Patent Publication Number: US-2021161603-A1

Title: Electromagnetic field generator alignment

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. application Ser. No. 16/017,906, filed Jun. 25, 3018, which claims the benefit of U.S. Provisional Application No. 62/526,348, filed Jun. 28, 2017, each of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to systems and methods for electromagnetic (EM) field generator alignment in robotically-enabled medical system, and more particularly to detecting the position of EM sensors for the EM field generator alignment. 
     BACKGROUND 
     Medical procedures such as endoscopy (e.g., bronchoscopy) may involve accessing and visualizing the inside of a patient&#39;s luminal network (e.g., airways) for diagnostic and/or therapeutic purposes. Surgical robotic systems may be used to control the insertion and/or manipulation of a surgical tool, such as, for example, an endoscope during an endoscopic procedure. The surgical robotic system may comprise at least one robotic arm including a manipulator assembly used to control the positioning of the surgical tool during the procedure. The surgical tool may be navigated through the patient&#39;s luminal network based on a detected electromagnetic (EM) field. 
     SUMMARY 
     The systems, methods and devices of this disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein. 
     In one aspect, a system is configured to detect electromagnetic (EM) distortion. The system may comprise a first EM sensor configured to generate a first set of one or more EM sensor signals in response to detection of the EM field, the first EM sensor configured for placement on a patient; a processor; and a memory storing computer-executable instructions to cause the processor to: calculate one or more baseline values of one or more metrics indicative of a position of the first EM sensor at a first time based on EM sensor signals from the first set of one or more EM sensor signals corresponding to the first time, calculate one or more updated values of the one or more metrics during a time period after the first time based on EM sensor signals from the first set of one or more EM sensor signals corresponding to the time period after the first time, determine that a difference between the one or more updated values and the one or more baseline values is greater than a threshold value, and determine that the EM field has been distorted in response to the difference being greater than the threshold value. 
     In another aspect, there is provided a non-transitory computer readable storage medium having stored thereon instructions that, when executed, cause at least one computing device to: calculate one or more baseline values of one or more metrics indicative of a position of a first EM sensor at a first time based on EM sensor signals from a first set of one or more EM sensor signals corresponding to the first time, the first EM sensor configured to generate the first set of one or more EM sensor signals in response to detection of an EM field; calculate one or more updated values of the one or more metrics during a time period after the first time based on EM sensor signals from the first set of one or more EM sensor signals corresponding to the time period after the first time; determine that a difference between the one or more updated values and the one or more baseline values is greater than a threshold value; and determine that the EM field has been distorted in response to the difference being greater than the threshold value. 
     In yet another aspect, there is provided a method of detecting EM distortion, the method comprising: calculating one or more baseline values of one or more metrics indicative of a position of a first EM sensor at a first time based on EM sensor signals from a first set of one or more EM sensor signals corresponding to the first time, the first EM sensor configured to generate the first set of one or more EM sensor signals in response to detection of an EM field; calculating one or more updated values of the one or more metrics during a time period after the first time based on EM sensor signals from the first set of one or more EM sensor signals corresponding to the time period after the first time; determining that a difference between the one or more updated values and the one or more baseline values is greater than a threshold value; and determining that the EM field has been distorted in response to the difference being greater than the threshold value. 
     In still yet another aspect, there is provided a system configured to detect EM distortion, comprising: an EM sensor at a distal end of an instrument, the EM sensor configured to generate one or more EM sensor signals in response to detection of an EM field; a processor; and a memory storing computer-executable instructions to cause the processor to: calculate one or more baseline values of one or more metrics indicative of a velocity of the distal end of the instrument at a first time based on EM sensor signals from the one or more EM sensor signals corresponding to the first time, calculate one or more updated values of the one or more metrics during a time period after the first time based on EM sensor signals from the one or more EM sensor signals corresponding to the time period after the first time, determine that a difference between the one or more updated values and the one or more baseline values is greater than a threshold value, and determine that the EM field has been distorted in response to the difference being greater than the threshold value. 
     In yet another aspect, there is provided a non-transitory computer readable storage medium having stored thereon instructions that, when executed, cause at least one computing device to: calculate one or more baseline values of one or more metrics indicative of a velocity of a distal end of an instrument at a first time based on EM sensor signals from one or more EM sensor signals corresponding to the first time, the instrument comprising an EM sensor located at the distal end of the instrument, the EM sensor configured to generate the one or more EM sensor signals in response to detection of an EM field; calculate one or more updated values of the one or more metrics during a time period after the first time based on EM sensor signals from the one or more EM sensor signals corresponding to the time period after the first time; determine that a difference between the one or more updated values and the one or more baseline values is greater than a threshold value; and determine that the EM field has been distorted in response to the difference being greater than the threshold value. 
     In still yet another aspect, there is provided a method of detecting EM distortion, the method comprising: calculating one or more baseline values of one or more metrics indicative of a velocity of a distal end of an instrument at a first time based on EM sensor signals from one or more EM sensor signals corresponding to the first time, the instrument comprising an EM sensor located at the distal end of the instrument, the EM sensor configured to generate the one or more EM sensor signals in response to detection of an EM field; calculating one or more updated values of the one or more metrics during a time period after the first time based on EM sensor signals from the one or more EM sensor signals corresponding to the time period after the first time; determining that a difference between the one or more updated values and the one or more baseline values is greater than a threshold value; and determining that the EM field has been distorted in response to the difference being greater than the threshold value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed aspects will hereinafter be described in conjunction with the appended drawings, provided to illustrate and not to limit the disclosed aspects, wherein like designations denote like elements. 
         FIG. 1  illustrates an embodiment of a cart-based robotic system arranged for diagnostic and/or therapeutic bronchoscopy procedure(s). 
         FIG. 2  depicts further aspects of the robotic system of  FIG. 1 . 
         FIG. 3  illustrates an embodiment of the robotic system of  FIG. 1  arranged for ureteroscopy. 
         FIG. 4  illustrates an embodiment of the robotic system of  FIG. 1  arranged for a vascular procedure. 
         FIG. 5  illustrates an embodiment of a table-based robotic system arranged for a bronchoscopy procedure. 
         FIG. 6  provides an alternative view of the robotic system of  FIG. 5 . 
         FIG. 7  illustrates an example system configured to stow robotic arm(s). 
         FIG. 8  illustrates an embodiment of a table-based robotic system configured for a ureteroscopy procedure. 
         FIG. 9  illustrates an embodiment of a table-based robotic system configured for a laparoscopic procedure. 
         FIG. 10  illustrates an embodiment of the table-based robotic system of  FIGS. 5-9  with pitch or tilt adjustment. 
         FIG. 11  provides a detailed illustration of the interface between the table and the column of the table-based robotic system of  FIGS. 5-10 . 
         FIG. 12  illustrates an exemplary instrument driver. 
         FIG. 13  illustrates an exemplary medical instrument with a paired instrument driver. 
         FIG. 14  illustrates an alternative design for an instrument driver and instrument where the axes of the drive units are parallel to the axis of the elongated shaft of the instrument. 
         FIG. 15  depicts a block diagram illustrating a localization system that estimates a location of one or more elements of the robotic systems of  FIGS. 1-10 , such as the location of the instrument of  FIGS. 13-14 , in accordance to an example embodiment. 
         FIG. 16  illustrates an example operating environment implementing one or more aspects of the disclosed navigation systems and techniques. 
         FIG. 17  illustrates an example luminal network  140  that can be navigated in the operating environment of  FIG. 16 . 
         FIG. 18  illustrates the distal end of an example endoscope having imaging and EM sensing capabilities as described herein. 
         FIGS. 19A-C  provide graphs of metrics which illustrate changes in the metrics which may be indicative of local EM distortion. 
         FIG. 20  provides a flowchart illustrating an example methodology of determining that local EM distortion has occurred. 
         FIG. 21  illustrates an embodiment of a system which may be used to detect global EM distortion in accordance with aspects of this disclosure. 
         FIG. 22  provides a flowchart illustrating an example methodology of determining that global EM distortion has occurred. 
         FIG. 23  provides a flowchart illustrating an example methodology of determining that one of a patient and an EM field generator has moved. 
         FIG. 24  provides an example in which EM patch sensors  105  are placed within a working volume of an EM field generator. 
         FIGS. 25A-25D  illustrate examples of visual feedback which may be provided to a user by the display during a setup and alignment procedure for EM field generator(s) and/or EM patch sensor(s) in accordance with aspects of this disclosure. 
         FIG. 26  depicts a block diagram illustrating an example of the EM tracking system which may perform various aspects of this disclosure. 
         FIG. 27  is a flowchart illustrating an example method operable by an EM tracking system, or component(s) thereof, for detecting EM distortion in accordance with aspects of this disclosure. 
         FIG. 28  is a flowchart illustrating another example method operable by an EM tracking system, or component(s) thereof, for detecting EM distortion in accordance with aspects of this disclosure. 
         FIG. 29  is a flowchart illustrating yet another example method operable by an EM tracking system, or component(s) thereof, for facilitating the positioning of an EM sensor within an EM field generated by a field generator in accordance with aspects of this disclosure. 
         FIG. 30  is a flowchart illustrating still yet another example method operable by an EM tracking system, or component(s) thereof, for detecting movement of at least one of a patient or an EM field generator in accordance with aspects of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of this disclosure relate to systems and techniques for the detection and/or mitigation of electromagnetic (EM) distortion which may cause errors in localization and/or navigation systems that rely on EM data. There are a number of possible sources of EM distortion, which may in extreme cases of distortion, cause the EM data to be unreliable. Additional embodiments of this disclosure relate to techniques for alignment of an EM generator with respect to a patient and/or one or more EM patch sensors placed on the patient. 
     As used herein, the term “approximately” refers to a range of measurements of a length, thickness, a quantity, time period, or other measurable value. Such range of measurements encompasses variations of +/−10% or less, preferably +/−5% or less, more preferably +/−1% or less, and still more preferably +/−0.1% or less, of and from the specified value, in so far as such variations are appropriate in order to function in the disclosed devices, systems, and techniques. 
     Various embodiments will be described below in conjunction with the drawings for purposes of illustration. It should be appreciated that many other implementations of the disclosed concepts are possible, and various advantages can be achieved with the disclosed implementations. Headings are included herein for reference and to aid in locating various sections. These headings are not intended to limit the scope of the concepts described with respect thereto. Such concepts may have applicability throughout the entire specification. 
     1. Overview. 
     Aspects of the present disclosure may be integrated into a robotically-enabled medical system capable of performing a variety of medical procedures, including both minimally invasive, such as laparoscopy, and non-invasive, such as endoscopy, procedures. Among endoscopy procedures, the system may be capable of performing bronchoscopy, ureteroscopy, gastroscopy, etc. 
     In addition to performing the breadth of procedures, the system may provide additional benefits, such as enhanced imaging and guidance to assist the physician. Additionally, the system may provide the physician with the ability to perform the procedure from an ergonomic position without the need for awkward arm motions and positions. Still further, the system may provide the physician with the ability to perform the procedure with improved ease of use such that one or more of the instruments of the system can be controlled by a single user. 
     Various embodiments will be described below in conjunction with the drawings for purposes of illustration. It should be appreciated that many other implementations of the disclosed concepts are possible, and various advantages can be achieved with the disclosed implementations. Headings are included herein for reference and to aid in locating various sections. These headings are not intended to limit the scope of the concepts described with respect thereto. Such concepts may have applicability throughout the entire specification. 
     A. Robotic System—Cart. 
     The robotically-enabled medical system may be configured in a variety of ways depending on the particular procedure.  FIG. 1  illustrates an embodiment of a cart-based robotically-enabled system  10  arranged for a diagnostic and/or therapeutic bronchoscopy procedure. During a bronchoscopy, the system  10  may comprise a cart  11  having one or more robotic arms  12  to deliver a medical instrument, such as a steerable endoscope  13 , which may be a procedure-specific bronchoscope for bronchoscopy, to a natural orifice access point (i.e., the mouth of the patient positioned on a table in the present example) to deliver diagnostic and/or therapeutic tools. As shown, the cart  11  may be positioned proximate to the patient&#39;s upper torso in order to provide access to the access point. Similarly, the robotic arms  12  may be actuated to position the bronchoscope relative to the access point. The arrangement in  FIG. 1  may also be utilized when performing a gastro-intestinal (GI) procedure with a gastroscope, a specialized endoscope for GI procedures.  FIG. 2  depicts an example embodiment of the cart in greater detail. 
