Patent Publication Number: US-10762627-B2

Title: Method and a system for registering a 3D pre acquired image coordinates system with a medical positioning system coordinate system and with a 2D image coordinate system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/449,116, filed 3 Mar. 2017 (the &#39;116 application), now U.S. Pat. No. 10,229,496, which is a continuation of U.S. application Ser. No. 14/815,319, filed 31 Jul. 2015 (the &#39;319 application), now U.S. Pat. No. 9,595,102, which is a continuation of U.S. application Ser. No. 14/204,536, filed 11 Mar. 2014 (the &#39;536 application), now U.S. Pat. No. 9,111,175, which is a continuation of U.S. application Ser. No. 13/569,043, filed 7 Aug. 2012 (the &#39;043 application), now U.S. Pat. No. 8,712,129, which is a continuation of U.S. application Ser. No. 12/014,498, filed 15 Jan. 2008 (the &#39;498 application), now U.S. Pat. No. 8,238,625, which claims the benefit of U.S. provisional application No. 60/880,877, filed 17 Jan. 2007 (the &#39;877 application). The &#39;116 application, the &#39;319 application, the &#39;536 application, the &#39;043 application, the &#39;498 application, and the &#39;877 application are all hereby incorporated by reference in their entirety as though fully set forth herein. 
    
    
     FIELD OF THE DISCLOSED TECHNIQUE 
     The disclosed technique relates to medical imaging and positioning systems in general, and in particular, to methods and systems for registering the coordinates of a three dimensional (3D) pre-acquired image with a Medical Positioning System (MPS), the MPS being registered with a two-dimensional (2D) real-time medical image. 
     BACKGROUND OF THE DISCLOSED TECHNIQUE 
     Superimposing a real-time representation of a medical device, such as a catheter or a biopsy needle, tracked by a Medical Positioning X-ray, Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET) and the like, during a medical procedure, is known in the art. This medical image serves as a map, aiding medical staff, performing a medical procedure, to navigate the medical device within a volume of interest in a body of a patient, subjected to this procedure. In order for that superposition to reflect the true position of the medical device within that volume of interest, it is required to register the coordinate system associated with the MPS with the coordinate system associated with the medical image. 
     U.S. Pat. No. 6,149,592 to Yanof et al, entitled “Integrated Flouroscopic Image Data, Volumetric Image Data, and Surgical Device Position Data” is directed to a system for integrating a CT scanner, fluoroscopic x-ray device and a mechanical arm type minimally invasive type surgical tool. In one embodiment, mechanical interconnections, between the CT scanner and the fluoroscopic device, provide a fixed and known offset there between. Mechanical interconnection between the surgical tool and the CT scanner measured by resolvers and encoders provide indication of the position and orientation of the surgical tool relative to the CT scanner. Because the fluoroscopic system is also mechanically constrained, the position and orientation of the surgical tool relative to the fluoroscopic system is also known. 
     In another embodiment, a plurality of transmitters, such as Light Emitting Diodes (LED), are mounted in a fixed and known relationship to the surgical tool or pointer. An array of receivers is mounted in a fixed relationship to the CT scanner. The surgical tool pointer is positioned on a plurality of markers, which are in a fixed relationship to the coordinate systems of the fluoroscopic scanner. Thus, the surgical tool coordinate system and the fluoroscopic scanner coordinate system are readily aligned. 
     U.S. Pat. No. 6,782,287 to Grzeszczuk et al, entitled “Method and Apparatus for Tracking a Medical Instrument Based on Image Registration” is directed to an apparatus, method and system for tracking a medical instrument, as it is moved in an operating space, by constructing a composite, 3-D rendition of at least a part of the operating space based on an algorithm that registers pre-operative 3-D diagnostic scans of the operating space with real-time, stereo x-ray or radiograph images of the operating space. An x-ray image intensifier, mounted on a C-arm, and the surgical instrument are equipped with emitters defining the local coordinate systems of each of them. The emitters may be LED markers which communicate with a tracking device or position sensor. The position sensor tracks these components within an operating space enabling the coordinate transformations between the various local coordinate systems. Image data acquired by the x-ray camera is used to register a pre-operative CT data set to a reference frame of a patient by taking at least two protocoled fluoroscopic views of the operating space, including a patient target site. These images are then used to compute the C-arm-to-CT registration. With the surgical tool being visible in at least two fluoroscopic views, the tool is then back-projected into the reference frame of the CT data set. The position and orientation of the tool can then be visualized with respect to a 3D image model of the region of interest. The surgical tool can also be tracked externally using the tracking device. 
