Abstract:
A method of determining the quality of contact between a remotely navigated medical device and a cyclically moving anatomical structure includes measuring movement of the device, and processing the measured movement of the device to determine the contact between the device and the moving anatomical structure.

Description:
CROSS-REFEENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 60/946,709, filed Jun. 27, 2007. The disclosure of the above-referenced application is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    This invention relates to remote navigation of medical devices, and in particular to improved control over the remote navigation of medical devices using real time location data. 
         [0003]    In remote medical navigation procedures, the distal end of an elongate medical device is introduced into the subject&#39;s body (typically the vessels of the subjects circulatory system), and the distal end of the device is oriented and advanced through the body to a desired location. An example of remote navigation system for conducting remote medical navigation procedures is the Niobe® magnetic navigation system available from Stereotaxis, Inc., St. Louis, Mo. This system allows the user to orient the distal end of a compatible device in a selected direction through the application of a magnetic field from one or more external source magnets, and selectively advance the medical device in the selected direction. These magnetic navigation systems allow for fast and easy navigation of a device through a subject&#39;s body. Other types of remote navigation systems could employ mechanical systems, electrostrictive systems, hydraulic systems, and pneumatic systems for remotely orienting the distal end of a medical device. 
         [0004]    Remote navigation systems are frequently used in conjunction with a localization system that can determine the position and the orientation of the medical device. These localization systems include magnetic localization systems and electropotential localization systems. The localization can be provided in real time, or it can be gated, for example in coordination with the respiratory or cardiac cycle. This localization information can be used in an iterative feedback algorithm for reaching an arbitrary target, or in an algorithm for making small moves along a desired user-defined path. 
       SUMMARY 
       [0005]    Embodiments of the present invention provide for enhanced remote control of remote medical device using real time location data. In accordance with one embodiment, when a catheter or other device is in contact with a structure such as a cardiac wall, the position is monitored in real time, and through analysis of the tip location data, it is possible to estimate the periodicities of the cardiac cycle and of the respiration cycle. The resulting identified periodicity can be used to gate the navigation system for respiration, for instance during targeting. 
         [0006]    In accordance with another embodiment of this invention, real-time tip location data can also be used to estimate the qualitative extent of contact with an anatomical structure. The inventors have discovered that a catheter with a soft shaft (such as magnetic catheters) that is engaged in wall contact will tend to maintain tip contact at a fixed location relative to the cardiac wall during motion of the wall, since the shaft can easily buckle or deform to permit this. Accordingly, the extent of motion of the tip over the cardiac cycle will be largest when contact is very consistent. This can be used in several ways to estimate the quality of contact of the device with the anatomical structure. 
         [0007]    One way of estimating the quality of contact is using the oscillation amplitude. Tracking the oscillation of the difference [x real −x gated ] over a few cardiac cycles, where the three dimensional vectors are defined such that x gated  is always the most recent gated location (gated to the cardiac cycle) and x real  is the real-time location. If contact is temporarily lost for a portion of time between successive cycles, the amplitude of the oscillation will display a “truncated” or flat behavior over such time intervals, where the catheter location will stay relatively constant, and this can be detected from signal analysis to assess stability of contact. 
         [0008]    Another way of estimating the quality of contact is using positional covariance. If x mean  is the mean location over a cardiac cycle, set X=(x−x mean ) and form the 3×3 covariance matrix C with entries C ij =&lt;X i X j &gt; where the average &lt; &gt;is taken over a cardiac cycle (or over several cycles). Let a, b and c be the eigenvectors of C, with corresponding eigenvalues e a , e b , e c . These eigenvectors and eigenvalues define the covariance ellipsoid associated with the motion of the catheter tip. A soft catheter that maintains good contact with a given target location on the cardiac wall over the cardiac cycle will have larger values of (e a , e b , e c ) than a catheter that is in inconsistent/unstable contact; thus for example, D=(e a +e b +e c ) can be used as a measure of contact. Alternatively or in addition, the difference between maximum and minimum eigenvalues (e max −e min ) can be used as a measure of contact; in some situations a larger eigenvalue difference can represent greater relative motion along one direction, thus characterizing the local cardiac wall motion. A catheter tip in stable contact with such a cardiac wall location would move with the cardiac wall through the cardiac cycle. 
