Patent Publication Number: US-2023138547-A1

Title: Oral patient tracking device and method of using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 16/401,456 filed on May 2, 2019. The entire disclosure of the above application is incorporated 
    
    
     FIELD 
     The present disclosure relates to registration between a patient, and image data and particularly to a system to track movement of a patient during a procedure. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     Image guided medical and surgical procedures utilize patient images obtained prior to or during a medical procedure to guide a physician performing the procedure. Recent advances in imaging technology, especially in imaging technologies that produce highly-detailed, two, three, and four dimensional images, such as computed tomography (CT), magnetic resonance imaging (MRI), fluoroscopic imaging (such as with a C-arm device), positron emission tomography (PET), and ultrasound imaging (US) has increased the interest in image guided medical procedures. 
     Typical image guided navigation systems generally require dynamic reference frames to track the position of the patient should patient movement occur during the assisted procedure. The dynamic reference frame is generally affixed to the patient in an immovable fashion. The dynamic reference frame may also be used as a fiducial marker and may, therefore, be attached to the patient during the acquisition of pre-operative images. This enables the image space to be aligned with patient space during the navigated procedure. For example, with relation to a cranial procedure, the dynamic reference frame can be attached to the skull by a bone screw. For other procedures, the dynamic reference frame may be fixed to other boney portions also with bone screws. Methods for affixing the dynamic reference frames to a patient can be invasive or inaccurate due to movement. Bone affixed dynamic reference frames require an incision that can often be more than two centimeters in length. Skin mobility can lead to undesirable movement when using a non-invasive dynamic reference frame attachment such as tape. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     In one aspect of the disclosure, a patient tracking device for insertion into a body cavity includes a flexible conformable sensor housing having a sensor cavity therein. The housing conforms to the body cavity when inserted therein. A sensor is disposed within the housing. 
     In another aspect of the disclosure, a method includes determining a body cavity for receiving a flexible resilient sensor housing; determining a sensor housing corresponding the body cavity; inserting the sensor housing having a sensor within a sensor cavity of the housing into the body cavity; conforming the sensor housing to a shape of the body cavity; collecting position data of the body cavity from the sensor; maintaining registration of an image space to a patient space in response to the position data; and displaying a navigated location. 
     In yet another aspect of the disclosure, a method of forming a patient tracking device includes forming a sensor cavity within a flexible conformable housing sized to fit within a body cavity; inserting an electromagnetic sensor within the sensor cavity; and retaining the electromagnetic sensor within the sensor cavity. 
     In still another aspect of the disclosure, a patient tracking device for insertion into an oral cavity includes a sensor housing comprising a first surface shaped to correspond to a pallet within the oral cavity. At least a portion of the first surface affixes the sensor housing to the oral cavity. An electromagnetic sensor is coupled to the sensor housing. 
     In another aspect of the disclosure, a method includes determining a shape of a pallet of an oral cavity of a patient, forming a sensor housing comprising a first surface based on the shape of the pallet, coupling an electromagnetic sensor to the sensor housing and affixing the sensor housing within the oral cavity. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG.  1    is an environmental view in an operating theatre that uses a DRF (endonasal or palatal patient tracker) of the present disclosure; 
         FIG.  2    is a front view of the endonasal dynamic tracking device of  FIG.  1   ; 
         FIG.  3 A  is a first example of a patient tracking device within a body cavity; 
         FIG.  3 B  is a second example of a patient tracking device within a body cavity; 
         FIG.  3 C  is a third example of a patient tracking device within a body cavity; 
         FIG.  4 A  is a partially exploded view of the patient tracking device; 
         FIG.  4 B  is an assembled view of the patient tracking device; 
         FIG.  5    is a perspective view of a sensor for the patient tracking device; 
         FIG.  6 A  is one example of a housing  310 ; 
         FIG.  6 B  is an end view of the housing of  FIG.  6 A ; 
         FIG.  6 C  is a side view of the housing having a cavity therein; 
         FIG.  6 D  is a cross-sectional view of a housing having a cavity that is only partially filled with a sensor. 
         FIG.  7 A  is a side view of an electromagnetic sensor with a cable  172 ; 
         FIG.  7 B  is a block diagrammatic view of an wireless EM sensor; 
         FIG.  8 A  is a cross-sectional view of an EM sensor within a housing with an adhesive retainer; 
         FIG.  8 B  is a cross-sectional view of an EM sensor within a housing with a clip or fastener; 
         FIG.  8 C  is a second cross-sectional view of an EM sensor within a housing with a clip or fastener; 
         FIG.  9 A  is a side view of the housing  310  having an RF EM sensor and the retainer being adhesive; 
         FIG.  9 B  is a side view of the housing  310  having an RF EM sensor and the retainer being a clip or fastener; 
         FIG.  9 C  is a second example of a side view of the housing  310  having an RF EM sensor and the retainer being a clip or fastener; 
         FIG.  10    is a flowchart of a method for forming the patient tracking device; and 
         FIG.  11    is a method for using the patient tracking device. 
