Abstract:
In a method and system for dual rendering of images, a first volume image is reconstructed with a computer. A second volume image is reconstructed with the computer. The first reconstructed volume image is adjusted for a desired rendering. The second volume image is adjusted for desired rendering. A dual rendering of the first and second volume images is displayed where one of the volume images can be seen through the other volume image. Also, a simultaneous generation of different types of reconstructed data sets out of a single acquisition run for those data sets corresponding to the dual rendered images is provided for. The diagnostic questions is shown in FIG.  5.

Description:
BACKGROUND 
     The 3-D reconstruction of volume data from 2-D x-ray projections has become important in many fields. In the medical imaging field, the reconstruction of vessel structures is especially helpful for treating aneurysms or stenoses in interventional angiography, for example. 
     A brief explanation of 3-D reconstruction of volume data from 2-D x-ray projections can be better understood by reference to prior art  FIG. 1 . In the x-ray process, a real-world object (the cube  10 ) is x-rayed. 2-D x-ray projection images are generated. On a computer, these projections are made up of discrete “pixels.” For the reconstruction process, from those 2-D x-ray projections a 3-D data volume (the cube  10 , which is now a computer object) is calculated. On a computer, this data cube is discrete, made up of “voxels”. In the volume rendering process, the data volume is viewed on a 2-D computer monitor, i.e. one generates 2-D views  12  of the 3-D data volume  10 . 
     The volume rendering process takes into account the following attributes which the voxels have: 
     1. Most important: the gray value (−CT value−Hounsfield value). Something that should be related to the original x-ray absorption of the object at this very spot. 
     2. Different gray values contribute according to the transfer function. Voxels having gray values where the transfer function is zero do not contribute to the rendering. 
     3. Opacity: Example—If there is a voxel with 100% opacity one will not see what is behind. The other way around: if the opacity is low, one can generate a glass-like impression. 
     4. Light effects, which are modeled with virtual light sources and are based on gray value gradients, etc. 
     Prior art  FIG. 2  shows a viewing ray for ray casting through volume/object  10 . The prior art algorithm for ray casting using a single volume is as follows: 
     for each image pixel ρ
         initial color of pixel to black: c ρ :=0   for each voxel v ρi  belonging to pixel ρ
           calculate the color value c(v ρi ) originating from voxel v ρi      determining opacity o(v ρi ) within a range 0 . . . 1
 
 c   ρ   :=c   ρ (1− o ( v   ρi ))+ c ( v   ρi ) o ( v   ρi ))
   
           end       

     end 
     For reconstructing vessel structures many x-ray images are taken from different positions around the object (e.g. a patient&#39;s head) and in two phases. During the first phase no contrast agent is applied whereas in the second phase contrast agent is used for making the vessels visible. The images from the first run (without contrast agent) are called mask images; and the images of the second run (with contrast agent being injected) are called fill images. The corresponding mask reconstruction for the mask images are shown in prior art  FIG. 3  and the corresponding fill reconstruction for the fill images is shown in prior art  FIG. 4 . 
     Four possible ways of reconstructing images using the Siemens, AG prior art product (InSpace 3D, DynaCT), are:
         From these images a volume data set can be reconstructed using only mask images. The resulting 3-D volume will show both bone  13  and metal implant structures  9  (e.g. coils in aneurysms, stents, wires). That reconstruction is called “native mask” (See 2-D  FIG. 3 ).   From these images a volume data set can be reconstructed using only fill images. The resulting 3-D volume will show both bone  13 , metal structures  9 , and vessels  14 . That reconstruction is called “native fill” (See 2-D  FIG. 4 ).   A third possibility is to subtract the information of fill and mask images. In this case only the vessel structures  14  are visible. That reconstruction is called “subtracted” (See 2-D  FIG. 5 ).   If the slice thickness is increased and a softer reconstruction kernel (a mathematical formula to transform data such as for different spatial resolution or contrast) is used, reconstructions with a better low contrast resolution can be achieved at the expense of spatial resolution. This type of reconstruction is usually only applied from a mask run in a native way, and is called “enhanced” (See 2-D  FIG. 6  showing enhanced bone  15 ). The enhanced reconstruction is available with the Siemens AG product Dyna CT. This reconstruction sacrifices resolution but gains gray value resolution. This enhances seeing a low contrast object like a hemorrhage.       

     When calculating the subtracted reconstruction described above as the third possibility, the method is done in three phases in the prior art. First, the reconstruction from the mask images is done. Secondly, the fill images is done. The reconstruction from the mask images is shown by the native mask reconstruction mentioned above in  FIG. 3 . The reconstruction from the fill images is shown by the native fill reconstruction explained above in  FIG. 4 . Thirdly, the subtracted reconstruction is finally computed by subtracting the native mask from the native fill reconstruction as shown in  FIG. 5 . 
     The reconstruction from the mask images as well as the reconstruction from the fill images in the prior art are stored as separate 3-D data sets for later use during visualization. The final subtracted result also is stored as a 3-D data set. 
     In the prior art the enhanced reconstruction is computed either with a modified reconstruction kernel starting from the same projection images as for the other reconstructions or by applying appropriate filter operations on the previously computed native reconstruction, as described above. 
     In the prior art, with the storage of all these data sets there exist several possibilities for visualization, which show vessel, bone, and low-contrast structures separately or in combination:
         The subtracted reconstruction is used for hiding occluding bones ( FIG. 5 );   The fill reconstruction is used to show both, bones and vessels ( FIG. 4 );   The mask reconstruction is used to show bones and other objects like coils and stents ( FIG. 3 ); and   The enhanced reconstruction is used to show low-contrast regions like tumors or bleedings ( FIG. 6 ).       

