Abstract:
The invention addresses the problem of correctly positioning a catheter and reducing radiation doses. It relates to an X-ray imaging system ( 1 ) for a robotic catheter, comprising said catheter ( 3 ), and a processing unit ( 5 ) for receiving X-ray images of a patient environment ( 15 ). By being adapted to receive one or more auxiliary information items and using said information for determining the catheter position, the processing unit does not entirely have to rely on a large number of scanned image data, thus helping to reduce radiation while correctly delivering the catheter position as a function of as few as a single image, preferably 2D, and said one auxiliary information items. Further, said processing unit allows for at least one of rendering an image and provide said image to a visualization device ( 21 ), and providing feedback, e.g. steering commands, to said robotic catheter.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to an X-ray imaging system for detecting the position and orientation of a robotic catheter inside a patient environment. The invention further relates to a method of determining the position and orientation of a robotic catheter inside said patient environment. Still further, the invention relates to the use of said system for determining said orientation and position of said catheter and a computer program for performing said method. 
       BACKGROUND OF THE INVENTION 
       [0002]    U.S. Pat. No. 8,046,054 B2 discloses an X-ray fluoroscopic imaging system mounted on a ring frame cooperating with a surgical robot. The patent addresses three-dimensional modeling of surgical instruments based on bi-planar imaging. Navigation of the surgical robot is allegedly accomplished by using a pre-planned path of movement and the three-dimensional model. A general problem related to known imaging systems which rely on fluoroscopy is that the patient is subjected to a large dosage of X-ray radiation, leading to an increased risk of cell damage. 
         [0003]    Said bi-planar imagining system uses relatively high radiation doses, which is perceived as disadvantageous. A further problem unsolved by the prior art is the circumstance that the catheter may possess an unknown amount of torque upon entry of the patient&#39;s body and is flexible which makes it all the more difficult to correctly assess its position and orientation. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the invention to provide an X-ray imaging system for a robotic catheter which allows for correct and reliable determination of the orientation and position of said catheter while limiting the patient&#39;s exposure to X-ray radiation. 
         [0005]    It is a further object of the invention to provide a method of determining the orientation and position of a robotic catheter inside a patient which provides for correct and reliable determination of the orientation and position of said catheter while limiting the patient&#39;s exposure to X-ray radiation. 
         [0006]    In a first aspect of the present invention an X-ray imaging system for a robotic catheter is presented, the system comprising a robotic catheter and a processing unit, said processing unit being connectable, preferably through a first data interface, to an X-ray image data source for receiving X-ray image data, preferably two-dimensional X-ray image data, of a patient environment. The processing unit further is connectable, preferably through a second data interface, to said robotic catheter, wherein said processing unit is adapted to 
         [0007]    a) receive one or more auxiliary information items received from at least one of said data source, preferably through said first data interface, and said catheter, preferably through said second data interface, 
         [0008]    b) determine a set of possible three-dimensional orientations and positions of the catheter (relative to said patient environment) as a function of said, preferably two-dimensional, image data and said one or more auxiliary information items. Further, said processing unit is adapted to perform at least one of: 
         [0009]    c) rendering an image of at least one of said set of possible orientations and positions of said catheter, and at least one of said, preferably two-dimensional, image data, and a three-dimensional representation of the patient environment, and providing said image to a visualization device; and 
         [0010]    d) providing feedback to the robotic catheter, said feedback being a function of the set of possible orientations and positions. 
         [0011]    The X-ray image data source preferentially is a C-arm apparatus, and could alternatively also be a computed tomography apparatus. 
         [0012]    If an image is rendered with respect to a three-dimensional representation of a patient environment, preferably said representation is provided in digital form either by the image data source directly or by means of conventional data storage means. The three-dimensional representation may have been provided by previously scanning the patient environment. 
