Abstract:
A method is provided for establishing contact of a medical device against a tissue surface within a subject body, the method comprising determination of the geometrical configuration of the distal portion of the medical device, and using this together with known control variable information to determine and control the contact force of the distal tip of the medical device against the tissue surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/700,269, filed Jul. 18, 2005, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD 
     This invention relates to control of medical devices in a subject body, and more particularly to estimation of contact force of a medical device against a tissue surface within the subject body. 
     BACKGROUND 
     Interventional medicine is the collection of medical procedures in which access to the site of treatment is made through one of the patient&#39;s blood vessels, body cavities or lumens. For example, electro-physiology mapping of the heart is most often performed using a catheter which may be inserted into a patient&#39;s arterial system through a puncture of the femoral artery in the groin area. Other interventional medical procedures include assessment and treatment of tissues on the inner surfaces of the heart (endocardial surfaces) accessed via peripheral veins or arteries, treatment of vascular defects such as cerebral aneurysms, removal of embolic clots and debris from vessels, treatment of tumors via vascular access, endoscopy of the intestinal tract, etc. 
     Interventional medicine technologies have been applied to manipulation of instruments which contact tissues during surgical procedures, making these procedures more precise, repeatable and less dependent of the device manipulation skills of the physician. Some presently available interventional medical systems for directing and manipulating the distal tip of a medical device by actuation of the distal portion of the device use computer assisted navigation and an imaging system for providing imaging of the device and blood vessels and tissues. Such systems can control the navigation of a medical device, such as a catheter, to a target destination in an operating region using a computer to orient and guide the distal tip through blood vessels and tissue. In some cases, when the computed direction for reaching the target destination is determined and the medical device is extended, it is desired to establish sufficient contact of the medical device with the intended target location on the three dimensional tissue surface. Adequate contact with the tissue surface within the subject body is important, for instance, in the analysis and treatment of cardiac arrhythmias. A method is therefore desired for controlling movement of a medical device that will establish adequate contact with the target tissue surface, estimate such contact force and will allow for treatment of the targeted area. 
     SUMMARY 
     The method and apparatus of the present invention facilitates the placement of the distal end of a medical device, such as a catheter or micro-catheter, against a target location on a three-dimensional curved surface within a subject body. Generally, the present invention provides a method for estimating the contact force of a medical device against a surface within a subject body, comprising obtaining three dimensional geometry information for the distal portion of the medical device, constructing a curve representative of the distal portion of the medical device from the pivot point to the tip of the known medical device, estimating the local rotation rate of the flexible portion of the distal portion of the medical device, and, estimating the contact force based on this data and the (known) bending stiffness and the total torque applied to the flexible portion of the medical device. 
     In one aspect of the present invention, a three-dimensional surface geometry is suitably rendered in an image model and registered with a known location within the subject body. A virtual model may be used in the estimation of the contact force of the medical device against the tissue surface, and in predicting a magnetic field to be applied to the medical device to establish a desired contact force against a target surface within the subject body. From the geometry of the medical device, a net bending moment may be estimated for the distal portion of the medical device. The estimated contact force may then be determined based on the net bending moment and the estimated torque applied to the medical device. The method may further provide the feature of determining an external magnetic field to be applied to the medical device for providing a desired estimated contact force against the tissue surface within the subject body. These and other features and advantages will be in part apparent, and in part pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a curved three dimensional tissue surface and a medical device held in contact with the surface through the over-torque method in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In a preferred embodiment of the present invention, a method for establishing and estimating the contact force of the tip of a medical device against a tissue surface within a subject body is provided in accordance with the principles of the present invention. In one embodiment, the method provides for estimating the contact force of a medical device with a tissue surface such as the heart, through the suitable estimation of the torque applied to the medical device via a magnetic field. While this embodiment is operable with magnetically navigable medical devices, other embodiments of a method in accordance with the present invention may be used with medical devices that are guided without magnetic navigation but instead use other control methods for remote navigation such as mechanical actuation, electrostrictive actuation, or hydraulic actuation. The method for estimating the contact force of a medical device against a surface within a subject body comprises obtaining three dimensional geometry information for the distal portion of the medical device, constructing a curve representative of the distal portion of the medical device from the pivot point to the tip of the known medical device, estimating the local rotation rate of the flexible portion of the distal portion of the medical device, determining a net bending moment for the distal portion of the medical device, estimating the contact force based on the (known) bending stiffness, the bending moment and the torque applied to the flexible portion of the medical device, and determining an external magnetic field to be applied to the medical device for providing a desired estimated contact force against the tissue surface within the subject body. 
     A medical device such as a catheter may be navigated to the interior of a subject body of a patient by various means, including but not limited to magnetic navigation. Once the medical device has been navigated to a target surface of the body, such as a heart wall, the tip of the medical device, the pivot point of the medical device, and at least two intermediate points may be defined in at least two X-ray projections by user-marking to construct (computationally, by interpolation) a three dimensional curve of the medical device as shown in  FIG. 1 . This curve may be written in the form of {right arrow over (x)}(s) where sε=[0,1]. Let s=0 correspond to the distal end of the medical device, and s=1 correspond to the pivot point. The interval [0,1] is then divided computationally into a predetermined number of increments so that ({right arrow over (x)} 1 , . . . {right arrow over (x)} n ), are a set of points from the distal tip of the medical device to the pivot point. The lengths of each individual segment between the points may be written as l i =|{right arrow over (x)} i+1 −{right arrow over (x)}x i | until the pivot point is reached. Each segment position that is near a magnet is preferably marked. We can let {right arrow over (x)} k  be the approximate location of a magnet on the medical device, and let: 
                   u   -&gt;     1     =         (         x   ⇀       k   -   1       -       x   ⇀     k       )                x   ⇀       k   +   1       -       x   ⇀     k              ≡       (         x   ⇀       k   -   1       -       x   ⇀     k       )       l   1   ′           ,   and                   u   -&gt;     2     =         (         x   ⇀     k     -       x   ⇀       k   +   1         )                x   ⇀     k     -       x   ⇀       k   +   1                ≡       (         x   ⇀     k     -       x   ⇀       k   +   1         )       l   2   ′               
to define the segments nearest the magnet at {right arrow over (x)} k  
 
