Locating guide

A locating guide for locating a target tissue site within a patient includes a body, a first substantially radiopaque reference marker on the body, and a second substantially radiopaque reference marker extending from a major surface of the body. The reference markers provide reference points that extend in at least two dimensions, and may be useful for registering a medical image of tissue with an actual location on or within the patient in order to more accurately and precisely locate a target tissue site within the patient. In some embodiments, the second reference marker is oriented at a predetermined angle relative to the major surface of the body, and provides a guide that a clinician may reference in order to orient a medical element introducer when introducing the introducer into the patient to access a particular target tissue site.

TECHNICAL FIELD

The invention relates to medical devices and, more particularly, the implantation of medical devices.

BACKGROUND

Medical systems for tissue therapy often require the implantation of one or more medical elements proximate to a target tissue site. The medical element may be, for example, a medical lead to deliver electrical stimulation energy to the tissue or a catheter to deliver a fluid to the tissue. In order to implant the medical element in a patient, a clinician may generate a medical image of the target tissue site for the medical element and guide the medical element into the patient based on the generated image.

For some tissue stimulation applications, an electrical stimulation lead is implanted near a sacral nerve, which is a nerve bundle within the sacrum, a large triangular bone situated at the lower part of the vertebral column and at the upper and back part of the human pelvic cavity. There are multiple sacral nerves that pass through anterior and posterior sacral foramina of the sacrum. The lead may include one or more stimulation electrodes, one or more sensing electrodes, or combinations thereof.

Electrical stimulation of one or more sacral nerves may eliminate or reduce some pelvic floor disorders by influencing the behavior of the relevant structures, such as the bladder, sphincter and pelvic floor muscles. Pelvic floor disorders include urinary incontinence, urinary urge/frequency, urinary retention, pelvic pain, bowel dysfunction, and male and female sexual dysfunction. The organs involved in bladder, bowel, and sexual function receive much of their control via the second, third, and fourth sacral nerves, commonly referred to as S2, S3 and S4 respectively. Electrical stimulation is typically delivered to at least one of the S2, S3, or S4 sacral nerves using an electrical stimulator, which is coupled to a stimulation lead that is implanted on a temporary or permanent basis proximate to the relevant sacral nerve.

In some cases, a medical element of a medical system is implanted through a sacral foramen (a single foramina) of a patient. For example, if selective stimulation of the S3 sacral nerve is desired, a medical lead may be introduced into the sacral foramen corresponding to the S3 sacral segment, which is commonly referred to as the “S3 sacral foramen.” In one technique, a hollow introducer needle is advanced through the S3 foramen and the lead is advanced through a lumen of the hollow introducer needle until one or more electrodes near a distal end of the lead are positioned near the S3 sacral nerve. Stimulation energy is applied through the lead to the electrode to test the S3 nerve response. If necessary, the one or more electrodes are moved back and forth to locate the most efficacious location.

Once in position, the lead may be secured by suturing the lead body to subcutaneous tissue posterior to the sacrum or using an anchoring mechanism such as tines carried by the lead, sutures or another technique. The lead may then be attached directly or indirectly (e.g., via an extension) to the output of an implantable electrical stimulator, although in some cases, the lead may be attached to an external pulse generator for temporary trial stimulation.

SUMMARY

In general, the invention is directed to a locating guide for locating a target tissue site. In some embodiments, the target tissue site includes a target nerve site, which may be, for example, a location of a target nerve or a location of an anatomical structure providing access to the nerve. The locating guides described herein include reference markers that are visible in a medical image, such as a radiographic image. In particular, a locating guide includes at least a first reference marker and a second, lateral reference marker protruding from a body of the locating guide in a different direction than the first reference marker, if the first reference marker protrudes from the body at all. The first and second reference markers provide reference points for registering a medical image of the patient, where the medical image may include an image of the target tissue site at an actual location on the patient. Together, the first and second reference markers provide visual reference points that extend in at least two dimensions, thereby increasing the possibility that a reference marker will be visible in a medical image. That is, the locating guide includes reference markers that are visible from at least two different perspectives.

The locating guide may be placed on a skin surface of a patient proximate to the target tissue site. In some embodiments, at least one reference marker of the locating guide may be used to align the locating guide with a boney landmark of a patient. A medical image of the locating guide and underlying tissue (proximate to the locating guide) may be generated to identify an approximate location of a target tissue site relative to the locating guide. Based on the medical image, the clinician may associate one or more reference markers of the locating guide with the location of the target tissue site. The clinician may then register the relevant portions of the medical image, such as the reference markers associated with the target tissue site and/or the target tissue site image, to an actual location on or within the patient. For example, the clinician may register the image of the reference markers associated with the target tissue site to the actual reference markers, and determine an approximate location of the target tissue site based on the actual location of the reference markers.

If desired, the clinician may introduce a medical element into the patient using the one or more reference markers associated with the target tissue site as a guide for locating the target tissue site. The medical element may be, for example, a medical lead, electrical stimulator or catheter. In some embodiments, a body of the locating guide defines an aperture that provides an opening in the locating guide through which the clinician may access the target tissue site. In some cases, the clinician may implant the medical element with the aid of an introducer needle, which may be percutaneously introduced into the patient or through an incision. In some embodiments, the second reference marker extends from the body of the locating guide at an angle and indicates the angle at which the clinician may orient the introducer needle relative to the skin surface of the patient in order to access a particular target tissue site, such as a sacral foramen. In this way, the second reference marker may also be a guide for orienting an introducer needle in order to more accurately access a target tissue site.

In one embodiment, the invention is directed to a target tissue site locating guide comprising a body defining a major surface, a first substantially radiopaque reference marker on the body, and a second substantially radiopaque reference marker extending from the major surface of the body.

In another embodiment, the invention is directed to a locating guide comprising a body configured to overlay a skin surface of a patient proximate to a sacrum of the patient, and a radiopaque reference marker on the body and configured to extend away from the skin surface of the patient at an angle when the body is positioned on the skin surface. The angle indicates an orientation for introducing an introducer into the patient to access a sacral foramen of the sacrum.

In another embodiment, the invention is directed to a method comprising positioning a locating guide on a skin surface of a patient. The locating guide comprises a body, a first radiopaque reference marker on the body, and a second radiopaque reference marker on the body and configured to extend away from the skin surface of the patient when the locating guide is positioned on the skin surface. The method further comprises generating an image of at least a portion of the locating guide and at least a portion of tissue of the patient proximate to the locating guide with a medical imaging device, and registering the image with an actual location on or within the patient based on at least one of the first or second reference markers.

DETAILED DESCRIPTION

The invention relates to a locating guide for locating a target tissue site within a patient. The target tissue site may be a target nerve site, which may refer to the location of a target nerve or a location of an anatomical structure providing access to the nerve. As one example, the target nerve site may be a sacral nerve or a sacral foramen, through which a sacral nerve is accessible. The locating guides described herein include reference markers that, together with a medical imaging device, such as fluoroscopy device, provide a clinician with information concerning the location of a target tissue site within the patient. In some embodiments, determining the location of a target tissue site may be useful for implanting a medical element, such as, but not limited to, a medical lead, electrical stimulator or catheter, proximate to the target tissue site.

Various embodiments of the invention may be applicable to different therapeutic applications requiring implantation of a medical element, such as neuromodulation by electrical stimulation of or drug delivery to one or more sacral nerves, pudendal nerves (including perineal and/or dorsal nerve branch), genitofemoral nerves (including genital and/or femoral nerve branch), ilioinguinal nerves, and iliohypogastric nerves. Such neuromodulation techniques may be provided to alleviate a variety of symptoms or disorders, including pelvic pain, urinary incontinence, fecal incontinence, and sexual dysfunction. The locating guides described herein may be applicable to a variety of other therapeutic applications and tissue sites, including a variety of peripheral nerve sites. For purposes of illustration, however, the disclosure will refer to electrical stimulation of one or more sacral nerves, and will refer to a “target nerve site,” although the locating guides described herein may also be useful for locating other target tissue sites.

