Abstract:
A medical system comprises a catheter having a first section, a second section and a main lumen extending through the first and second sections. The system also includes an imaging probe sized to extend through the main lumen of the catheter. The system also includes a first electromagnetic sensor extending along a longitudinal sensor axis at a proximal end of the first section and a second electromagnetic sensor extending along the longitudinal sensor axis at a distal end of the first section. The first section between the proximal and distal ends flexibly couples the first and second electromagnetic sensors so that the first electromagnetic sensor is movable with respect to the second electromagnetic sensor. The system also includes a third electromagnetic sensor positioned on the imaging probe and a fiber shape sensor system that extends through the first section between the proximal and distal ends and along the first electromagnetic sensor and the second electromagnetic sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent document is related to and incorporates by reference the following co-filed patent applications: U.S. Pat. App. No. Unknown, Attorney Docket No. ISRG02860/US, entitled “Catheters with Control Modes for Interchangeable Probes”; U.S. Pat. App. No. Unknown, Attorney Docket No. ISRG03170/US, entitled “Catheters with Control Modes for Interchangeable Probes”; and U.S. Pat. App. No. Unknown, Attorney Docket No. ISRG03230/US, entitled “Vision Probe and Catheter Systems.” 
     
    
     BACKGROUND 
       [0002]    Medical devices that navigate body lumens need to be physically 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 includes the mechanical and sensor structures suitable for remote or robotic operation and that has a diameter that is sufficiently small to navigate such small lumens can be challenging. In particular, one desirable configuration for a remotely operated catheter would provide a tool mounted on a steerable segment; tendons or pull wires that extend down the length of the catheter to an external drive system that pulls on the tendons to actuate the tool or steerable segment; lumens for suction and/or irrigation; a vision system for viewing of the target location; and sensors to identify the location of the instrument relative to the anatomy of a patient. Accommodating all of the desired features and elements of a lung catheter or other device that is robotically controlled and has a diameter about 3 mm or less can be difficult. 
       SUMMARY 
       [0003]    In accordance with an aspect of the invention, a robotic catheter system using distal feedback can provide a small diameter for a distal tip of the catheter and accurate measurements of the pose of the distal tip through use of a sensor system including both electromagnetic and fiber sensors. In accordance with an aspect of the present invention, a sensor system for a catheter has a thicker proximal section containing one or more electromagnetic (EM) sensors and a thinner distal section containing a fiber shape sensor. The EM sensor can provide an accurate measurement of a base point of the distal section relative to the anatomy of a patient, while the fiber sensor measures the shape of the distal section extending from the base point. Accordingly, the distal section of the catheter can be as small as a system using only a fiber shape sensor, but the catheter system does not suffer from the inaccuracy that is common to long fiber shape sensors. 
         [0004]    One specific embodiment of the invention is a medical system including a catheter, a first sensor system, and a second sensor system. The catheter has a first section and a second section with the second section being adjacent to the first section. The first sensor system is in the first section and configured for measurement of a pose of the first section. The second sensor system is in the second section and configured for measurement of a pose of the second section relative to the first section. 
         [0005]    Another embodiment of the invention is a method for sensing a pose of a distal tip of an elongated flexible structure such as a catheter in a medical instrument. The method includes applying a time-varying magnetic field to a space containing at least a portion of the flexible structure. An electric signal induced in a coil positioned at a location along the flexible structure of the medical instrument can then be analyzed as part of the pose measure. The location of the coil is separated from a proximal end and the distal tip of the flexible structure. In addition to analysis of the electrical signal from the coil, a shape of a portion of the flexible structure that extends from the location of the coil toward the distal end of the flexible section is measured. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows a robotic catheter system in accordance with an embodiment of the invention having multiple control modes. 
           [0007]      FIG. 2  shows an embodiment of a steerable segment that can be employed in the system of  FIG. 1 . 
           [0008]      FIGS. 3A and 3B  show cross-sectional views of proximal and distal sections of a catheter in accordance with an embodiment of the invention. 
           [0009]      FIG. 4  shows a cross-sectional view of a vision probe that may be deployed in the catheter of  FIGS. 3A and 3B  and swapped out for use of medical probes in the catheter shown in  FIGS. 3A and 3B . 
           [0010]      FIG. 5  is a flow diagram of a process for using the catheter system with a removable vision system and multiple control modes. 
           [0011]      FIG. 6  is a flow diagram of a catheter control process in a holding mode. 
           [0012]      FIG. 7  shows sensing coils that can be employed in electromagnetic sensors in medical systems in some embodiments of the invention. 
           [0013]      FIGS. 8A, 8B, 8C, and 8D  illustrate alternative configurations for sensor systems in accordance with embodiments of the invention including electromagnetic and shape sensors. 
           [0014]      FIG. 9  shows cross-sections of a catheter system containing a six-degree-of-freedom EM sensor and a catheter system containing two five-degree-of-freedom EM sensors. 