         [0013]    A central benefit of the invention is that while it would also work when using three-dimensional data as data input for the first data interface, detection of orientation and position of the catheter is enabled already on the basis of one single or multiple two-dimensional X-ray images. Instead of multiple further patient scans using X-ray radiation, auxiliary information items are provided to the processing unit through said second data interface which may be used by the processing unit to determine the orientation and position of the catheter. These auxiliary information items are provided without further X-ray scanning, thus lowering the radiation dosage submitted to the patient environment. Based upon this information, a set of orientations and positions is obtained. A set of orientations may comprise one or more orientations and positions of the catheter. Once the position and orientation of the catheter has been determined, the processing unit can be used to visualize the catheter alone, or in combination with a two- or in a three-dimensional representation of the patient environment. With the aid of this visualization, the system can provide feedback for the robotic catheter as a function of the catheter orientation and position. Said feedback may be triggered by user input or may be automatically generated by the processing unit. 
         [0014]    Preferably, said auxiliary information items represent at least one of: curvature of a portion of said catheter or of the entire catheter, a steering command previously transmitted to said catheter, a previous orientation/position/curvature of said catheter, a force feedback signal detected and transmitted by said catheter, and anatomical boundary information from a patient&#39;s three-dimensional model. The curvature of a catheter, or of a portion thereof, may be determined by analyzing the current steering setting of the catheter. Opionally, force feedback signals detected and transmitted by the catheter are considered for predecting the catheter curvature. Force feedback signals are in particular detected when the catheter due to its curvature setting touches a boundary wall of the patient environment, for instance a blood vessel wall. Once predicted, the curvature of the catheter can be matched by the processing unit to the curvature of the catheter as rendered in said image by the processing unit. Applying this curvature-matching technique will deliver one, two or a limited number of possible sets of position and orientation of the catheter. From the aforementioned it is clear that the auxiliary information items also provide for accurate determination of the orientation and position of the catheter. In particular the use of catheter-related information items such as curvature, previous position and steering commands of the catheter jointly or separately eliminates the uncertainty involved with initial torque and skew present along the catheter upon insertion into the body. The use of force-feedback signals and anatomical boundary conditions eliminates the uncertainty involved with an unknown deviation of the catheter&#39;s orientation from what the steering controls would effect when the catheter is in an “unbound” environment i.e. outside of the patient and unrestricted. Overall precision in determining the catheter position and orientation is achieved. 
         [0015]    Preferably, said auxiliary information items repesent at least one of a steering setting, previously transmitted to said catheter, and previous orientation/position/curvature of said catheter. By consideration of previously determined parameters such as the aforementioned, the system is able to select the most probable orientation and position for the catheter among the set of possible orientations and positions. This may be done by eliminating those orientations and positions from the set which deviate too significantly from the previously established positions and orientations to have likely been achieved by ordinary navigation of the catheter. 
         [0016]    The most probable orientation is selected by respectively taking the previously established curvature of the catheter, i.e. pose, into account and is preferentially determined using a similarity measure. Thus, the processing unit identifies one pose out of the set of possible orientations and positions which is most temporally consistent with the previously established pose. The pose with the highest similarity measure is then selected. One preferred similarity measure is performed by generating the sum of eucledian distances for each point in the test pose and their closest counterpart in the previously established pose. 
         [0017]    Preferentially, at the system comprises an input device adapted to receive user input, said user input representing a user choice of one out of the set of possible orientations and positions of the catheter displayed by the visualization device. This embodiment enables the user to manually confirm or select one of several orientations and positions suggested by the system. 
         [0018]    The user input preferentially represents one or more commands for moving the catheter. The commands for moving the catheter are preferentially displayed in display—or image—coordinates and may be entered by the user for example as commands to bend left, right, up, down, front or back, all with respect to the image which is being displayed. 
         [0019]    The input device preferentially is adapted to communicate said user input to the processing unit, the processing unit being adapted to process said user input into a set of steering commands, and said feedback being to provide said set of steering commands to the robotic catheter. The steering commands which are to be relayed to the robotic catheter are translated by the processing unit from the input “display” coordinates into “world” coordinates, i.e. coordinates in the catheter system. 
         [0020]    Alternatively, the processing unit is adapted preferentially to process the three-dimensional orientation and position of the catheter, once establised, relative to said patient environment as a function of said two-dimensional image data and said one or more auxiliary information items into a set of steering commands, wherein said feedback is to provide said set of steering commands to the robotic catheter. In case the orientation and position of the catheter is determined without user input being necessary, and in case the catheter navigation is predetermined, for example by a planning software, the system in this embodiment automatically feeds back steering information to the robotic catheter for advancing on the pre-planned path. 
