Patent Publication Number: US-2020275860-A1

Title: Electromagnetic sensor with probe and guide sensing elements

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent document claims the priority of U.S. provisional Pat. App. No. 61/646,619, filed May 14, 2012, and is a continuation-in-part and claims benefit of the earlier filing date of U.S. patent application Ser. No. 13/274,237, filed Oct. 14, 2011, both of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Minimally invasive medical devices that navigate natural body lumens need to be small enough to fit within the lumens. Lung catheters, for example, which may be used to perform minimally invasive lung biopsies or other medical procedures, may need to follow airways that decrease in size as the catheter navigates branching passages. To reach a target location in a lung, a catheter may follow passages having diameters as small as 3 mm or less. Manufacturing a catheter that is sufficiently small and includes the mechanical structures and sensors for remote or robotic operation can be challenging. 
     Electromagnetic sensors (EM) sensors can measure the position and orientation of a portion of a medical instrument. EM sensors are particularly suitable for minimally invasive medical instruments because EM sensors can combine high global accuracy with a small diameter package size. During EM sensor operation, a generator external to a patient can produce a well-controlled, time-varying magnetic field, and in response, one or more coils of an EM sensor in or on a portion of the medical instrument produce induced electrical signals. In particular, time variations in the magnetic field induce currents in the coils of the EM sensor, and the pose of each coil can be partially determined from knowledge of the generated magnetic field and the geometry of the coil. A single coil can be used, for example, to measure a position and a pointing direction, e.g., pitch and yaw angles, but a cylindrically symmetrical coil is unable to distinguish roll angles about the symmetry axis of the coil. Accordingly, EM sensors employing a single cylindrical coil have been used as 5-Degree-of-Freedom (5-DoF) sensors. To additionally measure the roll angle, a 6-DoF EM sensor generally requires two coils having symmetry axes that are not parallel, e.g., perpendicular symmetry axes. 
     The long, thin shape typical of 5-DoF EM sensors fits well with minimally invasive medical instruments or tools, which often have long and thin extensions. However, with the central axis of a single coil sensor aligned with the roll axis of an instrument, such 5-DoF EM sensors cannot measure the roll angle of the instrument. While some symmetric instruments such as needles may not require roll angle measurements, many instruments require knowledge of the roll angle of the instrument, particularly for robotic control. Measurement of the roll angle may require a 6-DoF sensor that includes two coils. For example, to create a 6-DoF EM sensor, two 5-DoF EM sensors may need to be placed perpendicular or at a non-zero angle to each other, which creates a much larger sensor package. If each 5-DoF sensor has a cylindrical shape about 1 mm in diameter and about 10 mm long, the 6-Dof sensor containing two 5-DoF sensors may be up to 10×10×1 mm. While the 1 mm diameter of a 5-DoF EM sensor may fit within a small, e.g., 3 mm diameter, instrument, a 10-mm wide 6-DoF EM sensor may not fit in a small instrument. 
     SUMMARY 
     In accordance with an aspect of the invention, a small diameter EM sensor can include a coil with windings that define areas with a normal direction at a significant angle to the symmetry or long axis of the coil. As a result, the magnetic axis of an EM sensor that extends along a length of an instrument may be at an angle to the roll axis of the instrument to enable the sensor to measure a roll angle of the instrument, while still providing a narrow diameter package. 
     In one specific embodiment, a sensing system uses a coil including wire that is wound in loops around an axis, and each of the loops defines an area that has a normal direction at a non-zero, angle relative to the axis of the coil. 
     In another embodiment, a sensing system includes a coil and sensor logic. The coil includes wire that is wound in loops about an axis, and the loops define respective areas that have a normal direction at a non-zero angle relative to the axis of the coil. The sensor logic is coupled to the coil and configured to use an electrical signal induced in the coil in determining a measurement of a roll angle about the axis of the coil. 