       
    
    
       [0015]    Use of the same reference symbols in different figures indicates similar or identical items. 
       DETAILED DESCRIPTION 
       [0016]    A robotic catheter for use in small lumens such as airways and passages in the respiratory tract employs combinations of one or more EM sensors and a fiber shape sensor to provide accurate measurements of the pose of a small-diameter distal tip.  FIG. 1  schematically illustrates a catheter system  100  in accordance with one embodiment of the invention. In the illustrated embodiment, catheter system  100  includes a catheter  110 , a drive interface  120 , control logic  140 , an operator interface  150 , and a sensor system  160 . 
         [0017]    Catheter  110  is a generally flexible device having one or more lumens including a main lumen that can accommodate interchangeable probes such as described further below. Flexible catheters can be made using a braided structure such as a woven wire tube with inner or outer layers of a flexible or low friction material such as polytetrafluoroethylene (PTFE). In one embodiment, catheter  110  includes a bundle of lumens or tubes held together by a braided jacket and a reflowed (i.e., fused by melting) jacket of a material such as Polyether Block Amide (Pebax). Alternatively, an extrusion of a material such as Pebax can similarly be used to form multiple lumens in catheter  110 . Catheter  110  particularly includes a main lumen for interchangeable probe systems and smaller lumens for pull wires and sensor lines. In the illustrated embodiment, catheter  110  has a proximal section  112  attached to drive interface  120  and a distal section  114  that extends from the proximal section  112 . An additional steerable segment  116  (e.g., a metal structure such as shown in  FIG. 2  and described further below) can form the distal subsection of distal section  114 . Pull wires extend from drive system  120  through proximal section  112  and distal section  114  and connect to steerable segment  116 . 
         [0018]    The overall length of catheter  110  may be about 60 to 80 cm or longer with distal section  114  being about 15 cm long and steerable segment  116  being about 4 to 5 cm long. In accordance with an aspect of the invention, distal section  114  has a smaller diameter than does proximal section  112  and thus can navigate smaller natural lumens or passages. During a medical procedure, at least a portion of proximal section  112  and all of distal section  114  may be inserted along a natural lumen such as an airway of a patient. The smaller diameter of distal section  114  permits use of distal section  114  in lumens that may be too small for proximal section  112 , but the larger diameter of distal section  114  facilitates inclusion of more or larger structures or devices such as electromagnetic (EM) sensors  162  that may not fit in distal section  114 . 
         [0019]    Steerable segment  116  is remotely controllable and particularly has a pitch and a yaw that can be controlled using pull wires. Steerable segment  116  may include all or part of distal section  114  and may be simply implemented as a multi-lumen tube of flexible material such as Pebax. In general, steerable segment  116  is more flexible than the remainder of catheter  110 , which assists in isolating actuation or bending to steerable segment  116  when drive interface  120  pulls on actuating tendons. Catheter  110  can also employ additional features or structures such as use of Bowden cables for actuating tendons to prevent actuation from bending proximal section  112  (or bending any portion the section of  114  other than steerable segment  116 ) of catheter  110 .  FIG. 2  shows one specific embodiment in which steerable segment  116  is made from a tube  210  that in catheter  110  of  FIG. 1  contains multiple tubes defining a main lumen for a probe system and smaller lumens for actuation tendons  230  and a shape sensor not shown in  FIG. 2 . In the illustrated embodiment, tendons  230  are placed 90° apart surrounding lumen  312  to facilitate steering catheter  110  in pitch and yaw directions defined by the locations of tendons  230 . A reflowed jacket, which is not shown in  FIG. 2  to better illustrate the internal structure of steerable segment  116 , may also cover tube  210 . As shown in  FIG. 2 , tube  210  is cut or formed to create a series of flexures  220 . Tendons  230  connect to a distal tip  215  of steerable segment  116  and extend back to a drive interface  120 . Tendons  230  can 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  and limit bending of proximal section  112  when drive interface  120  pulls on tendons  230 . 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. In operation, pulling harder on any one of tendons  230  tends to cause steerable segment  116  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. 
         [0020]    Drive interfaces  120  of  FIG. 1 , which pulls on tendons  230  to actuate steerable segment  116 , includes a mechanical system or transmission  124  that converts the movement of actuators  122 , e.g., electric motors, into movements of (or tensions in) tendons  230  that run through catheter  110  and connect to steerable segment  116 . (Push rods could conceivably be used in catheter  110  instead of tendons  230  but may not provide a desirable level of flexibility.) The movement and pose of steerable segment  116  can thus be controlled through selection of drive signals for actuators  122  in drive interface  120 . In addition to manipulating tendons  230 , drive interface  120  may also be able to control other movement of catheter  110  such as range of motion in an insertion direction and rotation or roll of the proximal end of catheter  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 . For example, some known drive systems for flexible instruments are described in U.S. Pat. App. Pub. No. 2010/0331820, entitled “Compliant Surgical Device,” which is hereby incorporated by reference in its entirety. Drive interface  120  in addition to actuating catheter  110  should allow removal and replacements of probes in catheter  110 , so that the drive structure should be out of the way during such operations. 