         FIG.  12    is a cutaway front view of the patient having an oral sensor housing 
         FIG.  13    is a cutaway side view of the patient having an oral sensor housing. 
         FIG.  14    is a top perspective view of the sensor housing. 
         FIG.  15    is a bottom view of the sensor housing. 
         FIG.  16    is a view of the sensor housing mounted within a palate of a patient. 
         FIG.  17    is a side view of the sensor housing with the sensor mounted thereto. 
         FIG.  18    is a view of a first alternate method for mounting a sensor housing. 
         FIG.  19    is a cutaway view of a sensor housing having a sensor mounted therein. 
         FIG.  20    is a side view of a sensor having a sensor over-molded therein. 
         FIG.  21    is a flowchart of a method for forming the sensor. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     The subject disclosure is directed to an exemplary embodiment of a surgical procedure on a subject, such as a human patient. It is understood, however, that the system and methods described herein are merely exemplary and not intended to limit the scope of the claims included herein. In various embodiments, it is understood, that the systems and methods may be incorporated into and/or used on non-animate objects. The systems may be used to, for example, to register coordinate systems between two systems for use on manufacturing systems, maintenance systems, and the like. For example, automotive assembly may use one or more robotic systems including individual coordinate systems that may be registered together for coordinated or consorted actions. Accordingly, the exemplary illustration of a surgical procedure herein is not intended to limit the scope of the appended claims. Various embodiments of a patient tracking device are disclosed in U.S. Pat. No. 11,446,094 issued Sep. 20, 2022, incorporated herein by reference. 
       FIG.  1    is a diagrammatic view illustrating an overview of a procedure room or arena. In various embodiments, the procedure room may include a surgical suite having a navigation system  26  that can be used relative to a patient or subject  30 . The navigation system  26  can be used to track the location of one or more tracking devices, tracking devices may include an imaging system tracking device  62 , and/or a tool tracking device  66 . A tool  68  may be any appropriate tool such as a drill, forceps, catheter, speculum or other tool operated by a user  72 . The tool  68  may also include an implant, such as a stent, a spinal implant or orthopedic implant. It should further be noted that the navigation system  26  may be used to navigate any type of instrument, implant, stent or delivery system, including: guide wires, arthroscopic systems, orthopedic implants, spinal implants, deep brain stimulation (DBS) probes, etc. Moreover, the instruments may be used to navigate or map any region of the body. The navigation system  26  and the various instruments may be used in any appropriate procedure, such as one that is generally minimally invasive or an open procedure including cranial procedures. 
     An imaging device  80  may be used to acquire pre-, intra-, or post-operative or real-time image data of a subject, such as the subject  30 . It will be understood, however, that any appropriate subject can be imaged and any appropriate procedure may be performed relative to the subject. In the example shown, the imaging device  80  comprises an O-Arm® imaging device sold by Medtronic Navigation, Inc. having a place of business in Louisville, Colo., USA. The imaging device  80  may have a generally annular gantry housing  82  in which an image capturing portion is moveably placed. The image capturing portion may include an x-ray source or emission portion and an x-ray receiving or image receiving portion located generally or as practically possible 180 degrees from each other and mounted on a rotor relative to a track or rail. The image capturing portion can be operable to rotate 360 degrees during image acquisition. The image capturing portion may rotate around a central point or axis, allowing image data of the subject  80  to be acquired from multiple directions or in multiple planes. The imaging device  80  can include those disclosed in U.S. Pat. Nos. 7,188,998; 7,108,421; 7,106,825; 7,001,045; and 6,940,941; all of which are incorporated herein by reference, or any appropriate portions thereof. In one example, the imaging device  80  can utilize flat plate technology having a 1,720 by 1,024 pixel viewing area. 
     The position of the imaging device  80 , and/or portions therein such as the image capturing portion, can be precisely known relative to any other portion of the imaging device  80 . The imaging device  80 , according to various embodiments, can know and recall precise coordinates relative to a fixed or selected coordinate system. This can allow the imaging system  80  to know its position relative to the patient  30  or other references. In addition, as discussed herein, the precise knowledge of the position of the image capturing portion can be used in conjunction with a tracking system to determine the position of the image capturing portion and the image data relative to the tracked subject, such as the patient  30 . 
     The imaging device  80  can also be tracked with a tracking device  62 . The image data defining an image space acquired of the patient  30  can, according to various embodiments, be inherently or automatically registered relative to an object space. The object or patient space can be the space defined by a patient  30  in the navigation system  26 . The automatic registration can be achieved by including the tracking device  62  on the imaging device  80  and/or the determinable precise location of the image capturing portion. According to various embodiments, as discussed herein, imageable portions, virtual fiducial points and other features can also be used to allow for registration, automatic or otherwise. It will be understood, however, that image data can be acquired of any subject which will define the patient or subject space. Patient space is an exemplary subject space. Registration allows for a translation between patient space and image space. 