         [0021]    It is preferred that the catheter tip is adapted to perform small movements along a predetermined path, and the processing unit is adapted to detect said movements from a series of two-dimensional X-ray image data or the patient environment and to deduct herefrom the orientation of said catheter. The series of X-ray image data is preferentially obtained through the X-ray image data source, e.g. a C-arm. The serial detection of small movements of the catheter tip, the so-called catheter wiggle, enables the system to determine which orientation and position of the catheter is the most probable among the set of possible orientations and positions. This is especially true for situations in which the curvature of the catheter is (essentially) parallel to the viewing plane, i.e. the two-dimensional X-ray image. It is then difficult to robustly select the most likely of the several plausible solutions. 
         [0022]    Preferentially, the system is adapted to provide feedback to the user of this uncertainty and to propose either to select a different view (i.e. different scanning angle) or to apply catheter wiggling for resolving the inconsistency. Under a “small” movement, such a movement of the catheter tip is understood which is smaller than the confining space surrounding said catheter tip. As confining space, inter alia the vessel diameter or heart chamber volume may be considered. 
         [0023]    Preferably, said catheter movement, also referred to as “catheter wiggle”, is at first performed in a first plane of movement one or more times, and afterwards, a catheter movement is performed in a second plane which is oriented in an angle with respect to said first plane of movement. If required, it is preferred to repeat this two-step procedure, preferably under variation of the angle between first and second plane. The result herefrom is seen in the projected image, which will indicate differing amplitudes for differing rotation planes and allows calculation of the catheter orientation. 
         [0024]    In a preferred embodiment, the catheter tip comprises multiple radio-opaque markers which are arranged along the tip, preferably in a fixed spatial relation to the wires for sealing said catheter in order to deduct herefrom the orientation of said catheter. 
         [0025]    According to a first preferred alternative, said markers are arranged symmetrically along the tip of the catheter. 
         [0026]    According to a second preferred alternative, said markers are arranged asymmetrically along the tip of the catheter. 
         [0027]    Optionally, also the steering wires of the catheter are provided with multiple radion-opaque markers. The effect of adding these assymetric markers to the catheter tip, and optionally the catheter, is that the steering wire which is being actuated to achieve curvature of the catheter can be related to the catheter in the two-dimensional X-ray image. Due to the assymetry it can be determined from the image if said marker is on the source side or on the detector side of the catheter, the source side specifying that side of the catheter which is closer to the source of radiation, and the detector side being opposite thereof. This allows for determining whether the pulled steering wire causes movement of the catheter tip towards or away from the radioation source. 
         [0028]    In a preferred embodiment, the processing unit is adapted to initialize the aforementioned procedures of determining the pose of said catheter based upon two or more two-dimensional images obtained by the image data source and to subsequently register, i.e. track the orientations and positions being determined. 
         [0029]    Preferentially, the first data interface, by way of which the processing unit is adapted to communicate with the robotic catheter, is adapted to provide steering commands to said X-ray image data source, said steering commands being a function of the orientation and position of the catheter. 
         [0030]    According to a further aspect of the invention, a method of determining the position and orientation of a robotic catheter inside a patient environment is presented, wherein the method comprises the steps of
       obtaining X-ray image data, in particular two-dimensional image data, of said patient environment and said catheter, from an X-ray image data source       
 
         [0032]    a) obtaining one or more auxiliary information items from at least one of the catheter and the image data source, 
         [0033]    b) determining a set of possible three-dimensional orientations and positions of the catheter relative to said patient environment as a function of said, preferably two-dimensional, image data and said auxiliary information item or items, and at least one of: 
         [0034]    c) rendering an image of said set of possible orientations and positions of said catheter and at least one of said, preferably two-dimensional, image data and a three-dimensional representation of the patient environment and providing said image to a visualization device; and 
         [0035]    d) providing feedback to the robotic catheter, said feedback being a function of the set of possible orientations and positions. 
         [0036]    With regard to the benefits and advantages embodiments of this method, it is referred to the benefits and embodiments of the system according to the invention presented hereinabove. 