     In yet another embodiment, a medical system includes a probe and optionally a guide instrument (e.g., catheter, bronchoscope, or endoscope) with a lumen sized for guiding the probe. A probe coil is in the probe and includes wire that is wound in loops collectively defining a first core that extends in a lengthwise direction of the probe. However, each of the loops in the probe coil defines an area that has a normal direction at a non-zero angle relative to the length of the probe. A secondary sensor (e.g., an electromagnetic sensor, shape sensor, gravity sensor, visualization sensor, and/or angular sensor(s), among others) included in the medical system can provide supplemental orientation information to be used with the probe coil signals to determine a roll angle of the probe. For example, a secondary sensor such as a coil could be positioned in a wall of a guide instrument for the probe, such that each of the loops of the guide instrument coil defines an area that has a second normal direction. Sensor logic that is coupled to receive induced signals from the probe coil and the guide instrument coil can then determine a roll angle of the probe from the induced signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a minimally invasive medical instrument that uses an electromagnetic sensor that includes an off-axis coil. 
         FIG. 2  shows an embodiment of a steerable segment that can be employed in the system of  FIG. 1 . 
         FIG. 3  shows sensing coils that can be employed in electromagnetic sensors in medical systems in some embodiments of the invention. 
         FIG. 4  shows a cross-section of an off-axis coil that can be used in an electromagnetic sensor. 
         FIG. 5  illustrates the geometry of one embodiment of an electromagnetic sensing system that uses an off-axis coil in a magnetic field for measurements of five degrees of freedom. 
         FIGS. 6A and 6B  show alternative configurations of electromagnetic sensor systems using at least one off-axis coil for measurement of six degrees of freedom. 
         FIG. 7  shows a medical system capable of using a coil in a probe and a secondary sensor to measure six degrees of freedom including a roll angle of the probe. 
     
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION 
     An EM sensor can employ an off-axis coil, which is a coil wound so that areas respectively defined by individual loops have a normal direction that is off-axis from the length of the coil. As a result, an effective area for magnetic flux in the off-axis coil has a normal direction that is also off-axis from the length of the coil. A magnetic field applied to an off-axis coil can be varied to induce an electrical signal that depends on the normal direction instead of about the long axis of the coil. Such a coil can thus be used in a small-diameter medical instrument to measure a roll angle about the long axis of the coil. 
       FIG. 1  schematically illustrates a medical system  100  in accordance with one embodiment of the invention. In the illustrated embodiment, medical system  100  includes a medical device  110 , a drive interface  120 , a control system  140 , an operator interface  150 , and a field generator  160  for a sensing system. 
     Medical device  110 , in the illustrated embodiment, may be a flexible device such as a catheter, bronchoscope, endoscope, or cannula that includes a main shaft  112  with one or more lumens. For example, main shaft  112  may include a main lumen sized to accommodate interchangeable probes. Such probes can include a variety of a camera or vision systems, biopsy tools, cutting tools, clamping tools, sealing tools, suturing tools, stapling tools, cautery tools, therapeutic or diagnostic material delivery tools, or any other surgical instruments. The probes used in device  110  may be robotically operated, for example, using actuating tendons (not shown) that run the length of the probe. Additionally, main shaft  112  may incorporate one or more steerable sections  114  that are similarly operable using actuating tendons that attach to steerable section  114  and run from steerable section at the distal end of main shaft  112 , through main shaft  112 , to the proximal end of main shaft  112 . 
     An exemplary embodiment of device  110  may be a lung catheter, bronchoscope or endoscope, where device  110  would typically be about 60 to 80 cm or longer. During a medical procedure, at least a portion of main shaft  112  and all of steerable section  114  may be inserted along a natural lumen such as an airway of a patient, and drive interface  120  may operate steerable section  114  by pulling on actuating tendons, e.g., to steer device  110  during insertion or to position steerable section  114  for a procedure. 
