of the therapy contacts  126 . In a particular aspect of this embodiment, the second signal transmission device  131   b  doubles as the distal-most therapy contact  126 , located at or near the distal tip of the lead  112 . In other embodiments, the second signal transmission device  131   b  can double as any of the other therapy contacts  126 . The third signal transmission device  131   c  is carried by the implanting tool  160 , rather than the lead  112 . For example, the third signal transmission device  131   c  can be located at the distal-most tip of the implanting tool  160 . 
     An advantageous feature of the first signal transmission device  131   a  is that its independence of therapy contacts  126  frees it from being limited by the particular geometry and arrangement of the therapy contacts  126 , which are typically sized, configured and arranged to provide optimal or highly effective and efficient therapy (e.g., modulation) signals. Instead, the first signal transmission device  131   a  can be tailored to provide effective and efficient first locator signals  132   a , e.g., in cases where the locator signals differ significantly from the therapy/modulation signals. Conversely, an advantage of combining the functions of the second signal transmission device  131   b  with one of the therapy contacts  126  is that it reduces the need for an additional element in the overall patient system  100 . An advantage of the third signal transmission device  131   c  is that it can be removed from the patient&#39;s body when it is no longer needed for locating the lead  112 . Although the configuration and individual features of the three signal transmission devices  131   a ,  131   b , and  131   c  in the embodiment of  FIG. 2  affords unique advantages, signal transmission system  130  may comprise only a single or single type of signal transmission device  131 , two such devices or types of devices  131 , or more than three such devices or types of devices  131  in any combination of locations, configurations, and types as herein described. 
     The locator signals  132  transmitted by the signal transmission device  131  can have any of a variety of characteristics suitable for conveying location information wirelessly through the patient&#39;s skin  187  to the signal detector device  141 . For example, in a particular embodiment, the locator signal  132  can include a radio frequency (RF) signal having a frequency in the range of from about 10 kHz to about 30 GHz. In other embodiments, the frequency of the locator signal  132  is outside the foregoing range. In still further embodiments, the signal transmission device  131  can be a magnetic device (e.g., a permanent magnet and/or an electromagnet) and can accordingly transmit locator signals  132  by virtue of magnetic fields, which are detected by the signal detector device  141 . Accordingly, the term “locator signal” as used herein includes a wide variety of electromagnetic fields and transmissions that can be received or otherwise detected by an appropriate detector device  141 . The signal can be generally constant, as in the case of a magnetic field produced by a permanent magnet, or varying, as in the case of an RF signal. In still a further embodiment, the locator signal  132  can be an acoustic signal (e.g., ultrasound) that is transmitted by the signal transmission device  131  and received by the signal detector device  141 . In yet another aspect of this embodiment, the locator signal can actually be emitted from a location external to the patient&#39;s body, and the signal detector device  141  can receive or detect an echo or return signal, as indicated by fourth (two-way) locator signals  132   d . Accordingly, unless otherwise indicated, the term “signal transmission device” includes devices that emit (e.g., actively generate) signals, and devices that reflect signals, with both types of signals selected to be detected by the signal detector device  141 . 
     When the signal includes a reflected ultrasound signal, the signal emitter can be co-housed with the signal detector  141  to simplify use. The signal delivery device  111  and/or the implanting tool  160  can be constructed from materials specifically selected to be highly reflective to ultrasound signals and/or surface treatments to optimize ultrasound reflectivity. Materials having densities different than the densities of the adjacent tissue (which has a significant water content) typically have a higher acoustic impedance and accordingly generate reflections that can be readily distinguished from those produced by the adjacent tissue. Such materials can include polymers such as polyethylene or polyurethane. In other embodiments, the materials can include compositions having higher densities and/or materials that are also radiopaque, so that they can be used with a fluoroscopic detection technique and/or an ultrasonic detection technique. Suitable materials include platinum, iridium, tantalum, titanium and/or alloys of the foregoing materials. The materials can be applied to one or more of several elements of the signal delivery system  110 , including the therapy contacts  126 , the stylet stop  128 , and/or the end of the stylet shaft  162 , which can have a ball shape (e.g., a welded ball) to inhibit penetration into the distal end of the lead  112 . In other embodiments, a radiopaque and acoustically reflective ink or other coating can be applied to any of the foregoing elements and/or to the outer surface of the stylet shaft  162  and/or to the outer surface of the lead  112 . Suitable materials include radiopaque inks available from CJ Medical of Norton, Mass., and sputtered tantalum available from Isoflex Biomed of Rochester, N.Y. 