     With continued reference to  FIG. 1 , once the cart  11  is properly positioned, the robotic arms  12  may insert the steerable endoscope  13  into the patient robotically, manually, or a combination thereof. As shown, the steerable endoscope  13  may comprise at least two telescoping parts, such as an inner leader portion and an outer sheath portion, each portion coupled to a separate instrument driver from the set of instrument drivers  28 , each instrument driver coupled to the distal end of an individual robotic arm. This linear arrangement of the instrument drivers  28 , which facilitates coaxially aligning the leader portion with the sheath portion, creates a “virtual rail”  29  that may be repositioned in space by manipulating the one or more robotic arms  12  into different angles and/or positions. The virtual rails described herein are depicted in the Figures using dashed lines, and accordingly the dashed lines do not depict any physical structure of the system. Translation of the instrument drivers  28  along the virtual rail  29  telescopes the inner leader portion relative to the outer sheath portion or advances or retracts the endoscope  13  from the patient. The angle of the virtual rail  29  may be adjusted, translated, and pivoted based on clinical application or physician preference. For example, in bronchoscopy, the angle and position of the virtual rail  29  as shown represents a compromise between providing physician access to the endoscope  13  while minimizing friction that results from bending the endoscope  13  into the patient&#39;s mouth. 
     The endoscope  13  may be directed down the patient&#39;s trachea and lungs after insertion using precise commands from the robotic system until reaching the target destination or operative site. In order to enhance navigation through the patient&#39;s lung network and/or reach the desired target, the endoscope  13  may be manipulated to telescopically extend the inner leader portion from the outer sheath portion to obtain enhanced articulation and greater bend radius. The use of separate instrument drivers  28  also allows the leader portion and sheath portion to be driven independent of each other. 
     For example, the endoscope  13  may be directed to deliver a biopsy needle to a target, such as, for example, a lesion or nodule within the lungs of a patient. The needle may be deployed down a working channel that runs the length of the endoscope to obtain a tissue sample to be analyzed by a pathologist. Depending on the pathology results, additional tools may be deployed down the working channel of the endoscope for additional biopsies. After identifying a nodule to be malignant, the endoscope  13  may endoscopically deliver tools to resect the potentially cancerous tissue. In some instances, diagnostic and therapeutic treatments may need to be delivered in separate procedures. In those circumstances, the endoscope  13  may also be used to deliver a fiducial to “mark” the location of the target nodule as well. In other instances, diagnostic and therapeutic treatments may be delivered during the same procedure. 
     The system  10  may also include a movable tower  30 , which may be connected via support cables to the cart  11  to provide support for controls, electronics, fluidics, optics, sensors, and/or power to the cart  11 . Placing such functionality in the tower  30  allows for a smaller form factor cart  11  that may be more easily adjusted and/or repositioned by an operating physician and his/her staff. Additionally, the division of functionality between the cart/table and the support tower  30  reduces operating room clutter and facilitates improving clinical workflow. While the cart  11  may be positioned close to the patient, the tower  30  may be stowed in a remote location to stay out of the way during a procedure. 
     In support of the robotic systems described above, the tower  30  may include component(s) of a computer-based control system that stores computer program instructions, for example, within a non-transitory computer-readable storage medium such as a persistent magnetic storage drive, solid state drive, etc. The execution of those instructions, whether the execution occurs in the tower  30  or the cart  11 , may control the entire system or sub-system(s) thereof. For example, when executed by a processor of the computer system, the instructions may cause the components of the robotics system to actuate the relevant carriages and arm mounts, actuate the robotics arms, and control the medical instruments. For example, in response to receiving the control signal, the motors in the joints of the robotics arms may position the arms into a certain posture. 
     The tower  30  may also include a pump, flow meter, valve control, and/or fluid access in order to provide controlled irrigation and aspiration capabilities to system that may be deployed through the endoscope  13 . These components may also be controlled using the computer system of tower  30 . In some embodiments, irrigation and aspiration capabilities may be delivered directly to the endoscope  13  through separate cable(s). 
     The tower  30  may include a voltage and surge protector designed to provide filtered and protected electrical power to the cart  11 , thereby avoiding placement of a power transformer and other auxiliary power components in the cart  11 , resulting in a smaller, more moveable cart  11 . 
     The tower  30  may also include support equipment for the sensors deployed throughout the robotic system  10 . For example, the tower  30  may include opto-electronics equipment for detecting, receiving, and processing data received from the optical sensors or cameras throughout the robotic system  10 . In combination with the control system, such opto-electronics equipment may be used to generate real-time images for display in any number of consoles deployed throughout the system, including in the tower  30 . Similarly, the tower  30  may also include an electronic subsystem for receiving and processing signals received from deployed EM sensors. The tower  30  may also be used to house and position an EM field generator for detection by EM sensors in or on the medical instrument. 
     The tower  30  may also include a console  31  in addition to other consoles available in the rest of the system, e.g., console mounted on top of the cart. The console  31  may include a user interface and a display screen, such as a touchscreen, for the physician operator. Consoles in system  10  are generally designed to provide both robotic controls as well as pre-operative and real-time information of the procedure, such as navigational and localization information of the endoscope  13 . When the console  31  is not the only console available to the physician, it may be used by a second operator, such as a nurse, to monitor the health or vitals of the patient and the operation of system, as well as provide procedure-specific data, such as navigational and localization information. 
     The tower  30  may be coupled to the cart  11  and endoscope  13  through one or more cables or connections (not shown). In some embodiments, the support functionality from the tower  30  may be provided through a single cable to the cart  11 , simplifying and de-cluttering the operating room. In other embodiments, specific functionality may be coupled in separate cabling and connections. For example, while power may be provided through a single power cable to the cart, the support for controls, optics, fluidics, and/or navigation may be provided through a separate cable. 
       FIG. 2  provides a detailed illustration of an embodiment of the cart from the cart-based robotically-enabled system shown in  FIG. 1 . The cart  11  generally includes an elongated support structure  14  (often referred to as a “column”), a cart base  15 , and a console  16  at the top of the column  14 . The column  14  may include one or more carriages, such as a carriage  17  (alternatively “arm support”) for supporting the deployment of one or more robotic arms  12  (three shown in  FIG. 2 ). The carriage  17  may include individually configurable arm mounts that rotate along a perpendicular axis to adjust the base of the robotic arms  12  for better positioning relative to the patient. The carriage  17  also includes a carriage interface  19  that allows the carriage  17  to vertically translate along the column  14 . 
     The carriage interface  19  is connected to the column  14  through slots, such as slot  20 , that are positioned on opposite sides of the column  14  to guide the vertical translation of the carriage  17 . The slot  20  contains a vertical translation interface to position and hold the carriage at various vertical heights relative to the cart base  15 . Vertical translation of the carriage  17  allows the cart  11  to adjust the reach of the robotic arms  12  to meet a variety of table heights, patient sizes, and physician preferences. Similarly, the individually configurable arm mounts on the carriage  17  allow the robotic arm base  21  of robotic arms  12  to be angled in a variety of configurations. 
     In some embodiments, the slot  20  may be supplemented with slot covers that are flush and parallel to the slot surface to prevent dirt and fluid ingress into the internal chambers of the column  14  and the vertical translation interface as the carriage  17  vertically translates. The slot covers may be deployed through pairs of spring spools positioned near the vertical top and bottom of the slot  20 . The covers are coiled within the spools until deployed to extend and retract from their coiled state as the carriage  17  vertically translates up and down. The spring-loading of the spools provides force to retract the cover into a spool when carriage  17  translates towards the spool, while also maintaining a tight seal when the carriage  17  translates away from the spool. The covers may be connected to the carriage  17  using, for example, brackets in the carriage interface  19  to ensure proper extension and retraction of the cover as the carriage  17  translates. 
     The column  14  may internally comprise mechanisms, such as gears and motors, that are designed to use a vertically aligned lead screw to translate the carriage  17  in a mechanized fashion in response to control signals generated in response to user inputs, e.g., inputs from the console  16 . 
     The robotic arms  12  may generally comprise robotic arm bases  21  and end effectors  22 , separated by a series of linkages  23  that are connected by a series of joints  24 , each joint comprising an independent actuator, each actuator comprising an independently controllable motor. Each independently controllable joint represents an independent degree of freedom available to the robotic arm. Each of the arms  12  have seven joints, and thus provide seven degrees of freedom. A multitude of joints result in a multitude of degrees of freedom, allowing for “redundant” degrees of freedom. Redundant degrees of freedom allow the robotic arms  12  to position their respective end effectors  22  at a specific position, orientation, and trajectory in space using different linkage positions and joint angles. This allows for the system to position and direct a medical instrument from a desired point in space while allowing the physician to move the arm joints into a clinically advantageous position away from the patient to create greater access, while avoiding arm collisions. 
     The cart base  15  balances the weight of the column  14 , carriage  17 , and arms  12  over the floor. Accordingly, the cart base  15  houses heavier components, such as electronics, motors, power supply, as well as components that either enable movement and/or immobilize the cart. For example, the cart base  15  includes rollable wheel-shaped casters  25  that allow for the cart to easily move around the room prior to a procedure. After reaching the appropriate position, the casters  25  may be immobilized using wheel locks to hold the cart  11  in place during the procedure. 
     Positioned at the vertical end of column  14 , the console  16  allows for both a user interface for receiving user input and a display screen (or a dual-purpose device such as, for example, a touchscreen  26 ) to provide the physician user with both pre-operative and intra-operative data. Potential pre-operative data on the touchscreen  26  may include pre-operative plans, navigation and mapping data derived from pre-operative computerized tomography (CT) scans, and/or notes from pre-operative patient interviews. Intra-operative data on display may include optical information provided from the tool, sensor and coordinate information from sensors, as well as vital patient statistics, such as respiration, heart rate, and/or pulse. The console  16  may be positioned and tilted to allow a physician to access the console from the side of the column  14  opposite carriage  17 . From this position the physician may view the console  16 , robotic arms  12 , and patient while operating the console  16  from behind the cart  11 . As shown, the console  16  also includes a handle  27  to assist with maneuvering and stabilizing cart  11 . 
       FIG. 3  illustrates an embodiment of a robotically-enabled system  10  arranged for ureteroscopy. In a ureteroscopic procedure, the cart  11  may be positioned to deliver a ureteroscope  32 , a procedure-specific endoscope designed to traverse a patient&#39;s urethra and ureter, to the lower abdominal area of the patient. In a ureteroscopy, it may be desirable for the ureteroscope  32  to be directly aligned with the patient&#39;s urethra to reduce friction and forces on the sensitive anatomy in the area. As shown, the cart  11  may be aligned at the foot of the table to allow the robotic arms  12  to position the ureteroscope  32  for direct linear access to the patient&#39;s urethra. From the foot of the table, the robotic arms  12  may insert ureteroscope  32  along the virtual rail  33  directly into the patient&#39;s lower abdomen through the urethra. 
     After insertion into the urethra, using similar control techniques as in bronchoscopy, the ureteroscope  32  may be navigated into the bladder, ureters, and/or kidneys for diagnostic and/or therapeutic applications. For example, the ureteroscope  32  may be directed into the ureter and kidneys to break up kidney stone build up using laser or ultrasonic lithotripsy device deployed down the working channel of the ureteroscope  32 . After lithotripsy is complete, the resulting stone fragments may be removed using baskets deployed down the ureteroscope  32 . 
       FIG. 4  illustrates an embodiment of a robotically-enabled system similarly arranged for a vascular procedure. In a vascular procedure, the system  10  may be configured such the cart  11  may deliver a medical instrument  34 , such as a steerable catheter, to an access point in the femoral artery in the patient&#39;s leg. The femoral artery presents both a larger diameter for navigation as well as relatively less circuitous and tortuous path to the patient&#39;s heart, which simplifies navigation. As in a ureteroscopic procedure, the cart  11  may be positioned towards the patient&#39;s legs and lower abdomen to allow the robotic arms  12  to provide a virtual rail  35  with direct linear access to the femoral artery access point in the patient&#39;s thigh/hip region. After insertion into the artery, the medical instrument  34  may be directed and inserted by translating the instrument drivers  28 . Alternatively, the cart may be positioned around the patient&#39;s upper abdomen in order to reach alternative vascular access points, such as, for example, the carotid and brachial arteries near the shoulder and wrist. 
     B. Robotic System—Table. 
     Embodiments of the robotically-enabled medical system may also incorporate the patient&#39;s table. Incorporation of the table reduces the amount of capital equipment within the operating room by removing the cart, which allows greater access to the patient.  FIG. 5  illustrates an embodiment of such a robotically-enabled system arranged for a bronchoscopy procedure. System  36  includes a support structure or column  37  for supporting platform  38  (shown as a “table” or “bed”) over the floor. Much like in the cart-based systems, the end effectors of the robotic arms  39  of the system  36  comprise instrument drivers  42  that are designed to manipulate an elongated medical instrument, such as a bronchoscope  40  in  FIG. 5 , through or along a virtual rail  41  formed from the linear alignment of the instrument drivers  42 . In practice, a C-arm for providing fluoroscopic imaging may be positioned over the patient&#39;s upper abdominal area by placing the emitter and detector around table  38 . 