FIG. 1is a schematic perspective view of therapy system10, which includes medical device12coupled to implanted medical element14. Implanted medical element14has been implanted proximate to target nerve site16, which has been located with the aid of a locating guide (described in further detail below). In the embodiment of therapy system10shown inFIG. 1, target nerve site16is sacral foramen20of sacrum22. Sacral foramen20provides access to sacral nerve24. However, in alternate embodiments, target nerve site16may be any suitable nerve site in patient18, whether it is a nerve or an anatomical structure providing access to the nerve, and may be selected based on, for example, a therapy program selected for a particular patient18. Medical device12may be subcutaneously implanted in the body of a patient18(e.g., in a chest cavity, lower back, lower abdomen, or buttocks). The type of medical device12and implanted medical element14incorporated into therapy system10may also depend upon the therapeutic application.

For example, in the embodiment shown inFIG. 1, medical device12is an electrical stimulator (either implantable or external), which is directly or indirectly (e.g., via an extension) coupled to medical element14, which is a stimulation lead. Medical device12provides a programmable stimulation signal (e.g., in the form of electrical pulses or a continuous signal) that is delivered to target nerve site16by stimulation lead14, and more particularly, via one or more stimulation electrodes carried by lead14. In some cases, an electrical stimulator may also be referred to as a signal generator or a neurostimulator. In another embodiment, lead14may include one or more sense electrodes to permit medical device12to sense electrical signals from target nerve site19, such as to sense one or more physiological parameters of patient18(e.g., blood pressure, temperature or electrical activity). Furthermore, in some embodiments, medical12may be coupled to two or more leads, e.g., for bilateral or multi-lateral stimulation and/or sensing.

While medical element14is primarily referred to as a “medical lead” for the remainder of the description, in other embodiments, medical element14may be any suitable medical element. For example, in another embodiments, medical device12may be a fluid delivery device, such as a drug pump, and medical element14may be a catheter that is placed to deliver a fluid (e.g., pharmaceutical agents, insulin, pain relieving agents, gene therapy agents or the like), from medical device12to target nerve site16. In yet other embodiments, medical element14may be a substantially self-contained device that is not coupled to medical device12, and therapy system10may not include medical device12. For example, in some embodiments, medical element14may be a leadless microstimulator, which includes a substantially self-contained device that includes stimulation and/or sensing electrodes and the associated electronics (e.g., controls, power source, and etc.) or a microstimulator with a lead that includes at least one stimulation and/or sensing electrode.

In some cases, a clinician may generate a real-time or still image of the relevant tissue of patient18using a medical imaging device, such as an x-ray machine or a fluoroscope, in order to locate target nerve site16. For example, a fluoroscopic image may be used to generate a real-time image of at least a portion of sacrum22of patient18in order to locate the target sacral foramen20. The clinician may locate target nerve site16and guide medical lead14proximate to sacral nerve24with the aid of the fluoroscopic image. Due at least in part to the curvature of sacrum22and/or the positioning of patient18, and in particular, the positioning of sacrum22relative to the fluoroscopic imager, the image of sacrum22may not clearly illustrate the target sacral foramen20. In some cases, the fluoroscopic image of sacrum22may not illustrate the target sacral foramen20or insufficiently illustrate target sacral foramen20(e.g., illustrate foramen20from a poor angle), such that the fluoroscopic image provides the clinician with insufficient guidance as to the location of target nerve site16.

Even if the target sacral foramen20is visible in the fluoroscopic image, the image of sacral foramen20may not be representative of the size or shape of the sacral foramen20or the relative location of the target sacral foramen20relative to a boney landmark or another internal reference point. In particular, the fluoroscopic image angle and/or the curvature of sacrum22may impart a skew to the size and shape of the imaged foramen20. Again, this may provide the clinician with little guidance as to the location of target nerve site16, and, in some cases, the clinician may inaccurately read the medical image and incorrectly determine a location of the target sacral foramen20. The inaccuracies in reading the medical image may arise from, for example, how the clinician relates the medical image to an actual location on the patient. The fluoroscopic image angle and/or the curvature of sacrum22may misguide the clinician in identifying the location of sacral foramen20relative to other portions of sacrum22or patient18.

A locating guide described herein provides one or more useful, visible reference markers that the clinician may use to more accurately register a medical image (e.g., a fluoroscopic image) to an actual location on or within the patient. The locating guide may help minimize any degradation in the usefulness of the medical image because of possible misalignment between the imaging device (e.g., a fluoroscope) and target nerve site16and/or a possible misregistration between the image and the patient by the clinician. Target nerve site16may be sacral foramen20or another site within patient18. A clinician may place the locating guide on a skin surface of patient18proximate to target nerve site16and a medical image may be taken of the locating guide and portions of tissue underlying the skin proximate to the locating guide. The clinician may associate one or more features of the locating guide, such as distance reference markers, lateral markers that protrude from the locating guide body, or centerline reference markers, shown in the medical image with the image of target nerve site16. For example, the clinician may identify one or more of the reference markers as being closest to the target nerve site. The clinician may then approximate the location of target nerve site16based on the relevant reference markers of the locating guide. In this way, the one or more features of the locating guide provide externally visible reference points that a clinician may use to register a medical image, and in particular, a target nerve site shown in the medical image, to an actual location on or within patient18.

FIGS. 2A and 2Billustrate a top view and a side view, respectively, of locating guide30in accordance with an embodiment of the invention. Reference to a “top view” and “side view” herein refers generally to the orientation of the views relative to each other, and is not intended to indicate that the views necessarily illustrate a top and side of the shown locating guide. The “top” and “side” views are merely orthographic projection views.

Locating guide30may be used to guide a clinician during a target nerve site16location technique. In general, locating guide30is placed on a patient's skin surface and provides an externally visible reference point that is also visible in a medical image (e.g., a fluoroscopic image). For example, a clinician may position locating guide30on a skin surface of patient18proximate to sacrum22in order to aid in locating a target sacral foramen20(FIG. 1). Locating guide30includes body32, which defines first portion32A and second portion32B. Locating guide30further includes distance reference markers34,36, centerline reference markers38, and lateral reference markers40along first portion32A of body32, and distance reference markers42,44, centerline reference markers46, and lateral reference markers48along second portion32B. Reference markers34,36,38,40,42,44,46, and48may each be integrally formed with body32(e.g., molded, casted, extruded, stamped or punched from body32, printed onto body32, etc.) or fixed to body32(e.g., via an adhesive, ultrasonic welding, or otherwise).

Locating guide30is preferably formed of a material including a color that allows locating guide30to be easily visually distinguished from skin on which it is placed. In addition, one or both sides of locating guide30may define a “skin contact side,” which is a surface of locating guide30that is intended to contact a patient's skin. The skin contact side of locating guide30may be chemically and biologically inert to limit any reaction with skin or other surfaces and materials (e.g., common sterilizing agents such as isopropyl alcohol or iodine) that may be contacted by locating guide30. In some embodiments, locating guide30may be disposable after use with a single patient. In other embodiments, locating guide30may be reusable. Of course, if locating guide30is reused for multiple patients, it may be desirable to sterilize locating guide30.

Body32may be constructed of any suitable material. In one embodiment, body32comprises a substantially radiopaque material, which enables body32to be visible in a radiographic image, such as an x-ray or fluoroscopic image. For example, body32may be constructed of stainless steel, titanium, barium sulfate (BaSO4) loaded silicone or polyimide (e.g., Kapton brand polyimide film, which is available from E. I. du Pont de Nemours and Company of Wilmington, Del.). In another embodiment, body32comprises a substantially non-radiopaque material and distance markers34,36, lateral markers40,48, and/or centerline markers38,46are formed of a radiopaque material, such that distance markers34,36, lateral markers40,48and/or centerline markers38,46are primarily visible in a radiographic image and any patient structures underlying body32are substantially visible in the radiographic image.