     In any of the foregoing embodiments, locator signals are generally transmitted (e.g., actively or by reflection) from the signal transmission device  131  to the signal detector device  141 . As discussed above, signals can travel in both directions when the detected signal is a reflected signal. In other embodiments, the signal detector device  141  can transmit additional signals to the signal transmission device  131 , e.g., to power the signal transmission device  131 , and/or to query the signal transmission device  131  for additional information. 
     In at least some embodiments, the signal detector system  140  can include a single detector device  141 , as shown in  FIG. 2 . In other embodiments (for example, as illustrated in  FIG. 3 ), the signal detector system  140  can include an array  142  or other plurality of detector devices or elements  143 , with each detector element  143  capable of detecting and responding to a locator signal. In a particular embodiment, the array  142  is positioned on the patient&#39;s skin  187  over the patient&#39;s spine  184 . Each detector element  143  can be individually placed on the patient&#39;s skin  187 , or the array  142  can include a flexible support member  139  (e.g., a thin plastic or fabric member or single- or multi-layer plastic or fabric composite, etc.) in which all the detector elements  143  are incorporated or located. For example, if support member  139  is a multi-layer construction of fabric and/or plastic, some or all detector elements  143  may be incorporated between one or more layers thereof and/or affixed to one or both outer surfaces of support member  139 . If support member  139  is a single layer of material, detector elements  143  may be affixed to one or both surfaces of member  139 . The support member  139  can be releasably attached to the patient&#39;s skin  187 , e.g., via an adhesive, straps, or another suitable, releasable attachment mechanism. The support member  139  can accordingly maintain a constant or generally constant spacing between neighboring detector elements  143  of the array  142 . The array  142  can include one or more index markers  146  that allow the practitioner to locate the array  142  properly relative to the patient&#39;s anatomy. For example, the practitioner can palpate and/or visually identify an anatomic feature  186  of the patient (e.g., the spinous process  197  of a specific vertebra  195 ) and locate the one or more index markers  146  relative to the anatomic feature  186 . The detector elements  143  can be coupled to a power source  144  that powers the detector elements  143 , and the detector elements  143  can communicate information to other elements of the overall system  100  via a detector output  145 . 
     As the practitioner inserts the signal delivery system  110  along the patient&#39;s spine  184 , the signal delivery system  110  (e.g., the signal delivery device  111  and associated implanting tool  160 ) can move in several directions. For example, as shown in  FIG. 3 , the signal delivery system  110  can move axially (or in a rostral/caudal direction) as indicated by arrows  101 , laterally as indicated by arrows  102  and/or in a direct ventral/dorsal direction  103  (viewed end-on in  FIG. 3 ). The direct ventral/dorsal direction  103  corresponds to a direction directly toward or away from the spinal cord. In at least some cases, the lead may tend to migrate around the spinal cord in a spiral fashion, as indicated by arrows  104 . 
     Because the detector elements  143  shown in the array  142  are positioned in a plane (e.g., a generally flat plane) that contains the axial and lateral axes  101 ,  102 , the detector elements  143  tend to be most sensitive to the location of the signal delivery system  110  in these generally orthogonal directions. The detector elements  143  may not be as sensitive to motion along the ventral/dorsal axis  103 , and/or motion of the signal delivery system  110  wrapping around the spinal cord. Accordingly, the overall system  100  can include other features that may supplement the information received from the detector elements  143 . In a particular embodiment, the overall system  100  can include an insertion tracker  150  (shown schematically in  FIG. 3 ) that tracks the length of the signal delivery device  111  that has been inserted into the patient. In a first embodiment, the insertion tracker  150  can include markings (e.g., a scale) on the signal delivery device  111  or on the implanting tool  160  that the practitioner observes to track the length of the signal delivery device  111  that has been inserted. In another embodiment, the insertion tracker  150  includes a wheel  151  or other suitable mechanical, electromechanical or electro-optic device that automatically determines the length of the signal delivery device  111  inserted into the patient. The inserted length can be presented at a display  152  and/or directed remotely via an output signal  153 . 
