Patent Publication Number: US-8523787-B2

Title: Detection of tenting

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to medical procedures, and specifically to detection of tenting during a procedure. 
     BACKGROUND OF THE INVENTION 
     Invasive medical procedures using a catheter probe typically involve the probe contacting internal tissue of the patient undergoing the procedure. Such contact typically involves the probe applying force to the tissue, and the force in turn may cause unwanted tenting of the tissue. 
     U.S. Patent Application 2006/0173480 to Zhang, whose disclosure is incorporated herein by reference, describes a system which is stated to more accurately control insertion of penetrating instruments (e.g., trocars, needles, or the like) into a body cavity, organ, or potential space. The disclosure describes coupling an accelerometer to the penetrating instrument, so as to achieve the control. 
     Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
     The description above is presented as a general overview of related art in this field and should not be construed as an admission that any of the information it contains constitutes prior art against the present patent application. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a method, including: 
     measuring a force exerted by a probe on tissue of a patient; 
     measuring a displacement of the probe while measuring the force; and 
     detecting a tenting of the tissue responsively to a relation between the measured force and the measured displacement. 
     Typically, detecting the tenting includes confirming that the relation consists of a mathematically direct relationship between a first magnitude of a change in the measured force and a second magnitude of the measured displacement. The method may further include measuring the change in the measured force in a direction defined by the measured displacement. 
     In a disclosed embodiment measuring the force includes measuring a change in the force, and detecting the tenting includes determining that the change in the force is greater than a preset force change range. 
     In a further disclosed embodiment detecting the tenting includes determining that the displacement is greater than a preset displacement range. 
     The method may include measuring a size of the tenting in response to the measured displacement. 
     Typically, the method includes issuing a warning to an operator of the probe in response to detecting the tenting. 
     In an alternative embodiment the method includes adjusting a map of coordinates of the tissue in response to detecting the tenting. Typically, the tenting of the tissue includes a conical formation in the tissue, and adjusting the map includes preparing the map absent a location of an apex of the conical formation. Typically, preparing the map includes determining a location of a base of the conical formation and using coordinates of the location of the base in preparing the map. 
     In another alternative embodiment the method includes correcting the measured force in response to at least one of a heartbeat and a respiration of the patient. 
     There is further provided, according to an embodiment of the present invention, apparatus, including: 
     a probe including: 
     a force sensor configured to measure a force exerted by the probe on tissue of a patient, and 
     a position transducer configured to measure a displacement of the probe while the force sensor is measuring the force; and 
     a processor which is configured to detect a tenting of the tissue responsively to a relation between the measured force and the measured displacement. 
     There is further provided, according to an embodiment of the present invention, a computer software product including a tangible computer-readable medium having non-transitory computer program instructions recorded therein, which instructions, when read by a computer, cause the computer to: 
     measure a force exerted by a probe on tissue of a patient; 
     measure a displacement of the probe while measuring the force; and 
     detect a tenting of the tissue responsively to a relation between the measured force and the measured displacement. 
     The present disclosure will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a tenting detection system, according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a distal end of a probe used in the system, according to an embodiment of the present invention; 
         FIG. 3  illustrates a tenting situation that may be generated during the manipulation of a probe, according to an embodiment of the present invention; 
         FIG. 4  illustrates another tenting situation that may be generated during the manipulation of a probe, according to an embodiment of the present invention; and 
         FIG. 5  is a flow chart of a process for detecting tenting, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     An embodiment of the present invention provides a method for detection of tenting in body tissue of a patient. The method may typically be applied while a patient is undergoing a medical procedure comprising insertion of a probe into a chamber of the patient&#39;s heart. The method comprises measuring the force exerted by the probe on the body tissue. In the case of the heart procedure the tissue is typically the endocardium. While the force is being measured, the displacement of the tissue is also measured. Both measurements may be made using respective sensors in the probe, one measuring the position of the probe, the other measuring the force exerted by the probe on the tissue. 
     Tenting may be detected by observing the behavior of the measured force compared to that of the measured displacement, i.e., by observing how the two parameters are related. Typically, if the force, measured in the direction of the displacement, increases as the displacement increases, i.e., if there is a mathematically direct relationship between the magnitude of the force and the magnitude of the displacement, tenting is occurring. 