     As an alternative to user-marking of the device on 2 X-ray projections, image processing could be employed to identify the distal portion of the medical device in each projection and thence to determine computationally the three dimensional curve corresponding to the distal portion of the medical device. 
     Defining the vector {right arrow over (V)}′ at the magnet location {right arrow over (x)} k  as shown below, we can define the unit vector {right arrow over (V)} k  that gives the orientation of the magnet at location {right arrow over (x)} k  as follows:
 
 {right arrow over (V)} ′=( l   2   ′{right arrow over (u)}   1   +l   1   ′{right arrow over (u)}   2 )  (1)
 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       ⇀ 
                     
                     k 
                   
                   = 
                   
                     
                       
                         V 
                         ⇀ 
                       
                       ′ 
                     
                     
                        
                       
                         
                           V 
                           ⇀ 
                         
                         ′ 
                       
                        
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Let m k  be the known magnetic moment of the magnet at location {right arrow over (x)} k , which may be any of the first, second, or n-th magnet from the distal tip of the given medical device. The torque resulting from the magnet at {right arrow over (x)} k  having an orientation {right arrow over (V)} k  may be written as the product shown below, where {right arrow over (B)} is the applied external magnetic field.
 
{right arrow over (τ)} k   =m   k ( {right arrow over (V)}   k   ×{right arrow over (B)} )  (3)
 
     Let the total magnetic torque acting on the medical device due to all of the magnets be:
 
{right arrow over (τ)} magnet =Σ magnets   1 {right arrow over (τ)} k   (4)
 
     Let (n+1) index the pivot point (n=50 in the present example). Let: 
                           V   ⇀       n   +   1       =       (         x   ⇀     n     -       x   ⇀       n   +   1         )                x   ⇀     n     -       x   ⇀       n   +   1                  ,   and     ⁢     
     ⁢           V   ⇀       n   -   1       =       (         x   ⇀       n   -   1       -       x   ⇀     n       )                x   ⇀       n   -   1       -       x   ⇀     n                ,     then   ⁢           ⁢   let       ⁢     
     ⁢       Δθ   =       cos     -   1       ⁡     (         V   ⇀       n   +   1       ·       V   ⇀       n   -   1         )         ,     and   ⁢           ⁢   let               (   5   )               Δ l=|{right arrow over (x)}   n   −{right arrow over (x)}   n+1   |+|{right arrow over (x)}   n−1   −{right arrow over (x)}   n |  (6)
         to yield the local estimated rotation rate ω k =Δθ/Δl       

     If index (n+1) or {right arrow over (x)} n+1  corresponds to a magnet location of the catheter, use instead a nearest point {right arrow over (x)} m  on the medical device such that {right arrow over (x)} m  is on a flexible or non-magnet segment. Let EI be the bending stiffness of the flexible segment of the medical device corresponding to {right arrow over (x)} n+1 , or the bending stiffness of the flexible segment nearest to the magnet. Let the vector from {right arrow over (x)} n+1  to the distal tip of the medical device at {right arrow over (x)} 1  be
 
 {right arrow over (r)} =( {right arrow over (x)}   1   {right arrow over (x)}   n+1 ) and  (7)
 
 r=|{right arrow over (r)}| . Then  (8)
 
     The estimated magnitude of the medical device contact force at the distal tip is given below (assuming no other forces in a direction perpendicular to {right arrow over (r)}): 
     