     Steerable section  114  is remotely controllable and particularly has a pitch and a yaw that can be controlled using actuating tendons, e.g., pull wires or cables, and may be implemented as a multi-lumen tube of flexible material such as Pebax. In general, steerable section  114  may be more flexible than the remainder of main tube  112 , which assists in isolating actuation or bending to steerable section  114  when drive interface  120  pulls on the actuating tendons. Device  110  can also employ additional features or structures such as use of Bowden cables for actuating tendons to prevent actuation from bending the more proximal portion of main tube  112 . In general, the actuating tendons are located at different angles about a roll axis  170  of steerable section  114 . For example,  FIG. 2  shows one specific embodiment in which steerable section  114  is made from a tube  210  that may be cut to create flexures  220 . Tube  210  in the illustrated embodiment defines a main lumen for a probe system and smaller lumens for actuating tendons  230 . In the illustrated embodiment, four actuating tendons  230  attach to a distal tip  215  of steerable section  114  at locations that are 90° apart around a roll axis  170  of steerable section  114 . In operation, pulling harder on any one of tendons  230  tends to cause steerable section  114  to bend in the direction of that tendon  230 . To accommodate repeated bending, tube  210  may be made of a material such as Nitinol, which is a metal alloy that can be repeatedly bent with little or no damage. 
     Actuating tendons  230  extend back through main tube  112  to drive interface  120  and may be coated or uncoated, single filament or multi strand wires, cables, Bowden cables, hypotubes, or any other structures that are able to transfer force from drive interface  120  to distal tip  215 . (Push rods could conceivably be used in device  110  instead of tendons  230  but may not provide a desirable level of flexibility needed in some medical instruments.) Tendons  230  can be made of any material of sufficient strength including but not limited to a metal such as steel or a polymer such as Kevlar. 
     Drive interface  120  of  FIG. 1 , which pulls on actuating tendons  230  to operate steerable section  114 , includes a mechanical system or transmission  124  that converts the movement of actuators  122 , e.g., electric motors, into movements of (or tensions in) actuating tendons  230 . The movement and pose of steerable section  114  can thus be controlled through selection of drive signals for actuators  122  in drive interface  120 . In addition to manipulating the actuating tendons, drive interface  120  may also be able to control other movement of device  110  such as a range of motion in an insertion direction and rotation or roll of the proximal end of device  110 , which may also be powered through actuators  122  and transmission  124 . Backend mechanisms or transmissions that are known for flexible-shaft instruments could in general be used or modified for drive interface  120 . 
     A dock  126  in drive interface  120  of  FIG. 1  can provide a mechanical coupling between drive interface  120  and device  110  and link the actuating tendons  230  to transmission  124 . Dock  126  may additionally contain an electronic or optical system for receiving, converting, and/or relaying sensor signals from one or more EM sensors  116  and contain an electronic or mechanical system for identifying the specific probe or the type of probe deployed in device  110 . 
     Control system  140  controls actuators  122  in drive interface  120  to selectively pull on the actuating tendons as needed to actuate or steer steerable section  114 . In general, control system  140  operates in response to commands from a user, e.g., a surgeon or other medical personnel using operator interface  150 , and in response to measurement signals such as from EM sensors  116 . Control system  140  may in particular include or execute sensor logic that analyzes signals (or digitized versions signals) from EM sensors  116  to determine measurement of the position and orientation of the distal end of device  110 . Control system  140  may be implemented using a general purpose computer with suitable software, firmware, and/or interface hardware to interpret signals from operator interface  150  and EM sensors  116  and to generate control signals for drive interface  120 . 
     Operator interface  150  may include standard input/output hardware such as a display, a keyboard, a mouse, a joystick, or other pointing device or similar I/O hardware that may be customized or optimized for a surgical environment. In general, operator interface  150  provides information to the user and receives instructions from the user. For example, operator interface  150  may indicate the status of system  100  and provide the user with data including images and measurements made by system  100 . One type of instruction that the user may provide through operator interface  150 , e.g., using a joystick or similar controller, indicates the desired movement or position of steerable section  114 , and using such input, control system  140  can generate control signals for actuators in drive interface  120 . 
     Field generator  160  and one or more EM sensors  116  can be used to measure a pose of a distal portion of main tube  112  or of steerable section  114 . EM sensors  116  may particularly include an off-axis coil that field generator  160  may subject to a magnetic field that varies over space or time. The magnetic field produces magnetic flux through EM sensors  116 , and variation in time of that magnetic flux induces a voltage or electric current in EM sensors  116 . 