Body32is configured to overlay a skin surface of patient18. In some embodiments, such as embodiments in which body32is comprised of silicone, body32is substantially flexible and substantially conforms to a contour of patient18when locating guide30is placed on the skin surface. A substantially flexible body32may enable body32to customize locating guide30to the contour of a particular patient18and/or to the particular region of patient18on which locating guide30is placed. In other embodiments, such as some embodiments in which body32is formed of stainless steel or titanium, body32may be substantially inflexible and contoured to fit over a backside of patient16, or another surface of patient18near target nerve site16. While a contour of a backside may differ substantially between patients, a contour of body32may be based on an average contour of a backside of, for example, adult humans. However, in some embodiments in which body32is formed of stainless steel or titanium, body32may exhibit limited flexibility if the stainless steel or titanium is thin enough. In other embodiments, body32may be formed of a material that is pliable enough to adapt to different skin surface contours, while at the same time, substantially hold its shape.

As shown inFIG. 2B, body32has a thickness T1(measured along a z-axis direction, where orthogonal x-y-z axes are shown inFIGS. 2A and 2B). In one embodiment, thickness T1is about 0.04 cm to about 1.0 cm, such as about 0.06 cm. Thickness T1may be selected to provide locating guide30with sufficient integrity to be handled and positioned on patient18with some degree of control by a clinician. If thickness T1is too small, the clinician may find it difficult to control body32and, accordingly, difficult to place locating guide30on patient18with accuracy and precision. On the other hand, if thickness T1is too great and body32is intended to be flexible, body32may not substantially conform to a contour of patient18when locating guide30is positioned on a skin surface of patient18. In some cases, the suitable thickness T1of body32may depend upon the type of material used to form body32. For example, if body32is formed of a substantially flexible material, thickness T1may be greater than when body32is formed of a substantially inflexible material.

An adhesive may be placed along a skin contact surface of locating guide30in order to couple locating guide30to a patient's skin at a desired position. The adhesive may be provided as a layer on one side of body32(e.g., an adhesive layer that includes a removable backing) or a clinician may manually apply the adhesive to locating guide30, e.g., the adhesive may be provided in a kit with locating guide30. In either case, the adhesive may not cover the entire skin contact surface of locating guide30. For example, a sufficient amount of adhesive to enable locating guide30to remain substantially in place during the target nerve site16location procedure may be applied to locating guide30. In some embodiments, the adhesive is removable and reusable, such that a clinician may attach locating guide30to patient16and subsequently adjust a position of locating guide30and reuse the same adhesive.

Examples of suitable adhesives include adhesives used for ground electrode pads and electrocardiogram (ECG) electrode pads, which may be, for example, tragacanth gum, karaya gum, or acrylates. The adhesive may be selected based on a preparation procedure for the medical lead14implantation technique. For example, the clinician may sterilize the skin surface of patient18near target nerve site16with iodine or another sterilizing agent. Locating guide30may be placed over the sterilized skin, and accordingly, it may be desirable for the selected adhesive to be compatible with the sterilizing agent, such that the adhesive maintains its adhesive properties when applied to the sterilized skin.

In other embodiments, other modes of attachment that enable locating guide30to be placed on skin of patient18and remain substantially in place may be used. For example, locating guide30may include a belt that extends around a waist of a patient or connects to a surface on which patient18is placed (e.g., a table or bed), or alternatively, locating guide30may include Velcro attachments that mate with strips already attached to patient18. In yet another alternative embodiment, locating guide30may be formed of a material that exhibits a high coefficient of friction with human skin, which enables locating guide30to remain substantially in place when placed on skin.

Preferably, a size of locating guide30remains the same for all patients. Of course, if desired, a clinician may be given the option to choose from a variety of locating guides30having different sizes for different patients. In the embodiment shown inFIGS. 2A and 2B, width W1(measured along the x-axis direction) of locating guide30is about 13 centimeters (cm) to about 15 cm and length L1(measured along the y-axis direction) is about 13 cm to about 15 cm. The L1and W1values are merely one example embodiment, and in other embodiments, locating guide30may be any suitable size.

Distance reference markers34,36extend from first portion32A of body32(“first body portion32A”) and distance reference markers42,44extend from second portion32B of body32(“second body portion32B”). While rectangular distance markers34,36,42,44are shown inFIG. 2A, in other embodiments, distance markers34,36,42,44may have any suitable shape, such as square, triangular, elliptical, and so forth. Furthermore, in the embodiment shown inFIGS. 2A-B, distance markers34,36,42,44extend away from body32. In other embodiments, distance markers34,36,42,44may extend into body, e.g., may defined by a plurality of slots or cuts defined by body32that are also visible in a radiographic medical image. In addition, as shown inFIG. 4and described below, in one embodiment, a locating guide may be substantially integral with body32.

Distance reference markers34,42may be referred to as “major markers,” while distance markers36,44may be referred to as “minor markers.” Major markers34,42are substantially evenly spaced and the respective minor markers36,44are positioned between major markers34,42. For example, in one embodiment, major markers34are spaced about 1 centimeter (cm) apart, while minor markers36are spaced between adjacent major markers34, such that a distance between a major marker34and an adjacent minor marker36is about 0.5 cm. However, in other embodiments, major markers34,42may be spaced from the respective minor markers36,44by any suitable distance and markers34,36along first body portion32A may be spaced differently than markers42,44. In other embodiments, locating guide30may include other distance reference marker configurations. For example, in one embodiment, locating guide30may include only one set of distance reference markers along each body portion32A,32B, while in another embodiment, locating guide30may include more than one minor marker36,44between two major markers34,42, respectively.

In general, locating guide30may include labeled distance markers34,36,42,44to indicate distance and/or provide a visual reference marker. In one embodiment, distance markers34,36,42,44may be labeled with numbers representing units of measurement. Any suitable unit of measurement may be used, including, but not limited to, millimeters, centimeters, and inches. Alternatively, distance marks34,36,42,44may merely be numbered consecutively, rather than representing units of measurement.

In one embodiment, distance markers34,36along first body portion32A represent x-axis coordinates and distance markers42,44along second portion32B represent y-axis coordinates. Together, distance marks34,36,42,44define an x-y coordinate system for identifying a specific location within area of interest49framed by locating guide30. An x-y coordinate system, or another type of coordinate system, may be useful for identifying a specific location within area of interest49. A clinician may register a location within a medical image to an actual location within area of interest by matching the x-y coordinates from the image to the x-y coordinates within area of interest49.

Centerline marker38are substantially aligned with along a longitudinal axis50of first body portion32A and centerline markers46are substantially aligned with longitudinal axis52of second body portion32B. In some embodiments, centerline markers38,46are radiopaque. Centerline markers38,46provide visual feedback to a clinician when the clinician positions locating guide30on patient18. For example, the clinician may align at least one of centerline markers38,46with a boney landmark of patient18. As described in further detail below, the boney landmark may be identified via a medical image, such as a fluoroscopic image or an x-ray image, and/or the clinician may identify the relevant boney landmarks based on a physical examination of the patient. In the embodiment shown inFIG. 2A, centerline marker38comprises a plurality of substantially aligned openings defined by first body portion32A and centerline markers46comprise a plurality of substantially aligned openings defined by second body portion32B. Centerline markers38,46defined as openings in body32may be visible in a radiographic image if body32is radiopaque because centerline markers38,46may be shown in the image to be voids in the radiopaque body32. In other embodiments, centerline markers38,46may comprise one or more visible marks on the respective body portion32A,32B, where the markers may be radiopaque, and if body32is also radiopaque, centerline markers38,46may have a different radiopacity than body32. In other embodiments, locating guide30does not include centerline markers38,46, and the clinician may estimate the centerline of body portions32A,32B.