     The patient  80  can also be tracked as the patient moves with an optical tracker  88 . Alternatively, or in addition thereto, the patient  30  may be fixed within navigation space defined by the navigation system  26  to allow for registration. As discussed further herein, registration of the image space to the patient space or subject space allows for navigation of the instrument  68  with the image data. When navigating the instrument  68 , a position of the instrument  68  can be illustrated relative to image data acquired of the patient  30  on a display device  84 . Various tracking systems, such as one including an optical localizer  88  or an electromagnetic (EM) localizer  94  can be used to track the instrument  68 . 
     More than one tracking system can be used to track the instrument  68  in the navigation system  26 . According to various embodiments, these can include an electromagnetic tracking (EM) system having the EM localizer  94  and/or an optical tracking system having the optical localizer  88 . Either or both of the tracking systems can be used to track selected tracking devices, as discussed herein. It will be understood, unless discussed otherwise, that a tracking device can be a portion trackable with a selected tracking system. A tracking device need not refer to the entire member or structure to which the tracking device is affixed or associated. 
     It is further appreciated that the imaging device  80  may be an imaging device other than the O-Arm® imaging device and may include in addition or alternatively a fluoroscopic C-arm. Other exemplary imaging devices may include fluoroscopes such as bi-plane fluoroscopic systems, ceiling mounted fluoroscopic systems, cath-lab fluoroscopic systems, fixed C-arm fluoroscopic systems, isocentric C-arm fluoroscopic systems, 3D fluoroscopic systems, etc. Other appropriate imaging devices can also include MRI, CT, ultrasound, etc. 
     In various embodiments, an imaging device controller  96  may control the imaging device  80  and can receive the image data generated at the image capturing portion and store the images for later use. The controller  96  can also control the rotation of the image capturing portion of the imaging device  80 . It will be understood that the controller  96  need not be integral with the gantry housing  82 , but may be separate therefrom. For example, the controller may be a portions of the navigation system  26  that may include a processing and/or control system including a processing unit or processing system  102 . The controller  96 , however, may be integral with the gantry housing  82  and may include a second and separate processor, such as that in a portable computer. 
     The patient  30  can be fixed onto an operating table  104 . According to one example, the table  104  can be an Axis Jackson® operating table sold by OSI, a subsidiary of Mizuho Ikakogyo Co., Ltd., having a place of business in Tokyo, Japan or Mizuho Orthopedic Systems, Inc. having a place of business in California, USA. Patient positioning devices can be used with the table, and include a Mayfield® clamp or those set forth in commonly assigned U.S. patent application Ser. No. 10/405,068 entitled “An Integrated Electromagnetic Navigation And Patient Positioning Device”, filed Apr. 1, 2003 which is hereby incorporated by reference. 
     The position of the patient  30  relative to the imaging device  80  can be determined by the navigation system  26 . The tracking device  62  can be used to track and locate at least a portion of the imaging device  80 , for example the gantry housing  82 . The patient  30  can be tracked with a non-invasive dynamic reference frame  170 , as discussed further herein. That is, a patient tracking device  170  may be used to receive or generate electromagnetic signals that are communicated through a cable  172  to an interface portion  110 . As is discussed below wireless communication to the interface portion  110  may also be used. The patient tracking device  170  may also be referred to as a dynamic reference frame. The patient tracking device  170  is located within a substantially rigid body cavity. In the following example, the body cavity is a nasal cavity or palate as will be described in more detail below. A piece of tape  174  may be used to secure the cable  172  to the patient  30 . 
     Accordingly, the position of the patient  30  relative to the imaging device  80  can be determined. Further, the location of the imaging portion can be determined relative to the housing  82  due to its precise position on the rail within the housing  82 , substantially inflexible rotor, etc. The imaging device  80  can include an accuracy of within 10 microns, for example, if the imaging device  80  is an O-Arm® imaging device sold by Medtronic Navigation, Inc. having a place of business in Louisville, Colo. Precise positioning of the imaging portion is further described in U.S. Pat. Nos. 7,188,998; 7,108,421; 7,106,825; 7,001,045; and 6,940,941; all of which are incorporated herein by reference, 
     According to various embodiments, the imaging device  80  can generate and/or emit x-rays from the x-ray source that propagate through the patient  30  and are received by the x-ray imaging receiving portion. The image capturing portion generates image data representing the intensities of the received x-rays. Typically, the image capturing portion can include an image intensifier that first converts the x-rays to visible light and a camera (e.g. a charge couple device) that converts the visible light into digital image data. The image capturing portion may also be a digital device that converts x-rays directly to digital image data for forming images, thus potentially avoiding distortion introduced by first converting to visible light. 