         [0037]    In particular, it is preferred that the set of possible three-dimensional orientations and positions of the catheter relative to said patient environment is determined by use of information representing at least one of:
       the curvature of said catheter, or of a portion of said catheter,   a steering command previously transmitted to said catheter,   a previous orientation of said catheter,   a previous position of said catheter,   a previous curvature of at least a portion of said catheter,   a force feedback signal detected and transmitted by said catheter, and   anatomical boundary information from a patient&#39;s three-dimensional model.       
 
         [0045]    Further, the method preferably comprises one, several or all of the steps of:
       registering user input, said user input representing at least one of a user choice of one out of the set of possible orientations and positions of the catheter, and one or more commands for moving the catheter;   processing said user input into a set of steering commands for the catheter;   providing said set of steering commands to said catheter.       
 
         [0049]    Preferentially, the method according to the invention further comprises one, several or all of the steps of
       processing the three-dimensional orientation and position of the catheter relative to said patient environment as a function of said, preferably two-dimensional, image data and said one or more auxiliary information items into a set of steering commands,   and providing said set of steering commands to the robotic catheter.       
 
         [0052]    It is preferred that the method according to the invention comprises one, several or all of the steps of
       performing small movements of the catheter tip along a predetermined path,   detecting said movements in a series of X-ray image data, and   deducting the catheter orientation from said movements.       
 
         [0056]    In a further aspect of the present invention, a use of an X-ray imaging system for a robotic catheter is presented for determining the orientation and position of said catheter inside a patient environment, wherein the system is a system according to one of the preferred embodiments of the invention described hereinabove. 
         [0057]    In a further aspect, a computer program for operating an X-ray imaging system according to the invention as described herinabove, in particular according to one of claims  1  to  10  is presented, the computer program comprising program code means for performing the method steps of the method of determining the orientation and position of a robotic catheter inside a patient environment according to the invention as described herinabove, in particular according to one of claims  10  to  13 , when the computer program is run on a computer controlling said X-ray imaging system and comprising said processor unit. 
         [0058]    It shall be understood that the X-ray imaging system according to the invention, in particular of one of claims  1  to  9 , the method according to the invention, in particular of one of claims  10  to  13 , the use according to the invention, in particular of claim  14  and the computer product according to the invention, in particular of claim  15 , have similar or identical preferred embodiments, in particular, as defined in the dependent claims. 
         [0059]    It shall further be understood that a preferred embodiment of the invention can also be any combination of the dependent claims with the respective independent claim. 
         [0060]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0061]    In the following drawings: 
           [0062]      FIG. 1  shows schematically and exemplarily an embodiment of an X-ray imaging system, 
           [0063]      FIG. 2  shows schematically and exemplarily a projection of a catheter pose on an X-ray detector, 
           [0064]      FIG. 3  shows schematically and exemplarily a user input with respect to an X-ray image, 
           [0065]      FIG. 4  shows schematically and exemplarily a flow chart of a method of determining the position and orientation of a robotic catheter, 
           [0066]      FIG. 5  shows schematically and exemplarily a detail of the flow chart according to  FIG. 4 , and 
           [0067]      FIG. 6  shows schematically and exemplarily a further detail flow chart according to  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0068]      FIG. 1  shows an X-ray imaging system  1  according to the invention. The system  1  comprises a robotic catheter  3  and a processing unit  5 . Said processing unit  5  is connected to an X-ray image data source  9  through a first data interface  7 . The image data source  9  is an X-ray detector in this embodiment. The catheter  3  is connected to the processing unit  5  through a second data interface  11 . The processing unit  5  is further connected to an X-ray source  17  which is adapted to emit X-rays  19  towards the image data source  9 . Positioned in between the X-ray source  17  and the image data source  9  is a patient  20 . The patient  20  comprises a region through which the X-rays  19  pass the body of the patient  20 . This region is denominated patient environment  15 . The catheter  3  comprises a catheter tip  13  which is in the state according to  FIG. 1  introduced into the patient environment  15  of the patient  20 . 