       FIG. 6  provides an alternative view of the system  36  without the patient and medical instrument for discussion purposes. As shown, the column  37  may include one or more carriages  43  shown as ring-shaped in the system  36 , from which the one or more robotic arms  39  may be based. The carriages  43  may translate along a vertical column interface  44  that runs the length of the column  37  to provide different vantage points from which the robotic arms  39  may be positioned to reach the patient. The carriage(s)  43  may rotate around the column  37  using a mechanical motor positioned within the column  37  to allow the robotic arms  39  to have access to multiples sides of the table  38 , such as, for example, both sides of the patient. In embodiments with multiple carriages, the carriages may be individually positioned on the column and may translate and/or rotate independent of the other carriages. While carriages  43  need not surround the column  37  or even be circular, the ring-shape as shown facilitates rotation of the carriages  43  around the column  37  while maintaining structural balance. Rotation and translation of the carriages  43  allows the system to align the medical instruments, such as endoscopes and laparoscopes, into different access points on the patient. 
     The arms  39  may be mounted on the carriages through a set of arm mounts  45  comprising a series of joints that may individually rotate and/or telescopically extend to provide additional configurability to the robotic arms  39 . Additionally, the arm mounts  45  may be positioned on the carriages  43  such that, when the carriages  43  are appropriately rotated, the arm mounts  45  may be positioned on either the same side of table  38  (as shown in  FIG. 6 ), on opposite sides of table  38  (as shown in  FIG. 9 ), or on adjacent sides of the table  38  (not shown). 
     The column  37  structurally provides support for the table  38 , and a path for vertical translation of the carriages. Internally, the column  37  may be equipped with lead screws for guiding vertical translation of the carriages  43 , and motors to mechanize the translation of said carriages based the lead screws. The column  37  may also convey power and control signals to the carriage  43  and robotic arms  39  mounted thereon. 
     The table base  46  serves a similar function as the cart base  15  in cart  11  shown in  FIG. 2 , housing heavier components to balance the table/bed  38 , the column  37 , the carriages  43 , and the robotic arms  39 . The table base  46  may also incorporate rigid casters to provide stability during procedures. Deployed from the bottom of the table base  46 , the casters may extend in opposite directions on both sides of the base  46  and retract when the system  36  needs to be moved. 
     Continuing with  FIG. 6 , the system  36  may also include a tower (not shown) that divides the functionality of system  36  between table and tower to reduce the form factor and bulk of the table. As in earlier disclosed embodiments, the tower may be provide a variety of support functionalities to table, such as processing, computing, and control capabilities, power, fluidics, and/or optical and sensor processing. The tower may also be movable to be positioned away from the patient to improve physician access and de-clutter the operating room. Additionally, placing components in the tower allows for more storage space in the table base for potential stowage of the robotic arms. The tower may also include a console that provides both a user interface for user input, such as keyboard and/or pendant, as well as a display screen (or touchscreen) for pre-operative and intra-operative information, such as real-time imaging, navigation, and tracking information. 
     In some embodiments, a table base may stow and store the robotic arms when not in use.  FIG. 7  illustrates a system  47  that stows robotic arms in an embodiment of the table-based system. In system  47 , carriages  48  may be vertically translated into base  49  to stow robotic arms  50 , arm mounts  51 , and the carriages  48  within the base  49 . Base covers  52  may be translated and retracted open to deploy the carriages  48 , arm mounts  51 , and arms  50  around column  53 , and closed to stow to protect them when not in use. The base covers  52  may be sealed with a membrane  54  along the edges of its opening to prevent dirt and fluid ingress when closed. 
       FIG. 8  illustrates an embodiment of a robotically-enabled table-based system configured for a ureteroscopy procedure. In a ureteroscopy, the table  38  may include a swivel portion  55  for positioning a patient off-angle from the column  37  and table base  46 . The swivel portion  55  may rotate or pivot around a pivot point (e.g., located below the patient&#39;s head) in order to position the bottom portion of the swivel portion  55  away from the column  37 . For example, the pivoting of the swivel portion  55  allows a C-arm (not shown) to be positioned over the patient&#39;s lower abdomen without competing for space with the column (not shown) below table  38 . By rotating the carriage  35  (not shown) around the column  37 , the robotic arms  39  may directly insert a ureteroscope  56  along a virtual rail  57  into the patient&#39;s groin area to reach the urethra. In a ureteroscopy, stirrups  58  may also be fixed to the swivel portion  55  of the table  38  to support the position of the patient&#39;s legs during the procedure and allow clear access to the patient&#39;s groin area. 
     In a laparoscopic procedure, through small incision(s) in the patient&#39;s abdominal wall, minimally invasive instruments (elongated in shape to accommodate the size of the one or more incisions) may be inserted into the patient&#39;s anatomy. After inflation of the patient&#39;s abdominal cavity, the instruments, often referred to as laparoscopes, may be directed to perform surgical tasks, such as grasping, cutting, ablating, suturing, etc.  FIG. 9  illustrates an embodiment of a robotically-enabled table-based system configured for a laparoscopic procedure. As shown in  FIG. 9 , the carriages  43  of the system  36  may be rotated and vertically adjusted to position pairs of the robotic arms  39  on opposite sides of the table  38 , such that laparoscopes  59  may be positioned using the arm mounts  45  to be passed through minimal incisions on both sides of the patient to reach his/her abdominal cavity. 
     To accommodate laparoscopic procedures, the robotically-enabled table system may also tilt the platform to a desired angle.  FIG. 10  illustrates an embodiment of the robotically-enabled medical system with pitch or tilt adjustment. As shown in  FIG. 10 , the system  36  may accommodate tilt of the table  38  to position one portion of the table at a greater distance from the floor than the other. Additionally, the arm mounts  45  may rotate to match the tilt such that the arms  39  maintain the same planar relationship with table  38 . To accommodate steeper angles, the column  37  may also include telescoping portions  60  that allow vertical extension of column  37  to keep the table  38  from touching the floor or colliding with base  46 . 
       FIG. 11  provides a detailed illustration of the interface between the table  38  and the column  37 . Pitch rotation mechanism  61  may be configured to alter the pitch angle of the table  38  relative to the column  37  in multiple degrees of freedom. The pitch rotation mechanism  61  may be enabled by the positioning of orthogonal axes  1 ,  2 , at the column-table interface, each axis actuated by a separate motor  3 ,  4 , responsive to an electrical pitch angle command. Rotation along one screw  5  would enable tilt adjustments in one axis  1 , while rotation along the other screw  6  would enable tilt adjustments along the other axis  2 . 
     For example, pitch adjustments are particularly useful when trying to position the table in a Trendelenburg position, i.e., position the patient&#39;s lower abdomen at a higher position from the floor than the patient&#39;s lower abdomen, for lower abdominal surgery. The Trendelenburg position causes the patient&#39;s internal organs to slide towards his/her upper abdomen through the force of gravity, clearing out the abdominal cavity for minimally invasive tools to enter and perform lower abdominal surgical procedures, such as laparoscopic prostatectomy. 
     C. Instrument Driver &amp; Interface. 
     The end effectors of the system&#39;s robotic arms comprise (i) an instrument driver (alternatively referred to as “instrument drive mechanism” or “instrument device manipulator”) that incorporate electro-mechanical means for actuating the medical instrument and (ii) a removable or detachable medical instrument which may be devoid of any electro-mechanical components, such as motors. This dichotomy may be driven by the need to sterilize medical instruments used in medical procedures, and the inability to adequately sterilize expensive capital equipment due to their intricate mechanical assemblies and sensitive electronics. Accordingly, the medical instruments may be designed to be detached, removed, and interchanged from the instrument driver (and thus the system) for individual sterilization or disposal by the physician or the physician&#39;s staff. In contrast, the instrument drivers need not be changed or sterilized, and may be draped for protection. 
       FIG. 12  illustrates an example instrument driver. Positioned at the distal end of a robotic arm, instrument driver  62  comprises of one or more drive units  63  arranged with parallel axes to provide controlled torque to a medical instrument via drive shafts  64 . Each drive unit  63  comprises an individual drive shaft  64  for interacting with the instrument, a gear head  65  for converting the motor shaft rotation to a desired torque, a motor  66  for generating the drive torque, an encoder  67  to measure the speed of the motor shaft and provide feedback to the control circuitry, and control circuitry  68  for receiving control signals and actuating the drive unit. Each drive unit  63  being independent controlled and motorized, the instrument driver  62  may provide multiple (four as shown in  FIG. 12 ) independent drive outputs to the medical instrument. In operation, the control circuitry  68  would receive a control signal, transmit a motor signal to the motor  66 , compare the resulting motor speed as measured by the encoder  67  with the desired speed, and modulate the motor signal to generate the desired torque. 
     For procedures that require a sterile environment, the robotic system may incorporate a drive interface, such as a sterile adapter connected to a sterile drape, that sits between the instrument driver and the medical instrument. The chief purpose of the sterile adapter is to transfer angular motion from the drive shafts of the instrument driver to the drive inputs of the instrument while maintaining physical separation, and thus sterility, between the drive shafts and drive inputs. Accordingly, an example sterile adapter may comprise of a series of rotational inputs and outputs intended to be mated with the drive shafts of the instrument driver and drive inputs on the instrument. Connected to the sterile adapter, the sterile drape, comprised of a thin, flexible material such as transparent or translucent plastic, is designed to cover the capital equipment, such as the instrument driver, robotic arm, and cart (in a cart-based system) or table (in a table-based system). Use of the drape would allow the capital equipment to be positioned proximate to the patient while still being located in an area not requiring sterilization (i.e., non-sterile field). On the other side of the sterile drape, the medical instrument may interface with the patient in an area requiring sterilization (i.e., sterile field). 
     D. Medical Instrument. 
       FIG. 13  illustrates an example medical instrument with a paired instrument driver. Like other instruments designed for use with a robotic system, medical instrument  70  comprises an elongated shaft  71  (or elongate body) and an instrument base  72 . The instrument base  72 , also referred to as an “instrument handle” due to its intended design for manual interaction by the physician, may generally comprise rotatable drive inputs  73 , e.g., receptacles, pulleys or spools, that are designed to be mated with drive outputs  74  that extend through a drive interface on instrument driver  75  at the distal end of robotic arm  76 . When physically connected, latched, and/or coupled, the mated drive inputs  73  of instrument base  72  may share axes of rotation with the drive outputs  74  in the instrument driver  75  to allow the transfer of torque from drive outputs  74  to drive inputs  73 . In some embodiments, the drive outputs  74  may comprise splines that are designed to mate with receptacles on the drive inputs  73 . 
     The elongated shaft  71  is designed to be delivered through either an anatomical opening or lumen, e.g., as in endoscopy, or a minimally invasive incision, e.g., as in laparoscopy. The elongated shaft  66  may be either flexible (e.g., having properties similar to an endoscope) or rigid (e.g., having properties similar to a laparoscope) or contain a customized combination of both flexible and rigid portions. When designed for laparoscopy, the distal end of a rigid elongated shaft may be connected to an end effector comprising a jointed wrist formed from a clevis with an axis of rotation and a surgical tool, such as, for example, a grasper or scissors, that may be actuated based on force from the tendons as the drive inputs rotate in response to torque received from the drive outputs  74  of the instrument driver  75 . When designed for endoscopy, the distal end of a flexible elongated shaft may include a steerable or controllable bending section that may be articulated and bent based on torque received from the drive outputs  74  of the instrument driver  75 . 
     Torque from the instrument driver  75  is transmitted down the elongated shaft  71  using tendons within the shaft  71 . These individual tendons, such as pull wires, may be individually anchored to individual drive inputs  73  within the instrument handle  72 . From the handle  72 , the tendons are directed down one or more pull lumens within the elongated shaft  71  and anchored at the distal portion of the elongated shaft  71 . In laparoscopy, these tendons may be coupled to a distally mounted end effector, such as a wrist, grasper, or scissor. Under such an arrangement, torque exerted on drive inputs  73  would transfer tension to the tendon, thereby causing the end effector to actuate in some way. In laparoscopy, the tendon may cause a joint to rotate about an axis, thereby causing the end effector to move in one direction or another. Alternatively, the tendon may be connected to one or more jaws of a grasper at distal end of the elongated shaft  71 , where tension from the tendon cause the grasper to close. 
     In endoscopy, the tendons may be coupled to a bending or articulating section positioned along the elongated shaft  71  (e.g., at the distal end) via adhesive, control ring, or other mechanical fixation. When fixedly attached to the distal end of a bending section, torque exerted on drive inputs  73  would be transmitted down the tendons, causing the softer, bending section (sometimes referred to as the articulable section or region) to bend or articulate. Along the non-bending sections, it may be advantageous to spiral or helix the individual pull lumens that direct the individual tendons along (or inside) the walls of the endoscope shaft to balance the radial forces that result from tension in the pull wires. The angle of the spiraling and/or spacing there between may be altered or engineered for specific purposes, wherein tighter spiraling exhibits lesser shaft compression under load forces, while lower amounts of spiraling results in greater shaft compression under load forces, but also exhibits limits bending. On the other end of the spectrum, the pull lumens may be directed parallel to the longitudinal axis of the elongated shaft  71  to allow for controlled articulation in the desired bending or articulable sections. 