     U.S. Pat. No. 6,246,898 to Vesely et al. entitled “Method for Carrying Out a Medical Procedure Using a Three-Dimensional Tracking and Imaging System”, is directed to a system including a 3D tracking module, an imaging modality, a registration module, an instrument (e.g., catheter), reference transducers and mobile transducers. The transducers may be ultrasonic or electromagnetic transducers. The mobile transducers are coupled with the instrument and with the 3D tracking module. The registration module is coupled with the 3D tracking module and with the imaging modality. The 3D tracking module transforms the measurements of the transducers into XYZ coordinates relative to a reference axis, indicating the position of the instrument. A 3D image, representing the position, size and shape of the instrument, based on the 3D coordinates, is constructed. The imaging modality acquires 2D, 3D or 4D image data sets from an imaging source (e.g., MRI, CT, US). The registration module registers the position of the instrument with the spatial coordinates of the image data set by registering features in the image, such as the reference transducers, with their position in the measuring coordinate system (i.e., 3D tracking module coordinate system). 
     U.S Patent application publication 2005/0182319 to Glossop entitled “Method and Apparatus for Registration, Verification, and Referencing of Internal Organs”, is directed to a method for registering image information of an anatomical region (image space) with position information of a path within the anatomical region (patient space). One or more images of the anatomical region, are obtained (e.g., CT, PET, MRI). A three dimensional model of the anatomical region is constructed. The position information of the path within the anatomical region is obtained by inserting a registration device into a conduit, while a tracking device simultaneously samples the coordinates of the position indicating element coupled to the registration device. A three dimensional path (“centerline”) of the registration device, in the anatomical region, is determined. The registration device includes at least one position indicating element (e.g., a coil that detects a magnetic field that is emitted by an electromagnetic tracking device). The image coordinate system is registered with the coordinate system of the tracking device, using the 3D image model and the 3D path of the registration device. Thus, it is possible to represent on the image, a graphical representation of an instrument, equipped with a position indicating element. However, in the method directed to by Glossop, there is no guarantee that the three dimensional path, obtained by the tracking device, is indeed the path of the center of the conduit. It may be that the tracking device traced a path close to the edges of the conduit or a sinusoidal path within the conduit. Therefore, the registration between the image coordinate system, with the coordinate system of the tracking device, may be rendered inaccurate. 
     U.S. Patent Application Publication 2006/0262970, to Boese et al, entitled “Method and Device for Registering 2D Projection Images Relative to a 3D Image Data Record” directs to a method for registering 2D projection images of an object relative to a 3D image data record of the same object. In the method to Boese et al, a pre-operative 3D data is recorded and a 3D feature (e.g., a model of a vessel tree) is extracted. The same 3D feature is recorded in at least two 2D fluoroscopy images from different C-arm angulations). A 3D symbolic reconstruction of the feature is determined from the two 2D fluoroscopy images. The coordinate systems of the 2D images and the 3D data are registered according to the reconstructed 2D feature from the 2D images and the extracted 3D feature from the 3D data. 
     SUMMARY OF THE PRESENT DISCLOSED TECHNIQUE 
     It is an object of the disclosed technique to provide a novel method and system for registering a three dimensional (3D) pre-acquired image coordinates system with a Medical Positioning System coordinate system and with a two dimensional (2D) image coordinate system. 