As shown inFIG. 2B, lateral markers40,48protrude from major surface54of body32. Lateral markers40,48are stippled inFIGS. 2A and 2Bto illustrate the protrusion from major surface54, and the cross-hatching is not intended to limit the scope of the present invention in any way. Major surface54substantially lies in the plane of the image shown inFIG. 2Aand extends substantially perpendicular to the plane of the image shown inFIG. 2B. That is, major surface54generally extends the x-y plane, where orthogonal x-y-z axes are shown inFIGS. 2A and 2B. However, major surface54is not necessarily planar, although in some embodiments, major surface54may be substantially planar. For example, in embodiments in which body32is formed with a predefined curvature to match a contour of patient18, major surface54of body32may not be substantially planar. However, even if body32exhibits some curvature or is otherwise nonplanar, lateral markers40,48may still protrude substantially away from body32and, may be referred to as “protruding” substantially along a z-axis direction and away from major surface54of body32. Although inFIGS. 2A and 2B, lateral markers40,48are substantially perpendicular to major surface54of body32, in other embodiments, lateral markers40,48may extend substantially perpendicular to major surface54of body32. In addition, lateral markers40,48may extend away from a skin surface of patient18when locating guide30is placed on the skin surface. The x-y-z axes are shown inFIGS. 2A and 2Bto aid the description of locating guide30, and are not intended to limit the present invention in any way.

Lateral markers40,48in the embodiment shown inFIGS. 2A and 2B, are spaced about 2 cm from an adjacent lateral marker. However, in other embodiments, lateral markers40,48may have any suitable spacing. Lateral markers40,48protrude from major surface54substantially the same distance in the embodiments shown inFIGS. 2A and 2B. That is, each lateral marker40,48has substantially the same height (measured along the z-axis direction). In other embodiments, lateral markers40,48may have different heights. In one embodiment, lateral markers40,48have a height of about 0.25 cm to about 2 cm, such as about 0.5 cm to about 1 cm, although lateral markers40,48may have any suitable height. Furthermore, a top surface40A,48A of each lateral marker does not necessarily need to be substantially parallel with major surface54. An embodiment of a locating guide including an angled lateral marker is shown inFIGS. 8A and 8Band described below.

Lateral markers40,48provide a reference point that is visible from a different perspective than distance reference markers34,36,42, and44because lateral markers40,48protrude from major surface54of body32, while distance reference markers34,36,42, and44substantially do not. In this way, locating guide30includes reference markers that extend in at least two dimensions. In some cases, distance reference markers34,36,42, and44may be formed from the same material as body32, in which case distance reference markers34,36,42, and44may not be discernable from body32in a medical image, such as a fluoroscopic image. For example, if the medical image is taken from a x-axis perspective, e.g., the perspective shown inFIG. 2B, distance reference markers34,36,42, and44may not be easily distinguishable from body32, and in some cases, may not be distinguished from body32. In contrast, lateral markers40,48may easily be seen in the medical image taken from the x-axis perspective, even if lateral markers40,48are formed of the same material as body32, because lateral markers40,48physically protrude from body32. However, in some cases, distance reference markers34,36,42, and44may be formed from a material having a different radiopacity than body32such that distance reference markers34,36,42, and44are visible in a medical image taken from the x-axis perspective.

If the medical image is taken from a z-axis perspective, e.g., the perspective shown inFIG. 2A, distance reference markers34,36,42, and44may be visible in the medical image, and, therefore, provide a visible reference point for the clinician to associate with the medical image. In contrast, lateral markers40,48may blend in with body32in the medical image, and, thus, may not be visible in the medical image taken from the z-axis perspective. However, in some cases, lateral markers40,48may be formed from a material having a different radiopacity than body32such that lateral markers40are visible in a medical image taken from the z-axis perspective.

In other embodiments, locating guide30may include other configurations of reference markers. For example, in some cases, distance markers34,36,42,44may have a greater thickness (measured along the z-axis direction) than body32, and therefore, protrude from body32, distance markers34,36,42,44may provide markers visible from at least two different perspectives. In embodiments in which distance markers34,36,42,44protrude from body32in a z-axis direction, distance markers34,36,42,44may provide two different types of reference markers, i.e., distance markers and lateral markers.

FIG. 3illustrates one technique for identifying a location of a target sacral foramen with the aid of locating guide30. As previously described, it may be useful to locate a sacral foramen in order to implant a medical device, such as a medical lead or a catheter, near a sacral nerve.FIG. 3illustrates a backside of patient18, and in particular illustrates skin60, Ilium bone62, sacrum22, and a part of spinal canal66. Ilium bone62, sacrum22, and spinal canal66are shown in phantom lines. Sacrum22includes S2, S3, and S4 foramen68,70,72, respectively, through which the S2, S3, and S4 sacral nerves, respectively, are accessible. The S1 foramen is not shown inFIG. 3. If a clinician wishes to implant a lead or another medical element near the S3 sacral nerve, the clinician typically locates S3 foramen70because as previously stated, the S3 sacral nerve is accessible through S3 foramen70. The clinician may use locating guide30to locate the S3 foramen70(or any of the other foramen of sacrum54). While a technique for locating the S3 foramen70is described below, in other embodiments, locating guide30may be used to locate other target nerve sites within patient18.

Preferably, locating guide30is positioned on a skin surface of patient18to frame the general area in which the S3 sacral foramen70is located. The clinician may use the boney landmarks of patient18to estimate the general location of the S3 foramen. In the present example, the boney landmarks are the posterior protuberance (or “crest”) of Ilium bone62and/or the lower part of spinal canal66, which runs through sacrum22. For example, the clinician may align first portion32A of body32to overlay the crest of the patient's Ilium bone62(i.e., where the sacroiliac ligament attaches to the Ilium) and align second portion32B of body32to overlay a midline of patient18, which may also substantially correspond to a spinal canal66. The clinician may identify the boney landmarks with the aid of a medical image taken before or after locating guide30is placed on skin60of patient18and/or with the aid of a physical examination.

In one embodiment, the clinician may initially place locating guide30on skin surface60relative to Ilium bone62and spinal canal66, image the patient18and locating guide30, and adjust the position of locating guide if desired based on the imaged location of Ilium bone62and spinal canal66(or other relevant boney landmarks) relative to the locating guide30. Based on the image, for example, the clinician may determine the magnitude and direction in which locating guide30should be moved in order to align centerline markers38with the crest of Ilium bone62and centerline markers46with spinal canal66. The clinician may then reposition locating guide30accordingly. However, it may not be necessary to align centerline markers38with the crest of Ilium bone62and centerline markers46with spinal canal66in all cases.

It may be desirable for skin60to be dry and unbroken in the regions in which locating guide30is applied. Patient18may be in any position that provides the clinician access to sacrum22. However, the prone position is preferred in some embodiments because some types of neurostimulation require testing of toe flexure, foot rotation, or a bellows response of the anus.

After locating guide30is attached to skin60of patient18, locating guide30frames a general area in which a target nerve site may be located. The framed area is referred to as area of interest49, which is outlined with phantom lines inFIG. 3. In the embodiment shown inFIG. 3, area of interest49frames the general area in which the S3 foramen70is most likely located. Of course, there may be certain patients in which area of interest49is inaccurate because of the patient's anatomy, in which case, the clinician may need to readjust the position of locating guide30if, for example, a medical image indicates the target nerve site is not located within area of interest49. In alternate embodiments, the size and shape of locating guide30are adjusted to accommodate to frame an area of interest for other target nerve sites, which may affect the boney landmarks locating guide30overlays. The relevant boney landmarks depend upon the medical lead implantation site, which is typically dictated by the target nerve site location.

After locating guide30is placed on skin60of patient18, the clinician may obtain a medical image of locating guide30and the underlying tissue proximate to locating guide30. The clinician may use the resulting image to locate S3 sacral foramen70. A technique for locating the S3 sacral foramen70or another target nerve site using locating guide30and a medical image of locating guide30and patient18is described in further detail below with reference toFIG. 5. After locating the S3 sacral foramen70in the medical image and registering the medical image to a location on the skin surface60of patient suitable for accessing the S3 foramen70with the aid of locating guide30, the clinician may guide an introducer needle (not shown) into patient18near the target nerve site and guide a medical lead, catheter, or other implantable medical element through the introducer needle for implantation proximate to the S3 sacral nerve70.