     In endoscopy, the elongated shaft  71  houses a number of components to assist with the robotic procedure. The shaft may comprise of a working channel for deploying surgical tools, irrigation, and/or aspiration to the operative region at the distal end of the shaft  71 . The shaft  71  may also accommodate wires and/or optical fibers to transfer signals to/from an optical assembly at the distal tip, which may include of an optical camera. The shaft  71  may also accommodate optical fibers to carry light from proximally-located light sources, such as light emitting diodes, to the distal end of the shaft. 
     At the distal end of the instrument  70 , the distal tip may also comprise the opening of a working channel for delivering tools for diagnostic and/or therapy, irrigation, and aspiration to an operative site. The distal tip may also include a port for a camera, such as a fiberscope or a digital camera, to capture images of an internal anatomical space. Relatedly, the distal tip may also include ports for light sources for illuminating the anatomical space when using the camera. 
     In the example of  FIG. 13 , the drive shaft axes, and thus the drive input axes, are orthogonal to the axis of the elongated shaft. This arrangement, however, complicates roll capabilities for the elongated shaft  71 . Rolling the elongated shaft  71  along its axis while keeping the drive inputs  73  static results in undesirable tangling of the tendons as they extend off the drive inputs  73  and enter pull lumens within the elongate shaft  71 . The resulting entanglement of such tendons may disrupt any control algorithms intended to predict movement of the flexible elongate shaft during an endoscopic procedure. 
       FIG. 14  illustrates an alternative design for an instrument driver and instrument where the axes of the drive units are parallel to the axis of the elongated shaft of the instrument. As shown, a circular instrument driver  80  comprises four drive units with their drive outputs  81  aligned in parallel at the end of a robotic arm  82 . The drive units, and their respective drive outputs  81 , are housed in a rotational assembly  83  of the instrument driver  80  that is driven by one of the drive units within the assembly  83 . In response to torque provided by the rotational drive unit, the rotational assembly  83  rotates along a circular bearing that connects the rotational assembly  83  to the non-rotational portion  84  of the instrument driver. Power and controls signals may be communicated from the non-rotational portion  84  of the instrument driver  80  to the rotational assembly  83  through electrical contacts may be maintained through rotation by a brushed slip ring connection (not shown). In other embodiments, the rotational assembly  83  may be responsive to a separate drive unit that is integrated into the non-rotatable portion  84 , and thus not in parallel to the other drive units. The rotational mechanism  83  allows the instrument driver  80  to rotate the drive units, and their respective drive outputs  81 , as a single unit around an instrument driver axis  85 . 
     Like earlier disclosed embodiments, an instrument  86  may comprise of an elongated shaft portion  88  and an instrument base  87  (shown with a transparent external skin for discussion purposes) comprising a plurality of drive inputs  89  (such as receptacles, pulleys, and spools) that are configured to receive the drive outputs  81  in the instrument driver  80 . Unlike prior disclosed embodiments, instrument shaft  88  extends from the center of instrument base  87  with an axis substantially parallel to the axes of the drive inputs  89 , rather than orthogonal as in the design of  FIG. 13 . 
     When coupled to the rotational assembly  83  of the instrument driver  80 , the medical instrument  86 , comprising instrument base  87  and instrument shaft  88 , rotates in combination with the rotational assembly  83  about the instrument driver axis  85 . Since the instrument shaft  88  is positioned at the center of instrument base  87 , the instrument shaft  88  is coaxial with instrument driver axis  85  when attached. Thus, rotation of the rotational assembly  83  causes the instrument shaft  88  to rotate about its own longitudinal axis. Moreover, as the instrument base  87  rotates with the instrument shaft  88 , any tendons connected to the drive inputs  89  in the instrument base  87  are not tangled during rotation. Accordingly, the parallelism of the axes of the drive outputs  81 , drive inputs  89 , and instrument shaft  88  allows for the shaft rotation without tangling any control tendons. 
     E. Navigation and Control. 
     Traditional endoscopy may involve the use of fluoroscopy (e.g., as may be delivered through a C-arm) and other forms of radiation-based imaging modalities to provide endoluminal guidance to an operator physician. In contrast, the robotic systems contemplated by this disclosure can provide for non-radiation-based navigational and localization means to reduce physician exposure to radiation and reduce the amount of equipment within the operating room. As used herein, the term “localization” may refer to determining and/or monitoring the position of objects in a reference coordinate system. Technologies such as pre-operative mapping, computer vision, real-time EM tracking, and robot command data may be used individually or in combination to achieve a radiation-free operating environment. In other cases, where radiation-based imaging modalities are still used, the pre-operative mapping, computer vision, real-time EM tracking, and robot command data may be used individually or in combination to improve upon the information obtained solely through radiation-based imaging modalities. 
       FIG. 15  is a block diagram illustrating a localization system  90  that estimates a location of one or more elements of the robotic system, such as the location of the instrument, in accordance to an example embodiment. The localization system  90  may be a set of one or more computer devices configured to execute one or more instructions. The computer devices may be embodied by a processor (or processors) and computer-readable memory in one or more components discussed above. By way of example and not limitation, the computer devices may be in the tower  30  shown in  FIG. 1 , the cart shown in  FIGS. 1-4 , the beds shown in  FIGS. 5-10 , etc. 
     As shown in  FIG. 15 , the localization system  90  may include a localization module  95  that processes input data  91 - 94  to generate location data  96  for the distal tip of a medical instrument. The location data  96  may be data or logic that represents a location and/or orientation of the distal end of the instrument relative to a frame of reference. The frame of reference can be a frame of reference relative to the anatomy of the patient or to a known object, such as an EM field generator (see discussion below for the EM field generator). 
     The various input data  91 - 94  are now described in greater detail. Pre-operative mapping may be accomplished through the use of the collection of low dose CT scans. Pre-operative CT scans generate two-dimensional images, each representing a “slice” of a cutaway view of the patient&#39;s internal anatomy. When analyzed in the aggregate, image-based models for anatomical cavities, spaces and structures of the patient&#39;s anatomy, such as a patient lung network, may be generated. Techniques such as center-line geometry may be determined and approximated from the CT images to develop a three-dimensional volume of the patient&#39;s anatomy, referred to as preoperative model data  91 . The use of center-line geometry is discussed in U.S. patent application Ser. No. 14/523,760, the contents of which are herein incorporated in its entirety. Network topological models may also be derived from the CT-images, and are particularly appropriate for bronchoscopy. 
     In some embodiments, the instrument may be equipped with a camera to provide vision data  92 . The localization module  95  may process the vision data to enable one or more vision-based location tracking. For example, the preoperative model data may be used in conjunction with the vision data  92  to enable computer vision-based tracking of the medical instrument (e.g., an endoscope or an instrument advance through a working channel of the endoscope). For example, using the preoperative model data  91 , the robotic system may generate a library of expected endoscopic images from the model based on the expected path of travel of the endoscope, each image linked to a location within the model. Intra-operatively, this library may be referenced by the robotic system in order to compare real-time images captured at the camera (e.g., a camera at a distal end of the endoscope) to those in the image library to assist localization. 
     Other computer vision-based tracking techniques use feature tracking to determine motion of the camera, and thus the endoscope. Some feature of the localization module  95  may identify circular geometries in the preoperative model data  91  that correspond to anatomical lumens and track the change of those geometries to determine which anatomical lumen was selected, as well as the relative rotational and/or translational motion of the camera. Use of a topological map may further enhance vision-based algorithms or techniques. 
     Optical flow, another computer vision-based technique, may analyze the displacement and translation of image pixels in a video sequence in the vision data  92  to infer camera movement. Examples of optical flow techniques may include motion detection, object segmentation calculations, luminance, motion compensated encoding, stereo disparity measurement, etc. Through the comparison of multiple frames over multiple iterations, movement and location of the camera (and thus the endoscope) may be determined. 
     The localization module  95  may use real-time EM tracking to generate a real-time location of the endoscope in a global coordinate system that may be registered to the patient&#39;s anatomy, represented by the preoperative model. In EM tracking, an EM sensor (or tracker) comprising of one or more sensor coils embedded in one or more locations and orientations in a medical instrument (e.g., an endoscopic tool) measures the variation in the EM field created by one or more static EM field generators positioned at a known location. The location information detected by the EM sensors is stored as EM data  93 . The EM field generator (or transmitter), may be placed close to the patient to create a low intensity magnetic field that the embedded sensor may detect. The magnetic field induces small currents in the sensor coils of the EM sensor, which may be analyzed to determine the distance and angle between the EM sensor and the EM field generator. These distances and orientations may be intra-operatively “registered” to the patient anatomy (e.g., the preoperative model) in order to determine the geometric transformation that aligns a single location in the coordinate system with a position in the pre-operative model of the patient&#39;s anatomy. Once registered, an embedded EM tracker in one or more positions of the medical instrument (e.g., the distal tip of an endoscope) may provide real-time indications of the progression of the medical instrument through the patient&#39;s anatomy. 
     Robotic command and kinematics data  94  may also be used by the localization module  95  to provide localization data  96  for the robotic system. Device pitch and yaw resulting from articulation commands may be determined during pre-operative calibration. Intra-operatively, these calibration measurements may be used in combination with known insertion depth information to estimate the position of the instrument. Alternatively, these calculations may be analyzed in combination with EM, vision, and/or topological modeling to estimate the position of the medical instrument within the network. 
     As  FIG. 15  shows, a number of other input data can be used by the localization module  95 . For example, although not shown in  FIG. 15 , an instrument utilizing shape-sensing fiber can provide shape data that the localization module  95  can use to determine the location and shape of the instrument. 
     The localization module  95  may use the input data  91 - 94  in combination(s). In some cases, such a combination may use a probabilistic approach where the localization module  95  assigns a confidence weight to the location determined from each of the input data  91 - 94 . Thus, where the EM data may not be reliable (as may be the case where there is EM interference) the confidence of the location determined by the EM data  93  can be decrease and the localization module  95  may rely more heavily on the vision data  92  and/or the robotic command and kinematics data  94 . 
     As discussed above, the robotic systems discussed herein may be designed to incorporate a combination of one or more of the technologies above. The robotic system&#39;s computer-based control system, based in the tower, bed and/or cart, may store computer program instructions, for example, within a non-transitory computer-readable storage medium such as a persistent magnetic storage drive, solid state drive, or the like, that, upon execution, cause the system to receive and analyze sensor data and user commands, generate control signals throughout the system, and display the navigational and localization data, such as the position of the instrument within the global coordinate system, anatomical map, etc. 
     2. Electromagnetic (EM) Distortion—Navigation and Localization 
     As discussed above, EM data may be used by embodiments discussed herein for navigation and localization of a surgical instrument (e.g. a steerable instrument). EM data may be generated by one or more EM sensors located within the medical instrument and/or one or more EM patch sensors placed on a patient.  FIG. 16  illustrates an example operating environment  100  implementing one or more aspects of the disclosed navigation systems and techniques. The operating environment  100  includes a table  38  supporting a patient, EM sensors  105  (also referred to as “EM patch sensor” so as to be distinguished from EM instrument sensors located on a medical instrument as discussed below), and an EM field generator  110 . Certain additional devices/elements may also be included, but have not been illustrated in  FIG. 16 . For example, the environment  100  may also include: a robotic system configured to guide movement of medical instrument, a command center for controlling operations of the surgical robotic system, and an EM controller. The EM controller may be electrically connected to EM patch sensors  105  to receive EM sensor signals therefrom. The EM controller may further be connected to the EM field generator  110  to provide control signals thereto for generating the EM field. However, in certain embodiments, the EM controller may be partially or completely incorporated into one or more of the other processing device of the system, including the EM field generator  110 , the cart  11  (see  FIG. 1 ), and/or the tower  30  (see  FIG. 1 ). 
     When included, the EM controller may control EM field generator  110  to produce a varying EM field. The EM field may be time-varying and/or spatially varying, depending upon the embodiment. The EM field generator  110  may be located on a cart, similar to the cart  11  illustrated in  FIG. 2 , or may be attached to a rail of the table  38  via one or more supporting columns. In other embodiments, an EM field generator  110  may be mounted on a robotic arm, for example similar to those shown in surgical robotic system  10  of  FIG. 1 , which can offer flexible setup options around the patient. 
     The EM field generator  110  may have an associated working volume in which the EM patch sensors  105  may be placed when in use. For example, the EM sensor signals produced by the EM patch sensors  105  may be sufficiently reliable for use in EM field detection (e.g., EM distortion detection) when they are positioned within the working volume. 
