Abstract:
Method for pre-interventional planning of a 2D fluoroscopy projection for an interventional entry using a fixed instrument, comprising the following steps:
   a) Recording a 3D data set,   b) Planning the intervention,   c) Planning the optimum projection direction,   d) Registering the 3D data set with a navigation system and a 2D fluoroscopy system,   e) Transmission of the intervention data to the navigation system,   f) Computing the position of the fluoroscopy system, and   g) Executing the interventional entry under fluoroscopy.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of German application No. 10 2005 032 523.8 filed Jul. 12, 2005, which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a method for pre-interventional planning of a 2D fluoroscopy projection for an interventional entry using a fixed instrument. 
     BACKGROUND OF THE INVENTION 
     The process of planning interventions with a fixed instrument, for example an aspiration needle, based on pre-interventionally recorded data sets, is already known. These data sets can be obtained by 3D angiography, CT or MR investigation. The planning undertaken in advance can then be transferred to a navigation system or a system for orientation of the fixed instrument or the aspiration needle. Such a system is for example a stereotactic frame, which allows pinpoint alignment and positioning of the aspiration needle. The case can arise however in which the aspiration needle or another fixed instrument has to pass within a very short distance of critical structures such as vertebrae, organs or blood vessels. In order not to endanger the patient a continuous monitoring of the position of the needle is necessary. The direction of projection must be defined within the context of pre-interventional planning. The direction of projection, which can be imagined as a straight line through the tube focus and the center of the detector of the fluoroscopy system, is set by corresponding positioning of the C-arm of the fluoroscopy device. This setting is made manually, based on the experience of the operator, with a number of steps sometimes being required. The disadvantage in this case however is that the choice of the best direction of projection and the setting of the C-arm position and the multi-stage optimization which may be required are undertaken during fluoroscopy so that the patient is subjected to a certain dose of radiation even before the actual intervention. 
     SUMMARY OF THE INVENTION 
     The problem underlying the invention is thus that of specifying a method of the type mentioned at the start in which the radiation dose affecting the patient is reduced and the time required for the procedure is reduced. 
     To resolve this problem a method for pre-interventional planning of a 2D fluoroscopy projection for an interventional entry using a fixed instrument with the features of the claims is provided. 
     With the inventive method a 3D data set is first recorded. This can usefully be done using computer tomography, magnetic resonance tomography or 3D angiography. This is followed in the inventive method by the planning of the intervention. This planning can include the definition of the target area for the instrument, the definition of the path and the entry point. When the planning of the instrument channel is completed, the optimum direction of projection is planned. Depending on the complexity of the intervention and of the access, a number of optimum directions of projection can also be defined, between which a switch is made during the intervention. The significant aspect here is that the planning of the optimum direction of projection is undertaken without subjecting the patient to extra radiation. Within the context of this planning the best possible visualizations of the instrument as well as of the critical structure are determined. 
     Subsequently the 3D data set is registered with a navigation system and with a 2D fluoroscopy system. The navigation system can for example be a stereotactic frame which allows a precise application of the instrument. In the next step the data of the planned direction of projection is transmitted to the navigation system. On the basis of this intervention data the required position of the fluoroscopy system is calculated. This definition of the position of the fluoroscopy system is undertaken automatically on the basis of the intervention data, accordingly the step-by-step adjustment for the optimum position for the fluoroscopy usual in the prior art is not necessary. 
     If both the navigation system and also the fluoroscopy system are correctly set, the intervention with the Instrument under fluoroscopy in the previously planned angulation can be started. With the inventive method there can be provision for the fluoroscopy system to feature a C-arm of which the position is calculated and set automatically. 
     To visualize critical structures such as vessels, organs and vertebrae, the projection of the 3D data set can be presented overlaid with the 2D fluoroscopy image. This is especially useful if the investigation area has low contrast. In these cases a manual optimization of the projection under fluoroscopy can only be achieved with great difficulty. 
     With the inventive method the position of the instrument can be checked using computer tomography, magnetic resonance tomography or 3D angiography, if necessary the method can be continued with the planning of the intervention. Thus the optimum direction of projection and setting of the position of the fluoroscopy system is planned once again. 