FIG. 4illustrates a top view of locating guide76in accordance with another embodiment. Locating guide76is substantially similar to locating guide30ofFIGS. 2A-2B, and includes body32defining first portion32A and second portion32B, major distance reference markers80along first body portion32A, minor distance reference markers82along first body portion32A, major distance reference markers84along second body portion32B, minor distance reference markers86along second body portion32B, lateral markers40along first body portion32A, and lateral markers48along second body portion32B. Although not shown inFIG. 4, in other embodiments, locating guide76may include centerline markers along first and second body portions32A,32B comprised of a radiopaque material, such as a radiopaque ink.

In the embodiment shown inFIG. 4, body32comprises a radio-transparent material, such that when a radiographic image is taken of body32positioned on skin60(FIG. 3) of patient18, body32is not visible in the radiographic image and the structure underlying body32is visible. However, it is desirable for distance reference markers80,82,84,86, and lateral markers40,48to be visible in the radiographic image in order to add useful features to the medical image that may be related back to an actual location on patient18. Thus, distance reference markers80,82,84,86, and lateral markers40,48may be formed of a radiopaque material. In one embodiment, distance markers80,82,84,86are printed onto body32with a radiopaque ink, and lateral markers40,48are comprised of a radiopaque material, such as silicone embedded with barium sulfate.

In some cases, it may be useful for the structure underlying body32to be visible, such as in cases in which locating guide76is inadvertently placed over the target nerve site. Body32may be formed of material that is relatively easily pierced by an introducer needle for implanting lead14in patient18. In such cases, the clinician does not need to adjust a position of locating guide76relative to patient18to introduce lead14(or another medical element) into patient18, even if body32overlays the target nerve site within patient18. If locating guide76is purposefully or inadvertently placed over the target nerve site and body32is relatively easy to pierce through with an introducer needle, distance reference markers80,82,84,86and/or lateral marker40,48may be positioned relatively close to the target nerve site and the clinician may identify which distance reference markers80,82,84,86, or lateral marker40,48, if any, directly overlay the target nerve site. A reference marker80,82,84,86, or lateral marker40,48directly overlaying the target nerve site may help the clinician implant lead14(FIG. 1) with improved accuracy and precision compared to a situation in which markers80,82,84,86,40,48do not directly overlay a target nerve site.

FIG. 5is a flow diagram illustrating a technique for locating a target nerve site with the aid of locating guide30. A clinician may place locating guide30on patient18near the approximate location of target nerve site16(FIG. 1) to frame area of interest49(FIG. 2) that may provide access to target nerve site16(110). In one embodiment, the clinician places locating guide30on patient18relative to one or more boney landmarks. The clinician may locate the relevant boney landmarks with the aid of a medical image taken before or after locating guide30is placed on patient18and/or with the aid of a physical examination. In an alternate embodiment, the clinician may physically examine patient18, such as by manual palpation, to locate Ilium bone62and spinal canal66, and initially places first body portion32A along the crest of Ilium bone62and second body portion32B along spinal canal66(110).

The clinician may then obtain a medical image at least a locating guide30and patient18(112). In some embodiments, the clinician may obtain more than one image, where different images are taken from more than one perspective, such as from a lateral perspective relative to the patient (i.e., a substantially x-axis perspective of locating guide30when locating guide30is positioned proximate to sacrum22of patient18) and an anterior-posterior perspective (i.e., a substantially z-axis perspective of locating guide30when locating guide30is positioned on a skin surface proximate to sacrum22of patient18). The clinician may determine whether locating guide30is properly positioned relative to the relevant boney landmarks based on the generated image (113). If locating guide30is not properly positioned relative to the boney landmarks, the clinician may adjust the position of locating guide30on patient18(110) and obtain a medical image of the repositioned locating guide (112). However, it is not necessary for the clinician to do so. The clinician may instead leave locating guide30in place because locating guide30may still provide one or more visual reference markers for locating target nerve site16even if locating guide30is not positioned to overlay the desired boney landmarks.

If locating guide30is properly positioned relative to the boney landmarks, the clinician may determine whether target nerve site16is visible in the image (114). If target nerve site16is not visible, the clinician may reposition locating guide on patient (110). If target nerve site16is visible in the generated medical image, the clinician may identify target nerve site16within the generated medical image and associate the imaged target nerve site16with at least one distance reference marker34,36,42, and44and/or lateral marker40,48of locating guide30that is also shown in the generated medical image (116). For example, the clinician may review the medical image and identify the reference markers that are closest to target nerve site16and/or the relative location of target nerve site16and at least one of the reference markers of locating guide30. As another example, the distance markers34,36,42, and/or44shown in the image may also indicate a distance of the target tissue site relative to a portion of locating guide30.

As previously described, if the medical image is taken from the z-axis perspective relative to the general plane of the patient's back, at least some of distance reference marker34,36,42, and44are visible in the medical image. In some cases, such as when lateral markers40,48are formed of a material having a different radiopacity than body32, at least some of lateral markers40,48may also be visible in the medical image taken from the z-axis perspective. If the medical image is taken from the x-axis perspective, at least some of lateral markers40,48are visible in the medical image. In some cases, such as when distance reference marker34,36,42, and44are formed of a material having a different radiopacity than body32, at least some of distance reference marker34,36,42, and44may also be visible in the medical image taken from the x-axis perspective. Locating guide30that include reference markers in at least two different dimensions or planes increases the visibility of reference markers in the medical image, regardless of the angle the image is taken from.

After associating the imaged target nerve site16with at least one imaged marker (116), the clinician may register the medical image of target nerve site16and locating guide30with an actual location on patient18based on the one or more associated reference markers (117). That is, the clinician may align the medical image of target nerve site16and locating guide30with an actual location on or within patient18using at least one of the reference markers of locating guide30. When “registering” the image to an actual location on patient18, the clinician may use the relative position between locating guide30, which is placed on patient18, and the position of locating guide30(and any of its reference markers) in the image to locate the actual target nerve site16.

Registering the medical image to an actual location at patient18may help the clinician interpret the medical image (e.g., what angle the medical image is taken from, what scale, etc.) and approximate an actual location of target nerve site16. In particular, the clinician may approximate an actual location of target nerve site16based on the actual location of the reference markers associated with the target nerve site16. If desired, the clinician may implant a medical element proximate to target nerve site16within patient18based on the approximated location of target nerve site16(118). For example, the clinician may guide a distal end of an introducer needle to target nerve site16using the reference marker(s) associated with target nerve site16as a guide. Locating guide30may remain on patient18as the clinician guides the introducer needle to target nerve site18to provide a substantially stable reference point that is also visible in the medical image.

In some cases, the clinician may not be able to guide the introducer needle to target nerve site16despite the use of locating guide30. For example, the clinician may misread the reference markers in the generated image or incorrectly associate the target nerve site16with at least one reference marker. Using S3 foramen70(FIG. 3) as an example, if the clinician does not locate the S3 foramen70and cannot introduce the distal end of the introducer needle through the S3 foramen70, the clinician may leave the needle within patient18, obtain another medical image of locating guide30, tissue of patient18proximate to locating guide30, and the needle, which is typically radiopaque. The clinician may register the location of the needle shown in the image with the actual location of the needle with the aid of locating guide30. The clinician may then reguide the needle toward foramen70with the aid of the image and locating guide30. Thus, in some embodiments, both an introducer needle and guide30may provide external reference points for identifying the location of S3 foramen70.

Once the introducer needle is introduced into the S3 foramen70, lead14(FIG. 1) may be inserted into patient18through the introducer needle and implanted proximate to the S3 sacral nerve. The exact location of the S3 sacral nerve may be determined by testing the patient's responses to electrical stimulation that is delivered through lead14, e.g., from an external stimulator that is coupled to a proximal end of the lead.