     An EM spatial measurement system may determine the location of objects within the EM field that are embedded or provided with EM sensor coils, for example EM patch sensors  105  or EM instrument sensors  305  (as shown in  FIG. 18  and discussed below). When an EM sensor is placed inside a controlled, varying EM field as described herein, voltages are induced in sensor coil(s) included in the EM sensor. These induced voltages can be used by the EM spatial measurement system to calculate the position and orientation of the EM sensor and thus the object having the EM sensor. As the EM fields are of a low field strength and can safely pass through human tissue, location measurement of an object is possible without the line-of-sight constraints of an optical spatial measurement system. 
     The EM field may be defined relative to a coordinate frame of the EM field generator  110 , and a coordinate frame of a 3D model of the luminal network can be mapped to the coordinate frame of the EM field. However, the EM field may be affected by one or more sources of EM distortion in the environment  100 . For example, the presence of a ferromagnetic material within working volume of the EM field generator  110  or within the environment  100  may distort the EM field. This effect may depend on the distance between the ferromagnetic material and the working volume of the EM field as well as on the properties of the ferromagnetic material. However, other materials may also affect the EM field, such as paramagnetic materials, etc. Examples of common sources of EM distortion which may be present in the environment  100  include: fluoroscopes, tools, instruments, beds, and tables. 
     The effects of an EM field distortion source may be tolerable for certain applications when the EM field distortion source is stationary. That is, the EM field may be substantially static when a stationary EM distortion source is present. However, the movement of an EM distortion source may cause changes in the EM sensor signals that would otherwise be interpreted as movement of the EM sensors. Thus, it is desirable to detect EM field distortion to prevent such distortions from being incorrectly interpreted by the EM spatial measurement system as movement of the EM sensors. 
     As shown in  FIG. 16 , a number of EM patch sensors  105  may be placed on the body of the patient (e.g., in the region of a luminal network  140 ). These EM patch sensors  105  may be used to track displacement of the patient&#39;s body caused by respiration as well as to track EM field distortion. A number of different EM patch sensors  105  may be spaced apart on the body surface in order to track the different displacements at these locations. For example, the periphery of the lungs may exhibit greater motion due to respiration than the central airways, and providing a number of EM patch sensors  105  as shown can enable more precise analysis of these motion effects. To illustrate, the distal end of an endoscope may travel through different regions of the luminal network  140  and thus experience varying levels of displacement due to patient respiration as it travels through these different regions. 
     Additionally, as the number of EM patch sensors  105  increases, the robustness of EM field distortion detection may be increased since more complex analysis of the movement of the EM patch sensors  105  may be performed using the additional EM sensor signals produced. As will be described in greater detail below, the EM sensor signals received from an EM patch sensor  105  may be used to determine the position and orientation of the EM patch sensor  105  with respect to the EM field generator  110 . In certain embodiments, an EM patch sensor  105  may provide 5 degrees-of-freedom (DoF) of movement data (e.g., 3 positional DoF and 2 angular DoF) or 6 DoF data (e.g., 3 positional DoF and 3 angular DoF). When only a single EM patch sensor  105  is present, it may be difficult to distinguish EM distortion from movement of the EM patch sensor  105 . However, with additional EM patch sensors  105 , additional metrics may be calculated, such as the relative distance between the EM patch sensors  105 . Since the relative distance between EM patch sensors  105  is substantially fixed (e.g., the EM patch sensors  105  are fixed to locations on the patient&#39;s body and the relative distance will only change due to respiration or removal from the patient), changes in the relative distance that are inconsistent with the patient&#39;s respiration may be identified as due to EM distortion. 
       FIG. 17  illustrates an example luminal network  140  that can be navigated in the operating environment  100  of  FIG. 16 . The luminal network  140  includes the branched structure of the airways  150  of the patient and a nodule  155  that can be accessed as described herein for diagnosis and/or treatment. As illustrated, the nodule  155  is located at the periphery of the airways  150 . The endoscope  115  has a first diameter and thus its distal end is not able to be positioned through the smaller-diameter airways around the nodule  155 . Accordingly, a steerable catheter  145  extends from the working channel of the endoscope  115  the remaining distance to the nodule  155 . The steerable catheter  145  may have a lumen through which instruments, for example biopsy needles, cytology brushes, and/or tissue sampling forceps, can be passed to the target tissue site of nodule  155 . In such implementations, both the distal end of the endoscope  115  and the distal end of the steerable catheter  145  can be provided with EM instrument sensors for tracking their position within the airways  150 . In other embodiments, the overall diameter of the endoscope  115  may be small enough to reach the periphery without the steerable catheter  145 , or may be small enough to get close to the periphery (e.g., within 2.5-3 cm) to deploy medical instruments through a non-steerable catheter. The medical instruments deployed through the endoscope  115  may be equipped with EM instrument sensors, and the position filtering and safety-mode navigation techniques described below can be applied to such medical instruments. 
     In some embodiments, a 2D display of a 3D luminal network model as described herein, or a cross-section of a 3D model, can resemble  FIG. 17 . Navigation safety zones and/or navigation path information can be overlaid onto such a representation. 
       FIG. 18  illustrates the distal end  300  of an example endoscope having imaging and EM sensing capabilities as described herein, for example the endoscope  13  of  FIG. 1 . However, aspects of this disclosure may relate to the use of other steerable instruments, such as ureteroscope  32  of  FIG. 3 , laparoscope  59  of  FIG. 9 , etc. In  FIG. 18 , the distal end  300  of the endoscope includes an imaging device  315 , illumination sources  310 , and ends of EM sensor coils  305 , which form an EM instrument sensor. The distal end  300  further includes an opening to a working channel  320  of the endoscope through which surgical instruments, such as biopsy needles, cytology brushes, and forceps, may be inserted along the endoscope shaft, allowing access to the area near the endoscope tip. 
     EM coils  305  located on the distal end  300  may be used with an EM tracking system to detect the position and orientation of the distal end  300  of the endoscope while it is disposed within an anatomical system. In some embodiments, the coils  305  may be angled to provide sensitivity to EM fields along different axes, giving the disclosed navigational systems the ability to measure a full 6 DoF: 3 positional DoF and 3 angular DoF. In other embodiments, only a single coil may be disposed on or within the distal end  300  with its axis oriented along the endoscope shaft of the endoscope. Due to the rotational symmetry of such a system, it is insensitive to roll about its axis, so only 5 degrees of freedom may be detected in such an implementation. 
     A. Local Distortion. 
     An example of the detection of local EM distortion will be described with reference to an embodiment of this disclosure that includes the navigation and localization of an endoscope. However, aspects of this disclosure also relate to the detection of EM distortion with respect to the navigation and localization of any type of surgical instrument, e.g., a gastroscope, laparoscope, etc. As used herein, local EM distortion generally refers to EM distortion caused due to a distortion source that is located adjacent to or within an instrument. 
     One example of a local EM distortion source is a radial endobronchial ultrasound (REBUS) probe. A REBUS probe may be used to provide a 360° image of the parabronchial structures and enable visualization of structures from the probe. A REBUS probe may include components which can cause local EM distortion that may affect an EM sensor provided on an instrument. For example, a REBUS probe may include a transducer in a conductive head, the transducer being bonded to a torque coil. The REBUS probe may also include a fluid-filled closed catheter. Each of these components may cause distortions to the EM field near the REBUS probe, which when the REBUS probe is moved through a working channel in the instrument, may cause local EM distortion with the EM sensor on the instrument. 
     As discussed above, surgical instruments such as biopsy needles, cytology brushes, and forceps, may be inserted and passed through the working channel  320  of an endoscope to allow the surgical instrument access to the area near the tip of the endoscope. These surgical instruments may be formed of material(s) or include components that distort the EM field when the surgical instrument is moved. Typically, the endoscope is substantially stationary while the surgical instrument is passed through the working channel or navigated within the area adjacent to the endoscope tip (e.g., the physician user does not navigate the endoscope while simultaneously moving the surgical instrument). 
     The EM instrument sensor may be configured to generate one or more EM sensor signals in response to detection of the EM field generated by the EM field generator  110 . Distortions in the EM field may be detectable by the EM instrument sensor (e.g., by the EM sensor coils  305 ) located on the distal end  300  of the endoscope based on the EM sensor signals. Since the EM instrument sensor is used for navigation and localization of the endoscope tip, changes in the EM field detected by the EM instrument sensor are interpreted by the EM spatial measurement system as movement of the endoscope tip. However, since the endoscope is typically stationary during movement of the surgical instrument, changes in the EM field as detected by the EM instrument sensor may be determined to be indicative of distortion in the EM field rather than movement of the endoscope when the endoscope is known to be stationary. 
     There are a number of methods by which the surgical robotic system may be able to determine that the endoscope is stationary. For example, the endoscope position and movement may be controlled by the user, and thus, when the system is not actively receiving command data for repositioning, controlling, or otherwise navigating the endoscope, the system can determine that the endoscope is stationary. The system may use additional navigation and control data to confirm whether the endoscope is stationary. For example, the vision data  92  and robotic command and kinematics data  94  may be analyzed to determine that the endoscope is stationary. 
     The system may be able to detect local EM distortion based on the EM sensor signals generated by the EM instrument sensor. For example, the system may calculate one or more baseline values of one or more metrics related to the position and/or movement the distal end of the instrument. The baseline values may be calculated at a first time based on the EM sensor signals corresponding to the first time generated by the EM instrument sensor. In one embodiment, the first time may be prior to insertion of the endoscope into the patient (e.g., the baseline metric may be a preoperative measurement). In one example, the first time at which the baseline measurement is calculated is after the environment  100  has been set up for a surgical procedure. For example, one or more of the cart  11 , tower  30 , robotic arms  12 , EM field generator  110 , and C-arm may be initially positioned in preparation for a surgical operation. Since the movement of one or more of the cart  11 , tower  30 , robotic arms  12 , EM field generator  110 , and C-arm may affect the EM field generated by the EM field generator  110 , the baseline metric(s) may be measured after positioning of the various devices within the environment  100  so that further movement of the devices may be minimized, thereby minimizing distortions to the EM field that would be introduced due to the movement of these devices. 
     However, the baseline metric may be calculated and/or updated at various times other than prior to the surgical operation in other embodiments. For example, it may be desirable to calculate and/or update the baseline measurement after movement of the C-arm to reduce the effects of the movement and/or repositioning of the C-arm on the measured EM field. In another embodiment, the baseline metric(s) may be automatically calculated in response to the start of the surgical procedure. Since the baseline measurements may be calculated in a relatively short time period (e.g., in a number of seconds), the baseline metric(s) may be sufficiently accurate when calculated as the endoscope is inserted into the patient via a patient introducer. 
     There are a number of different metrics which may be calculated by the system based on the EM sensor signals, each of which may be used to detect local EM distortion. Example metrics which may be calculated include: a linear velocity of the distal end  300  of the instrument, an angular velocity of the distal end  300  of the instrument, and a change in an indicator value.  FIGS. 19A-C  provide graphs of these metrics which illustrate changes in the metrics which may be indicative of local EM distortion. In particular,  FIG. 19A  illustrates a change in indicator value metric,  FIG. 19B  illustrates a linear velocity metric, and  FIG. 19C  illustrates an angular velocity metric. 
     In certain implementations, the system may calculate one or more of: an indicator value Ind, a position {right arrow over (P)} of the distal end  300  of the instrument, and an angular orientation {right arrow over (Q)} of the distal end  300  of the instrument. These values may be used by the system in the navigation and localization of the instrument. In certain implementations, the indicator value Ind, position P, and angular position {right arrow over (Q)} values may be calculated based on 5DoF measurements (e.g., 3 positional DoF and 2 angular DoF) generated based on the EM sensor signals received from the coil(s)  305 . The indicator value Ind may be a value that is representative of the quality of the position {right arrow over (P)} and angular orientation {right arrow over (Q)} measurements. Thus, the indicator value Ind may be compared to a threshold value by the system to determine whether the position {right arrow over (P)} and angular orientation {right arrow over (Q)} measurements are sufficiently accurate to be used in navigation and localization. In certain embodiments, the indicator value Ind may be calculated using a goodness of fit (GOF) algorithm between the 5DoF measurements received from the coil(s)  305  and a model of the endoscope tip as a rigid body. 
     Each of the graphs illustrated in  FIGS. 19A-19C  illustrate certain metrics which may be determined as a surgical instrument (e.g., forceps) is passed through an endoscope. These graphs were generated based on the same events, where the forceps were passed through the endoscope five times while the endoscope remained stationary. 
     Specifically,  FIG. 19A  illustrates a change in indicator value metric ΔInd, which is measured in Hz (e.g., 1/s). The five events where the forceps were passed through the endoscope are visible where the change in indicator value metric ΔInd increases to a level that is significantly higher than the noise in the change in indicator value metric ΔInd. The change in indicator value metric may be calculated as a time change in the indicator value using the following equation: 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     Ind 
                   