     In accordance with the disclosed technique, there is thus provided a method for registering a 3D pre-acquired image coordinates system with a MPS coordinate system and with a 2D image coordinate system. The method comprises the procedure of acquiring at least one 2D image of a volume of interest, acquiring a plurality of MPS points, within the at least one tubular organ, extracting a 3D image model of the at least one tubular organ, estimating a volumetric model of the at least one tubular organ and the 3D coordinate system with the MPS coordinate system and with the 2D coordinate system. The volume of interest includes at least one tubular organ. The 2D image is associated with the 2D coordinate system. The MPS points are associated with the MPS coordinate system. The MPS coordinate system is registered with the 2D coordinate system. The 3D image model is extracted form a pre-acquired 3D image of the volume of interest. The 3D image is associated with the 3D coordinate system. The volumetric model is estimated from the 2D image and the acquired MPS points. The 3D coordinate system is registered with the MPS coordinate system and with the 2D coordinate system by matching the extracted 3D image model and the estimated volumetric model of the tubular organ. 
     In accordance with another aspect of the disclosed technique, there is thus provided a system for registering a three dimensional (3D) pre-acquired image coordinates system with a Medical Positioning System (MPS) coordinate system and with a two dimensional (2D) image coordinate system. The system includes a medical imaging for acquiring at least one 2D image of a volume of interest, and a 3D medical images database for storing pre-acquired 3D images of the volume of interest. The 2D image is associated with the 2D coordinate system. The pre-acquired 3D images are associated with the 3D coordinate systems. The volume of interest includes at least one tubular organ. The system comprises an MPS and a coordinate system registration processor. The MPS is associated with the MPS coordinate system. The MPS coordinate system is registered with the 2D coordinate system. The MPS includes MPS transmitters and an MPS sensor for acquiring a plurality of MPS points within the at least one tubular organ. The coordinate systems registration processor is coupled the MPS, with the medical imaging system and with the 3D medical images database. The coordinate systems registration processor extracts a 3D image model of the tubular organ estimates a volumetric model of the tubular organ and registers the 3D coordinate system with the MPS coordinate system and with the 2D coordinate. The coordinate systems registration processor extracts the 3D image model form the pre-acquired 3D image. The coordinate systems registration processor estimates the volumetric model according to the 2D image and the acquired MPS points and registers the 3D coordinate system with the MPS coordinate system and with the 2D coordinate by matching the extracted 3D image model and the estimated volumetric model of the tubular organ. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
         FIG. 1A  is a schematic illustration of a 3D pre-acquired image  100  associated with a 3D coordinate system  104  in accordance with the disclosed technique; 
         FIG. 1B  is a schematic illustration of a 3D image model  106 , of tubular organ  102 , extracted from 3D pre-acquired image  100  ( FIG. 1A ); 
         FIG. 2A  is a schematic illustration of a trace  122  of a medical device (not shown) in accordance with the disclosed technique; 
         FIG. 2B  is a schematic illustration of a 2D image  112  of the volume of interest; 
         FIG. 2C  is a schematic illustration of estimated volumetric model  124  determined from trace  122  ( FIG. 2A ) and 2D representation  114  ( FIG. 2B ) of the tubular organ; 
         FIG. 3  is a schematic illustration of a registration process in accordance with the disclosed technique; 
         FIG. 4  is a schematic illustration of a system for registering a 3D coordinate system with an MPS coordinate system and with a 2D coordinate system, constructed and operative in accordance with another embodiment of the disclosed technique; 
         FIG. 5  is a schematic illustration of a method for registering a 3D coordinate system (e.g., of a pre-acquired volumetric image) with a 3D MPS coordinate system and with a 2D coordinate system (e.g. of a real-time image), operative in accordance with a further embodiment of the disclosed technique; and 
         FIGS. 6A and 6B  are schematic illustrations of three 2D images, of tubular organ in the body of a patient, acquired at three different activity states of the organ, and the MPS points acquired during these three different activity states, in accordance with another embodiment of the disclosed technique. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The disclosed technique overcomes the disadvantages of the prior art by providing a method and a system for registering a coordinate system associated with a three dimensional (3D) pre-acquired medical image with a 3D coordinate system associated with an MPS and with a 2D coordinate system associated with a 2D image. The system, according to the disclosed technique, pre-acquires a 3D image of the volume of interest, and extracts a 3D image model of at least one tubular organ, within the volume of interest, from that 3D image (e.g., the coronary vessel of the heart). The system further obtains an estimated volumetric model of the same tubular organ. The system obtains this estimated volumetric model, using a trace of a medical device (i.e., a set of locations representing the trajectory of the medical device), which is inserted into the tubular organ, and at least one 2D image of that same organ. The medical device is fitted with an MPS sensor. The system uses these models to register the above mentioned coordinate systems, thus achieving registration with a higher degree of accuracy. 