         [0009]    To enhance contact between the medical device and the tissue, the surface normal n to the cardiac wall can be used in order to torque the device tip into the wall. For example, with a magnetic navigation system, the magnetic field can be rotated about an axis defined by v=t×n in order to push the device tip into the wall, where t is the device tip orientation. In this case, let a be the eigenvector of C that has the largest dot product with n; then a can be used in some cases in place of n to define the axis of rotation v. 
         [0010]    Another way of estimating the quality of contact is using orientational covariance. If t mean  is the mean tip orientation over a cardiac cycle, and T=(t−t mean ), a covariance matrix M can be formed with entries M ij =&lt;M i M j &gt; (average taken over a cardiac cycle), and as with the positional covariance matrix discussed above, the eigenvalues and eigenvectors of M can be used to assess extent of contact. Analogously to the contact measures based on positional covariance eigenvalues, measures based on the orientational covariance eigenvalues such as the magnitude of the summed eigenvalues, or the difference between maximum and minimum eigenvalues, can be used to assess stability of cardiac contact. 
         [0011]    Another method of measuring contact stability is to perform a frequency analysis of catheter tip motion. The frequency spectrum of the motion of a catheter tip that is in firm contact with the cardiac wall will have a dominant peak at the frequency of the cardiac cycle. Thus, the ratio of the peak (maximum) in the Fourier transform of the catheter tip location over a period of time to the next smaller peaks in the frequency spectrum can provide a measure of contact. Other measures based on the frequency spectrum such as the spread in frequency between specified peak threshold values, or a variety of other such possible measures can also be used. 
         [0012]    Another way of estimating the quality of contact is using perturbation. A control variable of the navigation system (for example magnetic field direction, or catheter insertion length) can be perturbed or changed by a small amount, and the resulting change in a contact measure (such as one of the eigenvalue-based measures discussed above) can be determined. Thus for instance if contact is found to be enhanced by a small amount, a larger change in the same sense of the corresponding control variables can be applied to further enhance contact. 
         [0013]    Another way of estimating the quality of contact is using unipolar voltage: If unipolar intracardiac ECG voltage information is available in real-time, its magnitude can be used to sense proximity to the wall. In particular, its behavior over a cardiac cycle or a several cycles can be analyzed as in the methods above (except that voltage values would be used instead of positional data) in order to assess stability, with the difference that the variation in voltage will be the smaller when contact is better. 
         [0014]    Still another way of estimating the quality of contact is using a mapped mechanical model. A cardiac mechanical motion model is constructed by mapping over a “coarse” set of cardiac wall locations with a localized mapping catheter, so that the range of wall motion at every map point is recorded, the range of mechanical motion at intermediate points can be estimated, for example by interpolation. When the catheter is later placed at such an intermediate location, the range of motion as seen in the real-time location data can be compared with the expected range of motion in order to assess quality of contact. 