     The inventive method is especially suitable for fixed instruments such as a biopsy needle, an aspiration needle or an ablation device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and details of the invention will be explained on the basis of an exemplary embodiment which refers to the figures. The figures are schematic diagrams and show: 
         FIG. 1  a flowchart with the most important steps of the method in accordance with the invention; 
         FIG. 2  a planned needle channel running alongside a critical structure; 
         FIG. 3  the diagram shown in  FIG. 2  with the optimum projection directions indicated; 
         FIG. 4  two projections of a critical structure; 
         FIG. 5  a graphical presentation of the angulation and the associated needle depth of the projections of  FIG. 4 ; and 
         FIG. 6  a graphical presentation of the angulation and the needle depth with the tolerance range indicated. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The method  1  shown in  FIG. 1  in the form of a schematic flowchart for pre-interventional planning of a 2D fluoroscopy projection for an interventional entry using a fixed instrument comprises the following steps: Before the intervention a 3D data set is recorded in step  2  using a method such as 3D angiography, computer tomography or magnetic resonance tomography. Subsequently the intervention is planned  3 , in particular the projection directions of the instrument, for example an aspiration needle, are planned  4 . 
     Reference is made simultaneously to  FIG. 2 . 
       FIG. 2  shows a planned needle channel running alongside a critical structure. 
     A critical structure  11 , in the exemplary embodiment depicted, an organ of a patient, is located in the vicinity of a needle channel  12  indicated by an arrow. The structure  11  and the needle channel  12  are objects in a three-dimensional space. The task within the context of pre-interventional planning consists of finding an optimum direction of projection for the fluoroscopy. This direction or the associated plane which lies perpendicular to the direction of projection, is defined by the best visualization of the critical structure  11  in relation to the needle channel  12  and allows an especially secure monitoring of the needle movement during the intervention. 
     The optimum direction of projection is defined and planned interactively by the user. The optimum direction of projection is a straight line which is perpendicular to the needle channel  12 .  FIG. 2  shows two typical projection directions  13 ,  14 . The user can move away from these projection directions which are determined from the recorded 3D data and visualized on a screen, and selects an optimum visualization. Projections which cannot be set on the C-arm of the fluoroscopy device or projections at an extreme angle are not presented or a warning is displayed. In this case the user can manually deviate from the optimum values. 
     Again referring to  FIG. 1 , in the next step a registration  5  of the 2D fluoroscopy system with 3D data set is performed in order to match the coordinate systems to each other. Provided the patient is located in the same spatial position as during the intervention when the 3D data set is recorded, the registration can be dispensed with if necessary. In this case the coordinate systems coincide at least approximately. 
     If the 3D data set has been recorded on the same C-arm by means of 3D rotation angiography, the required registration of the patient with the 3D data set and the fluoroscopy system is automatically provided by the device calibration. 
     Where 3D data from other sources such as magnetic resonance tomography or computer tomography is used, automatic or interactive image registration methods can be used. It is also possible, to initially register an external data set with 3D/3D registration with the 3D rotation angiography data set, in order to implicitly establish the registration with the fluoroscopy system in this way. 
     In the next step shown in  FIG. 1  the defined intervention data  6  is transmitted to a navigation system. In this exemplary embodiment the navigation system is a stereotactic frame. Subsequently the data determined within the context of the projection planning  7  previously undertaken is transmitted to the fluoroscopy system. This enables the required position of the C-arm to be determined, so that the critical structure  11  and the needle can be seen on the fluoroscopy image in the optimum direction of projection. This position can be reached manually by restricting the movement options of the C-arm which are entered via the user Interface. Through this restriction the C-arm can only to be moved up to the optimum position direction and is then stopped. The fact that the optimum position has been reached is indicated by an optical or audible signal. The unit can however also be moved to the optimum position automatically. This automation is only intended to assist the user in reaching the optimum position, but it is however possible at any time to depart from the predetermined settings and move the C-arm as required. 
     If the optimum direction of projection is to be determined automatically, the normal to the surface must be determined on a plane which is produced by the needle channel  12  and the point on the critical structure closest to the needle  11 . This plane  15  is shown in  FIG. 3 . At right angles to this plane  15  are two projection directions  16 ,  17 , which are aligned opposite to each other and which both produce an optimum presentation. Since the projection directions  16 ,  17  are normals on the plane  15  they can be obtained by defining the plane  15  by the shortest connection between the critical structure  11  and the needle channel  12 . As shown in  FIG. 3 , two possible projection directions  16 ,  17  are produced of which one is selected manually or automatically. If a technically impossible or critical angulation is present, a warning is displayed at the user interface. 