     Two dimensional and/or three dimensional fluoroscopic image data that may be taken by the imaging device  80  can be captured and stored in the imaging device controller  96 . Multiple image data taken by the imaging device  80  may also be captured and assembled to provide a larger view or image of a whole region of a patient  30 , as opposed to being directed to only a portion of a region of the patient  30 . For example, multiple image data of the patient&#39;s  30  spine may be appended together to provide a full view or complete set of image data of the spine. 
     The image data can then be forwarded from the image device controller  96  to the navigation computer and/or processor system  102  that can be a part of a controller or work station  98  having the display  84  and a user interface  106 . It will also be understood that the image data is not necessarily first retained in the controller  96 , but may also be directly transmitted to the work station  98 . The work station  98  can provide facilities for displaying the image data as an image  108  on the display  84 , saving, digitally manipulating, or printing a hard copy image of the received image data. The user interface  106 , which may be a keyboard, mouse, touch pen, touch screen or other suitable device, allows the user  72  to provide inputs to control the imaging device  80 , via the image device controller  96 , or adjust the display settings of the display  84 . The work station  98  may also direct the image device controller  96  to adjust the image capturing portion of the imaging device  80  to obtain various two-dimensional images along different planes in order to generate representative two-dimensional and three-dimensional image data. 
     With continuing reference to  FIG.  1   , the navigation system  26  can further include the tracking system including either or both of the electromagnetic (EM) localizer  94  and/or the optical localizer  88 . The tracking systems may include a controller and interface portion  110 . The interface portion  110  can be connected to the processor system  102 , which can include a processor included within a computer. The EM tracking system may include the STEALTHSTATION® AXIEM™ Navigation System, sold by Medtronic Navigation, Inc. having a place of business in Louisville, Colo.; or can be the EM tracking system described in U.S. Pat. No. 7,751,865 issued Jul. 6, 2010, and entitled “METHOD AND APPARATUS FOR SURGICAL NAVIGATION”; U.S. Pat. No. 5,913,820, entitled “Position Location System,” issued Jun. 22, 1999; and U.S. Pat. No. 5,592,939, entitled “Method and System for Navigating a Catheter Probe,” issued Jan. 14, 1997; all of which are herein incorporated by reference. It will be understood that the navigation system  26  may also be or include any appropriate tracking system, including a STEALTHSTATION® TREON® or S7™ tracking systems having an optical localizer, that may be used as the optical localizer  88 , and sold by Medtronic Navigation, Inc. of Louisville, Colo. Other tracking systems include an acoustic, radiation, radar, etc. The tracking systems can be used according to generally known or described techniques in the above incorporated references. Details will not be included herein except when to clarify selected operation of the subject disclosure. 
     Wired or physical connections can interconnect the tracking systems, imaging device  80 , etc. Alternatively, various portions, such as the instrument  68  may employ a wireless communications channel, such as that disclosed in U.S. Pat. No. 6,474,341, entitled “Surgical Communication Power System,” issued Nov. 5, 2002, herein incorporated by reference, as opposed to being coupled directly to the processor system  102 . Also, the tracking devices  62 ,  66 ,  170  can generate a field and/or signal that is sensed by the localizer(s)  88 ,  94 . 
     Various portions of the navigation system  26 , such as the instrument  68 , and others as will be described in detail below, can be equipped with at least one, and generally multiple, of the tracking devices  66 . The instrument can also include more than one type or modality of tracking device  66 , such as an EM tracking device and/or an optical tracking device. The instrument  68  can include a graspable or manipulable portion at a proximal end and the tracking devices may be fixed near the manipulable portion of the instrument  68 . 
     Additional representative or alternative localization and tracking system is set forth in U.S. Pat. No. 5,983,126, entitled “Catheter Location System and Method,” issued Nov. 9, 1999, which is hereby incorporated by reference. The navigation system  26  may be a hybrid system that includes components from various tracking systems. 
     According to various embodiments, the navigation system  26  can be used to track the instrument  68  relative to the patient  30 . The instrument  68  can be tracked with the tracking system, as discussed above. Image data of the patient  30 , or an appropriate subject, can be used to assist the user  72  in guiding the instrument  68 . The image data, however, is registered to the patient  30 . The image data defines an image space that is registered to the patient space defined by the patient  30 . The registration can be performed as discussed herein, automatically, manually, or combinations thereof. 
     Generally, registration allows a translation map to be generated of the physical location of the instrument  68  relative to the image space of the image data. The translation map allows the tracked position of the instrument  68  to be displayed on the display device  84  relative to the image data  108 . A graphical representation  68   i , also referred to as an icon, can be used to illustrate the location of the instrument  68  relative to the image data  108 . 