         [0015]    Thus the various embodiments of this invention provide for the improved control, and in particular for enhanced measure of contact, and thus control of contact, of a remote medical navigation system. These and other features and advantages will be in part apparent, and in part pointed out hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1A  is a schematic diagram showing a medical device in contact with a cyclically moving anatomical structure (e.g., a heart wall) illustrating good contact as the device moves a distance d 1  to remain in contact over the cycle of movement; 
           [0017]      FIG. 1B  is a schematic diagram showing a medical device in contact with a cyclically moving anatomical structure (e.g., a heart wall) illustrating poor contact as the device moves a distance d 2  and loses contact over the cycle of movement; 
           [0018]      FIG. 2A  is a graph of x real −x gated  for a medical device in contact with an cyclically moving anatomical structure, over several motion cycles, showing substantially continuous movement indicative of good device contact; 
           [0019]      FIG. 2B  is a graph of x real −x gated  for a medical device in contact with an cyclically moving anatomical structure, over several motion cycles, showing discontinuous or truncated movement indicative of poor device contact; 
           [0020]      FIG. 3  is a schematic diagram showing the movement of the distal tip of a medical device that is in contact with a cyclically moving anatomical structure, together with the constructed covariance ellipsoid in dashed lines; 
           [0021]      FIG. 4  is a schematic diagram showing a co-variance ellipsoid and two of its axes {right arrow over (v)} 1  and {right arrow over (v)} 2  are shown; 
           [0022]      FIG. 5A  is a schematic diagram showing a magnetically navigated medical device with an applied magnetic field {right arrow over (B)} 1  with a tip orientation {right arrow over (t)} 1 , against the cardiac wall; 
           [0023]      FIG. 5B  is a schematic diagram showing a magnetically navigated medical device with an applied magnetic field {right arrow over (B)} 2 , different from field {right arrow over (B)} 1 , resulting in a tip orientation {right arrow over (t)} 2 ; and 
           [0024]      FIG. 6  is a schematic diagram of a remote navigation system, also showing a localization system or sub-system, that implements the real-time contact evaluation and control specified in the disclosure. 
       
    
    
       [0025]    Corresponding reference numerals indicated corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0026]    Remote navigation systems have been used in recent years in the interventional treatment of various medical conditions such as, for example, cardiac arrythmias. An example of remote navigation system for conducting remote medical navigation procedures is the Niobe® magnetic navigation system available from Stereotaxis Inc., St. Louis, Mo. This system allows the user to orient the distal end of a compatible device in a selected direction through the application of a magnetic field from one or more external source magnets, and selectively advance the medical device in the selected direction. These magnetic navigation systems allow for fast and easy navigation of a device through a subject&#39;s body. Other types of remote navigation systems could employ mechanical systems, electrostrictive systems, hydraulic systems, and pneumatic systems for remotely orienting the distal end of a medical device. Often, during the use of a medical device navigated with a remote navigation system, it is useful to determine the extent of contact of the medical device with an anatomical organ such as the heart. Such a measure can be useful in further enhancing contact so that the medical procedure, for example intracardiac Radio-Frequency ablation, can be better performed. 
         [0027]    Embodiments of the present invention provide for enhanced remote control of remote medical device using real time location data. In accordance with one embodiment, when a catheter or other device is in contact with a structure such as a cardiac wall, the position is monitored in real time, and through analysis of the tip location data, it is possible to estimate the periodicities of the cardiac cycle and of the respiration cycle. The resulting identified periodicity can be used to gate the navigation system for respiration, for instance during targeting. The remote navigation system interfaces with a localization system that determines real-time location data and provides it to the remote navigation system. The localization system can be an electromagnetic localization system (such as the Carto™ system manufactured by Biosense Webster Inc.) or an electric field-based localization system (such as the NavX™ system manufactured by St. Jude Medical), or be based on other modalities such as ultrasound localization. The remote navigation system can use the real-time device location data that it receives from the localization system in several ways. In particular, the present invention discusses methods of determining and enhancing contact stability based on such real-time location data (as generally used here, “location data” can be a combination of positional and orientational data). 
         [0028]      FIG. 6  illustrates in schematic form a remote navigation system for performing a remote navigation interventional procedure. As shown there, a patient  110  is positioned within a remotely actuated, computer controlled interventional system  100 . An elongated navigable medical device  120  having a proximal end  122  and a distal end  124  is provided for use in the interventional system  100  and the medical device is inserted into a blood vessel of the patient and navigated to an intervention volume  130 . A means of applying force or torque to advance or orient the device distal end  124  is provided, as illustrated by actuation block  140  comprising a component  142  capable of precise proximal device advance and retraction and a tip deflection component  144 . The actuation sub-system for tip deflection may be one of (i) a robotically controlled mechanical pull-wire system; (ii) a magnetic system, consisting of external magnets for applying a magnetic field and at least one magnet in the distal portion of the interventional device for steering the device in response to the applied magnetic field; (iii) an electrostrictive system; (iv) a magnetostrictive system; (v) a hydraulic or pneumatic system; or (vi) other navigation system as known in the art. For illustration of a preferred embodiment, in magnetic navigation a magnetic field externally generated by magnet(s) assembly  146  orients a small magnetically responsive element (not shown) located at or near the device distal end  124 . Real time information is provided to the physician by an imaging sub-system  150 , for example an x-ray imaging chain comprising an x-ray tube  152  and a digital x-ray detector  154 , to facilitate planning and guidance of the procedure. 