     The direct relationship occurring during tenting is in contrast to the typical relationship if no tenting occurs. In the case of a probe contacting the endocardium, typically the beating of the heart, and/or the respiration of the patient, cause both the displacement of the probe and the force measured by the probe to change. However, in a “normal,” non-tenting situation, the force typically decreases as the displacement increases, so that the two quantities have a mathematically inverse relationship. 
     The direct relationship between the force and the displacement that occurs during tenting thus provides a clear, simple indication for tenting detection. 
     System Description 
     Reference is now made to  FIG. 1 , which is a schematic illustration of a tenting detection system  20 , and to  FIG. 2 , which is a schematic diagram of a distal end of a probe used in the system, according to embodiments of the present invention. System  20  comprises a probe  22 , herein assumed to be a catheter, and a control console  24 . In the embodiment described herein, it is assumed by way of example that probe  22  may be used for mapping electrical potentials in a heart  26  of a patient  28 . Alternatively or additionally, probe  22  may be used for other therapeutic and/or diagnostic purposes, such as for ablation, in the heart or in another body organ. 
     Console  24  comprises a processor  42 , typically a general-purpose computer, with suitable front end and interface circuits for receiving signals from probe  22  and for controlling the other components of system  20  described herein. Processor  42  may be programmed in software to carry out the functions that are used by the system, and the processor stores data for the software in a memory  50 . The software may be downloaded to console  24  in electronic form, over a network, for example, or it may be provided on non-transitory tangible media, such as optical, magnetic or electronic memory media. Alternatively, some or all of the functions of processor  42  may be carried out by dedicated or programmable digital hardware components. 
     An operator  30  inserts probe  22  through the vascular system of patient  28  so that a distal end  32  of probe  22  enters a chamber of heart  26 . System  20  typically uses magnetic position sensing to determine position coordinates of the distal end inside heart  26 . In this case console  24  comprises a driver circuit  34 , which drives magnetic field generators  36  placed at known positions external to patient  28 , e.g., below the patient&#39;s torso. A magnetic field sensor  38  within the distal end of the probe generates electrical position signals in response to the magnetic fields from the coils, thereby enabling processor  42  to determine the position, i.e., the location and typically also the orientation, of distal end  32  within the chamber. Sensor  38 , also referred to herein as sensor “P,” typically comprises one or more coils, usually three coils orthogonal to each other. This method of position sensing is implemented, for example, in the CARTO™ system, produced by Biosense Webster Inc. (Diamond Bar, Calif.) and is described in detail in U.S. Pat. Nos. 5,391,199, 6,690,963, 6,484,118, 6,239,724, 6,618,612 and 6,332,089, in PCT Patent Publication WO 96/05768, and in U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are all incorporated herein by reference. 
     In an alternative embodiment, the roles of position sensor  38  and magnetic field generators  36  may be reversed. In other words, driver circuit  34  may drive a magnetic field generator in distal end  32  to generate one or more magnetic fields. The coils in generator  36  may be configured to sense the fields and generate signals indicative of the amplitudes of the components of these magnetic fields. Processor  42  receives and processes these signals in order to determine the position of distal end  32  within heart  26 . 
     Although in the present example system  20  is assumed to measure the position of distal end  32  using magnetic-based sensors, embodiments of the present invention may use other position tracking techniques, for example, tracking systems based on impedance measurements. Impedance-based position tracking techniques are described, for example, in U.S. Pat. Nos. 5,983,126, 6,456,864 and 5,944,022, whose disclosures are also incorporated herein by reference. Other position tracking techniques, known to one having ordinary skill in the art, may be used to determine the position of distal end  32 . Thus, in the present application, the term “position transducer” is used to refer to any element which provides signals, according to the location and orientation of a probe or a section of a probe, such as the probe&#39;s distal end, to console  24 . 
     Distal end  32  also comprises a force sensor  48 , also referred to herein as sensor “F,” which is able provide electrical force signals to processor  42  in order to measure the magnitude and direction of the force on the distal end. The direction of the force is typically measured relative to a symmetry axis  52  of the distal end. Various techniques may be used in measuring the force. Components and methods that may be used for this purpose are described, for example, in U.S. Patent Application Publications 2009/0093806 and 2009/0138007, whose disclosures are incorporated herein by reference and which are assigned to the assignee of the present patent application. These patent applications describe a probe whose distal tip is coupled to the distal end of the probe by a spring-loaded joint, which deforms in response to pressure exerted on the distal tip when it engages tissue. A magnetic position sensing assembly within the probe, comprising transmitting and receiving coils on opposite sides of the joint, senses the position of the distal tip relative to the distal end of the probe. Changes in this relative position are indicative of deformation of the spring and thus give an indication of the magnitude and direction of the force on the probe, i.e., on its distal tip. 