FIGS. 6A and 6Billustrate a top and a side view, respectively, of locating guide120in accordance with another embodiment of the invention. Locating guide120provides a stationary reference point for identifying a target nerve site for implantation of a medical element. Just as with locating guide30, locating guide120may be placed on a patient's skin surface to provide one or more externally visible reference markers that are also visible in a medical image (e.g., a fluoroscopic image). Locating guide120may be coupled to a patient's skin using any suitable mechanism, such as the ones described above with reference to locating guide30.

Locating guide120includes body122, which is approximates the shape of sacrum22(FIG. 3). In the embodiment shown inFIGS. 6A and 6B, body122is shaped to resemble an orthographic projection of sacrum22. Body122may be formed of a material similar to body32. Locating guide120further includes reference markers124A,124B, which are substantially aligned along the y-axis direction (orthogonal x-y-z axes are shown inFIGS. 6A and 6B), reference markers126A,126B, which are substantially aligned along the x-axis direction, centerline reference markers128,130, and lateral reference markers132. Reference markers124A,124B,126A,126B,128,130, and132may each be integrally formed with body122(e.g., molded, casted, extruded, stamped or punched from body122, printed onto body122, etc.) or fixed to body122(e.g., via an adhesive, ultrasonic welding, or otherwise).

Lateral reference markers132are stippled inFIGS. 6A and 6Bto illustrate the protrusion in the z-axis direction from body122, and the cross-hatching is not intended to limit the scope of the present invention in any way. InFIGS. 6A and 6B, lateral markers132protrude from major surface134of body122. Major surface134substantially lies in the plane of the image shown inFIG. 6Aand extends substantially perpendicular to the plane of the image shown inFIG. 6B. That is, major surface134generally extends along the x-y plane, where orthogonal x-y-z axes are shown inFIGS. 6A and 6B. However, major surface134is not necessarily planar, although in some embodiments, major surface134may be substantially planar. The x-y-z axes are shown inFIGS. 6A and 6Bto aid the description of locating guide30, and are not intended to limit the present invention in any way.

Centerline reference markers128,130are similar to centerline markers38,46of locating guide30, and provide a reference point for positioning locating guide120on patient18, and may also help register a medical image with an actual location on patient18. In the embodiment shown inFIGS. 6A and 6B, centerline markers128,130designate a center of locating guide120along both the x-axis direction and y-axis direction. Centerline markers128,130are visible in both a radiographic image and to the clinician's eye without the aid of an imaging device. While centerline markers128,130are shown to be apertures in body122, in other embodiments, such as in embodiments in which body122is formed of a radio-transparent material, centerline markers128,130may be formed of a radiopaque material, such as radiopaque ink printed on body122. Furthermore, in some embodiments, locating guide120does not include centerline markers128and/or130.

As shown inFIG. 6B, body122has a thickness T2(measured along a z-axis direction). In one embodiment, thickness T2is about 0.04 cm to about 1.0 cm, such as about 0.06 cm. In general, thickness T2may be selected to provide locating guide120with sufficient integrity to be handled and positioned on patient18with some degree of control by a clinician. Body122defines S-I junction markers145A and145B. A sacral-iliac (S-I) junction is the region in which the Ilium bone62(FIG. 3) meets the sacrum22. S-I junction markers145A and145B provide another reference point that a clinician may use to align locating guide120with a patient's sacrum. In other embodiments, S-I junction markers145A and145B may extend from body122, rather than being defined by a cut-out in body122.

Body122defines foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B. Foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B provide openings in body122through which the clinician may introduce an introducer needle to implant a medical element into patient18proximate to target nerve site16(FIG. 1). Foramen apertures136A,138A,140A,142A, and144A are configured to be located on an opposite side of a midline of patient18from foramen apertures136B,128B,140B,142B, and144B when locating guide120is placed on patient18such that reference markers124A, B are substantially aligned with spinal canal66(FIG. 3) of patient18.

A plurality of foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B enable a clinician to implant medical elements into more than one foramen at a time without having to adjust a position of locating guide120. For example, at least one set of apertures may be configured to substantially align with S4 sacral foramen68(FIG. 3) when locating guide120is positioned over sacrum22(FIG. 3), while another set of apertures may substantially align with S3 sacral foramen70(FIG. 3), and another set of apertures may substantially align with S2 sacral foramen72. The particular apertures136A-B,138A-B,140A-B,142A-B, and144A-B that correspond to sacral foramen68,70,72may differ, depending upon the size of sacrum22(FIG. 2) of patient18. For example, in one patient, apertures136A, B may correspond to the S4 sacral foramen68, while in another patient, apertures138A, B may correspond to the S4 sacral foramen.

In embodiments in which foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B are configured to substantially align with sacral foramen (e.g., based on the average distance between foramen), foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B may also provide general guidance to a clinician when implanting a medical element into patient18. If apertures138A,138B are configured to align with the S3 sacral foramen70, for example, the clinician may introduce a medical element through apertures138A,138B in order to access the S3 sacral foramen70.

Although five sets of circular foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B are shown inFIG. 6A, in other embodiments, locating guide120may include any suitable number of foramen apertures having any suitable shape. For example, as shown inFIG. 7, locating guide146may include two substantially elliptical apertures148A and148B instead of a plurality of circular apertures136A-B,138A-B,140A-B,142A-B, and144A-B. Elliptical apertures148A,148B are surrounded by lateral markers132that extend in a z-axis direction away from body122. Lateral markers132are arranged in a substantially curved pattern around elliptical apertures148A,148B. In other embodiments, lateral markers132may have another arrangement relative to elliptical apertures148A,148B. In other aspects, locating guide146is substantially similar to locating guide120. Alternatively, a locating guide may include three sets of substantially circular apertures, where each set corresponds to one of the S4, S3, and S2 sacral foramen68,70,72, as shown inFIGS. 8A and 8Band described below.

In embodiments in which body122is comprised of a radiopaque material, foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B further provide reference points that are visible in a radiographic medical image. In other embodiments, foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B may not be apertures defined by body122, but may be, for example, formed of a different material than body122that is radiopaque or has a different level of radiopacity than body122and/or outlined by a radiopaque and visible material (e.g., a radiopaque ink). In embodiments in which foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B are covered by material, it may be desirable for the material to be configured such that an introducer needle may puncture directly through the material to reach target nerve site16within patient18.

In the embodiment shown inFIGS. 6A-6B, reference markers124A, B and126A, B extend substantially along a major surface134of body122, i.e., do not protrude significantly from body122in a z-axis direction. A clinician may align locating guide120relative to a sacrum22of patient18with the aid of reference markers124A, B and126A, B. In particular, reference markers124A, B and126A, B are configured to overlay a crest of Ilium bone62and spinal canal66(FIG. 3), respectively. Just as with locating guide30ofFIGS. 2A-2B, a size of locating guide120may remain the same for all patients. In some embodiments, however, a clinician may be given the option to choose from a variety of locating guides120having different sizes for different patients.

Lateral reference markers132are similar to lateral markers40,48of locating guide30. Lateral reference markers132provide reference points that are visible from a perspective that reference markers124A, B and126A, B, centerline reference markers128,130, and foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B may not be visible from. In this way, lateral markers132, reference markers124A, B and126A, B, centerline reference markers128,130, and foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B provide reference points that extend in more than one dimension.

In the embodiment shown inFIGS. 6A-6B, lateral markers132surround each foramen aperture136A-B,138A-B,140A-B,142A-B, and144A-B. In particular, three lateral markers132surround each foramen aperture136A-B,138A-B,140A-B,142A-B, and144A-B such that a center point between the three lateral markers132also indicate a center of the respective foramen aperture136A-B,138A-B,140A-B,142A-B, and144A-B. The clinician may estimate a center of each foramen aperture136A-B,138A-B,140A-B,142A-B, and144A-B based on a location of lateral markers132.