         [0021]    A dock  126  in drive interface  120  can provide a mechanical coupling between drive interface  120  and catheter  110  and link actuation tendons  230  to transmission  124 . Dock  126  may additionally contain an electronic or optical system for receiving, converting, and/or relaying sensor signals from portions of sensor system  160  in catheter  110  and contain an electronic or mechanical system for identifying the probe or the type of probe deployed in catheter  110 . 
         [0022]    Control logic  140  controls the actuators in drive interface  120  to selectively pull on the tendons as needed to actuate and steer steerable segment  116 . In general, control logic  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 from sensor system  160 . However, in holding modes as described further below, control logic  140  operates in response to measurement signals from sensor system  160  to maintain or acquire a previously identified working configuration. Control logic  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 sensor system  160  and to generate control signals for drive interface  120 . 
         [0023]    In the illustrated embodiment, control logic  140  includes multiple modules  141 ,  142 ,  143 , and  144  that implement different processes for controlling the actuation of catheter  110 . In particular, modules  141 ,  142 ,  143 , and  144  respectively implement a position stiffening mode, an orientation stiffening mode, a target position mode, and a target axial mode, which are described further below. A module  146  selects which control process will be used and may base the selection on user input, the type or status of the probe deployed in catheter  110 , and the task being performed. Control logic  140  also includes memory storing parameters  148  of a working configuration of steerable segment  116  that is desired for a task, and each of the modules  141 ,  142 ,  143 , and  144  can use their different control processes to actively maintain or hold the desired working configuration. 
         [0024]    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 segment  116 , and using such input, control logic  140  can generate control signals for actuators in drive interface  120 . Other instructions from the user can, for example, select an operating mode of control logic  140 . 
         [0025]    Sensor system  160  generally measures a pose of steerable segment  116 . In the illustrated embodiment, sensor system  160  includes EM sensors  162  and a shape sensor  164 . EM sensors  162  include one or more conductive coils that may be subjected to an externally generated electromagnetic field. Each coil of EM sensors  162  then produces an induced electrical signal having characteristics that depend on the position and orientation of the coil relative to the externally generated electromagnetic field. In an exemplary embodiment, EM sensors  162  are configured and positioned to measure six degrees of freedom, e.g., three position coordinates X, Y, and Z and three orientation angles indicating pitch, yaw, and roll of a base point. The base point in system  100  is at or near the end of proximal section  112  and the start of distal section  114  of catheter  110 . Shape sensor  164  in the exemplary embodiment of the invention includes a fiber grating that permits determination of the shape of a portion of catheter  110  extending from the base point, e.g., the shape of distal section  114  or steerable segment  116 . Such shape sensors using fiber gratings are further described in U.S. Pat. No. 7,720,322, entitled “Fiber Optic Shape Sensor,” which is hereby incorporated by reference in its entirety. An advantage of the illustrated type of sensor system  160  is that EM sensors  162  can provide measurements relative to the externally generated magnetic field, which can be calibrated relative to a patient&#39;s body. Thus, system  160  can use EM sensors  162  to reliably measure the position and orientation of a base point for shape sensor  164 , and shape sensor  164  need only provide shape measurement for a relatively short distance. Additionally, distal section  114  only contains shape sensor  164  and may have a diameter that is smaller than the diameter of proximal section  112 . More generally, sensor system  160  need only be able to measure the pose of steerable segment  116 , and other types of sensors could be employed. 
         [0026]      FIGS. 3A and 3B  respectively show cross-sections of the proximal and distal sections  112  and  114  of catheter  110  in one embodiment of the invention.  FIG. 3A  shows an embodiment of catheter  110  having a body  310  that includes a main lumen  312  for a vision or medical probe, lumens  314  containing tendons  230 , lumens  316  containing EM sensors  162  or associated signal wires, and a lumen  318  containing a fiber shape sensor  164 . Main lumen  312 , tendon lumens  314 , and a shape sensor lumen  318  extend into distal section  114  as shown in  FIG. 3B , but lumens  316  for EM sensors  162  are not needed in distal section  114  because EM sensors  162  are only in proximal section  112 . Accordingly, distal section  114  can be smaller than proximal section  112  particularly because the lumen  318  for fiber shape sensor  164  fits between two lumens  314  for pull wires and does not negatively affect the outside diameter of distal section  114 . In an exemplary embodiment, body  310  in proximal section  112  has an outer diameter of about 4 mm (e.g., in a range from 3 to 6 mm) and provides main lumen  312  with a diameter of about 2 mm (e.g., in a range from 1 to 3 mm) and in distal section  114  has an outer diameter of about 3 mm (e.g., in a range from 2 to 4 mm) while maintaining the diameter of main lumen  312  at about 2 mm. A smooth taper (as shown in  FIG. 1 ) or an abrupt step in body  310  can be used at the transition from the larger diameter of proximal section  112  to the smaller diameter of distal section  114 . 