     In order to map the chamber of heart  26 , operator  30  manipulates probe  22  so that distal end  32  is at multiple locations on (or in close proximity to) the inner surface of the chamber. At each location, an electrode  40  coupled to the distal end measures a certain physiological property (e.g., the local surface electrical potential). Processor  42  correlates the location measurements, derived from the position signals of sensor  38 , and the electrical potential measurements. Thus, the system collects multiple map points, with each map point comprising a coordinate on the inner chamber surface and a respective physiological property measurement at this coordinate. 
     Processor  42  uses the coordinates of the map points to construct a simulated surface of the cardiac chamber in question. An example method for constructing the simulated surface is described further below. Processor  42  then combines the electrical potential measurements of the map points with the simulated surface to produce a map of the potentials overlaid on the simulated surface. Processor  42  displays an image  44  of the map to operator  30  on a display  46 . 
       FIG. 3  and  FIG. 4  respectively illustrate first and second tenting situations that may be generated during the manipulation of probe  22  by operator  30 , according to embodiments of the present invention. Tenting is the formation of a local generally conical structure, or “tent,” in tissue, herein assumed to be a heart wall  102 , and is typically caused by excessive force on a region  104  of the tissue, causing the region to form a tenting cone. The excessive force is typically caused by the distal tip of the probe pushing against region  104 , the contact location of the tip with the region forming an apex of the tent. During reconstruction of the region, the tenting effect may also be observed as a conical formation, or tent, in the reconstruction. 
     Processor  42  may use a surface reconstruction algorithm, which typically connects the outermost points of a set of mapped locations of the heart wall, to generate the surface map of the wall. By way of example, points  106  and  108  are assumed to be comprised within the set of mapped points. In this case, a tented region such as region  104  may cause significant deformation in the map, as described above. More seriously, excessive tenting may lead to perforation of the heart wall at the tenting site. As is described herein, embodiments of the present invention provide a warning to operator  30  that tenting is occurring, and also correct for any deformation in the surface map caused by the tenting. 
     The inventors have observed that tenting typically occurs when the force between a probe and tissue contacted by the probe grows as the probe moves forward in the direction of the force. Such a scenario typically occurs if the distal end of the probe engages the tissue head-on. An alternative scenario occurs when a guiding sheath around the probe constrains the probe to engage the tissue in a non-head-on, or oblique, direction. In this case, while the orientation of the probe to the tissue is oblique, the motion of the probe is in the same direction as the resolved force on the tissue. In both cases, the magnitude of the force and the magnitude of the displacement are in a mathematically direct relationship with each other, i.e., as the magnitude of the force increases, the magnitude of the displacement also increases. 
     The latter property contrasts with the typical case of a probe in contact with a “normal” heart wall, wherein, as the wall moves away from the probe, due to the heart beating and/or due to respiration, the magnitude of the force measured by the probe decreases while the magnitude of the displacement increases. Such a mathematically inverse relationship, i.e., where the force decreases as the displacement increases, occurs regardless of whether the contact between the probe and the heart wall is head-on or oblique. 
     A diagram  110  ( FIG. 3 ) illustrates a first tenting situation wherein distal end  32  is in contact with, and exerting a force on, heart wall  102 . In this situation, probe  22  engages wall  102  in a head-on manner. An arrow  112  represents the force vector exerted by the probe on region  104 , as measured by force sensor F. An arrow  114  represents the displacement vector of the probe from a position  116 , where tenting begins to occur, to a position  118 , in region  104 , where the tenting terminates. By way of example, the direction of the displacement is assumed to define the direction of a local x-axis for region  104 . Position  118  corresponds to the apex of the tenting cone formed in region  104 , and the displacement vector may be derived from locations measured by position sensor P. As is illustrated in the diagram, the force vector and the displacement vector are parallel. 
     A schematic graph  120  plots the magnitude of the force |F| vs. the magnitude of the displacement |D|, as the tenting situation develops, i.e., as the distal tip of the probe moves from position  116  to position  118 . As is illustrated by the graph sloping upward to the right, in the case of tenting the two magnitudes are directly related. 
     For comparison, a schematic graph  125  plots the magnitude of the force |F| vs. the magnitude of the displacement |D|, when no tenting is present, i.e., during motion of the heart wall due to the heart beating and/or respiration. In this no tenting case, as the displacement magnitude increases the force magnitude decreases, so the two magnitudes are inversely related. This is illustrated by the graph sloping downward to the right. 