                   = 
                   
                     
                       
                         Ind 
                          
                         
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                             t 
                             i 
                           
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                       - 
                       
                         Ind 
                          
                         
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     Where ΔInd is the change in indicator value metric, Ind is the indictor value, t i  is a current time (e.g., a time at which the indicator value is sampled and/or determined), and t i-1  is a previous time. 
     Similarly,  FIG. 19B  illustrates a linear velocity metric v, which is measured in mm/s. Here, each of the forceps movement events is visible as linear velocity values which are greater than noise in the baseline linear velocity value. The linear velocity metric may be calculated as a time change in position of the endoscope using the following equation: 
     
       
         
           
             
               
                 
                   
                     v 
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     Where v is the linear velocity metric and {right arrow over (P)} is the position of the distal end  300  of the instrument. 
     Finally,  FIG. 19C  illustrates an angular velocity metric ω, which is measured in rad/s. The angular velocity metric may be calculated as a time change in the orientation of the endoscope using the following equation: 
     
       
         
           
             
               
                 
                   
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     Where ω is the angular velocity metric and {right arrow over (Q)} is the angular orientation of the distal end  300  of the instrument. 
     As shown in  FIGS. 19A-19C , each of the calculated metrics illustrates a deviation from a baseline value (e.g., where the baseline value is set to 0) for each of the five individual endoscope movement events. By selecting appropriate threshold values, these deviations from the baseline can be detected. 
     After the baseline value(s) of the metric(s) have been calculated, the system may periodically calculate one or more updated values of the one or more metrics during a time period after the first time based on EM sensor signals from the one or more EM sensor signals corresponding to the time period after the first time. For example, the system may periodically calculate updated values of the metric(s) in order to determine whether local EM distortion is occurring. When the system has determined that the instrument is stationary, changes in one or more of the metric(s) may be indicative of local EM distortion. 
     Accordingly, the system may determine whether a difference between the one or more updated values and the one or more baseline values is greater than a threshold value. A different threshold value may be set for each of the metric(s) being calculated. When the difference is greater than the threshold value, the system may determine that the EM field has been distorted. 
       FIG. 20  provides a flowchart illustrating an example methodology of determining that local EM distortion has occurred. The method  2000  begins at block  2001 . At block  2005 , the system determines whether a debounce period is active. As used herein, the debounce period may generally refer to a predetermined period of time which limits the frequency at which EM distortion can be determined to have occurred. For example, in certain implementations, while the debounce period is active, the system will not calculate new metrics and/or evaluate metrics to determine whether EM distortion has occurred. The system may determine that EM distortion has effectively occurred for the entire debounce period and resume determining whether EM distortion has occurred once the debounce period has expired. A debounce flag, stored as data in the system, may be used to indicate that the debounce period is active. The debounce period may be set as an interval that defines how often EM distortion may be flagged. For example, a new occurrence of EM distortion may not be set while the debounce period is active. 
     If the debounce period is active, the method  2000  continues at block  2030 , where local EM distortion is determined to have occurred. When the debounce period is not active, the method  2000  continues at block  2010  where the system calculates a number of metrics. In one example, the system calculates a linear velocity metric, an angular velocity metric, and a change in indicator value metric. At block  2015 , the system analyzes the calculated metrics that have been stored over a window of time, including determining the standard deviation of each of the metrics. At block  2020 , the system determines whether the analyzed metrics are indicative of local EM distortion. This may include comparing each of the metrics against a corresponding threshold value and comparing the standard deviations against corresponding threshold values. In some cases, the system may attempt to limit the occurrences of false positives by comparing the occurrences of local distortion events with some criteria over time. For example, in one embodiment, when a quorum or some number of the comparisons in a given time window are indicative of local EM distortion, the system may determine that the metrics are indicative of local EM distortion. It is to be appreciated that such an approach is merely one approach and other embodiments may employ any suitable approach, such as determining that a local EM distortion has occurred when the metrics are indicative for some number of consecutive comparisons. 
     At block  2025 , in response to determining that the metrics are indicative of local EM distortion, the system activates the debounce period, which may include activating the debounce flag. At block  2030 , the system determines that local EM distortion has occurred, which may include setting an EM distortion flag and/or a local EM distortion flag. The method ends at block  2035 . It is to be appreciated that the system may perform a number of actions in response to detecting local EM distortion. Some exemplary responses are described below. 
     B. Global Distortion. 
     Another possible source of EM distortion is global EM distortion. As used herein, global EM distortion generally refers to EM distortion caused by sources that are located within the environment  100  but are not directly adjacent to the distal end of an instrument. For example, certain surgical procedures may be performed with the use of fluoroscopic imaging, which may include the placement of a C-arm next to the patient. An example setup for a fluoroscopic procedure is shown in  FIG. 5  in which the C-arm is positioned such that an emitter and detector are placed to be positioned on opposing sides of the patient. The C-arm may be positioned in anteroposterior (AP) position as an initial position for the surgical procedure. 
     Due to the technical requirements of fluoroscopy, the C-arm typically includes a number of components which may cause distortion in the EM field generated by the EM field generator  110 . For example, the production of X-rays by the emitter may require components which produce and/or affect EM fields as a byproduct of generating the X-rays. However, while the C-arm remains in the same position, the EM field distortions caused by the C-arm may be relatively static. That is, while the EM field distortions caused by the C-arm may distort the EM field measured by EM sensors (e.g., EM patch sensors  105  and EM instrument sensors  305 ), the EM spatial measurement system may still be able to effectively navigate and localize the instrument if the EM field is stable. However, when the position of the C-arm is moved during navigation and/or localization, the EM field may be dynamically distorted, causing the position and/or orientation of the instrument as calculated by the EM spatial measurement system to shift from the instrument&#39;s actual position and orientation. Thus, detection of such global EM distortion events is desirable in order to enable the EM spatial measurement system to act on global EM distortion events. While a C-arm has been provided as an example of a global EM distortion source, other global EM distortion sources may also be detected. Other materials which may be sources of global EM distortion include electrically conductive materials and magnetic materials as well as any EM field source. 
       FIG. 21  illustrates an embodiment of a system which may be used to detect global EM distortion in accordance with aspects of this disclosure. The  FIG. 21  embodiment includes an EM field generator  110  and three EM patch sensors  105  positioned within a working volume of the EM field generator  110 . As discussed above, the EM patch sensors  110  may be used to detect respiration of the patient, which can be used to correct the navigation and localization of an instrument via an EM instrument sensor located thereon. In addition, the patch sensors  105  may be used to detect global EM distortion, which will be described in greater detail below. 
     In the embodiment of  FIG. 21 , the patch sensors  105  include three patch sensor P 0 , P 1 , and P 2 . However, other implementations may include more or fewer patch sensors  105 . When the EM spatial measurement system includes a greater number of patch sensor  105 , the system may be able to calculate a greater number of metrics which may be used to track global EM distortion, improving the robustness of the distortion tracking. 
     When placed on a patient, each of the EM patch sensors  105  may be configured to generate a one or more EM sensor signals in response to detection of the EM field. Similar to the coil(s)  305 , the EM spatial measurement system may be able to generate 5DoF measurements based on the EM sensor signals received from the EM patch sensors  105 . When at least two EM patch sensors  105  are available, the system may be able to calculate a relative position metric and a relative angle metric. Further, when at least three EM patch sensors  105  are available, the system may be able to calculate a patch area metric and a patch space 6DoF metric. 
     The EM patch sensors are attached to various locations on the patient&#39;s body. As such, the relative distance, relative angle, patch space, and patch area metrics are relatively stable and may vary based only on the user&#39;s respiration. By tracking the user&#39;s respiration, the system can filter out changes in the calculated metrics caused to respiration. Once respiration variations have been filtered from the metrics any remaining changes may therefore be attributed to global EM distortion. 
     The relative position metric may be representative of the relative position between two of the EM patch sensors (e.g., P 1  and P 2 ). The relative position metric for EM patch sensors P 1  and P 2  may be calculated using the following equation: 
         dP 1 P 2 rel =√{square root over (( P 1 x   −P 2 x ) 2 +( P 1 y   −P 2 y ) 2 +( P 1 z   −P 2 z ) 2 )}  (4)
 
     Where dP 1 P 2   rel  is the relative position metric, P 1   x  and P 2   x  are the respective X-coordinates of the EM patch sensors P 1  and P 2 , P 1   y  and P 2   y  are the respective Y-coordinates of the EM patch sensors P 1  and P 2 , and P 1   z  and P 2   z  are the respective Z-coordinates of the EM patch sensors P 1  and P 2 . 
     The relative angle metric may the relative angle between the Z-axis of two of the EM patch sensors (e.g., P 1  and P 2 ). The relative angle metric may be calculated using the following equation: 
       θ rel =cos −1 (dot( P 1 Rz   ,P 2 Rz ))  (5)
 
     Where θ rel  is the relative angle metric, P 1   Rz  is the Z-axis of the EM patch sensor P 1 , and P 2   RZ  is the Z-axis of the EM patch sensor P 2 . 
     The patch area metric may be the area created by the EM patch sensors and may be calculated using the following equation: 
       area=√{square root over (( s *( s−dP 1 P 2 rel )+( s−dP 1 P 3 rel )+( s−dP 2 P 3 rel ))}  (6)
 
     Where area is the patch area metric, the relative positions are calculated according to equation (4), and s may be calculated using the following equation: 
     
       
         
           
             
               
                 
                   s 
                   = 
                   
                     
                       
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     The patch space 6DoF metric may be the 6DoF position and orientation of the space created by the EM patch sensors and may be calculated using the following equations: 
     
       
         
           
             
               
                 
                   
                     X 
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     Where P 0  is the position of EM patch sensor P 0  in EM field generator  110  space and is used as the origin, P 1  is the position of EM patch sensor P 1  in EM field generator  110  space, and P 2  is the position of EM patch sensor P 2  in EM field generator  110  space. Examples of the X axis , Y axis , and Z axis  of the patch space metric calculated by equations (8)-(10) are illustrated in  FIG. 21 . 
     After the baseline value(s) of the metric(s) have been calculated, the system may periodically calculate one or more updated values of the one or more metrics during a time period after the first time based on EM sensor signals from the one or more EM sensor signals corresponding to the time period after the first time. For example, the system may periodically calculate updated values of the metric(s) in order to determine whether global EM distortion is occurring. Since changes in the values of the metrics are affected only by the patient&#39;s respiration, when the difference between one or more of the updated metrics and the baseline values of the one or more metrics is greater than a threshold value, the system may determine that global EM distortion has occurred. Further, in certain embodiments, the respiration can be filtered out of the calculated metrics, and thus, any remaining changes in the metric(s) can be determined to be caused by distortions in the EM field. 
     Accordingly, the system may determine whether a difference between the one or more updated values and the one or more baseline values is greater than a threshold value. A different threshold value may be set for each of the metric(s) being calculated. When the difference is greater than the threshold value, the system may determine that the EM field has been distorted. 
       FIG. 22  provides a flowchart illustrating an example methodology of determining that global EM distortion has occurred. The method  2200  begins at block  2201 . At block  2205 , the system determines baseline metrics for each of the calculated metrics. This may include retrieving baseline values for the metrics from memory or calculating baseline metrics based on EM sensor signals received from EM patch sensors  105 . At block  2210 , the system determines whether the baseline metric quality is greater than a threshold quality. When the baseline metric quality is not greater than the threshold quality, the method  2200  ends and the method  2200  may repeat by attempting to collect better quality baseline metrics. 
     When the baseline metric quality is greater than the threshold quality, the method  2200  continues at block  2215 , where the system calculates a number of metrics. In one example, the system calculates a relative distance metric, a relative angle, metric, a 6DoF patch space metric, and a patch area metric. At block  2220 , the system analyzes the calculated metrics that have been stored over a window of time, including determining the standard deviation of each of the metrics. At block  2225 , the system determines whether the analyzed metrics are indicative of global EM distortion. This may include comparing each of the metrics against a corresponding threshold value and comparing the standard deviations against corresponding threshold values. When a quorum of the comparisons are indicative of global EM distortion, the system may determine that the metrics are indicative of global EM distortion. 
     At block  2230 , in response to determining that the metrics are indicative of global EM distortion, the system determines that global EM distortion has occurred, which may include setting an EM distortion flag and/or a global EM distortion flag. The method ends at block  2235 . It is to be appreciated that the system may perform a number of actions in response to detecting global EM distortion. Some exemplary responses are described below. 
     C. Motion Detection. 
     The navigation and localization of an instrument based on EM data may also be negatively affected when one or more of the patient and the EM field generator  110  is moved. There are generally two scenarios for movement of the EM field generator  110  or the patient. First, the EM field generator  110  or patient may be moved and settle at a new position. Second, the EM field generator  110  or patient may receive an impulse force (e.g., be bumped) and experience a temporary oscillation in place before returning to approximately the same position as before receiving the impulse force. Since the movement of either the patient or the EM field detector  110  may be incorrectly interpreted as movement of an instrument, local EM distortion, and/or global EM distortion, it may be desirable to detect the movement of the EM field generator  110  or patient. 
     Since the relative distance between the EM patch sensors  105  on the patient is relatively stable, the movement of the EM field generator  110  or patient will result in a change in the calculated absolute distance between each of the EM patch sensors  105  and the EM field generator  110 . Such movement may also result in a change in the calculated absolute angle between the EM patch sensors  105  and the EM field generator  110 . 
     When at least one EM patch sensors  105  is available, the system may be able to calculate an absolute position metric and an absolute angle metric. Further, when at least three EM patch sensors  105  are available, the system may to use the patch space 6DoF metric as described in connection with equations (8)-(10). Additional examples of the at least one metric include: an absolute position of each of the EM sensors with respect to the field generator, the root of the sum of the squares of the absolute positions of the EM sensors with respect to the field generator, the absolute angle of each of the EM sensors with respect to the field generator, the root of the sum of the squares of the absolute angles of the EM sensors with respect to the field generator, and the position and orientation of a space created by the EM sensors. 
     The absolute position metric may be representative of the absolute distance between a given one of the EM patch sensors  105  and the EM field generator  110 . The absolute position metric may be calculated using the following equation: 
         D   abs =√{square root over ( P   x   2   +P   y   2   +P   z   2 )}  (11)
 
     Where D abs  is the absolute position metric, P x  is the position of the EM patch sensor  105  with respect to the EM field generator  110  in the X-axis, P y  is the position of the EM patch sensor  105  with respect to the EM field generator  110  in the Y-axis, and P z  is the position of the EM patch sensor  105  with respect to the EM field generator  110  in the Z-axis. 
     The absolute angle metric may be representative of the absolute angle between a given one of the EM patch sensors  105  and the EM field generator  110 . The absolute angle metric may be calculated using the following equation: 
       θ abs =cos −1 (dot( P   Rz   ,FG   Rz ))  (12)
 