     The coordinate system associated with the 3D pre-acquired image will be referred to herein as 3D coordinate system. The coordinate system associated with the 2D image will be referred to herein as 2D coordinate system. The coordinate system associated with the MPS will be referred to herein as MPS coordinate system. It is noted that the MPS coordinate system is a 3D coordinate system. The term “registration” refers to finding a transformation associating the coordinates of each point in one coordinate system to the coordinates of the same point in another coordinate system. The terms “trace” and “centerline”, both refer herein to a set of locations representing the trajectory of the medical device. 
     A 3D pre-acquired image (e.g., CT, MRI, PET, 3D Ultra Sound) of a volume of interest can serve as a 3D reference road-map for navigating a minimal invasive medical device, such as a catheter, in that volume. Superimposing a minimal invasive medical device, operative in an MPS coordinate system and fitted with an MPS sensor, on the 3D pre-acquired image, requires registering the 3D coordinate system with the MPS coordinate system. 
     To achieve the registration, prior to a medical procedure, the system according to the disclosed technique, processes (e.g., segments) the 3D pre-acquired image and extracts a 3D model of a tubular organ. The tubular organ is situated within the imaged volume of interest. During a medical procedure (e.g., minimal invasive procedure), the medical staff inserts a medical device, fitted with an MPS sensor, into the tubular organ. An MPS acquires a plurality of MPS points (i.e., a plurality of locations of the MPS sensor within and along the tubular organ), and determines a 3D MPS trace of the shape of the same tubular organ. These MPS points are represented by coordinates in the MPS coordinate system. 
     When the medical staff inserts the medical device fitted with an MPS sensor into the tubular organ, the system obtains a 2D real-time image (e.g., X-ray, 2D Ultra Sound) of that organ. The MPS coordinate system is registered with the 2D coordinate system (e.g., by mechanically coupling the MPS transmitters to the imager). Using the 3D MPS trace and at least one 2D image, the system estimates a volumetric model of the tubular organ, and registers the MPS coordinate system and the 3D coordinate system by matching the extracted image model with the estimated volumetric model. The system achieves this registration with a high degree of accuracy, (i.e., since a volumetric model represents the tubular organ with a high degree of accuracy, than a simple trace of the trajectory of the MPS sensor within the tubular organ). Since the 2D coordinate system is registered with the MPS coordinate system, and the MPS coordinate system is registered with the 3D coordinate system, the 2D coordinate system is also registered with the 3D coordinate system. 
     During the medical procedure, the position and orientation of a patient might change. Consequently, the 2D real-time representation of the volume of interest may also change. These changes may affect the registration between the 3D coordinate system and the 2D coordinate system. Therefore, an MPS reference sensor, placed on the patient during the medical procedure, is operative to detect these changes in the patient position and orientation. The information about these changes may be used either for triggering a registration process or as input for such a registration process. 
     Reference is now made to  FIG. 1A  and to  FIG. 1B .  FIG. 1A  is a schematic illustration of a 3D pre-acquired image  100  associated with a 3D coordinate system  104  in accordance with the disclosed technique. Image  100  is a 3D image of a volume of interest which includes tubular organ  102 .  FIG. 1B  is a schematic illustration of a 3D image model  106 , of tubular organ  102 , extracted from 3D pre-acquired image  100  ( FIG. 1A ). Extracted image model  106  is also associated with 3D coordinate system  104 . 