     A diagram  130  ( FIG. 4 ) illustrates a second tenting situation wherein distal end  32  is in contact with, and exerting a force on, heart wall  102 . In this second situation, probe  22  is constrained by a sheath  132  to engage wall  102  obliquely. An arrow  134 , substantially the same as arrow  114 , represents the displacement vector of the probe from initial tenting position  116 . An arrow  136  represents the overall force vector exerted by the probe on region  104 , and an arrow  138  represents the force vector resolved in the direction of the displacement, i.e., parallel to the x-axis. In the second tenting situation the direction of the overall force is not parallel to the displacement, and in one embodiment the magnitude of the resolved force in the direction of the displacement is typically approximately of the order of 80%, depending on the degree of obliquity, of the value of the magnitude of the overall force. 80% corresponds to an obliquity of approximately 30°, but embodiments of the present invention encompass other angles, which may be more or less than 30°, such as 45°. 
     A schematic graph  140  plots the magnitude of the resolved force |F x | vs. the magnitude of the displacement |D|, as the second tenting situation develops. As is illustrated by the graph, the two magnitudes in the second tenting situation are also directly related. 
     The quantities F range , D range , |ΔD|, |ΔF|, and |ΔF x |, shown in graphs  120  and  140 , are described below, with reference to the flow chart of  FIG. 5 . 
       FIG. 5  is a flow chart  150  of a process for detecting tenting, according to an embodiment of the present invention. The process uses the characteristics described above with reference to  FIGS. 3 and 4 , concerning the relationship between the force and the displacement, and by way of example is directed towards detecting tenting in the endocardium. 
     In a first step  152 , operator  30  inserts probe  22  into patient  28  so that distal end  32  of the probe enters a chamber of heart  26 . The probe is inserted until it contacts the endocardium. The contact with the endocardium may be detected by a number of different methods, such as by observing that the potential on electrode  40  corresponds to that generated by the endocardium, determining that the force measured by force sensor  48  is above a zero level of the sensor, and/or determining that the position registered by position sensor  38  corresponds to coordinates of the endocardium. The endocardium coordinates may be determined from prior measurements with position sensor  38 , and/or by imaging heart  26  with systems using ultrasound, fluoroscopy, or magnetic resonance imaging. 
     In step  152  the probe, without a surrounding sheath, may be inserted to contact the endocardium, as is illustrated schematically in  FIG. 3 . Alternatively, the probe may have a surrounding sheath, as is illustrated in  FIG. 4 . 
     In a force measurement step  154 , processor  42  uses the signals from force sensor  48  to calculate a magnitude of the force exerted by the distal end of probe  22  on the tissue of the endocardium. The processor also evaluates the direction of the force, relative to symmetry axis  52  of the distal end ( FIG. 2 ), from the signals. 
     In a first comparison step  156 , the processor checks if the magnitude of the force is greater than or equal to a preset contact threshold value. A typical value for the contact threshold is approximately 3 g. If the magnitude is less than the contact threshold, the process returns to step  154 . If the magnitude exceeds the threshold, processor  42  continues to a force and displacement measurement step  158 . 
     In force and displacement measurement step  158 , the processor calculates, while the magnitude of the force is greater than the contact threshold used in step  156 , values of the magnitude and the direction of the force. Simultaneously, processor  42  uses the signals from position sensor  38  to evaluate locations of the distal end of the probe. The values are assumed to be measured over a period of time herein termed the measurement period. The processor stores the values of the force magnitude and direction, and the values of the locations, in memory  50 . 
     In an evaluation step  160 , the processor analyzes the values stored in memory  50 . 
     From analysis of the location values the processor determines the overall displacement vector, {right arrow over (ΔD)}, of the distal end, from the difference between the final location and the initial location of the distal end for the measurement period. Displacement vector {right arrow over (ΔD)} has a direction and magnitude, and the direction is herein assumed to define the direction of a local x-axis (as illustrated in  FIGS. 3 and 4 ). The processor also calculates the magnitude, |ΔD|, of the overall displacement. The displacement magnitude |ΔD| is used in graphs  120  and  140 . 
     From analysis of the force measurement values, the processor determines directions of the force during the measurement period. Typically, for an unsheathed probe the force directions are parallel to the direction of the overall displacement, i.e., are parallel to the local x-axis, as is illustrated in  FIG. 3 . Typically, for a sheathed probe, the force directions are parallel to the sheath and are oblique to the local x-axis, as is illustrated in  FIG. 4 . 
     For each force measurement taken in the measurement period, the processor calculates a force vector, {right arrow over (F)}, as a direction and as a magnitude |F|. The processor resolves the force vector {right arrow over (F)} along the local x-axis, and determines resolved magnitudes of the force, |F x |. (For the head-on case of  FIG. 3  the resolved and unresolved forces are equal; however, for the oblique case of  FIG. 4  the resolved force is less than the unresolved force.) From the final and initial resolved force magnitudes, respectively corresponding to the final and initial locations of the distal end, the processor calculates the value of an overall change in resolved force magnitude, |ΔF x |. Graph  140  illustrates the change in resolved force magnitude |ΔF x |. Graph  120  illustrates the change in overall force magnitude |ΔF|; since the graph is for a head on situation, |ΔF|≡|ΔF x |. 
     In a second comparison step  162 , the processor checks if the following inequalities are valid:
 