Other arrangements of lateral markers132are also contemplated, as long as lateral markers132provide some guidance to a clinician when registering a medical image of locating guide120and tissue of patient18to an actual location on patient18. For example, in other embodiments, a single lateral marker132may be placed relative to each foramen aperture136A-B,138A-B,140A-B,142A-B, and144A-B, or lateral markers132may be placed independently of foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B.

FIG. 6Cillustrates locating guide120and one embodiment of the dimensions of locating guide120. The dimensions provided below with reference toFIG. 6Care merely one embodiment, and in other embodiments, locating guide120may have any suitable dimensions. The overall length L2(measured along the y-axis direction) and width W2(measured along the x-axis direction) of body122of locating guide120is based on the size of a human adult sacrum. While the relative size of a human adult sacrum may differ between each patient, locating guide120may be representative of an average size of a human adult sacrum. In the embodiment of locating guide120shown inFIG. 6C, length L2of body122is about 15 cm to about 18 cm, such as about 16.5 cm (about 5.9 inches to about 7.0 inches, such as about 6.5 inches), and width W2of body122is about 14 cm to about 16 cm, such as about 15.24 cm (about 5.5 inches to about 6.3 inches, such as about 4.0 inches).

Markers124A,124B may extend from body122at any suitable length L3that is selected to provide a clinician with sufficient markers124A,124B with a center of sacrum22. In the embodiment shown inFIG. 6C, length L3is about 1.5 cm to about 3.5 cm, such as about 2.54 cm (0.59 inches to about 1.38 inches, such as about 1.0 inches). Markers124A,124B each have a width W3of about 0.5 cm to about 0.8 cm, such as about 0.635 cm (about 0.05 inches to about 0.35 inches, such as about 0.25 inches). Similarly, markers126A,126B extend from body122at any suitable width W4, which is selected to provide a clinician with sufficient markers126A,126B with a crest of Ilium bone62(FIG. 3). In the embodiment shown inFIG. 6C, width W4is about 1.0 cm to about 4.0 cm, such as about 2.54 cm (0.04 inches to about 1.5 inches, such as about 1.0 inch). A center of each markers126A,126B is distanced from an edge of body122by distance L4, which may be 1.0 cm to about 4.0 cm, such as about 2.54 cm (0.04 inches to about 1.5 inches, such as about 1.0 inch). Markers126A,126B each have a length L5of 0.4 cm to about 1.27 cm, such as about 0.635 cm (about 0.15 inches to about 0.5 inches, such as about 0.25 inches).

Each lateral marker132has a diameter of about 0.0127 cm to about 0.4 cm, such as about 0.254 cm (about 0.05 inches to about 0.15 inches, such as about 0.10 inches). In other embodiments, however, lateral marker132may have a shape other than a circle. Sacral foramen apertures136A,136B each have a diameter of about 1.27 cm to about 1.78 cm, such as about 1.5 cm (about 0.50 inches to about 0.70 inches, such as about 0.60 inches), and do not necessarily have the same diameter. Sacral foramen apertures138A,136B each have a diameter of about 1.14 cm to about 1.65 cm, such as about 1.4 cm (about 0.45 inches to about 0.65 inches, such as about 0.55 inches), and do not necessarily have the same diameter. Sacral foramen apertures140A,140B each have a diameter of about 1.0 cm to about 1.5 cm, such as about 1.27 cm (about 0.40 inches to about 0.60 inches, such as about 0.50 inches), and do not necessarily have the same diameter. Sacral foramen apertures142A,142B each have a diameter of about 0.95 cm to about 1.46 cm, such as about 1.21 cm (about 0.375 inches to about 0.575 inches, such as about 0.475 inches), and do not necessarily have the same diameter. Sacral foramen apertures144A and144B each have a diameter of about 0.90 cm to about 1.40 cm, such as about 1.14 cm (about 0.35 inches to about 0.55 inches, such as about 0.45 inches), and do not necessarily have the same diameter.

The distances between each sacral foramen may differ based on the patient, thus, a distance between a center of foramen apertures136A,136B and138A,136B, respectively, a distance D2between apertures138A,138B and140A,140B, respectively, a distance between apertures140A,140B and142A,142B, respectively, and a distance between apertures142A,142B and144A,144B, respectively may not accurately represent the distances between a particular patient's foramen. However, the distances between adjacent foramen apertures may be generic enough to match a large range of patients. For example, if apertures136A, B are configured to substantially align with S4 foramen68(on opposite sides of spinal canal66), apertures138A, B are configured to substantially align with S3 foramen70(on opposite sides of spinal canal66), a distance between a center of apertures136A, B and a center of an adjacent aperture138A, B may be calculated based on the average distance between an adult human's S1 and S2 sacral foramina.

Based on one measurement of the average distance between an adult human's sacral foramina, the distance between a center of apertures136A, B and a center of an adjacent aperture138A, B is about 1.9 cm to about 3.175 cm, such as about 2.6 cm (about 0.75 inches to about 1.25 inches, such as about 1.024 inches), the distance between a center of apertures138A, B and a center of an adjacent aperture140A, B is about 1.65 cm to about 2.92 cm, such as about 2.30 cm (about 0.65 inches to about 1.15 inches, such as about 0.906 inches), the distance between a center of apertures140A, B and a center of an adjacent aperture142A, B is about 1.27 cm to about 2.86 cm, such as about 1.8 cm (about 0.50 inches to about 0.90 inches, such as about 0.709 inches), and a distance between a center of apertures142A, B and a center of an adjacent aperture144A, B is about 1.0 cm to about 2.0 cm, such as about 1.5 cm (about 0.40 inches to about 0.80 inches, such as about 0.591 inches). In the embodiment shown inFIG. 6C, the center of each aperture136A, B is located length L6from an edge of body122. In one embodiment, length L6is about 1.27 cm to about 2.54 cm, such as about 1.90 cm (about 0.50 inches to about 1.0 inches, such as about 0.75 inches).

FIGS. 8A and 8Bare perspective and side views, respectively, of locating guide150in accordance with another embodiment. Also shown inFIGS. 8A and 8Bis a schematic view of sacrum22, including the S2 foramen68, S3 foramen70, S4 foramen72, and S5 foramen74, and introducer needle152. Sacrum22includes a set of the S2-S5 foramen68,70,72, and74on opposite sides of a midline of patient18. The illustration of sacrum22inFIGS. 8A and 8Bis greatly simplified for purposes of illustration, and is not intended to be limiting in any way.

Locating guide150is substantially similar to locating guide120ofFIGS. 6A and 6B, and includes body122, reference markers124A-B,126A-B, and midline reference markers128,130. However, locating guide150includes three sets of foramen apertures136A-138B,138A-138B, and140A-140B, rather than five sets, as locating guide120included. In addition, rather than including a plurality of lateral markers132positioned around each foramen aperture, locating guide150includes lateral markers154and156. Lateral marker154is positioned between sacral foramen aperture138A and138B, and lateral marker156is positioned between sacral foramen apertures140A and140B. In alternate embodiments, however, lateral markers154and156are not between foramen apertures138A-B and140A-B, respectively, and may be placed at any suitable location on body122. In the embodiment shown inFIGS. 8A and 8B, lateral markers154and156are substantially aligned with midline reference markers128. However, in other embodiments, lateral markers154and156may be placed at any suitable location on body122.

As previously described, sacrum22is curved relative to a skin surface60(FIG. 3) of patient18, and each foramen68,70,72, and74may also include walls68A,70A,72A, and74A, respectively, that are angled relative to skin surface60. In order to guide introducer needle152through a foramen68,70,72or74to reach a respective sacral nerve, the clinician may need to orient introducer needle152through the respective foramen at an angle relative to the skin surface60. The angle may be selected to be substantially compatible with the curved walls of the particular foramen. For example, as shown inFIG. 8B, in order to introduce needle152into the S3 foramen72, it may be desirable to orient needle152at around angle J or an angle similar to angle J relative to major surface134of body122of locating guide150. In some embodiments, angle J is about 45 degrees to about 75 degrees, such as about 60 degrees, although the angle may differ between patients.