         [0027]    The specific dimensions described in above are primarily for a catheter that accommodates probes having a diameter of 2 mm, which is a standard size for existing medical tools such as lung biopsy probes. However, alternative embodiments of the invention could be made larger or smaller to accommodate medical probes with a larger or smaller diameter, e.g., 1 mm diameter probes. A particular advantage of such embodiments is that a high level of functionality is provided in a catheter with relative small outer diameter when compared to the size of probe used in the catheter. 
         [0028]      FIGS. 3A and 3B  also show a sheath  360  that may be employed between catheter body  310  and a probe in main lumen  312 . In one embodiment of catheter  110 , sheath  360  is movable relative to body  310  and can be extended beyond the end of steerable segment  116 . This may be advantageous in some medical procedures because sheath  360  is even smaller than distal section  114  and therefore may fit into smaller natural lumens or passages. For example, if catheter  110  reaches a branching of lumens that are too small to accommodate steerable segment  116 , steerable segment  116  may be pointed in the direction of the desired branch, so that sheath  360  can be pushed beyond the end of steerable segment  116  and into that branch. Sheath  360  could thus reliably guide a medical probe into the desired branch. However, sheath  360  is passive in that it is not directly actuated or steerable. In contrast, distal section  114  accommodates actuation tendons  230  that connect to steerable segment  116  and can be manipulated to steer or pose steerable segment  116 . In some medical applications, the active control of steerable segment  116  is desirable or necessary during a medical procedure, and passive sheath  360  may not be used in some embodiments of the invention. 
         [0029]    Main lumen  312  is sized to accommodate a variety of medical probes. One specific probe is a vision probe  400  such as illustrated in  FIG. 4 . Vision probe  400  has a flexible body  410  with an outer diameter (e.g., about 2 mm) that fits within the main lumen of catheter  110  and with multiple inner lumens that contain the structures of vision probe  400 . Body  410  may be formed using an extruded flexible material such as Pebax or another polymer, which allows creation of multiple lumens and thin walls for maximal utility in minimal cross-sectional area. A multi-lumen extrusion also neatly organizes the location of the components. The length of body  410  may optionally include a combination of two multi-lumen extrusions, for example, a distal extrusion “butt-welded” to a proximal extrusion. This may be done, for example, so that the proximal or distal extrusion has desired shape, e.g., a clover-leaf or oval outside shape, to mate with a complementary keying feature in catheter  110 . These mating shapes or keying structures can prevent the probe from rotating within the catheter and assure a known orientation of camera  320  relative to catheter  110 . 
         [0030]    In the illustrated embodiment, the structure of vision probe  400  includes a CMOS camera  420 , which is at the distal end of the probe and connected through one or more signal wires (not shown) that extend along the length of vision probe  400 , e.g., to provide a video signal to control logic  140  or operator interface  150  as shown in  FIG. 1 . Alternatively, a fiber bundle imaging system could be employed, but CMOS cameras  420  can typically provide images of higher quality than can be achieved with fiber bundle imaging systems. Vision probe  400  also includes illumination fibers  430  that surround camera  420  and provide light for imaging within a body lumen. In an exemplary embodiment, illumination fibers  430  are made of a flexible material such as plastic, which tends to be more flexible than glass fibers. Oblong fluid ports  440  are provided in body  410  for suction and irrigation that may be useful, for example, for rinsing of a lens of camera  420 . Fluid ports  440  can also be used for delivering drugs, e.g., for numbing, before vision probe  400  is removed from catheter  110  and replaced with a biopsy probe. Although the illustrated embodiment of vision probe  400  includes multiple fluid ports  440 , a single fluid port could be used for both irrigation and suction, and vision probe  400  could alternatively have only a single fluid port to save space. Vision probe  400  may additionally include an electromagnetic sensor (not shown) embedded just proximally to CMOS camera  420  to provide additional pose information about the tip of vision probe  400 . 
         [0031]    Vision probe  400  is adapted to be inserted or removed from catheter  110  while catheter  110  is in use for a medical procedure. Accordingly, vision probe  400  is generally free to move relative to catheter  110 . While movement relative to catheter  110  is necessary or desirable during insertion or removal of vision probe  400 , the orientation of a vision probe  400  (and some medical probes) may need to be known for optimal or easier use. For example, a user viewing video from vision probe  400  and operating a controller similar to a joystick to steer catheter  110  generally expects the directions of movement of the controller to correspond to the response of steerable segment  116  and the resulting change in the image from vision probe  400 . Operator interface  150  needs (or at least can use) information on the orientation of vision probe  400  relative to tendons  230  in order to provide a consistency in directions used in the user interface. In accordance with an aspect of the invention, a keying system (not shown) can fix vision probe  400  into a known orientation relative to catheter  110  and tendons  230 . The keying system may, for example, be implemented through the shape of a proximal or distal section of probe  400  or include a spring, fixed protrusion, or latch on vision probe  400  or steerable segment  116  and a complementary notch or feature in steerable segment  116  or vision probe  400 . 