     The induced signals can be used to measure the pose of EM sensor  116 . For example, U.S. Pat. No. 7,197,354, entitled “System for Determining the Position and Orientation of a Catheter”; U.S. Pat. No. 6,833,814, entitled “Intrabody Navigation System for Medical Applications”; and U.S. Pat. No. 6,188,355, entitled “Wireless Six-Degree-of-Freedom Locator” describe the operation of some EM sensor systems and are hereby incorporated by reference in their entirety. U.S. Pat. No. 7,398,116, entitled “Methods, Apparatuses, and Systems useful in Conducting Image Guided Interventions,” U.S. Pat. No. 7,920,909, entitled “Apparatus and Method for Automatic Image Guided Accuracy Verification,” U.S. Pat. No. 7,853,307, entitled “Methods, Apparatuses, and Systems Useful in Conducting Image Guided Interventions,” and U.S. Pat. No. 7,962,193, entitled “Apparatus and Method for Image Guided Accuracy Verification” further describe systems and methods that can use electromagnetic sensing coils in guiding medical procedures and are also incorporated by reference in their entirety. 
       FIG. 3  illustrates three different types of sensing coils  310 ,  320 , and  330  that could be used in an EM sensor. Coil  310  is a helical coil containing individual loops defining areas that are substantially perpendicular to a lengthwise axis  312  of coil  310 . A field generator can vary the direction and magnitude of the magnetic field in a systematic manner that enables at least partial determination of the pose of coil  310  from the induced electric signal. In particular, up to five degrees of freedom can be measured using sensing coil  310 . However, sensing coil  310  is cylindrically symmetric, so that a roll angle, i.e., an angle indicating orientation about axis  312  of coil  310 , cannot be determined from an electric signal induced in coil  310 . However, the position and the pointing direction of coil  310  can be determined from the induced electrical signal and knowledge of the generated magnetic field. Accordingly, coil  310  can be used for a 5-DoF sensor that measures position X, Y, and Z and pointing angles θ and φ, but a 5-DoF sensor using coil  310  alone cannot measure a roll angle ψ. 
     Coils  320  and  330  of  FIG. 3  are off-axis coils. In particular, coil  320  (or  330 ) includes wire loops with a normal direction  322  (or  332 ) that is at a non-zero angle to lengthwise axis  312  passing through the loops of coil  320  (or  330 ). As a result, even when the lengths of coils  310 ,  320 , and  330  are parallel or aligned, coils  320  and  330  are capable of measuring five degrees of freedom that differ from the five degrees of freedom that coil  310  can measure. EM sensor  116  of system  100  can include one or more off-axis coils such as coil  320  or  330  oriented along the length of device  110  to enable measurement of a roll angle of the distal tip of device  110 . 
       FIG. 4  shows a cross-sectional view of an off-axis coil  400  that may be used in measuring a roll angle. Coil  400  is a winding of wire  410  that may be considered to form multiple loops that define respective areas with a normal direction Â. Coil  400  is wound so that normal direction Â is off-axis by an angle α from the length (i.e., from an axis  470 ) of coil  400 . Coil  400  may be formed, for example, by wrapping insulated conductive wire around a core  420  at an angle (90°−α) to axis  470  of core  420  and coil  400  for about one half of each loop and at an angle −(90°−α) for the other half of each loop. As a result, an effective area |A| for magnetic flux in off-axis coil  400  has normal direction A that is at angle α relative to lengthwise axis  470  of coil  400  and has a magnitude |A| that is equal to the product of the area of a single loop and the number of loops in coil  400 . In general, each loop may define an area having any desired shape and may have a shape that depends on angle α and the shape of core  420 . For example, each loop area can be elliptical when core  420  is circular cylindrical and angle α is non-zero. For an EM sensor in a medical device, coil  400  may have a diameter of about 1 mm and a length of about 10 mm. The off-axis angle α can be any angle greater than zero and less than 180°, but for roll angle measurement as described further below, angle α may be between about 5° and about 175°. A range for angle α between about 45° to 70° or 110° to 135° for an EM sensor could provide accurate data and avoid difficulties in wrapping a coil when angle α is near 90°. 