     For visualization, the 3-D data sets must be loaded into an application (InSpace 3D, Dyna CT) that is capable of producing 2-D projections or sections of the 3-D volume (e.g. Volume Rendering Technique (VRT), a way to visualize medical volume data, Maximum Intensity Projection (MIP)—good for showing bones, Multi-planar Reformatting (MPR) used for visualizing arbitrary plane slices through a cube in any direction, etc.). If the user is interested in vessel structures, he usually visualizes a subtracted reconstruction which has the advantage that no bone structures hide vessels. However, a big disadvantage is that the missing bone structures prevent from providing orientation help. If the user is interested in bone structures the user visualizes the native reconstruction. If the user is interested in tumors, bleedings or other low-contrast objects, he uses the enhanced reconstruction for visualization. 
     A problem in the above prior art is the simultaneous usage and visualization of the information contained in these data sets. Prior art techniques are known for attempting to solve this problem. For visualizing both bone and vessel structures the user previously had two possibilities which also can be combined:
         The user edits the volume such that bone structures that would hide vessel structures are removed manually. This approach is very time consuming and requires a lot of experience.   The user tries to separate the bones from the vessels by applying different visualization parameters (e.g. color, transparency) to different ranges of x-ray density (or gray values). This approach is also very time consuming, and usually no clear separation is possible as the density values of bones and vessels overlap.       

     In many cases the above approaches do not lead to a satisfactory result because either bony structures are spatially very close to vessel structures or the density of vessels does not differ much from the density of bones. 
     The visualization of enhanced reconstructions has not been used in the prior art in combination with native or subtracted reconstructions. 
     It is also known to invoke each type of reconstruction separately. It was not previously known to simultaneously generate different types of reconstructed data sets out of a single acquisition run. 
     SUMMARY 
     In a method and system for dual rendering of images, a first volume image is reconstructed with a computer. A second volume image is reconstructed with the computer. The first reconstructed volume image is adjusted for a desired rendering. The second volume image is adjusted for desired rendering. A dual rendering of the first and second volume images is displayed where one of the volume images can be seen through the other volume image. Also, a simultaneous generation of different types of reconstructed data sets out of a single acquisition run where those data sets correspond to the dual rendered images is provided for. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a chart of techniques explaining 3-D reconstruction of volume data from 2-D x-ray projections with respect to a volume/object; 
         FIG. 2  shows a viewing ray for ray casting through the volume/object  10  of  FIG. 1  for explaining an algorithm for ray casting; 
         FIG. 3  shows a native reconstruction image from a mask run-bones and three coil baskets in aneurysms are visible-vessels are not visible; 
         FIG. 4  shows a native reconstruction image from a fill run-bones and three coil baskets (or at least one of them) in aneurysms are visible-vessels are also visible; 
         FIG. 5  shows a subtracted reconstruction image from the mask and the fill run-only vessels are visible; 
         FIG. 6  shows an image of an enhanced data set; 
         FIG. 7  shows, according to the preferred embodiment, a dual volume rendering image display of native mask (bones and metal) and sub-volumes (vessels) where bones are more faded in with a percentage slider; 
         FIG. 8  shows, according to the preferred embodiment, a dual volume rendering image display of native mask (bones and metal) and sub-volumes (vessels) where bones are more faded out with the percentage slider; 
         FIG. 9  shows, according to the preferred embodiment, a dual volume display of a sub-volume (vessels) and of an enhanced contrast data set (MPR display); and 
         FIG. 10  shows a flowchart of a method of a preferred embodiment for a dual rendering of images. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and/or method, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates. 
     With the improvement of the dual rendering of the preferred embodiment, the following additional visualizations are possible:
         As shown in  FIGS. 7 and 8 , the subtracted reconstruction is used in combination with the mask reconstruction. In this case for each data set another viewing setting is chosen (e.g. different color and different structures are covered in different data sets. By interactively modifying the transparency of the mask reconstruction with the fading tool  16  with slider  16 A, bone structures  13 A ( FIG. 7) and 13B  ( FIG. 8 ) are faded in or out in real-time relative to vessels  14 . Alternatively, reconstruction is used to see bones, coils, and vessels. Then the mask reconstruction may be subtracted in order to remove bones and/or coils by   The enhanced reconstruction ( FIG. 6 ) may be used in combination with the subtracted reconstruction as shown in  FIG. 5 , e.g. for showing the position of tumors or bleedings in relation to vessel structures. The vessel structures  14  can be visualized as VRT or MIP and the enhanced reconstruction is overlaid as described above or as an MPR plane  15 A as shown in  FIG. 9 .   If the user is interested in an exact reconstruction of bones and also in low-contrast regions the user may simultaneously visualize the native with the enhanced reconstruction. For visualization the method mentioned in the last item can be applied again.       