         [0032]    Vision probe  400  is only one example of a probe system that may be deployed in catheter  110  or guided through catheter  110  to a work site. Other probe systems that may be used include, but are not limited to, biopsy forceps, biopsy needles, biopsy brushes, ablation lasers, and radial ultrasound probes. In general, catheter  110  can be used with existing manual medical probes that are commercially available from medical companies such as Olympus Europa Holding GmbH. 
         [0033]    The catheter system  100  of  FIG. 1  can be used in procedures that swap a vision probe and a medical probe.  FIG. 5  is a flow diagram of one embodiment of a process  500  for using the catheter system  100  of  FIG. 1 . In process  500 , vision probe  400  is deployed in catheter  110  in step  510 , and catheter  110  is inserted along a path including a natural lumen of a patient. For example, for a lung biopsy, steerable segment  116  of catheter  110  may be introduced through the mouth of a patient into the respiratory tract of the patient. Vision probe  400  when fully deployed in catheter  110  may fit into a keying structure that keeps vision probe  400  in a desired orientation at or even extending beyond steerable segment  116  to provide a good forward view from steerable segment  116  of catheter  110 . As noted above, steerable segment  116  of catheter  110  is steerable, and vision probe  320  can provide video of the respiratory tract that helps a user when navigating catheter  110  toward a target work site. However, use of vision probe  400  during navigation is not strictly necessary since navigation of catheter  110  may be possible using measurements of sensor system  160  or some other system with or without vision probe  400  being deployed or used in catheter  110 . The path followed to the work site may be entirely within natural lumens such as the airways of the respiratory track or may pierce and pass through tissue at one or more points. 
         [0034]    When steerable segment  116  reaches the target work site, vision probe  400  can be used to view the work site as in step  530  and to pose steerable segment  116  for performance of a task at the target work site as in step  540 . Posing of steerable segment  116  may use images or visual information from vision probe  400  and measurements from sensor system  160  to characterize the work site and determine the desired working configuration. The desired working configuration may also depend on the type of tool that will be used or the next medical task. For example, reaching a desired working configuration of catheter  110  may bring the distal tip of steerable segment  116  into contact with tissue to be treated, sampled, or removed with a medical tool that replaces vision probe  400  in catheter  110 . Another type of working configuration may point steerable segment  116  at target tissue to be removed using an ablation laser. For example, tissue could be targeted in one or more 2D camera views while vision probe  400  is still in place in catheter  110 , or target tissue can be located on a virtual view of the work site using pre-operative 3D imaging data together with the position sensing relative to patient anatomy. Still another type of working configuration may define a line for the insertion of a needle or other medical tool into tissue, and the working configuration includes poses in which the distal tip of steerable segment  116  is along the target line. In general, the desired working configuration defines constraints on the position or the orientation of the distal tip of steerable segment  116 , and the shape of more proximal sections of catheter  110  is not similarly constrained and may vary as necessary to accommodate the patient. 
         [0035]    Step  550  stores in memory of the control logic parameters that identify the desired working configuration. For example, the position of a distal tip or target tissue can be defined using three coordinates. A target line for a need can be defined using the coordinates of a point on the line and angles indicating the direction of the line from that point. In general, control logic  120  uses the stored parameters that define the desired working configuration when operating in a holding mode that maintains steerable segment  116  of catheter  110  in the desired working configuration as described further below. 
         [0036]    Step  560  selects and activates a holding mode of the catheter system after the desired working configuration has been established and recorded. Control logic  140  for catheter  110  of  FIG. 1  may have one or more modules  141 ,  142 ,  143 , and  144  implementing multiple stiffening modes that may be used as holding modes when the desired configuration of steerable segment  116  has fixed constraints. The available control modes may include one or more of the following. 
         [0037]    1.) A position stiffness mode compares the position of the distal tip of steerable segment  116  as measured by sensor system  160  to a desired tip position and controls the actuators to minimize the difference in desired and measured tip positions. The position stiffness mode may particularly be suitable for general manipulation tasks in which the user tries to precisely control the position of the tip and for situations where the distal tip contacts tissue. 
         [0038]    2.) An orientation stiffness mode compares the measured orientation or pointing direction of the distal tip to a desired pointing direction of the distal tip and controls the actuators to minimize the difference in desired and actual tip pointing direction. This orientation stiffening that may be suitable, e.g., when controlling an imaging device such as vision probe  400  attached steerable segment  116 , in which case the viewing direction is kept as desired, while the exact position of steerable segment  116  may be less important. 
