Abstract:
A method and a device for reference image adapting in the field of fluoroscopy-controlled interventional repair of abdominal aortic aneurisms on angiography systems are proposed. Displacements which can be brought about as a result of introducing instruments, such as when a stent is deployed in an aorta, are automatically corrected. It is also possible to correct such displacements which initially cannot be perceived in the image due to the angle of view.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of German application No. 10 2010 012 621.7 filed Mar. 24, 2010, which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to the field of fluoroscopy-controlled interventional repair of abdominal aortic aneurisms (=AAA) on angiography systems. 
     BACKGROUND OF THE INVENTION 
     An abdominal aortic aneurism is a dilatation of a vessel on the abdominal aorta. This condition is treated by insertion of a so-called stent graft, i.e. composite angioplasty devices. Guide wires and catheters are inserted into the aorta via both groins and one or more stent grafts are introduced via said guide wires and catheters. An abdominal aortic aneurism A of this type is shown by way of example in  FIG. 1   a . It is treated by insertion of a stent graft S. Guide wires and/or catheters K are inserted into the aorta via both groins and the stent grafts are introduced with the aid of said guide wires and catheters. 
     The aim when inserting said stent grafts is to place the “landing zone” of the vascular graft as far as possible in the healthy vessel wall region, though at the same time taking care that no important branch vessels are covered. In particular the branches of the renal arteries, of the superior mesenteric artery (arteria mesenterica superior), of the truncus coeliacus, and of the internal iliac arteries (a. iliaca interna) must be kept free. A sensitive point is the placement of the “main stent” in the aorta, in which case the cited branch vessels must not be occluded. In the case of complex stents which include the leg arteries, as shown for example in  FIG. 1   c , the final stent must sometimes be composed of “partial stents” (e.g. an aortic stent (I) to which the stent for the leg artery (II) is attached through what is termed a fenestration). 
     In order to avoid the necessity of injecting contrast agent to allow constant vessel visualization for monitoring purposes during the complex stent positioning procedure it is possible to overlay a reference image (anatomically correctly) in the manner of a positioning aid, which reference image renders the vessels (in this case aorta and branch vessels). As shown in  FIG. 2   a , said reference image can either be a 2D angiogram (DSA) or, as shown in  FIG. 2   b , it can beneficially be a previously recorded 3D dataset (e.g. a CT angiography sequence) of the aneurysm. These show more details and can be overlaid at any angulation of the C-arm. 
     As shown in  FIG. 3 , it is possible to pre-segment the aneurysm from the reference images. In this case the course of the vessel (centerline) or the contours of the vessels can be determined, for example. This can happen both in the case of the 2D reference image, as shown in  FIG. 3   a , and in the case of the 3D reference image, as shown in  FIG. 3   b . Furthermore it is possible to identify and track instruments (e.g. catheters or guidance devices) in 2D images. As already shown in  FIG. 2 , a partially flexible 2D-3D or 3D-3D registration, e.g. of 2D and 3D angiography images, is possible. 
     A problem with said overlays is that the reference image (2D or 3D) shows the vessel anatomy at a specific instant in time. If, as shown e.g. in  FIG. 4 , the physician introduces very inflexible or rigid instruments, e.g. a catheter K, the anatomy is deformed. If said deformation is not corrected in the overlaid reference image (see  FIGS. 4   a  and  4   b ), an imprecision or a discrepancy arises when the reference image is superimposed. This can lead to uncertainties in navigation during an intervention in which the overlay serves as a navigation aid. 
     SUMMARY OF THE INVENTION 
     As mentioned in the introduction, it is the object of the invention to correct deformations of said kind. 
     The object is achieved by means of the method and the device as claimed in the independent claims. Advantageous embodiments of the method and of the device are the subject matter of the dependent claims or can be derived from the following description and the exemplary embodiments. 
     The subject matter of the invention is a method for adapting at least one reference image, suitable in particular for inserting a stent into an aorta, which automatically corrects displacements which can be brought about as a result of introducing instruments, e.g. a catheter. At the same time it is preferred if displacements of the kind which initially cannot be perceived or are not visible in the image at corresponding viewing angles can also be corrected. 
     A further aspect of the invention is a device for the above-described adapting method, the device having means for performing the said adaption. The embodiment variants characterized in the dependent claims in relation to a method according to the invention also apply in respect of the device according to the invention. 
     The invention thereby increases the precision in the overlaid reference image which serves as a navigation aid during a medical intervention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages, details and developments of the invention will emerge from the following description of exemplary embodiments in conjunction with the drawings, in which: 
         FIGS. 1 ,  2 ,  3  and  4  cited in the introduction show the possible prior art and the above-described problem of deformation, and 
         FIGS. 5   a  and  5   b  show the correction according to the invention in the image plane, and 
         FIGS. 6   a ,  6   b  and  6   c  show the correction according to the invention in a three-dimensional representation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following the principle of the correction according to the invention is described with reference to the repair of an aortic aneurysm in the following embodiment variants: 
     Ideally the following preconditions are established:
     1. A reference dataset, registered with respect to the C-arm (or, as the case may be, the respective fluoroscopic images), which represents either a 3D volume, e.g. a previously performed CT angiography, or a C-arm CT recorded during the intervention, or else 2D images, e.g. angiography sequences (DSA) of the corresponding vessels,   2. information relating to the course (e.g. the so-called centerline) of the vessels or the course of vessel contours and/or other corresponding information, as shown for example in  FIG. 3 , in the reference images, e.g. relating to a (semi-)automatic 2D or 3D segmentation (depending on dataset used), and   3. a means of detecting and tracking the instrument(s) introduced, e.g. the instrument for introducing the stent. This can happen e.g. by way of a corresponding identification or tracking of the instruments in the fluoroscopic images.   