     Reference is now made to  FIGS. 2A, 2B and 2C .  FIG. 2A  is a schematic illustration of a trace  122  of a medical device (not shown) in accordance with the disclosed technique. Trace  122  is constructed from a plurality of MPS points, such as MPS point  120 , representing the locations of the MPS sensor, fitted on the medical device, acquired when the medical device moves along the tubular organ (i.e., pushed forward or pulled back). These points are represented as coordinates in MPS coordinate system  118 .  FIG. 2B  is a schematic illustration of a 2D image  112  of the volume of interest. 2D image  112  includes a 2D representation  114  of the tubular organ, and the trajectory  116  of the medical device inside this tubular organ. 2D image  112  is associated with 2D coordinate system  110 . When the system according to the disclosed technique, uses an X-ray imager to obtain 2D image  112 , it is desirable to inject the tubular organ with a dye to increase the apparentness of 2D representation  114  of the tubular organ in image  112 . Since MPS coordinate system  118  is registered with 2D coordinate system  110 , each of the MPS points, such as MPS point  120 , has a corresponding point in 2D coordinate system  110 . Using image processing techniques, such as segmentation or edge detection, the system determines the width of 2D representation  114  of the tubular organ for each MPS point. The system uses this width, together with trace  122  of the medical device (i.e., not necessarily the centerline of the tubular organ), to determine an estimated volumetric model of the tubular organ. For example, the width of 2D representation  114  of the tubular organ, at each MPS point, determines the diameter of a circle encircling that point.  FIG. 2C  is a schematic illustration of estimated volumetric model  124  determined from trace  122  ( FIG. 2A ) and 2D representation  114  ( FIG. 2B ) of the tubular organ. Estimated volumetric model  124  is associated with MPS coordinate system  118  and with 2D coordinate system  110 . 
     Reference is now made to  FIG. 3  which is a schematic illustration of a registration process in accordance with the disclosed technique. In  FIG. 3 , the system registers MPS coordinate system  118  with 3D coordinate system  104 , for example, by matching extracted 3D model  106  with estimated volumetric model  124 . Consequent to this registration, 2D coordinate system  110  is also registered with coordinate system  104 . Thus, each point, in each one of coordinate systems  110 ,  118  and  104 , has a corresponding point in each of the other coordinate systems. This registration, between coordinate systems  110 ,  118  and  104 , enables superimposing MPS points of interest, at their respective locations on the 3D image. For example, the 3D pre-acquired image may now serve, for example, as a roadmap for the medical staff, during medical procedures (e.g., treating structural heart disease, deployment of percutaneous valves, ablation, mapping, drug delivery, ICD/CRT lead placement, deploying a stent and other PCI procedures, surgery, biopsy). On this 3D reference roadmap, the system superimposes the 3D trace of the medical device within the tubular organ. This registration further enables superimposing points of interest included in the 3D image, at their respective location on the 2D image. As a further example, the 3D image model of the tubular organ may be projected on the 2D image. Thus, the projected 3D image may serve as a virtual dye, instead of injecting a fluoroscopic dye to the tubular organ prior to obtaining the 2D image. 
     Reference is now made to  FIG. 4 , which is a schematic illustration of a system, generally referenced  150 , for registering a 3D coordinate system with an MPS coordinate system and with a 2D coordinate system, constructed and operative in accordance with another embodiment of the disclosed technique. System  150  includes medical imaging system  168 , a Medical Positioning System (MPS)  174 , a 3D medical images database  176 , a registration processor  178 , a catheter  156 , a display unit  172  and a table  154 . Medical imaging system  168  includes an imaging radiation transmitter  170  and an imaging radiation detector  166 . Medical positioning system  174  includes MPS transmitters  160 ,  162  and  164 , attached to imaging radiation detector  166 , patient reference position sensor  180  and MPS sensor  158 . 