     Referring now to  FIGS.  2  and  3 A , the endonasal tracking device or patient tracking device  170  and cable  172  are illustrated disposed within a sensor housing  310  within a body cavity  312  of a patient  30 . The patient tracking device  170  may be used as a dynamic reference frame (DRF) for various procedures such as but no limited to cranial procedures. The housing  310  may be formed of a conformable material such as a nasal tamponade (e.g. Merocel® nasal dressing by Medtronic). The cavity  312  may be a cranial cavity such as a nasal cavity having an irregular shape. The housing  310  may be conformable to the shape of the cavity  312 . The housing  310  may transform its shape to be secured within the cavity  312  automatically or automatically in the presence of moisture such as water or bodily fluids. As will be described in more detail below, the size of the housing  310  may vary depending upon the particular body cavity  312  into which the patient tracking device  170  is located. 
     In  FIGS.  2  and  3 A- 3 C , the body cavity  312 , in this example, is a nasal cavity such as the inferior nasal meatus. The body cavity  312  may be rigid. Other nasal cavities such as the middle and superior meatus may also receive a sensor housing  310  with appropriate changes to dimensions. Of course, the disclosure should not be limited to cranial, nasal cavities, oral cavities or surgical procedures. The sensor housing  310  is substantially, non-invasively fixed relative to the patient  30  and particularly fixed relative to the skull or cranium. A nasal speculum  180  may be used to insert the housing  310  and sensor  320  within the cavity  312 . The housing  310  may be compressed during insertion. The housing  310  may expand into and conform to the cavity  312 . The housing  310  may also be removed by way of a tool such as the nasal speculum. The housing  310  may have a first form at rest, a second form as it is being inserted and a third form within the body cavity. 
     The cable  172  is coupled to an electromagnetic sensor  320  that either generates an electromagnetic field or receives electromagnetic field from the EM localizer  94  and generates a current in the presence of the electromagnetic field. The cable  172  communicates signals to or from the sensor  320  depending on the mode of operation mentioned previously. The signals from the sensor  320  allow precise position of the sensor  320  to be determined relative to the components within the operating environment. One example of a suitable sensor  320  is an AxiEM sensor. 
     The housing  310  may optionally have a removing member or handle  326  coupled thereto. The handle  326  may be one or more strings or other flexible device that is used for removing the housing  310  from the subject  30 . The handles  326  are particularly useful if the patient tracking device  170  is wireless. 
     Referring now to  FIG.  3 B , a different shape of housing  310  is set forth. In this example the housing  310  is shorter in length and has a dimension to fit within the entry to the nasal passage. 
     Referring now to  FIG.  3 C , a larger housing  310  is illustrated having the sensor  320  disposed therein. In this example, the housing  310  is thicker and longer than those set forth in  FIGS.  3 A and  3 B . As mentioned above, the shape of the housing  310  may vary depending upon the particular body cavity  312  as well as the size of the body cavity  312  and the size of the patient. 
     The housing  310  may automatically take the shape of the body cavity to substantially fix the sensor  320  in relation to the subject  30  in a non-invasive manner. The housing  310  may conform to an opening such as a nasal passage as it is being inserted. The sensor is fixed in shape and small enough to enter the desired opening. The housing  310  conforms to the cavity shape without the cavity having to conform to the housing. In this manner the housing  310  and sensor  320  are held in place during a procedure. The housing is non-invasively fixed to the subject. 
     Referring now to  FIGS.  4 A and  4 B , a detailed view of one example of a patient tracking device  170  is set forth. In this example a connector  410  is used for connecting the patient tracking device  170  to the interface  110  set forth above. In  FIG.  4 A , the housing  310  is illustrated disassembled from the sensor  320 . In  FIG.  4 B , the sensor  320  is illustrated within the housing  310 . Details of the assembly of the housing  310  and the sensor  320  are set forth in greater detail below. 
     Referring now to  FIG.  5   , an exemplary electromagnetic EM coil configuration for the sensor is illustrated. An electromagnetic sensor bobbin  510  or multiple coil members may be positioned in a cavity of the patient tracking device  170 . The sensor bobbin  510  includes a body  512  that is generally formed from material that is not conductive to allow the coils to operate and sense a position in a field. In addition, the body  512  may be manipulated by a manipulable portion or handle  514  extending from the body  512 . In addition, the handle  514  may allow cable  172  to be interconnected to the body portion  512  into multiple coils. In this example, three coils  516 ,  518  and  520  are illustrated. However, fewer coils or more coils may be used in a sensor. 
     The first coil  516 , the second coil  518  and third coil  520  are generally positioned at angles relative to one another. The angles may be any appropriate angle such as a generally orthogonal angle or other appropriate angle. The three coils  516 ,  518 ,  520  being positioned at angles relative to one another, allow for six degrees of freedom sensing including translation, angle, pitch, yaw, and rotation. Therefore, the position or movement of the patient tracking device  170  can be determined by sensing the electromagnetic field of the electromagnetic localizer  138 . 
     Generally, the body  512  of the bobbin  510  and the exterior or the bodies of the patient tracking device  170  are formed of an appropriate material. For example, the material may be a non-metallic and non-conducting material such as an appropriate ceramic, plastic, and the like. The material may be selected from a material that will not interfere with either transmitting or receiving information regarding the magnetic field and not interfere with imaging of the subject  30 . 