     In operation, the information received by the detector elements  143  can be used to estimate a length of the signal delivery device  111  projected into the plane of the array  142 . This estimated length can be compared to the length indicated by the insertion tracker  150 , either by the practitioner, or in an automated manner by the overall system  100 , based on the output signal  153 . If the location of the signal delivery device  111  as indicated by the detector elements  143  corresponds to (e.g., is identical or nearly identical to) the inserted length of the signal delivery device  111  identified by the insertion tracker  150 , then the signal delivery device  111  has not likely deviated significantly from a plane located just above the spinal cord. Alternatively, if the detector elements  143  indicate that the signal delivery device  111  is not progressing (or progressing slowly) in the lateral or axial directions, but the insertion tracker  150  indicates that the signal delivery device  111  is in fact progressing (or progressing quickly), this can indicate to the practitioner that the signal delivery device  111  is traveling out of the plane of the array  142 , e.g., either penetrating toward or into the spinal cord, or wrapping around the spinal cord. Further aspects of this operation are described later with reference to  FIG. 5 . 
       FIG. 4  is a partially schematic illustration of the dorsal region of the patient&#39;s spinal cord  191 , with the vertebrae  195  cut away (as shown in cross-section) and with the array  142  of detector elements  143  shown in position over the patient&#39;s spine  184 . As discussed above, the array  142  is typically located on the patient&#39;s skin, external to the patient&#39;s body, but for purposes of clarity, the patient&#39;s skin is not shown in  FIG. 4 . In a particular aspect of this embodiment, the detector elements  143  present information corresponding to a characteristic of the detected locator signals, in addition to detecting/receiving the locator signals. For example, the detector elements  143  can each be co-located with a display element  147  and the display elements  143  together can form a display device  170 . The display device  170  presents information corresponding to the strength of the signal received at individual detector elements  143 . In one aspect of this embodiment, the individual display elements  147  include an LED or other light source that presents light to the practitioner having a characteristic indicating the signal strength detected at that location. For example, the light can be brighter at a location where the signal is stronger, and dimmer where the signal is weaker. In other embodiments, the light can have one color where the signal is strong and a different color where the signal is weak. In still other embodiments, the light can flash intermittently where the signal is weak and remain steady where the signal is strong (or vice-versa). Combinations of the foregoing characteristics of the light can also be used, with or without other features such as an aural signal indicative of a strong or weak signal. For purposes of illustration, light corresponding to strong signals is indicated in  FIG. 4  with a heavier shading. 
     As shown in  FIG. 4 , the signal delivery device  111  has been advanced along the patient&#39;s spinal cord  191  via an implanting tool  160  that carries a signal transmission device  131  at its distal tip. Accordingly, the display elements  147  located closest to the signal transmission device  131  indicate the highest strength signal, and those further away from the signal transmission device  131  identify weaker signals. In some cases, the practitioner may position the signal delivery device  111  so that the signal transmission device  131  is aligned at a selected target location (e.g., a first target location  185   a ). In other cases, the target location (e.g., a second target location  185   b ) may be located apart from the signal emission device  131 , for example, in cases for which the practitioner deliberately wishes to have a part of the signal delivery device  111  other than the distal-most tip aligned with the second target location  185   b . In either embodiment, the practitioner can use the information presented by the display elements  147  to locate the signal transmission device  131  and, by knowing the relative spacing between the signal transmission device  131  and each of the therapy contacts  126 , can locate any given therapy contact  126  with equal or generally equal accuracy. 