         [0039]    3.) A target position stiffness mode uses a combination of the measured tip position and pointing direction to control catheter  110  to always point the distal tip of steerable segment  116  towards a specified target point some distance in front of steerable segment  116 . In case of external disturbances, control logic  140  may control the actuators to implement this target position stiffening behavior, which may be suitable, e.g., when a medical probe inserted though the catheter contains an ablation laser that should always be aimed at a target ablation point in tissue. 
         [0040]    4.) A target axial motion stiffness mode uses a combination of the measured tip position and pointing direction to ensure that the distal tip of steerable segment  116  is always on a line in space and has a pointing direction that is also along that line. This mode can be useful, e.g., when inserting a biopsy needle along a specified line into tissue. Tissue reaction forces could cause the flexible section of catheter  110  to bend while inserting the needle, but this control strategy would ensure that the needle is always along the right line. 
         [0041]    The selection of a mode in step  560  could be made through manual selection by the user, based on the type of probe that is being used (e.g., grasper, camera, laser, or needle) in catheter  110 , or based on the activity catheter  110  is performing. For example, when a laser is deployed in catheter  110 , control logic  120  may operate in position stiffness mode when the laser deployed in catheter  110  is off and operate in target position stiffness mode to focus the laser on a desired target when the laser is on. When “holding” is activated, control logic  140  uses the stored parameters of the working configuration (instead of immediate input from operator interface  150 ) in generating control signals for drive interface  120 . 
         [0042]    The vision probe is removed from the catheter in step  570 , which clears the main lumen of catheter  110  for the step  580  of inserting a medical probe or tool through catheter  110 . For the specific step order shown in  FIG. 5 , control logic  140  operates in holding mode and maintains steerable segment  116  in the desired working configuration while the vision system is removed (step  570 ) and the medical probe is inserted (step  580 ). Accordingly, when the medical probe is fully deployed, e.g., reaches the end of steerable segment  116 , the medical probe will be in the desired working configuration, and performance of the medical task as in step  590  can be then performed without further need or use of the removed vision probe. Once the medical task is completed, the catheter can be taken out of holding mode or otherwise relaxed so that the medical probe can be removed. The catheter can then be removed from the patient if the medical procedure is complete, or the vision or another probe can be inserted through the catheter if further medical tasks are desired. 
         [0043]    In one alternative for the step order of process  500 , catheter  110  may not be in a holding mode while the medical probe is inserted but can be switched to holding mode after the medical probe is fully deployed. For example, catheter  110  may be relaxed or straightened for easy remove of vision probe  400  (step  570 ) and insertion of the medical probe (step  580 ). Once holding mode is initiated, e.g., after insertion of the medical probe, control logic  140  will control the drive interface  130  to return steerable segment  116  to the desired working configuration if steerable segment  116  has moved since being posed in the desired working configuration. Thereafter, control logic  140  monitors the pose of steerable segment  116  and actively maintains steerable segment  116  in the desired working configuration while the medical task is performed in step  590 . 
         [0044]      FIG. 6  shows a flow diagram of a process  600  of a holding mode that can be implemented in control logic  140  of  FIG. 1 . Process  600  begins in step  610  with receipt of measurement signals from sensor system  160 . The particular measurements required depend on the type of holding mode being implemented, but as an example, the measurements can indicate position coordinates, e.g., rectangular coordinates X, Y, and Z, of the distal tip of steerable segment  116  and orientation angles, e.g., angles θ X , θ Y , and θ Z  of a center axis of the distal tip of steerable segment  116  relative to coordinate axes X, Y, and Z. Other coordinate systems and methods for representing the pose of steerable segment  116  could be used, and measurements of all coordinates and direction angles may not be necessary. However, in an exemplary embodiment, sensor system  160  is capable of measuring six degrees of freedom (DoF) of the distal tip of steerable segment  116  and of providing those measurements to control logic  140  in step  610 . 
         [0045]    Control logic  140  in step  620  determines a desired pose of steerable segment  116 . For example, control logic  140  can determine desired position coordinates, e.g., X′, Y′, and Z′, of the end of steerable segment  116  and desired orientation angles, e.g., angles θ′ X , θ′ Y , and θ′ Z  of the center axis of steerable segment  116  relative to coordinate axes X, Y, and Z. The holding modes described above generally provide fewer than six constraints on the desired coordinates. For example, position stiffness operates to constrain three degrees of freedom, the position of the end of steerable segment  116  but not the orientation angles. In contrast, orientation stiffness mode constrains one or more orientation angles but not the position of end of steerable segment  116 . Target position stiffness mode constrains four degrees of freedom, and axial stiffness mode constrains five degrees of freedom. Control logic  610  can impose further constraints to select one of set of parameters, e.g., X′, Y′, and Z′ and angles θ′ X , θ′ Y , and θ′ Z , that provides the desired working configuration. Such further constraints include but are not limited to mechanical constraints required by the capabilities of steerable segment  116  and of catheter  110  generally and utilitarian constraints such as minimizing movement of steerable segment  116  or providing desired operating characteristics such as smooth, non-oscillating, and predictable movement with controlled stress in catheter  110 . Step  620  possibly includes just keeping a set pose steerable segment  116  by finding smallest movement from the measured pose to a pose satisfying the constraints, e.g., finding the point on the target line closest to the measure position for axial motion stiffness or finding some suitable pose from registered pre-op data that is close to the current pose. 