       
         
           
             
               
                 
                   f 
                   = 
                   
                     
                       1 
                       r 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                            
                           
                             
                               τ 
                               ⇀ 
                             
                             magnet 
                           
                            
                         
                         - 
                         
                           EI 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ω 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     The second term in equation (9) above represents the net bending moment of the distal portion of the device. Referring to  FIG. 1 , the tissue surface of a three dimensional object in a subject body is represented by curve  20  having an interior surface normal vector {right arrow over (n)} at a target point indicated at  22 . The local surface geometry of the surface may be obtained from a three-dimensional pre-operative image of the anatomy, or from geometric mapping and anatomical 3D reconstruction that may be performed by reconstructing an interpolated anatomical surface based on endocardial surface locations that have been visited with a catheter device and a localization system that is suitably registered with the computer-controlled navigation system. Since the three-dimensional data of the surface is available, the interior surface normal vector {right arrow over (n)} at the target location may be determined from this data. The tip of the actual medical device {right arrow over (x)} 1 , or a virtual medical device where localization data is available, is positioned against the tissue surface  20  near the target location  22 . Assuming there is no tangential contact force at the tip, let F be the normal contact force. Defining r′=−r/|r|, and θ=cos −1 ({right arrow over (r)}′•{right arrow over (n)}), then the normal contact force F is:
 
 F=f /sin θ  (10)
 
     Where a magnetically navigable medical device is used, an applied external field {right arrow over (B)} can also be determined for providing an over-torque of the medical device against the tissue surface at a desired estimated contact force F, within certain physically feasible bounds. This may be accomplished by applying a magnetic moment in a direction that provides the over torque (i.e., leads the orientation of the catheter tip by an angle of approximately 90° as measured about an axis that is normal to the plane defined by the catheter tip orientation and the local surface normal), where a suitable torque τ magnet  can be determined from equations (10) and (9). 
     The rotation rate of the flexible portion of the medical device resulting from the applied torque may also be determined using a virtual medical device within a computational model. In the case where the device actuation system is magnetic, this estimation of the rotation rate of the distal portion of the medical device may be used to estimate the contact force based on a computed magnetic torque applied to the tip of the medical device, based on the model. A subsequent navigational movement of the medical device may be determined to obtain a desired estimated contact force for improved electro-physiology electrical readings, or to apply improved ablation treatment. A suitable magnetic field for producing a desired force for the medical device can be estimated using the local surface geometry of the target location within the body. Likewise, a virtual representation of the medical device may be suitably rendered in a three-dimensional model of the surface geometry. Such virtual modeling of the medical device may be used to predict the rotation of the medical device prior to movement of the actual medical device. In the above example, we describe the particular case where magnetic field actuation is used to remotely navigate the medical device, as a non-limiting example of an actuation method. Other actuation techniques could be employed as would be familiar to persons skilled in the art of remote surgical navigation, for example a mechanically actuated system where the actuation is based on a system of pull-wires and electronically controlled servo motors. In such a case where this type of mechanical actuation is used, for example equation (9) above would be replaced by a similar equation involving mechanically applied bending torque. 
     In use, the medical device may be moved in incremental steps towards the target location at an increment of about 1-5 millimeters. The incremental step is made in association with a three-dimensional image model of the surface geometry, which may determine whether the incremental step results in an image threshold crossing. The above distances are suitable for applications of determining the curvature of certain surfaces such as the interior of a heart. It should be noted that the above distances and increments are exemplary in nature, and may be varied for a variety of applications. The magnet system is controlled to apply a magnetic field in a direction that causes the tip or distal portion of the medical device to be rotated to provide the torque for establishing a desired contact force with the tissue surface. Once the tip has established a desired estimated contact against the tissue surface, the lag between the field vector {right arrow over (B)} 0  and the actual orientation of the tip  24  can provide an indication that the tip of the medical device is in firm contact with the surface at  24 . Likewise, where an imaging system is used, the prolapse or buckling in the distal portion of the medical device  24  that can be seen in the acquired images, or the observation that the device tip has not changed position may also indicate that the tip has established the desired contact with the surface  20 . The method of estimating the contact force can be used to predict and drive navigation controls. Additionally the estimated contact force could be displayed to inform the user. 
     The advantages of the above described embodiment and improvements for estimating contact force, enabling over-torque of a medical device and thereby enhancing device-tissue contact against a three dimensional surface within a subject body, when the device is controlled by a remote navigation system, should be readily apparent to one skilled in the art. The actual controls used by the remote navigation system could comprise actuation schemes employing any one or more of magnetic, mechanical, electrostrictive, hydraulic or other actuation means familiar to those skilled in the art. Additional design considerations may be incorporated without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited by the particular embodiment or form described above, but by the appended claims.