     Display unit  172  is coupled with coordinate systems registration processor  178 . Coordinate systems registration processor  178  is further coupled with 3D medical images database  176 , with MPS  174  and with imaging radiation detector  166 . MPS sensor  158  is fitted on the distal end catheter  156 . MPS transmitters  160 ,  162  and  164  are mechanically coupled with imaging radiation detector  166 . 
     The 3D pre-acquired medical images, stored in 3D medical images database  176 , are associated with a 3D coordinate system. The images acquired by medical imaging system  168  are associated with a 2D coordinate system. MPS  174  is associated with an MPS coordinate system. As mentioned above, since MPS transmitters  160 ,  162  and  164  are mechanically coupled to imaging radiation detector  166 , the MPS coordinate system is registered with the 2D coordinate system. However, when MPS transmitters  160 ,  162  and  164  are not mechanically coupled with imaging radiation detector  166 , the MPS coordinate system may be registered with the 2D coordinate system by placing an MPS sensor, at the 2D image space as a fiducial mark, at pre-determined positions and acquiring a 2D image of the MPS sensor at these locations. MPS  174  determines the location of the MPS sensor in the MPS coordinate system. Registration processor  178  determines the position of the MPS sensor in a plurality of 2D images and registers the 2D coordinate system with the MPS coordinate system. 
     A member of the medical staff inserts catheter  156  in to a patient  152 , lying on table  154  and subjected to a treatment, and navigates the catheter, inside a tubular organ toward a volume of interest (e.g., the cardiovascular system). MPS transmitters  160 ,  162  and  164  transmit magnetic fields which are mutually orthogonal, corresponding to axes of the MPS coordinate system. MPS sensor  158  detects the magnetic fields generated by MPS transmitters  160 ,  162  and  164 . The detected signals are related to the positions of distal end  158 , in the MPS coordinate system, for example, by the Biot Savart law. Medical positioning system  174  obtains a trace of catheter  156  within a tubular organ, situated within the volume of interest. MPS  174  provides this trace to registration processor  178 . 
     Imaging radiation transmitter  170  transmits radiation that passes through patient  152 . This radiation, detected by imaging radiation detector  166 , is a 2D projection of the anatomy of a volume of interest of patient  152 . Imaging radiation detector  166  provides the 2D image to coordinate systems registration processor  178 . 
     Using the MPS trace of catheter  156  and the 2D image, registration processor  178  constructs an estimated volumetric model of the tubular organ. 3D images database  176  provides coordinate systems registration processor  178  a 3D pre-acquired image of the same volume of interest of patient  156 . Registration processor  178  extracts a 3D image model of the tubular organ. The two models are, for example, 3D triangulated mesh representations of the tubular organ. Registration processor  178  registers the 3D coordinate system with the MPS coordinate system, for example, by matching the two models. 
     Reference is now made to  FIG. 5 , which is a schematic illustration of a method for registering a 3D coordinate system (e.g., of a pre-acquired volumetric image) with a 3D MPS coordinate system and with a 2D coordinate system (e.g. of a real-time image), operative in accordance with a further embodiment of the disclosed technique. In procedure  200 , a volume of interest is selected. The volume of interest includes at least one tubular organ. After procedure  200  the method proceeds to procedures  202 ,  206  and  208 . 
     In procedure  202 , a 3D image of the selected volume of interest is pre-acquired. The 3D image is associated with a 3D coordinate system. This 3D image may be, for example, an MRI image, a PET image, a 3D reconstructed Ultrasound image, and the like. The pre-acquired 3D image is stored in a database. With reference to  FIG. 1A , image  100  is an exemplary 3D image. With reference to  FIG. 4 , 3D medical image database  176  stores the 3D pre-acquired image. After procedure  202  the method proceeds to procedure  210 . 