         [0046]    Control logic  140  in step  630  uses the desired and/or measured poses to determine corrected control signals that will cause drive interface  120  to move steerable segment  116  to the desired pose. For example, the mechanics of catheter  110  and drive interface  120  may permit development of mappings from the desired coordinates X′, Y′, and Z′ and angles θ′ X , θ′ Y , and θ′ Z  to actuator control signals that provide the desired pose. Other embodiments may use differences between the measured and desired pose to determine corrected control signals. In general, the control signals may be used not only to control actuators connected through tendons to steerable segment  116  but may also control (to some degree) insertion or roll of catheter  110  as a whole. 
         [0047]    A branch step  650  completes a feedback loop by causing process  600  to return to measurement step  610  after control system  140  applies new control signals drive interface  120 . The pose of distal tip is thus actively monitored and controlled according to fixed constraints as long as control system  120  remains in the holding mode. It may be noted, however, that some degrees of freedom of steerable segment  116  may not require active control. For example, in orientation stiffness mode, feedback control could actively maintain pitch and yaw of steerable segment  116 , while the mechanical torsional stiffness of catheter  110  is relied on hold the roll angle fixed. However, catheter  110  in general may be subject to unpredictable external forces or patient movement that would otherwise cause catheter  110  to move relative to the work site, and active control as in process  600  is needed to maintain or hold the desired working configuration. 
         [0048]    The sensor system  160  of a catheter  100  as noted above can employ both an EM sensor  162  and a fiber shape sensor  164 . EM sensors or trackers are state-of-the-art position and orientation sensors that combine high global accuracy with small package size (e.g., about 1×10 mm). EM sensors are commercially available from companies such as Ascension Technology Corporation and Northern Digital Inc. Shape sensing technology, which may be used in the above described embodiments, commonly employ reflections and interference within an optical fiber to measure the shape along the length of the optical fiber. This shape sensing technology is good for giving 6-DoF relative measurements between two points along the fiber as well as measuring bend angles of controllable joints or providing full three-dimensional shape information. A typical fiber shape sensor of this type may have a diameter of about 0.2 mm, which is considerably smaller than a typical EM sensor. 
         [0049]      FIG. 7  illustrates three different types of sensing coils  710 ,  720 , and  730  that could be used in an EM sensor. In operation, the sensing coil, e.g., coil  710 , in the catheter or other device is placed in a well-controlled magnetic field that an external EM generator produces. The EM generator typically has the form of a square or cylindrical box of 20-60 cm wide and several cm thick and may have a fixed position relative to the patient. The magnetic field produced by the generator varies in time and induces a voltage and electric current in the sensing coil  710 . 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 suitable for in medical environment 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. In general, the induced voltage in a sensing coil depends on time derivative the magnetic flux, which in turn depends on the strength of the magnetic field and the direction of the magnetic field relative to a normal to the areas of loops in the coil. The 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  710  from the induced electric signal. Up to five degrees of freedom can be determined using a sensor  162  containing a single sensing coil  710 . However, sensing coil  710  is cylindrically symmetric, so that a roll angle, i.e., an angle indicating orientation about a normal  712  to the inductive areas of coil  710 , cannot be determined. Only the position and the pointing direction can be determined using a single coil  710 . Even so, a 5-Degree-of-Freedom (5-DoF) sensor containing a single sensing coil  710  is useful in many medical systems. In particular, the mechanical shape of a typical sensing coil (long and slender) fits well with the mechanical shape of minimally invasive medical tools, and if the tool is rotationally symmetrical (e.g. in the case of a needle or laser fiber), the roll angle is not relevant. 
         [0050]    A robotic control catheter such as catheter  110  may need a 6-DoF measurement including a measurement of the roll angle so that the positions of actuating tendons are known. If measurement of the roll angle is of interest, two 5-DoF EM sensors can be combined to create a 6-DoF EM sensor. One specific configuration of a 6-DoF EM sensor uses two coils such as  710  with the inductive areas of two coils having normal vectors that are askew, e.g., perpendicular to each other. More generally, the two coils need to be arranged so that the normal vectors to inductive areas are not along the same axis, and larger angles between the normal vectors generally provide better measurement accuracy. Coils  720  and  730  illustrate how a coil  720  or  730  that may have wire loops with a normal  722  or  732  that is at a non-zero angle to the axes of a cylinder containing the coil  720  or  730 . Coils  720  and  730  can thus be oriented along the same direction, e.g., along the length of a catheter or other medical tool, and still be used to measure six degrees of freedom. 