         [0069]    The processing unit  5  according to  FIG. 1  is further connected to a visualization device  21 . Optionally, an input device (not shown) is connected to the processing unit  5 . The visualization device  21  and said input device may be integrated into one device, for example comprising a touch screen. The system  1  optionally comprises at least one of the visualization device  21 , the image data source  9 , the X-ray source  17  and the input device to form an integrated system. 
         [0070]    In operation of said system  1 , the X-ray source  17  emits X-rays  19  which pass through the patient environment  15  and are recorded by the image data source  9 . In addition to anatomical information of the patient  20 , the image data source  9  also records geometrical information of the catheter inside the patient environment  15 . Image data is transmitted from the image data source  9  through the first data interface  7  to the processing unit  5 . Furthermore, auxiliary information items in the form of information representing at least one of the curvature of the catheter  3 , steering commands previously transmitted to said catheter  3 , and force feedback signals detected and transmitted by said catheter  3  are transmitted from the catheter  3  to the processing unit  5 . The catheter  3  is either directly coupled to the processing unit  5  or coupled to a catheter control device (not shown) which in turn is connected to the processing unit  5  for data communication. 
         [0071]    The processing unit  5  uses as many auxiliary information items as necessary for determining the curvature of the catheter  3 . Additionally, the processing unit  5  matches the curvature of the catheter, once determined, to the image data transmitted from the image data source  9 . Once the curvature has been successfully mapped to the image data, or at least once a set of possible orientations and positions of the catheter with respect to the image data have been established, an image of the catheter and at least one of said, preferably two-dimensional, image data and a three-dimensional model of the patient environment and the catheter are rendered and transmitted to the visualization device  21 . The three-dimensional model of the patient environment is preferentially supplied in a previous scanning procedure under use of the X-ray source  17  and the image data source  9  or through data storage means. 
         [0072]      FIG. 2  schematically depicts a preferred way of matching the curvature of the catheter  3  to the image data detected by the image data source  9 . In  FIG. 2 , C(t) is the specific description of the curved catheter, called pose, in a given coordinate system. C(t) is obtained from the auxiliary information items as described herein-above. {right arrow over (p)} i  is a set of points representing the projection of the detected catheter on an X-ray image of the image data source  9 . {right arrow over (s)} is the vector leading from the center of the coordinate system to the point of emission of the X-ray source. From the X-ray source, a set of lines L i  are emitted towards the image data source  9 . L i  is determined as 
         [0000]        L   i   ={right arrow over (s)}+τ· ( {right arrow over (p)}   i   −{right arrow over (s)} ). 
         [0073]    Herein, τ is a scaling variable employed to describe a straight line from {right arrow over (s)} (τ is 0) to {right arrow over (p)} i  (τ is 1). 
         [0074]    The coordinates of {right arrow over (s)}, and the coordinates of the projection {right arrow over (p)} i  are known to the processing unit. What is not known and has to be determined is the orientation and position of C(t) in the coordinate system. The processing unit  5  is adapted to transpose, i.e. move the known curvature C(t) within the coordinate system until its projection along L i  resembles the shape of the projection in the plane of the image recorded by the image data source  9 . In addition, by varying the distance between the curvature C(t) and the source of radiation, due to the conical orientation of the rays L i , the position of the curvature with respect to the size ratio shown on the projection {right arrow over (p)} i  may be varied. In mathematical terms, what the processing unit does is minimizing the distance between C(t) and the set of lines L i  along the algorithm 
         [0000]      L i : argmindist(C(t),L i ). 
         [0075]    Following this approach will lead to one or more minima which represent plausible solutions to the algorithm and thus represent a set of possible orientations and positions of the catheter in the coordinate system. 
         [0076]      FIG. 3  shows a schematic visualization of the catheter projection of  FIG. 2  on a display. A coordinate system indicates options for user input  100  in three dimensions. The user can input steering commands based upon the two-dimensional view which is being displayed. The processing unit adapted to translate the user input from the coordinates in the image plane, which is the plane of the detector, into the coordinates of the catheter system and generate a set of steering commands which will cause the catheter to move in the three-dimensional catheter coordinate system. In those embodiments in which the catheter contains radio-opaque markers which preferably are located asymetrically on the catheter, the marker detection in the X-ray image being displayed can help determine the pose of the catheter, or help determine which plausible solution of the set of possible orientations and positions is most likely. In a closed feedback loop it will further be possible to move the X-ray source  17  ( FIG. 1 ) to a different viewing angle relative to the patient environment  15  which is selected as a function of the catheter orientation and position in case the latter is changed during the use of the catheter. 