     Optionally the selection of the optimum projection direction can be undertaken on the basis of minimum x-ray absorption. The x-ray absorption is determined from the 3D angiography, the CT or the MR investigation and the corresponding 3D data set. This criterion in particular excludes projections at extreme angles or unfavorable projections because of bone structures or at least identifies them as such, so that the user can change the planning. 
     If a structure with low contrast is involved, as is shown in  FIG. 1 , after the beginning of the intervention  8  an overlaying  9  of the projection from the 3D recording onto the 2D fluoroscopy image can be undertaken. 
       FIG. 4  shows two projections of a critical structure. 
     The critical structures can be determined automatically. After a critical distance to the needle channel  12  has been specified, with the distance amounting to 5 mm for example, the structures are determined automatically by a computer for which this critical distance is undershot and these are displayed to the user. The user can then decide for himself whether the structure is critical or not. Before the beginning of the intervention the user can create a first fluoroscopy image to ensure that the critical structure with the virtually displayed planned needle channel and where necessary the target structure for the needle can be seen as well as possible. If equally valid needle channels are present, the planning of the intervention can be optimized in order to define the optimum direction of projection or the optimum directions of projection. Following a position check  10  an update of the planning and thereby of the optimum direction of projection can be undertaken. In the flowchart shown in  FIG. 1  this process is represented by the branch from the position check  10  back to the planning  3  of the intervention. The optimized direction of projection is passed on to the fluoroscopy system, the C-arm is moved accordingly and the intervention can be continued. 
     The optimum projection can change during the course of the intervention at different penetration depths of the needle and with different critical structures which are passed. The critical points in the two projections are indicated in this case by “x” or “y”. Referring to the diagram shown in  FIG. 4 , it can be seen that with specific structure geometries, two or more projection directions are necessary. In advance of the intervention the relevant direction of projection can be correlated with the depth of penetration of the needle. This is shown schematically in  FIG. 5  which shows the needle depth on the horizontal axis and the angulation of the C-arm of the fluoroscopy device on the vertical axis. The information about the depth of penetration of the needle from the navigation system is used, to refer the user to the depth-dependent optimum projection plane, so that this optimum projection plane can be adapted automatically or semi-automatically. This can be done interactively on the C-arm, with the C-arm only rotating around the needle channel and the rest being blocked. The left-hand section of  FIG. 5  corresponds to the left-hand projection shown in  FIG. 4 . The angle of angulation initially remains constant as the needle depth increases, until a limit value is reached and a switch is made to another angle of angulation, which is assigned to the projection shown on the right in  FIG. 4 . 
     The needle tip position defined, measured or calculated by the navigation system can be used to update the optimum direction of projection. Alternatively the current needle tip position can be determined manually, by clicking on it with the mouse for example, or through automatic detection from images which have been recorded in two different directions of projection. 
     Taking into account critical structures and where necessary further criteria, varying optimum projection directions are produced along the needle channel depending on the current needle tip position. In an extreme case the direction can be changing constantly during navigation through a network of vessels, but this is time-consuming and inconvenient. In this case an attempt is made to keep the change of the angulation of the C-arm as small as possible during the advance, this value can for example be quantified as an overall angle covered. Simultaneously during the advance a specific degree of sub-optimality of the projections can be allowed, with such a value for example being able to be set as ±20°. 
       FIG. 6 , in a similar way to  FIG. 5 , shows a graphical presentation of the angulation over the needle depth. The dashed line specifies the averaged angulation  18  which is surrounded by a tolerance range with a lower and an upper limit value  19 ,  20 . An iterative optimization algorithm then finds the optimum path, with the criteria “overall change of angle” and “optimality of the projection” being able to be weighted separately. In this way a compromise between perfect angulation and frequent angulation changes can be achieved. This restriction in the changes of angulation makes operation easier for the user, since few spatial reorientations are required, in addition there is a saving in contrast media for vessels and the movement time of the C-arm is reduced.