         [0029]    Additional real-time information such as distal tip position and orientation may be supplied by use of a three-dimensional (3D) device localization sub-system such as comprising one or more electromagnetic wave receivers located at the device distal end (not shown), and associated external electromagnetic wave emitters (not shown); or other localization device with similar effect such as an electric field-based localization system that measures local fields induced by an externally applied voltage gradient. In the latter case the conducting body of a wire within the device itself carries the signal recorded by the tip electrode to a proximally located localization system. The real-time location information provided by the localization sub-system can be used for device contact assessment and control, as described further below. 
         [0030]    The physician provides inputs to the navigation system through a user interface (UIF) sub-system  160  comprising user interfaces devices such as keyboard  162 , mouse  164 , joystick  166 , display  168 , and similar input or output devices. Display  168  also shows real-time image information acquired by the imaging system  150  and localization information acquired by the three-dimensional localization system. UIF sub-system  160  relays inputs from the user to a navigation sub-system  170  comprising 3D localization block  172 , feedback block  174 , planning block  176 , and controller  178 . Navigation control sequences are determined by the planning block  176  based on inputs from the user, and also possibly determined from pre-operative or intra-operative image data and localization data from a localization device and sub-system and processed by localization block  172 , and alternatively or additionally real-time imaging or additional feedback data processed by feedback block  174 . The feedback block  174  can in some cases include one or more of the various contact estimation and optimization methods described in the following. The navigation control sequence instructions are then sent to controller  178  that actuates interventional device  120  through actuation block  140  to effect device advance or retraction and tip deflection. Other navigation sensors might include an ultrasound device or other device appropriate for the determination of distances from the device tip to surrounding tissues, or for tissue characterization. Further device tip feedback data may include relative tip and tissue positional information provided by a local intra-operative imaging system, and predictive device modeling and representation. Such device feedback in particular enables remote control of the intervention. 
         [0031]    In closed-loop implementations, the navigation sub-system  170  automatically provides input commands to the device advance/retraction  142  and tip orientation  144  actuation components based on feedback data and previously provided input instructions; in semi closed-loop implementations, the physician fine-tunes the navigation control, based in part upon displayed information and possibly other feedback data, such as haptic force feedback. Control commands and feedback data may be communicated from the user interface  160  and navigation sub-system  170  to the device and from the device back to navigation sub-system  170  and the user through cables or other means, such as wireless communications and interfaces. Additionally,  FIG. 6  schematically shows “remote navigation interventional procedure” block  180  that performs specific functions in various embodiments of the present invention. Block  180  applies to magnetic navigation system such as that illustrated in  FIG. 6 , and more generally to any medical navigation device and actuation method that also comprises contact sensing/estimation and enhancement, as described in this disclosure. 