     Where θ abs  is the absolute angle metric, P Rz  is the Z-axis of the EM patch sensor P 1 , and FG Rz  is the Z-axis of the EM field generator  110 . 
     Since movement of the EM field generator  110  and/or the patient is temporary, the EM spatial measurement system may be configured to determine the period of time for which the patient and/or the EM field generator  110  is moving. 
     Thus, the EM tracking system may be able to detect movement of the patient and/or the EM field generator  110  based on the EM sensor signals generated by the EM patch sensor(s). For example, the system may calculate a baseline value of at least one metric based on the one or more EM sensor signals. The baseline value of the at least one metric may correspond to a first time. In one embodiment, the first time may be prior to insertion of the endoscope into the patient (e.g., the baseline metric may be a preoperative measurement). However, for movement detection, the baseline value may be the most recent stable value for the metric (e.g., changes to the metric are less than a threshold value for a period of time). 
     The EM tracking system may calculate an updated value of the at least one metric based on the one or more EM sensor signals. The updated value of the at least one metric may correspond to a second time after the first time. The system may then compare the updated value of the metric to the baseline value of the metric. When the difference between the updated value and the baseline value of the metric is greater than a threshold value, the system may determine that at least one of the patient and the field generator has moved during a time period that includes the first time and the second time. 
     Once the system has determined that one of the patient and the EM field generator  110  has moved, the system may determine whether one of the patient or the EM field generator  110  has changed its pose (e.g., has moved to a new position). For example, in response to determining that at least one of the patient and the field generator has moved, the system may calculate a frequency value of the at least one metric based on the one or more EM sensor signals corresponding to a frequency of a change in positioning of the EM sensor at a third time, subsequent to the second time. The system may then compare the frequency value to the threshold frequency value. When the frequency value is greater than the threshold frequency value, the system may determine that at least one of the patient and the field generator has changed its pose. 
     The EM tracking system may also determine whether one of the patient and the EM field generator  110  receives an impulse force and returns to an initial state. For example, the system may, in response to determining that at least one of the patient and the field generator has moved, calculate a subsequent value of the at least one metric based on the one or more EM sensor signals. The subsequent value of the at least one metric may correspond to a positioning of the EM sensor at a third time, subsequent to the second time. The system may then determine that the field generator received an impulse force and returned to an initial state after receiving the impulse force, in response to the subsequent value being within an error threshold of the baseline value. 
     Prior to selecting the third time for calculating the subsequent value, the system may determine that an interval value of the at least one metric has stabilized for an interval of time prior to the third time and select the third time in response to determining that the interval value of the at least one metric has stabilized. Thus, the system may determine that the patient or the EM field generator  110  has settled at a final pose before determining whether the patient or EM field generator  110  has moved to a new pose or has settled to its initial pose. 
     In one implementation, the system may determine that the pose of the patient or the EM field generator  110  has stabilized based on the maximum and minimum values of the at least one metric during the interval of time. For example, the system may calculate a maximum value and a minimum value of the at least one metric during the interval of time, calculate the difference between the maximum and minimum values of the at least one metric, and determine that that the interval value of the at least one metric has stabilized for the interval of time in response to the difference between the maximum and minimum values of the at least one metric being less than a threshold difference value. When changes to the at least one metric are determined to be less than the threshold difference value, the system may determine that the changes in the metric are due to noise and not oscillation of the patient or the EM field generator  110 . 
     In another example, the system may calculate a subsequent value of the at least one metric based on the one or more EM sensor signals in response to determining that at least one of the patient and the field generator has moved. The subsequent value of the at least one metric may correspond to a positioning of the EM sensor at a third time, subsequent to the second time. The system may then determine that at least one of the patient and the field generator has changed its pose in response to the subsequent value being outside an error threshold of the baseline value. For example, as discussed above, the metric may be the absolute position or absolute angle of one or more of the EM patch sensors  105 . If the baseline value for the absolute difference or absolute angle changes and is stable at a new value, this is indicative of at least one of the patient and the EM field generator  110  being moved and settling at a new position. 
       FIG. 23  provides a flowchart illustrating an example methodology of determining that one of a patient and an EM field generator has moved. The method  2300  begins at block  2301 . At block  2305 , the system determines baseline metrics for each of the calculated metrics. This may include retrieving baseline values for the metrics from memory or calculating baseline metrics based on EM sensor signals received from EM patch sensors  105 . At block  2310 , the system determines whether the baseline metric quality is greater than a threshold quality. When the baseline metric quality is not greater than the threshold quality, the method  2300  ends and the method  2300  may repeat by attempting to collect better quality baseline metrics. 
     When the baseline metric quality is greater than the threshold quality, the method  2230  continues at block  2315 , where the system calculates a number of metrics. In one example, the system calculates an absolute difference metric, an absolute angle metric, and a 6DoF patch space metric. At block  2320 , the system analyzes the calculated metrics that have been stored over a window of time, including determining the standard deviation of each of the metrics. At block  2325 , the system determines whether the analyzed metrics are indicative of at least one of the patient and the EM field generator being moved or at least one of the patient and the EM field generator receiving an impulse force. This may include comparing each of the metrics against a corresponding threshold value and comparing the standard deviations against corresponding threshold values. When a quorum or some threshold number of the comparisons are indicative of at least one of the patient and the EM field generator being moved, the method continues at block  2330 . When a quorum or some threshold number of the comparisons are indicative of at least one of the patient and the EM field generator receiving an impulse force, the method  2300  continues at block  2335 . 
     At block  2330 , in response to determining that the metrics are indicative of at least one of the patient and the EM field generator being moved, the system may set an EM distortion flag and/or a movement flag. At block  2330 , in response to determining that the metrics are indicative of at least one of the patient and the EM field generator receiving an impulse force, the system may set an EM distortion flag and/or an impulse force flag. The method ends at block  2235 . It is to be appreciated that the system may perform a number of action in response to detecting movement of the EM field generator. Some exemplary responses are described below. 
     D. Responses to Detection of EM Distortion 
     The EM tracking system may perform one or more of a number of techniques in response to detection EM distortion. The specific technique performed may depend on one or more of: the type of EM distortion detected (e.g., local or global EM distortion, distortion due to movement, etc.), the magnitude of the EM distortion, the location of the EM distortion, etc. 
     In one implementation, the system may refrain from using or otherwise limit the weight given to EM data in navigation and/or localization of an instrument. When refraining from using EM data, the navigation and/or localization performed by the system may rely on other types of data during EM distortion. Specifically, in one embodiment, the system may detect that the EM distortion flag has been set and then as a consequence of the EM distortion flag being set, refrain from or otherwise limit the weight given to determining the position of the distal end of an instrument based on EM sensor signals by lowering a confidence value (or any other suitable weighting) corresponding to an EM location based algorithm. The use of confidence values and weighting to different location algorithms is discussed in U.S. patent application Ser. No. 15/268,238, filed on Sep. 16, 2016, the contents of which are herein incorporated in its entirety. 
     In some implementations, in response to determining that the EM field is distorted, the system may calculate the amount of distortion. The amount of EM field distortion may be proportional to the change in one or more of the calculated metrics. In this implementation, the system may calculate an amount of the distortion in the EM field based on one or more updated values calculated at a second time and one or more baseline values calculated a first time prior to the second time. The system may encode an indication of the amount of the distortion and provide the encoded indication of the amount of distortion to a display configured to render encoded data. Accordingly, the user may be notified of the amount of the EM field distortion. The user may then be able to determine whether to use navigation based on EM data during the surgical procedure. 
     In certain embodiments, the system may use the amount of distortion to alter the weight of the EM data used in the navigation and/or localization techniques. As the EM distortion increases, the system may assign a lower weight to the EM data when generating location data  96  for the distal tip of a medical instrument. 
     The system may also be able to determine an area in which the distortion in the EM field is greater than a threshold distortion value. For example, the relative distance metrics may be used to determine that the area surrounding one of the EM patch sensors is experiencing EM distortion. That is, if the relative distance between EM patch sensor P 1  and each of EM patch sensors P 0  and P 2  has changed by more than a threshold value, but the relative distance between EM patch sensors P 0  and P 2  is substantially unchanged, the system may determine that the EM field in the area near EM patch sensor P 1  has been distorted. 
     In response to determining that the EM field near one of the EM patch sensors  105  has been distorted, the system may adjust (e.g., reduce) a weight applied to EM data received from the identified EM patch sensor  105 . The system may also indicate to the user the area in which EM field distortion is occurring. The user may then be able to determine whether to continue with navigation using EM data based on whether the target site is within the distorted area. Alternatively, the system may automatically determine whether to continue using EM data for navigation based on the current location of the instrument with respect to the EM distorted area. 
     In certain embodiments the system may also access a model representative of a luminal network of the patient and calculate a mapping between a coordinate frame of the EM field and a coordinate frame of the model based on at least one of: (i) the one or more baseline values and (ii) the one or more updated values. The system may further refrain from using the one or more updated values in calculating the mapping in response to determining that the EM field has been distorted. 
     E. Alignment. 
     Prior to performing a surgical procedure that uses EM data for navigation and/or localization, it is desirable to align the patient with the EM field generator  110 . More precisely, it is desirable to align the EM field generator  110  with an anatomical feature of the patient on which the surgical procedure is to be performed. One advantage to performing such an alignment procedure is that the EM field generator  110  may have a working volume in which EM sensors are able to more accurately measure the EM field. That is, when one or more of the EM sensors are outside of the working volume, the EM sensor signals generated by the EM sensors may not be sufficiently reliable for navigation and/or localization, respiration tracking, and/or EM distortion detection. 
     As discussed above, a number of EM patch sensors  105  may be placed on the patient at prescribed locations which surround, or at least partially overlap, an area of interest. The area of interest may be an anatomical feature of the patient on which the surgical procedure is to be performed. One example of an anatomical feature is a luminal network, such as luminal network  140 . The EM tracking system may provide guidance to the user on where to position the EM patch sensors  105  on the patient and where to position the EM field generator  110  such that the EM patch sensors  105  are within a working volume of the EM field generator  110 . When the EM patch sensors  105  are appropriately positioned, the positioning of the EM patch sensor&#39;s within the working volume may guarantee that the patient&#39;s area of interest is aligned with the EM field generator  110 . 
     An example procedure for aligning the EM field generator  110  with a patient will be described in connection with a bronchoscopy procedure. However, this procedure may be modified for any type of robotic-assisted surgical procedure in which EM data is used for navigation and/or localization. 
     Initially, the user may position one or more EM patch sensors  105  on the patient. For bronchoscopy, the user places the EM patch sensors  105  to surround, or at least partially overlap, the area of interest (e.g., the patient&#39;s lungs). When the system includes three EM patch sensors  105 , the user may place a first EM patch sensor on the patient&#39;s mid sternum, a second EM patch sensor on the patient&#39;s left lateral 8 th  rib, and a third EM patch sensor on the patient&#39;s right lateral 8 th  rib. The above-described placement of the EM patch sensors  105  is merely exemplary, and the EM patch sensors  105  may be placed in other locations that overlap the area of interest. 
       FIG. 24  provides an example in which EM patch sensors  105  are placed within a working volume of an EM field generator. The EM field generator  110  and the EM patch sensors  105  may be aligned with respect to each other such that the EM patch sensors  105  can generate a signal indicative of the position of the EM patch sensors  105  with respect to the EM field generator  110 . The alignment may involve the user positioning of one or more of the field generator  105 , the EM patch sensors  105 , and/or the table such that at least some of the EM patch sensors  105  are positioned with a working volume of the EM field generator  110 . In certain embodiments, after the EM patch sensors  105  have been placed, the user may position the EM field generator  110  such that the EM patch sensors  105  are located within the working volume  400  of the EM field generator  110 . Although  FIG. 24  illustrates a working volume  400  when viewed from above, the working volume  400  may define a three-dimensional volume in which the EM patch sensors  105  are to be placed during alignment. 
     The user may attach the EM field generator  110  to a holder, which may be attached to a bed rail. Using guidance provided by the EM tracking system, the user may rotate the EM field generator  110  such that all of the EM patch sensors  105  are located within the working volume  400 . In order to provide feedback via a display (e.g., via the touchscreen  26 ), the EM tracking system may determine a position of the EM patch sensors  105  with respect to the EM field generator  110  based one or more EM patch sensor signals generated by the EM patch sensors  105 . The system may encode a representation of the position of the EM patch sensors  105  with respect to the working volume of the EM field. Encoding of the representation of the position of the EM patch sensors  105  may include generating an image (or series of images to form a video) in which the relative position of the EM patch sensors  105  is displayed with respect to a representation of the working volume. The encoding may further include encoding the image (or video) using an image or video codec such that the image can be decoded and rendered by a display. The system may then provide the encoded representation of the position to a display configured to render encoded data. 
     The user may use the visual feedback provided by the display in rotating the EM field generator  110  such that the EM patch sensors  105  are positioned within the working volume. Once the EM patch sensors  105  are rotationally aligned with the EM field generator  110 , the user may position the field generator closer to the EM patch sensors  105  such that the EM patch sensors  105  are within a predefined distance from the EM field generator  110  as defined by the visually displayed working volume. With reference to  FIG. 24 , the working volume may include a plurality of sub-volumes, which may define preferred  405 , acceptable,  410 , and at risk  415  sub-volumes. Since the strength of the EM field may decay at greater distanced from the EM field generator  110 , it may be desirable to position the EM patch sensors  105  within the preferred  405  or acceptable  410  sub-volumes over the at risk  415  sub-volume. 