     As shown in  FIG. 5 , the procedure according to the invention is preferably performed in the following sequence:
         The position detected in the fluoroscopic image identifies the current course of the vessel, since the instrument is located inside the vessel.   The reference image (or the centerline of the segmentation of the reference image) is then adapted (updated) accordingly or, as the case may be, distorted or, as the case may be, displaced or, as the case may be, deformed so that the current vessel course and assumed vessel course are consistent once again. In this case the corresponding part of the vessel in the reference image is brought into congruence with the current vessel course resulting from the position of the detected instrument, e.g. a catheter K, as shown for example in  FIG. 5   a . The reference image is adapted differently according to position and penetration depth of the instrument; see e.g. also  FIG. 5   b.      The remainder of the vessel course (i.e. the part in which no instrument has yet been introduced) is extrapolated e.g. on the assumption of “smoothness conditions”, i.e. generally the vessels have no abrupt bends or similar. In this case regions remote from the detected instrument are not deformed (e.g. the renal arteries when an instrument is introduced into the leg arteries) and closer regions are deformed in such a way that a smooth vessel course is preserved.   In the deformation of the reference image (in particular of a 3D vessel) attention can also be devoted in particular to a “trueness to length”, since an introduced instrument does not change the length of the centerline of the vessel. By way of these boundary conditions it is possible in particular to correct deviations which initially are not visible in the image on account of the viewing angle, as shown for example in  FIG. 6   a . If the vessel is e.g. curved “in the viewing direction”, i.e. perpendicular to the image plane, an introduced instrument initially creates no visible displacement between overlaid reference image and fluoroscopic image. However, if the curvature of the centerline is known by way of the segmentation, the stretching produced by the instrument can be computed and consequently e.g. the displacement of a branch vessel from its actual location computed, and the overlay can then be adapted accordingly, as shown for example in  FIGS. 6   b  and  6   c.          

     Optionally, the following embodiment variants are conceivable. The information relating to the course of the vessel (in 2D or in 3D)
         can also be defined manually by the user (e.g. by means of a marker),   can also be given by means of a mathematical description, e.g. a higher-order polynomial or another suitable function. The overlay can then be adapted e.g. by way of the updating of the function parameters according to the position of the detected instrument.       

     Optionally or alternatively, the position of the introduced instrument can be
         defined manually by the user,   determined via a position-transmitting sensor,   determined or reconstructed three-dimensionally with the aid of two or more X-ray images from a number of angles.       

     It is advantageous that not just one, but a plurality of instruments are detected or tracked. This enables e.g. other stationary instruments (e.g. guide wires introduced into the renal arteries) to be identified and tracked as well in order to ensure the consistency of the overlay at a plurality of points. 
     It is also conceivable for the method to be applied in other interventional procedures that benefit from the overlaying of preferably pre-segmented reference images. The replacement of aortic valves, interventions in coronary blood vessels, etc. are conceivable as other interventional procedures. 
     LIST OF REFERENCE SIGNS 
     
         
         A Abdominal aortic aneurysm 
         S Stent graft 
         K Catheter