     Referring now to  FIG.  6 A , the housing  310  is illustrated in further detail. The housing  310  may be formed of a flexible, resilient and conformable material such as nasal dressing (e.g. a nasal tamponade). The housing  410  is autoconformable and resilient in that the shape of the housing  410  automatically conforms to the shape of the cavity when inserted therein. In one example, the material is compressible and expandable when in contact with a fluid such as water. In either case, the housing material automatically fills and conforms to the cavity shape. The housing  310  is illustrated in its expanded form. The material of the housing  310  may also be sterile and disposable. In one example, the housing  310  is formed of hydroxylated polyvinyl acetate which is a compressed foam polymer. The material is conformable and compliant so that when the housing  310  is placed within a body cavity, the irregularities of the body cavity  312  are compensated for by the conforming housing  310 . That is, the housing  310  may be retained in a fixed position within the body cavity  312  due to the conformable and compliant nature. The housing  310  is also resilient in that it may be compressed before or during placement within the body cavity  312  and expands after positioning within the body cavity  312  from a first configuration to a second configuration. As mentioned above, the material forming the housing  310  may be expandable when in contact with moisture or water. The housing  310  may include a longitudinally extending slot  610 . In the present example the slot  610  is about half way between the thickness T and extends a length L less than the length of the housing  310 . 
     Referring now to  FIG.  6 B , the slot  610  is illustrated in a lateral end side  612 . The slot  610  extends a width W less than the width of the housing  310 . The width W of the slot  610  is sized to receive the sensor  320  and cable  172  therein. 
     Referring now to  FIG.  6 C , the interior of the housing  310  may be pre-formed with a cavity  614  for receiving the sensor  320  therein. The cavity  614  may also be formed by placing the sensor  320  into the slot  610  within the housing  310 . That is, because the material of the housing  310  is compressible, when the sensor  320  is inserted within the slot  610 , the cavity  614  may be formed. The material directly adjacent to the cavity  614  may be more highly compressed than the material further from the cavity  614 . 
     In a set of patient tracking devices, the length, width and thickness of multiple devices may vary to allow a number of options depending on the patient and cavity characteristics. 
     A retainer may optionally be used to secure the sensor within the cavity  614 . In this example, the retainer may be an adhesive  616  that is disposed on at least some of the surfaces of the cavity  614 . In this manner, when the sensor  320  is disposed within the cavity  614 , the sensor  320  remains engaged with the housing  310 . In one example, a drop or two of an adhesive material may be communicated through the slot  610  in to the cavity  614  prior to the insertion of the sensor  320  within the cavity  614 . Ultimately, the adhesive  616  is forced toward the surfaces of the cavity  614 . The types of retainers may be flexible and conformable to allow the patient tracker to conform to the cavity. 
     Referring now to  FIG.  6 D , the sensor  320  may not completely fill the cavity  614 . A biocompatible material  620  may be injected into the unfilled portion of the cavity  614  to securely lodge the electromagnetic sensor  320  and the cable  172  therein. A suitable biocompatible material is polylactic acid. 
     Referring now to  FIG.  7 A , the electromagnetic sensor  320  is illustrated with a cable  172  thereon. Adhesive  710  may be applied to the outer surface of the sensor  320  so that upon insertion within the cavity  614  of  FIG.  6 B  the sensor is maintained therein. 
     Referring now to  FIG.  7 B , a wireless electromagnetic sensor  320 ′ may also be provided. In this example, the wireless EM sensor may be formed with sensing coils  720  as described above. However, a wireless communication may be formed with the interface using a transmitter  722 . Thus, the wireless EM sensor  320 ′ may be fully enclosed within the housing  310  and thus no wire or cable  172 , such as that illustrated above, is required for communication with the interface  110 . 
     Referring now to  FIG.  8 A , one example of an assembled patient tracking device  170  is set forth. The handles  326  which are optional in all of the figures, are illustrated. The handles  326  may be drawstrings that are embedded within the material of the housing  310 . The handles  326  may be adhesively bonded within the housing  310  so that they may be used to remove the patient tracking device  170  when the patient tracking device is no longer needed. In this example, the sensor  320  is held within the housing  310  by adhesive  810  disposed at the interface between the sensor housing  310  and cable  172 . 
     Referring now to  FIG.  8 B , the same reference numerals are used as those set forth in  FIG.  8 A . However, in this example a clip or fastener  820  is used for securing the cable  172  to the housing and thus the sensor  320  is secured within the cavity of the housing  310 . 
     Referring now to  FIG.  170   , another type of clip or fastener  822  is illustrated. In this example the clip or fastener  822  extends from an upper surface  824  to a lower surface  826 . The clip or fastener  822  has arms  828  that are directly adjacent to the respective surfaces  824  and  826  and provide a clamping force to secure the cable  172  therein. As mentioned above, the clips or fasteners may be flexible so that they allow the housing to be inserted into the body cavity. 