     In an embodiment shown in  FIG. 4 , the display elements  147  present information corresponding to the location of the signal transmission device  131  in situ, directly on the patient&#39;s skin overlying the spine. In other embodiments, this information can be presented at a remote location, in addition to, or in lieu of being presented in situ. For example,  FIG. 5  illustrates a signal detector system  140  that displays information at a position spaced apart from the detector elements  143  (shown in  FIG. 4 ). In a particular embodiment, the signal detector system  140  includes a processor  148  and a memory  149  that receive and store signal detector output  145  from the detector elements  143 , and receive and store insertion tracker output  153  from the insertion tracker  150  ( FIG. 4 ). This information is processed (e.g., via instructions contained by a computer-readable medium) and presented at a display device  170   a , e.g., an LCD or LED screen. The display device  170   a  can include a graphical depiction of the patient&#39;s spinal cord midline via a midline indicator  171  and can graphically display detector element indicators  172  which are spaced apart from each other in a manner that corresponds to the spacing of the detector elements  143  on the patient&#39;s back. The detector element indicators  172  can be illuminated or otherwise presented in a manner that distinguishes strong detected signals from weaker detected signals, e.g, as previously described with respect to the embodiment of  FIG. 4 . In addition to or in lieu of presenting the detector element indicators  172 , the display device  170   a  can present a signal delivery device indicator  173  that represents the location of the signal delivery device as determined by the signal detector system  140 , based on the information received at the detector elements  143  (e.g., using a suitable interpolation scheme). Accordingly, the practitioner can view the display device  170   a  to obtain a graphical presentation of the location of the signal delivery device (which is not visible) relative to the patient&#39;s midline and the receiver elements  143  (which are visible). 
     The foregoing information received from the detector elements  143  can be combined with information received via the insertion tracker output  153  to indicate when the signal delivery device  111  ( FIG. 4 ) moves out of plane. As discussed above, it is expected that the signal delivery device  111  will be out of plane when the lead length determined via the array  142  of detector elements  143  is less than the lead length determined by the insertion tracker  150 . This information can be presented via an out-of-plane indicator  174  that illuminates when the lead is out of plane. This information can also be conveyed to the practitioner via an inserted length display  176  which compares the calculated length of the signal delivery device in the plane of the array  142 , with the measured length of the signal delivery device actually inserted into the patient. The information presented at the display device  170   a  can still further include an intrathecal penetration indicator  175 , which indicates that the dura around the spinal cord has been penetrated. Further information corresponding to this aspect of the system is described further below with reference to  FIG. 6 . 
       FIG. 6  schematically illustrates an intrathecal penetration detector  180  that can be used alone or in conjunction with other features of the overall patient system  100  described above to aid the practitioner in identifying the location of the signal delivery device  111 . The intrathecal penetration detector  180  take advantage of the low impedance that cerebral spinal fluid (CSF) has relative to the surrounding tissue in the spinal cord region. In particular, the intrathecal penetration detector  180  can use the impedance difference between CSF and the surrounding tissue to determine whether the dura  199  around the spinal cord  191  has been penetrated. In a particular embodiment, the intrathecal penetration detector  180  can detect an unexpectedly low impedance of a circuit that includes the signal delivery device  111  and the adjacent patient tissue, and identify this event to the practitioner as an indication that the signal delivery device  111  has potentially damaged or penetrated through the dura  199  of the patient&#39;s spinal cord  191 . In most applications, it is undesirable to penetrate the dura  199  and therefore providing an indication of intrathecal penetration can allow the practitioner to withdraw and reposition the signal delivery device  111 , optionally repair the damaged dura  199 , and complete the process of implanting the signal delivery device  111 . 
     In a particular embodiment, the intrathecal penetration detector  180  includes a power source  181  that applies a detection signal to a detection circuit  183 . The detection circuit  183  includes patient tissue, and can further include one or more of the therapy contacts  126  in contact with the patient tissue. Using the therapy contacts  126  as part of the impedance circuit  183  reduces the need for adding additional features to the signal delivery device  111 ; nevertheless, in another embodiment, the signal delivery device  111  can carry contacts that are dedicated to impedance detection. In a particular embodiment, the detection circuit  183  can include two selected therapy contacts  126  and the patient tissue located between the two therapy contacts  126 . In another embodiment, the detection circuit  183  can include a single therapy contact  126  and ground (e.g., a patient ground pad). 
     The intrathecal penetration detector  180  further includes an impedance detector  182  in the detection circuit  183  that identifies the impedance of the circuit  183 . The impedance detector  182  can be connected to the processor  148 , memory  149 , and display device  170   a  described above with reference to  FIG. 5  or to another processor and/or output device. In operation, the power source  181  provides a detection signal (e.g., a pulsed subthreshold signal with a current amplitude of about 0.2 milliamps, a pulse width of about 80 microseconds). The detection signal can be subthreshold to avoid inadvertently stimulating the patients&#39; motor and/or sensory neural pathways. The pulses can be delivered in bursts at any suitable frequency, e.g., a frequency provided by the external programmer  120  ( FIG. 1A ). In a representative embodiment, the frequency can coincide with a representative therapy frequency (e.g., about 3 kHz to about 50 kHz) and in other embodiments, can have other values. 