     In procedure  204 , an MPS coordinate system is registered with a 2D coordinate system. This registration is achieved, for example, by mechanically coupling the MPS transmitters to the imaging system. Alternatively, for example, an MPS sensor is placed in the 2D image space as a fiducial mark, at pre-determined positions and 2D images of the MPS sensor are acquired at these locations. MPS  174  determines the location of the MP sensor in the MPS coordinate system. Registration processor  178  determines positions of MPS sensor in the 2D image and registers the 2D coordinate system with the MPS coordinate system. Consequent to this registration, each point in the MPS coordinate system has a corresponding point in the 2D coordinate system. With reference to  FIG. 4 , the MPS coordinate system is registered with the 2D coordinate system by mechanically coupling MPS transmitters  160 ,  162  and  164  to imaging radiation detector  166 . After procedure  204  the method proceeds to procedure  208 . 
     In procedure  206 , at least one 2D real-time medical image is acquired. This 2D real-time medical image is, for example, an X-ray image, of a projection of a volume of interest in a body of a patient. The 2D real-time image is acquired, for example, during a medical procedure involving the use of a medical device, such as a catheter. With reference to  FIG. 4 , 2D medical imaging system  168  acquires at least one 2D real-time medical image. With reference to  FIG. 2B , image  112  is, for example, a 2D real-time medical image. After procedure  206  the method proceeds to procedure  212 . 
     In procedure  208 , a plurality of MPS points, within at least one tubular organ, are acquired in real-time, the tubular organ being within the selected volume of interest. These MPS points are associated with the MPS coordinate system. The MPS points are acquired with a catheter, fitted with an MPS sensor, inserted into the tubular organ. The MPS points are acquired during the insertion of the catheter or during a manual or automatic pullback of the catheter. These MPS points form a trace of the trajectory of the catheter within the tubular organ. With reference to  FIG. 4 , MPS  174  acquires a plurality of MPS points, within the tubular organ. MPS  174  acquires these points with MPS sensor  158  fitted on catheter  156 . With reference to  FIG. 2A , trace  122  is formed from a plurality of MPS points such as point  120 . After procedure  208  the method proceeds to procedure  212 . 
     In procedure  210 , at least one 3D image model of the at least one tubular organ is extracted from the pre-acquired 3D image. This 3D image model is, for example, a 3D triangulated mesh representation of the tubular organ. The 3D image model of the tubular organ is extracted for example by segmenting the 3D pre-acquired image. With reference to  FIGS. 1A, 1B and 4 , registration processor  178  ( FIG. 4 ) extracts a 3D image model, such as 3D image model  106  ( FIG. 1B ), from a 3D pre-acquired image, such as 3D pre-acquired image  100  ( FIG. 1A ). The 3D pre-acquired image is stored in a 3D medical image database  176  ( FIG. 4 ). After procedure  210  the method proceeds to procedure  214 . 
     In procedure  212 , a volumetric model of the at least one tubular organ is estimated according to the at least one 2D image and the acquired MPS points. This volumetric model of the tubular organ is estimated by detecting the border points of the tubular organ, for each point in the at least one 2D image, corresponding to an MPS point. These border points determine the constraints of a closed curve generated around each point on the at least on 2D image (e.g., the circumference of the closed curve must include these border points). In the case wherein one 2D image, of the tubular organ, was acquired from one perspective, the closed curve is a circle. The diameter of that circle is the distance between the detected 2D borders of the tubular organ. When, for example, two 2D images of the tubular organ, were acquired from two different perspectives, the refined contour will have the shape of an ellipse. In the case wherein more than two 2D images were acquired, the shape of the closed curve changes accordingly. The estimated volumetric model is also represented as a 3D triangulated mesh. It is noted that the MPS point need not be at the center of the closed curve. With reference to  FIGS. 2A, 2B, 2C and 4 , registration processor  178  estimates a volumetric model of the tubular organ such as volumetric model  124  ( FIG. 2C ). Registration processor  178  estimates this volumetric model form at least one 2D image such as 2D image  112  ( FIG. 2B ) and a plurality of MPS points such as MPS point  120  ( FIG. 2A ). After procedure  212  the method proceeds to procedure  214 . 