     In  FIGS.  8 A and  8 B , the clips or fasteners  820 ,  822  may be formed of a dielectric material so that they do not interfere with the electromagnetic fields sensed or generated by the electromagnetic sensor  320 . 
     The sensor  320  may also be secured by heat sealing or by overmolding the sensor  320  within the material of the housing  310  during the forming of the housing  310 . The handles  326  may also form a drawstring that are used to secure the cable  172  to the housing or close the slot so that the sensor  320  is retained within the housing  310 . 
     The handles  326  may not be required should the housing  310  be removed with a separate instrument. The housing  310  may also be removed by pulling on the cable  172 . In such a case, the retainer such as the adhesive  810 , or the clips or fasteners  820 ,  822  are stronger than the force required to pull the housing  310  from the body cavity of the subject  30 . 
     Referring now to  FIG.  9 A , the wireless EM sensor  320 ′ is illustrated in further detail. In this example, the handles  326  may be required to allow removal of the housing  310  from the body cavity of the subject. In FIGS.  9 A- 9 C the slot  610  illustrated above in  FIGS.  6 A- 6 B  are illustrated. In  FIGS.  8 A- 8 C  the slot has been filled by the cable  172 . However, in the case of the RF EM sensor  320 ′ no wire is required. Thus, when the sensor  320 ′ is inserted within the cavity  614 , the slot  610  may be sealed with adhesive  910 .  FIGS.  9 B and  9 C  may use clips or fasteners  820 ,  822  to enclose the sensor  320 ′ within the cavity of the housing  310 . 
     Referring now to  FIG.  10   , a method for forming the patient tracking device  170  is set forth. In block  1010 , a cavity is formed within the housing. As mentioned above, the housing may be conformable and the cavity may be pre-formed or automatically formed as the sensor  320  is inserted within the housing  310 . In block  1012 , the sensor  320  is positioned within the cavity  614 . The sensor may be positioned within the cavity  614  by insertion through the slot  610  illustrated above. In block  1014 , the sensor is secured within the housing using a retainer such as but not limited to adhesive, clips, fasteners, overmolding, or heat staking, et cetera. 
     Referring now to  FIG.  11   , the use of the patient tracking device for a procedure is set forth. In block  1110 , the subject is imaged. During the imaging of the subject, the body cavity within which the patient tracking device is to be secured during the procedure may be determined. In block  1112 , the cavity size is determined for the body cavity based upon the image determined in block  1110 . 
     An optional block  1114  may also be performed. In block  1114 , the patient tracking device may be coupled to an instrument such as a nasal speculum for insertion. However, an instrument such as a nasal speculum may not be required if the patient tracking device is in a compressed state before insertion. The use and type of the instrument in block  1114  depends upon the particular body cavity and the type of sensor and the material of the housing. 
     In block  1116 , the sensor housing may be compressed manually or using an instrument. This is an optional block since the sensor housing may also be compressed prior to insertion or during insertion by the rigid body cavity walls. In block  1118  the patient tracking device is inserted into the body cavity. In block  1120 , the patient tracking device housing is automatically expanded to conform, nest, deform or otherwise be fixed into to the body cavity. That is, the housing  310  is changed from a first configuration or shape into a second configuration or shape. In block  1122 , the cable is secured to the patient using an adhesive or tape. In block  1124 , the procedure is performed. During the procedure, block  1126  collects data from the patient position sensor (DRF) and adjusts the navigation location in block  1128 . The navigation location in block  1128  maintains the registration of the patient and the image in response to any movement of the patient. The correlation of the patient and the images is maintained. 
     In block  1130 , the patient tracking device may be removed from the patient and disposed of. The conforming housing may allow the patient tracking device to easily be removed by handle, cable or using an instrument. In this manner, the insertion and the removal of the patient tracking device is non-invasive to the subject. 
     The sensor assembly set forth herein may be used as a dynamic reference frame and is particularly suited for body cavity insertion such as a nasal cavity during cranial procedures. The housing  310  may be compressed manually or by tool (or not at all). The housing material conforms to the rigid body cavity without deformation of the body cavity. The sensor assembly can be used interopertively without incisions or fixing to the subject with screws or other invasive methods. 
     Referring now to  FIGS.  12  and  13   , a sensor housing  1310  having an electromagnetic sensor  320  coupled thereto may be mounted within an oral cavity  1320  of patient  30 . The housing  1310  may be formed in various ways including three-dimensional printing. One suitable biocompatible material for housing  1310  is polylactic acid. The housing  310  may be affixed to the palate  1330  or teeth  1332  or both. The sensor housing  1310  and electromagnetic sensor  320  may be used in the system of claim  1 . The use of the oral sensor housing  1310  is suitable for cranial procedures when access to or through the nasal passages are required. 