         [0032]    In accordance with another embodiment of this invention, real-time tip location data can also be used to estimate the qualitative extent of contact with an anatomical structure. The inventors have discovered that a catheter with a soft shaft (such as magnetic catheters) that is engaged in wall contact will tend to maintain tip contact at a fixed location relative to the cardiac wall during motion of the wall, since the shaft can easily buckle or deform to permit this. Accordingly, the extent of motion of the tip over the cardiac cycle will be largest when contact is very consistent. This is illustrated in  FIGS. 1A and 1B , where in  FIG. 1A , the medical device  20  is in good contact and remains in contact with the moving anatomical surface  22  during the entire cycle. Thus midway through the cycle, the medical device  20 ′ is still in contact with the surface  22 ′, and the device  20  has a total displacement d 1  corresponding to the movement of the surface  22 . As shown in  FIG. 1B  where the medical device is not in good contact, the tip loses contact with the anatomical surface. At the start of the cycle the tip  20  is in contact with anatomical surface  22 , and midway through the cycle medical device  20 ′ loses contact with the surface  22 , and the device has a total displacement d 2  that is less than the total movement of the surface  22 . The motion of the medical device is such that d 1 &gt;d 2 , so that greater device motion corresponds to better contact, and this can be used in several ways to estimate the quality of contact of the device with the anatomical structure. 
         [0033]    One way of estimating the quality of contact is using the oscillation amplitude. The oscillation of the difference [x real −x gated ] can be tracked over a few cardiac cycles, where the three dimensional vectors are defined such that x gated  is always the most recent gated location (gated to the cardiac cycle) and x real  is the realtime location. If the contact is good, the distal tip of the device moves with the movement of the surface, and the graph of the oscillation shows a relatively smooth and continuous movement (as shown in  FIG. 2A ). If contact is temporarily lost for a portion of time between successive cycles, the amplitude of the oscillation will display a “truncated” or flat behavior over such time intervals (as shown in  FIG. 2B ), where the catheter location will stay relatively constant, and this can be detected from signal analysis to assess stability of contact. 
         [0034]    Another way of estimating the quality of contact is using positional covariance. If x mean  is the mean location over a cardiac cycle, set X=(x−x mean ) and form the 3×3 covariance matrix C with entries C ij =&lt;X i X j &gt; where the average &lt; &gt;is taken over a cardiac cycle (or over several cycles). Let a, b and c be the eigenvectors of C, with corresponding eigenvalues e a , e b , e c . These eigenvectors and eigenvalues define the covariance ellipsoid associated with the motion of the catheter tip (as shown in  FIG. 3 ). A soft catheter that maintains good contact with a given target location on the cardiac wall over the cardiac cycle will have larger values of (e a , e b , e c ) than a catheter that is in inconsistent/unstable contact; thus for example, D=(e a +e b +e c ) can be used as a measure of contact. 
         [0035]    Alternative measures based on the eigenvalues of the positional covariance matrix can also be constructed. Thus, alternatively or in addition to the measure D given above, the difference between maximum and minimum eigenvalues (e max −e min ) can be used as a measure of contact; in some situations a larger eigenvalue difference can represent greater relative motion along one direction, thus characterizing the local cardiac wall motion. For example the cardiac ventricles exhibit expansion and contraction in volume together with a strong twisting wall motion; this leads to a local wall motion pattern that can be generally termed an elongated approximate ellipse. A catheter tip in stable contact with such a cardiac wall location would move with the cardiac wall through the cardiac cycle. 
         [0036]    To enhance contact between the medical device and the tissue, the surface normal n to the cardiac wall can be used in order to torque the device tip into the wall. For example, with a magnetic navigation system, the magnetic field can be rotated about an axis defined by v=t×n in order to push the device tip into the wall, where t is the device tip orientation. In this case, let a be the eigenvector of C that has the largest dot product with n ({right arrow over (v)} 1  in  FIG. 4 ); then a can be used in some cases place of n to define the axis of rotation v. While this is useful in magnetic navigation, it can also be exploited with other navigation systems. 
         [0037]    Another way of estimating the quality of contact is using orientational covariance. If t mean  is the mean tip orientation over a cardiac cycle, and T=(t−t mean ), a covariance matrix M can be formed with entries M ij =&lt;M i M j &gt; (average taken over a cardiac cycle), and as with the positional covariance matrix discussed above, the eigenvalues and eigenvectors of M can be used to assess extent of contact. Analogously to the contact measures based on positional covariance eigenvalues, various measures based on the orientational covariance eigenvalues such as the magnitude of the summed eigenvalues, or the difference between maximum and minimum eigenvalues, can be used to assess stability of cardiac contact. 