     With the preferred embodiment, during one reconstruction process three data sets can be generated, be saved to the database, and be loaded into the dual (multiple) volume rendering application. 
     Thus with the preferred embodiment, first, the simultaneous generation of different types of reconstructed data sets out of a single acquisition run occurs. Second, the simultaneous rendering of two (or more) of these data sets, on top of each other, but with different rendering settings, is provided for. 
     The dual rendering of the preferred embodiment allows the user to retrieve several types of information from a single acquisition run. One has the advantage of being able to display image information, which is shown in four volume data sets that are generated separately (native mask, native fill, subtracted, and enhanced reconstruction as in the prior art). These volumes are displayed as described in prior art  FIGS. 3 ,  4 ,  5 , and  6 . With the preferred embodiment, however, a user can also display different information parts simultaneously with different colors and with different transparencies overlaying two visualizations ( FIGS. 7 ,  8 , and  9 ). A modification of the color and transparency values allows the user to show or hide parts of the 3-D data sets dynamically without big editing efforts as in the prior art. Finally, in  FIG. 9 , there is an example for a dual rendering with an enhanced reconstruction (sub and enhanced). Also here it is preferable to select the type and the rendering settings for both data sets separately. 
     The user gets more information to orientate himself via anatomic and topological details and he can easily hide parts he is not interested in. 
     The concept of dual rendering involves: 
     1. Two cubes. 
     2. For each cube the settings i), ii) . . . etc. may be completely different. 
     3. On the monitor one shows an image that is generated by sending virtual rays through cube  1  with first set of settings AND cube  2  with a second set of settings simultaneously. This is more difficult than just adding two 2-D views. Example: Opacity of cube  1  may make it impossible to see the contributions of cube  2  (example: the bones cover the vessels). 
     As an analogy the following is helpful. The computation of a glass animal sitting in a glass house would also be a dual volume rendering. 
     The extended algorithm for ray casting using multiple volumes (dual rendering) is as follows: 
     for each image pixel ρ
         set initial color of pixel to black: c ρ :=0   for each voxel v ρi  belonging to pixel ρ
           for each volume j
               calculate the color value c(v ρi , j) originating from voxel v ρi  in volume j   determine opacity o(v ρ,J ) within a range 0 . . . 1   
               end   for all volumes j in ascending order of opacities o(v ρi , j)
 
 c   ρ   :=cp (1− o ( v   ρi   ,j ))+ c ( v   ρi   ,j ) o ( v   ρi   ,j ))
   end   
           end       

     end 
     As shown in  FIGS. 7 and 8 , with the fading (blending) tool  16  with slider  16 A, a dual-volume property&#39;s window is provided by which image a and image b may be volume blended (faded) such as in a percentage from 0 to 100 percent with slider  16 A (0 is in the middle with a positive percent to the right and a negative percent to the left). Thus in  FIG. 7 , the bone  13 A relative to the vessels  14  has a stronger appearance whereas in  FIG. 8  the bone  13 B has a lesser appearance compared to the vessels  14 . 
     In  FIG. 9 , a dual volume display is provided with an enhanced contrast data set for the bone portion  15  taken from  FIG. 6 . 
     In summary, with the dual rendering of the preferred embodiment, in the first example shown in  FIGS. 7 and 8 , the native mask ( FIG. 3 ) as a first reconstruction and the fill mask ( FIG. 4 ) as a second reconstruction are provided. Those two volumes are blended with the appropriate fading being chosen between the two images. 
     As a second example, the first reconstruction could be enhanced, and the second reconstruction could be subtracted. 
     In a third example, the first reconstruction could be native fill and the second reconstruction can be subtracted. 
     In other words, for the four different types of reconstruction, any two of them can be combined for the dual volume rendering. 
       FIG. 10  summarizes the method steps involved. In the first step  17 , volume  1  is reconstructed. In a second step  18  volume  2  is reconstructed. As described previously, a simultaneous generation of different types of reconstructed data sets out of a single acquisition run may be employed. Thus steps one and two may be simultaneous. In third step  19  both volumes are loaded into the volume rendering application in the computer. In fifth step  20 , adjustments are made for a desired (pleasing) rendering of volume  1 . In fifth step  21 , adjustments are made for a desired (pleasing) rendering of volume  2 . In sixth step  22 , viewing on the computer screen is switched over to dual rendering. In seventh step  23 , as described above with reference to window  16 , the fading percent is selected for the dual rendering. The seventh step determines an extent to which one volume can be seen through the other volume. 
     While a preferred embodiment has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention both now or in the future are desired to be protected.