         [0051]      FIG. 8A  shows a configuration of a catheter system  810  having a proximal section  812  containing a 6-DoF EM sensor  820  and a distal section  814  containing a fiber shape sensor  822 . EM sensor  820  terminates at or near a distal end of proximal section  812 . Accordingly, distal section  814  can have a diameter (e.g., about 3 mm to accommodate a probe diameter of about 2 mm) that is smaller than the diameter (e.g., about 4 mm) of proximal section  812  because EM sensor  820  does not extend into distal section  814 . The pose of distal tip of section  814  can be determined using EM sensor  820  to measure or determine the global position and orientation of a point along shape sensor  822  and using shape sensor  822  to determine the shape of distal section  814  extending from the measured point. The accuracy of shape sensor  822  can be relatively high because shape sensor  822  only needs to measure the shape of a relatively short section  814  of catheter  810 , rather than the entire length of catheter  810 . For example, in one case, the accuracy of the position measurement for the distal tip of section  814  is a function of the position and orientation accuracy of the EM sensor  820  (typically about 1 mm and 0.01 radians respectively) and the position accuracy of the shape sensor (0.15% of the length of section  814 ). If 6-DoF EM sensor  820  is about 115 mm away from the distal tip, the typical tip position accuracy would be about 2.5 mm. 
         [0052]      FIG. 8B  shows a catheter  830  that uses two 5-DoF EM sensors  840  and  841  in a proximal section  832  to measure six degrees of freedom of a base point along a shape sensor  842  that extends into a distal section  834  of catheter  830 . Coils of EM sensors  840  and  841  are within the same cross-section of proximal section  832  and therefore are rigidly fixed relative to each other. EM sensors  840  and  841  can also contain sensing coils such as coils  720  and  730  having wire loops with different orientations to measure different degrees of freedom of a point along shape sensor  842 . The roll angle can thus be determined using the two measured pointing directions of sensors  840  and  841  to define a reference frame. The use of 5-DoF sensors  840  and  841  may allow a reduction in the diameter of proximal section  832 . In particular, 6-DoF EM sensors that are available commercially generally have diameters that are larger than the diameters of similar 5-DoF EM sensors. In accordance with an aspect of the current invention, the diameter of a catheter may be decreased through use of 5-DoF EM sensors.  FIG. 9 , for example, illustrates how a distal section  832  of catheter  830  can accommodate two 5-DoF EM sensors  840  and  841  and a main lumen  910  within a circular cross-sectional area that is smaller than the area of distal section  812  of catheter  810 . In particular, section  812  is larger because section  812  must accommodate the main lumen and a 6-DoF sensor that has a larger diameter than do 5-DoF sensors  840  and  841 . 
         [0053]      FIG. 8C  shows an embodiment of the invention using a sensor system that may allow a proximal section  852  of catheter  850  to be even smaller by using two 5-DoF EM sensors  860  and  861  that are separated along the length of the proximal section  852 . Accordingly, only one 5-DoF EM sensor  860  or  861  needs to be accommodated within the cross-section of proximal section  852 . However, since distal section  852  is flexible and may be bent when in use, EM sensors  860  and  861  are not rigidly fixed relative to each other, and a shape sensor  862  is used to measure the shape of the portion of proximal section  852  between EM sensors  861  and  860  and the relative orientation of EM sensors  860  and  861 . The shape measurement between EM sensors  861  and  860  indicates the position and orientation of sensor  860  relative to sensor  861 , and the relative configuration is needed for determination of a 6-DoF measurement from the two 5-DoF measurement. Shape sensor  862  also measures the shape of distal section  854 , which indicates the position and orientation of the distal tip relative to the global position and orientation measurements determined using EM sensors  860  and  861 . 
         [0054]      FIG. 8D  shows yet another catheter  870  using two 5-DoF EM sensors  880  and  881  that are separate along the length of catheter  870 . The sensing system in catheter  870  of  FIG. 8D  differs from the sensing systems of  FIGS. 8A, 8B, and 8C  in that one EM sensor  880  is located in a proximal section  872  of catheter  870  and the other EM sensor  881  is located in a distal section  874  of catheter  870 . Accordingly, distal section  874  must be large enough to include sensor  881 , but still may allow a reduction in the diameter of catheter  870  when compared to a catheter having a 6-DoF EM sensor at a distal tip. 
         [0055]    The use of two 5-DoF EM sensors in embodiments of  FIGS. 8B, 8C, and 8D  provides more information than is strictly required for a 6-DoF measurement. In accordance with a further aspect of the invention, one of the two 5-DoF EM sensors in catheter  830 ,  850 , or  870  of  FIG. 8B, 8C , or  8 D could be replaced with another type of sensor that may not measure five degrees of freedom. For example, an accelerometer could be employed in place of one of the two EM sensors and provide a measurement of the direction of gravity, i.e., down. Provided that the symmetry axis of the 5-DoF sensor is not vertical, the combination of the measurements of a 5-DoF sensor and a measurement of the orientation relative to the vertical direction is sufficient to indicate measurements for six degrees of freedom. 
         [0056]    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.