       FIGS. 25A-25D  illustrate examples of visual feedback which may be provided to a user by the display during a setup and alignment procedure for EM field generator(s) and/or EM patch sensor(s) for a medical procedure in accordance with aspects of this disclosure. In particular,  FIG. 25A  includes an example view  2500  including instructions  2505  for setting up an EM field generator  2501  (e.g., the EM field generator  110  of  FIG. 16 ) and instructions  2510  for setting up one or more patient sensors  2507  (e.g., one or more EM patch sensors). As shown in the view  2500 , the instructions  2505  may involve plugging the EM field generator  2501  into a tower  2503  (e.g., the movable tower  30  of  FIG. 1 ). The instructions  2510  may involve plugging the patient sensors  2507  into the tower  2503 . In addition to the instructions, the view  2500  may include graphical representations of the locations in the tower  2503  in which the EM field generator  2501  and the patient sensors  2507  can be plugged into the tower  2503 . As shown in the example of  FIG. 25A , the instructions  2505 ,  2510  may include both graphical/illustrative component and a textual component; however, in other examples the instructions may be one of the graphical/illustrative component and the textual component, and may include (in addition to or in lieu of the graphical and/or textual components) audible instructions. 
     In the view  2515  illustrated in  FIG. 25B , the procedure may continue with instructions  2520  and  2525  to place the patient sensors  2507  on the patient. In the illustrated example, the patient sensors  2507  may include a first sensor marked “M,” a second sensor marked “L,” and a third sensor marked “R.” The instructions  2520  included in the view  2515  may include instructions  2520  to place the patient sensor marked “M” at, e.g., the patient&#39;s sternal notch, and instructions  2525  to place the patient sensors marked “L” and “R” along, e.g., the mid-auxiliary line located at the 8 th  rib on the patient&#39;s left and right sides, respectively. The view  2515  may further include accompanying illustrations which show the approximate locations to place the patient sensors  2507  on a reference body  2521 . 
     In  FIG. 25C , the view  2530  may include instructions  2535  to the user to position the EM field generator  2501 , e.g., adjacent to the patient&#39;s mid-chest on the same side of the body as a target site. The view  2530  may include an illustration of the location at which the EM field generator  2501  should be placed with respect to the reference body  2521  representing the patient. The illustration may further include an indication of the working volume  2540  of the EM field generator  2501 . 
       FIG. 25D  illustrates a view  2545  which may include real-time information or feedback regarding the locations of the patient sensors  2507 , e.g., with respect to a working volume  2540  of the EM field generator  2501 . The view  2545  of  FIG. 25D  may be one implementation of the example described above in connection with  FIG. 24 . The system may determine the position of each of the patient sensors marked “M,” L,” and “R” and display the positions of the sensors with respect to a top view  2541  of the working volume  2540  and a lateral view  2543  of the working volume  2540 . In the view  2545 , the sensor marked “M” may be located within a preferred sub-volume within the working volume  2540  (as viewed from both the top and lateral views  2541 ,  2543 ), while the sensor marked “R” may be located within an acceptable sub-volume within the working volume  2540 . The sensor marked “L” may be located outside of the top view  2541  of the working volume  240 , and may be too far from the lateral view  2543  of the working volume  2540  to be displayed, which is indicative of the need to reposition the sensors marked “L” and/or “R.” This way, the user may use the information in the view  2545  as feedback to adjust the position of the sensor “L” such that the sensor “L” falls within at least the acceptable sub-volume within the working volume  2540 . 
     In at least one implementation, the system may encode the representation of the position of the EM patch sensors  105  with respect to each of first and second sub-volumes of the field generator. The second sub-volume larger than and enveloping the first sub-volume, and thus, in at least one implementation, the second sub-volume may be an at-risk  415  sub-volume. The system may provide the encoded representation of the position of the EM patch sensors  105  with respect to each of the first and second sub-volumes to the display so that the user can reposition the EM patch sensors  105  within the first sub-volume by moving the EM field generator  110 . 
     In other implementations, the first and second sub-volumes may correspond to the preferred  405  and acceptable  410  sub-volumes. In these implementations, the system may encode user instructions to the user to position the EM field generator  110  such that the EM patch sensors  105  is positioned within at least one of the first and second sub-volumes and provide the encoded user instructions to the display. 
     The user may repeat the rotation of the EM field generator  110  and adjusting the distance of the EM field generator  110  until all of the EM patch sensors  105  are within the working volume. Thereafter, the user may lock the position of the EM field generator  110  in preparation for the surgical procedure. 
     In certain implementations, it may not be possible to place all of the EM patch sensors  105  within the working volume. For example, the EM field generator  110  may not produce a large enough working volume to encompass all of the EM patch sensors  105  for patients having a large area of interest. In these implementations, the system may encode user instructions to position the field generator such that a defined number of the EM sensors are positioned within the first working volume and provide the encoded user instructions to the display. For example, when three EM patch sensors  105  are used, the system may encode instructions to the user such that at least two of the EM patch sensors  105  are positioned within the working volume. 
     In one implementation, the system may encode user instructions to position: (i) a first one of the EM sensors on the patient&#39;s mid sternum, (ii) a second one of the EM sensors on the patient&#39;s left lateral eighth rib, and (iii) a third one of the EM sensors on the patient&#39;s left lateral eighth rib. Thus, prior to positioning of the EM field generation  110 , the system may provide the user with instructions for placement of the EM patch sensors  105 . The system may provide the encoded user instructions to position the first to third EM sensors on the patient to the display. 
     In another implementation, the system may be configured to receive input from the user that one of the second and third EM sensors cannot be positioned with the working volume, for example, via the touchscreen  26 . In response, the system may encode user instructions to reposition the one of the second and third EM sensors closer to the field generator than the one of the second and third EM sensors&#39; current position. For example, the instruction may encode instructions to reposition the second EM patch sensor on the patient&#39;s 6 th  left lateral rib. The system may provide the encoded user instructions to reposition the one of the second and third EM sensors to the display. 
     It is to be appreciated that some embodiments of the systems described above relating to the technical features for aligning the field generator with the patient anatomy can have a number of advantages. For example, providing feedback to the user on the placement and alignment of the field generator can simplify the setup of the system. Such a simplified setup can avoid user frustration in whether the system is properly aligned. Still further, feedback of the alignment may produce more accurate reading and, as a result, provide better input to the navigation and/or localization systems. 
     E. EM Tracking System and Example Flowcharts. 
       FIG. 26  depicts a block diagram illustrating an example of the EM tracking system which may perform various aspects of this disclosure. The EM tracking system  2600  may include one or more EM sensor(s)  2603 , a processor  2610 , and a memory  2615 . The one or more EM sensor(s)  2603  may be embodied as the EM patch sensors  105  and/or the EM instrument sensor(s)  305 . The EM tracking system  2600  may be incorporated into one or more of the tower  30 , the console  16 , the EM field generator  110 , and/or any other component within the environment  100 . Additionally, the EM tracking system  2600  may be configured to perform one or more of the methods and/or techniques described above in connection with  FIGS. 20-24  or described below in connection with  FIGS. 27 and 28 . 
       FIG. 27  is a flowchart illustrating an example method operable by an EM tracking system  2600 , or component(s) thereof, for detecting EM distortion in accordance with aspects of this disclosure. For example, the steps of method  2700  illustrated in  FIG. 27  may be performed by a processor  2610  of the EM tracking system  2600 . For convenience, the method  2700  is described as performed by the processor  2610  of the EM tracking system  2600 . 
     The method  2700  begins at block  2701 . At block  2705 , the processor  2610  calculates one or more baseline values of one or more metrics indicative of a position of a first EM sensor at a first time. The calculation of the one or more baseline values may be based on EM sensor signals received from a first set of one or more EM sensor signals corresponding to the first time. Additionally, the first EM sensor may be configured to generate the first set of one or more EM sensor signals in response to detection of an EM field. At block  2710 , the processor  2610  calculates one or more updated values of the one or more metrics during a time period after the first time. The calculation of the one or more updated values may be based on EM sensor signals from the first set of one or more EM sensor signals corresponding to the time period after the first time. 
     At block  2715 , the processor  2610  determines that a difference between the one or more updated values and the one or more baseline values is greater than a threshold value. At block  2720 , the processor  2610  determines that the EM field has been distorted in response to the difference being greater than the threshold value. The method  2700  ends at block  2725 . 
       FIG. 28  is a flowchart illustrating another example method operable by an EM tracking system  2600 , or component(s) thereof, for detecting EM distortion in accordance with aspects of this disclosure. For example, the steps of method  2800  illustrated in  FIG. 28  may be performed by a processor  2610  of the EM tracking system  2600 . For convenience, the method  2800  is described as performed by the processor  2610  of the EM tracking system  2600 . 
     The method  2800  begins at block  2801 . At block  2805 , the processor  2610  calculates one or more baseline values of one or more metrics indicative of a velocity of a distal end of an instrument at a first time. The calculation of the one or more baseline values may be based on EM sensor signals received from one or more EM sensor signals corresponding to the first time. The instrument may include an EM sensor located at the distal end of the instrument. The EM sensor may be configured to generate the one or more EM sensor signals in response to detection of an EM field. 
     At block  2810 , the processor  2610  calculates one or more updated values of the one or more metrics during a time period after the first time. The calculation of the one or more updated values may be based on EM sensor signals from the one or more EM sensor signals corresponding to the time period after the first time. At block  2815 , the processor  2610  determines that a difference between the one or more updated values and the one or more baseline values is greater than a threshold value. At block  2820 , the processor  2610  determines that the EM field has been distorted in response to the difference being greater than the threshold value. The method  2800  ends at block  2825 . 
       FIG. 29  is a flowchart illustrating yet another example method operable by an EM tracking system  2600 , or component(s) thereof, for facilitating the positioning of an EM sensor within an EM field generated by a field generator in accordance with aspects of this disclosure. For example, the steps of method  2900  illustrated in  FIG. 29  may be performed by a processor  2610  of the EM tracking system  2600 . For convenience, the method  2900  is described as performed by the processor  2610  of the EM tracking system  2600 . 
     The method  2900  begins at block  2901 . At block  2905 , the processor  2610  determines a position of the EM sensor with respect to the field generator based on one or more EM sensor signals. The EM sensor may be configured to generate, when positioned in a working volume of the EM field, the one or more EM sensor signals based on detection of the EM field. Additionally, the EM sensor may be configured for placement on a patient. At block  2910 , the processor  2610  encodes a representation of the position of the EM sensor with respect to the working volume of the EM field. At block  2915 , the processor  2610  provides the encoded representation of the position to a display configured to render encoded data. The method  2900  ends at block  2920 . 
       FIG. 30  is a flowchart illustrating still yet another example method operable by an EM tracking system  2600 , or component(s) thereof, for detecting movement of at least one of a patient or an EM field generator in accordance with aspects of this disclosure. For example, the steps of method  3000  illustrated in  FIG. 30  may be performed by a processor  2610  of the EM tracking system  2600 . For convenience, the method  3000  is described as performed by the processor  2610  of the EM tracking system  2600 . 
     The method  3000  begins at block  3001 . At block  3005 , the processor  2610  calculates a baseline value of at least one metric based on one or more EM sensor signals generated by an EM sensor. The baseline value of the at least one metric may correspond to a positioning of the EM sensor at a first time. The EM sensor may be configured to generate the one or more EM sensor signals in response to detection of an EM field. The EM sensor may be configured for placement on a patient. At block  3010 , the processor  2610  calculates an updated value of the at least one metric based on the one or more EM sensor signals. The updated value of the at least one metric may correspond to a positioning of the EM sensor at a second time. At block  3015 , the processor  2610  determines, based on the baseline value and the updated value, that at least one of the patient and the field generator has moved during a time period that includes the first time and the second time. The method  3000  ends at block  3020 . 
     Implementing Systems and Terminology 
     Implementations disclosed herein provide systems, methods and apparatus for detection EM distortion. 
     It should be noted that the terms “couple,” “coupling,” “coupled” or other variations of the word couple as used herein may indicate either an indirect connection or a direct connection. For example, if a first component is “coupled” to a second component, the first component may be either indirectly connected to the second component via another component or directly connected to the second component. 
     The robotic motion actuation functions described herein may be stored as one or more instructions on a processor-readable or computer-readable medium. The term “computer-readable medium” refers to any available medium that can be accessed by a computer or processor. By way of example, and not limitation, such a medium may comprise random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory, compact disc read-only memory (CD-ROM) or other optical disk storage, magnetic disk storage or other magnetic storage devices, 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. It should be noted that a computer-readable medium may be tangible and non-transitory. As used herein, the term “code” may refer to software, instructions, code or data that is/are executable by a computing device or processor. 
     The methods disclosed herein comprise one or more steps or actions for achieving the described method. The method steps and/or actions may be interchanged with one another without departing from the scope of the claims. In other words, unless a specific order of steps or actions is required for proper operation of the method that is being described, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the claims. 
     As used herein, the term “plurality” denotes two or more. For example, a plurality of components indicates two or more components. The term “determining” encompasses a wide variety of actions and, therefore, “determining” can include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining and the like. Also, “determining” can include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory) and the like. Also, “determining” can include resolving, selecting, choosing, establishing and the like. 
     The phrase “based on” does not mean “based only on,” unless expressly specified otherwise. In other words, the phrase “based on” describes both “based only on” and “based at least on.” 
     The previous description of the disclosed implementations is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these implementations will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the scope of the invention. For example, it will be appreciated that one of ordinary skill in the art will be able to employ a number corresponding alternative and equivalent structural details, such as equivalent ways of fastening, mounting, coupling, or engaging tool components, equivalent mechanisms for producing particular actuation motions, and equivalent mechanisms for delivering electrical energy. Thus, the present invention is not intended to be limited to the implementations shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.