|ΔD|&gt;D range   (1)
 
|ΔF x |&gt;F range   (2)
 
     
       
         
           
             
               
                 
                   
                     
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     D range  and F range  are preset minimum ranges of |ΔD| and |ΔF x | that are used by processor  42 , and that are illustrated in graphs  120  and  140 . Typical values for D range  and F range  are approximately 4 mm and approximately 8 g respectively. The processor uses inequalities (1) and (2) to ensure that the values used to check inequality (3) are not too small. Using values that are too small could cause the check of inequality (3) to be adversely affected, e.g., by noise. 
     The validity of inequality (3) determines that the relationship between |ΔD| and |ΔF x | is a mathematically direct relationship, so that as the magnitude of the displacement increases the magnitude of the resolved force also increases. The direct relationship is illustrated by graphs  120  and  140 . 
     It will be understood that inequality (3) is typically invalid during normal beating of the heart and respiration of the patient, wherein as the magnitude of the displacement increases the magnitude of the resolved force decreases, so that the relationship is an inverse relationship. Such an inverse relationship is illustrated by graph  125 . Thus the validity of inequality (3) confirms that tenting is occurring, and that the changes in force and displacement are not typical of the normal behavior of the heart. 
     If any of inequalities (1), (2), and (3) are invalid, the flow chart returns to step  154 . 
     If all inequalities (1), (2), and (3) are valid, the processor proceeds to a warning step  164 . 
     In warning step  164 , the processor assumes that tenting may be occurring, and issues a visual and/or audible warning to operator  30 , for instance, by placing a notice on display  46 , that tenting may be occurring. The processor may also calculate a size of the tenting, by using the results stored in memory  50  to find initial location  116  ( FIGS. 3 and 4 ) of the tented tissue (the point at which the tenting began), and final location  118  of the tented tissue, the apex of the tent formed. The size may be included in the warning. In some embodiments, if the tenting size is greater than a preset value, the warning may be enhanced to reflect a possible dangerous situation. A dangerous tenting size typically depends on the thickness of the tissue that is undergoing the tenting. The thickness of the tissue may be known, or may be estimated, for example from a knowledge of location  116 . Alternatively or additionally, a dangerous situation may be assumed if 
                        Δ   ⁢           ⁢     F   X                   Δ   ⁢           ⁢   D            &gt;   Q     ,         
where Q is a positive value, typically greater than 2 g/mm.
 
     An optional mapping step  166  (shown as optional by broken lines in the flow chart) is typically implemented if the processor is generating a map of the locations of the endocardium using a mapping algorithm. In step  166 , the processor replaces location  118  of the tent apex with initial location  116  of the tented tissue, calculated in step  164 , and uses this value as the location of the tissue. The replaced location is used for recalculating the map using the mapping algorithm. 
     Flow chart  150  then ends. 
     The description of the steps of flow chart  150  has assumed that the forces measured by force sensor F have not undergone any correction due to heartbeat and/or respiration of the patient. Some embodiments of the present invention may apply such a correction, for example, by measuring or estimating the forces applied to the force sensor from a “typical” heart, over a number of heartbeats and respiration cycles, so as to determine a typical force vs. time relationship for the force sensor. The processor may use the relationship to find the expected typical force measurement at times when tenting may be occurring, and subtract these typical force measurements from the actual forces measured by the force sensor. The corrected forces may then be used in inequalities (2) and (3) above. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.