     It is generally expected that the impedance of a circuit that includes two therapy contacts  126 , as shown schematically in  FIG. 6 , will have an impedance of less than 1000 ohms, and typically an impedance in the range of about 300 ohms to about 600 ohms. If the impedance falls below a first threshold (e.g., about 200 ohms), the detector  180  and/or other elements of the system  100  can issue a warning, calling the practitioner&#39;s attention to the possibility of a CSF leak. If the impedance falls below a second threshold, e.g., about 50 ohms, the detector  180  and/or other elements of the system  100  can indicate a likely intrathecal penetration by the contact(s)  126   a  that are included in the detection circuit  183 . As discussed above with reference to  FIG. 5 , this indication can be presented at the display device  170 . In other embodiments, the indication can be presented in other manners, e.g., aurally. In still further embodiments, the foregoing threshold levels may have different values. For example, if the implant procedure includes using large amounts of saline, the “typical” impedance may fall from 300-600 ohms to 180 ohms, in which case the practitioner may require a lower threshold level (e.g., 150 ohms rather than 200 ohms) for an indication of CSF leakage. In other patients, e.g., patients with a significant amount of scar tissue, the “typical” impedance may be much larger than 1000 ohms, e.g., 200 ohms. 
     In a particular embodiment, the practitioner can select from any of the therapy contacts  126  to be included in the impedance detection circuit  183 . In at least some embodiments, the practitioner may wish to include the distal-most therapy contact (e.g., at the distal tip of the signal delivery device  111 ) in the detection circuit  183  to provide an early indication that the signal delivery device  111  has penetrated the dura  199 . If the signal delivery device  111  does not include a therapy contact  126  at the tip, a special-purpose contact can be added to the signal delivery device  111 , or the practitioner can use the therapy contact  126  closest to the tip. In other embodiments, the practitioner may wish to include one or more of the other therapy contacts  126  in the circuit, for example, to identify the extent and/or rate of a cerebral spinal fluid leak, and/or for other diagnostic purposes. 
     As discussed above, the information received from the impedance detector  182  can be processed to indicate to the practitioner whether or not the dura  199  has been penetrated. The information can be provided in a fairly straightforward manner, e.g., by indicating either no intrathecal penetration or intrathecal penetration, optionally with an intermediate indication of likely CSF leakage. In other embodiments, the intrathecal penetration detector  180  can provide more sophisticated information. For example, the intrathecal penetration detector  180  can employ a multiplex arrangement or other suitable signal processing arrangement to scan over the therapy contacts  126  and identify issues or insipient issues associated with any of the contacts  126 . The intrathecal penetration detector  180  can track a rate at which a drop in impedance passes along the signal delivery device  111  (e.g., as detected by multiple therapy contacts  126 ) to provide the practitioner with an indication of the rate at which CSF is leaking from the dura  199 . In other embodiments, the intrathecal penetration detector  180  can include other arrangements. For example, the intrathecal penetration detector  180  can indicate which contacts(s)  126  have lower than expected associated impedance. In a particular example, the tip of the signal delivery device may penetrate the dura  199  by breaking the continuity of the dura  199  without actually proceeding into the subdural space. Leaking CSF may then be indicated by low impedances at proximal therapy contacts  126  as they pass by the break in the dura  199 , and/or as CSF flows in a proximal direction, but a normal impedance (at least for a period of time) at the distalmost therapy contact  126 . 
       FIGS. 7A-7D  illustrate flow diagrams of methods in accordance with several embodiments of the disclosure described above. As described above, many of the steps in these methods may be performed automatically by instructions contained in one or more computer readable media.  FIG. 7A  illustrates a process  700   a  that includes, for each of a plurality of patients, receiving a target location from which to deliver a modulation signal to the patient&#39;s spinal cord (process portion  701   a ). The target location can be a single location or one of multiple locations, and can have axial and lateral coordinates selected to produce a desired patient effect. The process  700   a  can further include implanting a signal delivery device within a vertebral foramen of the patient, and positioning an electrical contact carried by the signal delivery device to be within ±5 mm. of the target location (process portion  702   a ). In particular embodiments, this accuracy level can be obtained in the axial and/or lateral directions via a single signal detector or via an array of detector elements. The same level of accuracy can be obtained in the dorsal/ventral direction, e.g., via an insertion tracker or other suitable methodology. 