     Referring now to  FIGS.  14  and  15   , the sensor housing  1310  is illustrated in further detail. The sensor housing  1310  includes a first surface  1410  and a second surface  1412 . The first surface  1410  is based upon the surface of the palate  1330 . As will be further described below, a computed tomography (CT) image may be formed of the oral cavity of the patient  30 . The housing  1310  may be formed by three-dimensional printing or the like and may be based on the CT image in order for the first surface to rest with the palate. The housing may also be molded from a negative of the palate surface as is described below. The second surface  1412  may be of an arbitrary shape and include a planer portion as will be illustrated below. A third and fourth surface  1414  may be shaped to conform to the profile of the teeth so that ultimately an adhesive may be used on the surfaces  1414  to adhesively join the teeth and the housing  1310 . 
     The surface  1410  may be formed especially for an individual patient. Also, the surface  1410  may be formed by a general patient size to be relatively close to various types of patients. That is, a set of various size housings  1310  may be formed for a doctor to select from. The material of the housing may be flexible to allow a fit within the oral cavity. For example, a large, medium or small adult and a large, medium or small child may all be sized differently. The surface  1410  may be rounded generally to conform to the size. The surfaces  1414  may be used to adhesively join one of the selected housings from the set of housings to the patient  30 . 
     Referring now to  FIG.  16   , a plurality of teeth  1332  are illustrated. The teeth  1332  may be adhesively coupled to the sensor housing  1310  by an adhesive  1610 . In addition to an adhesive on the teeth  1332 , adhesive  1610  on the palate or merely saliva from the patient may be used to secure the sensor housing  1310  within the oral cavity  1320  of the patient  30 . 
     Referring now to  FIG.  17   , a side view of an assembled sensor housing  1310  is set forth. The sensor housing  1310  is set forth in great detail in  FIGS.  4 A,  4 B and  5   . In all instances the electromagnetic sensor  320  may be replaced with the wireless electromagnetic sensor  320 ′ illustrated in FIG.  7 B. The sensor  320  illustrated in  FIG.  17   , is secured to the surface  1412  with adhesive  1710 . 
     Referring now to  FIG.  18   , the sensor  310  may be coupled to a planer surface  1810  of the sensor housing  1310 ′. In this example, the shape of the housing  1310 ′ is a convex shape and the sensor  310  is coupled to the planer surface  1810 . For this example, adhesive may also be used to affix the housing  1310 ′ to the teeth  1332 . 
     Referring now to  FIG.  19   , the sensor  310  may also be formed within a cavity  1910  of the sensor housing  1310 ″. The cavity  1910  may be sized to receive the sensor  310  therein as well as allowing the wire  172  to extend therefrom. A biocompatible material  1912  may be used to retain the electromagnetic sensor  310  within the cavity  1910 . The biocompatible material  1912  may be an adhesive or may be formed from the same material as the housing  1310 ″. 
     Referring now  FIG.  20   , the electromagnetic sensor  310  may be over-molded within the material of the housing  1310 ′″. The first surface  1410  and the second surface  1412  may be formed in a similar manner. 
     Referring now to  FIG.  21   , a method for forming the palate housing is set forth. In step  2110  a shape of the patient&#39;s palate is determined. The shape of the patient&#39;s palate may be formed by a noninvasive method such as using a CT image. In step  2112 , the CT image may be used to form the sensor housing and in particular the first surface of the sensor housing to correspond to the shape of the palate. An exact mirror image of the palate surface of the patient may be obtained using the CT image. However, a completely exact match is not necessary. The sensor housing may be three-dimensionally printed based on the CT scan. Alternatively, a negative print of the palate may be obtained. The sensor housing may use the negative print as a surface of a mold. That is the housing is molded based on the negative print. The sensor housing may then be adhesively coupled to the sensor using adhesive heat staking or the like. As mentioned above, over-molding the sensor may also be over-molded within the material of the sensor. The sensor may be affixed within the oral cavity to perform a procedure as described above. 
     In all cases, the material of the sensor housing may have a slight flex so that, when inserted into the palate of a patient the housing flexes and provides force against the teeth, the palate or both for retaining or helping retain the housing within the oral cavity. For example, in  FIG.  18    arrows  1820  represent inward flexing for insertion and arrows  1830  illustrate the direction of flex after release into the patient. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention. 
     It should be understood that various aspects disclosed herein may be combined in different combinations than the combinations specifically presented in the description and accompanying drawings. It should also be understood that, depending on the example, certain acts or events of any of the processes or methods described herein may be performed in a different sequence, may be added, merged, or left out altogether (e.g., all described acts or events may not be necessary to carry out the techniques). In addition, while certain aspects of this disclosure are described as being performed by a single module or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or modules associated with, for example, a medical device. 
     In one or more examples, the described techniques may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include non-transitory computer-readable media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer). 
     Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor” as used herein may refer to any of the foregoing structure or any other physical structure suitable for implementation of the described techniques. Also, the techniques could be fully implemented in one or more circuits or logic elements.