As shown inFIG. 8B, lateral marker154extends from major surface134of body122at an angle J. When introducing needle152into S3 foramen72, the clinician may orient needle152to substantially match the orientation of lateral marker154in order to access S3 foramen72through apertures138A and138B (or another aperture). In this way, lateral marker154provides a visual guide that indicates an introduction angle for introducer needle152. Positioning lateral marker154near foramen apertures138A and138B may also suggest to the clinician that needle152may be introduced into apertures138A and138B at the angle denoted by lateral marker154in order to access the S3 foramen72(or another sacral foramen). Foramen apertures138A-138B may be positioned on locating guide150such that when reference markers124A-124B are substantially aligned with spinal canal66(FIG. 3) and reference markers126A-126B are substantially aligned with a crest of Ilium bone62, a clinician may introducer needle152into apertures138A and138B at angle J and reach the S3 foramen70. However, because anatomy may differ between patients, apertures138A-138B may not be aligned with the S3 foramen70for all patients.

Lateral marker156may also provide a visual guide that indicates an introduction angle at which introducer needle152may be introduced into foramen apertures140A and140B of locating guide150. Lateral marker156extends from major surface134of body122at an angle K. Needle152may be oriented at an angle of about 80 to about 100 degrees, such as about 90 degrees, relative to major surface134of locating guide150in order to guide needle152through S4 foramen72. Accordingly, angle K may be about 80 degrees to about 100 degrees. Foramen apertures140A-140B may be positioned on locating guide150such that when reference markers124A-124B are substantially aligned with spinal canal66(FIG. 3) and reference markers126A-126B are substantially aligned with a crest of Ilium bone62, a clinician may introducer needle152into apertures140A and140B at angle J and reach the S4 foramen72. However, because anatomy may differ between patients, apertures140A-140B may not necessarily be aligned the S4 foramen72for all patients.

In other embodiments, locating guide150may include other lateral reference markers that are oriented at an angle relative to major surface134of body122, where the angle may be selected to provide guidance to a clinician when the clinician introduces needle152or another device into a particular sacral foramen. In some embodiments, the lateral reference markers may extend from major surface134of body at an angle of about 45 degrees to about 100 degrees.

A clinician may employ the technique shown inFIG. 5to locate a target nerve site16with the aid of locating guide120.FIG. 9is a flow diagram of another technique that a clinician may use to locate a target nerve site with any of locating guides120,146, and150. Locating guide120is primarily referred to throughout the description ofFIG. 9for ease of description. The clinician may position locating guide120on a skin surface of patient18near an approximate location for accessing target nerve site16(FIG. 1) (160). In one embodiment, the clinician places locating guide120on patient18relative to one or more boney landmarks. The clinician may locate the relevant boney landmarks with the aid of a medical image taken before or after locating guide120is placed on patient18and/or with the aid of a physical examination. For example, in the embodiment shown inFIG. 9, the clinician physically examines patient18to locate Ilium bone62and spinal canal66, and initially aligns reference markers126A, B with the crest of Ilium bone62and reference markers124A, B with spinal canal66(160). The clinician may also align centerline markers128,130with the crest of Ilium bone62and reference markers124A, B with spinal canal66when positioning locating guide120on patient18.

The clinician may then obtain a radiographic image of at least a portion of locating guide120and underlying tissue of patient18near locating guide120(162). Upon obtaining the image, the clinician may determine whether one of the foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B is aligned with target nerve site16(164). As previously described, foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B provide an opening in locating guide120through which the clinician may access target nerve site16. If one of the foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B is not aligned with target nerve site16, the clinician may adjust a position of locating guide120(160) and generate another medical image. The image and reference markers of locating guide120may help indicate the magnitude and direction in which locating guide120should be adjusted.

If the medical image indicates that one of the foramen apertures136A-B,138A-B,140A-B,142A-B, and144A-B is aligned with target nerve site16, the clinician may associate at least one of the reference markers of locating guide120with target nerve site16(166) in order to relate the image to an actual location on patient18. For example, the clinician may determine which reference markers (e.g., centerline markers128,130, lateral markers132, and/or at least one of foramen aperture136A, B,138A, B,140A, B,142A, B, and144A) are closest to target nerve site16. Based on the associate reference marker(s), the clinician may register the image of target nerve site16with an actual location on or within patient18(167). Using the associated reference markers to associate the imaged target nerve site16with an actual location within patient18, the clinician may introduce a medical element into patient18, e.g., with the aid of an introducer needle (168).

FIG. 10is a top view of locating guide180in accordance with another embodiment. Locating guide180includes body182, a plurality of x-axis reference lines184, a plurality of y-axis reference lines186, and lateral markers188that extend from body182in a general z-axis direction. Orthogonal x-y axes are shown inFIG. 10, and the z-axis extends substantially perpendicular to the plane of the image shown inFIG. 10. In other embodiments, lateral markers188may not extend substantially along the z-axis, but extend from body182in a direction other than the x-axis and y-axis directions.

Body182is formed of a substantially radio-transparent material and reference lines184,186are formed of a substantially radiopaque material, such that body182is substantially invisible in a radiographic image and reference lines184,186are substantially visible in the image. Body182may be coupled to patient18in any of the ways described above with respect to locating guides30and120ofFIGS. 2A-2BandFIGS. 6A-6B, respectively. Body182may be any suitable size, which may depend upon the target nerve site16that locating guide180is used to locate. In embodiments in which target nerve site16is a sacral foramen, body182may be a quadrilateral having a length and width of about 13 cm to about 15 cm, although other dimensions and shapes are also possible. While the description of locating guide180refers primarily to a technique for locating sacral foramen70, in other embodiments, locating guide180may be useful for locating other target nerve sites within patient18.

In the embodiment shown inFIG. 10, reference lines184are spaced about 0.25 cm to about 1.5 cm from an adjacent reference line184. Similarly, reference lines186are spaced about 0.25 cm to about 1.5 cm from an adjacent reference line186. Reference lines184,186may be separated by other distances in other embodiments. In some embodiments, reference lines184,186may be numbered, e.g., with distance indicators. Lateral markers188may be similar to lateral markers40,48of locating guide30(FIGS. 2A-B), and are visible in a radiographic image.

Reference lines184,186define a grid that a clinician may refer to when registering a medical image to an actual location on or within patient18. For example, if a medical image indicates that sacral foramen70is located by an intersection190between reference lines184,186, the clinician may introduce a medical element through body182at the relevant intersection190on the actual locating guide180. If, however, the relevant intersection190between reference lines184,186is not visible in the medical image, the clinician may associate at least one of lateral markers188with the imaged target nerve site. In general, the clinician may use a combination of any of reference lines184,186and lateral markers188to relate a medical image to an actual location on patient18. Reference lines184,186and lateral markers188that may directly overlay a target nerve site16may provide addition guidance to the clinician when locating target nerve site16as compared to locating guide30which provides reference markers that surround, rather than overlay, target nerve site16.

Various embodiments of the invention have been described. Other embodiments of locating guides are also contemplated. For example, while a T-shaped locating guide, a sacral-shaped locating guide, and a substantially quadrilateral locating guide are primarily described above, in other embodiments, a locating guide may include other shapes. In some embodiments, the locating guide may have an L-shape, a substantially circular shape, a substantially elliptical shape, and so forth. In addition, reference markers have any suitable shape, such as substantially rectangular, substantially square, substantially circular, substantially triangular, and so forth. Furthermore, while locating guides (e.g., guide30ofFIGS. 2A-2Band guide120ofFIGS. 6A-6B) are described primarily with reference to locating a sacral foramen, in other embodiments, a locating guide may be used to locate other target nerve sites. As one example, a locating guide, such as locating guide30, may be used to locate a target nerve site within a pelvis, such as the obturator foramen, which may provide access to a pelvic nerve (e.g., the pudendal nerve).

These and other embodiments are within the scope of the following claims.