     EM sensors coils such as coil  310  may employ helical coils that are wound so that the normal to the magnetic flux areas are along the lengthwise axis of the coil. In particular, such coils may be helically wound with a constant, slight angle, i.e., the helix angle. For example, the sine of a wrap angle for coil  310  may be about equal to the ratio of the wire thickness to the diameter of coil  310 . However, the effects of the wire being at the helix angle around a full loop cancel, and the normal for each loop of coil  310  is along the lengthwise axis. In contrast, the magnitude of the wrap angle (90°−α) in coil  410  can be much greater than the ratio of the diameter of wire  410 . Further, for coil  410 , the sign of the wrap angle reverses at some point in each loop. As a result, each loop of coil  410  has a part in which the wire angles down core  420  and a part in which the wire angles up core  420 . 
     A magnetic field B applied to off-axis coil  400  can be varied to induce an electrical signal that depends on the normal direction A to the areas of the loops forming coil  400 . In particular, according to Faradays law, an induced voltage in coil  400  is proportional to the time derivative of the dot product of magnetic field B and an effective area vector |A|Â.  FIG. 5  shows one specific geometry for magnetic field B and effective normal vector Â. In  FIG. 5 , magnetic field B is along the x axis of a Cartesian coordinate system that may be defined relative to a field generator that generates magnetic field B. With this configuration, if only the magnitude |B| of magnetic field B varies with time, the induced signal in coil  400  will have a voltage V given by Equation 1, wherein C is a constant that depends on the magnetic permeability inside coil  400 . Since the induced voltage V for coil  400  depends on the direction Â, i.e., angles θ and φ, the direction Â can be determined or measured, by varying the magnitude and direction of magnetic field B and analyzing the change in the induced voltage V. For example, Cartesian coordinates B x , B y , and B z  of the magnetic field B applied to coil  400  can be varied with different frequencies, and the different frequency components of the resulting induced voltage in coil  400  can be analyzed to determine measurements of up to five degrees of freedom of coil  400 , including direction angles θ and φ. (Determining a roll angle ψ may further require knowledge the direction of a roll axis, which may, for example, be measured using a second coil.) 
     
       
         
           
             
               
                 
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     Off-axis coils can be employed in small diameter 6-DoF sensors that are well adapted for use in minimally invasive medical instruments, e.g., as EM sensor  116  of  FIG. 1 .  FIG. 6A , for example, shows a sensing system  600 A employing coils  611  and  612  having lengths aligned along the same axis  170 . Each coil  611  and  612  may have a diameter of about 1 mm or less, so that 6-DoF sensor  610 A may similarly have a diameter of about 1 mm or less. One or both of coils  611  and  612  can be an off-axis coil such as coil  410 , which is described above with reference to  FIG. 4 . For coil  611 , a normal direction Â 1  of the effective area for magnetic flux is at an angle α to axis  170 . For coil  612 , a normal direction Â 2  of the effective area for magnetic flux is at an angle β to axis  170 . At least one of coils  611  and  612  are off-axis coils, i.e., α≠0 or β≠0, which enables measurement of a roll angle about axis  170 . 
     EM sensing using a single coil can generally only measure a set of five degrees of freedom because a single-coil EM sensor cannot distinguish rotations about the normal direction associated with the effective area of its coil. Two coils  611  and  612  with different normal directions Â 1  and Â 2  are used in sensor system  600 A, so that each of coils  611  and  612  measures a different set of five degrees of freedom. In particular, a field generator  620  can produce a variable magnetic field that passes through coils  611  and  612 . Coils  611  and  612  then produce respective induced voltages V 1  and V 2 , and sensor logic  630  can process signal V 1  to determine measurements of one set of five degrees of freedom and process signal V 2  to determine measurements of a different set of five degrees of freedom. Sensor logic  630 , which may include software for analyzing digitized versions of signals V 1  and V 2 , can account for the difference in position of coils  611  and  612  and generate measurements of six degrees of freedom, e.g., position coordinates X, Y, and Z and pitch, yaw, and roll angles. 
     Coils  611  and  612  in the specific configuration illustrated in  FIG. 6A  are identical off-axis coils, but are oriented so coil  612  is rotated by 180°, e.g., about a yaw axis of sensor  610 A, relative to coil  611 . As a result, angle β of a normal direction Â 2  to axis  170  is the supplement to angle α, i.e., β=180°−α. Sensing system  600 A may be able to achieve highest accuracy measurements if normal directions Â 1  and Â 2  are perpendicular to each other, and in one particular configuration of sensor  610 A, angle α is 45° to make normal directions Â 1  and Â 2  perpendicular. If coils  611  and  612  are not identical, a wide range of combinations of angles α and β are possible that make normal directions Â 1  and Â 2  perpendicular, e.g., configurations where |β−α|−90°. 