         [0038]    Another method of measuring contact stability is to perform a frequency analysis of catheter tip motion. The frequency spectrum of the motion of a catheter tip that is in firm contact with the cardiac wall will have a dominant peak at the frequency of the cardiac cycle. Thus, the ratio of the peak (maximum, or dominant frequency) in the Fourier transform of the catheter tip location over a period of time to the next smaller peaks (the sub-dominant peaks) in the frequency spectrum can provide a measure of contact and can in a sense be interpreted as a Signal-to-Noise ratio. The frequency spectrum of the tip position will have to be obtained in general from motion over multiple cardiac cycles, and can be obtained from applying the Fourier transform to the tip position data. The amplitude of the heart rate frequency should be greater with more consistent contact and less with less consistent myocardial contact. More consistent contact should be conveyed by a greater signal-to-noise ratio as the tip moves with the myocardium, and vice-versa. Filtering the tip position data prior to implementing these algorithms can be implemented as a preprocessing step and can lead to improved quantification of contact stability. Of course, it will be important to apply the filter at least several beats prior to the cardiac cycle being analyzed to allow the transient effects of the filter to decay sufficiently. Other measures based on the frequency spectrum such as the spread in frequency between specified peak threshold values, or a variety of other such possible measures that can be determined by those skilled in the art can also be used without limitation and the specific examples of measures detailed here are provided for illustrative purposes only. 
         [0039]    Another way of estimating the quality of contact is using perturbation. A control variable of the navigation system (for example magnetic field direction, or catheter insertion length) can be perturbed or changed by a small amount, and the resulting change in a contact measure (such as one of the eigenvalue-based measures discussed above) can be determined. Thus for instance if contact is found to be enhanced by a small amount, a larger change in the same sense of the corresponding control variables can be applied to further enhance contact. As shown in  FIG. 5A , the tip orientation {right arrow over (t)} 1  is determined with a particular control value, for example the applied magnetic field {right arrow over (B)} 1 . As shown in  FIG. 5B , the control variable is changed, for example from {right arrow over (B)} 1  to {right arrow over (B)} 2 , and the resulting changing in tip orientation {right arrow over (t)} 2  is evaluated, to determine the control variable that provides the optimum, or at least the local maximum, contact. 
         [0040]    Another way of estimating the quality of contact is using unipolar voltage: If unipolar intracardiac ECG voltage information is available in real-time, its magnitude can be used to sense proximity to the wall. In particular, its behavior over a cardiac cycle or a several cycles can be analyzed as in the methods above (except that voltage values would be used instead of positional data) in order to assess stability, with the difference that the variation in voltage will be the smaller when contact is better. 
         [0041]    Still another way of estimating the quality of contact is using a mapped mechanical model. A cardiac mechanical motion model is constructed by mapping over a “coarse” set of cardiac wall locations with a localized mapping catheter, so that the range of wall motion at every map point is recorded at each of these locations. Then the range of mechanical motion at intermediate points can be estimated, for example by interpolation. When the catheter is later placed at such an intermediate location, the range of motion as seen in the real-time location data can be compared with the expected range of motion in order to assess quality of contact, so that the closer the actual range is to the expected range, the better the contact quality. 
         [0042]    While one method of remote actuation involving applied magnetic fields has been described for illustrative purposes, other methods of remote actuation can be involved in the remote navigation control, such as mechanical operation using a set of motorized pull-wires, electrostrictive actuation, hydraulic actuation, and various other actuation schemes known to those skilled in the art. In one embodiment the directly actuated device can be a hollow sheath that carries through it another device or devices. 
         [0043]    Thus the various embodiments of this invention provide for the improved control, and in particular for enhanced measure of contact, and thus control of contact, of a remote medical navigation system. Any method of remote navigation can be implemented. Likewise a variety of contact measures can be constructed by those skilled in the art according to the concepts taught in this invention. These and other features and advantages will be in part apparent, and in part pointed out hereinafter, and the scope of the invention is limited only by the appended claims.