     The foregoing process can be performed without the use of fluoroscopy (process portion  703   a ). For example, in particular embodiments, the practitioner can use electromagnetic techniques (e.g., RF or magnetic techniques) or ultrasound techniques to accurately implant the signal delivery device on a consistent, repeatable basis over multiple patients (e.g., a patient population numbering in the tens or hundreds or more). In further particular embodiments, the accuracy of this method can be better than ±5 mm., e.g., ±2 mm. or ±1 mm., depending on factors that include, but are not limited to, the sensitivity of the signal detector or signal detector elements, the unidirectionality of the signal transmitters, and the spacing between signal detector elements. In any of these embodiments, the ability to locate the signal delivery device within the foregoing ranges without the use of fluoroscopy can simplify the implanting process, and can reduce the patient&#39;s exposure to x-ray radiation. In addition, fluoroscopy devices can be cumbersome and, due to the protective gear worn by the practitioner, can interfere with the practitioners freedom of movement. Still further, fluoroscopy equipment is generally expensive and not generally available in remote and/or developing parts of the world. The current technology can reduce or eliminate the dependence on fluoroscopy for accurate device placement which can in turn allow the device and associated therapy to be used in a larger number of treatment centers (i.e., those without fluoroscopic equipment) and a concomitant potentially greater number of patients in need of such therapy. 
     Optionally, the process  700   a  can be performed with less or no patient feedback (process portion  704   a ). For example, the increased accuracy with which the signal delivery device is implanted in the first instance can reduce the number of subsequent iterations the practitioner and patient engage in to identify an effective location for the signal delivery device and associated therapy contacts. Such iterations can include moving the signal delivery device and/or selecting different active contacts on the signal delivery device. 
     Once the signal delivery device is implanted, it can be activated to modulate neural activity at the spinal cord (process portion  705   a ). In a particular embodiment, the therapy includes RF signals delivered to the patient&#39;s spinal cord at a frequency of from about 3 kHz to about 50 kHz to address patient pain. Further details of suitable signal delivery parameters are included in pending U.S. patent application Ser. No. 12/765,747, filed on Apr. 22, 2010 and incorporated herein by reference in its entirety. In other embodiments, the signal delivery device can provide signals in accordance with other signal delivery parameters to treat the same or other patient indications, at the same or other implantation sites. 
       FIG. 7B  is a flow diagram illustrating a process  700   b  in accordance with another embodiment of the disclosure, which includes implanting an implantable signal delivery system beneath the patient&#39;s skin and into a vertebral foramen of at least one of the patient&#39;s vertebrae (process portion  701   b ). The signal delivery system includes a signal delivery device having at least one electrical contact. In process portion  702   b , a locator signal is emitted from the signal delivery system. As discussed above, the locator signal can be emitted from the signal delivery device and/or from an implanting tool that temporarily carries the signal delivery device during an implanting process. In process portion  703   b , the locator signal is detected from a position external to the patient. In particular embodiments, the locator signal can be detected at a plurality of locations via an array of signal detectors (process portion  704   b ). In such embodiments, the results can be presented at a display indicating the relative signal strength received at the signal detectors (process portion  705   b ). Based at least in part on detecting the locator signal, the practitioner can adjust a position of the signal delivery device relative to the patient&#39;s spinal cord (process portion  706   b ) and the practitioner can then direct a therapy signal from the electrical contact (process portion  707   b ). As discussed above, an advantage of methods performed in accordance with the flow diagram shown in  FIG. 7B  is that they can allow the practitioner to accurately position the signal delivery device, e.g., without using fluoroscopy. 