       FIG. 6B  shows another sensing system  600 B using a 6-DoF sensor  610 B containing two identical off-axis coils  611  and  612 . Coils  611  and  612  in  FIG. 6B  have respective normal directions Â 1  and Â 2 , both of which are at angle α with roll axis  170 . However, coil  612  is rotated by an angle θr about roll axis  170  relative to coil  611 . In this configuration, normal directions Â 1  and Â 2  are at an angle to each other that depends on angles α and θr. If angle α is greater than or equal to 45°, at least one value for angle θr exists that will make normal directions Â 1  and Â 2  perpendicular. For example, in one configuration, angle α is 45°, angle θr is 180°, and normal directions Â 1  and Â 2  are perpendicular. 
       FIG. 7  shows a medical system  700  capable of measuring six degrees of freedom using a sensing element  715  in an instrument  710  and a coil  725  in a probe  720  that fits within instrument  710 . Instrument  710  may be or may include a catheter, a cannula, bronchoscope, endoscope, cannula, or similar instrument through which a probe-like object with unknown roll angle may fit. Sensing element  715  is a device suitable for measurement of at least a pointing direction of the distal tip of instrument  710 . As described above, a conventional helical coil can be used to measure five degrees of freedom including a pointing direction of a distal tip of system  700  when such a coil is oriented along a lengthwise axis of system  700 , and sensing element  715  could be a coil. Alternatively, sensing element  715  could be another type of sensing device such as a shape sensor, a gravity sensor, a joint angle sensor (for jointed rigid-link instruments), or a vision-based sensor. Note that although described as a system including both a guide instrument  710  and a corresponding probe  720  for exemplary purposes, in various other embodiments, both sensing element  715  and coil  725  can be incorporated into a single instrument. 
     Coil  725  is an off-axis coil, which can measure five degrees of freedom and when combined with a measurement of a pointing direction of the roll axis can be used to determine a roll angle as described above. Accordingly, the combination of sensing element  715  in instrument  710  and off-axis coil  725  in probe  725  can provide a 6-DoF measurement of probe  720  including measurement of a roll angle of probe  720 . An advantage of system  700  is that the use of a single sensing element  715  in instrument  710  may provide additional space in instrument  710  and probe  720  for other structures, which is particularly important for small diameter devices such as lung catheters. Additionally, in system  700 , coil  725 , which is in probe  720 , may be closer to the center of the distal tip than is sensing element  715 , which is in the wall of instrument  710 . As a result, the roll axis of coil  725  may closely correspond to the roll axis of system  700  and probe  720 . Sensing element  715  in instrument  710  may as indicated above be a conventional helical coil so that a measurement of the direction of the area normal of sensing element  715  indicates the direction of the roll axis, and a measurement of the area normal direction of coil  725  can then give the roll angle of probe  720 . Alternatively, sensing element  715  could be an off-axis coil, and if the normal direction of the areas defined by the loops in sensing element  715  differs from the normal direction of the areas defined by the loops in coil  725 . 
     System  700  may be used by inserting probe  720  through instrument  710  until the distal ends of instrument  710  and probe  720  are aligned. Probe  720  may, for example, be a camera or vision system that is inserted in instrument  710  for navigation of natural lumens such as lung airways. Instrument  710  with the vision probe may then be steered to a worksite where measurements determined using sensing element  715  and coil  725  are used when orienting the distal tip of instrument  700  for a medical function such as biopsying tissue. The vision probe can then be removed and a probe such as a biopsy needle may be inserted in instrument  710  in place of the vision probe. The biopsy probe may similarly contain a coil or EM sensor, but the EM sensor used then may or may not need to be an off-axis coil or a coil intended for use with sensing element  715 . For example, a biopsy needle may be inserted past the distal tip of instrument  710 , and the position of the tip of the biopsy needle may be important to measure while the roll angle of a symmetric needle does not need to be measured. 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.