     In procedure  214 , the 3D coordinate system is registered with the MPS coordinate system and with the 2D coordinate system, by matching the extracted 3D image model with the estimated volumetric model of the tubular organ. This registration is performed, for example, by matching the 3D representations of the two models, thus achieving registration with a high degree of accuracy (i.e., since a volumetric model represents the tubular organ with a higher degree of accuracy, than a simple trace of the trajectory of the MPS sensor within the tubular organ). With reference to  FIG. 4 , registration processor  178  registers the MPS coordinate system with the 3D coordinate system and with the 2D coordinate system. 
     It is noted that the system and the method described in conjunction with  FIG. 4  and  FIG. 5 , relate to the case wherein the 3D pre-acquired image, the MPS model and the 2D image are static. However, the disclosed technique is readily extended to the case where the 3D and 2D images change with time, for example, as a result of respiration and cardiac motion due to the cyclic motion of the heart and lungs. A cardiac cycle is defined as the time between two subsequent heart contractions, and the respiratory cycle is defined as the time between two subsequent lung contractions. It is noted that a time changing 3D or 2D image, is composed of a plurality of static 3D or 2D images respectively, each visually representing the organ at a different state. Furthermore, it is noted that for each static 3D or 2D image there is an organ activity state (i.e., a point within the cardiac or respiratory cycles) associated therewith. 
     As mentioned above, during acquisition of the MPS points, the inspected tubular organ may move (e.g., due to the cardiac and respiratory motion). This motion affects the MPS sensor readings (e.g., position and orientation). Therefore, while acquiring MPS point readings, and 3D and 2D time changing images, the system simultaneously acquires organ timing signals of the organ (e.g., the heart, the lungs). These organ timing signals represent the activity states in the cycle of the organ. Accordingly, each MPS point, and each of the static 3D and 2D images, (i.e., the static 3D and 2D images composing the time changing 3D and 2D images), is associated with a respective organ timing signal phase (i.e., an activity state of the organ), for example, an electrocardiogram (ECG) signal. Consequently, each MPS point is associated with a respective static 3D and 2D image, according to the respective organ timing signals thereof. A plurality of MPS points, acquired during the same activity state, define a unique 3D trajectory of the MPS sensor associated with the respective activity state. Thus, the unique 3D trajectory, of the MPS sensor, is also defined for each static 3D and 2D image. 
     Reference is now made to  FIGS. 6A and 6B , which are schematic illustrations of three 2D images, of a tubular organ in the body of a patient, acquired at three different activity states of the organ, and the MPS points acquired during these three different activity states, in accordance with another embodiment of the disclosed technique. The first image of the organ, designated  250   1 , was acquired at a first activity state T 1 . The second image of the organ, designated  250   2 , was acquired at a second activity state T 2 . The third image of the organ, designated  250   3 , was acquired at a third activity state T 3 . During activity state T 1 , MPS points  252 ,  258  and  264  were acquired. During activity state T 2 , MPS points  254 ,  260  and  266  were acquired. During activity state T 3 , MPS points  256 ,  262  and  268  were acquired. Thus, referring to  FIG. 6B , centerline  270  is the projection of the catheter 3D trajectory on image  250   1 . Centerline  272  is the projection of the catheter 3D trajectory on image  250   2 . Centerline  274  is the projection of the catheter 3D trajectory on image  250   3 . Consequent to associating the MPS points, the 2D images and the 3D images with a respective organ timing signal, the system according to the disclosed technique can superimpose the unique 3D trajectory on the respective static 3D or 2D image (i.e., the image associated with the same activity state). Furthermore, only a trajectory, a 2D image and a 3D image, associated with a single organ timing signal reading of interest can be considered. For example, referring back to  FIG. 6B , only image  250   3 , with centerline  274  is considered. A patient reference sensor, such as patient reference sensor  180  ( FIG. 4 ), compensates respiration artifacts as well as patient movements during the acquisition of the 3D, 2D images or the organ timing signal. It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.