         [0077]    The method according to the invention is described hereinafter with reference to  FIGS. 4 through 6 . 
         [0078]      FIG. 4  shows a flow chart of a method for determining the orientation and position of a catheter inside a patient environment. In step  201 , a robotic catheter is introduced into the body of a patient, in particular into the patient environment. In step  203 , X-ray image data is obtained, in particular two-dimensional image data from said patient environment and said catheter. Preferably, this is done with an X-ray image data source. 
         [0079]    In step  205 , one or more auxiliary information items are obtained from the catheter and/or the image data source. 
         [0080]    In step  207 , a set of possible three-dimensional orientations and positions of the catheter relative to said patient environment are determined as a function of said, preferably two-dimensional, image data and said auxiliary information item or items. 
         [0081]    In step  209 , the set of possible orientations and positions is reduced to one definite orientation and position of the catheter by performing the procedure of catheter wiggling ( FIG. 6 , described hereinabove and hereinafter). 
         [0082]    After establishing at least said set of possible orientations and positions of the catheter, in step  211   a  an image of said set of possible orientations and positions of said catheter is rendered. Alternatively or additionally, in step  211   b  an image of the two-dimensional image data is rendered. Alternatively or additionally, in step  211   c  an image of a three-dimensional representation of the patient environment is rendered. Steps  211   a - c  may be performed simultaneously or jointly. In step  213 , the three-dimensional orientation and position of the catheter is being processed relative to said patient environment as a function of said two-dimensional image data and said one or more auxiliary information items into a set of steering commands, and said steering commands are provided to the robotic catheter in step  219 . Alternatively or additionally, in step  215  said image is provided to a visualization device. Alternatively or additionally to step  213  and  215 , feedback is provided immediately to the robotic catheter, said feedback being a function of the set of possible orientations and positions. 
         [0083]    Following step  215 , if performed, preferably the steps as shown in  FIG. 5 , are followed through.  FIG. 5  shows the substeps of step  217 . At first, in step  301  user input is registered, wherein said user input represents a user choice of one out of the set of possible orientations and positions of the catheter, and/or one or more commands for moving the catheter. 
         [0084]    Step  303  consists of processing said user input into a set of steering commands for the catheter, and in step  305 , said set of steering commands is provided to said catheter, leading to step  219 . 
         [0085]      FIG. 6  shows in more detail the method step of the “catheter wiggle” procedure. Herein, at first small movements of the catheter tip are performed along a predetermined path in step  401 . Preferably, the catheter tip is at first moved in a first plane for a predetermined number of times, before it is moved in a second plane for a predetermined number of times. The first plane and second plane are oriented in an angle relative to each other. 
         [0086]    Step  401  may be repeated multiple times, until the orientation and position of the catheter has been successfully and definitely been determined. Herein, it is preferred if the angle between the first plane and second plane of movement of the catheter tips varies with each repetition. 
         [0087]    In step  403 , the movements performed by the catheter tip are respectively detected in a series of X-ray image data. In step  405 , the catheter orientation is deducted from said movements. 
         [0088]    The system and method described herein-above are provided for use in endovascular interventional treatment, such as AAA-procedures, TAVI procedures, EP procedures, structural heart disease procedures and others. 
         [0089]    Procedures like the determination of the catheter orientation and position, the obtaining of X-ray image data, obtaining one or more auxiliary items, rendering of images, providing feedback to the catheter, registering user input, processing said input into steering commands for the catheter, et cetera performed by one or several units or devices may be performed by any other number of units or devices. The procedures and/or the control of the X-ray imaging system in accordance with the method for determining a catheter position and the procedure of navigating said catheter can be implemented as program code means of a computer program and/or as dedicated hardware. 
         [0090]    A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. 
         [0091]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
         [0092]    In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. 
         [0093]    A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
         [0094]    Any reference signs in the claims should not be construed as limiting the scope.