       FIG. 7C  illustrates another process  700   c  in accordance with an embodiment of the disclosure that includes receiving a plurality of indications of the strength of a locator signal from a corresponding plurality of signals detectors (process portion  701   c ). The signal detectors are positioned in an array external to the patient to form a plane extending laterally and in a rostral/caudal direction (e.g., an axial direction). In process portion  702   c , a first length (e.g., a projected length of an implanted portion of the signal delivery device in the plane of the array) is identified, based upon the plurality of indications received in process portion  701   c . In process portion  703   c , an indication of the second length (e.g., an actual length) of the implanted portion of the signal delivery device is received. For example, process portion  703   c  can include receiving an indication of the actual implanted length via the insertion tracker discussed above with reference to  FIG. 6 . In process portion  704   c , the second length of the implanted portion of the lead is compared with the first length and, based on a difference between the actual length and the projected length, an indication of a ventral/dorsal location of the signal delivery system is provided. For example, if the actual and projected lengths differ by more than a threshold amount (e.g., one mm. in one embodiment and other values in other embodiments), the foregoing indication can be triggered. The foregoing arrangement can be used to account for the fact that the signal delivery device may move along three different axes, while the detector array is positioned in, or generally in, a two-dimensional plane. 
       FIG. 7D  illustrates a process  700   d  that may be used independently of or in conjunction with any of the foregoing methods described above with reference to  FIGS. 7A-7C . In particular, the process  700   d  includes introducing an electrical contact into a patient, proximate to the patient&#39;s dura (process portion  701   d ). In process portion  702   d , an impedance of an electrical circuit that includes the electrical contact and patient tissue adjacent to the electrical contact is detected. Process portion  703   d  includes comparing the detected impedance to a predetermined criterion (e.g., a threshold impedance value). If the detected impedance meets the predetermined criterion, then the process  700   d  can include identifying penetration of the patient&#39;s dura based at least in part on the detected impedance (process portion  704   d ). As discussed above, penetration can include breaking the continuity of the dura, whether or not the electrical contact actually passes through the dura to an intrathecal location. As was also discussed above, the predetermined criterion can include an impedance value at or below which detected impedances correspond to exposure to cerebral spinal fluid. An advantage of the foregoing methodology (and associated computer readable media and methods for programming the computer readable media) is that the practitioner can receive an early indication that the dura has been penetrated, and can take appropriate corrective action. Corrective actions include re-positioning the signal delivery device and possibly repairing damaged dural tissue. 
     From the foregoing, it will be appreciated that specific embodiments of the technology have been described herein for purposes of illustration, but that various modifications may be made without deviating from the technology. For example, in other embodiments, the foregoing systems and methods can be used to locate devices other than spinal cord implants. In a particular embodiment, the intrathecal detection device and methodology described above can be applied to other areas of the patient&#39;s body that are surrounded by the dura and contain cerebral spinal fluid, for example, the brain. In still further embodiments, these devices and methodologies can be applied to implantable patient devices other than neural modulators (e.g., other elements configured for patient implantation, with therapy contacts in at least some cases). The implanting tools described above can have configurations other than a stylet (e.g., a catheter) in other embodiments. The locator signal emitters and/detectors can be omnidirectional in certain embodiments or can be unidirectional in other embodiments. In certain embodiments, phase shift and/or phased array techniques can be implemented to enhance system efficacy. The signal delivery system can include one transmission device in certain embodiments, and more than one transmission device in other embodiments. 
     Certain aspects of the technology described in the context of particular embodiments may be combined or eliminated in other embodiments. For example, the display  170   a  described above with reference to  FIG. 5  may in some embodiments be made thin and flexible enough to be placed directly on the patient&#39;s body, with the detector elements integrated into the display medium. Accordingly, the practitioner can obtain the benefit of a graphical representation of the implanted signal delivery device, together with the proximity of the display to the actual location of the signal delivery device. The use of an aural indicator described above in the context of the intrathecal penetration detector can be applied to the technique for locating the signal delivery device relative to other motion axes, in addition to or in lieu of presenting the information via a visual display. For example, the aural indication can be triggered if the signal delivery device exceeds a threshold distance from the patient&#39;s midline. Further, while advantages associated with certain embodiments have been described in the context of those embodiments, other embodiments may also exhibit such advantages and not all embodiments need necessarily exhibit such advantages to fall within the scope of the present disclosure. Accordingly, the disclosure and associated technology can encompass other embodiments not expressly described or shown herein.